functionalization of spherical carbons with metal complexes and … · 2016-01-13 · the role of...

Functionalization of Spherical Carbons

with Metal Complexes and Ionic Liquids

for Application in Catalysis and Gas Purification

Funktionalisierung sphärischer Aktivkohlen

mit Metallkomplexen und ionischen Flüssigkeiten

zur Anwendung in Katalyse und Gasreinigung

Der Technischen Fakultät

der Friedrich-Alexander-Universität

Erlangen-Nürnberg

zur

Erlangung des Doktorgrades Dr.-Ing.

vorgelegt von

Heiko Klefer

aus Jever

Als Dissertation genehmigt

von der Technischen Fakultät

der Friedrich-Alexander-Universität Erlangen-Nürnberg.

Tag der mündlichen Prüfung: 11. Dezember 2015

Vorsitzender des Promotionsorgans: Prof. Dr. Peter Greil

Gutachter: Prof. Dr. Peter Wasserscheid

Prof. Dr. Martin Hartmann

Vorwort

Das Vorwort ist in der elektronischen Version der Dissertation nicht verfügbar.

Publications

Parts of this work have been previously published in the following publications listed.

Patent

• B. Böhringer, S. Fichtner, C. Schrage, J.-M. Giebelhausen, P.Wasserscheid, B.J.M. Etzold

andH. Klefer. “Katalysatorsystem und dessen Verwendung”. DE Patent 102,014,103,351

A1. 2014

Publication

• H. Klefer, M. Munoz, A. Modrow, B. Böhringer, P. Wasserscheid and B.J.M. Etzold.

“Polymer-based spherical activated carbon as easy-to-handle catalyst support for hy-

drogenation reactions”. Chem. Eng. Technol. 2015

i

Contents

1. Introduction 1

2. Theoretical and technical background 62.1. Activated carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1.1. Activated carbon materials . . . . . . . . . . . . . . . . . . . . . . . . 6

2.1.2. Polymer-based activated carbon particulates . . . . . . . . . . . . . . 9

2.2. Activated carbon as catalyst support . . . . . . . . . . . . . . . . . . . . . . . 15

2.2.1. Activated carbon support materials in catalysis . . . . . . . . . . . . 15

2.2.2. Mass transport in catalysis - aspects of di�usion . . . . . . . . . . . . 16

2.2.3. High-purity polymer-based activated carbon particulates . . . . . . . 20

2.2.4. Post-treatment of catalyst - catalyst separation . . . . . . . . . . . . 22

2.2.5. Carbon surface functionalization . . . . . . . . . . . . . . . . . . . . 23

2.2.5.1. Surface oxidation . . . . . . . . . . . . . . . . . . . . . . . 24

2.2.5.2. Deposition of noble metal . . . . . . . . . . . . . . . . . . 28

2.2.5.3. The role of metal precursors in catalyst preparation . . . . 33

2.2.5.4. The role of C-π sites in catalyst preparation . . . . . . . . 34

2.2.6. Reactions studied in this work . . . . . . . . . . . . . . . . . . . . . . 35

2.3. Activated carbon in gas puri�cation . . . . . . . . . . . . . . . . . . . . . . . 40

2.3.1. Activated carbon adsorbents in gas puri�cation . . . . . . . . . . . . 40

2.3.2. Functionalization of activated carbon with reactive surfaces . . . . . 44

2.3.3. Application of activated carbon adsorbents in gas puri�cation . . . . 47

2.3.3.1. Air �ltration in clean rooms and for personal protection . 47

2.3.3.2. Industrial exhaust gas cleaning . . . . . . . . . . . . . . . . 49

2.3.4. Functionalization of activated carbon with ionic liquids . . . . . . . 50

2.3.4.1. Supported ionic liquid phase (SILP) concept . . . . . . . . 50

2.3.4.2. Properties of ionic liquids . . . . . . . . . . . . . . . . . . . 52

2.3.4.3. Reactive metal complexes dissolved in ionic liquids . . . . 53

ii

Contents

2.3.5. Application of SILP adsorbents in gas puri�cation . . . . . . . . . . . 53

2.3.5.1. SILP adsorbents based on inorganic supports [9, 10] . . . . 53

2.3.5.2. SILP adsorbents based on spherical carbon . . . . . . . . . 55

2.3.6. Gases studied in this work . . . . . . . . . . . . . . . . . . . . . . . . 58

3. Objective of this work 62

4. Experimental 664.1. Catalyst preparation and characterization . . . . . . . . . . . . . . . . . . . . 66

4.1.1. Oxidation of spherical carbon . . . . . . . . . . . . . . . . . . . . . . 66

4.1.2. Deposition of noble metal . . . . . . . . . . . . . . . . . . . . . . . . 66

4.1.3. Evaluation of catalytic activities . . . . . . . . . . . . . . . . . . . . . 67

4.1.4. Evaluation of �ltration rates . . . . . . . . . . . . . . . . . . . . . . . 69

4.2. SILP material preparation and characterization . . . . . . . . . . . . . . . . . 70

4.2.1. Preparation of SILP materials . . . . . . . . . . . . . . . . . . . . . . 70

4.2.2. Evaluation of SILP corrosivity . . . . . . . . . . . . . . . . . . . . . . 71

4.2.3. Evaluation of gas puri�cation performance . . . . . . . . . . . . . . 71

4.3. Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

4.3.1. Overview of applied spherical carbon materials . . . . . . . . . . . . 73

4.3.2. Synthesis of metal salts and ionic liquids . . . . . . . . . . . . . . . . 75

4.4. Analytical methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

5. Results and discussion 795.1. Filtration and pressure drop of spherical carbon . . . . . . . . . . . . . . . . 79

5.1.1. Spherical carbon in lab-scale �ltration . . . . . . . . . . . . . . . . . 79

5.1.2. Spherical carbon in large-scale �ltration . . . . . . . . . . . . . . . . 81

5.1.3. Spherical carbon in �ow chemistry applications . . . . . . . . . . . . 82

5.2. Spherical carbon as novel catalyst support material . . . . . . . . . . . . . . 84

5.2.1. Carbon surface functionalization . . . . . . . . . . . . . . . . . . . . 84

5.2.1.1. In�uences on the amount of functional surface groups . . 84

5.2.1.2. In�uences on the chemical composition of functional sur-

face groups . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

5.2.1.3. In�uences on spherical carbon acidity . . . . . . . . . . . . 90

5.2.1.4. In�uences on carbon surface wettability . . . . . . . . . . 94

5.2.1.5. In�uences on carbon pore structure . . . . . . . . . . . . . 95

iii

Contents

5.2.1.6. Important di�erences between nitric acid and sulfuric acid

oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

5.2.2. Active metal deposition . . . . . . . . . . . . . . . . . . . . . . . . . 99

5.2.2.1. Palladium: In�uences on metal loading and dispersion . . 99

5.2.2.2. Ruthenium: In�uences on metal loading and dispersion . . 107

5.2.2.3. Platinum: In�uences on metal loading and dispersion . . . 109

5.2.3. Catalytic performance . . . . . . . . . . . . . . . . . . . . . . . . . . 114

5.2.3.1. Palladium catalysts: In�uences on catalytic activity . . . . 114

5.2.3.2. Ruthenium catalysts: In�uences on catalytic activity . . . 124

5.2.3.3. Platinum catalysts: In�uences on catalytic activity . . . . . 129

5.2.4. The role of C-π sites . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

5.2.5. Stability tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142

5.2.5.1. Catalyst leaching . . . . . . . . . . . . . . . . . . . . . . . 142

5.2.5.2. Catalyst recycling . . . . . . . . . . . . . . . . . . . . . . . 143

5.2.6. Proof of concept: SILP catalyst in slurry phase reaction . . . . . . . . 146

5.3. Spherical carbon as support for SILP �lter materials . . . . . . . . . . . . . . 149

5.3.1. Preliminary investigations for the advancement of SILP technology

in gas puri�cation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

5.3.1.1. In�uence of impregnation solvent on SILP product quality 149

5.3.1.2. In�uence of metal salt species on ammonia and hydrogen

sul�de adsorption . . . . . . . . . . . . . . . . . . . . . . . 149

5.3.1.3. Corrosion investigations of halide-containing SILP materials 151

5.3.2. Filter material improvements for irreversible adsorption of ammonia

and other hazardous gases . . . . . . . . . . . . . . . . . . . . . . . . 152

5.3.2.1. Development of halide-free SILP materials . . . . . . . . . 152

5.3.2.2. Alternative coating techniques of spherical carbon . . . . 159

5.3.2.3. Organic copper salts in SILP materials . . . . . . . . . . . 161

5.3.2.4. Development of a �lter material for formaldehyde removal 163

5.3.3. Design of SILP materials for pressure and temperature swing adsorp-

tion processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

5.3.3.1. Reversible adsorption of ammonia . . . . . . . . . . . . . . 166

5.3.3.2. Reversible adsorption of hydrogen sul�de . . . . . . . . . 169

iv

Contents

6. Summary and Outlook 1766.1. Summary / Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176

6.2. Zusammenfassung / Abstrakt . . . . . . . . . . . . . . . . . . . . . . . . . . 183

6.3. Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190

A. Lists of chemical substances and suppliers II

B. Carbon surface functionalization VIB.1. In�uences on the amount of functional surface groups . . . . . . . . . . . . VI

B.2. In�uences on the chemical composition of functional surface groups . . . . VII

B.3. In�uences on spherical carbon acidity . . . . . . . . . . . . . . . . . . . . . . VIII

B.4. In�uences on the pore structure of spherical carbon . . . . . . . . . . . . . . IX

C. Catalytic experiments XC.1. Catalytic experiments concerning carbon pore structure . . . . . . . . . . . X

C.2. Catalytic experiments concerning carbon surface functionalization . . . . . XII

D. List of Algorithms XIV

E. List of Figures XV

F. List of Tables XX

References XXV

v

1. Introduction

Activated carbon is a highly porous, carbonaceous substance with a characteristically large

internal surface area. Society bene�ts from the adsorptive properties of activated carbon for

more than 3500 years. In medicine, for instance, a variety of poisonous substances are ad-

sorbed by activated carbon. Adsorption to activated carbon is technically applied in product

puri�cation by the food, chemical, pharmaceutical and hydrometallurgy industries, as well

as in water treatment and air puri�cation.[1–3] Furthermore, modi�ed activated carbon ma-

terials �nd use in catalysis, sensors, batteries, fuel cells, capacitors, hydrogen storage, and

magnetic materials.[4]

Polymer-based spherical activated carbon (PBSAC, spherical carbon) is an advanced ma-

terial with combined features derived from its polymer origin, spherical shape, and car-

bonaceous nature. The material is industrially produced on a large scale. Spherical poly-

styrene-divinylbenzene precursor material from a suspension polymerization process is clas-

si�ed into the desired particle size distribution by sieving. After the polymer is thermally

stabilized by a sulfuric acid treatment, a carbonization step follows at elevated temperatures.

A �nal steam and carbon dioxide activation process produces the spherical activated carbon

material.[5, 6]

The pore size distribution of spherical carbons is tunable in a broad range. Total pore vol-

umes up to 1.3 cm3g-1and BET surface areas as large as 1946 m

2g-1are realized. Due to

its particulate character, spherical carbon is easy to handle and exhibits low pressure drops

in �ow applications. The constant chemical composition of the polymer precursor improves

production reproducibility and results in high purity carbonwith very low ash content. Thus,

high mechanical stability is achieved. The large internal surface area and good �ow char-

acteristics render spherical carbon a very suitable �lter material for organic molecules and

metal ions.[5, 7]

Due to its carbonaceous nature, spherical carbon features a simple disposal of spent material,

resistance to high temperatures [8], chemical inertness [1], and hydrolysis stability [4]. Nev-

ertheless, utilization of conventional activated carbon is economically more feasible in many

1

1. Introduction

application scenarios. Spherical carbon excels in advanced processes pro�ting from most of

its outstanding features. One current example is continuous gas puri�cation for clean rooms

and personal protection.[1, 5]

In this work, two novel applications of spherical carbon are investigated, in the �elds of catal-

ysis and advanced gas puri�cation. In the �rst part of this thesis, the development of catalysts

based on metal-loaded spherical carbon is presented. The second part deals with continuous

gas puri�cation using spherical carbons coated with thin �lms of ionic liquids, so called sup-

ported ionic liquid phase (SILP) materials. In both cases, a fundamental understanding of the

carbon surface and subsequent surface modi�cations are essential for successful application

of the �nal products. The resulting materials are then screened for their performance in a

broad variety of chemical reactions and gas adsorption/desorption processes, respectively.

Spherical carbon as novel catalyst support material

In heterogeneous catalysis, catalytically active species are immobilized in a second, solid

phase and are contacted with the �uid reactant mixture at reaction conditions. The catalytic

active species interacts with the reactants, reducing required activation energies and enhanc-

ing reaction rates. The advantage of heterogeneous catalysis is the ease of separating catalyst

and product by �ltration, which allows recycling of the catalyst. The selection of support me-

dia for catalytically active species depends on various process parameters. Activated carbon

and polymer-based spherical activated carbon, in particular, are very interesting support ma-

terials for many reasons. The most relevant aspects are pointed out in the following. Here,

the bene�ts of activated carbons compared to inorganic support materials as well as spherical

carbon compared to conventional activated carbon powder are elaborated.

Advantages of activated carbon compared to inorganic supports:

• The exceptionally large surface area of activated carbon materials enables the catalyt-

ically active species to be highly dispersed and the number of principally accessible

catalytic sites to be maximized. This results in large intrinsic catalytic activities.

• Carbon materials can be functionalized, e.g. with oxygen surface groups, to facilitate

deposition of catalytically active species. So, the activated carbon surface can be tuned

to irreversibly retain the catalytically active species.

• With emerging green chemical technology, an increasing amount of sustainable plat-

form chemicals and derived compounds are catalytically produced. These reactants

are usually accompanied with traces of water. Water negatively a�ects the integrity of

many today’s catalysts, which are based on inorganicmetal oxides. So, more hydrolysis-

2

1. Introduction

stable materials need to replace conventional catalysts. Carbon supported catalysts are

the prospective alternative.

Advantages of spherical carbon compared to activated carbon powder:

• The particle size and pore structure of the spherical carbon support can be adjusted

to �t the reaction system best. Smaller particles and a hierarchically structured pore

system, for instance, enhance pore di�usion and mass transport of reactant molecules

between catalytically active sites and bulk solution.

• In the industrial production of chemical commodities, as well as in analytical chem-

istry, reaction and post processing times are both economically important. Today’s

application of powdered activated carbon catalysts in chemical reactions makes prod-

uct separation needlessly tedious. The particulate nature of spherical carbon allows for

easy catalyst �ltration and e�cient product separation. Thus, post processing times

are minimized. Catalyst recycling is possible, as well.

• In �ow chemistry applications, the low pressure drop of the spherical catalyst beds

is of great advantage. Fluids can pass the bed of spherical catalyst material with low

resistance, thus enlarging the window of operation. Additionally, spherical shape and

particulate character improve material handling during reactor (un-)loading and cata-

lyst preparation due to good and dust-free material pourability.

• The reliably high purity of polymer-based carbon evidenced by the very low ash con-

tent is ideal for pharmaceutical and �ne chemical processes. Thus, the reaction solution

isn’t contaminated with catalyst impurities. The good batch-to-batch reproducibility

of spherical carbon further strengthens reliability of the �nal catalyst material to gen-

erate the desired outcome in terms of product selectivity and catalytic activity.

• Because one spherical carbon particulate is produced from one polymer particulate,

mechanical stability is larger than that of traditionally molded carbon pellets based

on activated carbon powder. The large mechanical stability and the resulting high

integrity of the particulate material allow the application in stirred reactor setups with

high-impact �ow characteristics.

In this research work, spherical carbon materials with di�erent particle size and pore struc-

ture are functionalized with oxygen surface groups using various oxidation conditions. The

materials are then loaded with noble metals applying the principle of electrostatic adsorption

of metal ions and subsequent metal transformation. The spherical carbon surface modi�ca-

tions are characterized in detail. Catalytic activities are determined in hydrogenation test

3

1. Introduction

reactions. The dehydrogenation of an organic liquid hydrogen carrier molecule using these

PBSAC-based catalysts is investigated as technical relevant test reaction.

Spherical carbon as support for SILP filter materials

In continuous gas puri�cation, as discussed in the second part of this thesis, speci�cmolecules

are removed from gas streams passing through �lter materials. In this process, the speci�c

molecules are physically or chemically bound to adsorption sites of the �lter material. In

certain applications, such as air �ltration in clean rooms and for personal protection, the gas

stream has to be completely puri�ed from very low concentrations of hazardous molecules,

usually by irreversible reaction with the �lter material. In many industrial applications, on

the other hand, a weaker interaction of speci�c molecules with adsorption sites is desired in

order to economically regenerate the �lter material.

The engineering objectives to improve �lter materials for continuous gas puri�cation are

increasing the number of adsorption sites for speci�c molecules and tuning adsorption pro-

cesses for reversible or irreversible adsorption. The SILP technology is a novel approach

to signi�cantly improve �ltration performance. SILP materials are porous solid materials

coated with thin �lms of ionic liquids. The ionic liquids incorporate solubility and reactivity

for speci�c molecules. The di�usion of molecules into the ionic liquid volume signi�cantly

increases the number of available sorption sites compared to adsorption to a plain solid sur-

face. The strength of interaction with speci�c molecules is tuned by modifying the ionic

liquid composition allowing for both irreversible and reversible �ltration processes. With

di�erent reactive sites in the ionic liquid and at the solid support, SILP materials can retain

a broad variety of molecules utilizing di�erent reaction pathways.[9, 10]

Several distinct material properties render spherical carbon a well suitable support material

for ionic liquids in continuous gas puri�cation compared to inorganic support materials and

activated carbon powder. The majority of the following aspects have already been pointed

out to apply to catalyst support materials, as well.

Advantages of spherical carbon compared to inorganic supports:

• Due to the very large surface area of activated carbon, the gas/liquid interface area

resulting from the ionic liquid coating is similarly large. Thus, mass transfer from the

gas phase into the ionic liquid �lm is facilitated.

• The activated carbon material itself o�ers additional adsorption capacity for di�erent

hazardous gases. This adsorption capacity is currently utilized to realize SILP broad-

band �lters. Other hazardous gases are removed by the thin �lm of ionic liquid partially

4

1. Introduction

covering the carbon surface.

Advantages of spherical carbon compared to activated carbon powder:

• Spherical carbon �lter materials exhibit low pressure drops. The pressure drop isn’t af-

fected by the ionic liquid coating. Even smaller pressure drops result from open porous

foam �lter media, furnished with spherical carbon. Low pressure drops in �lter materi-

als are highly important for applications with a limited maximum pressure di�erence,

e.g. personal protection and process extensions. For economic reasons, low pressure

drops are generally desirable in industrial �ltration processes.

• The handling bene�ts of spherical carbon are signi�cant. Both, loading and unloading

of the adsorber units, as well as SILP material preparation, are simpli�ed.

In the past years, the SILP group at the Institute of Chemical Reaction Engineering in Erlan-

gen and other research groups worldwide have developed SILP �lter materials based on in-

organic support materials for a variety of applications.[11–15] The investigations focused on

the reversible and irreversible adsorption of mercury compounds, organic sulfur compounds,

sulfur dioxide, and carbon dioxide from a �uid phase. More recently, the research group in

Erlangen investigated spherical carbon as a novel support material.[16, 17] Research started

with the successful development of an ammonia �lter for personal protection purposes with

additional broadband capacities for other types of hazardous gases. In an ongoing project,

this work continues with product optimizations and advances into other �elds of applica-

tion, i.e. industrial gas separation and clean room �ltration. In this research work, spherical

carbon is coated with ionic liquids that contain reactive properties for a variety of hazardous

gases. These reactive properties are mainly introduced by dissolving metal salts within the

ionic liquid. Already at room temperature, the metal ions selectively react with dissolved

acidic gases like hydrogen sul�de or ammonia species forming metal complexes. For certain

gases and ionic liquid impregnations, reducing pressure or increasing temperature regener-

ates the SILP materials.

5

2. Theoretical and technicalbackground

2.1. Activated carbon

2.1.1. Activated carbon materials

Activated carbon materials are porous, carbonaceous substances with pore volumes of more

than 0.2 ml g-1and internal surface areas of at least 400 m

2g-1. Pore sizes can range over

�ve orders of magnitude, starting as low as 0.3 nm. Activated carbons exhibit adsorptive

surface properties for a broad range of substances. They can be thermally and chemically

well durable. Also, they can possess a good electrical conductivity and capacity. These prop-

erties render activated carbons an attractive material for many applications. In gas-phase

and liquid-phase adsorption processes, activated carbons adsorb speci�c substances. For in-

stance, they are technically applied in the puri�cation of air and water, the decolorization of

sugar solutions and the recovery of gold and silver. In catalysis, activated carbons are cata-

lyst support or the catalyst itself. The electrical properties of activated carbons are utilized

in capacitors and fuel cells. Here, activated carbon is the electrode material. Activated car-

bon products still under development are hydrogen storage materials, batteries and sensors

[4].[1, 2, 8]

Synthesis of activated carbons

Commercially available activated carbon is typically produced from organic raw materials

such as coal, wood, coconut shells and other fruit kernels. The raw materials are carbonized

at temperatures between 600-850°C in inert atmosphere, yielding a slightly porous material

with a carbon content of more than 80 wt%. Afterwards, the carbon material is further acti-

vated by carbon gasi�cation, thus creating the �nal pore structure. Physical activation with

steam, carbon dioxide or air takes place at temperatures between 750-1100°C. The respec-

tive carbon gasi�cation reactions are presented in scheme 2.1. Compared to the exothermic

6

2. Theoretical and technical background

Scheme 2.1 Carbon gasi�cation by steam, carbon dioxide and air [2]

C + H2O CO + H2 ∆H = 121k Jmol−1

C +CO2 2CO ∆H = 163k Jmol−1

C +O2 CO2 ∆H = −406k Jmol−1

air oxidation process, the endothermic gasi�cation reactions with steam and carbon diox-

ide are easier to control by heat management. To enhance the steam gasi�cation process,

catalytically active species (e.g. alkali metal oxides, iron, copper) are added. Alternatively,

raw materials are carbonized and activated chemically in a single process step. Therefore,

the raw materials are mixed with dehydration agents like phosphoric acid, zinc chloride or

sulfuric acid and treated in inert atmosphere at temperatures between 200-650°C. Compared

to physical activation, lower temperatures are applied in chemical activation processes. On

the other hand, chemical activation involves corrosive chemicals that need to be washed out

after the activation process.[2, 8, 18–20]

Structure and composition of activated carbons

Forms of activated carbon, that are commercially available, include pulverized and granu-

lar activated carbon as well as carbon extrudates. Extrudates consist of pulverized activated

carbon mixed with a binder. The mixture is formed into pellets and the binder is carbonized

in an additional carbonization step. However, mechanical stress is induced between the ac-

tivated carbon and the binder during this process, which reduces mechanical stability of

extrudates.[1, 2, 21]

The overall structure of activated carbon is highly disorganized. Activated carbon is as-

sembled of crystalline, graphitic microstructures and amorphous carbon. The degree of

graphitization generally increases with increasing activation temperature. The edges of these

graphitic microstructures consist of unsaturated carbon-carbon bonds. These so called C-π

sites are important adsorption sites in activated carbon materials. Voids between the carbon

layers resemble the slit-shaped pore system. A schematic representation of the activated car-

bon structure is shown in �gure 2.1. A certain amount of heteroatoms like oxygen is incorpo-

rated into the activated carbon structure, as well. The fraction of noncombustible impurities

is noted as ash content. These impurities, originating from the raw material, make precise

control of the activation process more di�cult and negatively a�ect batch-to-batch repro-

ducibility. Also, a large ash content reduces mechanical stability of the activated carbon.[1,

2, 4, 22]

Due to the carbon activation process, the bulkmaterial is perforated by a hierarchically struc-

7


Figure 2.1.: Schematic structure of activated carbons ([22], modi�ed)

tured pore system. The extended, slit-shaped pore network starts withmacropores andmeso-

pores leading into micropores. Depending on the pore width, pores are classi�ed into three

categories: macropores (dpore > 50 nm), mesopores (2 ≤ dpore ≤ 50 nm), and micropores (dpore

< 2 nm) [23]. Especially the large quantity of small micropores contributes to the outstand-

ingly large internal surface area of activated carbon materials. Internal surface areas up to

2500 m2g-1are reached. The resulting pore structure depends on the raw material, as well

as the carbonization and activation process conditions. Figure 2.2 shows typical pore size

distributions of physically activated carbon derived from coconut shell and coal. For the co-

conut shell material, the pore size distribution is monodisperse around a pore diameter of

2 nm. In case of the coal-based material, the pore system is more hierarchically structured.

Here, micropores, mesopores and macropores are present in signi�cant quantities. The co-

conut shell activated carbon is ideal for the adsorption of small molecules. In applications

dealing with larger molecules, e.g. catalysis, the coal-based material with a larger meso- and

macropore content is more suitable.[1, 2]

Characterization of activated carbons

A qualitative impression of the bulk carbon material is observable in electron microscopy

images. The degree of carbon graphitization is elucidated by X-ray di�raction. Thermo-

gravimetric analysis allows the quanti�cation of volatile compounds and the ash content.

The pore structure is typically analyzed by nitrogen sorption measurements. Applying DFT

calculations to sorption isotherms [24], pore size distributions between 0.3-36 nm are gen-

8


1 10 100 100002468101214

dpore / nm

dV /

a.u

.

Coconut shell Coal

Figure 2.2.: Pore size distributions of physically activated carbons derived from coconut shell

and coal ([2], modi�ed)

erated, for instance. In carbon dioxide sorption experiments, the micropore structure is illu-

minated with higher resolution. Mercury porosimetry is a suitable analysis method for the

characterization of mesopores and macropores. By these means, relevant parameters such as

pore size distribution, surface area, total pore volume, and micropore volume are derived.[1]

2.1.2. Polymer-based activated carbon particulates

Polymer-based activated carbon particulates are a group of activated carbon materials with

synthetic polymers used as raw material. The �nal activated carbon product derived from

polymer materials features a particularly low ash content. The constantly high material

purity results from the de�ned chemical composition of the polymer precursor. Particle size

and pore geometry of the resulting activated carbon are speci�cally tailored by modifying

the polymer structure. Batch-to-batch production of polymer-based carbon materials results

in reproducible physicochemical properties.[4, 21]

The complete production sequence of polymer-based activated carbon particulates, as shown

in �gure 2.3, generally consists of four main process steps: polymerization, chemical treat-

ment, carbonization and physical activation. From the polymer precursor solution, the pro-

cess chain begins with a polymerization reaction forming the polymer. The polymer particles

are classi�ed into size fractions. A chemical treatment follows to improve its thermal stabil-

9


Polymer precursor solution

Polymerization

Classification(Sieving)

Chemical stabilization?

Chemical activation?

Carbonization

Chemical activation

Physical activation

Classification(Sieving)

Activated carbon particulates

Chemical treatment

Yes

Yes

No

No

Figure 2.3.: Production sequence of polymer-based activated carbon particulates

ity. The polymer is then carbonized by pyrolysis and further activated via partial oxidation

and carbon gasi�cation. Finally, the activated carbon particulates are classi�ed into size frac-

tions. The chemical stabilization treatment can be omitted, if the polymer material is stable

enough. Instead of the carbonization and physical activation sequence, the polymer can be

transformed into activated carbons with chemical dehydration agents. The �nal pore struc-

ture is established during the activation process. Nevertheless, in many cases, controlled

polymerization already induces a pore structure. Porosity further increases with every pro-

cess step.[2, 5, 6, 25]

Polymer formation

Several di�erent polymer precursors have been successfully applied in the preparation of ac-

tivated carbon particulates. Prominent examples are phenole-formaldehyde resins, resorcinol-

formaldehyde resins, acrylonitrile-divinylbenzene copolymers, and polystyrene-divinylben-

zene copolymers (see �gure 2.4). In the following, the formation of these polymer precur-

sors and corresponding stabilization measures are discussed. Afterwards, an overview of

carbonization and carbon activation procedures is given.

Phenolic resins are synthesized in a polymerization process at very acidic conditions. The

phenol and formaldehyde reactants are provided at a molar stoichiometry of e.g. 1 to 0.75-

0.85. After polymer chain growth and partial crosslinking, a macroporous resin is obtained.

The resin can be further hardened with a curing agent like hexamethylene tetramine, intro-

ducing some nitrogen crosslinks. The critical step during polymerization is the controlled

10


partial crosslinking of single polymer molecules. Controlled partial crosslinking guarantees

successful particulate formation and the preservation of the resin’s macroporous structure. If

the resin is too soft, it will melt during thermal treatments and loose its porosity. On the other

hand, if the resin is completely hardened, particulate formation by thermal sintering will be

di�cult. For particulate formation, the phenolic resin is at �rst pulverized and classi�ed

into size fractions of e.g. 1-500 μm. The desired form of the material is then produced with

these small particles, which are thermally sintered together. The sintered polymer particu-

lates determine the shape of the �nal carbon particulates. By this method, activated carbon

particulates are produced in various forms and sizes. As no additional binder is necessary,

mechanical stability of the material is high.[21, 26]

Spherical resorcinol-formaldehyde polymers are obtained in an emulsion polymerization

process. Therefore, a resorcinol-formaldehyde molar ratio generally between 1:1 and 1:4

is provided in an alkaline environment and mixed with an organic solvent like cyclohexane

and a surface agent at temperatures between 25-90°C for between 1-7 days. Sodium carbon-

ate or potassium carbonate acts as polymerization catalyst. Emulsion viscosity and stirring

speed determine the polymer particle size. Particle diameters are adjustable to values be-

tween 10-500 μm. The polymer drying process signi�cantly in�uences the carbon structure.

One approach is to exchange water inside the polymer spheres with acetone and to then

dry the material in supercritical carbon dioxide. This results in mesoporous activated carbon

spheres.[20, 27]

Another starting material for the production of polymer-based activated carbon is poly-

styrene-divinylbenzene. In a suspension polymerization process, the monomer mixture is

stirred in an aqueous solution containing dispersing agents and polymerization initiator at

around 80-90°C. The monomer mixture is dispersed into small droplets and polymerizes into

individual beads. So, spherical copolymers are formed consisting of polystyrene cross-linked

with divinylbenzene. Crosslinking increases the thermal stability of the material. A sulfona-

tion treatment with sulfuric acid further improves the thermal stability of the copolymer.

An alternative to sulfonation is phosphorylation of the polymer material. Thus, the polymer

precursor remains sulfur-free, but thermal stability lies between the untreated polymer and

the sulfonated material. Also, air oxidation of the polymer material at 250°C increases ther-

mal stability by introducing additional crosslinks.[25, 28–31] The addition of di- and trivalent

metal ions to the sulfonated polymer resin via ion exchange results in microporous spherical

carbon. The multivalent metal ions establish crosslinks between di�erent functional groups,

thus stabilizing the structure.[32]

11


m n

m n

N

H2C

OH

n

H2C

OH

OH

n

(a) (c)

(b) (d)

Figure 2.4.: General molecular structures of polymer resins: phenol-formaldehyde resin (a),

resorcinol-formaldehyde resin (b), polystyrene-divinylbenzene copolymer (c),

acrylonitril-divinylbenzene copolymer (d)

Acrylonitrile-divinylbenzene is an alternative starting material for spherical carbon. Spheri-

cal acrylonitrile-divinylbenzene copolymers are also formed in a suspension polymerization

applying pore structure building solvents like toluene and nonane. The polymer structure

is stabilized by partial oxidation in air at around 250-350°C, with 250°C giving best carbon

yields.[33–35]

Polymer carbonization and carbon activation

Carbonization and physical activation processes are similar for all applicable raw materials,

in principle. In an oven, the materials are heated in an inert atmosphere to temperatures be-

tween 500-900°C applying a certain temperature program. The exact temperature program

of carbonization, including heating rates and durations of constant temperature, varies in

dependence of the raw material and the desired carbon composition. Afterwards, the pore

system is extended by partial oxidation with steam or carbon dioxide. These endothermic

oxidation reactions require increased temperatures of around 800-900°C. As an alternative,

chemical activation of polymers is carried out to establish an extended pore structure. As

chemical dehydration agent, potassium hydroxide or sodium hydroxide is applied. The ma-

terials are treated in inert atmosphere at temperatures between 700-900°C. In a washing step,

12


excess dehydration agents in the activated carbon are removed. A nitric acid treatment of

polymers at room temperature also results in activated carbons.[20, 36–39]

Phenolic resins are carbonized with a yield of around 40-50% by heating up to 900°C and acti-

vated with carbon dioxide at 800-900°C. Depending on the degree of activation, BET surface

areas up to 2130 m2g-1and pore volumes of 2.13 ml g

-1are reached. The pore size is directly

dependent on the size of the small, sintered particles. Pore geometry is in�uenced by the

formaldehyde-phenol ratio, with a ratio below 0.5 leading to mesopores.[21, 40] Mesoporous

activated carbon spheres are also produced by carbonization and steam activation at 800°C

of polymer spheres containing ferrocene as pore-forming agent.[26]

Heating spherical resorcinol-formaldehyde polymers up to temperatures between 500-900°C,

BET surfaces areas up to 779 m2g-1and total pore volumes up to 0.68 ml g

-1are obtained af-

ter carbonization.[27] Highly-activated carbons with a dominating micropore content result

from chemical activation with potassium hydroxide at temperatures up to 700°C. With in-

creasing amount of applied potassium hydroxide, pore volume and BET surface area increase

up to 1.35 cm3g-1and 2760 m

2g-1, respectively.[36, 37]

After carbonization by heating up to 900°C, the de�ned molecular structure of the poly-

styrene-divinylbenzene copolymer remains preserved.[41] Thus, bimodal pore size distribu-

tions are obtained with large mesopore volumes of up to 0.44 cm3g-1.[31] With the help of

steam and carbon dioxide activations at 800°C, the pore system is extended on demand and

BET surface areas increase up to 2000 m2g-1. Chemical activation with potassium hydrox-

ide at 770°C after carbonization of polymer spheres at 500-900°C results in highly-activated

spherical carbons. With increasing carbonization temperature, spherical carbon becomes

more compact and its hardness increases. Increasing the amount of potassium hydroxide,

total pore volumes up to 0.78 cm-3g-1and BET surface areas up to 2022 m

2g-1are reached.

Carbon hardness slightly decreases with increasing pore volume. Still, mechanical stability

of this polymer-based spherical activated carbon is signi�cantly larger than that of tradi-

tional activated carbon materials. Particle size of spherical activated carbon is determined

by the particle size of the starting material.[39] Technical considerations allowing e�cient

large-scale production of spherical carbon based on the polystyrene-divinylbenzene copoly-

mer have been proposed, as well.[6] Polystyrene-divinylbenzene based spherical activated

carbon with di�erent particle size fractions is depicted in �gure 2.5.

Acrylonitrile-divinylbenzene-based material, carbonized at 850°C, exhibits BET surface ar-

eas between 250-440 m2g-1, depending on the amount of pore structure building substances

added during polymerization. Meso- and macroporous carbon materials result for less inten-

13


Figure 2.5.: Light microscopy images of di�erently sized polystyrene-divinylbenzene based

spherical activated carbons ([5], modi�ed)

sively cross-linked polymers.[35]

Polymer-based activated carbon particulates used in this work

As shown in this section, a variety of di�erent polymers and copolymers are successfully

transformed in subsequent process steps into activated carbon particulates. Polymer-based

activated carbon materials excel due to their high material purity and large mechanical sta-

bility. Also, the pore system can be structured in detail, already starting at the polymerization

step. Material properties are reproducible in batch-to-batch productions, as well.[42]

In this work, the spherical activated carbon particulates are based on polystyrene-divinyl-

benzene polymers (see �gure 2.5). A variety of particle sizes and pore structures is available.

These spherical carbons are already produced at a larger scale and are thus readily available.

The applications of activated carbon as catalyst support and in advanced gas puri�cation are

discussed speci�cally in the following sections of this chapter.

14


2.2. Activated carbon as catalyst support

2.2.1. Activated carbon support materials in catalysis

In heterogeneous catalysis, activated carbon is the catalyst support material of choice in the

synthesis of many �ne chemical and pharmaceutical products. Chemical reactions such as

hydrogenations, dehydrogenations, oxidations, hydrogenolysis reactions and hydrodehalo-

genations are e�ectively and selectively promoted by carbon supported noble metal cata-

lysts.[42–44]

For instance, the hydrogenation of alkines and alkenes is catalyzed by palladium on carbon.

Rhodium and ruthenium on carbon are generally the most active catalysts in the hydro-

genation of carbocyclic rings. Heterocyclic compounds are easily hydrogenated by a vari-

ety of noble metal catalysts, with palladium and ruthenium being ideal in case of nitrogen-

containing heterocycles. Platinum on carbon performs well in the selective hydrogenation of

α,β-unsaturated aldehydes to the corresponding alcohols. Concerning dehydrogenation re-

actions, palladium and platinum on carbon are generally the most active catalysts. Palladium

on carbon is applied in oxidations, hydrogenolysis and hydrodehalogenation reactions.[42–

45] Technical applications of carbon supported noble metal catalysts are, for example, the

hydrogenation of thymol and menthone to produce menthol [46, 47], the hydrogenation of

cinnamaldehyde and crotonaldehyde [44], and the hydrogenation and dehydrogenation of

liquid organic hydrogen carriers [48, 49].

Several unique features render activated carbons a very well suitable catalyst support ma-

terial. Their large speci�c surface area allows for large dispersions of noble metals. Metal

dispersion is the ratio of surface atoms to total metal content. Large dispersions are usually

desired for good catalytic activities, i.e. many noble metal atoms are accessible to reactants.

Also, the internal surface of activated carbons can be speci�cally functionalized or remain

unmodi�ed. Thus, metal-support and reactant-support interactions are tunable.[8]

As activated carbon is durable to a certain degree in acidic and alkaline solutions [42] and

withstands temperatures up to 1000 K in an oxygen-free atmosphere [8], chemical reactions

can be catalyzed at harsh conditions. Compared to alumina and silica supports, the much

higher hydrolytic stability of activated carbon is an advantage in water-containing reaction

systems [4, 50–52]. Also, the absence of Lewis acid sites can prevent unwanted side reactions

that would be catalyzed by an oxide catalyst support [1]. After their application in catalysis,

the precious noble metals are easily recovered by burning the carbon [8, 42].

15


Support

Macropore

Mesopore

Micropore &

metal clusters

Figure 2.6.: Schematic pore system of activated carbon catalysts

2.2.2. Mass transport in catalysis - aspects of di�usion

Activated carbon catalysts are commercially available in powder and granular form, or as

extrudates. If the catalytically active species is not explicitly present at the outer particle

surface, but dispersed throughout the inner carbon surface, mass transport e�ects inside the

pore system need to be taken into account.[21] These mass transport e�ects signi�cantly

in�uence the overall reaction kinetics of the catalyst. Both pore structure and particle size of

the activated carbon support a�ect the extent of mass transport limitation. In the following,

after illustrating the general pathway of reactant molecules for heterogeneously catalyzed

reactions, these two parameters are examined more closely.

In the pathway of reactant molecules in heterogeneous catalysis, the molecules di�use from

the bulk solution through a boundary layer to the outer carbon surface. With further dif-

fusion, the molecules are transported along the extended pore system to the catalytically

active sites (compare �gure 2.6). There, the reactants adsorb to the surface, react and desorb

as products. The product molecules then di�use through the pores and the boundary layer

into the bulk solution. Di�usion through the boundary layer between bulk solution and the

outer carbon surface is known as �lm di�usion, while pore di�usion describes the mobility

inside the porous activated carbon material.[53, 54]

16


Pore di�usion

Depending on the pore structure of the catalyst support, reactant molecules are transported

more or less easily. Larger pore diameters accelerate mass transport. If the pores leading

to the catalytically active sites are dimensioned smaller than the reactant molecules, the

catalytically active sites won’t be accessible or the formed products won’t reach the bulk

solution. This e�ect is utilized by shape selective catalysts [2, 54, 55]. Mass transport inside

the pore system by di�usion is quanti�ed by the e�ective di�usion constant De f f [54]. As

shown in equation 2.1, it includes the porosity ϵ and tortuosity τ of the porous support,

as well as the pore di�usivity Dpore . The tortuosity is an empirical factor considering the

random movement of reactant molecules in the pore system [53, 54]. The pore di�usivity

can be expressed as a function of molecular di�usivity Dmol and Knudsen di�usivity DKnu

(see equation 2.2). Knudsen di�usion is taken into account, if the pore diameter is smaller

than the mean free path of reactant molecules. An simpli�ed estimation of the e�ective

di�usion constant is given in equation 2.3, which solely depends on the molecular di�usion

constant, neglecting Knudsen di�usion.[53]

De f f =ϵ

τ· Dpore (2.1)

Dpore = (1

Dmol+

1

DKnu)−1 (2.2)

De f f = 0.1 · Dmol (2.3)

The hierarchically structured pore system of activated carbon, with macropores and meso-

pores branching o� into micropores, greatly bene�ts the overall performance of catalyzed

reactions. The larger pores allow for su�cient mass transport, while the micropores provide

a large surface area to �nely disperse the catalytic species.

17


Kinetics on pore di�usion limitation

Concerning particle size, with increasing particle size, mass transport e�ects increasingly

in�uence the overall reaction kinetics of the catalyst. The statistic pathway of reactant

molecules between bulk solution and catalytically active site increases. The Thiele modulus

ϕ is frequently applied to estimate the extent of pore di�usion limitation [56]. It describes

the ratio of intrinsic surface reaction rate over e�ective pore di�usion and incorporates the

particle size of the catalyst. Equation 2.4 presents the Thiele modulus for spherically shaped

catalysts.

ϕ = R ·

√kintr · c

n−1

b

De f f(2.4)

Here, R is the particle radius, kintr the intrinsic catalytic activity of the active site without

mass transport limitation, cb the bulk reactant concentration, n the reaction order, and De f f

the e�ective di�usion coe�cient as de�ned in equation 2.1. The intrinsic reaction rate con-

stant is determined experimentally. For large Thiele moduli, surface reaction is faster than

the transport of reactants through the pore system, thus leading to a shortage of reactants at

the active sites, especially near the center of the catalyst pellet.

The intensity of mass transport limitation is also described by the e�ectiveness factor η [45,

54]. Looking at the catalytically active sites, the e�ectiveness factor represents the ratio

of actually reacting molecules over the maximum possible reaction rate under isothermal

conditions. It can be expressed to be solely a function of the Thiele modulus. Equation 2.5

shows the e�ectiveness factor for an irreversible �rst order reaction in a spherical catalyst

pellet [54].

η =3

ϕ(

1

tanhϕ−

1

ϕ) (2.5)

Calculation of Thiele modulus and e�ectiveness factor both require knowledge of the in-

trinsic reaction rate constant. Therefore, the catalytic reaction has to be studied in absence

of di�usion limitation. An alternative approach is the application of a-priori criteria. Here,

only immediately measurable parameters are applied. The extent of pore di�usion limita-

tion inside a spherical catalyst pellet is estimated a-priori by the Weisz-Prater criterion Φ

(see equation 2.6) [54, 57].

18


Φ =re f f · R

2

cb · De f f= ϕ2 · η

< 1 f or n = 1

< 6 f or n = 0

< 0.3 f or n = 2

(2.6)

As equation 2.6 also shows, the Weisz-Prater criterion is a function of Thiele modulus and

e�ectiveness factor. Critical values, at which the absence of pore di�usion limitation is ex-

pected, are given for individual reaction orders.

Overall progress of catalyzed reactions

The e�ective reaction rate re f f can be expressed by a power law approach [54] (see equation

2.7), including the e�ective reaction rate constant ke f f , the reactant concentration c and the

e�ective reaction order ne f f .

re f f = ke f f · cnef f

(2.7)

The e�ective reaction rate constant ke f f quantitatively describes the performance of a cata-

lyst concerning catalytic activity, thus allowing quick comparison of di�erent catalysts. The

e�ective reaction rate constant incorporates the intrinsic performance of the catalytically

active sites and mass transport limitations due to pore di�usion. The strong temperature

dependence of the reaction rate constant is represented by the Arrhenius law (see equation

2.8).

ke f f = k0 · e−EA,appR ·T (2.8)

In equation 2.8, k0 is the pre-exponential factor, EA,app the apparent activation energy, R the

ideal gas constant and T the temperature. Apparent activation energies below 30 kJ mol-1

can result from mass transport limitation.[53, 54]

Integrating the power law expression results in the course of conversion X over time t . The

conversion function in dependence of the modi�ed residence time τmod and the e�ective

reaction rate constant is shown in equation 2.9, assuming a �rst order reaction. The modi�ed

19


residence time accounts for the mass of applied noble metalmmetal and the reaction volume

Vreaction (see equation 2.10) [58].

X (τmod ) = 1 − ekef f ·τmod(2.9)

τmod =t ·mmetal

Vreaction(2.10)

Another characteristic �gure to compare catalytic activities is the turnover frequency TOF

of reactant molecules at catalytically active sites [59, 60]. It’s the rate of converted number of

substrate molecules ∆nsubstrate with regard to the number of catalytically active sites ncatalyst

(see equation 2.11). For supported noble metal catalysts, where only the surface metal atoms

are accessible to the reactants, equation 2.12 can be applied. This equation incorporates

molar metal massMmetal and metal dispersion Dmetal .

TOF =∆nsubstratencatalyst · t

(2.11)

TOF =∆nsubstrate ·Mmetal

τmod ·Vreaction · Dmetal(2.12)

2.2.3. High-purity polymer-based activated carbon particulates

Polymer-based activated carbon pellets exhibit superior properties over traditional activated

carbon materials for application as catalyst support. These advantages are presented in the

following, together with examples of successful application in catalysis.

Advantages as catalyst support

Due to the complex chemical composition of natural raw materials, conventional activated

carbons exhibit a disordered inner structure with large ash contents and volatile impurities.

In catalysis, impurities on catalysts support can have negative e�ects regarding selectivity

and activity of the reaction. The impurities can also contaminate the product.[8] Especially

20


in pharmaceutical applications, product purity is essential [61]. Legislative guidelines limit

the amount of di�erent impurities within pharmaceutical products [62]. Application of purer

activated carbon with known chemical composition is highly desired.

Activated carbon materials derived from synthetic polymers are a potential solution, as the

chemical composition of synthetic polymers is well-known. Thus, polymer-based activated

carbon particulates are synthesized with reproducible structure and low ash content (see

section 2.1.2 for details). This reproducible chemical composition renders polymer-based

activated carbon particulates a potentially well suitable group of catalyst support materials.

Furthermore, the pore structure of these materials is widely adjustable to �t the catalyzed

reaction best [42]. The pore system can already be established during polymerization and is

further extended in carbonization and carbon activation treatments (see also section 2.1.2).

The particulate character of the material has the advantage of an easier catalyst handling, but

induces the risk of mass transport limitation issues. So, as pointed out in the previous section,

polymer-based activated carbon catalysts need to be evaluated for pore di�usion limitation

for a given reaction. To mitigate poor pore di�usion and allow for su�cient catalytic activity,

the most suitable pellet size and pore structure need to be determined for a given reaction.

Previous applications in catalysis

Despite the advantages of polymer-based activated carbon particulates as catalyst support,

in literature, only a few noble metal catalysts of this type were prepared and characterized.

Mesoporous carbon was produced by carbonizing resorcinol-formaldehyde-based polymer

gels. Compared to activated carbons derived from organic raw materials, the mesopore con-

tent was exceptionally large and the amount of micropores very low. This mesoporous car-

bon material was deemed highly suitable for application as catalyst support. Pore di�usion

and accessibility of catalytically active sites was proposed to be improved in this carbon ma-

terial.[63]

Another mesoporous carbon material containing nickel nanoparticles was prepared by car-

bonization of a phenolic resin containing nickel nitrate. The size of the nickel nanoparticles

varied between 30–200 nm, depending on carbonization temperature and metal loading. Ab-

solute metal loadings between 1-15 wt% were realized. But, only a limited amount of nickel

was located at the carbon surface. This magnetic material was easily separable and was

proposed to �nd application as nickel catalyst or catalyst support.[64]

Spherical carbon with diameters between 1-40 μmwere derived from petroleum residue as a

platinum-ruthenium catalyst support. The carbonwas treated with potassium hydroxide and

21


impregnated with an aqueous solution of chloroplatinic acid and ruthenium chloride at 80°C.

By addition of sodium dithionite, the metal salts were transformed to elemental platinum and

ruthenium. The catalyst performed well in methanol oxidation reactions.[65]

A pure, phenolic resin based, spherical carbon catalyst was prepared and successfully tested

in the oxidation of cyclohexanone to C4-C6 dicarboxylic acids. The spherical carbon was

activated with carbon dioxide at 850°C and air at 450°C, which signi�cantly increased pore

volume and BET surface area to values up to 1477 m2g-1and 1.33 ml g

-1. Oxygen surface

groups were formed in the process. The type of oxygen surface groups had a signi�cant

in�uence on the selectivity of cyclohexanone oxidation.[66]

2.2.4. Post-treatment of catalyst - catalyst separation

An e�cient and economically feasible catalyzed chemical production is not solely deter-

mined by catalytic performance, but by the whole process including post-processing. If the

catalyst is suspended in a liquid phase, post-processing includes separation of reactant so-

lution and catalyst material by �ltration. To evaluate �ltration performance, the Darcy law

can be applied. The Darcy law is a simpli�ed expression of the Navier-Stokes equation, as

shown in equation 2.13.

J =V

A=

∆p

η · R(2.13)

It describes the �uid �ux J through a porous medium. The �uid �ux is the volume �ow rate

V of the �uid over the �lter area A. The Darcy law incorporates the pressure drop ∆p of the

�ow, the �uid viscosity η and the speci�c �lter resistance R of the porous �lter medium. The

resistance can be seen as the sum of the resistances of �lter membrane Rm and �lter cake Rc .

The �lter cake resistance can be described according to the Carman-Kozeney extension with

empirical constants and material properties (see equations 2.14-2.17).

Rc = α · L · (1 − ϵ ) · ρc (2.14)

22


α =k′′ · S2p · (1 − ϵ )

ϵ3 · ρc(2.15)

Sp =6

dp(2.16)

L =4 ·mc

π · ρc · d2

c

(2.17)

Where α is the speci�c cake resistance, k′′ an empirical �lter factor, Sp the speci�c particle

surface, ϵ the �lter cake porosity, ρc the packed bed density, dp the particle diameter, L the

height of the �lter cake, mc the mass of the �lter cake, and dc the diameter of the �lter

cake.[67]

2.2.5. Carbon surface functionalization

The surface of carbon materials can be functionalized in many di�erent ways. By surface

engineering, carbon surfaces are modi�ed with speci�c surface functionalities in order to in-

herit certain surface properties. Means of speci�c surface functionalization are heteroatom

inclusion, oxidation, halogenation, sulfonation, grafting, nanoparticle attachment or poly-

mer coating.[4]

Heteroatoms are included during carbon synthesis. For instance, polyacrylonitrile polymer

carbonization results in nitrogen functional surface groups. Various oxygen surface groups

are created by carbon treatment with oxidation agents. Nitric acid oxidation is most com-

mon, predominantly resulting in carboxylic acid groups and is controlled by acid concentra-

tion, temperature and treatment time. Sulfonation with sulfuric acid at elevated temperature

results in solid-acid catalysts. Fluorination with �uorine gas at 25-250°C leads to hydropho-

bic carbon surfaces. Grafting of oxidized surfaces with organic reactants further modi�es

the surface. Impregnation enhances functionality of the material by incorporating e.g. cat-

alytically active metals and metal oxides. Polymer coating by wet impregnation enhances

mechanical strength, while preserving carbon porosity.[4]

23


The two carbon functionalization methods applied in this work are surface oxidation and

noble metal deposition by impregnation. These methods are eleborated in the following

subsections.

2.2.5.1. Surface oxidation

For preparation and application of activated carbon as catalyst support, formation of oxygen

surface groups is most relevant. Depending on the production process of activated carbon

(see sections 2.1.1 and 2.1.2), some functional surface groups are already present. In the fol-

lowing, an overview of the types and features of oxygen surface groups is given. Afterwards,

applicable functionalization procedures and characterization methods are described. Finally,

previously conducted surface modi�cations of polymer-based spherical activated carbon are

discussed.

Types and features of oxygen surface groups

Important oxygen surface groups are carboxylic acids, lactones, carboxylic anhydrides, phe-

nols, quinones, and cyclic peroxides. Figure 2.7 schematically depicts these di�erent types

of oxygen functional groups on a generic activated carbon surface. Surface oxidation occurs

at the edges of graphitic microstructures. The integration of these oxygen surface groups

in�uences wettability, acidity and adsorption properties of activated carbons. Oxygen sur-

face groups decrease the carbon’s hydrophobicity, so that wettability of polar solvents is

improved. In catalyst preparation, especially during the impregnation of activated carbon

particulates, a polar metal precursor solution more easily penetrates a pore system with sur-

face oxide functionalities. Also, oxygen surface groups act as adsorption sites. During cata-

lyst impregnation, metal precursor compounds can adsorb to these surface sites (see section

2.2.5.2 for details). Acidic or basic functional surface groups modify the alkaline nature of

activated carbon. Carboxylic acids, lactones, carboxyl anhydrides and phenols are known to

contribute to surface acidity. Carboxylic acids are the most acidic functional groups. Classi�-

cation of other surface groups regarding their e�ect on carbon basicity is still ambiguous and

under research [68, 69]. The acidity of a carbon suspension a�ects the strength of precursor-

support interactions during metal impregnation (also see section 2.2.5.2 for details).[4, 8, 63,

70, 71]

24


O

O

O

OOH

OHO

O

O

O

Carboxylic acidLactone

Cyclic peroxide

Quinone

Phenol

Carboxylanhydride

O

O

O

Ketone

Figure 2.7.: Types of oxygen functional groups on activated carbon surfaces ([8], modi�ed)

25


Methods of surface oxidation

Applicable functionalization procedures for carbon surface oxidation are conducted with ei-

ther liquid-phase or gas-phase oxidation agents. Nitric acid, sulfuric acid and hydrogen per-

oxide are suitable liquid oxidizers. Oxygen, air, carbon dioxide, steam or ozone are applied

in gas-phase oxidations. Nitric acid treatment creates mainly carboxylic acid groups and

some carbonyls, as well as phenols.[68, 72–74] A mixture of nitric acid and sulfuric acid re-

sults in the introduction of nitrogen surface groups by amination, with sulfuric acid inducing

the required nitronium ion formation [74]. Oxygen treatment generally produces carbonyl

and phenol surface groups. The intensity of oxidation depends on the concentration of the

oxidation agent, temperature, treatment time and the carbon amount.[7, 63, 73, 75, 76]

Surface characterization

A variety of analytical methods are available to characterize surface functionalization of car-

bon materials. Oxygen surface groups, in particular, are analyzed with titration methods and

spectroscopic investigations. Additionally, thermogravimetric analysis and sorption mea-

surements reveal information on oxygen surface modi�cations.

The materials point of zero charge (PZC), resembling the sum of all surface charges, is deter-

mined by mass titration. With increasing amount of carbon material, the pH of an aqueous

suspension approaches the pHPZC. Acidic surface groups decrease the PZC of carbon mate-

rials.[74]

A classi�cation into individual types of acidic and basic surface groups is possible by Böhm

titration. Titration solutions of sodium hydroxide, sodium carbonate, sodium hydrogen car-

bonate and hydrochloric acid are applied. Sodium hydrogen carbonate neutralizes carboxylic

acid and carboxylic anhydride groups. Sodium carbonate reacts with lactonic and carboxylic

groups. Sodium hydroxide neutralizes four types of oxygen surface groups: carboxylic acid,

carboxylic anhydride, lactonic and phenolic groups. Hydrochloric acid reacts with basic sites.

Other types of surface oxides, i.e. quinones, carbonyls, and ethers, cannot be determined by

Böhm titration.[68, 71, 77–80]

A more advanced method to quantify di�erent acidic and basic surface groups is potentio-

metric titration. Starting from a suspension’s initial pH, two suspensions are titrated with

an acid and a base, respectively. With the assumption that acid sites are characterized by an

acid constant Ka, titration curves are mathematically transformed into populations of sur-

face sites pKa. As a result, a distribution function of pKa is obtained. Peaks below a pKa of

5 correspond to strongly acidic carboxylic acid groups. Peaks in the pKa range of 5-7 are

26


Surface group Desorbing gas species Temperature range / °C

Carboxylic acid CO2 100-400

Carboxyl anhydride CO, CO2 350-600

Lactone CO2 400-650

Phenol CO 600-700

Ethers CO 600-700

Carbonyls, quinones CO 700-1000

Table 2.1.: Desorption temperatures and desorbing gas species of oxygen surface groups [86,

87]

assigned to carboxylic acids, lactones and carboxylic anhydrides. Weak acids like phenolic

groups appear in in the pKa range from 7-11.[71, 72, 81–85]

A di�erent carbon surface characterization approach is temperature programmed desorption

with o�-gas analysis bymass spectroscopy (TPD-MS). In the temperature range between 100-

1000°C, surface groups desorb successively in distinct temperature regions. Also, surface

groups decompose into characteristic gases like carbon dioxide and carbon monoxide, but

also nitrogen or sulfur species. A comprehensive list of desorption regions of surface oxides

with corresponding decomposition products is given in table 2.1. Strong acids desorb at

lower temperatures, while weak acids, neutral groups and alkaline groups decompose at

very high temperatures. Compared to the titration methods, TPD-MS distinguishes more

surface groups such as neutral oxygen groups, but also nitrogen, phosphorous, and sulfur

functional groups.[68, 71]

Applicable, purely spectroscopic methods in carbon surface science are, for instance, Fourier

transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and dy-

namic light scattering (DLS).[68, 74, 88] With FTIR, characteristic peaks in each spectrum

can be assigned to speci�c surface interactions. Thus, surface functionalities are qualita-

tively determined. But, only the outer carbon surface is spectroscopically analyzed. XPS

measurements clarify the chemical composition of surfaces and sub-surfaces by monitoring

the electrons being released from the material [89]. These spectroscopic methods are useful

to validate results of TPD-MS measurements, for instance. DLS can be applied to determine

the isoelectric point (IEP) of carbon suspensions by Zeta potential measurements at di�erent

pH values. In contrast to the pHPZC, the pHIEP of porous materials resembles solely the ex-

27


ternal surface charges. The di�erence between both values indicates the charge distribution

across carbon particles and, thus, the distribution of acidic and basic groups between the

internal and external carbon surface.[71, 74]

Nitrogen sorption experiments illuminate the structural in�uence of functionalization on

pore geometry. Carbon material that was etched away or blocked micropores result in a

noticeable change of total pore volume, micropore volume and BET surface area. Sorption

measurements with water vapor characterize wettability of the carbon material. From water

vapor isotherms, information such as a surface-speci�c oxygen number and the distance

between oxygen atoms can be derived.[24, 45, 72, 75]

Previous surface functionalization of PBSAC materials

Polymer-based spherical activated carbon with 1 wt% of volatile components was previously

oxidized in air at 450°C for 15-120 min. With increasing processing time, the amount of

volatile components increased up to 15 wt%. Correspondingly, the material’s PZC decreased

from pH 8-9 to around 6. Water vapor isotherms supported the decrease in hydrophobicity.

After intensive oxidation, mechanical properties were a�ected. Functionalization of oxi-

dized spherical carbon with nitrogen functional groups was also successful. The material

was loaded with urea and post-treated at 500-920°C in an inert atmosphere.[7]

2.2.5.2. Deposition of noble metal

Metal on carbon catalyst preparation generally consists of two succeeding steps. Impreg-

nation of carbon support material with metal species. And, metal transformation into the

catalytically active metal species by calcination or reduction of metal precursors. Typical

metals applied in heterogeneous catalysis are palladium, platinum, ruthenium, rhodium and

nickel [44]. Di�erent impregnation and metal transformation techniques are described in the

following. Afterwards, analysis methods for catalyst characterization are described.

Impregnation of activated carbon with metal species

The basic principles of most impregnation techniques involve the attraction of metal species

to the inner surface of the support materials and the transition of dissolved metal species into

small solid clusters. The physico-chemical reason, sequence and timing of these two steps

vary for each impregnation technique.[90]

In a standard impregnation, a metal precursor solution is introduced into the pore system of

the catalyst support. The metal species adsorb to the inner surface of the support material

28


due to di�erent attractive forces. After the solvent is evaporated in a drying process, the

metal species are fully deposited to the catalyst support. Attractive forces are for instance

electrostatic interactions between the surface of the support material and ionic metal species

in the impregnation solution. The surface of the support bears charges that are neutralized

by adsorbing ions. Countercharged metal ions adsorb to the surface, thus replacing the pre-

viously adsorbed ions. In a washing step, non-adsorbing ions are removed from the catalyst.

Besides electrostatic adsorption by ion-exchange, metal species can be chemically attached

to surface functional groups.[91–93]

In the deposition-precipitation process, transition of dissolved metal species into small solid

clusters takes place in the pores that are still �lled with impregnation solution. Here, nucle-

ation is initiated by reducing the solubility of the metal precursor. Solubility can be decreased

by changing the pH value of the solution. Alternatively, the metal precursor can be trans-

formed to a less soluble species by changing its valency or by complexing/decomplexing the

precursor adding/removing a compexing agent. The solid clusters are then attracted to the

surface of the support. As an excess of impregnation solution is applied, the precipitation

process needs to be controlled to prevent precursor nucleation in bulk solution. Deposition-

precipitation works well for particulate support materials. In electrochemical deposition-

precipitation, metal species dissolve from a metal anode and are deposited to the support.

The electrochemical setup can also be used to induce pH or valency changes resulting in

precipitation of metal species. These electrochemical processes are di�cult to realize at a

larger scale, though.[90]

Instead of a metal precursor solution, metal nanoparticles are deposited in colloidal impreg-

nations. The metal nanoparticles have either been previously prepared or are precipitated

in-situ in the impregnation process.[94, 95]

Depending on the amount of impregnation solution, catalyst preparation techniques are clas-

si�ed into incipient wetness and wet impregnation processes. In the incipient wetness im-

pregnation, only the pore system is �lled with impregnation solution. The volume of impreg-

nation solution is limited to the total pore volume of the catalyst support. Wet impregnation

utilizes an excess of impregnation solution. So, advantages of the incipient wetness technique

are the complete deposition of precursor species inside the pore system and reduced solvent

waste. In wet impregnations, chemical agents for precipitation or metal transformation can

be added during the process.[90–93]

Concerning the preparation of noble metal catalysts based on activated carbon support ma-

terials, the carbon surface can be either oxidized or non-functionalized, depending on the

29


C-π site

PdCl42-

OH

OH+

HO OH+

Figure 2.8.: Adsorption of metal anions to protonated carbon surface groups by electrostatic

interactions; C-π sites for alternative metal adsorption ([8], modi�ed)

applied impregnation mechanism and solvent. But, in order to fully utilize the outstand-

ing diversity of carbon surface oxidations, as presented in section 2.2.5.1, the following part

focuses on the electrostatic adsorption of metal ions to charged carbon surfaces.

In a wet impregnation process applying the concept of electrostatic adsorption, an oxidized

carbon support is suspended in an aqueous metal salt solution. The pH of the impregnation

solution is adjusted by adding an acid or base, so that the pH di�erence to the carbon’s PZC is

su�ciently large. In consequence, metal ions adsorb to protonated or deprotonated surface

sites (see �gure 2.8). In case of a metal anion, the surface sites need to be protonated for

successful ion adsorption by electrostatic forces. Here, the impregnation solution’s pH needs

to be smaller than the pHPZC. Metal cations adsorb to negatively charged surface sites.[96]

It needs to be noted that metal salts can also electrostatically adsorb to basic C-π sites (see

�gure 2.8). These are non-oxidized, unsaturated carbon surface structures that provide free

electrons to interact with the precursor. For more details on the in�uence of C-π sites see

section 2.2.5.4. Furthermore, metal ions can create new surface groups by oxidation, thus

reducing themselves.[63, 76, 96, 97]

Metal transformation of impregnated carbon materials

Metal transformation of suspended catalysts after wet impregnation, reducing the metal pre-

cursor to the catalytically active species, is carried out by addition of alkaline formaldehyde,

hydrazine, sodium borohydride, sodium formate, or hydrogen gas [98]. Alternatively, dried

catalyst material is placed in hydrogen atmosphere at elevated temperatures until themetal is

fully reduced. Here, the applied metal transformation temperature has signi�cant in�uence

on metal cluster size. For each noble metal, an optimum temperature exists, resulting in min-

30


imum cluster sizes. Above this optimum, with increasing metal transformation temperature,

metal clusters grow due to agglomeration and sintering.[76, 93, 99–101] The metal speci�c

Hüttig temperature, empirically derived from the melting temperature of the noble metal

(see equation 2.18), points out the onset of thermal sintering by di�usion of metal atoms.

For metal nanoparticles, metal di�usion and sintering can already occur at lower tempera-

tures due to melting-point depression e�ects. Many other factors are also described to have

an e�ect on the onset temperature of sintering such as metal-support and metal-chloride

interactions.[44, 102, 103].

THuttiд [K] = 0.3 · (Tmeltinд [K]) (2.18)

C-π sites can spontaneously reduce some metal salts, as well (see section 2.2.5.4).

Catalyst characterization

Heading to catalyst characterization, the three most important properties of supported metal

catalysts are metal loading, metal dispersion and metal distribution. Besides, pore geometry

and surface composition of the catalyst support material are relevant (see sections 2.2.1 and

2.2.5.1). These properties signi�cantly in�uence catalytic activity.[63]

The total amount of metal loaded to the carbon support is determined by e.g. ash analysis

after thermal carbon decomposition. Also, a multi-stage metal extraction with aqua regia is

possible. Alternatively, mass balances of metal concentrations in impregnation and washing

solutions indirectly give information on the catalyst’s metal loading.

Metal dispersion refers to the number of catalytically active sites that are principally accessi-

ble to reactant molecules [104]. As shown in equation 2.19, it’s de�ned as the ratio of surface

metal atoms nsur f ace atoms to the total number of metal atoms ntotal in a catalyst. With increas-

ing metal dispersion, the metal cluster size decreases. For spherical metal clusters, the metal

cluster size dcluster is de�ned in equation 2.20 withvmetal atom being the volume and smetal atom

the surface area of a metal atom.

Dmetal =nsur f ace atoms

ntotal(2.19)

dcluster =6 · vmetal atom

Dmetal · smetal atom(2.20)

31


C

O

C

O

C

O

C O

C

O

C

O

(a) (b) (c)

(d) (e)

Figure 2.9.: Adsorption of carbon monoxide to metal surfaces: linear (a), bridged (b & c),

triple bond (d), dissociative bonding (e) ([104], modi�ed)

Metal dispersion is determined in chemisorption experiments. Hydrogen or carbon monox-

ide gas adsorbs to surface metal atoms and the amount is correlated to the catalyst’s metal

loading. Hydrogen chemisorption on carbon supports can be di�cult to interpret due to

hydrogen spillover e�ects. In case of carbon monoxide chemisorption, it needs to be noted

that �ve di�erent adsorption behaviors are known, as �gure 2.9 demonstrates. Stoichiomet-

rically, a carbon monoxide molecule can adsorb to between one and three metal atoms or

dissociatively adsorb to two metal atoms. Temperature programmed desorption of carbon

monoxide loaded catalysts gives insight into these adsorption phenomena. At a palladium

surface, for instance, linearly adsorbed carbon monoxide desorbs at lower temperatures than

bridge bonded and triple bonded carbon monoxide, respectively. It was found that carbon

monoxide chemisorption stoichiometry depends on particle size and decreases from 2 to 1

with decreasing particle size.[104–106]

An overview of metal cluster sizes is apparent from electron microscopy images (SEM or

TEM). Alternatively, crystallite sizes are derived from X-ray di�raction (XRD) spectra using

the Scherrer equation [104].

The third property of noble metal catalysts is the metal distribution over the cross-section

of the material. Metal distribution in particulate catalysts is classi�ed into uniform, egg

shell, egg white, and egg yolk distributions (see �gure 2.10). Egg yolk type catalysts per-

form well in kinetically controlled reactions, while egg white and egg shell distributions are

preferable in di�usion limited reactions. Uniform type catalysts exhibit the largest metal

dispersions, because the area for metal deposition is largest. The type of metal distribution

is controlled during catalyst impregnation and drying. Tomographic methods are applied to

32


(a) (b)

(c) (d)

Figure 2.10.: Metal distributions in catalyst particulates: uniform (a), egg-shell (b), egg-white

(c), egg-yolk (d) ([108], modi�ed)

determine metal dispersion. The easiest way is to embed the catalyst particulate in a resin,

polish the sample and scan the cross section using energy-dispersive X-ray spectroscopy

(SEM-EDX).[107, 108]

2.2.5.3. The role of metal precursors in catalyst preparation

Palladium on activated carbon

In case of palladium catalyst preparation to a microporous carbon support, the steric demand

of the metal precursor in�uences the �nal position of the metal clusters in the pore system.

Planar precursor molecules like tetrachloropalladic acid adsorb into slit-shaped micropores,

while more bulky metal precursors like palladium acetate don’t. This was indicated by dif-

ferences in catalytic hydrogenations of cyclohexene, as cyclohexene doesn’t adsorb well to

palladium in micropores. Comparison with a non-microporous carbon support didn’t exhibit

these di�erences.[76]

Concerning transformation of the palladate precursors to metallic palladium by reduction

in hydrogen atmosphere, the reduction temperature in�uences the �nal metal dispersion.

The highest palladium dispersions are observed at 80°C. Metal dispersion decreases with

increasing reduction temperature.[76, 99]

Halide-free metal precursors are also of particular interest, because some catalyzed reac-

33


tions are a�ected by residual halides.[109] Chloride-free and highly dispersed palladium cat-

alysts are prepared in an incipient wetness impregnation with palladium acetylacetonate in

tetrahydrofuran.[92] The palladium complex of tetramethylammonium hydroxide is a suit-

able, halide-free precursor, as well, which is precipitated to activated carbon by increasing

the impregnation solution’s pH value.[110] Halide-free, colloidal palladium on carbon cat-

alysts is prepared with electrostatically and sterically stabilized nanoparticles. Traditional

palladium on carbon catalysts are outperformed by the colloidal palladium on carbon cata-

lyst in terms of cinnamic acid hydrogenation activity and catalyst lifespan (see also section

2.2.6 for details).[94]

Platinum on activated carbon

Platinum on activated carbon catalysts are successfully prepared in wet impregnation pro-

cesses applying electrostatic adsorption of hexachloroplatinic acid or platinum tetramine

chloride precursors. After metal transformation, platinum clusters are very small and nar-

rowly distributed (1-2 nm) over a wide range of metal loadings (0.5-13.5 wt%). During im-

pregnation, it is important to account for pH bu�ering of the carbon. The platinum tetramine

precursor is most suitable for impregnating carbon materials of low PZC applying neutral or

alkaline impregnation solutions. The anionic platinate of hexachloroplatinic acid is ideally

deposited to carbon materials of a higher PZC using an acidic impregnation solution.[96]

In incipient wetness impregnations, these two platinum precursors, dissolved in a benzene-

ethanol mixture or in water, are also successfully deposited to carbon materials.[93] Here,

surface functionalization doesn’t strongly a�ect catalyst quality.[63] Metal transformations

are performed in hydrogen atmosphere at 400-500°C. In this temperature range, platinum

dispersion decreases with increasing reduction temperature. Pretreatment of the impreg-

nated materials in helium atmosphere at reduction temperature improves metal dispersion

after metal transformation.[93]

2.2.5.4. The role of C-π sites in catalyst preparation

For graphitic carbon materials containing many surface C-π sites, two pathways of metal salt

deposition were proposed by Simonov et al. correlating experimental results with electro-

chemical theory (see scheme 2.2).

In the �rst case, tetrachloropalladic acid is spontaneously reduced to elemental palladium

at the outer surface of the carbon material by the electrical double layer of the material.

The electrical double layer, formed by adsorbing counterions to the charged carbon particle,

34


Scheme 2.2 Reaction pathways of palladate deposition on graphitic carbon [97]

H2PdCl4 +C → Pd/C + 2Cl/C + 2HClH2PdCl4 +C → PdCl2/C + 2HCl

increases the reductive strength of the graphitic carbon. Alternatively, palladium chloride

adsorbs to the C-π sites. The di�erent oxidation states of deposited palladium can be deter-

mined by XPS analysis. The formation of metallic palladium at the outer carbon surface is

immediately visible as the black carbon turns gray. An egg-shell metal distribution results.

Ion adsorption of palladate to C-π sites is more uniform across carbon pellets. A broad size

range of metal clusters are formed in these processes with small clusters in the range of 1-3

nm and larger palladium clusters in the range of 6-100 nm.[97] The in-situ reduction of metal

salts at C-π sites generally results in larger cluster sizes than pure ion adsorption to oxygen

surface groups.[63, 73, 99]

Also, at longer impregnation times, hexachloroplatinic acid reacts with C-π sites in an ad-

sorptive reduction reaction.[96]

The degree of graphitization can be in�uenced by heat treatment. Heat treatment of acti-

vated carbon at 1300-1500°C forms a graphitized surface consisting of C-π sites and lacking

oxygen groups. With this support, a good platinum dispersion is obtained due to adsorption

of metal precursor to C-π sites.[111] For oxidized graphitized carbon loaded with platinum,

the presence of surface oxides negatively a�ects resistance to sintering and thus platinum

dispersion.[112]

2.2.6. Reactions studied in this work

A variety of model reactions have been used in this work. All are slurry-phase hydrogena-

tion or dehydrogenation reactions, i.e. the reaction systems are comprised of suspended

catalyst material in reactant solutions with dissolved hydrogen gas. In these catalyst screen-

ing experiments, catalytic performance has been determined by monitoring the conversion

of the substrate and deriving reaction rate constants or turnover frequencies. Besides cat-

alytic activity, product selectivity was another relevant parameter studied in this work. In

the following, the di�erent model reactions are presented together with turnover frequencies

of noble metal carbon catalyst found in literature.

35


Scheme 2.3 Hydrogenation of cinnamic acid

OH

O

OH

O

H2

Cinnamic acid Hydrocinnamic acid

Palladium catalysts screening

In accordance to literature, the hydrogenation activity of palladium on carbon catalysts is

evaluated in the hydrogenation of cinnamic acid to hydrocinnamic acid (see scheme 2.3).[94,

110, 113–115] Only the unsaturated carbon-carbon bond is reduced at ambient reaction con-

ditions. Reaction rates at ambient conditions regarding temperature and hydrogen pressure

are already signi�cant, so that palladium catalysts can be screened under mild conditions

within reasonable reaction times.

As found in literature, the cinnamic acid turnover frequency of a supported palladiumnanopar-

ticle catalyst amounted to 0.51molCinnamic acid molPd-1s-1at 25°C and 1 bar hydrogen pressure.

Here, stabilized palladium nanoparticles, with particle sizes of 3.2±0.98 nm, were deposited

on activated carbon with a metal loading of 4.83 wt%.[114] For 5 wt% palladium on activated

carbon materials, cinnamic acid turnover frequencies of up to 0.92 molCinnamic acid molPd-1s-1

were determined at the same ambient process conditions.[113]

Platinum catalysts screening

Cinnamaldehyde hydrogenation is a model reaction to characterize platinum on carbon cat-

alysts.[44, 116–118] As scheme 2.4 shows, the unsaturated carbon-carbon bond and the alde-

hyde group is reduced at reaction conditions. Thus, product selectivity can also be studied

for this reaction.

Referring to literature reports, activated carbon loaded with 4 wt% platinum was tested in

the hydrogenation of cinnamaldehyde at 100°C and 60 bar hydrogen pressure. It was demon-

strated that the addition of iron chloride to the reactant solution up to an iron to platinum

ratio of 0.2 signi�cant improved catalytic activity and selectivity towards cinnamic alcohol.

Initial turnover frequencies up to 0.02 molCinnamaldehyde molPt-1s-1and an initial cinnamic

alcohol selectivity up to 70% were reached. After reaction, iron was found to be deposited

on the platinum clusters and cluster sizes between 1.5-5 nm were determined.[116] A mono-

lithic structure with 1.23 wt% platinum on carbon exhibited a cinnamaldehyde turnover fre-

quency of 0.07 molCinnamaldehyde molPt-1s-1at 30°C and 50 bar hydrogen pressure. The average

36


Scheme 2.4 Hydrogenation of cinnamaldehyde

H

O

OH

H

O

OH

Cinnamaldehyde

Hydrocinnamaldehyde Cinnamic alcohol

Hydrocinnamic alcohol

H2 H2

H2 H2

platinum cluster size amounted to 4.1 nm.[117] The cinnamaldehyde turnover frequency of

carbon nano�bers loaded with 5 wt% platinum reached values up to 0.83 molCinnamaldehyde

molPt-1s-1at 30°C and 48 bar hydrogen pressure. Thermal treatment of the catalyst material

at 700°C under nitrogen atmosphere signi�cantly improved catalytic performance. Platinum

cluster sizes of 1.8±0.7 nm were obtained.[118]

Ruthenium catalysts screening

Ruthenium on carbon is suitable for the hydrogenation of carbocyclic rings.[44, 119] In this

work, multiple benzene derivatives have been used as reactants with such catalysts, such as

toluene, m-cresol and thymol. Additionally, hydrogenation of the linear molecule 1-octene

with ruthenium on carbon has been analyzed in this work. The molecule structures of the

four substates are shown in �gure 2.11.

In literature, toluene turnover frequencies up to 2.72 molToluene molRu-1s-1were reported

for ruthenium on di�erent carbon materials. Ruthenium metal loadings varied between

3.3-5.7 wt% and toluene hydrogenation took place at 110°C and 40 bar hydrogen pressure.

The most active ruthenium catalysts were prepared by thermal reduction of impregnated

ruthenium chloride to ruthenium clusters at 900°C in nitrogen atmosphere. It was hypothe-

37


Toluene

OH OH

m-Cresol Thymol

1-Octene

Figure 2.11.: Substrates for ruthenium catalyst screening - molecule structures of toluene,

m-cresol, thymol, and 1-octene

sized that thermal reduction signi�cantly increased metal-support interactions, which posi-

tively a�ected toluene hydrogenation activity.[120] Ruthenium on carbon nano�bers exhib-

ited turnover frequencies of up to 3.94 molToluene molRu-1s-1at 100°C and 30 bar hydrogen

pressure. Here, metal loading varied between 1.1-3.8 wt% and ruthenium clusters were be-

tween 2-4 nm in size.[121]

Rhodium catalysts screening

Rhodium metal complexes are, for instance, applied as catalyst species in the selective hy-

drogenation of cycloalkenes and its derivatives.[122, 123] In this work, the consecutive hy-

drogenation of 1,5-cyclooctadiene to cyclooctene and cyclooctane (see scheme 2.5) has been

investigated asmodel reaction to test the performance of a rhodiummetal complex supported

on spherical carbon.

According to literature, di�erent cationic rhodium(I) complexes have previously hydrogenated

1,5-cyclooctadiene with high selectivity (62-92%) towards cyclooctene under ambient condi-

tions (room temperature, 1 bar hydrogen pressure).[122]

Platinum catalysts in a technical relevant test reaction

A technical application of activated carbon based noble metal catalysts is the hydrogenation

and dehydrogenation of liquid organic hydrogen carriers (LOHCs). LOHCs are of high eco-

nomic interest as they provide a viable option to store excess electrical energy. To shortly

38


Scheme 2.5 Hydrogenation of 1,5-cyclooctadiene

H2 H2

1,5-Cyclooctadiene Cyclooctene Cyclooctane

Scheme 2.6 (De-)hydrogenation of Marlotherm SH

9 H2

DibenzyltolueneH18-dibenzyltoluene

describe the entire process, �rst of all, excess electrical energy is used to produce hydrogen by

electrolysis. Then, in a hydrogenation step, unsaturated LOHC compounds are catalytically

loaded with hydrogen. On demand, hydrogen is released and converted back to electrical

energy within a fuel cell.[124–126]

A promising LOHC candidate is the commercially available heat-transfer oil known by the

trade name Marlotherm SH. It is a mixture of isomeric dibenzyltoluenes and is toxicolog-

ically comparable to diesel fuel.[48] The (de-)hydrogenation scheme of Marlotherm SH is

shown in scheme 2.6. In this work, the dehydrogenation activity of fully hydrogenated H18-

dibenzyltoluene using platinum catalysts has been investigated.

Previous catalyst screenings published in literature show that for a 1 wt% platinum on carbon

catalyst, the dehydrogenation degree after 3.5 h reaction time at 170°C amounted to 71%. A

0.5 wt% platinum on aluminum oxide catalyst only reached 51% dehydrogenation degree at

the same platinum to LOHC ratio of 0.15 mol%.[48]

39


Table 2.2.: Selected gases and volatile organic compounds adsorbing to activated carbon sur-

faces

Gas classi�cation Gaseous species Molecularformula

Organic

Organic compounds in general [127–129]

Cyclohexane C6H12 [1]

Benzene C6H6 [1, 130]

Chloropicrin CCl3NO2 [1]

Carbon dioxide CO2 [131]

Acidic

Sulfur dioxide SO2 [128, 129]

Hydrogen chloride HCl [128, 129]

Inorganic

Chlorine Cl2 [130]

Heavy metals [128, 129]

2.3. Activated carbon in gas purification

2.3.1. Activated carbon adsorbents in gas purification

In gas puri�cation, activated carbons are applied as adsorbents for a variety of gases and

volatile organic compounds. The gaseous species accumulate at the internal surface of ac-

tivated carbons by adsorption to active surface sites or condensation in the pore structure.

So, the large internal surface area of activated carbons and the extensive pore structure are

highly bene�cial to achieve large adsorption capacities. The inherent surface hydrophobic-

ity and the resulting low wettability render activated carbon materials well suitable for �lter

applications at humid conditions. An incomplete list of species adsorbing to activated carbon

is presented in table 2.2.[1, 7, 127]

Adsorption kinetics

Active adsorption sites are located across the internal surface area of activated carbons.

These C-π adsorption sites, consisting of unsaturated carbon-carbon bonds, attract the gas-

eous species. Molecules are either adsorbed physically by van-der-Waals forces (physisorp-

tion) or more strongly by chemical bonding (chemisorption). Upon adsorption, adsorption

enthalpies between 5-65 kJ mol-1are released. Adsorption enthalpy increases with increasing

interaction strength of adsorbate and adsorption site.[1]

40


The equilibrium of a reversibly adsorbing gaseous species is in�uenced by temperature and

pressure, as well as partial pressure and adsorption enthalpy of the adsorbate. The adsorp-

tion equilibrium immediately a�ects the adsorption capacity of a gaseous species. With in-

creasing temperature, adsorption equilibrium is thermodynamically shifted to the gas-phase.

Larger pressures increase surface coverage. The presence of other compounds, competing

for adsorption sites, negatively a�ects the adsorption capacity of an adsorbate. A larger ad-

sorption enthalpy results in a preferred adsorption.[1, 22, 132, 133]

Additionally, if the adsorption temperature is lower than the critical temperature of the ad-

sorbate, capillary condensation in the pore structure takes place. Thus, not only the surface

area, but the whole pore volume is utilized to retain the adsorbate. This e�ect allows for sig-

ni�cantly enhanced adsorption capacities, especially in microporous activated carbons.[1]

Several models are available to describe isothermal adsorption processes of gaseous species

to porous adsorbents. For activated carbon materials, the Dubinin model generally �ts best.

Here, micropores are �lled with adsorbate at �rst, followed by multi-layer adsorption in

meso- and macropores.[1, 132]

Adsorption capacity is an important characteristic of adsorbents in gas puri�cation applica-

tions. It describes the total amount of an adsorbate that can be retained to an adsorbent at

de�ned process conditions. But also, e�ective adsorption kinetics are highly relevant. The

gaseous species need to e�ectively di�use through the pore system to the active site and

adsorb to the surface. Pore di�usion limitation and slow intrinsic adsorption rates decrease

the performance of a �lter material. The hierarchical pore structure of activated carbon ma-

terials promotes e�cient mass transport. The intrinsic adsorption rate is determined by the

type of adsorbate and the carbon surface composition.[1]

Reversible and irreversible adsorption modes

Depending on the �eld of application, adsorption of gaseous species is either reversible or

irreversible. Adsorbents are regenerated in closed systems, low-maintenance systems or in

bulk �ltrations. Regeneration of the adsorbents is realized in pressure swing adsorption

(PSA) or temperature swing adsorption (TSA) setups. The adsorbate is desorbed by either

decreasing the pressure of the adsorption column or increasing temperature. A PSA setup is

less energy intensive than a TSA setup for adsorbates exhibiting adsorption enthalpies below

30 kJ mol-1. In case of adsorbent regeneration, multiple adsorption and desorption cycles

need to be monitored to determine long-term adsorption capacities. Irreversible adsorption

of the gaseous species is applied in �ne �ltrations. The adsorbed species permanently remain

41


in the �lter material and are not released at changing process conditions, e.g. temperature

increase or a strip gas stream.[1, 134]

Adsorption performance measurements of filter materials

The performance of adsorbents is characterized by measuring adsorption isotherms of spe-

ci�c compounds or in continuous breakthrough experiments. Information about the adsorp-

tion capacity, the type of adsorption isotherm and the adsorption enthalpy is derived from the

discontinuous adsorption of speci�c compounds. At di�erent relative pressures and isother-

mal conditions, the equilibrium surface coverage is determined. Using multiple adsorption

isotherms and applying the Clausius-Clapeyron equation, the adsorption enthalpy is calcu-

lated from pressure and temperature points having equal surface coverages (see equation

2.21).

(d ln(p · p−1

0)

dT −1)θ=const =

∆Hads

R(2.21)

Here, p is the pressure, T the temperature, θ the adsorbate surface coverage, ∆Hads the ad-

sorption enthalpy and R the ideal gas constant.[1, 134]

In breakthrough experiments, a gas stream with the adsorbing species continuously �ows

through an adsorber unit. This adsorber unit contains a bed of adsorbents with de�ned height

and diameter. Temperature and pressure in the adsorber unit are regulated according to the

desired process conditions. Concentration di�erences of the adsorbing species between inlet

and outlet of the adsorber unit are monitored over the process time. Optionally, the pressure

di�erence is also determined for investigations concerning �ow characteristics of di�erent

adsorbents. Results of a typical breakthrough experiment are shown in �gure 2.12.

In the �rst section of the breakthrough curve, the outlet concentration of the adsorbing

species is ideally zero. The gaseous compound is completely adsorbed to the �lter mate-

rial. At the breakthrough time, a certain threshold concentration is detected in the outlet

stream. The breakthrough capacity has been reached and the �lter material no longer ad-

sorbs all tested gas compounds. In the following saturation phase, the outlet concentration

increases until inlet and outlet concentration are equal. At this point of complete saturation,

the saturation capacity of the �lter material is reached. For most applications, breakthrough

capacity is much more relevant than the saturation capacity, though. The slope of the sat-

uration phase correlates to the e�ective adsorption rate. This is due to the conception that,

42


0 200 400 6000.00.20.40.60.81.0 (3)

(2)

c c

-1 0 /

-

t / min

(1)

Figure 2.12.: Typical breakthrough curve of a continuous breakthrough experiment: (1) ex-

emplary breakthrough point, (2) curve in�ection point, (3) saturation point

within a certain adsorption zone, gaseous species passing the �lter bed adsorb to consecutive

active sites. The adsorption zone moves through the �lter bed. When the adsorption zone

reaches the end of the �lter bed, concentration gradients are visible by measuring the outlet

gas concentration and adsorption kinetics can be derived.[134, 135]

Various mathematical models have been postulated to describe breakthrough curves of con-

tinuous, gas-phase adsorption experiments.[136] For instance, simpli�ed mass balances of

isothermal �xed-bed reactors were derived and integrated with respective boundary con-

ditions. Steady decrease of active sites was also implemented. Some models ended up with

more complex solutions that had to be applied iteratively.[137] One-dimensional, continuous,

time-dependent concentration functions were also reported.[138] In this work, an empirical

approach has been applied to �t individual data points of breakthrough experiments. The

function of hyperbolic tangent type is shown in equation 2.22.

c · c−10= A · tanh(B · t +C ) +A (2.22)

In this time-dependent function of normalized test gas concentration c · c−10, parameter A

43


represents the normalized concentration regime. Ideally, A = 0.5, so that 0 ≤ c · c−10≤ 1.

Due to measurement uncertainties, often A , 0.5 and the data series are normalized by the

factor 0.5 · A−1. E�ective adsorption kinetics are essentially described by parameter B. Fast

adsorption and �lter material saturation is indicated by large values of B. ParameterC math-

ematically describes the function’s horizontal alignment. This o�set is directly in�uenced

by breakthrough time and adsorption rate. Applying this �t function, breakthrough times

can be estimated for any breakthrough concentration. Also, adsorption behavior of di�erent

�lter materials can be compared. The slope m at the in�ection point of the breakthrough

curve e�ectively quanti�es the adsorption rate (see equation 2.23).[16]

m = A · B (2.23)

2.3.2. Functionalization of activated carbon with reactive surfaces

The adsorption capability of pure activated carbon is limited to a certain range of compounds

that interact with the unsaturated carbon-carbon active surface sites (see section 2.3.1). Ad-

sorption of other species is realized by carbon surface functionalization. The surface of ac-

tivated carbon is e�ectively modi�ed by introducing surface functional groups or by im-

pregnation with reactive components. Oxygen and nitrogen surface functional groups are

the most prevailing carbon surface modi�cations that are directly integrated into the carbon

structure (see also section 2.2.5.1). Concerning impregnations, metals, metal salts, metal ox-

ides and non-metal salts are predominantly applied.[1] The impregnations are located inside

the pore system of activated carbon, dispersed at the carbon surface.

In the following, these di�erent surface modi�cations enhancing adsorption capabilities of

activated carbon are described and summarized in table 2.3. The presented surface modi�ca-

tions allow for reactive adsorption of targeted gas compounds under ambient conditions. The

combination of active sites with reactivity for di�erent gaseous species is utilized in broad-

band �lters. The activated carbon support also extends adsorption capacity for other gases

that don’t react with the impregnation, e.g. unpolar organic compounds like cyclohexane

(see section 2.3.1). Nevertheless, the absolute gas adsorption capacity cannot be signi�cantly

increased by these types of surface modi�cations. This is, because the absolute number of ac-

cessible surface adsorption sites doesn’t increase after surface group introduction or impreg-

nation.[139] Under humid conditions, though, water dissolves salt impregnations increasing

the number of accessible reaction sites by absorption into the liquid phase.[7]

44


Activated carbons with oxygen functional groups

Activated carbon functionalizedwith oxygen surface groups signi�cantly increases ammonia

adsorption capacity. An activated carbon material, functionalized by air oxidation at 300°C

and then treated in a nitrogen atmosphere at 500°C exhibited an average ammonia capacity

of 8.9 mg g-1. Compared to pure activated carbon, ammonia capacity increases by a factor of

6.4. Humid conditions promote the overall ammonia adsorption, likely because water allows

the formation of ammonium ions. The ammonium ions then adsorb to deprotonated acidic

surface groups.[81, 140] Oxidized carbon adsorbents also adsorb sulfur dioxide better than

the pure carbon variant.[141]

Activated carbons with nitrogen functional groups

Nitrogen surface functionalization of polymer-based spherical activated carbon improves

sulfur dioxide, hydrogen sul�de and formaldehyde adsorption capacities. With increasing

nitrogen content, the adsorption capacities of sulfur dioxide and hydrogen sul�de increase.

Formaldehyde adsorption performance strongly depends on the presence of water. The base-

catalyzed polymerization to react away formaldehyde seems to require an aqueous environ-

ment at the carbon surface.[7]

Activated carbons with alkali metal salt impregnations

Activated carbon containing potassium hydroxide, potassium carbonate, potassium iodide,

sodiumhydroxide or sodium carbonate is applied to remove acidic gases like hydrogen sul�de

from air. The reaction mechanism of hydrogen sul�de with potassium iodide is an oxidation

to elemental sulfur. Also catalyzed by potassium iodide, phosphine is oxidized to phospho-

ric acid. Boron species are also removed using potassium hydroxide impregnated activated

carbon.[1, 127, 139, 142, 143]

Biological �lters are comprised of antibacterial potassium iodide impregnated activated car-

bon.[142]

Activated carbons with acid impregnations

Phosphoric acid, sulfuric acid and citric acid impregnations adsorb and neutralize amine

species.[127, 142]

Activated carbons with transition metal oxide impregnations

Porous carbon materials impregnated with molybdenum trioxide or vanadium pentoxide

adsorb ammonia. Ammonia capacities up to 4 wt% were reached with 5-10 wt% metal oxide

45


Scheme 2.7 Ammonia chemisorption by amine-complex formation with metal cations

Cu2+ + 4NH3 Cu (NH3)2+4

Zn2+ + 4NH3 Zn(NH3)2+4

impregnations in continuous operation in air at 1 bar and 25°C.[81]

Activated carbons with metal salt impregnations

Metal salt impregnations with copper(II) chloride or zinc(II) chloride perform well in the

removal of ammonia and other amine species. With an ammonia breakthrough capacity

of 7 wt% at 1 bar and 25°C, metal salt impregnations performed even better than equally-

loaded metal oxide materials. Correlating surface pH values to material acidities, the metal

chloride impregnated carbon systems exhibited highest acidities. Thus, metal chloride im-

pregnations o�ered the largest reactivities for ammonia. It was proposed that water enables

Brønsted acid formation of inorganic metal salts, thus facilitating ammonia adsorption. Un-

der dry conditions, Lewis sites are mostly responsible for ammonia adsorption. Spectro-

scopic measurements allowed in-depth investigation of ammonia adsorption mechanisms. It

was demonstrated that metal chlorides form amine-complexes upon ammonia adsorption.

In scheme 2.7, this complexation is presented for copper(II) and zinc(II) salts.[81, 142, 144]

Zinc acetate impregnations also facilitate the adsorption of ammonia species. Concerning

hydrogen sul�de removal, copper sulfate or lead acetate impregnated materials are suitable

adsorbents. Arsine is adsorbed by a copper tetraamine salt impregnation via exchange of the

complexing ligand.[127]

Activated carbon impregnations with copper(II) salts generally perform better than other

metal salt impregnations due to the larger stability of copper complexes with ligands of

all types. Due to the reciprocal ionic radii and second ionization potentials, zinc, nickel,

or cobalt salts exhibit lower formation constants of metal-ligand systems.[145] In essence,

copper(II) salts form the most stable complexes with ligands such as ammonia species (e.g.

ammonia, ethylendiamine, diethylentriamine, triethylentetramine), most amino acids, and

cyanide. Of these systems, ammonia exhibits much larger formation constants than cyanide.

Even though, most copper salts form complexes with water, ammonia displaces water as a

ligand, because it’s more polarizable than water.

The good performance of copper(II) salt impregnations in hydrogen sul�de adsorption can be

explained similarly. It’s well known that e.g. copper(II) chloride forms more stable products

with hydrogen sul�de than cadmium or zinc chloride. The metal sul�de solubility constant is

the largest for zinc sul�de. It decreases by a factor of 250 for cadmium sul�de and by a factor

46


Scheme 2.8 Hydrogen sul�de chemisorption by sul�de-complex formation with metal

cations [147]

Cu2+ + H2S � Cu (HS )+ + H+

Cu (HS )+ + H2S � Cu (HS )2 + H+

of 3x1011

for copper sul�de. It’s proposed that copper(II) salts are reduced by sul�des to

result in copper(I) hydrogen sul�de and later in copper(0) dihydrogen disul�de (see scheme

2.8).[145–148]

Activated carbons with metal impregnations

Elemental copper on polymer-based spherical activated carbon shows very large capacities

for hydrogen sul�de and sulfur dioxide removal. It’s proposed that hydrogen sul�de more

e�ectively yields copper(II) sul�de with elemental copper impregnations than with copper

salts.[7]

Silver impregnated activated carbons are used in biological �lters due to the antibacterial

properties of silver ions.[142]

2.3.3. Application of activated carbon adsorbents in gas purification

2.3.3.1. Air filtration in clean rooms and for personal protection

Activated carbon adsorbents are applied in clean rooms and for personal protection to �lter

airborne contaminants. In both applications, the air �ltration takes place at ambient condi-

tions, i.e. room temperature and atmospheric pressure. The concentration of contaminants

in air is usually very low, within the ppm-level. To mitigate desorption of hazardous com-

pounds, the adsorption process needs to be irreversible, at least under operation conditions.

These process requirements necessitate speci�cally designed �lter materials. In the follow-

ing, air �ltration in clean rooms and for personal protection is presented, including the role

of activated carbon adsorbents in these two processes.

Air filtration in clean rooms

Clean rooms are intended to protect products (e.g. semiconductors) from environmental con-

tamination or the environment from chemically or biologically hazardous substances. Air

continuously circulates through the clean room and is primarily �ltered for particulate mat-

ter. Particles down to 0.3 μmare separated by high-e�ciency particulate air (HEPA) �lters. In

order to remove chemical substances, activated carbon �lters are installed, additionally.[142]

47


Table 2.3.: Overview of the adsorption capabilities of di�erent carbon surface modi�cations

Type of surfacemodi�cation

Active sites Examples oftargeted gascompounds

Carbon surface

functionalization

Oxygen surface

groups

NH3, SO2 [81, 141]

Nitrogen surface

groups

SO2, H2S, CH2O [7]

Non-metal salt

impregnations

KOH H2S, H3BO3 [127,

142]

NaOH, Na2CO3,

K2CO3

H2S, HCl, HF [1, 127,

139, 142,

143]

KI H2S, PH3, AsH3 [1]

H3PO4, H2SO4,

C6H8O7

NH3 [127,

142]

Metal oxide

impregnations

MoO3, V2O5 NH3 [81]

Metal salt

impregnations

CuCl2, ZnCl2,

Zn(OAc)2

NH3 [81, 127,

142, 144]

CuSO4, Pb(OAc)2 H2S [127]

Cu(NH3)4SO4 AsH3 [127]

Metal

impregnations

Cu H2S, SO2 [7]

48


In semiconductor processing, typically done in clean rooms, air contamination by boron

compounds is an important issue. Airborne boron species a�ect doping levels of semicon-

ductors. Traces of boron species are present in the atmosphere and standard HEPA clean

room �lters consisting of borosilicate glass also release boric acid. As already presented in

section 2.3.2, boron species are removed using potassium hydroxide impregnated activated

carbon. Furthermore, activated carbons containing speci�c salt impregnations are applied

to remove acid gases, amine species and airborne minerals from air. Biological �lters are

comprised of antibacterial potassium iodide or silver impregnated activated carbons.[142]

Air filtration for personal protection

The other application of activated carbon adsorbents in air �ltration is the protection of �rst

responders from hazardous gases.[1, 5] A few broadband �lters exist covering all toxic gases.

But, most �lters are made for a speci�c group of airborne substance, like particles, organic,

inorganic, and acidic gases, as well as ammonia species.[16, 75, 149]

Standardized test procedures exist to determine gas adsorption capacities. Also, minimum

adsorption capacities are speci�ed in product requirements. This allows for comparison of

�lter materials. Examples of standardized procedures are the DIN EN 14387 ABEK1 standard

and the US NIOSH test parameters.[150, 151]

Substances with high boiling points, i.e. organic gases, directly adsorb to carbon materi-

als (see section 2.3.1). This behavior is also present for silica and alumina, but not at high

relative humidity. This is why activated carbons are mainly used in air �lters for personal

protection. The �lter materials need to perform well in environments with di�erent relative

humidity. With salt impregnations, similar to clean room �lters, additional functionality for

other gases is realized (see section 2.3.2 for details). Acidic gases, ammonia species and in-

organic gases are e�ectively removed by salt impregnated activated carbons. Mixing carbon

adsorbents with di�erent impregnations to allow broadband capabilities is possible without

negatively a�ecting adsorption capacities. Irreversibility of adsorption until breakthrough is

also important in �lter materials for personal protection.[81, 127]

2.3.3.2. Industrial exhaust gas cleaning

European and national regulations limit emission of toxic gases to protect the environment.

Threshold concentrations of many substances are speci�ed in the European Union Industrial

Emissions Directive 2010/75/EU and the German technical instructions for the maintenance

of air purity (“TA Luft”).[152, 153] These values are oriented on environmental toxicity of

49


a substance and the capability of available �ltration technologies to remove this substance.

E�ective equipment and operation costs are also accounted for. With emerging �ltration

technologies or toxicological knowledge, threshold concentrations are reevaluated. These

emission regulations apply to all industrial and agricultural processes. Common substances

in o�-gases, that are known to be hazardous to the environment, are hydrogen sul�de, sulfur

dioxide, nitrogen oxides, ammonia species, heavy-metal compounds, organic compounds,

and particles smaller 10 μm.[154]

Solid particles are separated by electrical precipitation, for instance. By total oxidation with

air, organic compounds are converted to inorganic oxides like carbon dioxide, sulfur dioxide,

nitrogen oxides and metal oxides.[128, 129]

Sulfur dioxide gas is industrially removed by absorption in calcium carbonate solutions at pH

values between 6.5 and 7. Calcium sulfate is irreversible formed. Activated carbons can also

be applied in sulfur dioxide removal. With activated carbons, sulfur dioxide is oxidized by

air in the presence of water, resulting in the deposition of sulfuric acid in the pore system. At

400-500°C, these carbon adsorbers can then be thermally regenerated yielding sulfur dioxide,

steam and carbon dioxide. The carbon atoms of carbon dioxide originate from the activated

carbon support itself.[1, 128, 129]

Nitrogen oxides are catalytically reduced to nitrogen gas at temperatures between 200-600°C.

Therefore, e.g. ammonia is added as reducing agent in front of highly selective metal/metal

oxide catalysts. Activated carbon also catalyzes the reduction of nitrogen oxide in the pres-

ence of ammonia and air.[1, 128, 129]

Heavy metals and organic substances are removed by adsorption to activated carbon. Hy-

drogen chloride adsorbs to activated carbon, as well.[128, 129]

Impregnated activated carbons are not yet reported to be applied in the �ne �ltration of

o�-gas streams, thus also removing the last remaining hazardous molecules. Possibly, �ne

�ltration with impregnated adsorbents isn’t currently economical due to limited adsorption

capacities and limited regeneration abilities.

2.3.4. Functionalization of activated carbon with ionic liquids

2.3.4.1. Supported ionic liquid phase (SILP) concept

In the supported ionic liquid phase concept, a thin �lm of ionic liquid is dispersed on a porous

support material. Macroscopically, a dry and pourable solid results, with the ionic liquid

50


immobilized inside the pore system. SILP materials were initially developed for applications

in catalysis. Homogeneous, catalytically active complexes were immobilized in the ionic

liquid �lm. These SILP catalysts were successfully tested in hydroformylation and watergas-

shift reactions. The small �lm thicknesses of around 1-3 nm and the large exchange surface

areas allow for fast di�usion and mass transfer. Next to pore volume and BET surface area,

the ionic liquid loading αIL is an important parameter as de�ned in equation 2.24. It’s the

ratio of ionic liquid volume VIL over the total pore volume Vpore .

αIL =VILVpore

(2.24)

Compared to traditional two-phase liquid-liquid processes, only small amounts of ionic liq-

uid are necessary, rendering the process e�cient and economically feasible. Applying sup-

ported ionic liquid phase technology to gas puri�cation is a fundamentally new approach to

increase the capacity of solid adsorbents.[10] Compared to traditional solid impregnations

(see section 2.3.2), not just surface adsorption sites are available, but absorption of gases into

a liquid volume takes place. The gaseous substances dissolve in the supported, liquid salt �lm

or undergo a chemical reaction in the liquid salt (see �gure 2.13). It needs to be noted that

gas puri�cation using SILP materials is macroscopically an adsorption process, but micro-

scopically an absorption process. In the following, important properties of ionic liquids are

presented, as well as the e�ects of metal salt dissolution into ionic liquids.[9, 10, 16, 155–158]

51


M

M

M

+ - + - + -

+ -

+ - + -

+ -

Support

PoreIonic liquid

Hazardous gas

Figure 2.13.: SILP concept ([16] - adapted by permission of The Royal Society of Chemistry)

2.3.4.2. Properties of ionic liquids

Ionic liquids are made of cations and anions. They have melting points below 100°C. Char-

acteristically, ionic liquids exhibit very low vapor pressures and are thermally stable up to

250°C.[159–161] By selectively combining cations and anions, ionic liquids can adopt a broad

range of physicochemical properties. For application in gas puri�cation, melting point, solu-

bility, reactivity, and thermal stability are the most important parameters. If the ionic liquid

is in fact liquid at operating temperature, mass transfer from the gas phase into the ionic liq-

uid phase takes place. Solubility of certain gases can be selectively increased or decreased to

facilitate gas separation. Ionic liquids can be tuned to physically solve signi�cant amounts of

speci�c gases. These physically loaded ionic liquids can be easily regenerated at decreased

pressure or increased temperature.[162] Additionally, reactive functionalities can be inte-

grated into the used ionic liquids. So, substances can be removed from the gas phase by

forming chemical bonds. Ionic liquids can also solve regular salts in large quantities, while

still remaining an ionic liquid. Thus, reactive functionalities of these regular salts are in-

herited.[17] Solubility parameters of ionic liquids are predictable by a priori calculations, for

instance with COSMO-RS. Thus, the immense number of ion combinations is systematically

screened to �nd ionic liquids with suitable properties.[163]

The fact that ionic liquids have extremely low vapor pressures is a huge advantage in con-

tinuous gas-phase processes. Release of ionic liquid into the product gas feed is e�ectively

mitigated. Very high product purities are achieved. For immobilized ionic liquids, long-term

stable adsorbents result with large gas-liquid interfaces for e�cient mass transfer.[10, 16]

52


2.3.4.3. Reactive metal complexes dissolved in ionic liquids

In conventional �lter materials with metal salt dispersed on a solid support (see section 2.3.2),

only metal salts at the solid-gas interface are accessible to the hazardous gases. This signi�-

cantly limits adsorption capacity of thesematerials to roughly the surface area of the support.

Dissolving metal salts in ionic liquids and preparing a SILP material results in increased gas

adsorption capacities.[17] All metal species are accessible by absorption of the gaseous sub-

stances into the ionic liquid. For good accessibility and utilization, the metal salts need to be

dissolved, or at least �nely suspended, in the ionic liquid. Reactivity of the ionic liquid �lm

is greatly enhanced by incorporating reactive metal salts.

As presented in section 2.3.2, metal salts are known to readily reactwith hazardous gases such

as hydrogen sul�de and ammonia, e.g. by complex formation. Complex formation already

takes place at room temperature. Reactivity depends on complex formation constants of

metal salts. The stability of the formed complex is characterized by product dissociation

constants.

2.3.5. Application of SILP adsorbents in gas purification

2.3.5.1. SILP adsorbents based on inorganic supports [9, 10]

In 2012, the Malaysian company Petronas reported the application of a SILP material for the

removal of mercury compounds from natural gas. An industrial adsorber unit utilizing 60 t of

SILP material was presented. The exact SILP composition and reaction mechanisms weren’t

published, though. This is the �rst published, large-scale application of SILP technology in

gas puri�cation.[12]

Concerning the �ue gases sulfur dioxide and carbon dioxide, suitable ionic liquids with large

absorption capacities were found. Several research groups immobilized these ionic liquids to

study e�ects like mass transfer, stability and regeneration ability.

Zhang et al. characterized a SILP material based on 1,1,3,3-tetramethylguanidinium lactate

and silica in the adsorption of sulfur dioxide at 20-30°C and 1 bar. With increasing ionic liq-

uid loading, pore volume, speci�c surface area and porosity of the material decreased. Also,

apparent density and average pore diameter increased. The average pore diameter increased

as primarily the smaller pores were �lled with ionic liquid. The mechanism of sulfur diox-

ide absorption in the ionic liquid was proposed to be a reaction with the amine functional

group on the cation [164]. Due to the large gas-liquid interface, sorption equilibrium of SILP

53


Scheme 2.9 Carbon dioxide chemisorption in amino-functional ionic liquids [15, 165]

(Reprinted with permission from [15]. Copyright 2012 American Chemical Society)

−NH2 +CO2 � −NHCOOH

−NHCOOH + −NH2 � −NHCOO− + −NH+3

materials was reached in less than 15 min, while the bulk ionic liquid required 45 min. After

decreasing adsorption temperature from 30 to 20°C, sulfur dioxide capacity increased by 40%.

Interestingly, SILP materials with lower ionic liquid loadings exhibited larger sulfur dioxide

capacities of up to 0.95 gSO2 gIL-1. Sulfur dioxide could have condensed in the smaller pores

prevailing at low ionic liquid loadings. Regeneration and recycling of SILP material was

carried out at 90°C and 0.1 bar without loss in capacity.[13] Reversible adsorption of sulfur

dioxide and carbon dioxide using di�erent SILP materials was also reported by Shunmugavel

et al..[14]

For carbon dioxide adsorption, Ren et al. [15] immobilized liquid amine-functionalized amino

acid salts on silica. The e�ects of both pure carbon dioxide and �ue gas (14% CO2 in N2) were

investigated. In experiments with a lysine-based SILP material at 1 bar and 25°C, the adsorp-

tion capacity decreased from 1.87 mmolCO2 gSILP-1using pure carbon dioxide to 1.54 mmolCO2

gSILP-1using �ue gas. Contrary to previous assumptions, carbon dioxide sorption stoichiome-

try didn’t appear to depend on the amine groups’ location, but rather on ion pair size. Carbon

dioxide chemically reacted with the ionic liquid in a 1:2 mechanism. At �rst, carbamic acid

was formed, which protonated a second amine functional group forming carbamate and an

ammonium ion (see scheme 2.9).

The in�uence ofwaterwas investigated, aswell. Largewater contents signi�cantly decreased

carbon dioxide capacity due to competition in hydrogen bonding. The SILP materials based

on glycine and lysine anions were successfully regenerated at 0.08 bar and 90°C. Because

the ionic liquids were thermally very stable, only minor losses in adsorption capacity (3-6%)

were observed during recycling experiments.

To remove sulfur species such as n-butyl mercaptan from nitrogen in a continuous pro-

cess, SILP materials containing reactive metal salts were developed. Porous aluminum ox-

ides coated with 1-alkyl-3-methylimidazilium chlorides and equimolarly dissolved zinc or

tin chlorides were applied. The normalized breakthrough curves are presented in �gure

2.14. Until breakthrough, n-butyl mercaptan was always completely removed at 90°C ad-

54


0 400 800 12000.00.20.40.60.81.0

[C4C1IM]Cl-ZnCl2 [C4C1IM]Cl-SnCl2 [C8C1IM]Cl-SnCl2 [C12C1IM]Cl-SnCl2

c c

-1 0 /

-

time / min

Figure 2.14.: Normalized breakthrough curves of n-butyl mercaptan using di�erent SILP ma-

terials; T = 90°C, p = 1.05 bar, mSILP = 10.6 g, h�lling = 9 cm, VN2 = 40 mlN min-1,

Vn-butyl mercaptan = 0.2 ml min-1(adapted from Ref. [11] with permission from

The Royal Society of Chemistry)

sorption temperature and atmospheric pressure. High capacities were observed for acidic

chlorostannate-based ionic liquids. The capacity increasedwith increasing alkyl-chain length

of the cation, likely due to a lower packing density of the ionic liquid. The SILP material was

e�ectively regenerated by a temperature increase to 130°C and a pressure decrease to 0.025

mbar. A decrease in adsorption capacity was caused by dibutyldisul�de condensation during

regeneration experiments.[11]

2.3.5.2. SILP adsorbents based on spherical carbon

Kohler et al. [16] prepared SILP materials based on spherical carbon by wet impregnation

and tested these materials in the continuous removal of ammonia from nitrogen (1000 ppm

NH3 in N2) at 1 bar and 30°C. Before SILP preparation, suitable ionic liquids were screened

for ammonia solubility at 30°C. Solubility of ammonia was large in all tested samples, but

signi�cantly enhanced for metal salt containing systems, especially [C8C1IM]Cl-CuCl2 1:1.

This was due to the formation of metal-amine complexes, e.g. [Cu(NH3)4]2+

(see also section

2.3.2).

The chlorocuprate ionic liquid coated to spherical carbon exhibited noticeable ammonia

55


0 400 800 12000.00.20.40.60.81.0

[C8C1IM]Cl-CuCl2 [C4C1IM]Cl-CuCl2 [C2C1IM]Cl-CuCl2 CuCl2 fits according to model

c c

-1 0 /

-

t / min

Figure 2.15.: Normalized ammonia breakthrough curves of SILP materials based on spherical

carbon with chlorocuprate ionic liquids (xCuCl2 = 0.5, αIL = 0.4) and a copper

chloride impregnation as reference; T = 303 K, p = 1.21 bar, rh = 85%, h�lling = 2

cm, VN2 = 325.6 mlN min-1, VNH3 = 0.33 mlN min

-1([16] - adapted by permission

of The Royal Society of Chemistry)

breakthrough times, already with 10 vol% ionic liquid loading. With increasing relative hu-

midity from 0 to 80%, ammonia capacity increased by 74%. Varying the alkyl group of the

imidazolium cation, ammonia capacity increased with decreasing alkyl chain-length and in-

creasing copper content (see �gure 2.15). At 85% relative humidity, SILP materials performed

better than a pure copper chloride impregnation with comparable copper content (see �gure

2.15). The ionic liquid seemed to improve accessibility of copper ions for complex formation

and could have resulted in a more homogeneous impregnation. At dry and humid conditions,

ammonia breakthrough times increased with increasing ionic liquid loading.[16]

Irreversibility of a partially loaded SILP material (below breakthrough) was successfully

demonstrated at 30°C. A completely saturated SILP material desorbed 56% of adsorbed am-

monia at 85°C (see �gure 2.16). Target applications of these one-time SILP �lter materials are

for personal protection and gas polishing, i.e. the removal of ammonia traces.[16]

Broadband �ltration capabilities of these irreversible SILP adsorbers were also analyzed with

regard toDINEN 14387ABEK1. Spherical carbon coatedwith [C2C1IM]Cl-CuCl2 1:1 (αIL=0.2)

performed very well in the adsorption of ammonia, inorganic (chlorine) and acidic (hydrogen

56


0 400 800 1200012 Absorption (T = 30°C) Desorption (T = 85°C)

NH

3 flow

/ m

g m

in-1

t / min

Figure 2.16.: Comparison of absorbed and desorbed amount of ammonia for the SILP absorber

material coated with [C2C1IM]Cl-CuCl2 (xCuCl2 = 0.57, αIL = 0.2); p = 1.21 bar,

h�lling = 2 cm, VN2 = 325.6 mlN min-1, VNH3 = 0.33 mlN min

-1([16] - adapted by

permission of The Royal Society of Chemistry)

sul�de) gases. Organic (cyclohexane) adsorption capacity was below ABEK1 requirements.

This issue could be solved by decreasing ionic liquid loading, as organic gases directly adsorb

to the unoccupied spherical carbon surface.[16]

The resulting pressure drop of SILP materials based on spherical carbon was investigated,

as well. 500 μm highly activated spherical carbon was coated with [C2C1IM]Br-CuBr2 1:1

(αIL=0.2). Also, open porous foam �lter media [5] were prepared, furnished with the same

SILP material, and analyzed with regard to ammonia adsorption capacity and pressure drop.

It needs to be noted that several ammonia adsorption parameters, including the geometries

of the SILP �xed-bed and SILP foam adsorber, were di�erent for structural reasons. Still,

ammonia capacities of both �lters were signi�cant. Compared to the previous report of

Kohler et al., ammonia capacity of the bromocuprat �xed-bed system was 23% larger than

the chlorocuprat equivalent. The pressure drop of the SILP foam was substantially lower

than the pressure drop of the SILP �xed-bed adsorber. Due to large adsorption capacity and

low pressure drop, �lter materials made of SILP foams were deemed generally well suitable

for large-scale industrial application.[10]

57


2.3.6. Gases studied in this work

In this work, �lter materials for the continuous removal of ammonia, hydrogen sul�de,

formaldehyde and cyclohexane are investigated. Concerning applications in gas puri�cation,

the speci�c properties of these gases and their occurrences in the environment are presented

in the following. The resulting requirements for the �lter materials are highlighted, as well.

Ammonia (NH3) is a colorless gas with a pungent odor. It’s toxic to human health and

the environment. Ammonia corrodes copper-based metals like brass and disintegrates some

polymers like polypropylene. Ammonia gas is released in industries such as food, livestock

breeding, rubber, leather, and waste decomposition plants.[140, 166–168]

In order to protect humans from immediate ammonia exposure, personal protection �lter

materials for ammonia retention are speci�cally designed. Ammonia gas species inherit a

separate category in the classi�cation of �lter materials due to their alkaline properties, dis-

tinguishing ammonia from other gaseous compounds. Also, in broadband �lters, ammonia

adsorption capacity is a critical �gure. Filter requirements for personal protection are irre-

versible adsorption of ammonia, fast adsorption kinetics and large breakthrough capacities

at ambient adsorption conditions. Also, the �lter materials need to perform well at vary-

ing relative humidity. As only few broadband �lters with good ammonia capacity exist, yet,

the development of SILP materials with large ammonia capacity and broadband capability is

continued in this work.[7, 10, 16]

Ammonia removal from biogas is an important industrial application that is elaborated in

the following. Ammonia is a critical gas in the production of biogas for two reasons. Biogas

production by bacteria in anaerobic fermentation plants is inhibited by too large ammonia

concentrations in the plant. This is due to the toxic nature of ammonia to microorganisms.

To limit the amount of free ammonia, parameters like temperature and pH value of the fer-

mentation process are adjusted in dependence on the nitrogen content of the substrate.[169,

170] Also, biogas produced in an anaerobic fermentation plant contains of up to 0.05 vol%

ammonia (see table 2.4). Ammonia is typically removed from these gases by chemical ab-

sorption processes.[171, 172] In both scenarios, adsorbents with large ammonia adsorption

capacities could simplify the processes. Adsorption conditions vary between 37-55°C and

1-10 bar.[173, 174] The e�ects of large relative humidity need to be considered. In large bio-

gas production plants with signi�cant ammonia quantities to be removed, regeneration of

adsorbents is likely to be economically bene�cial, as well.

Consequentially, in this work, the reversible adsorption of ammonia in SILP materials for

industrial applications is also investigated. Previous investigations on ammonia desorption

58


Biogas compound Biogas fraction / vol%

Methane 55-80

Carbon dioxide 20-45

Hydrogen sul�de 0-1

Ammonia 0-0.05

Water (100% RH)

Table 2.4.: Typical composition of biogas from an anaerobic digestion energy plant [171, 176]

from copper-containing adsorbents and SILP materials indicated that adsorption is at least

partially reversible. In �lter materials saturated with ammonia, ammonia partially desorbed

by temperature increase or �ushing with inert gas.[16, 81] It’s known that metal-amine com-

plexes like Cu(NH3)4SO4 disintegrate at temperatures between 105-355°C.[175] Thus, SILP

materials are applied in temperature-swing and pressure-swing adsorption processes for fur-

ther analysis.

Hydrogen sul�de (H2S) is a toxic gas for human health and the environment. The gas

smells like rotten eggs at concentrations between 0.2-100 ppm, but isn’t noticeable at larger

concentrations due to failure of the olfactory sense. It’s easily in�ammable and forms explo-

sive mixtures with air. The combustion product of hydrogen sul�de with air, sulfur dioxide,

is similarly hazardous.[166, 167] Furthermore, hydrogen sul�de can act as catalyst poison

in the production of chemicals.[177, 178] Large quantities of hydrogen sul�de are released

by mankind in the processing of organic raw materials. Industrial examples are coal com-

bustion in power plants, petroleum re�neries, paper production, as well as natural gas and

biogas processing plants.[179, 180]

Concerning adsorption technologies to protect from hydrogen sul�de gas exposure, com-

mercial acidic gas �lters and broadband �lters for personal protection are available. They

include functionalities for hydrogen sul�de adsorption.[7, 16] In this work, hydrogen sul�de

is applied as a test gas to determine the broadband capability of SILP materials for acidic

gases. The gas needs to irreversibly adsorb to the SILP material at di�erent relative humidity

with su�cient breakthrough capacities. The process takes place at ambient conditions, i.e.

room temperature and atmospheric pressure.

As for ammonia adsorption, hydrogen sul�de removal is relevant in biogas puri�cation. Bio-

gas contains up to 1 vol% hydrogen sul�de (see table 2.4). Currently, hydrogen sul�de re-

59


moval in biogas plants is technologically realized by biological desulfurization using appro-

priate microorganisms.[174] Quantitatively the largest gas processing operation worldwide,

involving the removal of hydrogen sul�de, is the puri�cation of raw natural gas.[181] So,

besides irreversible adsorption, two potential applications of SILP materials adsorbing hy-

drogen sul�de, as discussed in this work, are natural gas and biogas puri�cation. Large

hydrogen sul�de capacities and the ability to regenerate the adsorber are demanded. Both,

pressure and temperature swing adsorption modes are assessed. Based on the results, the

competitiveness of SILP technology compared to conventional hydrogen sul�de scrubbers is

evaluated.

Cyclohexane (C6H12) is a volatile organic liquid. Cyclohexane is toxic for human health

and the environment. It’s colorless and has a characteristic odor. Mixtures of air and cyclo-

hexane vapor are explosive.[166] Cyclohexane vapor is the model compound to quantify the

organic adsorption capacity of �lter materials for personal protection purposes. In this work,

the broadband capabilities of SILP materials are evaluated by the cyclohexane breakthrough

capacity. As cyclohexane directly adsorbs to the activated carbon surface (see section 2.3.1),

the cyclohexane capacity also indicates the number of remaining carbon adsorption sites

after ionic liquid impregnation.[16]

Formaldehyde (CH2O) is a toxic gas for human health. It’s colorless, but has a pungent

odor. The gas is easily in�ammable and forms explosive mixtures with air.[166]

Formaldehyde exposure is an issue in newly built houses. Formaldehyde gas and other alde-

hydes are mainly released fromwood and wooden products. Composite materials containing

urea-formaldehyde resins are the most dominant sources of formaldehyde air contamination.

Water-based paints with Quaternium 15 are a source of formaldehyde exposure, as well. In-

doors, ozone chemistry was found to also contribute to formaldehyde formation.

Outdoors, formaldehyde is released from automotive exhaust gases. The formaldehyde con-

tent increases with increasing amount of oxygen species in the fuel, e.g. ethanol, methanol

and methyl tertiary butyl ether.[182–185]

The development of SILP materials to remove toxic formaldehyde gas from air is part of this

work. A �rst application scenario of irreversible formaldehyde adsorption is in household

room �lters for the Asianmarket. This market region has a climate with large relative humid-

ity. A possible reaction pathway for formaldehyde conversion is, for instance, the reaction of

aldehydes with alcohols forming acetale copolymers.[186] Such a speci�c functionality for

formaldehyde removal could be introduced into ionic liquids. For good customer acceptance,

ionic liquid impregnations with well-known non-toxicity are desired. A suitable salt with a

60


hydroxide functional group could be choline chloride. Choline chloride is listed as animal

feed additive and exhibits a well-known non-toxicity.[187, 188]

61

3. Objective of this work

This thesis intends to demonstrate the applicability of polymer-based spherical activated car-

bon materials in catalysis and advanced gas puri�cation. Furthermore, the inherent advan-

tages of spherical carbon in comparison to activated carbon powder and inorganic support

materials are discussed. Compared to powder materials, these advantages are, for instance,

the easy material handling, the low pressure drop and the high �ltration rate of spherical

carbon. In contrast to inorganic supports such as alumina and silica, the broadly modi�able

carbon surface for tailored catalyst preparation and the additional adsorption capability of

spherical carbon for organic and inorganic gases in broadband gas �lters are of high interest.

In the following, the strategy for these scienti�c investigations is elaborated.

Filtration and pressure drop of spherical carbons

Faster �ltration rates of spherical carbon over carbon powder can reduce the time needed for

catalyst separation from the product solution. Due to the larger particle size and its spheri-

cal shape, spherical carbon materials are expected to exhibit a lower pressure drop in liquid

phase �ltration processes than carbon powder and, thus, faster �uid �ow rates through spher-

ical carbon beds. A lower pressure drop of spherical carbon beds will also be bene�cial in

continuous processes such as �ow chemistry applications. The comparison of spherical car-

bon materials and carbon powder, as well as the in�uence of particle size on �uid �ow rates

and �ltration times are demonstrated with the help of �ltration experiments and numerical

investigations.


For successful application in catalysis, catalytically active species need to be well deposited

to spherical carbon. In�uencing parameters in catalyst preparation resulting in catalytically

active materials can be the particle size, pore structure and surface properties of spherical

carbon. In this work, noble metals such as palladium, ruthenium and platinum are chosen as

catalytically active species. Important properties of the �nal catalysts that characterize the

results of active metal deposition, i.e. metal dispersion and metal distribution, are analyzed

62


in detail. In screening experiments using a variety of test reactions, the performance of the

prepared spherical catalysts is compared to the catalytic activity of respective carbon powder

catalysts. Furthermore, correlations between catalytic activity and material properties, e.g.

metal dispersion, metal distribution, particle size, pore structure and surface functionaliza-

tion, are discussed.

In order to apply spherical carbons as support materials for di�erent noble metals, the spher-

ical carbons are oxidized, at �rst. Then, the oxidized materials are loaded with noble metals

by electrostatic ion adsorption and subsequent metal transformation. Parameters of catalyst

preparation are chosen that keep a potential technical scale-up to larger quantities in mind.

The spherical carbon materials, the oxidized spherical carbons and the prepared catalysts are

characterized in detail and compared to the respective powder reference material. Correla-

tions of experimental data are illuminated and conclusions are devised for each experimental

series.

A variety of spherical carbons are used as starting material, di�ering in particle size and pore

structure. Palladium catalysts based on these di�erently sized spherical carbons are charac-

terized regarding metal loading, metal dispersion and catalytic activity. To furthermore in-

vestigate the e�ects of di�erent particle sizes, comminution experiments are conducted. The

same characterization ofmetal loading, dispersion and catalytic activity is done for palladium

loaded spherical carbons with di�erent pore structures.

Concerning the surface composition of spherical carbon, oxidation parameters of spherical

carbon functionalization are varied. Oxidations are carried out with nitric acid and sulfuric

acid, respectively. For each acid, two acid concentrations and two oxidation temperatures are

chosen. The surface properties of these eight carbon samples are analyzed in detail. Impor-

tant di�erences between nitric acid and sulfuric acid oxidations are speci�cally illuminated.

The oxidized materials are loaded with palladium and characterized regarding metal loading,

metal dispersion and catalytic activity.

To study the in�uence of metal loading, palladium catalysts are prepared with desired metal

loadings of 2, 5 and 10 wt%. The obtained metal loading and resulting catalytic activities

are determined. Ruthenium catalysts with desired metal loadings of 0.5, 5 and 10 wt% are

also characterized regarding metal loading, metal dispersion and catalytic activity. Catalytic

activity of spherical ruthenium catalysts is tested in hydrogenation reactions of di�erent

molecules and compared to ruthenium on carbon powder.

In a stepwise optimization of the platinum catalyst preparation procedures, the in�uence of

various parameters is studied to yield a well performing catalyst for the technical dehydro-

63


genation of a liquid organic hydrogen carrier. Applying this derived preparation procedure,

oxidized spherical carbon is loaded with 0.5, 5 and 10 wt% platinum. The catalysts are char-

acterized regarding metal loading, metal dispersion and catalytic activity. Also, they are

compared to platinum on carbon powder and platinum on alumina.

In the end of the catalysis part, stability tests of palladium catalysts are conducted. Therefore,

the extent of palladium and sulfur leaching is analyzed for two prepared spherical palladium

catalysts and the palladium on carbon powder reference. Additionally, the recycling ability

of a spherical carbon catalyst is demonstrated. Also, a rhodium metal complex dissolved in

ionic liquid is supported on spherical carbon. In a proof of concept style, recycling ability

and activation energy are determined applying this SILP catalyst in a test reaction.


In the second part concerning advanced gas puri�cation, spherical carbon is gas adsorbent

and support material for di�erent liquid salts. Reactive metal salts are dissolved in ionic

liquids and coated to the internal surface of spherical carbon by wet impregnation. Speci�c

gases are either adsorbed to the carbon surface or absorbed into the ionic liquid �lm, where

they react with dissolved metal salts.

These SILP materials are prepared and tested for the applicability in two areas of gas puri�-

cation with di�erent functional requirements. In �ne �ltration processes, i.e. the removal

of trace contaminants in the ppm-level, a fast and irreversible reaction of the contaminant

with the �lter material is necessary. The contaminants need to be completely removed from

the gas stream. Applications of these �lter materials are, for instance, in clean rooms and

for personal protection. The second area, with large concentrations of contaminants, is the

bulk �ltration of gases, which is of particular interest in industrial gas puri�cation processes.

Here, the ability to regenerate the �lter materials is most important. To minimize the energy

input for regeneration, the contaminants need to weakly interact with the active sites of the

�lter material.

As the �lter capacity is of high interest in all applications, the SILP �lter materials are primar-

ily characterized regarding their breakthrough time in the continuous removal of hazardous

test gases. Corrosion experiments and surface analysis of SILP materials are also conducted.

In preliminary experiments, the in�uence of di�erent impregnation solvents on SILP product

quality is assessed. Also, SILP materials containing reactive cuprate and zincate salts are

compared in the adsorption of ammonia and hydrogen sul�de. Corrosion of stainless steel

and aluminum is determined for these halide-containing SILP materials, as well.

64


For the irreversible removal of hazardous gases, novel, halide-free SILP materials are pre-

pared with ionic liquid loadings of 0.1, 0.2 and 0.3. Both, the ionic liquid and the reactive

cuprate salt are halide-free. Ammonia breakthrough capacities are measured for a relative

humidity of 25, 50 and 80%. Broadband capability of the medium-loaded SILP material is

evaluated. Alternative coating techniques, i.e. incipient wetness impregnation and dropwise

impregnation, are examined.

Furthermore, organic metal salts are synthesized, dissolved in ionic liquid and coated to

spherical carbon. In breakthrough experiments, ammonia capacity of these SILP materials is

determined.

Hygroscopic salts with reactive functionality for formaldehyde are also coated to spherical

carbon. The resulting �lter materials are applied in the continuous removal of formaldehyde

gas at large relative humidity.

Finally, the regeneration of SILP materials is investigated in pressure swing and temperature

swing adsorption setups. Adsorption and desorption capacities of ammonia and hydrogen

sul�de are measured. Metal-free impregnations of spherical carbon with reactive ionic liq-

uids are also tested in the reversible adsorption of hydrogen sul�de.

65

4. Experimental

4.1. Catalyst preparation and characterization

4.1.1. Oxidation of spherical carbon

Standard oxidation parameters were 10% nitric acid with 2.5 h treatment time at RT. In detail,

3.2 ml of 65% nitric acid solution were mixed with 26.1 ml water in a round bottom �ask.

22 g of spherical carbon were quickly added. The suspension was stirred at RT for 2.5 h. The

carbon material was separated using suction �lter and feeding bottle and intensively washed

with 5 l water. In the end, the oxidized spherical carbon was dried at 120°C and 0.01 mbar

for 20 h.

Alternative functionalization conditions have also been applied varying oxidation agent (sul-

furic acid), acid concentration (15-50%), oxidation temperature (90°C) or amount of spherical

carbon (50 g). In all alternative functionalization processes, the volume of oxidizing solu-

tions was set to three times the materials pore volume. In case of oxidations at 90°C, a re�ux

condenser was mounted to the round bottom �ask. Oxidations with 15 and 36% nitric acid

were performed at Blücher GmbH, Erkrath.

4.1.2. Deposition of noble metal

Palladium, ruthenium and platinum salts were loaded to oxidized spherical carbon in a wet

impregnation process. Speci�cally, the principle of electrostatic adsorption of metal anions

to charged carbon surface groups was applied (see section 2.2.5.2).

5 g of palladium catalyst with metal loadings between 2-10 wt% were prepared by dissolving

palladium chloride in 5.5 ml 1 N hydrochloric acid and 29.5 ml water. Oxidized spherical

carbon (5 g) was quickly added and the suspension was slowly stirred at RT for 24 h. Using

suction �lter and feeding bottle, the solid material was separated and rinsed with 200 ml wa-

ter. The catalyst material was dried at 120°C and 0.01 mbar for 2 h. Metal transformation was

66

4. Experimental

carried out in a horizontal �ow reactor in hydrogen atmosphere. A temperature of 80°C was

chosen for maximum palladium dispersion with reference to literature (see section 2.2.5.2).

A thin layer of catalyst material (ca. 2 g) was activated by 3.5 vol% hydrogen in nitrogen

(reforming gas) at a �ow velocity of 0.032 m s-1for 1 h.

The procedure of ruthenium and platinum catalyst preparationwas very similar. The impreg-

nation solution for ruthenium catalysts only consisted of ruthenium chloride hydrate and

water. Hexachloroplatinic acid hexahydrate was used as a precursor for platinum catalysts

with metal loadings between 0.5-10 wt%. Metal transformation temperature of ruthenium

and platinum catalysts was set to 300°C, in accordance to literature (see section 2.2.5.2).

A rhodium SILP catalyst was also prepared. The SILP catalyst was based on spherical car-

bon coated with an ionic liquid containing a dissolved rhodium complex. 0.1 g precur-

sor rhodium(I) dicarbonyl acetylacetonate, 0.45 g ligand triphenylphosphine-3,3,3-trisulfonic

acid trisodium salt (TPPTS) and 4 ml ionic liquid [C4C1IM][PF6] were solved in 40 ml water.

32.3 g of slightly activated 500 μm spherical carbon (Vtot = 0.61 cm3g-1) were added. Water

was slowly removed by rotary evaporation and the SILP material was further dried at 120°C

and 0.1 mbar.

4.1.3. Evaluation of catalytic activities

Catalytic activities of the prepared catalysts were determined on the basis of standardized

hydrogenation experiments in a stirred-tank reactor. Reaction rates and turnover frequencies

were calculated from conversion curves applying reaction engineering models. The dehy-

drogenation activity of the liquid organic hydrogen carrier Marlotherm SH, demonstrating

the performance of prepared platinum catalysts in a technical application, was separately

carried out in a glass reactor. The studied reactions are presented in section 2.2.6.

The stainless steel reactor with a volume of 600 ml made by Parr Instruments consisted of

a gas entrainment impeller, a heating mantle, a cooling coil, temperature and pressure sen-

sors, valves for hydrogen, nitrogen and o�-gas, a safety relief valve, and a sampling pipe

with a �lter. Reactant, solvent, catalyst, and optionally, an internal standard were added into

the reactor. The internal standard was used for calibration purposes of the gas chromato-

graph. In general, the total reaction volume amounted to 200 ml. The reactor was mounted,

�ushed with nitrogen and heated to reaction temperature. The stirrer velocity was set to

1000-1500 rpm and a �rst sample was collected. The experiment was initiated by the addi-

tion of hydrogen gas with pressures between 25-50 bar. Hydrogen gas pressure was kept

67

4. Experimental

Table 4.1.: Standard hydrogenation parameters for testing catalyst performance

Catalyst Pd oncarbon

Pt oncarbon

Ru oncarbon

Rh-SILP

Eductmolecule

Cinnamic

acid

Cinnamalde-

hyde

Toluene 1,5-Cyclo-

octadiene

Solvent Ethanol Tetrahydro-

furane/Water

(3:1 v/v)

Cyclohexane Cyclohexane

Internalstandard

Ethane-1,2-

diole

- - n-Heptane

Eductconcentration/ mol m-3

236 189 180 229

Temperature /°C

40 100 150 60-90

Hydrogenpressure / bar

5-30 25 50 30

Stirrervelocity / rpm

1500 1500 1000 1500

GC columntype

FFAP FFAP Fused silica Fused silica

constant throughout the experiment. In certain time intervals, samples were manually taken

and analyzed in a gas chromatograph (GC) with FFAP or fused silica column. Using the frac-

tions of feedstock and products, reactant concentrations were calculated and veri�ed with

the known concentration of the internal standard. Standard parameters for each hydrogena-

tion test reaction are listed in table 4.1.

The course of conversion over the modi�ed residence time X (τmod ) was �tted using an in-

tegrated power law expression, assuming a pseudo �rst order reaction (see section 2.2.2).

Linear regressions of the logarithmized expression were performed in OriginLab Origin ap-

plying chi-square minimization, as well as further hypothesis tests and residuals plotting.

Data points up to a conversion of around 80% were taken into account. The e�ective re-

action rate constant ke f f was used to quantitatively describe the performance of a catalyst

concerning catalytic activity. Alternatively, catalytic activities were compared via turn over

68

4. Experimental

frequencies of substrate molecules at surface metal sites. Both parameters are explained in

section 2.2.2.

The extent of mass transport limitation due to pore di�usion was investigated with classi-

cal reaction engineering methods, calculating Thiele moduli and e�ectiveness factors (see

section 2.2.2). The required intrinsic reaction rate constant was determined in comminution

experiments of particulate catalysts. The e�ective di�usion coe�cient was estimated from

the molecular di�usion coe�cient of substrate molecules in solvent.

For application of catalysts in the dehydrogenation of fully hydrogenated Marlotherm SH,

experiments were conducted in a 100 ml glass reactor �lled with 10-30 ml of Marlotherm SH.

The system was stirred with a magnetic stirrer and heated to 310°C. Then, the reaction was

initiated by introducing the catalyst material. The molar ratio of noble metal to Marlotherm

SH was always 0.1 mol%. The total reaction time amounted to 120 min. In certain time

intervals, 0.1 ml samples were taken and quantitatively analyzed by 1H NMR to determine

the dehydrogenation degree of Marlotherm SH.

4.1.4. Evaluation of filtration rates

The �ltration rates of carbon materials were determined experimentally and mathematically.

The experimental setup consisted of a �ltration apparatus from Sartoriusmade of transparent

polycarbonate (Polycarbonate Filter Holder 16510). As a membrane, paper �lters with 1 μm

particle retention from Sartorius were used (Filter Discs Grade 393).

For each �ltration experiment, 1 g oxidized spherical carbon material or commercial palla-

dium on carbon powder was used (Sigma-Aldrich, 5 wt% palladium on activated carbon).

The carbon materials were suspended in 200 ml colored water (potassium permanganate) in

the upper reservoir of the �ltration apparatus. The closed system of the �ltration apparatus

allowed the application of 0.5 bar excess air pressure. The �ltration process was recorded

with a video camera and evaluated to result in a material-speci�c �ltration rate.

As a mathematical model, the Darcy law was applied to describe the �ltration process and

extrapolate �ltration rates of di�erent catalyst particle sizes for larger industrial reactors (see

section 2.2.4 for details).

69

4. Experimental

4.2. SILP material preparation and characterization

4.2.1. Preparation of SILP materials

SILPmaterials were usually prepared bywet impregnation and assisted ultrasonication. Ionic

liquids were fully deposited inside spherical carbon, so that dry and pourable material re-

sulted.[16] To calculate the required amount of ionic liquid melt, its density was determined

by helium pycnometry at 20°C. Ethanol, acetonitrile, methylenchloride or water was used as

impregnation solvent with a quantity of 1.9 times the spherical carbons’ total pore volume.

In general, highly activated spherical carbon from Blücher GmbH, Erkrath with a particle

diameter around 500 μm was applied (SBET = 2210 m2g-1, Vpore = 1.33 ml g

-1, ρbulk = 0.38 g

ml-1). In a round-bottom �ask, ionic liquid melt was dissolved in solvent and spherical carbon

was then suspended at room temperature. The system was treated in an ultrasonication bath

for two hours at room temperature. In a rotary evaporator, solvent was removed at 50°C

and pressures down to 20 mbar. Samples prepared with organic solvent were additionally

dried at 120°C and 0.01 mbar. The standard production amount was 200 g SILP material. Two

SILP materials based on copper bromide and zinc bromide were supplied by Blücher GmbH,

Erkrath.

Equation 4.1 describes the volume-speci�c molar metal content γ of a SILP material, with β

being the ionic liquid loading as de�ned in equation 4.2. Also, ν being the molar fraction of

each ionic liquid compound, zi,Cu the molar number of copper atoms in the ionic liquid, Mi

the molar mass of each ionic liquid compound,Vpore the pore volume of spherical carbon, αIL

the ionic liquid loading and ρ the bulk densities of spherical carbon, SILP or the density of

ionic liquid. In this equation of molar metal content, all parameters are usually known.

γ = β · ρSILP ·

∑i νi · zi,Cu∑i νi ·Mi

= β · (1 +β

1 − β) ·

∑i νi · zi,Cu∑i νi ·Mi

(4.1)

β =α ·Vpore · ρIL

1 + α ·Vpore · ρIL(4.2)

70

4. Experimental

4.2.2. Evaluation of SILP corrosivity

Rudimentary corrosion tests derived from industrial standards were applied to characterize

the general corrosion potential of SILP materials. Relative humidity was accounted for. Alu-

minum corrosion was analyzed by wrapping SILP material in aluminum foil and placing it

in a climate chamber at 65-75% rh and 20°C for 7 days. Then, the degree of corrosion was

optically evaluated. Aluminium corrosion tests were carried out by Blücher GmbH, Erkrath.

For stainless steel corrosion tests, a glass bottle was partially �lled with glass wool and 3 g

of SILP material on top. A stainless steel 1.4301 (X5CrNi18-10) platelet of known mass was

placed in each bottle. To account for the in�uence of relative humidity, 0.5 ml of water were

injected into the bottom part of the bottle. The corrosion test was conducted at RT for 7

days. After that time, the stainless steel platelet was analyzed for corrosion. Additionally, 2

ml water was added to each bottle and the corrosion test was continued for another 7 days at

RT. Finally, corrosion of stainless steel platelets was evaluated optically and gravimetrically.

4.2.3. Evaluation of gas purification performance

Continuous gas-phase removal of ammonia was performed in the test rig shown in �gure 4.1.

SILP material was placed in the adsorber unit (18 mm diameter) with a bed height of 20 mm.

The adsorber was heated to 30°C and operated at atmospheric pressure. Nitrogen gas and

ammonia were fed by mass �ow controllers (Bronkhorst), so that an ammonia concentration

of 1000 ppm resulted. Nitrogen gas was optionally humidi�ed in the saturator. Relative

humidity was controlled by the saturator’s water temperature and a humidity sensor. For

calibration purposes, a bypass to the adsorber was available. With a constant velocity of

0.02 m s-1, the gas mixture passed through the SILP material. Behind the adsorber, the gas

composition was analyzed by gas chromatography in short time intervals (Varian CP-3800,

Volamine fused silica 60 m x 0.31 mm capillary column, thermal conductivity detector). The

ammonia detection limit was around 100 ppm.[16]

In a typical breakthrough experiment, the SILP material was preconditioned for 15+ hours in

a nitrogen stream at adsorption conditions, but without ammonia. Then, using the adsorber

bypass, ammonia was dosed into the system and its concentration was monitored for 30 min.

The adsorption process was started by switching from bypass mode to the adsorber. When

the initial ammonia concentration was reached, the experiment was �nished.

For formaldehyde adsorption experiments, this test rig was slightly modi�ed. Nitrogen gas

was e�ectively enriched with formaldehyde by passing through the saturator unit containing

71

4. Experimental

PIC

TIC-6

TIC-4

TIC-3

N2

TIC-5

MFC-2

MFC-1

Mixer

NH3

TI-3

TIC-7

GC Filter 1Filter 2

Thermostat

cycle

Saturator

H2O

TIC-1

rh I

TIC-2

TI-1

TI-2

F

F

Adsorber

Figure 4.1.: Gas-phase test rig for continuous removal of ammonia ([16] - adapted by permis-

sion of The Royal Society of Chemistry)

a 4% formalin solution.

Breakthrough measurements of SILP materials with regard to DIN EN 14387 ABEK1 [150]

and US NIOSH [151] were conducted by ProQares B.V., the Netherlands. These test rigs were

similarly constructed to the previously described setup. Due to standardized speci�cations,

adsorber geometry and process parameters varied. The removal of ammonia, hydrogen sul-

�de, cyclohexane, and formaldehyde was analyzed for selected SILP materials.

72

4. Experimental

Table 4.2.: Particle size fractions and bulk densities of di�erent spherical carbon materials

Particle size/ μm

Carbonactivation

Particle sizefraction / μm

Bulkdensity /g l-1

500

wwwww�Increasing 450-500

628

412

411

380

200

wwwww�Increasing100-315

602

495

200-250

446

341

50

wwww�Increasing < 100

571

500

424

4.3. Materials

In the following, a characterization of the spherical carbon materials is given, as well as a

description of the synthesis of metal salts and ionic liquids. All other materials and chem-

icals were commercially available. Appendix A includes several tables listing the di�erent

chemicals, purities and suppliers.

4.3.1. Overview of applied spherical carbon materials

A broad selection of polymer-based spherical activated carbons was available for this study.

All spherical carbon materials were produced and supplied by Blücher GmbH, Erkrath. Poly-

styrene-divinylbenzene copolymer was the respective polymer origin. Process parameters

for sulfonation, carbonization and carbon activationweren’t disclosed. The spherical carbons

varied in particle size and carbon activation. Table 4.2 shows the particle size fractions after

product classi�cation, as well as bulk densities of the spherical carbon materials applied

in this work. Additionally, table 4.3 lists the respective material properties derived from

nitrogen sorption experiments.

73

4. Experimental

Table 4.3.: Pore characteristics derived from nitrogen sorption data of di�erent spherical car-

bon materials

Particlesize /μm

Carbonactivation

Vtot(0.990)*/cm3 g-1

Vmicro /cm3 g-1

Fraction ofmesopores/ %

MP BET(0.05-0.1)* /m2 g-1

Averageporediameter /nm

500

wwwww�Increasing0.61 0.55 10 1376 1.8

1.18 0.77 35 1916 2.5

1.19

(0.995)*

0.37 69 1159 4.1

1.33 1.00 25 2210 n/a

200

wwwww�Increasing0.69 0.56 18 1439 1.9

0.95 0.69 27 1810 2.1

1.13 0.77 32 2006 2.3

1.67 0.92 45 2350 2.9

50

wwww�Increasing 0.61 0.53 13 1358 1.8

0.89 0.68 24 1772 2.0

1.07 0.77 28 1942 2.2

* p p0-1

74

4. Experimental

Depending on sieves used for classi�cation, di�erent particle size fractions were obtained.

The large spherical carbon with 450-500 μm in particle size and the smaller, highly activated

material with 200-250 μm in size were most narrowly distributed. Particles of the �ne mate-

rial were classi�ed to be smaller than 100 μm. To allow for quick reference, the three major

particle size fractions were indicated with a general particle size of 500, 200, and 50 μm.

For each particle size, bulk density decreased with increasing carbon activation due to an

increase in carbon discharge by gasi�cation.

Regarding nitrogen sorption measurements, the total pore volume increased corresponding

to an increase in carbon activation. The total pore volume varied between 0.61-1.67 cm3g-1.

Micropore volume and BET surface area generally increased with increasing carbon acti-

vation, as well. One exception of this trend was observed for the highly activated 500 μm

spherical carbon. Here, the carbon activation procedure was modi�ed to result in a more

mesoporous material. The signi�cantly smaller ratio of micropore volume to total pore vol-

ume indicated that this modi�cation was successful. Overall, values of micropore volumes

between 0.53-0.92 cm3g-1and BET surface areas between 1159-2350 m

2g-1were determined.

The ratio of micropore volume to total pore volume decreased with increasing carbon acti-

vation, thus, leading to a larger fraction of mesopores. The average pore diameter also in-

creased with increasing carbon activation. Average pore diameters between 1.8-4.1 nm were

realized, with the largest average pore diameter corresponding to the strongly mesoporous

500 μm spherical carbon.

In essence, di�erent degrees of carbon activation resulted in di�erent pore geometries within

the spherical carbon material. A larger quantity of mesopores and generally larger surface

areas were formed after more intensive carbon activation.

4.3.2. Synthesis of metal salts and ionic liquids

Copper(II) octylsulfate was synthesized via two di�erent synthesis routes. In the �rst route

(see scheme 4.1), an aqueous solution of 10 g sodium octylsulfate was percolated through a

column containing Amberlite IR-120 ion exchange resin. The resulting acid was then neutral-

ized with an excess of 2.4 g copper(II) hydroxide/copper(II) carbonate mixture. The aqueous

copper(II) octylsulfate solution was �ltered and dried in a rotary evaporator. Alternatively,

as shown in scheme 4.2, 500 g copper octylsulfate were synthesized by sulfatization of 325.5

ml 1-octanol in 200 ml dichloromethane with 138 ml chlorosulfuric acid at room tempera-

ture and under slight vacuum. The solution was constantly stirred, chlorosulfuric acid was

added dropwise and hydrogen chloride vapor was removed by a water jet pump. Then, the

75

4. Experimental

Scheme 4.1 Synthesis of copper(II) octylsulfate by cation exchange of the octylsulfate salt

(1) and neutralization (2)

Na+[C8H17OSO3]− + R − SO3H H+[C8H17OSO3]

− + R − SO3Na (1)

4 · H+[C8H17OSO3]− +CuCO3 ·Cu (OH )2 → 2 ·Cu[C8H17OSO3]2 + H2CO3 + 2 · H2O (2)

Scheme 4.2 Synthesis of copper(II) octylsulfate by sulfatization of 1-octanol (1) and neutral-

ization (2)

C8H17OH + HSO3Cl → H+[C8H17OSO3]− + HCl (1)

4 · H+[C8H17OSO3]− +CuCO3 ·Cu (OH )2 → 2 ·Cu[C8H17OSO3]2 + H2CO3 + 2 · H2O (2)

aqueous phase was separated from the organic phase and 115 g copper(II) hydroxide/cop-

per(II) carbonate mixture was added. Finally, the solution was �ltered and dried in a rotary

evaporator.[189–191]

Copper octanoate was synthesized by neutralizing commercially available octanoic acid (50

g) with a copper(II) hydroxide/copper(II) carbonate mixture (19.5 g) at room temperature (see

scheme 4.3). Afterwards, copper octanoate was extracted using dichloromethane and dried

in a rotary evaporator.[192]

The hydrate ionic liquids tetramethylammonium acetate and tetramethylammonium mal-

onate were synthesized by neutralizing an aqueous tetramethylammonium hydroxide so-

lution with stoichiometric amounts of acetic acid and malonic acid, respectively. The sub-

stances were then dried in a rotary evaporator.

Scheme 4.3 Synthesis of copper(II) octanoate by neutralization of octanoic acid

4 · H+[C7H15COO]− +CuCO3 ·Cu (OH )2 → 2 ·Cu[C7H15COO]2 + H2CO3 + 2 · H2O

76

4. Experimental

4.4. Analytical methods

Nitrogen sorption experiments were conducted in a Quadrasorb SI from Quantachrome In-

struments at 77 K. Nitrogen adsorption and desorption isotherms were recorded. Quenched

solid density functional theory (QSDFT) modelling was applied to the obtained isotherms

using a ratio of slit to cylindrical pores of 1:1 for activated carbon materials. The total pore

volume was usually determined at a relative pressure of 0.990. BET surface area was derived

in the relative pressure region between 0.05-0.1. Dynamic water vapor sorption experiments

were carried out in Hydrosorb 1000 from Quantachrome Instruments.

The PZC of the carbon materials was determined by pH measurement after hot �ltration

according to CEFIC [193]. 4 g of carbon material was suspended in 100 ml degassed water

and boiled for 5 min. By �ltration, the carbon material was separated at a temperature above

60°C. Then, the permeate solution was cooled down to RT and the pH was measured under

constant stirring.

Potentiometric titrations with sodium hydroxide were donewith a Titrando 888 byMetrohm.

For sample preparation, special carbon was comminuted in a Retsch mixer mill. 0.2 g carbon

material was suspended in 100 ml 0.01 N sodium nitrate solution. With a 0.01 N hydrogen

chloride solution, the suspension was adjusted to a pH value of 3. Then, a 0.1 N sodium

hydroxide solution was automatically added and the pH continuously recorded. A pKa dis-

tribution was derived applying the numerical Saieus procedure [84].

Thermogravimetrical measurements were performed for more speci�c characterization. In

a quick analysis of volatile components, a known amount of sample was placed in an oven,

rapidly heated to 120°C and weighted. Afterwards, the sample was further heated to 800°C

and itsmasswas recorded again. This approach allowed an easy partitioning of water content

and amount of volatile components.

All these previously described measurements were conducted by Blücher GmbH, Erkrath.

Temperature programmed desorption with attached mass spectroscopy (TPD-MS) measure-

ments were conducted in a Carlo Erba QTMD instrument at Zeta Partikeltechnik GmbH,

Mainz. The helium �ow amounted to 15 ml min-1and the heating rate was 10 K min

-1from

RT to 1000°C. Sample masses were gravimetrically determined before and after a TPD-MS

measurement.

The actual metal loading of the prepared catalysts was indirectly determined. The concen-

tration of noble metal in the permeate solution after carbon impregnation was analyzed

77

4. Experimental

by inductively coupled plasma atomic emission spectroscopy (ICP-AES) at the Institute of

Chemical Reaction Engineering, Erlangen.

Pulse chemisorption experiments with carbon monoxide were carried out to analyze the

metal surface of carbon supported noble metal catalysts. In particular, metal dispersion was

determined. Around 0.2 g of catalyst sample was placed in a quartz glass tube of a Mi-

cromeritics AutoChem II 2920. The catalyst was preconditioned at its original metal trans-

formation temperature (see section 4.1.2) in a continuous �ow of 10 vol% hydrogen in argon

for 60 min. After �ushing with argon for 30 min, temperature was decreased to 40°C. At

40°C, de�ned amounts of carbon monoxide were added in pulse titration mode. The amount

of non-adsorbing carbon monoxide was monitored. Pulse titration was continued until no

carbon monoxide was adsorbed in succeeding pulse measurements. Metal dispersion and

average cluster diameter were derived assuming a 1:1 adsorption stoichiometry (see section

2.2.5.2). Measurements were conducted at the Institute of Chemical Reaction Engineering,

Erlangen.

Additionally, TEM images were taken of selected catalysts at the University of Erlangen-

Nuremberg. Therefore, spherical catalyst material was comminuted, dispersed in acetone

and applied to a TEM grid. An average metal cluster size was determined by image analysis.

Metal distribution over the cross section of spherical carbon was determined by SEM-EDX.

Spherical catalyst was embedded in proprietary epoxy resin (SpeciFix-20Kit) from Struers,

Denmark. The samples were then polished and sputtered with gold. Sample preparation and

analysis were carried out at the Institute of Glas and Ceramics (WW3), Erlangen.

Metal leaching tests were conducted in order to determine the degree of catalyst reduction

and to compare noble metal leaching of commercial catalysts. Concerning the degree of

reduction, the assumptionwas that unreducedmetal salts more readily dissolve in water than

reduced metals. 0.5 g of catalyst was suspended in 10 g water and treated by ultrasonication

at RT for 1-3 h. The catalyst was �ltered o� and metal concentration (and optionally sulfur

concentration) in aqueous solution was analyzed by ICP-AES.

Densities of ionic liquids and ionic liquid melts were determined by helium pycnometry

measurements at 20°Cusing Pycnomatic ATC fromThermo Scienti�c. Around 5 g ionic liquid

were weighted in a small crucible and placed in the pycnometry device. After �ushing the

sample with helium, an average ionic liquid density was determined after 20 measurements.

78

5. Results and discussion

5.1. Filtration and pressure drop of spherical carbon

5.1.1. Spherical carbon in lab-scale filtration

These sections take a look at the e�ciency of �ltration using spherical carbon materials. It is

expected that spherical carbons are �ltered o� signi�cantly faster than carbon powder. An

in�uence of spherical carbon particle size on �ltration time is also likely.

The spherical carbons used in the �ltration experiments had particle sizes of 500 and 200 μm.

They were highly activated with total pore volumes of 1.18 and 1.67 cm3g-1, respectively. See

section 4.3.1 for additional material properties. The applied commercial 5 wt% palladium on

carbon powder consisted of small particles between 0.7-1.9 μm in size and larger particles up

to 10 μm, as determined by analysis of light microscopy images. A timeline of the �ltration

experiments is shown in �gure 5.1 and the results are summarized in table 5.1.

Looking at the timeline, a clear trend of increasing �ltration timewith decreasing particle size

is observed. The spherical carbon materials quickly settled forming a �lter cake. Filtration of

the largest spherical carbon material with 500 μm particle size was �nished in less than two

minutes. It took shortly more time for the 200 μm material and even longer in case of the

powdered catalyst. While with the �ne catalyst material, the colored water turned yellowish,

the permeate color of the experiments with spherical carbon didn’t change. The color change

of the permeate stream in case of the powdered catalyst was due to a reduction reaction of the

manganese(VII)-containing dye to e.g. manganese(IV) and manganese(II) ions. Additionally,

the powdered catalyst adhered to the walls of the �ltration apparatus.

The exact �ltration times in table 5.1 show that the gap between the fastest �ltration with 1.6

min and the slowest �ltration with 7.5 min was large. The �ltration factors quantify these

di�erences. While �ltration of the 200 μm spherical carbon material took 50% longer, the

factor amounted to 4.7 and 3.1, respectively, for the powdered catalyst. Thus, it is apparent

79


Start 0.5min 1min 2min 4min

500µmPBSAC

200µmPBSAC

Catalystpowder

Figure 5.1.: Timeline of �ltration experiments applying spherical carbonmaterials and a com-

mercial powder catalyst (1 g material, 200 ml water, membrane with 1 μmparticle

retention, 0.5 bar excess pressure)

Table 5.1.: Filtration experiments using spherical carbon materials and a commercial powder

catalyst (1 g material, 200 ml water, membrane with 1 μm particle retention, 0.5

bar excess pressure)

Material Filtration time Filtration factorto 500 μmsphericalcarbon

Filtration factorto 200 μmsphericalcarbon

500 μm spherical

carbon

1 min 36 s 1 -

200 μm spherical

carbon

2 min 26 s 1.5 1

Catalyst powder 7 min 30 s 4.7 3.1

80


Table 5.2.: Speci�cation of constant parameters for large-scale �ltration

Parameter Symbol Value

Pressure di�erence ∆p 100,000 Pa

Fluid viscosity (water) η 0.001 Pa s

Membrane resistance Rm 1e11 m-1

Porosity ϵ 0.4

Filter factor k′′ 4.8

Filter cake diameter dc 0.1 m

Filter cake mass mc 0.1 kg

Packed bed density ρc 400 kg m-3

Filtrate volume Vf iltrate 0.1 m3

that systems with spherical adsorbents exhibit a signi�cant �ltration advantage compared

to powdered activated carbon. Spherical carbon is quickly �ltered o�. The �ltration advan-

tage increases with increasing particle size. Reaction and product separation equipment was

easily cleaned, because the spherical adsorbents weren’t very adhesive. This advantageous

behavior would also enable catalyst recovery and recycling. Primarily, these small-scale ex-

periments apply to �ltration processes in analytical chemistry. Generally, in analytical chem-

istry, the shortening of time consuming post-processing by application of spherical carbon

based catalysts cannot be neglected.

5.1.2. Spherical carbon in large-scale filtration

Large-scale �ltration rates of spherical carbon materials were estimated a-priori with the

Darcy equation and the Carman-Kozeney extension (see section 2.2.3). Here, it is assumed

that the spherical carbons have formed a �lter cake. Table 5.2 lists reasonable values chosen

for calculations.

In the calculations, the particle diameter of the spherical adsorbents dp was varied. The

in�uences of particle size on the �ltration rate J and �ltration time t are shown in �gure 5.2.

The particle sizes of 500, 200 and 50 μm are highlighted in the diagrams, because these three

particle sizes are used in this work.

81


100 200 300 400 5000.0000.0010.0020.0030.004

dp / µm

J /

l m

-2 h

-1

Filtration rate function Highlighted performance of PBSAC materials

100 200 300 400 5000510152025

dp / µm

t /

h

Filtration time function Highlighted performance of PBSAC materials

(a) (b)

Figure 5.2.: In�uence of spherical carbon particle size on �ltration rate (a) and �ltration time

(b)

It is obvious that the increase in �ltration rate and the respective decrease in �ltration time

is most pronounced for small particle sizes up to 50-100 μm. With further increasing parti-

cle size, a plateau is approached due to the dominating e�ect of the membrane resistance.

In these calculations, the in�uence of particle size on �ltration rate and �ltration time is

negligible for particle sizes larger than 100 μm.

Compared to the �ltration experiments in section 5.1.1, the �ltration advantage of spherical

carbon over carbon powder (with around 2 μm in particle size) is even more pronounced

on a large, industrial scale. For a 10 μm powder, however, the large-scale �ltration factor

compared to 200 μm spherical carbon is about 4.1 and, thus, within the same dimension as

the lab-scale �ltration factor (see section 5.1.1).

Catalysts based on spherical carbon materials can result in cost reduction of chemical pro-

ductions on any scale due to a signi�cantly enhanced catalyst/product separation. Reactor

cleaning and re�lling will also be much easier, as the spherical carbon materials are non-

adhesive and non-dusting.

5.1.3. Spherical carbon in flow chemistry applications

Another noteworthy aspect, already discussed for gas puri�cation in section 2.3.5.2, is the low

pressure resistance of spherical adsorbents. In continuously operated reactions, a reactant

82


100 200 300 400 50001234

dp / µm

J /

l m

-2 h

-1

Filtration rate function Highlighted performance of PBSAC materials

Figure 5.3.: In�uence of spherical carbon particle size on �uid �ux (Rm = 0)

�uid would more easily �ow through a spherical catalyst bed, exhibiting a smaller pressure

drop. In �gure 5.3, the �ow �ux through a �xed bed of spherical carbon is estimated for

di�erent particle sizes using the �ltration rate function and the parameters in table 5.2, but

neglecting membrane resistance.

Here, with increasing particle size, the �uid �ux continuously increases. The factor f , which

describes the increase of �uid �ux with increasing particle size, simpli�es to the expression

f =d2p,larдed2p,small

. It solely is a function of the particle diameters of the materials in comparison.

For instance, an increase of particle size from 200 to 500 μm increases the �uid �ux by a factor

of 6.25 for the chosen parameters.

So, in essence, an active catalyst based on spherical carbon can be very advantageous in �ow

chemistry applications of �ne chemical and pharmaceutical productions due to a decreased

�ow resistance and pressure drop.

83


5.2. Spherical carbon as novel catalyst support material

5.2.1. Carbon surface functionalization

A functionalized carbon surface is important for metal deposition by ion adsorption. In this

section, the experimental window between mild carbon functionalization and possible struc-

tural disintegration is investigated. So, functionalization of spherical carbon with nitric acid

and sulfuric acid was analyzed at di�erent acid concentrations and temperatures.

For the investigations in this section, 50 g of highly activated spherical carbon with an aver-

age particle diameter of 200 μm was oxidized in each experiment. Nitric acid concentrations

of 15 and 36% and sulfuric acid concentrations of 25 and 50% were applied. Oxidation tem-

peratures were set to RT and 90°C, respectively. To guarantee homogeneous oxidation, the

volumes of oxidizing solutions were set to three times the materials’ pore volume. The sus-

pensions with nitric acid were stirred by an external stirrer instead of a magnetic stirrer and

later dried at 100°C. Here, less extreme drying conditions were chosen, because this would

technically simplify a prospective scale-up of the oxidation process. Additionally, both nitric

acid oxidations at RT were repeated a second time.

5.2.1.1. Influences on the amount of functional surface groups

The total degree of surface functionalization was quantitatively determined by gravimetri-

cal analysis before and after a thermal treatment. The resulting fractions of volatile com-

ponents are compared to the manually performed gravimetrical measurements of TPD-MS

experiments. Here, a well-de�ned temperature program was used, but water content wasn’t

distinguishable from other volatile components, as the samples were only weighted before

and after TPD-MS measurements. The determined values were corrected by the separately

determined water contents. Figure 5.4 shows the amounts of volatile components for di�er-

ently oxidized spherical carbon. The table in appendix B.1 separately lists the water contents

and the amounts of other volatile components.

The water content of the nitric acid treated samples varied between 0.5-4.8 wt% due to tech-

nically simpli�ed drying conditions. This was acceptable and accounted for in the quanti�ca-

tion of volatile surface functionalities. Nevertheless, su�cient sample �ushing with water to

remove residual nitric acid is important for precise surface analysis. The sulfuric acid func-

tionalized materials were completely dry, with only one sample containing 0.4 wt% water.

84


RT 90°C RT 90°C RT 90°C RT 90°C02468

1012 Volatile components

mvo

lati

le c

ompo

nent

s / w

t%

Oxidation temperature

024681012 Volatile components (TPD-MS)

15% HNO3 36% HNO3 25% H2SO4 50% H2SO4Acid concentration

Figure 5.4.: Quantitative surface analyses of oxidized spherical carbon (amount of volatile

components excluding water content)

The fraction of volatile components was measured in order to directly determine the total

amount of surface groups. Values varied between 1.9-8.4 wt%. The amounts of volatile com-

ponents �uctuated more strongly for the nitric acid treated samples than for the sulfuric acid

treated materials. With increasing oxidation temperature, the amount of volatile compo-

nents increased for nitric acid and sulfuric acid treatments. This trend is also supported by

the TPD-MS gravimetrical results. The mass loss after TPD-MS was always larger than the

values determined in the rapid test method for volatile components. This is due to a higher

end temperature of 1000°C during TPD-MS (instead of 800°C), desorbing also the most stable

surface groups.

With increasing nitric acid concentration, there is a trend of decreasing surface functional-

ization. Though, in�uence of nitric acid concentration on the amount of volatile components

seems to be small, except for samples functionalized with nitric acid at RT. Here, the total

amount of surface groups signi�cantly decreased with increasing acid concentration. This

behavior is apparent in the reproduction samples, as well. So, oxidation with 36% nitric acid

at RT in fact resulted in a signi�cantly lower degree of surface functionalization. Possibly,

this e�ect is caused by the speci�c composition of the spherical carbon itself. The nitric acid

samples oxidized at 90°C exhibited at both acid concentrations a similarly high degree of

surface functionalization. Compared to room temperature experiments, the carbon surface

was more strongly oxidized.

85


In case of the sulfuric acid treated samples, the degree of oxidation increased with increasing

temperature, too. The in�uence of acid concentration was small. In contrast to most nitric

acid treated samples, sulfuric acid functionalization results in a lower absolute amount of

weakly adsorbing surface groups. Due to the larger surface-speci�c acid molarity during

oxidation treatment (see also the table in appendix B.1), more strongly adsorbing surface

groups were introduced. This indicates a more intense reaction of carbon with sulfuric acid

molecules compared to nitric acid.

5.2.1.2. Influences on the chemical composition of functional surface groups

In the previous section, it was already indicated that di�erent surface groups with di�erent

adsorption strength are present. The exact chemical composition can be derived from TPD-

MS signals.

Mass spectroscopy signals of released gases during temperature programmed desorption are

plotted in �gure 5.5 for nitric acid treatments and in �gure 5.6 for sulfuric acid treatments.

In all diagrams, signal intensity of carbon monoxide has been corrected to �t the baseline.

All nitric acid treated samples exhibit a characteristic trend of the carbon monoxide signal.

In the temperature regime between about 200-500°C, two smaller overlapping peaks occur,

followed by two larger overlapping peaks between 500-1000°C. For the material oxidized

with 35% nitric acid at RT, these peaks are much less pronounced. In case of the carbon

dioxide signal, the peak areas in the same temperature regimes are opposed to the carbon

monoxide signal. Here, two large peaks overlap between 200-500°C, followed by a decrease of

signal intensity until 1000°C. Again, for the material oxidized with 36% nitric acid at RT, these

peaks are much less pronounced. For nitrogen oxide, all nitric acid treated samples exhibit

an increasing peak that abruptly decreases at around 500°C. The absolute signal intensity

is much lower than the carbon oxide signal intensities. Nitrogen dioxide and sulfur dioxide

show a similar trend as nitrogen oxide, but with even lower signal intensities. The hydrogen

sul�de signal continually decreases from RT to 1000°C at very low absolute intensities. At

around 1000°C, no further desorption of surface groups occurred.

Sulfuric acid treated samples also featured characteristic trends. The carbon monoxide sig-

nal exhibits a small peak between 200-400°C, followed by a constant value and a sharp peak

around 900°C. The constant plateau between 400-700°C is of very low intensity for the sample

oxidized with 50% sulfuric acid at 90°C. The signal of carbon dioxide generally has a smaller

peak between 200-400°C and two larger overlapping peaks between 400-900°C. With a much

86


15% HNO3 36% HNO3

RT200 400 600 800 10000.0

2.0x10-6

4.0x10-6

6.0x10-6

8.0x10-6

1.0x10-5

inte

nsit

y /

a.u

.

temperature / °C

CO CO2 NO NO2 H2S SO2

200 400 600 800 10000.02.0x10-6

4.0x10-6

6.0x10-6

8.0x10-6

1.0x10-5

inte

nsit

y /

a.u

.

temperature / °C


(a) (b)

90°C200 400 600 800 10000.0

2.0x10-6

4.0x10-6

6.0x10-6

8.0x10-6

1.0x10-5

inte

nsit

y /

a.u

.

temperature / °C


200 400 600 800 10000.02.0x10-6

4.0x10-6

6.0x10-6

8.0x10-6

1.0x10-5

inte

nsit

y /

a.u

.

temperature / °C


(c) (d)

Figure 5.5.: TPD-MS spectra of nitric acid treated spherical carbon with 15% HNO3 @RT (a),

36% HNO3 @ RT (b), 15% HNO3 @ 90°C (c), and 36% HNO3 @ 90°C (d)

87


25% H2SO4 50% H2SO4

RT200 400 600 800 10000.0

2.0x10-6

4.0x10-6

6.0x10-6

inte

nsit

y /

a.u

.

temperature / °C

CO CO2 NO NO2 H2S

200 400 600 800 10000.0

2.0x10-6

4.0x10-6

6.0x10-6

inte

nsit

y /

a.u

.

temperature / °C


(a) (b)

90°C200 400 600 800 10000.0

2.0x10-6

4.0x10-6

6.0x10-6

inte

nsit

y /

a.u

.

temperature / °C


200 400 600 800 10000.0

2.0x10-6

4.0x10-6

6.0x10-6

inte

nsit

y /

a.u

.

temperature / °C


(c) (d)

Figure 5.6.: TPD-MS spectra of sulfuric acid treated spherical carbon with 25% H2SO4 @ RT

(a), 50% H2SO4 @ RT (b), 25% H2SO4 @ 90°C (c), and 50% H2SO4 @ 90°C (d)

88


lower absolute intensity, nitrogen oxide reaches a maximum between 200-400°C. Addition-

ally, nitrogen dioxide exhibits two more broad peaks between 400-900°C. Similarly to nitro-

gen oxide, both hydrogen sul�de and sulfur dioxide have a single peak between 200-400°C

at low absolute intensities. In case of the hydrogen sul�de signal, the absolute intensity in-

creases with increasing oxidation temperature and acid concentration. Like the nitric acid

treated material, no further desorption occurred at around 1000°C.

The TPD-MS studies show that most surface groups desorb as either carbonmonoxide or car-

bon dioxide. This is strongly indicated by the large absolute signal intensities of these two

gases compared to the much lower signal intensities of the remaining gases. This behavior is

observed for nitric acid and sulfuric acid treatments. The less pronounced signals of the sam-

ple oxidized with 36% nitric acid at RT indicate a lower degree of surface functionalization,

which is consistent with the quantitative analysis presented earlier. Also, signi�cant signal

intensities are observed in the temperature range between 800-1000°C, plausibly explaining

the deviations of the two thermo-gravimetrical analysis methods in section 5.2.1.1.

Together with the desorption temperature, the detected gases can be directly linked to spe-

ci�c surface groups (see section 2.2.5.1). So, the functionalized surface of spherical carbon

primarily consists of oxide surface groups. Based on absolute signal intensities, the nitric

acid treated samples contain many strongly adsorbing groups like carbonyls and quinones,

but also a signi�cant amount of phenols, ethers andweakly adsorbing carboxylic acid groups.

Sulfuric acid functionalized samples primarily consist of carbonyls, quinones, and less strongly

adsorbing carboxylic anhydrides. Figure 5.7 shows ratios of di�erently strong chemisorbing

oxide surface groups for each oxidized material, which are also listed in the table in appendix

B.2. This ratio of surface groups was extracted from �gures 5.5 and 5.6 by integration of the

carbonmonoxide and carbon dioxide signal intensities in the distinctive temperature regimes

100-500°C and 500-1000°C.

All oxidized materials exhibit a larger number of strongly adsorbing surface groups releasing

carbon monoxide. Again, the material treated with 36% nitric acid at RT shows deviating

properties with an enlarged ratio of strongly adsorbing surface oxides desorbing as carbon

monoxide. Except for this material, the other nitric acid functionalized spherical carbons

contain an excess of weakly adsorbing groups releasing carbon dioxide. The four sulfuric

acid oxidized carbons have more strongly than weakly adsorbing surface groups, which is

in accordance with the quantitative investigations of the previous section. Carboxylic acids

seem to be more prevalent on nitric acid functionalized carbons than on sulfuric acid treated

samples. Otherwise, surface composition in relation to adsorption strength appears to be

89


RT 90°C RT 90°C RT 90°C RT 90°C01238

10 Groups releasing CORa

tio

of s

tron

gly

to w

eakl

y ad

sorb

ing

grou

ps /

-


0123810

Groups releasing CO2 15% HNO3 36% HNO3 25% H2SO4 50% H2SO4Acid concentration

Figure 5.7.: Ratio of weakly adsorbing surface oxides desorbing between (100-500°C) and

strongly adsorbing oxides desorbing between (500-1000°C)

similar for almost all samples. The dominant prevalence of carboxylic acid groups in nitric

acid treated samples correlates well with literature reports (see section 2.2.5.1).

Next to pure oxygen chemisorption, nitric acid treatment incorporated some nitrogen func-

tional groups into the carbon surface, observable by nitrogen oxide desorption. A small

amount of sulfur is also found at the carbon surface of the nitric acid oxidized samples. The

sulfur originates from the production process of the spherical carbon itself (see section 2.1.2).

It’s important to address the fact that after oxidation treatment, only sulfate is present, which

is not a catalyst poison. Unexpectedly, nitrogen surface groups were detected in the samples

with sulfuric acid treatment. These traces of nitrogen heteroatoms can originate from the

production of spherical carbon, as well. An anticipated e�ect is that the sulfuric acid treat-

ment apparently introduces additional sulfur functional surface groups. The amount of sulfur

groups increases with intensifying reaction conditions, i.e. increasing acid concentration and

temperature.

5.2.1.3. Influences on spherical carbon acidity

Another analytic approach towards a diversi�ed understanding of surface characteristics is

possible by potentiometric titration. As described in section 2.2.5.1, this method provides

detailed information of the surface’s acid-base character. The results of potentiometric titra-

tion are shown in �gure 5.8 in the form of distribution functions derived from the titration

90


curves.

Pure spherical carbon exhibits three peaks, a larger one at a pKa of 10 and two smaller peaks

between pKa 6-9. After oxidation, the total number and area of peaks increase. For nitric

acid treated samples, three additional peaks between pKa 3-6 appear, except for the material

treated with 36% nitric acid at RT. The sample oxidized with 15% nitric acid at 90°C has

its �rst peak at a lower pKa of 3 compared to the other samples. Focusing on the larger

peak at pKa 10, the peak area increases with increasing oxidation temperature. At constant

temperature, this peak area decreases with increasing acid concentration. In case of the

sulfuric acid functionalized samples, the di�erences to pure spherical carbon are small. The

large peak at pKa 10 has a larger absolute area than the pure material, though the size then

only slightly increases with increasing oxidation intensity. Three peaks exist in total with

pKa values between 5-8, an exception of one additional peak being the oxidation with 50%

sulfuric acid at 90°C.

Interestingly, the pure spherical adsorbents already contain an observable amount of acidic

surface groups. These are especially weak acids like phenols, but also some stronger car-

boxylic groups and lactones with pKa values between 6-7. This surface modi�cation occurred

during carbon activation with steam and carbon dioxide (see section 2.1.1).

Nitric acid treatment introduces additional carboxylic groups and lactones with varying acid-

ity. The exception is the material oxidized with 36% nitric acid at RT. Here, no additional

surface groups were formed. This low degree of functionalization is consistent with TPD-

MS results. Very acidic carboxylic acid groups are present in the sample treated with 15%

nitric acid at 90°C. This behavior doesn’t correlate with the TPD-MS studies. An explanation

for especially acidic surface groups is the interaction of carboxylic acid groups with neigh-

boring oxide or nitro surface groups [71, 83, 85]. In this case, this oxidation treatment leads

to a particular surface distribution of functional groups. The amount of less acidic surface

groups (pKa 10) still exceeds the amount of carboxylic acids. With increasing temperature

these groups increase in number, correlating well with TPD-MS studies and the quantita-

tive increase in functional groups. Noteworthy is the trend of decreasing amount of surface

groups at pKa 10 with increasing acid concentration. This trend is also observed in the gravi-

metrical analysis. It is an indication of too excessive oxidation, resulting in carbon removal

and a decreased carbon surface area. Consequentially, this smaller surface area has a lower

capacity for functional groups.

In contrast, sulfuric acid oxidation is more intense, as already discussed in the previous sec-

tions. Only elevated temperatures and acid concentrations lead to the formation of a few

91


2 4 6 8 100123 2 4 6 8 100123 2 4 6 8 100123

f(pK

a) /

mm

ol g

-1

pKa / -

pure PBSAC

f(pK

a) /

mm

ol g

-1 15% HNO3 @ RT 36% HNO3 @ RT 15% HNO3 @ 90°C 36% HNO3 @ 90°C

f(pK

a) /

mm

ol g

-1 25% H2SO4 @ RT 50% H2SO4 @ RT 25% H2SO4 @ 90°C 50% H2SO4 @ 90°C

Figure 5.8.: Potentiometric titration of functionalized and non-functionalized spherical car-

bon

92


None RT 90°C RT 90°C RT 90°C RT 90°C02468

10 Pure PBSAC pH

PZC /

-


0246810 HNO3 oxidized PBSAC H2SO4 oxidized PBSAC

None 15% HNO3 36% HNO3 25% H2SO4 50% H2SO4Acid concentration

Figure 5.9.: Point of zero charge pH values of functionalized and non-functionalized spherical

carbon

more weakly adsorbing, acidic carboxylic acid groups. Nevertheless, already at mild oxi-

dation conditions, a signi�cant amount of strongly adsorbing, weakly acidic oxide groups

is created at pKa 10, correlating well with TPD-MS results. This amount doesn’t increase

further with increasing acid concentration or temperature.

As a short summary, the potentiometric titrations generally validate gravimetrical measure-

ments and TPD-MS results. Certainly, the acidic character of the oxidized spherical carbon

has been clari�ed in more detail.

In addition to potentiometric titrations, it’s interesting to look at the overall acid-base char-

acter of di�erently oxidized spherical carbon. This is possible by analyzing the point of zero

charge of each material. The point of zero charge measurements were conducted according

to section 4.4. The corresponding pH values are shown in �gure 5.9. Numerical data is listed

in appendix B.3.

Prior to oxidation, the pure spherical carbon had an initial point of zero charge at pH 9.1.

The point of zero charge decreases after oxidation. The nitric acid treated samples exhibited

zero charge at pH values between 3.6-4.4, with an exception for the sample oxidized with 36%

nitric acid at RT. It had its point of zero charge signi�cantly higher at pH 6.4. The samples

oxidized with sulfuric acid all had a similar point of zero charge between pH 3.3-3.4.

The pure spherical carbon exhibits a basic character in water. Surface functionalization re-

sults in acidic materials. Sulfuric acid treated samples are more acidic than nitric acid treated

93


0.0 0.2 0.4 0.6 0.8 1.00.00.51.00.00.51.01.5

p p-10

/ -

v ads (S

TP)

/ c

m3 g

-1

15% HNO3 @ RT 36% HNO3 @ RT 15% HNO3 @ 90°C 36% HNO3 @ 90°C pure PBSAC

25% H2SO4 @ RT 50% H2SO4 @ RT 25% H2SO4 @ 90°C 50% H2SO4 @ 90°C pure PBSAC

Figure 5.10.: Water vapor isotherms of di�erently oxidized spherical carbon

materials. Even though, nitric acid treated samples mostly have a larger number of surface

groups and according to potentiometric titration some very acidic groups, they also have a

large amount of less acidic groups. In sum of all surface charges, which represents the point

of zero charge, the sulfuric acid treated materials are still more acidic. This correlates well

with literature reports, where sulfuric acid treated carbons were applied as acid catalysts (see

section 2.2.5). The sample oxidized with 36% nitric acid at RT had a much higher point of

zero charge, because its overall degree of functionalization was lower.

5.2.1.4. Influences on carbon surface we�ability

Because carbon surface oxidation increases water a�nity of the material, water vapor sorp-

tion experiments were performed to look at the hydrophilic character of di�erently oxidized

spherical carbons. The results of water adsorption with increasing relative humidity and

desorption with decreasing humidity are plotted in �gure 5.10.

The bottom diagram shows isotherms of nitric acid pretreated samples and pure spherical

carbon. The pure spherical carbon slowly began to adsorb water at around 70% rh. At 100%

rh, the pore system was saturated with around 1.1 cm3g-1water. Desorption proceeded

slowly, but rapidly increased at around 60% rh. Oxidizing spherical carbon with nitric acid

94


signi�cantly shifts sorption isotherms to lower relative humidity and larger total water ad-

sorption. With increasing oxidation temperature and decreasing acid concentration, this

shift is more pronounced. The largest total water adsorption amounted to about 1.3 cm3g-1

for the sample oxidized with 15% nitric acid at 90°C. Already at around 30% rh, this material

began to adsorb water vapor.

Looking at the upper diagram, a similar shifting of isotherms from the pure carbon to sulfuric

acid treated samples is observed. With increasing temperature and increasing acid concen-

tration, the isotherms are slightly shifted further to lower humidity and larger total wa-

ter adsorption. Here, this e�ect is much less intense than for nitric acid treated materials.

The maximum water adsorption amounted to around 1.25 cm3g-1for the sample oxidized

with 50% sulfuric acid at 90°C. Besides this basic description, mathematical analysis of these

isotherms can be carried out. Thus, information such as surface-speci�c oxygen number and

distance between oxygen atoms can be derived.[72]

The described shifting of isotherms to lower relative humidity and larger water adsorption

demonstrates an increase in water a�nity of the material. Compared to pure spherical car-

bon, all oxidized samples are more hydrophilic. The hydrophilic nature of the materials

further increases with increasing degree of functionalization. The order of isotherm shift-

ing correlates exactly with the trends observed earlier in the previous sections by thermal

desorption experiments and potentiometric titration. For the nitric acid treated samples,

the surface functionalization increases with increasing oxidation temperature and decreas-

ing acid concentration due to an increasing quantity of surface functional groups. In case

of sulfuric acid treatment, more functional groups are introduced by increasing temperature

and acid concentration. Nevertheless, these e�ects on the hydrophilic character are small

compared to nitric acid treatments due to much smaller deviations in the quantity of surface

functional groups for di�erent oxidation treatments.

5.2.1.5. Influences on carbon pore structure

Finally, the in�uence of surface oxidation on pore geometry and pore integrity was also an-

alyzed in classical nitrogen sorption experiments. Out of all process steps in catalyst prepa-

ration, the oxidation procedure can most likely negatively a�ect stability of the support

material due to total carbon oxidation. Theoretically, the growth of metal clusters during

impregnation and metal transformation can also damage the pore structure.

Figure 5.11 shows the volume-speci�c pore size distributions of pure spherical carbon and the

oxidized samples. Additionally, �gure 5.12 presents the change in BET surface area and total

95


0 1 2 3 4 5 6 7 8 9 100.00.51.00.00.51.0

0 1 2 3 4 5 6 7 8 9 10

pore width / nm

dV(w

) /

cm

3 nm

-1 g

-1

15% HNO3 @ RT 36% HNO3 @ RT 15% HNO3 @ 90°C 36% HNO3 @ 90°C pure PBSAC

25% H2SO4 @ RT 50% H2SO4 @ RT 25% H2SO4 @ 90°C 50% H2SO4 @ 90°C pure PBSAC

Figure 5.11.: Volume-speci�c QSDFT pore size distribution of di�erently oxidized spherical

carbon

pore volume after di�erent surface oxidation treatments. Appendix B.4 includes a table of

pore geometry properties derived from nitrogen sorption isotherms, including the deviation

of BET surface area and pore volume from values of pure spherical carbon.

In the pore size distribution of pure spherical carbon, all pores appear to be smaller than

3 nm. A large volume-speci�c amount of pores exhibits pore sizes between 1-3 nm, but there

are also many pores smaller than 1 nm. Oxidation with nitric acid has a visible e�ect on the

total pore volume. In the pore size distribution, the individual peaks decrease in size. The

distribution of pores itself doesn’t change, except for some pores apparently being formed

between 3-4.5 nm for samples treated with 36% nitric acid at RT and 15% nitric acid at 90°C.

The pore size distributions of carbons treated with sulfuric acid at di�erent temperatures and

acid concentrations are very similar. Compared to pure spherical carbon, pore size distribu-

tion and pore volume aren’t a�ected by sulfuric acid oxidation.

Quantitatively looking at the pore geometry, nitric acid treatment has a signi�cant impact on

BET surface area and pore volume. With a 30% decrease at rather mild oxidation conditions,

this e�ect was most pronounced for the sample treated with 15% nitric acid at RT. Not too

surprisingly, as indicated by investigations of the previous sections, the sample oxidized with

96


RT 90°C RT 90°C RT 90°C RT 90°C

-30-20-10

0

BETDiff

eren

ce to

pur

e PB

SAC

/ %


-30-20-100

Vtot 15% HNO3 36% HNO3 25% H2SO4 50% H2SO4Acid concentration

Figure 5.12.: Change in BET surface area and total pore volume of di�erently oxidized spher-

ical carbon from values of pure spherical carbon

36% nitric acid at RT only shows very small deviations in pore geometry. The changes in

surface area and pore volume are generally much smaller in case of sulfuric acid treatment.

Here, the e�ect of decreasing deviations with increasing acid concentration is also observed.

Oxidation of spherical carbon noticeably decreases internal surface area and pore volume.

By nitric acid treatment, these quantities decrease by up to 31%. In case of sulfuric acid

treatment, the deviation only amount to 12% and less. The decrease in speci�c surface area

and pore volume is due to mass increase by heteroatom inclusion [194]. Consequently, using

sulfuric acid, the carbon structure is oxidized with fewer e�ects on the pore structure.

5.2.1.6. Important di�erences between nitric acid and sulfuric acid oxidation

In the previous sections, a variety of in�uences of di�erent oxidation treatments on the sur-

face properties of spherical carbon have been pointed out. The most important di�erences

between the two applied acids are the e�ects on quantity and type of surface oxides, as well

as on wettability, acidity, and pore geometry of spherical carbon. These di�erences are dis-

cussed in the following.

The quantity of surface functional groups was similar for all sulfuric acid treated samples.

For nitric acid treatments, on the other hand, the quantity of functional groups increased sig-

ni�cantly more strongly with increasing oxidation temperature. Due to the strongly increas-

97


ing quantity of surface functional groups, the hydrophilic nature of the materials strongly

increased for nitric acid treated samples with increasing temperature and decreasing acid

concentration. This e�ect of increasing carbon wettability was much smaller for sulfuric

acid treated samples with only small deviations in the degree of surface functionalization.

The type of surface groups present in spherical carbon also vary for nitric acid and sulfuric

acid treatments. While all oxidized materials consisted of many strongly adsorbing surface

groups such as carbonyls and quinones, nitric acid treated samples also had a signi�cant

amount of phenols, ethers and weakly adsorbing carboxylic acid groups. Instead sulfuric acid

functionalized samples contained additional, less strongly adsorbing carboxylic anhydrides.

In total, sulfuric acid treated samples generally exhibit a larger fraction of strongly adsorbing

oxides with thermal decomposition temperatures between 500-1000°C, due to a more intense

reaction with the carbon surface.

While the overall acidity of all oxidized spherical carbons was similar, most nitric acid treated

samples contained some very acidic functional groups, likely due to the interaction of car-

boxylic acid groups with neighboring oxide or nitro surface groups.

Concerning the pore geometry of spherical carbon, generally, pore volume and BET surface

area strongly decrease after nitric acid treatment due to heteroatom inclusion, while the pore

structure is much less a�ected by sulfuric acid treatments.

Concluding the investigations on surface functionalization using nitric acid and sulfuric acid,

the observed trends regarding the in�uences of oxidation temperature and acid concentra-

tions were similar, but generally much less pronounced for sulfuric acid treatments. In the

experimental window of oxidation temperature and acid concentration applied in this work,

the results were most consistent for sulfuric acid treatments.

98


5.2.2. Active metal deposition

5.2.2.1. Palladium: Influences on metal loading and dispersion

Influence of particle size

In this section, the prepared 5 wt% palladium catalysts based on spherical carbon are char-

acterized in order to study the in�uence of di�erently sized catalysts on metal loading and

dispersion. Furthermore, material properties of the prepared catalysts are compared to a

reference platinum on carbon powder catalyst.

Therefore, highly activated spherical carbon with a particle diameter around 500 μm and a

total pore volume of 1.18 cm3g-1was functionalized with a 10% nitric acid solution at RT

using standard oxidation parameters. Applying the ion adsorption method and subsequent

metal transformation in hydrogen atmosphere, palladium metal was deposited. In addition,

a 1 kg scale-up of the spherical catalyst was conducted by Blücher GmbH applying the very

same conditions with proportionally increased masses. So, a su�cient amount of material

was available for analysis. Moreover, experience of catalyst preparations on a larger scalewas

obtained. This material was also crushed and classi�ed into multiple particle size fractions

for further analysis in section 5.2.3.1.

In a parallel approach, the pure carbon material was intensively crushed in a ball mill. Then,

it was oxidized with nitric acid and loaded with 5 wt% palladium using standard parame-

ters. This crushed powder catalyst was prepared to better understand the in�uence of mass

transfer during catalyst preparation.

The metal loading was indirectly determined by analyzing the impregnation solution after

solid separation using ICP-AES elemental analysis. The metal dispersion was determined

by carbon monoxide pulse chemisorption analysis. TEM images of intensively crushed cat-

alyst material were taken to gather information about the palladium cluster size. Also, the

palladium distribution over the particle cross section was screened by SEM-EDX.

Upon addition of carbon to the amber precursor solution, the solution discolored. According

to ICP-AES results, the spherical carbonwas loaded with 4.98 wt% palladium. As the quantity

of non-adsorbed palladium salt was negligibly small, it can be stated that palladium chloride

was completely deposited onto oxidized spherical carbon. For the 1 kg catalyst scale-up,

almost no solid residues remained after the washing solution was evaporated, thus, demon-

strating full metal salt deposition. Metal loading of the crushed catalyst wasn’t explicitly

determined. Table 5.3 lists metal dispersions of the scale-up catalyst and commercial 5 wt%

palladium on carbon powder.

99


Table 5.3.: Metal dispersions of 5 wt% palladium catalysts based on spherical carbon and car-

bon powder

Catalyst DPd / % Derived metal clustersize / nm

5 wt% Pd @ 500 μm PBSAC n/a n/a

5 wt% Pd @ crushed PBSAC n/a n/a

5 wt% Pd @ 500 μm PBSAC (1 kg

scale-up)

52 2.2

5 wt% Pd @ PAC (commercial) 30 3.7

With 52%, palladium dispersion on spherical carbon was very large. Compared to 30%, pal-

ladium was more �nely dispersed on spherical carbon than on commercial carbon powder.

The average metal cluster size derived from metal dispersion was in the range of 2.2 nm.

A TEM image of crushed catalyst material, as shown in �gure 5.13, gave a more accurate

overview of metal cluster sizes. In �gure 5.14, the results of a SEM-EDX scan over the cross

section of a catalyst sphere is shown.

In TEM images, palladium clusters were visible only with low contrast to the carbon support.

Lattice planes of palladium weren’t distinguishable. The clusters had an average particle di-

ameter of 2.7±0.5 nm. With only 16 counts, sampling quantity was small, though. Compared

to the carbon monoxide chemisorption result, the cluster size determined from TEM images

was about 20% larger. This deviation is either due to a partially deviating carbon monoxide

adsorption stoichiometry or due to uncertainties in TEM image analysis. Generally, palla-

dium clusters were very small and homogeneously distributed across the sample, indicating

successful catalyst preparation.

In the SEM image, the cross section of the spherical catalysts showed a concentrical structure.

The catalyst shell with a thickness of about 30 μm appeared much brighter than the core ma-

terial. Also, the spherical carbons weren’t perfectly spherical. The palladium concentration,

determined by EDX at several positions across the radius, varied between 0.2 and 0.6%. With

the exception of an elevated concentration in the outer shell, palladium was homogeneously

distributed across the catalyst’s diameter. The outer concentrical band was likely caused by

epoxy resin partially penetrating the pore system. The elevated palladium content in this

outer shell can result from a di�ering focus of the EDX beam. Because the resin penetrated

shell had an increased hardness, a hollow can have been polished in the softer core. Never-

100


Figure 5.13.: TEM image of a crushed 5 wt% palladium on 500 μm spherical carbon catalyst

with palladium clusters highlighted

2 4 6 8 10 12 14 16 18 200.00.20.40.60.8

position / -

Pd-E

DX

cont

ent

/ %

Figure 5.14.: SEM image of a cross section of a 5 wt% palladium on 500 μm spherical carbon

catalyst including the results of an EDX scan

101


theless, these �ndings further demonstrated that the internal surface area of spherical carbon

likely was completely utilized in palladium deposition. A potential systematic error due to

palladium smearing over the cross section during polishing cannot be ruled out, though.

These results most importantly show that palladium was successfully deposited on spher-

ical carbon with good dispersion. With the 1 kg catalyst preparation, the methods of car-

bon oxidation, subsequent electrostatic palladate adsorption and metal transformation were

achieved on a signi�cantly larger scale. Metal dispersion was even larger than palladium

dispersion of the commercial powder catalyst.

Influence of pore structure

Spherical carbons with di�erent pore structures were loaded with palladium and character-

ized to study the in�uence of pore structure on metal loading and dispersion. Almost all

spherical carbon materials presented in section 4.3.1 were tested as catalyst support. Thus,

pore structure variations for three di�erent particle sizes were investigated.

The materials were oxidized with nitric acid and loaded with 5 wt% palladium according to

standard procedures (see section 4.1.1 and 4.1.2). The prepared catalysts were characterized

with carbon monoxide pulse chemisorption for metal dispersion. The results are listed in

table 5.4, together with indirectly determined metal loadings.

In all cases, palladium chloride fully adsorbed to spherical carbon resulting in complete metal

deposition. The spherical catalysts had metal dispersions in the range between 28-38%. A

slight trend of increasing metal dispersion with increasing total pore volume was apparent,

due to an increased BET surface area. Thus, more space was available for surface oxidation

and metal deposition. The mesoporous 500 μm exhibited a slightly larger metal dispersion

than the least activated 500 μmmaterial, despite its lower BET surface area, though, possibly

due to a lower degree of micropore �lling with noble metal.

Consequently, pore structure has a low in�uence on metal loading and metal dispersion.

The e�ects of pore structure and metal dispersion on catalytic performance are addressed

separately in section 5.2.3.1.

102


Table 5.4.: Palladiummetal loadings and dispersions of spherical catalysts with di�erent par-

ticle size and pore volume

Particle size / μm Vtot / cm3 g-1 Obtained Pdloading / wt%

DPd / %

500

0.61 4.97 30

1.18 5.00 n/a

1.19* 4.93 34

200

0.69 4.97 28

0.95 4.93 32

1.13 4.96 37

1.67 4.97 37

50

0.61 4.98 32

0.89 4.98 34

1.07 4.98 38

* spherical carbon with enhanced mesoporous pore structure

103


Table 5.5.: Palladium metal loadings and dispersions of di�erently oxidized spherical carbon

catalysts

Acid type Acidconcentration /wt%

Oxidationtemperature / °C

Obtained Pdloading / wt%

DPd / %

Nitric acid

15

RT 4.95 23

90 4.90 50

36

RT 4.98 37

90 4.90 55

Sulfuric acid

25

RT 4.99 58

90 4.99 31

50

RT 4.99 57

90 4.99 63

Influence of surface functionalization

Next to the mild standard oxidation of spherical carbon, carbon functionalization was carried

out with di�erent acids, acid concentrations and oxidation temperatures. In section 5.2.1,

these materials were analyzed in detail. After palladium deposition, the resulting materials

were characterized further in order to investigate the in�uence of surface functionalization

on metal loading and dispersion. The results are discussed in this section.

Each 5 g sample of functionalized material was loaded with 5 wt% palladium according to

standard procedures (see section 4.1.2). Metal dispersions were determined by pulse chemi-

sorption experiments using carbon monoxide. The results are listed in table 5.5, together

with indirectly determined metal loadings.

Considering measuring uncertainties, palladium chloride was always fully deposited onto

the di�erently functionalized carbon surfaces. Metal dispersions of the di�erently oxidized

catalysts varied strongly between 23 and 63%. For nitric acid pretreated materials, metal

dispersion increased with increasing acid concentration and oxidation temperature. Three

samples oxidized with sulfuric acid showed similarly large palladium dispersions of 57-63%.

The catalyst pretreated with 25% sulfuric acid at 90°C had a lower metal dispersion of 31%,

though.

The trend of increasing metal dispersion with intensifying nitric acid oxidation conditions

didn’t directly correlate with any material properties determined in section 5.2.1. That is,

104


Table 5.6.: Obtained metal loadings of spherical carbon catalysts loaded with di�erent

amounts of palladium

Expectedpalladiumloading / wt%

Obtainedpalladiumloading / wt%

2 1.90

5 4.94

10 9.58

neither the total amount of surface groups, the ratio of strongly to weakly adsorbing surface

groups, the PZC values nor the pore structure alone did determine dispersion of palladium

clusters. As a consequence, an interaction of antagonizing mechanisms is hypothesized. This

can be the interaction of ion adsorption to charged surface oxides and reactive adsorption

to unsaturated C-π surface sites. In case of poor surface oxidation, reactive C-π sites re-

main in large amounts on the carbon surface. Each C-π site can instantly reduce multiple

palladate ions, forming large palladium clusters. Those palladate ions are then no longer

available for ion adsorption. With increasing degree of surface functionalization, metal dis-

persion increases due to a lower number of C-π sites. Electrostatic adsorption of palladium

salt to protonated surface oxides now is the primary mechanism of metal deposition. In or-

der to clarify this hypothesis, further catalyst characterization, e.g. in the form of catalytic

experiments, are necessary. See also section 5.2.4 for concluding remarks on the role of C-π

sites.

Based on the same hypothesis, sulfuric acid apparently functionalizes spherical carbon well

at mild conditions, thus leading to large metal dispersions through homogeneous ion adsorp-

tion. The anomaly of the low metal dispersion for the material oxidized with 25% sulfuric

acid at 90°C is di�cult to explain, though.

Influence of metal loading

In these experimental series, palladiummetal loading of spherical carbonwas varied between

2-10 wt%. Therefore, 200 μm highly activated spherical carbon was oxidized and loaded

with the respective amount of palladium, applying standard procedures. Table 5.6 lists the

applied metal loadings for each catalyst, as determined by indirect ICP-AES analysis. Metal

dispersions have not been determined.

Between 95-99% of palladium chloride did adsorb to spherical carbon during the impregna-

105


tion process. It can be stated that palladium can be almost completely deposited on spherical

carbon up to a metal loading of 10 wt%.

106


5.2.2.2. Ruthenium: Influences on metal loading and dispersion

Another noble metal that catalyzes hydrogenation reactions is ruthenium (see section 2.2.1).

In a similar approach to the previous section, the deposition of ruthenium on spherical carbon

was studied. Speci�cally, ruthenium loading was varied, metal dispersions were determined

and compared to a reference ruthenium on carbon material.

As catalyst support, highly activated 200 μm spherical carbon was chosen and oxidized with

50% sulfuric acid solution at 90°C for 2.5 h. Impregnation with ruthenium chloride hydrate

and metal salt transformation were carried out according to standard procedures (see sec-

tion 4.1.2). Three catalysts were prepared with desired metal loadings of 0.5, 5, and 10 wt%.

The actual ruthenium loadings and ruthenium leaching were analyzed by ICP-AES measure-

ments. Metal dispersion was determined by carbon monoxide pulse titration.

In the beginning of the impregnation process, monitored pH values showed that aqueous

ruthenium chloride solutions were acidic. For example, the precursor solution for the 5 wt%

ruthenium catalyst had a pH value of 1.27. Thus, a signi�cant pH di�erence to the spherical

carbon’s PZC of 3.1 (see section 5.2.1) was given to allow for e�ective electrostatic adsorp-

tion. Nevertheless, after addition of carbon material and stirring for 24 h, the impregnation

solutions didn’t discolor, indicating incomplete adsorption of ruthenium ions.

The quantitatively determined ruthenium depositions are plotted in �gure 5.15 for di�erent

desired metal loadings. It should be noted that the impregnation time amounted to 65 h in

case of the 5 wt% ruthenium catalyst.

All three ruthenium catalysts with desired metal loadings between 0.5 and 10 wt% showed

incomplete ruthenium deposition. Only between 55 and 63% of ruthenium were deposited.

Metal deposition was already limited at a low desired metal loading of 0.5 wt%. Nevertheless,

with increasing desired metal loadings up to 10 wt%, larger absolute amounts of ruthenium

up to 5.54 wt% were still deposited. Consequentially, concerning the 0.5 and 5% ruthenium

catalyst systems, the total number of carbon surface groups was su�cient for complete ad-

sorption of ruthenium ions. Still, ruthenium deposition was incomplete. Also, a signi�cant

increase in impregnation time didn’t improve metal deposition. These observations strongly

indicated equilibrium of di�erently charged ruthenium species a�ecting ion adsorption. It

is known in literature that aqueous solutions of ruthenium chloride consist of cationic and

anionic ruthenate, as well as neutral ruthenium complexes [195, 196]. At a pH of 1, about

50% of ruthenium species were positively charged, more than one third was neutral and a

small remaining fraction was negatively charged [195]. Apparently, negatively and posi-

tively charged ruthenium ions adsorbed to carbon surface groups, while neutral ruthenium

107


0 2 4 6 8 1002468

10 Ru deposition Linear fit Ideal Ru depositon

obta

ined

Ru

load

ing

/ w

t%

expected Ru loading / wt%

Figure 5.15.: Comparison of desired and actual ruthenium loadings

species weren’t adsorbed. Adsorbed ruthenium ions still contributed to equilibrium. Other-

wise, additional ruthenium ions would have been formed and ruthenium deposition would

have eventually been completed. This equilibrium is likely responsible that, in contrast to

the palladium system, ruthenium wasn’t completely adsorbed by spherical carbon. How-

ever, ruthenium loadings of at least 5.54 wt% were accomplished by using the ion adsorption

method.

After metal salt transformation in hydrogen atmosphere, the prepared catalysts didn’t leach

ruthenium. It was assumed that the reduction parameters were su�cient to completely re-

duce the catalyst material. Table 5.7 lists corresponding metal dispersions and derived clus-

ter sizes of the di�erently loaded ruthenium catalysts and a commercial 5 wt% ruthenium on

carbon powder.

Metal dispersions varied strongly between 19 and 59%. The catalyst with the lowest ruthe-

nium loading exhibited the largest dispersion and the smallest derived cluster size. With 50%

metal dispersion, the 5.54 wt% ruthenium catalyst also consisted of small ruthenium clusters.

The 3.15 wt% ruthenium catalyst had a signi�cantly smaller metal dispersion and an average

cluster size of around 6.9 nm. The commercial powder catalyst with an actual metal loading

of 5.26 wt% exhibited ametal dispersion of 34%. The large deviation inmetal dispersion of the

3.15 wt% ruthenium catalyst can be due to the extended impregnation time. With increas-

ing impregnation time, agglomerates of ruthenium ions can have allocated. The 0.31 wt%

ruthenium catalyst exhibited a larger dispersion than the 5.54 wt% catalyst, because smaller

108


Table 5.7.: Metal dispersions and derived cluster sizes of ruthenium on spherical carbon, with

di�erent metal loadings and impregnation times, and commercial ruthenium on

carbon powder

Catalystsupportmaterial

Rutheniumloading / wt%

Impregnationtime / h

DRu / % Derived Rucluster size / nm

PBSAC

0.31 24 59 2.3

3.15 65 19 6.9

5.54 24 50 2.7

PAC 5.26 - 34 3.9

amounts of ruthenium can be more homogeneously distributed across the carbon surface

due to a lower metal surface concentration.

In essence, ruthenium was deposited on spherical carbon with generally good metal disper-

sion. With the presented impregnation method, only about two-third of the precursor salt is

deposited.

5.2.2.3. Platinum: Influences on metal loading and dispersion

Investigations on metal deposition of palladium and ruthenium on spherical carbon were

presented in detail in the previous sections. In case of spherical carbon loaded with plat-

inum, the focus was to develop a highly active catalyst applying this previously acquired

knowledge on catalyst preparation. Parameters, that a�ect catalytic activity most, were ad-

justed stepwise. Prepared materials were directly tested in catalytic experiments. Obtained

results were interpreted to further improve catalytic activity.

Influence of ion adsorption modification

This experiment takes a closer look at the role of hydrogen chloride addition in the impreg-

nation of platinum catalysts by ion adsorption. The additional chloride ions compete with

metal anions for adsorption sites potentially a�ecting metal loading and dispersion.

The �rst PBSAC-based platinum catalysts were produced in analogy to the established prepa-

ration method of palladium catalysts. A metal loading of 5 wt% was targeted. Highly acti-

vated spherical carbon was treated with nitric acid at standard oxidation parameters and im-

pregnated with an aqueous solution of hexachloroplatinic acid and hydrochloric acid. Devi-

109


Table 5.8.: Platinum metal loadings of di�erently impregnated spherical catalysts

Addition ofhydrogen chlorideto impregnationsolution

pH of impregnationsolution / -

Obtained Pt loading/ wt%

Yes 0.8 4.88

No 1.3 4.99

ating from standard procedures, platinum salt transformation in hydrogen atmosphere took

place at 200°C. Additionally, an impregnation was conducted without addition of hydrochlo-

ric acid.

In both cases, with and without hydrochloric acid, hexachloroplatinic acid readily dissolved

into yellowish aqueous solutions. Similar to the ruthenium chloride system and in contrast

to palladium chloride, hydrogen chloride addition wasn’t necessary for dissociation of hex-

achloroplatinic acid into protons and chloroplatinate anions. After addition of spherical car-

bon both yellowish precursor solutions discolored. The obtained platinum loadings were

indirectly determined by ICP-AES analysis (see table 5.8). Measurements of metal disper-

sions have not been conducted.

Platinum was fully deposited onto spherical carbon after impregnation without hydrogen

chloride addition. Likely due to chloride anion competitors at the adsorption sites, platinum

loading was slightly lower after impregnation with hydrogen chloride. Still, metal deposition

was almost complete. As expected, the pH value of the impregnation solution decreased with

hydrogen chloride addition. This larger pH di�erence to the material’s PZC can a�ect metal

dispersion and distribution, because of increased strong metal support interactions.


Catalysts with 0.5 wt% platinum loading were prepared on the basis of two spherical carbon

materials with di�erent pore structure. The idea was that less activated spherical carbon

already provides a su�ciently large surface area for good dispersion of 0.5 wt% platinum.

Spherical carbon materials with pore volumes of 1.67 and 0.95 cm3g-1were oxidized and

impregnated with platinum salt according to standard procedures. Salt transformation in

hydrogen atmosphere took place at 200°C. Platinum was completely deposited onto both

materials, as no platinumwas detected by indirect ICP-AESmeasurements. Metal dispersions

are listed in table 5.9.

110


Table 5.9.: Platinum metal dispersions of two spherical carbon catalysts with di�erent total

pore volume

Vtot / cm3 g-1 DPt / %

1.67 10

0.95 5

With values between 5 and 10%, platinum wasn’t dispersed well, at all. The catalyst with a

larger pore volume exhibited a larger metal dispersion. This could have originated from the

larger surface area of the highly activated carbon material, but is di�cult to explain without

further knowledge about the di�erences in surface composition. Catalytic performance is

expected to be low, in both cases. Most likely, the carbon surfaces weren’t su�ciently func-

tionalized to allow for homogeneous adsorption of platinate ions. This aspect is addressed

in the following section.


The cause of low metal dispersion of the catalysts prepared in the previous subsection likely

was due to insu�cient surface functionalization. If, for instance, C-π sites remain at the

carbon surface, each C-π-site can possibly adsorb multiple metal ions resulting in large metal

clusters (see section 5.2.4 for concluding remarks on the role of C-π sites).

In order to produce good 0.5 wt% platinum catalysts, the PBSAC surface had to be completely

oxidized. Sulfuric acid is a potentially well suitable oxidation agent. It was demonstrated in

section 5.2.1 that sulfuric acid intensively oxidizes spherical carbon with large degrees of

surface functionalization. Also, oxidation of spherical carbon with a 50% sulfuric acid at

90°C resulted in a very high palladium metal dispersion of 63% (see table 5.5).

Thus, a di�erent 0.5 wt% platinum catalyst was prepared with sulfuric acid functionalization.

The highly activated PBSAC material was oxidized with a 50% aqueous solution of sulfuric

acid at 90°C for 2.5 h. The parameters for impregnation and reduction remained unchanged.

As yet another in�uential parameter had to be adjusted to obtain a well performing catalyst,

metal loadings and dispersions of this material weren’t determined, but complete platinum

deposition was assumed. As indirect statement of metal dispersion, catalytic activity is dis-

cussed in section 5.2.3.3.

111


Table 5.10.: Metal loadings and dispersions of 0.5, 5, and 10 wt% platinum on spherical carbon

and 5 wt% platinum on pulverized activated carbon


Expected Ptmetal loading /wt%

Obtained Ptmetal loading /wt%

DPt / %

PBSAC

0.5 0.5 50

5 4.98 51

10 8.41 15

PAC 5 n/a 28

Influence of metal transformation temperature

To ensure complete reduction of the adsorbed platinum salt, reduction parameters were ad-

equately adjusted. The biggest impact was expected by increasing temperature. So, some

impregnated PBSAC material was reduced at 300°C, instead of the previously applied 200°C.

Metal dispersion of this 0.5 wt% platinum catalyst increased to 50%.


In the following, the platinum metal loading was varied between 0.5 and 10 wt% to study

the in�uence of obtained metal loading and dispersion. Also, these material properties are

compared to a reference platinum on carbon catalyst.

Therefore, 200 μm highly activated spherical carbon was oxidized with 50% sulfuric acid so-

lution at 90°C for 2.5 h. For the 0.5 wt% platinum catalyst, metal impregnation was conducted

with hexachloroplatinic acid and hydrochloric acid according to standard procedures. The

5 and the 10 wt% spherical catalysts were prepared without addition of hydrogen chloride.

Metal transformation was carried out in hydrogen atmosphere at 300°C. For details, see sec-

tions 4.1.1 and 4.1.2. Obtained metal loadings were indirectly determined by ICP-AES anal-

ysis. The spherical catalysts and commercial 5 wt% platinum on carbon powder were also

analyzed by carbon monoxide chemisorption. Table 5.10 lists metal loadings and dispersions.

In case of the 0.5 and 5 wt% metal loadings, chloroplatinate completely adsorbed to the oxi-

dized spherical carbon. A platinum loading of 10 wt% wasn’t achieved, though. Only 84% of

the desired platinum metal was deposited. Platinum metal dispersions of the 0.5 and 5 wt%

spherical catalysts were similarly large with 50-51% and signi�cantly decreased to 15% for

the 8.41 wt% platinum loading. The powder catalyst exhibited a metal dispersion of 28%.

112


The applied, sulfuric acid functionalized spherical carbon seems to have a maximum plat-

inum adsorption of 8.41 wt%. For larger platinum loadings, the surface likely needs to be

further optimized, increasing the number of available adsorption sites. A decrease of metal

dispersion at increased metal loading can be explained by the larger chloroplatinate ion sur-

face densities promoting sintering e�ects during the rapid temperature increase in the ap-

plied metal transformation [96].

In essence, metal dispersions of the 0.5 and 5 wt% spherical platinum catalysts were very

high and signi�cantly higher than the dispersion of the reference powder catalyst.

113


0 50 100 150020406080100

mod / s kgPd m-3

X /

%

5% Pd @ 500 µm PBSAC 5% Pd @ 500 µm PBSAC (1 kg Scale-Up) 5% Pd @ PAC fits according to model

Figure 5.16.: Conversion of cinnamic acid using 5 wt% palladium catalysts based on spher-

ical carbon and carbon powder, respectively; �ts according to model applying

equation 2.9 (T = 40°C, pH2 = 30 bar, c0, cinnamic acid = 236 mol m-3, mcatalyst = 0.5 g)

Table 5.11.: E�ective reaction rate constants and metal dispersions of 5 wt% palladium cata-

lysts based on spherical carbon and carbon powder

Catalyst ke� / m3 kg-1 s-1 DPd / %

5 wt% Pd @ 500 μm PBSAC 0.034 n/a

5 wt% Pd @ 500 μm PBSAC (1 kg scale-up) 0.024 52

5 wt% Pd @ PAC (commercial) 0.296 30

5.2.3. Catalytic performance

5.2.3.1. Palladium catalysts: Influences on catalytic activity

Influence of particle size

Catalytic performance of the prepared spherical palladium catalysts was analyzed in cin-

namic acid hydrogenations (see sections 2.2.6 and 4.1.3) and compared to commercially avail-

able palladium on activated carbon powder. Figure 5.16 shows the conversion of cinnamic

acid over time of two prepared 500 μm spherical catalysts and the reference catalyst. Table

5.11 lists the derived reaction rate constants and previously determined metal dispersions.

Concerning the spherical catalysts, the rate of reaction was larger for the laboratory-scale

114


0 50 100 150020406080100

mod / s kgPd m-3

X /

%

5% Pd @ 500 µm PBSAC 5% Pd @ crushed PBSAC 5% Pd @ PAC fits according to model

Figure 5.17.: Conversion of cinnamic acid using a 5 wt% spherical carbon palladium cata-

lyst and a 5 wt% palladium catalyst prepared on the basis of crushed spherical

carbon, and commercial 5 wt% palladium on carbon powder; �ts according to

model applying equation 2.9 (T = 40°C, pH2 = 30 bar, c0, cinnamic acid = 236 mol m-3,

mcatalyst = 0.5 g)

catalyst than the 1 kg scale-up material. The powder catalyst resulted in a very fast con-

version of cinnamic acid. The di�erence in e�ective reaction rate constant between the two

spherical catalysts was a factor of 1.4. The powder catalyst was 8.7 times more active than

the PBSAC-based catalyst. The di�erences in catalytic activity of the two prepared spheri-

cal catalysts can be explained by di�erent masses applied in the preparation procedure. The

lower catalytic activity of spherical carbon despite the larger metal dispersion compared to

the powder catalyst is an indication of mass transport limitation inside the 500 μm spherical

carbon.

To characterize the in�uence of particle size during catalyst preparation, cinnamic acid con-

version of the powder catalyst based on crushed 500 μm spherical carbon was compared

to the result of the 500 μm spherical palladium catalyst, as well as the commercial powder

catalyst (see �gure 5.17).

The palladium catalyst based on crushed spherical carbon fully converted cinnamic acid in

a very short time and, thus, was signi�cantly more active than the 500 μm spherical carbon

catalyst. Regarding the e�ective reaction rate constants, catalytic activity increased by a

factor of 3.2. The powder catalyst was still 2.7 times more active than the crushed PBSAC-

115


0 50 100 150 200 250 300020406080100

mod / s kgPd m-3

X /

% x = 500 µm 200 > x > 100 µm 100 > x > 80 µm 80 > x > 63 µm 63 > x > 30 µm x < 30 µm fits according to model

Figure 5.18.: Palladium on spherical carbon (5 wt%, 1 kg scale-up, wet) with varying particle

size fractions tested in cinnamic acid hydrogenations; �ts according tomodel ap-

plying equation 2.9 (T = 40°C, pH2 = 30 bar, c0, cinnamic acid = 236 mol m-3, mcatalyst

= 0.5 g)

based catalyst. This demonstrates that spherical carbon itself is a suitable catalyst support

material that can compete with commercial powder catalysts. Optimization of particle size

and catalyst preparation procedure can possibly further improve the performance of PBSAC-

based catalysts.

For traditional molded catalysts, it’s well known that mass transport limitation can occur

inside the porous network, thus a�ecting the overall catalytic activity (see section 2.2.1). In

order to scienti�cally determine, if particle size is limiting catalytic activity, comminution ex-

periments of the scale-up catalyst were conducted. The 500 μm scale-up catalyst was chosen,

because a su�cient amount of material was available for comminution and the metal distri-

bution is likely homogeneous due to the high metal dispersion. The material was crushed

in a porcelain mortar and sieved into di�erent size fractions. Catalyst material of each size

fraction was tested in cinnamic acid hydrogenation experiments to compare catalytic activ-

ities. Figure 5.18 shows the resulting cinnamic acid conversions as a function of modi�ed

residence time.

It’s apparent that the e�ective reaction rate increased with decreasing particle size fraction.

The increase in reaction rate was very pronounced between the size fractions x=500 μm,

200>x>100 μm, and 100>x>80 μm. The di�erences in cinnamic acid conversion were much

116


0.0574 0.05430.0437

0.0362

0.01370.0053

<30 30-63 63-80 80-100 100-200 5000.000.010.020.030.040.050.06

k eff /

m3 k

g-1 Pd s

-1

particle size fraction / µm

(a)

<30 30-63 63-80 80-100 100-200 5000.00.51.01.52.0

Thie

le m

odul

us /

-


(b)

99.9 99.8 99.5 99.2 97.8 81.3

<30 30-63 63-80 80-100 100-200 500050

100

effe

ctiv

enes

s fa

ctor

/ %


(c)

Figure 5.19.: E�ective reaction rate constants (a), Thiele moduli (b), and e�ectiveness factors

(c) of di�erent catalyst size fractions (kintr = 0.0072 s-1, De� = 1.18e-10 m

2s-1)

smaller for the remaining particle size fractions, with particle sizes below 80 μm. Compared

to the hydrogenation experiments in �gure 5.16, the e�ective reaction rate of the 500 μm

spherical scale-up catalyst was smaller, likely because here the wet catalyst wasn’t dried

before application. The increasing rate of cinnamic acid conversion with decreasing parti-

cle size demonstrates an in�uence of particle size on catalytic activity. It’s an indication of

internal mass transport limitation.

Mathematical models, i.e. the determination of Thiele moduli and e�ectiveness factors, were

applied using equations 2.4 and 2.5 to further analyze this e�ect and predict critical particle

sizes of limiting mass transport. Therefore, e�ective reaction rate constants of the exper-

imental data were derived by applying the power law (see equation 2.9). For very small

particle sizes, absence of mass transport limitation was assumed, so that the intrinsic reac-

tion rate can be estimated by the e�ective reaction rate. Figure 5.19 displays derived e�ective

reaction rate constants of each catalyst size fraction, as well as calculated Thiele moduli and

e�ectiveness factors.

With decreasing particle size, the e�ective reaction rate constant increased and asymptoti-

cally approached a maximum value of about 0.06 m3kgPd

-1s-1for catalyst material smaller

than 30 μm. In order to di�erentiate between intrinsic surface reaction and pore di�usion

rate, the Thiele modulus was introduced. As intrinsic reaction rate constant, the largest ef-

117


fective reaction rate constant of this speci�c catalytic system from �gure 5.19 (a) was applied.

The e�ective di�usion constant was estimated from a molecular di�usion constant of cin-

namic acid in toluene found in literature according to equation 2.3. The molecular di�usion

constant from literature was 1.18e-9 m2s-1[197, 198]. As seen in �gure 5.19 (b), the Thiele

modulus slowly increases from 0.12 to 0.58 with increasing particle size until a size fraction of

100-200 μm. The non-comminuted catalyst with a particle size of around 500 μm has a signif-

icantly larger Thiele modulus of 1.95. This means that the 500 μm spherical carbon catalyst

exhibits a stronger disparity between intrinsic reaction rate and e�ective pore di�usion in a

cinnamic acid hydrogenation. As, for spherical catalysts, a Thiele modulus larger than 3 in-

dicates strong pore di�usion limitation, the PBSAC-based catalyst wasn’t considered purely

di�usion controlled [53].

The critical particle size resulting in negligible pore di�usion limitation was determined with

the help of the e�ectiveness factor. The e�ectiveness factor was solely calculated from the

Thiele modulus, assuming a pseudo �rst order reaction. Absence of pore di�usion limitation

is de�ned for e�ectiveness factors between 0.95-1 [45, 53]. Particles smaller 200 μm already

complied with this criterion. Consequentially, catalytic systems can essentially be realized

without pore di�usion limitation by using spherical carbon with particle sizes smaller 200

μm.

However, as the wet palladium catalyst applied in this experimental study showed a reduced

e�ective reaction rate constant compared to the 500 μm PBSAC catalyst in table 5.11, due to a

decreased intrinsic reaction rate, the e�ect of pore di�usion limitation can be even stronger

in other PBSAC catalysts than assumed here. Also, the estimated e�ective di�usion con-

stant can be signi�cantly smaller due to Knudsen di�usion and molecular sieve e�ects in the

micropores of the PBSAC pore system [53].

Further investigations with di�erent particle sizes and pore structures will follow in the next

section.


Spherical catalysts with di�erent particle size and pore structure (see section 5.2.2.1) were

tested in cinnamic acid hydrogenation reactions to compare catalytic activities.

Figure 5.20 contains experimental results of all performed cinnamic acid hydrogenation re-

actions in form of e�ective reaction rate constants, together with the previously determined

palladium metal dispersions. Plots of cinnamic acid conversion and a table of e�ective reac-

tion rate constants can be found in appendix C.1.

118


0.61 1.18 1.19 0.69 0.95 1.13 1.67 0.61 0.89 1.070.000.020.040.060.080.10 keff

k eff /

m3 k

g-1 Pd s

-1

Vtot / cm3 g-1

0102030405060 DPd

DPd

/ %

500 200 50Particle size / µm

Figure 5.20.: E�ective reaction rate constants of cinnamic acid hydrogenations and palladium

metal dispersions of spherical catalysts with di�erent particle size and total pore

volume (T = 40°C, pH2= 30 bar, c0, cinnamic acid= 236 mol m-3, mcatalyst= 0.5 g)

In all cases of spherical carbon based catalysts with a particle diameter of 500, 200, and 50 μm,

reaction rate increasedwith increasing total pore volume. Metal dispersionwere very similar,

but also followed the observed trend in reaction rate.

Generally, with increasing degree of carbon activation, the e�ective reaction rate constant

also increased due to two reasons. On the one hand, the intrinsic reaction rate increases with

increasing metal dispersion. Additionally, pore di�usion is faster in more activated carbon

materials with larger pore diameters.

Comparing spherical catalysts with similar total pore volume, e.g. 0.61-0.69 cm3g-1, but

di�erent particle size, catalytic activity increased with decreasing particle size. This poten-

tial in�uence of mass transport in dependence on particle size was supported by previous

comminution experiments (see the previous subsection). The fact that the highly activated

200 μmmaterial exhibited enhanced catalytic activity over the second most activated 200 μm

material, despite equal metal dispersions, also indicates an in�uence of pore di�usion. The

same interpretation can be made for the mesoporous 500 μm spherical carbon that performed

better than the similarly activated material with a total pore volume of 1.18 cm3g-1, despite

similar pore volumes.

The characterization of palladium catalysts based on di�erent available spherical carbon ma-

terials shows that particle size and pore structure signi�cantly in�uence the overall catalytic

119


activity. As previously demonstrated in comminution experiments, with decreasing particle

size, the extent of internal mass transport limitation decreases. Increasing carbon activa-

tion results in larger surface areas leading to generally larger metal dispersions and catalytic

activity. The increasing average pore diameter also positively a�ects pore di�usion and ef-

fective reaction rates.

Of all tested materials, the catalysts based on highly activated 50 and 200 μm spherical carbon

performed best. Thus, the 200 μm spherical carbon is the proposed catalyst support material

of choice in the hydrogenation of cinnamic acid due to the ideal combination of good catalytic

activity and ease in catalyst handling.


Besides particle size and pore geometry, carbon surface functionalization is another param-

eter impacting catalytic performance, as the e�ects on metal dispersion were already signif-

icant.

Prepared palladium catalysts based on di�erently functionalized spherical carbon (see sec-

tion 5.2.2.1) were tested in the hydrogenation reaction of cinnamic acid to compare catalytic

activities. Note that in this experimental series, a hydrogen partial pressure of 5 bar was

applied. For each catalyst, e�ective reaction rate constants and previously determined pal-

ladium metal dispersions are shown in �gure 5.21. Plots of cinnamic acid conversion and a

table of e�ective reaction rate constants can be found in appendix C.2.

Regarding the nitric acid pretreated catalysts, three of the four catalysts exhibited similar

reaction rate constants between 0.015-0.02 m3kgPd

-1s-1. The catalyst material oxidized with

36% nitric acid at RT showed a signi�cantly higher reaction rate of about 0.029 m3kgPd

-1s-1.

So, in this nitric acid oxidation series, catalytic activity of the best catalyst was 82% larger

than the slowest catalyst. With increasing metal dispersion, catalytic performance of the 15%

nitric acid oxidized samples increased slightly. The two catalysts prepared using spherical

carbon oxidized with 36% nitric acid exhibited an opposing trend of decreasing catalytic

activity with increasing metal dispersion.

The best catalyst with sulfuric acid pretreatment was slightly more active than the best cat-

alyst with nitric acid pretreatment. Sulfuric acid pretreated catalysts showed a distinct trend

of increasing reaction rate with harsher oxidation conditions. Catalytic activity doubled by

doubling acid concentration from 25-50% at constant temperature. Also, with increasing

oxidation temperature from RT to 90°C at constant acid concentrations, catalytic activity

increased by around 80%. The fastest hydrogenation reaction occurred with a 50% sulfuric

120


RT 90°C RT 90°C RT 90°C RT 90°C0.000.010.020.03 keff

k eff /

m3 k

g-1 Pd s

-1


0204060 DPd

DPd

/ %

15% HNO3 36% HNO3 25% H2SO4 50% H2SO4Acid concentration

Figure 5.21.: E�ective reaction rate constants of cinnamic acid hydrogenation reactions and

palladium metal dispersions of di�erently oxidized spherical carbon catalysts

(T = 40°C, pH2= 5 bar, c0, cinnamic acid= 236 mol m-3, mcatalyst= 0.5 g)

acid pretreatment at 90°C. Reaction rate of the sample oxidized with 25% sulfuric acid at RT

was lowest. The most active catalyst happened to also exhibit the largest metal dispersion.

Otherwise, metal dispersion didn’t appear to directly correlate with reaction rate constants.

To determine the in�uence of carbon surface functionalization on catalytic activity, the re-

sults of hydrogenation and carbon monoxide chemisorption experiments were correlated to

the numerous surface characterizations performed in section 5.2.1.

In the series of nitric acid pretreated samples, an optimum in catalytic activity seemed to

occur. A plausible explanation is the interplay of three e�ects: surface functionalization,

metal dispersion, and metal distribution (see also section 2.2.5.2 for details).

C-π sites can remain in an insu�ciently oxidized carbon surface, competitively adsorbing

palladium ions and promoting the formation of egg-shell catalysts. These egg-shell catalysts

exhibit lower metal dispersions with lower intrinsic catalytic activity. But, this lower intrin-

sic activity can be compensated by a lower level of mass transfer restriction due to shorter

di�usion distances. The role of C-π sites is explicitly discussed in section 5.2.4.

While the number of C-π sites cannot directly be determined, an anomaly in carbon sur-

face functionalization, as discussed in section 5.2.1, can be another explanation for the opti-

mum in catalytic activity of this experimental series. The most active catalyst is surprisingly

based on an oxidized spherical carbon, which exhibits by far the lowest amount of func-

121


tional surface groups and a large point of zero charge. Possibly, the pH di�erence between

the material’s PZC and the pH of the impregnation solution impacted metal distribution and

metal dispersion. A larger pH di�erence increases electrostatic interactions and promotes

the formation of egg-shell catalysts. This hypothesis has already been discussed in the liter-

ature [96]. Another pronounced di�erence is the extent of strongly adsorbing surface oxides,

which is signi�cantly higher for the most active catalyst. So, the chemical surface composi-

tion can also impact the strength of electrostatic adsorption and thus metal dispersion and

metal distribution. In order to clearly state the predominant e�ect resulting in large catalytic

activity, further analysis is necessary. Comminution experiments, for instance, can lead to a

more detailed insight in metal distribution, as the e�ective reaction rate of crushed samples

is expected to increase more strongly for homogeneously distributed catalyst pellets than for

egg-shell catalysts.

Concerning sulfuric acid functionalization, the large metal dispersions indicate the absence

of most C-π sites and the dominance of electrostatic palladate adsorption to protonated sur-

face oxides. Thus, already at mild oxidation conditions, sulfuric acid has homogeneously

oxidized unsaturated carbon bonds. In this experimental series, the pronounced increase in

catalytic activity indicates a signi�cant e�ect of surface functionalization. Nevertheless, most

surface properties were similar, i.e. PZC, amount of volatile components, and pore geometry.

The in�uence of surface functionalization is not expected to be that large. The increase of

catalytic activity by 80-100% between each sample is more di�cult to explain, because the

di�erent samples appear to have only an insigni�cantly di�erent surface composition. But,

the ratio of strongly to weakly adsorbing surface oxides correlates well with catalytic activ-

ity. An increasing total amount of strongly adsorbing surface oxides can have resulted in a

more comprehensive egg shell metal distribution due to enhanced electrostatic adsorption.

This plausible hypothesis needs to be veri�ed by additional experiments.

In essence, carbon surface functionalization has a large impact on a catalyst’s metal disper-

sion, metal distribution and catalytic activity. Many observations regarding carbon func-

tionalization were already published in the literature (see section 2.2.5). Even though, they

generally apply to spherical carbon, too, this detailed characterization was necessary to de-

termine the most suitable oxidation conditions. Spherical carbon can be oxidized with either

nitric acid or sulfuric acid treatment. Because nitric acid strongly a�ects the carbon surface

due to heteroatom inclusion, large acid concentrations, but mild temperatures are ideal to

achieve a highly active catalyst with high cinnamic acid hydrogenation reaction rates.

In contrast, sulfuric acid intensively reacts with the surface of spherical carbon without af-

122


0 10 20 30 40 50020406080100

mod / s kgPd m-3

X /

%

2% Pd @ 200 µm PBSAC 5% Pd @ 200 µm PBSAC10% Pd @ 200 µm PBSAC fits according to model

Figure 5.22.: Di�erently loaded 200 μm spherical catalysts with 2, 5, and 10 wt% palladium in

the hydrogenation of cinnamic acid; �ts according to model applying equation

2.9 (T = 40°C, pH2= 30 bar, c0, cinnamic acid= 236 mol m-3, mcatalyst= 0.5 g)

fecting the pore structure. Large acid concentrations and high temperatures promote advan-

tageously functionalized surfaces without sacri�cing carbon surface area. Metal dispersions

and catalytic activities in the hydrogenation reaction of cinnamic acid were best for inten-

sively oxidized materials. Because the e�ects of surface functionalization are more easily

predictable, sulfuric acid treatment of spherical carbon is highly recommended for catalyst

preparations.


Concerning spherical carbon loaded with di�erent amounts of palladium, hydrogenation

experiments have been conducted to determine catalytic activities. Catalytic conversions of

cinnamic acid are plotted in �gure 5.22. Derived e�ective reaction rate constants are listed

in table 5.12.

In case of the prepared 2 wt% palladium on spherical carbon catalyst, conversion of cin-

namic acid was fastest. Rate of reaction decreased with increasing metal loading. It needs

to be noted that conversion is plotted in �gure 5.22 as function of modi�ed residence time,

which incorporates palladium mass. Di�erences in e�ective reaction rate constants were

most signi�cant between the 5 and 10 wt% spherical catalysts, with an increase in catalytic

activity of 43%. Between the 2 and 5 wt% catalysts, catalytic activity only increased by 19%.

123


Table 5.12.: E�ective reaction rate constants of cinnamic acid hydrogenations of spherical

catalysts with di�erent palladium loading

Palladium loading / wt% ke� / m3 s-1 kgPd-1

2 0.099

5 0.083

10 0.058

This e�ect of increasing catalytic activity with decreasingmetal loading is likely due to larger

metal dispersions at low metal loadings. During impregnation via ion adsorption, palladate

anions are homogeneously distributed across the internal carbon surface as they adsorb to

the positively charged surface groups. With increasing metal loading, palladate anions are

located more closely together, so that during metal transformation larger palladium clusters

are formed.

Generally, the application of palladium catalysts based on spherical carbon with low metal

loadings is recommended. For low metal loadings, utilization of noble metal to catalyze

cinnamic acid hydrogenation reactions is better. Also, the larger amount of applied spherical

carbon support material doesn’t a�ect product separation due to fast �ltration rates.

5.2.3.2. Ruthenium catalysts: Influences on catalytic activity


Spherical catalysts with di�erent ruthenium loadings (see section 5.2.2.2) were tested in the

conversion of toluene to methyl cyclohexane (see section 2.2.6). Results of these hydro-

genation experiments are plotted in �gure 5.23. Table 5.13 lists the corresponding turnover

frequencies and previously determined metal dispersions of these catalysts. For catalyst

screening purposes, turnover frequencies according to equation 2.12 were determined in-

stead of e�ective reaction rates. Fitting of data points using equation 2.9 requires further

investigations concerning the applied catalytic reaction systems. Turnover frequencies were

determined around a modi�ed residence time of 150 s kgRu m-3.

With the 3.15 wt% ruthenium catalyst, toluene was steadily hydrogenated until full conver-

sion. In the same residence time frame, almost no product was formed with the 0.31 wt%

ruthenium catalyst. Also, with the 5.54 wt% spherical catalyst, conversion of toluene was

very slow and reached around 15% in the end. The turnover frequencies quanti�ed these ob-

124


0 50 100 150 200020406080

100 0.31% Ru @ PBSAC 3.15 % Ru @ PBSAC 5.54 % Ru @ PBSACX

/ %

mod

/ s kgRu

m-3

Figure 5.23.: Ruthenium on spherical carbon with di�erent metal loadings in the hydrogena-

tion of toluene (T = 150°C, pH2 = 50 bar, c0, toluene = 180 mol m-3, mcatalyst = 0.1 g)

Table 5.13.: Turnover frequencies of toluene hydrogenations and metal dispersions of ruthe-

nium on spherical carbon with di�erent metal loadings

Rutheniumloading / wt%

TOF / s-1 DRu / %

0.31 0.01 59

3.15 2.55 19

5.54 0.02 50

125


servations. Toluene hydrogenation using the 3.15 wt% ruthenium catalyst was about a factor

37 faster than the second fastest hydrogenation. Looking at ruthenium metal dispersions,

catalytic activity increased with decreasing metal dispersion. So, large ruthenium clusters

appear to perform best in the hydrogenation of toluene. This thesis is supported by other re-

search groups. They previously reported that hydrogenation activity of aromatic functional

groups increases with increasing metal cluster size.[199, 200]

The 3.15 wt% ruthenium spherical catalyst was reproduced according to the same prepara-

tion procedure and catalytic performance was successfully validated in a toluene hydrogena-

tion experiment. Nevertheless, instead of varying impregnation duration, ruthenium cluster

sizes can be most easily increased by applying increased metal transformation temperatures

during catalyst preparation [100, 101].

In essence, the prepared ruthenium catalysts are catalytically active. As a specialty for ruthe-

nium catalysts in the hydrogenation reaction of toluene, small metal dispersions and thus

larger ruthenium clusters are necessary for good catalytic activities.

Influence of reactant molecules

The most active toluene hydrogenation catalyst, 3.15 wt% ruthenium on spherical carbon,

was also tested in the hydrogenation of 1-octene, m-cresol and thymol (see also section 2.2.6).

The results are presented in �gure 5.24-a. The performance of a commercial 5 wt% ruthe-

nium on carbon powder catalyst (5.26 wt% exact metal loading, 34% ruthenium dispersion) in

these hydrogenation reactions is shown in �gure 5.24-b, for comparison. Table 5.14 gives the

respective turnover frequencies. This table also includes molecule dimensions determined

using the Chem3D Pro modelling software by looking at the Cartesian coordinates. The

corresponding molecule structures are drawn in �gure 5.25.

With the prepared 3.15 wt% ruthenium on spherical carbon catalyst, 1-octene was very

quickly converted. The rate of toluene conversion was signi�cantly slower. m-Cresol was

only partially converted within the experiment’s time frame. Thymol hydrogenation was

negligibly small. In comparison, the powder catalyst rapidly converted 1-octene, as well.

Toluene was also quickly hydrogenated. The reaction rates of m-cresol and thymol were sim-

ilarly fast and much faster than with the PBSAC catalyst. In both cases, full conversion was

reached during the experiment. The spherical catalyst exhibited a larger 1-octene turnover

frequency than the powder catalyst. For toluene, m-cresol and thymol hydrogenations, the

powder catalyst had larger turnover frequencies. With the exception of the 1-octene hy-

drogenation reaction, the powder catalyst performed signi�cantly better than the spherical

ruthenium catalyst in the conversion of toluene, m-cresol and thymol.

126


0 50 100020406080

100 1-Octene Toluene m-Cresol Thymol

X /

%

mod

/ s kgRu

m-3

0 50 100020406080

100

1-Octene Toluene m-Cresol ThymolX

/ %

mod

/ s kgRu

m-3

(a) (b)

Figure 5.24.: Hydrogenation of 1-octene, toluene, m-cresol and thymol using 3.15 wt% ruthe-

nium on spherical carbon (a) and commercial 5.26 wt% ruthenium on powdered

carbon (b) (T = 150°C, pH2 = 50 bar, c0 = 180 mol m-3, mcatalyst = 0.1 g)

Table 5.14.: Turnover frequencies in the hydrogenation of di�erent educt molecules with

3.15 wt% ruthenium on spherical carbon and commercial 5.26 wt% ruthenium

on powdered carbon catalysts

Educt molecule Moleculedimensions / Å

TOFRu@PBSAC /s-1

TOFRu@PAC / s-1

1-Octene 11.0 x 3.2 x 2.4 15.03 5.67

Toluene 6.4 x 4.9 x 2.5 0.61 2.55

m-Cresol 6.4 x 5.8 x 2.5 0.18 0.78

Thymol 8.4 x 5.2 x 4.4 0.01 0.62

127


Figure 5.25.: Molecule structures of 1-octene, toluene, m-cresol and thymol (from left to right)

- view along the x-axis of the molecule

Comparing molecule geometries, 1-octene is the longest molecule, but is small and �exible

in the other two dimensions. Toluene and m-cresol exhibit a broader molecule width than

1-octene. Thymol is a molecule with larger width and height than 1-octene.

Unexpectedly, thymol wasn’t converted by the spherical catalyst, at all. Three highly in�u-

ential factors are discussed in the following to explain the catalytic inactivity of the PBSAC

catalyst for this speci�c reaction, i.e. metal dispersion, space con�nement issues and chlo-

rine/chloride inhibition.

It was already pointed out in the previous section that ruthenium cluster size matters. The

cluster size resulting from 34% metal dispersion of the commercial powder catalyst is appar-

ently suitable to catalyze thymol hydrogenation reactions. It’s unknown, if the larger cluster

sizes lead to a signi�cant decrease in catalytic activity. The hypothesis, that the larger cluster

sizes of the spherical ruthenium catalyst result in catalytic inactivity, cannot be completely

ruled out without an experiment applying a similarly dispersed ruthenium on spherical car-

bon catalyst. But, the spherical catalyst performed better in the hydrogenation of 1-octene

than the commercial catalyst. This indicates that the spherical catalyst was intrinsically more

active due to larger ruthenium clusters.

Thus, space con�nement issues of the spherical carbon pore geometry and chlorine/chloride

inhibition originating from the applied ruthenium chloride precursor are more likely rea-

sons for the catalytic inactivity against bulkier substrates. In the �rst case, a too restrictive

128


pore structure of spherical carbon hinders the thymol molecules from entering the pore sys-

tem and accessing the catalytically active sites [55]. Transfer of reactants through the pore

structure between bulk solution and active ruthenium clusters is prevented. Mass trans-

port limitation of spherical carbon has already been discussed in section 5.2.3.1 for cinnamic

acid. However, cinnamic acid is a planar molecule and, thus, �ts well into slit shaped, micro

porous carbons. Thymol has a signi�cantly larger molecule height than cinnamic acid and,

thus, more di�culties penetrating this spherical carbon material (see also section 2.2.5.3).

Another possible explanation for inactivity of the thymol hydrogenation reaction is that chlo-

rine/chloride inhibits this particular reaction. It was reported in literature that ruthenium

catalyzed reactions are strongly a�ected by chlorine species dissociating from the applied

chlorine-containing metal salt precursors [101, 201, 202]. The in�uence of chlorine/chloride

inhibition can be validated and mitigated by using chlorine-free ruthenium precursors in

catalyst preparation.

In conclusion, spherical ruthenium catalysts are generally well suitable for hydrogenation

reactions of planar and linear molecules such as carbocyclic rings and ole�nes. Further in-

vestigations and subsequent modi�cation of the catalyst preparation procedure is necessary

for the successful transformation of more complex and functionalized molecules.

5.2.3.3. Platinum catalysts: Influences on catalytic activity

As a technically relevant reaction, PBSAC-based catalysts were tested for catalytic activity

in the dehydrogenation of H18-dibenzyltoluene (see section 2.2.6 for details). It has already

been experimentally demonstrated that platinum catalysts performed best at hydrogen dis-

charge. Two commercial powder catalysts based on activated carbon and aluminum ox-

ide, respectively, were chosen as reference systems to benchmark catalytic activities of the

PBSAC-based platinum catalysts prepared in this work.

Influence of ion adsorption modification

The prepared catalysts with and without addition of hydrogen chloride during impregnation

were tested in dehydrogenation of H18-dibenzyltoluene in order to check, if the additional

chloride ions a�ect metal dispersion. In �gure 5.26, the results of two PBSAC-based 5 wt%

platinum catalysts are shown. The performance of the two commercial powder catalysts is

also plotted in �gure 5.26.

Both commercial platinum catalysts supported on activated carbon powder and aluminum

oxide powder showed similarly high catalytic activities. The dehydrogenation degrees were

129


0 20 40 60 80 100 120020406080

100

dehy

drog

enat

ion

degr

ee /

%

time / min

0.5% Pt @ Al2O3 5% Pt @ PAC 5% Pt @ PBSAC 5% Pt @ PBSAC (w/o HCl)

Figure 5.26.: Dehydrogenation of H18-dibenzyltoluene using 5 wt% platinum on spherical

carbon, commercial 0.5 wt% platinum on aluminum oxide and commercial 5

wt% platinum on activated carbon (T = 310°C, p = 1 bar, nMetal / nLOHC = 0.1

mol%)

78 and 83%, respectively, after 120 min. While the reaction rates were alike within the �rst

30 min, the 0.5 wt% platinum catalyst on aluminum oxide �attened out less strongly than

the 5 wt% platinum catalyst on activated carbon and, thus, �nally reached a higher plateau.

The conversion curves of the two spherical catalysts behaved similarly. In parallel, they

continuously proceeded with decreasing rate until they reached dehydrogenation degrees of

43 and 44%, respectively. The di�erence was within measurement uncertainties.

Apparently, the in�uence of the impregnations pH value on the catalytic activity was neg-

ligibly small. The additional chloride anions from the hydrochloric acid had no noticeable

e�ect on the dispersion of the chloroplatinate anions during impregnation, even though, the

anions compete with each other for adsorption sites. An excess of adsorption sites was avail-

able. Compared to the �nal dehydrogenation degree of the commercial platinum on carbon

catalyst, only 56% of that reference value was reached after 120 min with the PBSAC-based

system. The initial reaction rate was signi�cantly lower. After 60 min reaction time, reaction

rates of PBSAC-based and powder catalysts were similar. Admittedly, the �rst PBSAC-based

platinum catalysts didn’t reach the catalytic activities of the powdered commercial catalysts.

Nevertheless, these catalysts with particle diameters around 200 μm already showed reason-

able dehydrogenation performance together with the advantage in catalyst handling.

130


Due to the handling bene�t of spherical carbon materials, lower platinum loadings are still

technically interesting. A smaller platinum loading allows for a larger metal dispersion and,

consequentially, a better utilization of the expensive noble metal [96]. So, the strategy was

to develop highly active catalysts with a platinum loading of 0.5 wt%.

131


0 20 40 60 80 100 120020406080

100

dehy

drog

enat

ion

degr

ee /

%

time / min

0.5% Pt @ Al2O3 5% Pt @ PBSAC 0.5% Pt @ PBSAC 0.5% Pt @ PBSAC (low Vtot)

Figure 5.27.: Dehydrogenation of H18-dibenzyltoluene using two 0.5 wt% platinum on spher-

ical carbon catalystswith di�erent carbon activation, a 5wt% platinumon spher-

ical carbon catalyst and commercial 0.5 wt% platinum on aluminum oxide (T =

310°C, p = 1 bar, nMetal / nLOHC = 0.1 mol%)


The catalytic activity of two spherical catalysts loaded with 0.5 wt% platinum, but varying

total pore volume is investigated in this section to see, if the highly activated carbon sup-

port is necessary. The results of the H18-dibenzyltoluene dehydrogenation experiments are

presented in �gure 5.27, together with the best performing experiments already discussed in

the previous subsection.

Within the �rst 35 min, both catalysts only lead to a dehydrogenation degree of less than 3%.

Then, the catalyst based on the highly activated support showed increasing activity and a

dehydrogenation degree of 26% was reached after 120 min. With a total conversion of merely

7%, the second catalyst performed even worse. The results of the catalytic experiments of

these two PBSAC-based catalysts correlate well with the low platinum metal dispersions

determined in section 5.2.2.3, as only few platinum atoms are available and accessible for

reactants. Because reaction rates increased with time, a continuous activation process is

observed. The catalytically active platinum species is obviously formed during the reaction.

The catalysts are not completely reduced.

The very low activity of the catalyst based on the less activated PBSAC material with the

lower total pore volume can be explained by the small average pore diameter of 2.1 nm.

132


0 20 40 60 80 100 120020406080

100

dehy

drog

enat

ion

degr

ee /

%

time / min

0.5% Pt @ Al2O3 5% Pt @ PBSAC 0.5% Pt @ PBSAC (HNO3) 0.5% Pt @ PBSAC (H2SO4)


ical carbon catalysts with di�erent surface functionalization, a 5 wt% platinum

on spherical carbon catalyst and commercial 0.5 wt% platinum on aluminum

oxide (T = 310°C, p = 1 bar, nMetal / nLOHC= 0.1 mol%)

Thus, it is di�cult for the planar H18-dibenzyltoluene molecules with a steric demand of at

least 10.9 x 10.6 x 2.4 Å to reach the active sites within the catalyst. Mass transport limitation

due to restricted pore di�usion plays a signi�cant role in this catalytic system. In contrast,

the highly activated PBSACmaterial exhibits an average pore diameter of 2.9 nm. Here, pore

di�usion limitation didn’t appear to be critical.

Consequently, the pore structure of the highly activated spherical carbon material is su�-

cient to allow for pore di�usion of H18-dibenzyltoluene molecules. But, the low platinum

dispersion of the 0.5 wt% platinum catalysts needs to be addressed to further improve cat-

alytic activity.


Di�erently functionalized spherical carbon materials, loaded each with 0.5 wt% platinum,

were tested in H18-dibenzyltoluene dehydrogenation experiments to determine, whether the

sulfuric acid treatment improves metal dispersion. The results are presented in �gure 5.28,

together with the best performing experiments already discussed in the previous subsections.

During the �rst 5 min, catalytic activity of the catalyst oxidized with sulfuric acid was low.

Then, until the 35 min data point, the reaction rate continuously increased and declined af-

133


terwards in accordance with reaction kinetics. Compared to the commercial catalyst, the

catalytic activity was much lower in the region between 0 and 60 min reaction time. At

around 90 min, the spherical carbon catalyst caught up with and, shortly later, even outper-

formed the commercial catalyst. The total dehydrogenation degree after 120 min amounted

to 87%.

In contrast to the �rst dehydrogenation catalysts based on nitric acid oxidized spherical car-

bon, a huge improvement in catalytic activity has been accomplished. By changing oxidation

parameters, the number of remaining C-π sites was e�ectively reduced and strongly adsorb-

ing surface oxides increase the strength of electrostatic platinate ion adsorption for a more

suitable dispersion of platinum.

The activation behavior in the beginning of the experiment is due to an incomplete metal

transformation during catalyst preparation. In the course of the dehydrogenation experi-

ment, the catalyst was obviously further transformed to metallic platinum by the produced

hydrogen gas.

Influence of metal transformation temperature

To ensure complete transformation of the adsorbed platinum salt, metal transformation pa-

rameters were adjusted. The biggest impact is expected by increasing the metal transfor-

mation temperature. So, some impregnated spherical carbon material was treated at 300°C

in hydrogen atmosphere and tested thereafter in the H18-dibenzyltoluene dehydrogenation

reaction. Figure 5.29 presents the results of this catalytic experiment, together with the best

performing experiments already discussed in the previous subsection.

Conversion smoothly increased up to a total conversion of 94%. At any time, catalytic activity

superseded the commercial catalyst’s activity. This catalyst was reproduced according to the

same preparation parameters and catalytic performance was successfully validated.

The fact that this PBSAC-based catalyst outperforms the standard powder catalyst in the

dehydrogenation of H18-dibenzyltoluene further demonstrates that pore di�usion limitation

is not a general issue with PBSAC catalyst pellets. For this speci�c reaction, though, mass

transport is enhanced by volume expansion through hydrogen formation at the active sites,

subsequent �ow of hydrogen gas to the outside and inherent uptake of liquid reactants by

capillary forces of emptied pores [203, 204].

A high performance LOHC dehydrogenation catalyst has been successfully developed. Im-

portant steps in the preparation of the platinum catalyst were selecting a PBSAC starting

material with suitable pore structure, appropriately modifying the carbon surface applying

134


0 20 40 60 80 100 120020406080

100

dehy

drog

enat

ion

degr

ee /

%

time / min

0.5% Pt @ Al2O3 0.5% Pt @ PBSAC (200°C) 0.5% Pt @ PBSAC (300°C)


ical carbon catalysts with di�erent metal transformation temperatures and com-

mercial 0.5 wt% platinum on aluminum oxide (T = 310°C, p = 1 bar, nMetal /

nLOHC= 0.1 mol%)

sulfuric acid oxidation, impregnating the material with low platinum surface concentrations

and transforming the catalyst at elevated temperatures. This PBSAC-based material marks a

signi�cant advancement in catalytic activity and ease of handling compared to the platinum

on aluminum oxide standard catalyst.


The well performing 0.5 wt% platinum on spherical carbon dehydrogenation catalyst, as de-

veloped in the previous subsections, was also tested in a hydrogenation reaction. Addition-

ally, 5 and 8.4 wt% platinum on spherical carbon were characterized in the same model re-

action. So, in essence, this resulted in an experimental series concerning the variation of

platinum metal loading. The catalysts were tested in cinnamaldehyde hydrogenations. In

this reaction system, selectivity e�ects can also be studied. See section 2.2.6 for details on

this reaction.

In �gure 5.30, catalytic conversion of cinnamaldehyde is plotted for di�erently loaded spher-

ical carbon materials and a commercial 5 wt% platinum on pulverized activated carbon cata-

lyst. Figure 5.31 shows the selectivity to hydrocinnamic alcohol. Table 5.15 lists reaction rate

constants, �nal product selectivity and previously determined platinum metal dispersions.

135


0 100 200 300 400 500020406080100

mod / s kgPt m-3

X /

%

0.5% Pt @ PBSAC 5% Pt @ PBSAC 8.4% Pt @ PBSAC 5% Pt @ PAC fits according to model

Figure 5.30.: Hydrogenation of cinnamaldehyde using 0.5, 5 and 8.4 wt% platinum on spheri-

cal carbon and a commercial 5% platinum on pulverized activated carbon; �ts ac-

cording to model applying equation 2.9 (T = 100°C, pH2 = 25 bar, c0, cinnamaldehyde

= 189 mol m-3)

All prepared platinum catalysts were very active in the hydrogenation of cinnamaldehyde.

Normalized to the platinum loading, the 0.5 wt% platinum spherical catalyst performed best.

Cinnamaldehyde hydrogenation quickly approached full conversion. The 5 and 8.4 wt% plat-

inum on spherical carbon materials showed lower catalytic activities. Full conversion was

reached after longer modi�ed residence times. The commercial platinum catalyst was less

active than the equally loaded spherical carbon system. Interestingly, after 150 s kgPt m-3,

the reaction rate of the commercial catalyst strongly decreased before approaching full con-

version.

Between 50 and 80% conversion of cinnamaldehyde, selectivity towards hydrocinnamic al-

cohol remained constant with values between 34 and 48%. Approaching full conversion,

selectivity increased. In case of spherical catalysts, �nal selectivity at full conversion varied

between 49 and 65%. The 5 wt% platinum spherical catalyst exhibited a lower selectivity

towards hydrocinnamic alcohol than the two other spherical materials. With 43%, the com-

mercial catalyst had the lowest �nal selectivity. The concentration of cinnamic alcohol in-

creased in the beginning of the reaction and decreased towards full conversion. A depletion

of hydrocinnamic aldehyde wasn’t observed. Interestingly, besides the expected hydrogena-

tion products, propylbenzene was formed when using spherical catalysts. The amount of

136


20 40 60 80 10020406080100

X / %

S /

%

0.5% Pt @ PBSAC 5% Pt @ PBSAC 8.4% Pt @ PBSAC 5% Pt @ PAC

Figure 5.31.: Selectivity of cinnamaldehyde hydrogenation towards hydrocinnamic alco-

hol using 0.5, 5 and 8.4 wt% platinum on spherical carbon and a commer-

cial 5 wt% platinum on pulverized activated carbon (T = 100°C, pH2 = 25 bar,

c0, cinnamaldehyde = 189 mol m-3)

Table 5.15.: Catalytic activity, �nal selectivity towards hydrocinnamic alcohol and metal dis-

persions of 0.5, 5 and 8.4 wt% platinum on spherical carbon and 5 wt% platinum

on pulverized activated carbon


Platinum metalloading / wt%

ke� / m3 kgPt-1s-1

S�nal / % DPt / %

PBSAC

0.5 0.029 64 50

5 0.013 65 51

8.4 0.007 49 15

PAC 5 0.009 43 28

137


propylbenzene increased steadily up to values between 13 and 17%. The commercial catalyst

didn’t catalyze propylbenzene formation. The 0.5 wt% spherical catalyst was 2.2 times more

active than the 5 wt% spherical catalyst and 4.1 times more active than the 8.4 wt% spherical

catalyst, with respect to applied platinum.

Concluding this section on platinum-based PBSAC catalysts, it can be stated that spheri-

cal carbon loaded with 0.5 wt% platinum is an outstanding catalyst for both hydrogenation

and dehydrogenation reactions. In the variation of platinum metal loading on spherical car-

bon, a clear trend is apparent. With increasing metal loading between 0.5-8.4 wt% platinum,

catalytic activity decreases. This partially correlates with metal dispersions. Utilization of

metal is improved in highly dispersed systems. Also, the 5 wt% platinum spherical catalyst

performed better than the commercial catalyst, as metal dispersion was increased. The large

surface area of spherical carbon facilitates large metal dispersions.

It’s di�cult to explain the enhanced catalytic activity of the 0.5 wt% over the 5 wt% spher-

ical catalyst despite equal metal dispersions. A plausible e�ect is a di�erent extent of pore

di�usion limitation. Due to strong electrostatic interactions during catalyst preparation, the

small amount of platinate ions necessary for a 0.5 wt% spherical catalyst can be predom-

inantly adsorbed in the outer shell or the mesopores with large metal dispersion. Such a

catalyst exhibits a decreasing in�uence of mass transport due to shorter distances between

catalytically active sites and bulk solution, as well as easier di�usion in mesopores. For a

similarly well dispersed 5 wt% platinum on spherical carbon catalyst, metal distribution is

more homogeneous resulting in stronger pore di�usion limitations.

The commercial catalyst deactivated stronger during cinnamaldehyde hydrogenation reac-

tion than the PBSAC-based catalysts. According to literature, this deactivation behavior can

be due to carbon monoxide poisoning of the catalytically active sites resulting from a decar-

bonylation side reaction of cinnamic alcohol [205]. In this experimental series, water was

added to the reactant solution in order to vacate poisoned platinum sites by solvent e�ects.

While the spherical catalysts didn’t show signs of poisoning in this experimental series, pos-

sibly, the commercial catalyst exhibited deactivation due to di�erent support interactions.

In literature, a strong in�uence of carbon surface functionalization on cinnamaldehyde hy-

drogenation activity was reported, with the hypothesis that non-functionalized carbon sites

assist in the adsorption of aromatic substrates [118, 206].

Concerning selectivity of the cinnamaldehyde hydrogenation reactions, interesting observa-

tions were made. Similar cases were observed in the literature and explained with e�ects

discussed in the following. The intermediate formation and depletion of cinnamic alcohol

138


proved the conventional parallel reaction mechanism of cinnamaldehyde hydrogenation (see

section 2.2.6). Interestingly, hydrocinnamic aldehyde didn’t deplete, indicating stabilization

of the aldehyde functional group after carbon-carbon bond saturation. Selectivity towards

hydrocinnamic alcohol was enhanced in case of the 0.5 and 8.4 wt% platinum spherical cata-

lysts. Impurities originating from the preparation procedure, such as traces of chlorine/chlo-

ride in varying amounts from the hexachloroplatinate precursor, can be responsible for the

lower selectivity of the 5 wt% platinum spherical catalyst [116, 119]. Compared to spheri-

cal carbon, the lower aldehyde group hydrogenation activity of the carbon powder catalyst

can be due to di�erent steric e�ects within the pore system [117]. The reduction reaction

to propylbenzene, catalyzed by spherical catalysts, can only be explained by di�erences in

interactions between metal cluster, carbon support and reactant molecule.

In a short summary, spherical carbon loaded with platinummetal performed better in the hy-

drogenation of cinnamaldehyde than a respective catalyst based on activated carbon powder.

Catalytic activity and selectivity towards hydrocinnamic alcohol were generally enhanced.

A low platinum loading of spherical catalysts is recommended for catalyzed reactions carried

out in batch processes. Large metal loadings are desirable in �ow chemistry processes with

a limited catalyst bed volume [207].

5.2.4. The role of C-π sites

In some previous experimental series on active metal deposition and subsequent catalyst

screening, trends were observed that cannot be explained with available information on ma-

terial properties. As an explanation, it was already hypothesized in the individual sections

that another in�uential factor can have a�ected metal deposition and, thus, catalytic perfor-

mance. Besides metal ion adsorption to charged surface oxides by electrostatic interaction,

remaining C-π sites at the carbon surface can competitively adsorb metal ions (see section

2.2.5.2). As these C-π sites are known to instantly transform metal ions to elemental metal,

the metal will be immobilized at a certain position within the carbon spheres. As C-π sites in

the shell of the carbon spheres are reached at �rst, the presence of C-π sites will a�ect metal

distribution resulting in more distinct egg-shell catalysts. Furthermore, if C-π sites adsorb

multiple metal ions, metal dispersion will decrease as larger metal clusters are formed by

instantaneous metal transformation.

Experiments explicitly demonstrating the presence of C-π sites and their role in active metal

deposition and subsequent catalytic performance are not possible. But, the number of per-

formed experiments and previous reports in literature on graphitic carbon materials (see

139


sections 2.2.5.2 and 2.2.5.4) indicate that C-π sites are present and in�uence metal distribu-

tion, metal dispersion and, thus, catalytic activity of spherical catalysts. In the following,

the role of C-π sites in the preparation of palladium and platinum catalysts based on pure

and oxidized spherical carbon is discussed, also referring to and elaborating on experimental

work of this thesis.

Palladium deposition to pure spherical carbon

During a 5 wt% palladium catalyst impregnation of non-functionalized PBSAC material (500

μm, 1.18 cm3g-1) with palladium chloride solution, a very interesting observation was made.

The palladium salt was instantly transformed to elemental palladium when contacting the

PBSAC surface. A lustrous palladium shell catalyst resulted. The following explanation

was found, which directly correlates to literature reports on catalyst preparations based on

graphitic carbon materials (see section 2.2.5.4): As the non-functionalized PBSAC materials

exhibit a large number of unsaturated C-π sites at the surface, their signi�cant reduction

potential has obviously transformed virtually all metal ions into metal immediately at �rst

contact and, thus, immobilized the metal at the outer carbon shell.

Palladium deposition to oxidized spherical carbon

By oxidation treatment of the carbon material, the number of C-π sites is substantially re-

duced. After surface oxidation, the predominant mechanism of, for instance, palladium de-

position to spherical carbon is ion adsorption. This is is apparently indicated by the absence

of lustrous palladium metal at the outer surface of the �nal catalysts, which are prepared by

applying oxidized spherical carbons.

Furthermore, at least for nitric acid pretreated palladium catalysts, the results of catalytic

experiments on the in�uence of surface functionalization (see section 5.2.3.1) indicate that

a certain amount of C-π sites remained, thus noticeably in�uencing metal deposition with

regard to metal distribution and metal dispersion. In case of poor surface functionalization,

unsaturated C-π sites remain in larger amounts at the carbon surface. Because C-π sites in

the spherical carbon outer shell are reached at �rst, palladium distribution will primarily be

located within this outer shell. Metal dispersion will be lower than in case of homogeneously

distributed palladium. But, easier mass transport due to shorter di�usion distances in egg

shell catalysts can balance catalytic performance. With increasing degree of surface func-

tionalization, metal dispersion increases due to a lower number of C-π sites. Electrostatic

adsorption of palladium salt to protonated surface oxides then is the primary mechanism

of metal deposition. A more homogeneous metal distribution then negatively a�ects mass

140


transport and, thus, catalytic activity.

While the catalyst pretreated with 15% nitric acid at RT exhibited a poor metal dispersion

and a low catalytic activity (see section 5.2.3.1 on the in�uence of surface functionalization),

the amount of C-π sites likely was too large leading to large palladium clusters located in

the spherical shell. With increasing oxidation conditions, metal dispersion increased. Ox-

idation with 36% nitric acid at RT apparently formed the best compromise between metal

dispersion and metal distribution, because higher catalytic activity was achieved. Metal dis-

persions between 50-55% of more strongly oxidized samples indicate a more homogeneous

metal distribution utilizing the whole carbon surface for metal deposition with increasing

in�uence of pore di�usion limitation, thus decreasing the e�ective reaction rate.

By controlling the number of C-π sites through speci�c carbon surface treatments, these

additional adsorption sites can be used to tailor metal deposition by a�ecting metal distribu-

tion and metal dispersion. Such a variable is unique to graphitic carbon materials and poses

another advantage of spherical carbon over inorganic catalyst supports.

Platinum deposition to oxidized spherical carbon

Concerning the catalytic performance of 0.5 wt% platinum on spherical carbon, the in�uence

of surface functionalization was signi�cantly larger than for the prepared 5 wt% palladium

catalysts (compare sections 5.2.3.1 and 5.2.3.3). An interpretation is possible by discussing

the presence and absence of C-π sites.

For the 5 wt% palladium on spherical carbon materials, only a small fraction of palladium

salt is instantly reduced by remaining C-π sites, forming larger palladium clusters. Most

palladate ions adsorb to the charged functional surface groups, eventually leading to small

palladium clusters.

In the case of the 0.5 wt% platinum catalysts with the same oxidized support, the same abso-

lute amount of platinum salt will be sacri�ced at the reducing C-π sites. In a relative view, a

much smaller quantity of salt remains for ion adsorption. Consequently, as demonstrated for

a sulfuric acid treated material, a well oxidized carbon surface without any C-π sites allows

for high dispersions of noble metals, especially at low metal loadings.

141


Table 5.16.: Palladium and sulfur leaching of two di�erent spherical catalysts and a commer-

cial powder catalyst with two di�erent durations of ultrasonic treatment

Catalyst Ultrasonica-tion time /h

Leachedpalladiumfraction / ppmw

Leached sulfurconcentrationin solution /ppmw

5 wt% Pd @ 200 μm

PBSAC

1 1.7 0.7

3 1.6 0.1

5 wt% Pd @ 500 μm

PBSAC (1 kg scale-up)

1 9.9 33.7

3 9.4 32.5

5 wt% Pd @ PAC

(commercial)

1 2.0 11.0

3 2.1 10.8

5.2.5. Stability tests

5.2.5.1. Catalyst leaching

Palladium leaching tests have been established as a method to determine completeness of

metal reduction after hydrogen treatment. Leaching tests were conducted as described in

section 4.4. With 1 and 3 h, two di�erent ultrasonication durations were applied. In addi-

tion to metal concentration, the sulfur content was analyzed in order to investigate potential

e�ects of sulfur that is present in spherical carbon from PBSAC synthesis. Here, 5 wt% pal-

ladium on 200 μm highly activated spherical carbon (see section 5.2.2.1), the 500 μm scale-up

catalyst (also see section 5.2.2.1) and a commercial palladium powder catalyst were tested for

leaching. The PBSAC samples were oxidized with nitric acid. The results are listed in table

5.16.

Comparing palladium and sulfur leaching of each catalyst at di�erent ultrasonic treatments,

the di�erences were very small. The amount of leached substance remained almost constant

at increasing ultrasonication time. All catalysts exhibited very low palladium leaching with

ratios of less than 10 ppmw towards total palladium content. The catalyst based on 200 μm

spherical carbon leached the least amount of palladium, closely followed by the commercial

powder catalyst. The 500 μm spherical catalyst leached about six times more palladium than

the 200 μmmaterial. Concerning sulfur leaching, the amount was signi�cantly lower in case

of the 200 μm material compared to the other catalysts. The 500 μm spherical catalyst and

142


Table 5.17.: E�ective reaction rate constants of four subsequent hydrogenation experiments

with catalyst recycling

# of recycling experiment ke� / m3 kgPd-1 s-1

Hydrogenation #1 0.024




the commercial powder catalyst leached notable amounts of sulfur.

In conclusion, the e�ect of extended ultrasonication was negligibly small. All catalysts were

well reduced. The scale-up catalyst exhibited more leaching possibly due to nitric acid and

hydrochloric acid residues from catalyst preparation, as acids can dissolve palladium. To

mitigate this, the material needs to be more intensively washed and dried. The fact that the

200 μm spherical catalyst leached less palladium than the commercial powder catalyst is an

advantage in catalytic applications, in which palladium contamination is an issue, e.g. in

pharmaceutical synthesis [61, 62].

Also, due to intensive washing and drying, the 200 μm catalyst leached almost no sulfur

compared to the 500 μm material. TPD-MS measurements in section 5.2.1 also showed that

desorption of sulfur species from nitric acid treated surfaces was low. Interestingly, the

commercial powder catalyst leached sulfur, as well. But, considering the results in section

5.2.3, catalytic activity of the studied test reactions obviously wasn’t inhibited by these sulfur

species.

5.2.5.2. Catalyst recycling

The spent spherical scale-up catalyst (see section 5.2.3.1) was recycled and reused for three

further cinnamic acid hydrogenations to test its recycling capability. After each experiment,

the catalyst was separated from the reaction solution by �ltration. The catalyst material was

washed with acetone and dried at 120°C and 0.01 mbar vacuum. The reaction parameters of

cinnamic acid hydrogenation remained constant throughout the recycling experiment series.

The results of cinnamic acid conversion are shown in �gure 5.32 with e�ective reaction rate

constants listed in table 5.17.

A signi�cant decrease in rate of cinnamic acid reactionwas observed between the hydrogena-

143


0 50 100 150 200 250 300020406080100

mod / s kgPd m-3

X /

%

Hydrogenation #1 Hydrogenation #2 Hydrogenation #3 Hydrogenation #4 fits according to model

Figure 5.32.: Conversion of cinnamic acid in four subsequent hydrogenation experiments

with catalyst recycling; �ts according to model applying equation 2.9 (T = 40°C,

pH2 = 30 bar, c0, cinnamic acid = 236 mol m-3, mcatalyst = 0.5 g)

144


tion with fresh catalyst and the �rst recycling experiment. Concerning the e�ective reaction

rate constant, the decrease amounted to 58%. The third and fourth hydrogenations exhib-

ited similar, slightly decreasing, reaction rates than the second hydrogenation. Apparently,

catalytic activity only decreased after the �rst experiment and remained almost constant in

further reuses.

The initial decrease is di�cult to explain. It can be due to solvent e�ects or the observed

palladium leaching of this scale-up catalyst as demonstrated in the previous section. Never-

theless, this series of experiments demonstrates that palladium catalysts based on spherical

carbon can be recycled multiple times. The handling of spherical carbon in catalyst separa-

tion and drying is particularly easy, so that almost no precious material is lost in the recycling

process.

145


0.00 0.25 0.50 0.75 1.00020406080

100

mod / s molRh m-3

X /

%

Hydrogenation #1 Hydrogenation #2 Hydrogenation #3 Hydrogenation #4 fits according to model 0 20 40 60 80 100020406080100

X / %

S /

%

Hydrogenation #1 Hydrogenation #2 Hydrogenation #3 Hydrogenation #4 fits according to model

(a) (b)

Figure 5.33.: Conversion of 1,5-cyclooctadiene using a rhodium SILP catalyst that is recycled

in three additional experiments with �ts according to model applying equation

2.9 (a) and cyclooctene selectivity (b) (T = 60°C, pH2 = 30 bar, c0 = 229 mol m-3)

5.2.6. Proof of concept: SILP catalyst in slurry phase reaction

The SILP concept, as introduced for gas adsorption and catalysis applications, was also ap-

plied to prepare a homogeneous rhodium catalyst immobilized on spherical carbon.

The SILP catalyst preparation procedure is presented in section 4.1.2. The SILP catalyst with

an ionic liquid loading of 20 vol% with respect to the pore volume was tested in the hy-

drogenation of 1,5-cyclooctadiene. See section 2.2.6 for details of the test reaction. After

the initial hydrogenation at 60°C, the catalyst was separated from the reactant solution by

�ltration and dried at 50°C and 0.1 mbar. This SILP catalyst was reused in three recycling

experiments at 60°C and three further temperature variation experiments. See �gures 5.33

and 5.34 for respective results.

The initial hydrogenation experiment showed that 1,5-cyclooctadiene was readily converted.

Full conversion was quickly reached. After the �rst recycling, the initial reaction rate re-

mained in a similar range, but full conversion wasn’t approached anymore. The reaction

rate leveled o� at around 90% conversion. Though, full conversion was approached again

in the next recycling experiment. The last hydrogenation run exhibited a larger reaction

rate than the initial experiment. Here, full conversion was soon reached. So, the prepared

rhodium SILP catalyst remained fully active in four consecutive hydrogenation runs. Up to

80% substrate conversion, selectivity of the intermediate product cyclooctene varied between

146


0.0 0.5 1.0-4-20246

ln

c /

ln(

mol

m-3

)

mod / s molRh m-3

T = 60°C T = 70°C T = 80°C T = 90°C linear fits

0.0027 0.0028 0.0029 0.003001234

T-1 / K-1

ln k

eff /

ln(

m3 s

-1 m

ol-1 Rh

)

keff = f(T) linear fit

(a) (b)

Figure 5.34.: Conversion of 1,5-cyclooctadiene at temperatures between 60-90°C reusing the

rhodium SILP catalyst from previous recycling experiments (a) and Arrhenius

plot of reaction rate constants (b) (pH2 = 30 bar, c0 = 229 mol m-3)

47-67%. Then, cyclooctene molecules were also fully hydrogenated according to the reaction

mechanism.

This series of experiments demonstrates that spherical carbon immobilized well the ionic

liquid catalyst solution. Leaching of catalyst complex into the reactant solution didn’t oc-

cur. The large internal surface area of spherical carbon e�ectively immobilized the solution.

Also, in this reaction system, the nonpolar reactant solution doesn’t strongly interact with

the immobilized polar ionic liquid and the dissolved polar catalyst complex due to di�erent

molecule polarities, thus further preventing leaching of the ionic liquid solution. More de-

tailed future studies need to show that the homogeneous catalyst stays in a homogeneous

form inside of the spherical carbon.

The spherical SILP catalyst was easily separated from the reactant solution. The catalyst was

stable in air andwater atmosphere. Further analysis is necessary to explain the apparently re-

versible deactivation of catalyst in the second experiment. Also, the role of ionic liquid needs

to be investigated, i.e. leaching and the in�uence towards catalytic activity and selectivity.

In literature, a nickel on silica catalyst coated with the ionic liquid [C4C1IM][n-C8H17OSO3]

didn’t exhibit leaching of ionic liquid during hydrogenation of 1,5-cyclooctadiene [208]. Also,

ionic liquids are known to increase the yield of the intermediate cyclooctene [208].

Temperature variation experiments showed a trend of increasing catalytic activity with in-

147


creasing reaction temperature (see �gure 5.34-a). Due to the demonstrated ability of SILP

catalyst for recycling, the same catalyst was justi�ably reused in this experimental study.

The activation energy was derived from initial reaction rate constants applying Arrhenius

law (see �gure 5.34-b). Thus, the activation energy amounted to around 37 kJ mol-1. This

low value indicates pore di�usion in�uences due to the large particle size of spherical carbon

and its low activation degree.

The e�ect of particle size on pore di�usion limitation of 1,5-cyclooctadiene was investigated

in literature in more detail [208]. Nevertheless, the prepared rhodium SILP catalyst based on

500 μm spherical carbon performedwell in the hydrogenation of 1,5-cyclooctadiene. Material

handling was simple due to the large particle size, air and water stability, and the absence of

leaching. The product was easily separated from the catalyst by �ltration.

The SILP concept with PBSAC supports seems to be principally suitable to immobilize homo-

geneous catalysts by pure adsorption for application in slurry phase reactions. Previously,

homogeneous catalyst complexes have been successfully immobilized to solid support ma-

terials by covalent bonding, as well [123].

148


5.3. Spherical carbon as support for SILP filter materials

5.3.1. Preliminary investigations for the advancement of SILPtechnology in gas purification

5.3.1.1. Influence of impregnation solvent on SILP product quality

In some breakthrough measurements of SILP materials with ammonia and hydrogen sul�de

conducted by Blücher GmbH, Erkrath, with reference to DIN EN 14387 ABEK1, undesired

solvent artifacts appeared. Traces of residual ethanol were interfering with gas sensors and

negatively a�ecting breakthrough times. Apparently, ethanol wasn’t always su�ciently re-

moved during SILP preparation, despite intensive drying under vacuum and elevated temper-

ature. To mitigate solvent artifacts, acetonitrile was chosen as an alternative organic solvent.

Acetonitrile is a toxicologically well-known solvent in chemical industry. As an advantage,

acetonitrile exhibits a larger vapor pressure than ethanol (344 hPa instead of 293 hPa at 50°C),

so that drying of SILP materials is less energy-intensive.[166]

All tested ionic liquids (e.g. 1-ethyl-3-methylimidazolium chloride [C2C1IM]Cl, 1-ethyl-3-me-

thylimidazolium bromide [C2C1IM]Br) and applied metal salts (e.g. CuCl2, CuBr2, ZnCl2, and

ZnBr2) are well soluble in acetonitrile. After SILP preparation, the ionic liquid melts were

completely deposited within the pore system of spherical carbon. The ammonia and hydro-

gen sul�de breakthrough times remained una�ected, indicating a similar distribution of ionic

liquid. Solvent artifacts didn’t appear during breakthrough measurements. Consequently,

acetonitrile became the solvent of choice in SILP preparation for this application.

5.3.1.2. Influence of metal salt species on ammonia and hydrogen sulfideadsorption

Well performing SILP materials based on spherical carbon have already been previously pre-

pared for the irreversible adsorption of ammonia an hydrogen sul�de (see section 2.3.5.2).

Concerning further optimization of SILP materials for irreversible gas adsorption, many pa-

rameters can be tuned. But, attention was centered to parameters having the biggest poten-

tial impact. So, it was most relevant to understand the di�erent underlying mechanisms and

dependencies. One focus was set to the e�ectiveness of the ionic liquid �lm in gas clean-

ing. It was found that metal salts dissolved in ionic liquid were most responsible to react

away hazardous gases at ambient temperature (see section 2.3.4.3). Copper(II) salts generally

149


Table 5.18.: Ammonia and hydrogen sul�de adsorption capabilities of spherical carbon

coated with [C2C1IM]Br-CuBr2 1:1.3, [C2C1IM]Br-ZnBr2 1:1.3, and [C2C1IM]Br–

CuBr2-ZnBr2 1:0.65:0.65 in reference to previously tested [C2C1IM]Cl-CuCl2 1:1,

all with αIL=0.2 according to DIN EN 14387 ABEK1 (T=23°C, dadsorber=5 cm,

h�lling=2 cm, vair=0.1 m s-1, c=1000 ppm, rh=70%)

Metal salt Metalcontent /10-4 molmlSILP-1

Ammoniabreakthroughtime / min

Hydrogensul�debreakthroughtime / min

CuBr2 6.8 150 260

ZnBr2 5.9 104 4

CuBr2-ZnBr2 6.4 112 19

CuCl2 5.7 103 [16] 83 [16]

performed well in previous ammonia and hydrogen sul�de breakthrough experiments (see

section 2.3.5.2). But, are copper(II) salts the most suitable metal salts for application in SILP

materials? Several publications in the literature support this assumption (see section 2.3.2).

In a short experimental series, the proposed superiority of copper(II) salts was validated by

comparing two SILP materials containing copper(II) bromide and zinc(II) bromide, respec-

tively. A mixture of both salts was also tested for possible synergistic e�ects. SILP prepara-

tions were carried out according to standard procedures. Table 5.18 lists the corresponding

molar metal contents and breakthrough times of adsorption experiments with ammonia and

hydrogen sul�de. Also, the breakthrough times of a previously characterized SILP material

with copper chloride impregnation is included.

Ammonia capacities of the tested materials lay in a narrow range with breakthrough times

between 103-112 min, with the copper bromide material having a larger breakthrough time

of 150 min. Adsorption capacities of hydrogen sul�de were large for both SILP materials

containing only copper bromide or copper chloride with breakthrough after 260 and 83 min.

The zinc bromide material exhibited a signi�cantly lower breakthrough time of only 4 min.

The mixture of copper bromide and zinc bromide had a negligibly larger breakthrough after

19 min.

Apparently, ammonia complexationworks similarlywell with di�erent transitionmetal salts.

The metal salt mixture resulted in no signi�cant synergetic e�ects. Ammonia capacity de-

pends on the molar content of transition metal ions solved in ionic liquid. The copper bro-

150


mide material performed better than the copper chloride material due to a higher solubility

of the bromide salt in the melt hydrate and consequently a higher copper utilization.

On the contrary, hydrogen sul�de reactivity is more sensitive towards the transition metal

cation and its anion. As proposed in literature, copper(II) reacts more strongly with sul�des

than zinc(II) (see section 2.3.2). In fact, copper(II) salts generally are more reactive than other

transition metal salts due to their large complex formation constants. Interestingly, in a mix-

ture of copper bromide and zinc bromide, zinc bromide appeared to inhibit sul�de complex

formation. Also, copper bromide obviously is signi�cantly more reactive than copper chlo-

ride. Consequentially, further development of SILP materials for ammonia and hydrogen

sul�de adsorption was focused on copper(II)-containing systems.

5.3.1.3. Corrosion investigations of halide-containing SILP materials

In the following, the aspect of SILP corrosivity is discussed. It’s well known that halide salts

are corrosive to metals. Concerning the electrochemical series, a noble metal cation also

oxidizes a less noble metal. In industrial applications, adsorber materials must not corrode

instruments and process periphery usually made of stainless steel. Also, �lter media for

personal protection purposes are usually encapsulated in aluminum canisters. Because, in

SILPmaterials, the halide ionic liquids andmetal halides are immobilizedwithin the spherical

carbon, the critical compounds are not in immediate contact to the metal container. Thus,

it was experimentally analyzed, if these halide and copper containing SILP materials were

corrosive to aluminum and stainless steel. Corrosion tests of aluminum foil and stainless

steel were conducted with di�erent SILP materials at elevated relative humidity. The degree

of corrosion is listed in table 5.19.

Corrosion of aluminum was most pronounced in the case of the copper bromide based SILP

material. Here, the aluminum foil dissolved completely. The copper chloride system resulted

in a partial dissolution of aluminum foil, while the copper bromide and zinc bromide mixture

produced minor corrosion at the surface. The zinc bromide based material didn’t noticeably

a�ect the appearance of aluminum foil. In case of stainless steel, it was observed that, after

7 days, the spherical SILP particles with zinc bromide and a mixture of copper and zinc

bromide sticked to the stainless steel platelets. No corrosion was noticed for the other SILP

materials. After further addition of water and another 7 days, signs of corrosion accompanied

with a signi�cant mass loss were observed for the stainless steel platelets contacted with

copper bromide and copper chloride SILP materials.

In general, copper(II) salts appeared to be more corrosive than zinc(II) salts and bromide

151


Table 5.19.: Corrosion investigations of spherical carbon coated with [C2C1IM]Br-CuBr2

1:1.3, [C2C1IM]Br-ZnBr2 1:1.3, [C2C1IM]Br-CuBr2-ZnBr2 1:0.65:0.65, [C2C1IM]

Cl-CuCl2 1:1, and [C2C1IM]Cl-ZnCl2 1:1.3, all with αIL=0.2 in aluminum foil at

65-75% rh and 20°C for 7 days and with stainless steel 1.4301 (X5CrNi18-10) at

60-90% rh and 20°C for 14 days (+: no corrosion, -: slight corrosion, –: corrosion,

—: strong corrosion)

Metal salt Aluminum corrosion Stainless steel corrosion

CuBr2 — –

ZnBr2 + n/a

CuBr2-ZnBr2 - -

CuCl2 – –

more aggressive than chloride. The results of the corrosion experiments demonstrate that

halide-containing SILP materials were corrosive, even though the critical substances were

immobilized inside the spherical carbon. But, a minor amount of impregnation substance can

still be located on the outside of spherical carbon. Thus, it is in direct contact to aluminum

or stainless steel. This was strongly indicated by the sticking of spherical SILP material to

the stainless steel platelets.

Another possibility of corrosion is the release of hydrogen bromide or hydrogen chloride gas

traces due to ionic liquidmelt decomposition. This is another strong argument for halide-free

materials in personal protection applications. For other applications, the excellent perfor-

mance of this generation of SILP materials can still be leveraged by using plastic containers.

Even in case of undetectable corrosion, the psychological e�ect of the presence of halides to

process engineers would also be a relevant fact to be taken into account. Also, corrosion may

occur at elevated temperature and humidity, which has not yet been tested. Consequently,

alternative halide-free SILP materials are of high relevance and should be developed.

5.3.2. Filter material improvements for irreversible adsorption ofammonia and other hazardous gases

5.3.2.1. Development of halide-free SILP materials

For halide-free systems, the commercially-available ionic liquid 1-n-butyl-3-methyimidazo-

lium octylsulfate ([C4C1IM][n-C8H17OSO3]) was chosen, because it’s well characterized and

152


its physical properties suit the application. It has a melting point between 34 and 35°C. Ther-

mal decomposition starts at around 341°C. The ionic liquid is hydrolysis-stable at 80°C.[209]

The octyl-group of the anion decreases the hygroscopic character compared to previously

used halide ionic liquids. The ionic liquid is well soluble in many solvents, including ethanol,

water, acetonitrile and methylenechloride. The low melting point and good thermal and

chemical stabilities are advantageous in gas separation processes at room temperature and

above.

As metal salt, copper(II) sulfate pentahydrate was initially used. It is soluble in [C4C1IM]

[n-C8H17OSO3] up to a molar ratio of 1:1.4. The best solvent for this copper salt is water.

The limited solubility of copper sulfate in organic solvents necessitated the impregnation of

spherical carbon with water. This had an immediate advantage. In contrast to organic sol-

vents, water didn’t need to be completely removed from the SILP material. This eliminates

time and energy intensive drying processes in the �nal stage of SILP preparation. Never-

theless, water is known to penetrate hydrophobic activated carbon less deeply than organic

solvents like alcohols and acetone [8]. This results in an inhomogeneous distribution of the

ionic liquid within the spherical carbon. Surface areas for absorption and adsorption can be

a�ected.

For initial investigations, SILP materials with [C4C1IM][n-C8H17OSO3]-CuSO4(5H2O) 1:1.4

based on spherical carbon were prepared according to standard procedures using water as

solvent. SILP materials with di�erent ionic liquid loadings between 0.1-0.3 were obtained

and characterized. The quality of impregnation was determined by nitrogen sorption ex-

periments at Blücher GmbH, Erkrath (see table 5.20). Corrosion tests were carried out (see

table 5.21). The ammonia separation e�ciency was analyzed in breakthrough measurements

at di�erent relative humidity (see �gure 5.35 and table 5.22). Additionally, the materials’

broadband capacities were characterized by cyclohexane and hydrogen sul�de breakthrough

measurements (see table 5.23).

After preparation of SILP materials with ionic liquid loadings of 0.2 and 0.3, white particles

and powder were visible in the material. Also, the glass �ask was coated with a white sub-

stance. It appeared that ionic liquid was partially precipitating outside the spherical carbons

pore system during water removal in the rotary evaporator. Likely, the solubility of copper

sulfate in water was limiting ionic liquid deposition inside spherical carbon as copper sulfate

already precipitated in the bulk solution. In this case, a modi�cation of the SILP preparation

method should be developed to prevent ionic liquid precipitation to the outside of spherical

carbon and, thus, improve product quality. Also, access to the pore system possibly is al-

153


Table 5.20.: Pore characteristics derived from nitrogen sorption experiments of PBSAC

#1 coated with the halide-free ionic liquid melt [C4C1IM][n-C8H17OSO3]–

CuSO4(5H2O) 1:1.4 and of PBSAC #2 coated with the halide ionic liquid melt

[C2C1IM]Cl-CuCl2 1:1.3, both with αIL=0.2, in reference to the pure 500 μm PB-

SAC support materials PBSAC #1 and PBSAC #2 with varying pore structure

Material Vtot / cm3

cm-3Vmicro / cm3

cm-3MP BETarea / m2

cm-3

Loss in Vtot / %

PBSAC #1 (Vtot = 1.33

cm3g-1)

0.51 0.38 840 -

Halide-free IL @

PBSAC #1

0.39 0.30 631 24

PBSAC #2 (Vtot = 1.18

cm3g-1)

0.51 0.33 824 -

Halide IL @ PBSAC #2 0.33 0.19 479 35

ready blocked by solid copper sulfate preventing further inclusion of ionic liquid. The pore

�lling was investigated in nitrogen sorption experiments. Table 5.20 lists the remaining pore

volume and other properties derived from nitrogen sorption experiments, also for a halide-

based SILP reference material. The two SILP materials di�er in the used activated carbon

support material. To allow direct comparison, pore properties were correlated to the same

�lling volume applying bulk densities.

Normalized to the bulk density, the two PBSAC support materials varied only slightly in mi-

cro pore volume and BET surface area. After impregnation, pore volume and surface area

decreased. The loss in total pore volume was signi�cantly lower for the copper sulfate based

ionic liquid than for the copper chloride based melt. With a decrease of 24% in pore vol-

ume, the copper sulfate based melt occupied roughly its own volume. There is no indication

of blocked access to empty pores, as postulated earlier. Despite utilization of water instead

of ethanol during impregnation, the distribution of this ionic liquid in spherical carbon ap-

peared to be more homogeneous than the chloride-based melt. A clear conclusion of this

data is di�cult, because several parameters were varied, i.e. PBSAC support material, ionic

liquid and impregnation solvent.

Corrosion experiments of SILP material with [C4C1IM][n-C8H17OSO3]-CuSO4(5H2O) 1:1.4

were conducted as described in section 4.2.2. The results are shown in table 5.21.

154


Table 5.21.: Corrosion investigations of spherical carbon coated with [C4C1IM][n-C8H17-

OSO3]-CuSO4(5H2O) 1:1.4 with αIL=0.2 in aluminum foil at 65-75% rh and 20°C

for 7 days and with stainless steel 1.4301 (X5CrNi18-10) at 60-90% rh and 20°C for

14 days (+: no corrosion, -: slight corrosion, –: corrosion, —: strong corrosion)

Metal salt Aluminum corrosion Stainless steel corrosion

CuSO4 + +

At room temperature and elevated relative humidity, this halide-free SILP material exhibited

no corrosion on aluminium foil and stainless steel platelets. Thus, it’s potentially suitable for

industrial applications.

Results of ammonia breakthrough measurements are presented in �gure 5.35. Characteristic

quantities of each breakthrough measurement are listed in table 5.22. The adsorption rates

were derived from the slopes of breakthrough curves at the in�ection points. The molar

breakthrough capacity describes the utilization of copper species at breakthrough. It’s the

ratio of adsorbed ammonia molecules over the number of cuprate ions in the SILP material.

The assumption is that one cuprate ion forms complexes with four ammonia molecules.

In case of the 10 vol% ionic liquid loading, breakthrough of ammonia appeared between 150

and 200 min. The breakthrough was delayed with increasing relative humidity. The maxi-

mum ammonia concentration was quickly reached for all three curves. A very similar behav-

ior was observed with the SILP material loaded with 20 vol% ionic liquid. The breakthrough

times were between 300-400 min, increased with increasing humidity and were followed by

quick saturation. The 30 vol% loadedmaterial exhibited breakthrough times between 600-800

min. Here, breakthrough time and slope of the breakthrough curve decreasedwith increasing

relative humidity. Doubling the ionic liquid loading from 0.1 to 0.2 increased breakthrough

capacity by 66-126%. Further increasing the loading by 50%, led to an increase of capacity

by 90-148%. With increasing ionic liquid loading, absorption rates decreased. For each ionic

liquid loading, ammonia absorption rates were generally similar. Only for the 30 vol% loaded

spherical carbon, adsorption rates continuously decreased with increasing relative humidity.

The molar breakthrough capacity of ammonia was between 61-78% in case of ionic liquid

loadings between 0.1-0.2. For a loading of 0.3, the molar breakthrough capacity reached

maximum values between 86-99%.

Generally, these SILP materials exhibited large ammonia breakthrough capacities and a low

in�uence of relative humidity compared to previously tested halide-containing SILP materi-

als [16]. The fast ammonia saturations after breakthrough indicate fast adsorption kinetics

155


0 100 200 300 4000.00.20.40.60.81.0 rh = 25% rh = 50% rh = 80% fits

c c

-1 0 /

-

t / min0 200 400 600 8000.0

0.20.40.60.81.0 rh = 25% rh = 50% rh = 80% fits

c c

-1 0 /

-

t / min

(a) (b)

0 400 800 1200 16000.00.20.40.60.81.0 rh = 25% rh = 50% rh = 80% fits

c c

-1 0 /

-

t / min

(c)

Figure 5.35.: Ammonia breakthrough measurements of spherical carbon coated with [C4C1-

IM][n-C8H17OSO3]-CuSO4(5H2O) 1:1.4 with (a) αIL=0.1, (b) αIL=0.2 and (c)

αIL=0.3 at di�erent relative humidity; �ts according to equation 2.22 (T=30°C,

p=1.2 bar, dabsorber=1.8 cm, h�lling=2 cm, vN2=0.02 m s-1, cNH3=1000 ppm)

156


Table 5.22.: Ammonia absorption characteristics of spherical carbon coated with [C4C1IM]-

[n-C8H17OSO3]-CuSO4(5H2O) 1:1.4 at di�erent ionic liquid loadings and rela-

tive humidity (T=30°C, p=1.2 bar, dabsorber=1.8 cm, h�lling=2 cm, vN2=0.02 m s-1,

cNH3=1000 ppm)

Ionic liquidloading / -

Relativehumidity /%

Break-throughtime / min

Absorptionrate / 10-3min-1

Molarbreak-throughcapacity / %

0.1

25 162 13.9 66

50 169 24.2 68

80 194 22.9 78

0.2

25 299 7.8 61

50 382 4.1 77

80 321 6.7 65

0.3

25 740 6.1 99

50 725 3.5 98

80 674 1.8 86

157


and good mass transfer properties of the SILP materials in a broad range of relative humidi-

ties. Also, utilization of available copper species until breakthrough was very large. Increas-

ing ionic liquid loading from 0.1 to 0.2 led to a proportional increase of breakthrough time

with similar molar breakthrough capacities.

A further increase of ionic liquid loading to 0.3 resulted in an almost complete copper utiliza-

tion until breakthrough. It can be hypothesized that, at low ionic liquid loadings, ionic liquid

and copper sulfate partially segregate in the pore structure due to chromatographic e�ects.

The ionic liquid [C4C1IM][n-C8H17OSO3] adsorbes more deeply into micropores and/or the

center of the carbon spheres, while copper sulfate remains in the outer shell, as already ob-

served during SILP preparations. REM-EDX measurements over the cross section of SILP

carbon spheres could be conducted to show cuprate and sulfate distributions clarifying this

hypothesis. At an ionic liquid loading of 0.3, a su�ciently large amount of ionic liquid likely

dissolved copper sulfate best. The breakthrough capacity and absorption rate of this SILPma-

terial slightly decreased with increasing relative humidity due to water absorption, volume

expansion and thus a reduced boundary layer for ammonia absorption.

Concerning broadband capability, the copper sulfate based SILP material with an ionic liquid

loading of 0.2 was tested with multiple gases. The results are summarized in table 5.23. For

comparison purposes, the adsorption capacity of pure spherical carbon for each type of gas

and the DIN EN 14387 ABEK1 requirements are listed, as well.

The SILP material had capacities for cyclohexane, hydrogen sul�de and ammonia. Compared

to pure spherical carbon, breakthrough times of cyclohexane decreased and capacities of hy-

drogen sul�de and ammonia signi�cantly increased. Chlorine capacities weren’t determined,

but are expected to decrease by around 20%, in correlation to previous measurements [16].

The ABEK1 requirements were exceeded in case of hydrogen sul�de and ammonia adsorp-

tion and, even though not speci�cally tested for this SILP material, very likely for chlorine,

as well. For cyclohexane, only 79% of the required capacity was reached. While ammonia

and hydrogen sul�de react with the cuprate ions in the ionic liquid melt, cyclohexane phys-

ically adsorbs to the carbon surface. Because the ionic liquid melt occupies a certain amount

of carbon surface, cyclohexane capacity decreases with increasing ionic liquid loading. As

chlorine also adsorbs directly to the carbon surface, its capacity decreases with increasing

ionic liquid loading, as well. Thus, slightly decreasing ionic liquid loading will result in a

broadband �lter meeting the ABEK1 requirements of the four gas classes.

In summary, SILP materials based on copper sulfate adsorb large amounts of ammonia. They

perform equally well at di�erent relative humidities and are promising candidates for broad-

158


Table 5.23.: Broadband capabilities of spherical carbon coated with [C4C1IM][n-C8H17O-

SO3]-CuSO4(5H2O) 1:1.4 with αIL=0.2 in reference to pure spherical carbon

according to DIN EN 14387 ABEK1 (T=23°C, dadsorber=5 cm, h�lling=2 cm,

vair=0.1 m s-1, c=1000 ppm, rh=70%)

Class of gas Test gas ABEK1 re-quirement /min

Puresphericalcarbonbreak-throughtime / min

SILP break-throughtime / min

Organic Cyclohexane 70 133 55

Inorganic Chlorine 20 158 [16] n/a

Acidic Hydrogen sul�de 20 1 [16] 39

Ammonia Ammonia 50 1 [16] 63*

* Ammonia breakthrough time adapted from experiment with di�erent measurement

conditions

band �lters. Corrosion is no issue with these halide-free systems. SILP preparation still is a

challenge, because copper sulfate partially precipitates outside the spherical carbon pore sys-

tem. So, dust particles are released and the material cannot be processed to foam structures,

for instance.

5.3.2.2. Alternative coating techniques of spherical carbon

Alternative impregnation techniques of spherical carbon with [C4C1IM][n-C8H17OSO3]–

CuSO4(5H2O) 1:1.4 have been tested to prevent precipitation of copper sulfate outside the

spherical carbon. Two incipient wetness approaches were evaluated, with each utilizing a

signi�cantly lower amount of water compared to classical wet impregnation. See section

2.2.5.2 on catalyst impregnation for di�erences between incipient wetness and wet impreg-

nation processes. In the �rst approach, the standard impregnation procedure was slightly

modi�ed, so that water only amounted to 90% of the carbon’s total pore volume. In the

second approach, the impregnation solution (also: V = 0.9 · Vpore ) was slowly added drop-

wise to a continuously circulating mass of spherical carbon inside a rotary evaporator. The

impregnation results are depicted in �gure 5.36.

As already discussed earlier, white copper sulfate particles precipitated during solvent re-

moval in the wet impregnation process. With the incipient wetness impregnation, spherical

159


(a) (b)

(c)

Figure 5.36.: Images of spherical carbon coated with [C4C1IM][n-C8H17OSO3]-CuSO4(5H2O)

1:1.4 with αIL=0.2 applying di�erent impregnation approaches: standard wet im-

pregnation (a), incipient wetness impregnation (b) and dropwise impregnation

(c)

160


carbon was homogeneously coated with a white layer of copper sulfate on the outside. Ap-

plying the dropwise impregnation process, only a minor amount of copper sulfate was visible

on a small fraction of spherical carbon.

Consequently, a modi�cation of the impregnation process led to a signi�cant improvement

of product quality. The reduction of solvent below the carbon pore volume prevented pre-

cipitation of copper sulfate outside the carbon material. Nevertheless, copper sulfate visibly

deposited in the outer carbon shell, possibly aggravated by ultrasonication treatment. A

dropwise impregnation allowed the impregnation solution to thoroughly penetrate the pore

system. Further enhancement of the dropwise impregnation towards spray/spin coating can

result in a completely satisfying SILP product quality [210].

5.3.2.3. Organic copper salts in SILP materials

Copper octylsulfate (Cu[n-C8H17OSO3]2) is another halide-free copper salt that has the same

organic anion than the [C4C1IM][n-C8H17OSO3] ionic liquid. Thus, this copper salts is well

miscible with the ionic liquid and will likely penetrate the hydrophobic carbon material eas-

ily, without precipitating outside the spherical carbon.

Copper octylsulfate is verywell soluble inwater, slightly inmethanol and not soluble in other

organic solvents like ethanol or methylenechloride. The salt wasn’t commercially available,

but was produced by cation exchange of sodium octylsulfate to octylsulfate acid and subse-

quent neutralization with copper hydroxide carbonate (see section 4.3.2). Also, it was syn-

thesized from 1-octanol, chlorosulfuric acid and copper hydroxide carbonate by sulfatization

and neutralization (also see section 4.3.2). In the second synthesis route, isolation of copper

octylsulfate from reactant mixture was di�cult. So, only technical grade copper octylsulfate

with residues of e.g. copper sulfate were easily producible.

A similarly structured compound is copper octanoate (Cu[n-C7H15COO]2) [192]. See sec-

tion 4.3.2 for details on its synthesis. Copper octanoate is well soluble in methylenchlo-

ride, slightly soluble in acetone, but not soluble in water or ethanol. With the [C4C1IM]

[n-C8H17OSO3] ionic liquid, copper octanoate formed a homogenous mixture in a molar ra-

tio of 1:1.3. If octanoic acid should be formed in a reaction with a gas, it would remain within

the spherical carbon due to its low vapor pressure.

Two SILP materials were prepared according to standard procedures containing [C4C1IM]

[n-C8H17OSO3]-Cu[n-C8H17OSO3]2 1:1.3withwater and [C4C1IM][n-C8H17OSO3]-Cu[n-C7-

H15COO]2 1:1.3 with methylenchloride as solvent. The ionic liquid loadings amounted to 0.3.

161


Table 5.24.: Ammonia breakthrough measurements of spherical carbon coated with

[C4C1IM][n-C8H17OSO3]-Cu[n-C8H17OSO3]2 1:1.3, [C4C1IM][n-C8H17OSO3]–

Cu[n-C7H15COO]2 1:1.3, and [C4C1IM][n-C8H17OSO3]-CuSO4(5H2O) 1:1.4, all

with α=0.3 (T=23°C, dadsorber=5 cm, h�lling=2 cm, vair=0.1 m s-1, cNH3=1000 ppm,

rh=70%)

Metal salt Copper content /10-4 molCu mlSILP-1

Breakthroughtime / min

Copper octylsulfate 2.5 20*

Copper octanoate 2.6 16*

Copper sulfate pentahydrate 4.6 63*

* Ammonia breakthrough time adapted from experiment with di�erent measurement

conditions

The quality of SILP preparations was evaluated. Additionally, the materials were character-

ized in ammonia breakthrough experiments. In both cases, the ionic liquid was completely

deposited inside the spherical carbon. Stainless steel corrosion wasn’t observed for these

SILP materials. Aluminum corrosion tests still need to be conducted. The results of break-

through measurements with regard to DIN EN 14387 ABEK1 are listed in table 5.24 together

with results of the copper sulfate based SILP material.

The two alternative copper salts exhibited similar breakthrough times. The copper octylsul-

fate based SILP material had a slightly larger ammonia breakthrough capacity. Compared

to copper sulfate pentahydrate, ammonia capacity was decreased by a factor of 3.2. The

copper content of both alternative copper salts was similar, with the copper octanoate SILP

exhibiting a slightly larger metal content. The SILP metal content of the copper sulfate ma-

terial was about 1.9 times larger. The decreased copper contents of the two alternative SILP

materials were due to the more spacious anions. Varying the copper salt anion, ammonia

adsorption didn’t proportionally decrease with decreasing copper content. The in�uence of

the metal salt anion on ammonia adsorption seemed to be large. Copper utilization of the

alternative SILP materials was signi�cantly decreased. This result can interpreted taking wa-

ter management into account. Berberich et al. stated that undissolved copper sulfate greatly

contributes maintaining a stable water balance [211]. Likely due to the presence of crystal

water in copper sulfate pentahydrate, ammonia adsorption appeared to be greatly enhanced

in the previous section. Copper sulfate is also very hygroscopic in nature. The two alterna-

tive SILP systems containing copper octylsulfate and copper octanoate seem to lack proper

water management. The ocytlsulfate and octanoate anions render the metal salts more hy-

162


drophobic. Copper octanoate isn’t even water soluble. Nevertheless, the result with copper

octanoate likely demonstrates that ammonia adsorption by metal complex formation works

even without the presence of water in those ionic liquids. A well suitable salt for water man-

agement discussed in the report of Berberich et al. is sodium hydrogen phosphate [211].

This salt or derivatives thereof could be integrated in future SILP materials with weak native

water management for enhanced adsorption performance.

Both, copper octylsulfate and copper octanoate, were well miscible with the ionic liquid

[C4C1IM][n-C8H17OSO3] and were easily coated onto spherical carbon. These SILP materi-

als were non-corrosive to stainless steel. Utilization of water during impregnation of copper

octylsulfate based systems simpli�ed SILP production, because water didn’t have to be com-

pletely removed. Low boiling methylenchloride was also an appropriate solvent in the pro-

duction of copper octanoate based SILP materials. Nevertheless, ammonia capacities were

lower rendering these systems unsuitable for one-time applications, e.g. in clean rooms and

for personal protection. The spacious, hydrophobic anions constrained the number of ac-

tive copper ions in the SILP material and made water management more di�cult. Despite

low ammonia capacity, these SILP materials can be applied in temperature or pressure swing

adsorption units, because of their chemical inertness and economically-viable production.

Copper octanoate is more easily synthesized than copper octylsulfate, but a potential release

of octanoic acid during regeneration needs to be investigated.

5.3.2.4. Development of a filter material for formaldehyde removal

The development of SILP materials to remove toxic formaldehyde gas from air is the focus

of this chapter. Due to its well-known non-toxicity (see also section 2.3.6), choline chloride

impregrations are investigated at �rst. Choline chloride is no ionic liquid, but a hygroscopic

salt. Operating at high relative humidity, all choline chloride molecules dissolve in water

and are accessible to the gas stream when coated to spherical carbons. A closely related salt,

choline hydroxide, even possesses two functional hydroxide groups per molecule. This can

double molar formaldehyde capacity.

Choline chloride and choline hydroxide were successfully coated to spherical carbon apply-

ing standard SILP preparation techniques using water as solvent. The material’s formalde-

hyde capacity was measured in a modi�ed ammonia test rig, where the water saturator was

�lled with 200 ml of 4% stabilized formalin solution (see section 4.2.3 for details). Table

5.25 lists formaldehyde breakthrough times of pure spherical carbon, a previously developed

broadband SILP �lter with copper chloride, two choline impregnated materials, and an im-

163


Table 5.25.: Formaldehyde adsorption capabilities of pure spherical carbon and spherical

carbon coated with [C2C1IM]Cl-CuCl2 1:1.3 (αIL=0.2), [Choline]Cl-H2O 1:16

(αIL=0.3), [Choline]OH-H2O 1:16 (αIL=0.3), and [Choline]Cl-ZnCl2 1:1 (αIL=0.2);

T=30°C, dadsorber=1.8 cm, h�lling=2 cm, vN2=0.02 m s-1, c=600 ppm, rh=55%

SILP impregnation Hydroxidecontent /10-4 molmlSILP-1

Formalde-hydebreak-throughtime / min

Formalde-hydeabsorptionrate / 10-3min-1

None (αIL=0) 0 299 5.8

[C2C1IM]Cl-CuCl2 1:1.3 (αIL=0.2) 0 263 5.7

[Choline]Cl-H2O 1:16 (αIL=0.3) 3.8 607 4.2

[Choline]OH-H2O 1:16 (αIL=0.3) 7.9 819 5.9

[Choline]Cl-ZnCl2 1:1 (αIL=0.2) 5.8 622 4.9

pregnation containing choline chloride and zinc chloride. Unfortunately, the choline-based

SILP materials released undesired amine odors, which can be removed by a subsequent am-

monia polishing �lter, as described in section 5.3.2, or prevented by the addition of choline

stabilizing reagents. In literature, choline solutions have been stabilized by either formalde-

hyde or hydroxylamine [186, 212, 213]. Addition of formaldehyde would lower formaldehyde

capacity and hydroxylamine is unstable itself. Electrostatic stabilization using ionic liquids

like [C2C1IM][n-C2H5OSO3] was a promising approach, though. The ionic liquid formed a

homogeneous mixture of low viscosity with choline chloride exhibiting a certain odor inhi-

bition.

Spherical carbon itself exhibited a formaldehyde breakthrough time of 299 min. The refer-

ence SILP material with [C2C1IM]Cl-CuCl2, optimized for ammonia adsorption, showed a

slightly lower formaldehyde capacity than pure spherical carbon. With choline chloride im-

pregnation, breakthrough time signi�cantly increased up to 607 min. Formaldehyde break-

through was further delayed up to 819 min applying the choline hydroxide impregnation.

With a breakthrough time of 622 min, the material coated with a mixture of choline and zinc

chloride performed slightly better than the choline chloride impregnated material. The rate

of adsorption was similarly large in all experiments. Breakthrough curves increased most

steeply for choline hydroxide and copper chloride impregnations, as well as pure spherical

carbon.

164


In principle, formaldehyde gas already adsorbed to pure spherical carbon. An ionic liquid im-

pregnation without speci�c functionality for aldehyde conversion decreased formaldehyde

capacity of the SILP material. Due to ionic liquid impregnation, the carbon surface was par-

tially inaccessible for formaldehyde. As postulated, choline-based materials greatly enhance

formaldehyde capacity, because the aldehyde likely reacts with hydroxide functional groups.

But, even though the choline hydroxide material contained twice as many hydroxide groups

as the choline chloride material, absolute breakthrough capacity only increased by about

35%. This is possible, because a signi�cant amount of formaldehyde still adsorbed directly

to the carbon surface. The relative increase of formaldehyde capacity from choline chloride

to choline hydroxide was estimated to around 69%. So, formaldehyde capacity of salt im-

pregnations proportionally correlated to the number of hydroxide functional groups. This

behavior was di�erent in the case of the choline and zinc chloride impregnation. Despite a

53% increased amount of hydroxide groups, relative breakthrough capacity increased only

by around 17%. Apparently, in some systems, increase of capacity was disproportionate to

the amount of functional hydroxide groups. Also the rate of adsorption, corresponding to

the slope of the breakthrough curve, was largest for the choline hydroxide system. Conse-

quently, it appears that other e�ects such as basicity of a system play an important role. The

more alkaline a system, the more readily aldehydes react with alcohol groups. This hypothe-

sis was validated by a series of formaldehyde breakthrough experiments of spherical carbon

coated with salts of di�erent basicity. These salt impregnations were acidic choline and zinc

chloride, less acidic [Choline][n-C8H17OSO3]-ZnSO4(7H2O) 1:1.3 and alkaline choline hy-

droxide. The results of breakthrough experiments of this experimental series are listed in

table 5.26 and are compared to spherical carbon functionalized with nitrogen surface groups.

The NIOSH3 standard requires formaldehyde breakthrough times of at least 45 min. With

89 min, spherical carbon functionalized with nitrogen surface groups exceed this bench-

mark signi�cantly. At NIOSH conditions, the acidic choline and zinc chloride system ex-

hibited breakthrough after only 1 min. Spherical carbon coated with less acidic [Choline]

[n-C8H17OSO3]-ZnSO4(7H2O) 1:1.3 had a breakthrough time of 18 min. With 77 min, the

formaldehyde capacity of alkaline choline hydroxide impregnation was smaller than the ni-

trogen functionalized material, but still exceeded the NIOSH3 standard by a factor of 1.7.

With increasing basicity of the tested systems, formaldehyde breakthrough capacity increased.

Utilization of hydroxide functional groups improved absorption due to higher reactivity. In

nitrogen modi�ed spherical carbon, gases seemed to be simply solved in adsorbed water [7].

In contrast, formaldehyde appeared to irreversibly react with hydroxide functional groups

of impregnated salts. In summary, next to hydroxide functional groups, the basicity of the

165


Table 5.26.: Formaldehyde adsorption capabilities of nitrogen surface functionalized spher-

ical carbon and spherical carbon coated with [Choline]Cl-ZnCl2 1:1 (αIL=0.2),

[Choline][n-C8H17OSO3]-ZnSO4(7H2O) 1:1.3 (αIL=0.2) and [Choline]OH-H2O

1:16 (αIL=0.3); according to NIOSH conditions [151] (T=25°C, h�lling=2 cm,

vair=0.13 m s-1, rh=25%, c=500 ppm, cbreakthrough=1 ppm); basicity ratings: ++:

strongly alkaline system, +: alkaline system, -: more acidic system

Functionalization ofspherical carbon

Hydroxidecontent / 10-4mol mlSILP-1

Basicity Formaldehydebreakthroughtime / min

Nitrogen surface

functionalization (αIL=0)

0 ++ 89 [7]

[Choline]Cl-ZnCl2 1:1

(αIL=0.2)

5.8 - 1

[Choline][n-C8H17OSO3]

-ZnSO4(7H2O) 1:1.3

(αIL=0.2)

3.1 + 18

[Choline]OH-H2O 1:16

(αIL=0.3)

7.9 ++ 77

system is likely responsible for large formaldehyde capacities. With choline-based spherical

adsorbents, an e�cient formaldehyde �lter was found.

5.3.3. Design of SILP materials for pressure and temperature swingadsorption processes

5.3.3.1. Reversible adsorption of ammonia

Speci�c SILP materials for reversible adsorption of ammonia for application in industrial gas

separation were developed after observing that ammonia partially desorbed from SILP mate-

rials at certain conditions. In previous experiments, published in literature, irreversibility of

ammonia adsorption was investigated and some interesting e�ects were noticed. Spherical

carbon coated with [C2C1IM]Cl-CuCl2 1:1.3 (αIL=0.2) was loaded with ammonia at 30°C until

saturation. At 90°C, 56% of adsorbed ammonia was desorbed by �ushing with nitrogen (see

section 2.3.5.2). So, a signi�cant amount of ammonia was relatively weakly adsorbed to the

SILPmaterial. This chapter takes a closer look at this weakly adsorbed quantity. Even though

pressure swing adsorption is a more �exible mode of operation than temperature swing ad-

166


sorption, both pressure and temperature in�uences are investigated. In principle, a complete,

low-energy regeneration of SILP materials loaded with ammonia would necessitate metal-

free systems. Nevertheless, metal-based systems were tested as metal-amine complexes like

Cu(NH3)4SO4 are known to disintegrate at temperatures between 105-355°C [175].

The regenerative SILP materials were primarily intended for industrial applications (see sec-

tion 2.3.6). Thus, as already discussed in section 5.3.1.3, halide-based ionic liquids aren’t

suitable due to their stigma of corrosiveness or their actual corrosiveness. Nevertheless, to

understand the fundamental mechanisms, halide-based SILP materials were experimentally

analyzed and compared to the performance of their halide-free counterparts.

In the following experiment, three SILP materials based on copper bromide, copper octyl-

sulfate and copper octanoate were loaded with ammonia at 4 bar and 20°C until saturation.

Then, the �rst two samples were continuously �ushed with air at 20°C and, in a pressure

swing desorption, the pressure was released to 1 bar. After no further desorption of ammo-

nia was detected, the temperature of the adsorber unit was slowly increased until ammonia

desorption stopped. The results are summarized in table 5.27.

The copper bromide based SILP material possessed an ammonia adsorption capacity, which

was around 2.2 times larger than the ammonia capacity of the copper octylsulfate material

and 2.8 times larger than the copper octanoate system. With 52-59% for the copper bromide

and copper octylsulfate materials, more than half of the adsorbed ammonia was released by

simple pressure reduction. With a temperature ramp up to 91°C, additional 36% ammoniawas

desorbed from the copper bromide material. At higher temperatures, no further ammonia

was detected in the air stream, so that 12% of ammonia remained irreversibly adsorbed to

this SILP material. The copper octylsulfate based �lter material only released 22% ammonia

up to 160°C. Here, 19% of ammonia was irreversibly bound. The copper octanoate material

exhibited a lower ammonia capacity than the copper octylsulfate based system.

As shown in previous sections, ammonia capacities strongly correlate to the copper contents.

Copper octylsulfate exhibits a larger adsorption capacity in relation to the copper content.

This can be due to larger ammonia solubility in the ionic liquid melt and possibly a better

utilization of reactive copper sites. The amount of ammonia, which desorbed at pressure

reduction, was likely physically absorbed in the ionic liquid melt. Disintegration of metal-

amine complexes by pressure reduction wasn’t reported in literature. So, the copper octyl-

sulfate system seems to physically dissolve more ammonia than the copper chloride system.

The ammonia released by temperature increase can be attributed to copper-amine complex

decomposition. Copper bromide apparently releases bound ammonia at signi�cantly lower

167


Table 5.27.: Ammonia adsorption and desorption capabilities of spherical carbon coated with

[C2C1IM]Br-CuBr2 1:1.3 (αIL=0.2), [C4C1IM][n-C8H17OSO3]-Cu[n-C8H17OSO3]2

1:1.3 (αIL=0.3) and [C4C1IM][n-C8H17OSO3]-Cu[n-C7H15COO]2 1:1.3 (αIL=0.3);

adsorption conditions: T=20°C, p=4 bar, dadsorber=1.5 cm, h�lling=5 cm, vair=

0.02 m s-1, c=1000 ppm until beginning saturation, then c=5000 ppm until 90%

saturation, rh=15%; pressure swing desorption conditions: T=20°C, p=1 bar,

vair=0.02 m s-1; temperature swing desorption conditions: T=91-160°C, p=1 bar,

vair=0.02 m s-1

SILP impregnation Coppercontent /10-4 molCuml-1

Ammoniaadsorptioncapacity at 4bar and20°C / wt%

Fraction ofdesorbedammonia at1 bar and20°C / %

Fraction ofdesorbedammonia at1 bar and91-160°C / %

[C2C1IM]Br-CuBr2 1:1.3

(αIL=0.2)

6.8 6.7 52 36 (91°C)

[C4C1IM][n-C8H17OSO3]-

Cu[n-C8H17OSO3]2 1:1.3

(αIL=0.3)

2.5 3.0 59 22 (160°C)


Cu[n-C7H15COO]2 1:1.3

(αIL=0.3)

2.6 2.4 - -

168


temperatures than copper octylsulfate does. As already discussed in section 2.3.2, complex

decomposition temperatures vary over a wide range depending on complex stability.

The ammonia complex decomposition temperature is highly relevant for temperature swing

adsorption processes. Other halide-free metal salts need to be screened for complex decom-

position temperatures in order to reduce the amount of thermal energy for SILP regeneration.

For pure ammonia adsorption purposes, other transition metal cations next to copper(II) can

be applied as other metal salts perform comparably well in ammonia complexation (see sec-

tions 2.3.2 and 5.3.1.2). A lot of potential exists in �nding the ideal metal salt, which desorbs

ammonia at low temperatures, but still has good ammonia capacity at adsorption conditions.

Additionally, multiple ammonia adsorption and desorption cycles need to be performed to

investigate long-term capacity of SILP materials.

5.3.3.2. Reversible adsorption of hydrogen sulfide

In this section, the applicability of SILP materials for reversible hydrogen sul�de adsorption

is investigated. Both, pressure and temperature swing adsorption modes are applied. Due

to their good adsorption performance, spherical carbon materials coated with cuprate ionic

liquids are assessed (see section 5.3.1.2).

SILP materials based on copper sulfate, copper octylsulfate and pure ionic liquid were pre-

pared according to standard SILP preparation procedures using water as solvent. Afterwards,

the copper sulfate based material was homogeneously sprinkled with an ammonia solution,

so that about 50% of copper species formed tetraamine complexes. Thiswas done on the back-

ground that sul�de complexes could more easily displace ammonia than from a fresh copper-

hydrogen sul�de complex. The released ammonia would instantly form a new copper-amine

complex. In the following experiment, SILP materials were loaded with hydrogen sul�de at

4 bar and 20°C until saturation. Then, the samples were continuously �ushed with air at 20°C

and, in a pressure swing desorption, the pressure was released to 1 bar. After no further des-

orption of hydrogen sul�de was detected, the temperature of the adsorber unit was slowly

increased to temperatures between 64-160°C. The results are summarized in table 5.28.

Spherical carbon coated with the ionic liquid compound alone, didn’t adsorb any noticeable

amounts of hydrogen sul�de. On the other hand, both copper-based SILP materials exhib-

ited signi�cant hydrogen sul�de adsorption capacities. The copper sulfate material adsorbed

about 2.4 times more hydrogen sul�de than the copper octylsulfate material. But, hydrogen

sul�de wasn’t desorbed at decreased pressure or increased temperature.

169


Table 5.28.: Hydrogen sul�de adsorption and desorption capabilities of spherical carbon

coated with [C4C1IM][n-C8H17OSO3] (αIL=0.3), [C4C1IM][n-C8H17OSO3]–

CuSO4-Cu(NH3)4SO4 1:0.7:0.7 (αIL=0.2) and [C4C1IM][n-C8H17OSO3]-

Cu[n-C8H17OSO3]2 1:1.3 (αIL=0.3); adsorption conditions: T=20°C, p=4 bar,

dadsorber=1.5 cm, h�lling=5 cm, vair=0.02 m s-1, c=1000 ppm until beginning

saturation, then c=5000 ppm until 90% saturation, rh=15%; pressure swing

desorption conditions: T=20°C, p=1 bar, vair=0.02 m s-1; temperature swing

desorption conditions: T=64-160°C, p=1 bar, vair=0.02 m s-1

SILP impregnation Hydrogensul�deadsorptioncapacity at 4 barand 20°C / wt%

Fraction ofdesorbedhydrogensul�de at 1 barand 20°C / %

Fraction ofdesorbedhydrogensul�de at 1 barand 64-160°C / %

[C4C1IM][n-C8H17OSO3]

(αIL=0.3)

< 0.1 - -


CuSO4-Cu(NH3)4SO4

1:0.7:0.7 (αIL=0.2)

17 0 0 (64°C)


Cu[n-C8H17OSO3]2 1:1.3

(αIL=0.3)

7 0 0 (160°C)

170


Compared to similar measurements with ammonia (see section 5.3.3.1), hydrogen sul�de ad-

sorption capacity was very large. Consequently, next to complex formationwith copper salts,

other sorptionmechanismswere likely present. Hydrogen sul�de is either irreversibly solved

within the ionic liquid melt in signi�cant quantities or is reduced to elemental sulfur. Addi-

tionally, ammonia from copper-amine complexes react with hydrogen sul�de. As pure ionic

liquid [C4C1IM][n-C8H17OSO3] didn’t adsorb any hydrogen sul�de, the metal salt appears to

be an indispensable component. The metal salts allows complex formation, in�uences acid-

ity of the system, contributes to water management and possibly catalyzes hydrogen sul�de

transformation reactions. Complex formation, directly depending on the number of metal

ions, is easily accounted for. Over-stoichiometrically adding metal salts to ionic liquids in-

creased acidity. This is counterproductive, because alkaline systems are expected to collude

better with weak acids like hydrogen sul�de. Water management is expected to be posi-

tively a�ected by hygroscopic metal salts, especially with hardly hygroscopic ionic liquids

at low relative humidity. Within an alkaline environment, hydrogen sul�de dissolves and

dissociates in water.

Surprisingly, at elevated temperatures up to 160°C, no hydrogen sul�de was released. Both,

copper complexes and otherwise stored hydrogen sul�de were thermally very stable. These

copper(II)-based SILP materials cannot be regenerated.

Alkaline ionic liquid impregnations without metal salts are an alternative approach towards

reversible hydrogen sul�de adsorption. As no metal species need to be thermally decom-

posed, these SILP systems are ideally regenerated with minimal energy input by pressure

reduction. These ionic liquids need to possess a su�ciently strong alkaline character and

suitable water management. At very low relative humidity, any ionic liquid that relies on

its hygroscopic nature is limited by water equilibrium between gas and liquid phases. Ionic

liquids with chemically combined water are more likely to succeed at low humidities. In

literature, hydrated ionic liquids were already presented as good absorber liquids for acidic

gases [214, 215]. Upon solidi�cation, these ionic liquids were said to release the absorbed

gases again. Nevertheless, these ionic liquids have never been applied to porous adsorbents.

Spherical carbon was coated with a variety of hydrate ionic liquids applying standard SILP

preparation procedures using water as solvent. These materials were characterized in hy-

drogen sul�de adsorption and desorption experiments. To verify the observations of Quinn

et al. [215], desorption experiments were also conducted at -45°C. Figure 5.37 demonstrates

the result of an initial hydrogen sul�de adsorption experiment at 50°C utilizing a metal-free

SILP material with the ionic liquid tetramethylammonium �uoride [(CH3)4N]F·(3H2O). Ta-

171


0 20 40 60010002000300040005000 H2S input H2S output

c H2S

/ p

pm

time / h

measurementinterruptionfor 32 h 0246810

H2S adsorbed

mad

s / g

Figure 5.37.: Hydrogen sul�de adsorption of spherical carbon coatedwith [(CH3)4N]F·(3H2O)

(αIL = 0.3; T = 50°C, p = 1 bar, rh = 15%, dadsorber = 2.35 cm, h�lling = 5 cm, vair =

0.02 m s-1, cH2S = 1000-5000 ppm)

ble 5.29 summarizes the results of adsorption and desorption experiments for SILP materials

with tetramethylammonium �uoride and tetramethylammonium silicate [(CH3)4N]OH-SiO2

1:1.[214, 215]

The hydrogen sul�de breakthrough experiment of spherical carbon loaded with [(CH3)4N]F·

(3H2O) didn’t show breakthrough within 24 h. Only after increasing the input concentration

of hydrogen sul�de from 1000 to 5000 ppm, breakthrough occurred after about 27 h. Still,

more hydrogen sul�de was adsorbed for about 35 h without approaching saturation. After an

interruption of the measurement for 32 h, analysis was continued and adsorption saturation

was reached after a total measurement time of around 67 h.

This novel SILP material adsorbed a very large amount of hydrogen sul�de at low relative

humidity. The investigations show that, in e�ect, alkaline hydrate ionic liquids coated to

spherical carbon are highly potential adsorber materials for acidic gases. Also, in further

adsorption measurements, the �uoride ionic liquid had an unprecedentedly large hydrogen

sul�de capacity of 80 wt%. No desorption was noticed by cooling the material down to -45°C

and heating to 150°C. The silicate ionic liquid also showed a large adsorption capacity. No

desorption occurred during pressure release and only 2% of hydrogen sul�de desorbed up to

160°C.

Compared to copper-based SILP materials, adsorption performance of these hydrate ionic

172


Table 5.29.: Hydrogen sul�de adsorption and desorption capabilities of spherical carbon

coated with [(CH3)4N]F(3H2O) (αIL=0.3) and [(CH3)4N]OH-SiO2 1:1 (αIL = 0.3);

adsorption conditions: T = 20°C, p = 4 bar, dadsorber = 1.5 cm, h�lling = 5 cm, vair =

0.02 m s-1, c = 1000 ppm until beginning saturation, then c = 5000 ppm until 90%

saturation, rh = 15%; pressure swing desorption conditions: T = 20°C, p = 1 bar,

vair = 0.02 m s-1; temperature swing desorption conditions: T = -45-160°C, p = 1

bar, vair = 0.02 m s-1

SILPimpregnation

Hydrogensul�deadsorptioncapacity at 4 barand 20°C / wt%

Fraction ofdesorbedhydrogensul�de at 1 barand 20°C / %

Fraction ofdesorbedhydrogensul�de at 1 barand -45-160°C /%

[(CH3)4N]F(3H2O)

(αIL=0.3)

80 n/a < 1 (-45-150°C)

[(CH3)4N]OH-SiO2

1:1 (αIL=0.3)

27 0 2 (160°C)

liquids is signi�cantly enhanced. Still, the SILP materials cannot be regenerated, probably

due to the same reasons why the copper based materials didn’t release hydrogen sul�de.

Either elemental sulfur is formed or an enhanced physisorption process is present. In the

second case, hydrogen sul�de dissociates and dissolves within the hydrate water and is sta-

bilized via strong hydrogen bridge bonding by the alkaline ionic liquids. It is also possible

that the anion of the ionic liquid is exchanged by a sul�de, while the corresponding acid is

formed. Additionally, presul�ded carbon materials are known to adsorb sulfur species [216].

In that case, the ionic liquid can have initiated the adsorption process.

As tetramethylammonium ionic liquids tend to emit amine odors, their olfactory neutral-

ity was examined, as well. Besides the two ionic liquids already tested, three additional

ionic liquids were examined that were proposed to absorb acidic gases in the original lit-

erature [214, 215]. These ionic liquids are tetramethylammonium hydroxide [(CH3)4N]OH,

tetramethylammonium acetate [(CH3)4N][CH3COO] and tetramethylammonium malonate

[(CH3)4N]2[C(COO)2]. Table 5.30 summarizes general information on olfactory neutrality

and availability of selected hydrate ionic liquids.

The �uoride and silicate ionic liquids exhibited a certain, non-pungent amine odor. This

amine odor was signi�cantly more pronounced for the hydroxide ionic liquid. The acetate

and malonate salts were olfactory neutral. So, depending on the anion, tetramethylammo-

173


Table 5.30.: Olfactory neutrality and availability of selected hydrate ionic liquids

Hydrate ionic liquid Olfactory neutrality Availability

[(CH3)4N]F(3H2O) No Commercial

[(CH3)4N]OH-SiO2 1:1 No Commercial

[(CH3)4N]OH No Commercial

[(CH3)4N][CH3COO] Yes Simple synthesis

[(CH3)4N]2[C(COO)2] Yes Simple synthesis

nium salts emitted amine odors or were olfactory neutral. The release of amine species can

be thwarted by an ammonia polishing �lter as described in section 5.3.2. Thus, slightly odor

emitting tetramethylammonium salts with large capacities for hydrogen sul�de can still be

applied in SILP materials. A few tetramethylammonium ionic liquids are commercially avail-

able, i.e. �uoride, silicate and hydroxide salts. The remaining ionic liquids were easily synthe-

sized by neutralizing tetramethylammonium hydroxide with the acid of the corresponding

anion.

In summary, it was demonstrated that good water management is essential to realize ad-

sorption of hazardous gases. SILP materials based on hydrate ionic liquids exhibit signi�cant

hydrogen sul�de adsorption capacities at low relative humidity. The tetramethylammonium

�uoride impregnation performed 4.7 times better than the tested copper sulfate impreg-

nation. Spectroscopic analysis of the absorbed species can verify the proposed absorption

mechanism of enhanced physisorption, which is hydrogen sul�de dissolving in the hydrate

water and forming hydrogen bonds with the alkaline ionic liquid.

Large gas adsorption capacities are also expected for other acidic gases like sulfur dioxide,

carbon dioxide, carbonyl sul�de, and hydrogen cyanide [214]. Contrary to literature reports,

hydrogen sul�de didn’t desorb from the alkaline ionic liquids. The SILP materials cannot

be regenerated. Possibly, in�uences arising from water management, acidity of the systems

or the carbon support are critical. While the current adsorption mechanism appears to be

irreversible, at alternative process conditions, the ionic liquid can potentially be regenerated

[214, 215]. The disadvantage of hydrate ionic liquids depending heavily on good water man-

agement was also an argument for Bates et al. to develop task-speci�c ionic liquids [217].

Nevertheless, the extraordinary adsorption capacities of hydrogen sul�de make SILP mate-

rials based on alkaline hydrate ionic liquids a well suitable polishing �lter. In case other

hazardous, acidic gases are also adsorbed, a good broadband �lter for acidic gases has been

174


found.

The copper(II)-based SILP materials perform acceptably well in hydrogen sul�de adsorption,

too. SILP regeneration wasn’t possible due to strong product stability. Less strongly inter-

acting metal salts can result in a reversible SILP adsorber. Therefore, di�erent metal salts

need to be analyzed regarding product dissociation and complex formation. Thus, a SILP

impregnation can be found that reversibly absorbs hydrogen sul�de in su�cient amounts.

Such a SILP �lter material has the potential to replace traditional gas scrubbing technologies.

175

6. Summary and Outlook

6.1. Summary / Abstract

Due to their unique material properties, polymer-based spherical activated carbons were

used as novel support materials for applications in catalysis and advanced gas puri�cation.

The prepared spherical carbon materials were characterized in detail and tested in model

systems. Also, the advantages of spherical carbon compared to conventional carbon pow-

der and inorganic support materials such as alumina and silica were evaluated. Figure 6.1

schematically depicts the main process steps concerning the investigations performed in this

thesis.

Filtration and pressure drop of spherical carbons

The e�ects resulting from the low pressure drop of spherical carbon beds in liquid-phase

processes were demonstrated in �ltration experiments and �uid dynamics calculations. The

signi�cant �ltration advantage of spherical carbons over carbon powder is due to increased

�ltration rates and, thus, decreased �ltration times. In comparison to carbon powder, �ltra-

tion time in a laboratory-scale �ltration setup decreased by a factor of 4.7 for 500 μmspherical

carbon. In a large-scale �ltration simulation, the decrease in �ltration time with increasing

particle size was signi�cant for particles up to 100 μm. For larger particles, �ltration time

didn’t further decrease due to the in�uence of membrane resistance. Furthermore, in �ow

chemistry applications, larger spherical carbon materials excel as �ow rates in e.g. catalyst

beds ideally increase quadratically with increasing particle diameter.


A variety of spherical carbon materials with di�erent particle size and pore structure were

applied as catalyst support material. Therefore, the carbon was oxidized with nitric acid

or sulfuric acid in a �rst process step. Afterwards, noble metals such as palladium, ruthe-

nium and platinum were deposited by ion adsorption and subsequent metal transformation

in hydrogen atmosphere.

176


Spherical carbon

materials

Filtration andpressure dropinvestigations

Carbon surface oxidation

Ionic liquid loading

Noble metal deposition

Application in catalysis

Application in gas purification

HNO3 vs. H2SO4:Influences on surface function-alization, surface properties and pore structure

Pd, Ru, Pt:Influences on metal loading, dispersion and

distribution

Pore diffusion investigations

Comparison with powder catalysts

Comparison with carbon powder

Test reactions: e.g. cinnamic acid, toluene,

cinnamaldehyde hydrogenations; dehydrogenation of a liquid organic hydrogen carrier

Comparison with powder catalysts

Halides in SILP: Influences on

metal corrosion

Preparation of halide-free and metal-free SILP

materials

Irreversible adsorption of

NH3, H2S, C6H12, and CH2O

Reversible adsorption of

NH3 and H2S by temperature and pressure swing

adsorption

Figure 6.1.: Functionalization and characterization of spherical carbons for application in

catalysis and gas puri�cation as investigated in this thesis

177


Treatment with nitric acid and sulfuric acid predominantly resulted in the formation of oxy-

gen functional groups at the carbon surface. The degree of surface functionalization varied

between 2-11 wt%, depending on acid type, acid concentration and temperature. With in-

creasing acid concentration and increasing temperature, the degree of functionalization gen-

erally increased. Carboxylic acid functional groups prevailed in case of nitric acid treated

samples. These samples also contained many strongly adsorbing groups like carbonyls and

quinones and a signi�cant amount of phenols and ethers. Sulfuric acid functionalized sam-

ples primarily consisted of carbonyls, quinones, and less strongly adsorbing carboxylic an-

hydrides. Concerning di�erent surface characterization methods, TPD-MS data generally

correlated well with titration measurements. Also, sulfuric acid treatment didn’t a�ect the

pore structure. Intensive functionalization with nitric acid, on the other hand, resulted in a

decrease in pore volume and surface area mainly due to heteroatom inclusion.

Carbon surface functionalization in�uences metal dispersion and catalytic activities. Surface

functionalization with sulfuric acid provided the most consistent results. Metal dispersion

and catalytic performance increased with increasing oxidation temperature and sulfuric acid

concentration due to an increasing number of strongly adsorbing surface oxides. In case of

the nitric acid treatments, similar trends in metal dispersion and catalytic activity weren’t

observable likely due to in�uences of a varying metal distribution by competitive adsorption

of metal ions to non-oxygen surface sites. So, in contrast to inorganic support materials,

the inner surface of spherical activated carbon can be speci�cally modi�ed to in�uence the

properties of the �nal catalyst with respect to metal dispersion, metal distribution, support-

substrate interactions and catalytic performance.

Particle size and pore structure of the spherical carbons a�ected catalytic activity due to mass

transport limitations within the pore system. With decreasing particle size and increasing

carbon activation, catalytic performance improved.

Palladiumwas completely deposited to oxidized spherical carbon for metal loadings between

2-10 wt%. The metal clusters of 5 wt% palladium catalyst appeared to be homogeneously

distributed over the cross section of carbon spheres. Palladium metal dispersions up to 63%

resulted. The spherical palladium catalysts performed well in the hydrogenation reaction

of cinnamic acid. Spherical palladium catalysts with smaller metal loadings exhibited larger

metal-speci�c catalytic activities. A 2 wt% palladium on 200 μm spherical carbon catalyst

achieved about 33% the e�ective reaction rate of the reference carbon powder catalyst. The

di�erence in catalytic activity was mainly due to di�usion limitation in the pore structure of

the applied spherical carbon.

178


Ruthenium was only partially deposited to spherical carbon due to equilibrium of di�er-

ent metal charge states during electrostatic ion adsorption. Ruthenium loadings between

0.31-5.54 wt% were realized. The highest catalytic activities in the hydrogenation of toluene

resulted for a spherical ruthenium catalyst with a low metal dispersion of 19%. Compared

to the ruthenium on carbon powder reference, 24% of the toluene turnover frequency was

reached, mainly due to pore di�usion limitation. In the hydrogenation reaction of 1-octene,

the spherical ruthenium catalysts outperformed the reference catalyst by a factor of 2.7 with

regard to the turnover frequency. For the ruthenium system, a �rst indication was found that

sterically demanding reactant molecules like thymol are not hydrogenated by the applied

spherical catalysts. Planar and linear molecules have been well suitable to be catalytically

transformed with spherical catalysts, though. This possible space con�nement e�ect of the

spherical carbon pore structure needs to be investigated in more detail. Another possible

cause for catalytic inactivity of this speci�c reaction can be the inhibition by chlorine/chlo-

ride originating from the ruthenium chloride precursor.

Remarkably, a prepared 0.5 wt% platinum on spherical carbon catalyst performed exception-

ally well in the dehydrogenation of the liquid organic hydrogen carrier H18-dibenzyltoluene,

being speci�cally optimized for this technical reaction. The spherical catalyst outperformed

two platinum reference catalysts based on carbon powder and alumina in terms of catalytic

dehydrogenation activity. After 120 min reaction time, a dehydrogenation degree of 94% was

reached, compared to 78 and 83%, respectively, for the two reference catalysts. Platinum was

also fully deposited to spherical carbon with a metal loading of 5 wt% and to about 84% in

case of a 10 wt% metal loading. Large platinum metal dispersions up to 51% were achieved.

The 0.5 and 5 wt% spherical platinum catalysts outperformed a commercial 5 wt% platinum

on carbon powder catalyst in the hydrogenation reaction of cinnamaldehyde by a factor of

3.2 and 1.4, respectively, with regard to the e�ective reaction rate constant.

In catalyst stability tests, a spherical palladium catalyst exhibited less palladium and sulfur

leaching than a palladium on carbon powder reference catalyst. Thus, spherical catalysts

are potentially well suitable for e.g. pharmaceutical applications in which product purity is

essential. In addition, spherical catalysts were successfully recycled and reused in multiple

cinnamic acid hydrogenation experiments. Also remarkably, a rhodium SILP catalyst based

on spherical carbon was prepared and successfully applied for multiple times in slurry-phase

hydrogenation experiments of 1,5-cyclooctadien with constantly large catalytic activity.

In summary, the concept of spherical carbon as catalyst support was successfully demon-

strated and applied to a number of test reactions. The advantages of spherical catalysts over

179


powder catalysts in material handling are obvious. Flow characteristics and material purity

are excellent. Compared to catalysts based on activated carbon powder, catalytic hydrogena-

tion activities were already signi�cant with e�ective reaction rate constants and turnover

frequencies reaching between 24-320% of the reference. The performance of the platinum

spherical catalysts was very remarkable. Also, with 1 kg of 5 wt% palladium on spherical

carbon, spherical catalysts were successfully produced on a larger scale.


Concerning the second �eld of application treated in this work, pure spherical carbon was

used as support material for ionic liquids yielding advanced gas adsorbents.

In this supported ionic liquid phase (SILP) technology, a thin layer of ionic liquid is deposited

onto the inner surface of a porous support. Due to the very low vapor pressure of ionic

liquids, the ionic liquids remain immobilized on the support. Contamination of product gas

feed with ionic liquid is e�ectively mitigated.

For these advanced gas adsorbents, di�erent reactive metal salts were dissolved in the ionic

liquids. Thus, the absolute capacity of gas adsorbents is signi�cantly enhanced, because the

volume of a liquid �lm is available for gas removal instead of a plain surface, only.

In this work, SILP materials based on spherical carbon were prepared by wet impregna-

tion and tested in the continuous removal of hazardous compounds from a gas stream. The

prepared materials were assessed based on adsorption capacity, broadband capability and

applicability. Two main areas of application with di�erent SILP development strategies were

investigated. Filter materials in clean rooms and for personal protection require �ne �ltration

capabilities for irreversible chemisorption of various trace contaminants. For larger quanti-

ties of contaminants in industrial �ltration processes, the ability to economically regenerate

�lter materials is most important. Thus, weaker interactions between the gas species and the

adsorbent are necessary.

The impregnation process was optimized by applying lower-boiling acetonitrile or water as

solvent in SILP preparation. Evaluating the performance of di�erent reactive metal salts,

irreversible ammonia adsorption is independent of the applied metal cation, i.e. Cu2+

and

Zn2+. SILP materials based on copper(II) instead of zinc salts were found to irreversibly

adsorb hydrogen sul�de with up to 65 times larger capacities. Furthermore, it was observed

that halide containing SILP materials were corrosive to aluminum and stainless steel due to

direct metal-ionic liquid contact and a potential dissociation of ionic liquids into corrosive

gases. As such materials are not applicable for personal protection and industrial adsorption

180


purposes, halide-free, metal-containing ionic liquids were developed.

A non-corrosive SILP material with a copper sulfate based ionic liquid melt exhibited excep-

tionally large ammonia capacities and independence on relative humidity. With ammonia

breakthrough times up to 740 min with an ionic liquid loading of 30 vol%, previously in the

literature determined ammonia breakthrough times of up to 519 min for copper(II) chloride

based systemswere exceeded by a factor of 1.4. Besides irreversible ammonia adsorption, suf-

�cient adsorption of acidic gases and inorganic gases meeting DIN EN 14387 ABEK1 criteria

was also given for the copper sulfate based SILP material with an ionic liquid loading of 20

vol%. But, organic gas capacity was lacking for full broadband capability. Also unfortunately,

SILP preparation turned out to be di�cult, with small amounts of copper sulfate precipitating

on the outside of spherical carbon. A dropwise impregnation process signi�cantly improved

SILP quality, almost completely preventing undesired precipitation of copper sulfate.

With copper octylsulfate and copper octanoate, two alternative copper salts have been syn-

thesized that readily deposited inside spherical carbon during wet impregnation. Compared

to the copper sulfate based system, ammonia adsorption capacities were decreased due to a

lower absolute number of copper species in these SILP materials by a factor of 3.2 and 3.9,

respectively. Independently, formaldehyde �lter development based on the SILP technology

was initiated. It was successfully demonstrated that formaldehyde reacts with highly hygro-

scopic choline salt impregnations. Besides the absolute number of reactive functional groups,

in�uential factors such as basicity and hygroscopicity of the �lter systems were found. For

the most alkaline choline hydroxide impregnation, for instance, formaldehyde capacity nor-

malized to the number of functional groups increased by a factor of 1.7 compared to a less

alkaline impregnation based on a mixture of choline octylsulfate and zinc sulfate.

For selected SILP materials, the reversible adsorption of ammonia and hydrogen sul�de was

also investigated in pressure and temperature swing adsorption modes. The tested SILP ma-

terials exhibited signi�cant capacities for reversible adsorption of ammonia. For a copper(II)

octylsulfate based material, for instance, 59% and 22% of ammonia desorbed by pressure

decrease from 4 to 1 bar and temperature increase from 20 to 160°C, respectively. Thus,

these SILP materials are not only suitable in �ne �ltration of trace contaminants, but also in

the bulk scrubbing of ammonia. The prepared SILP materials loaded with hydrogen sul�de

couldn’t be regenerated due to irreversible chemisorption. Finally, novel, metal-free SILP

�lter materials were produced with remarkably large hydrogen sul�de capacities of up to

80 wt% for a tetramethylammonium �uorid impregnation. Desorption of hydrogen sul�de

wasn’t possible due do irreversible chemisorption, though. These SILP materials are poten-

181


tially also well suitable for the adsorption of other acidic gases due to the general acid-base

interactions of dissolved acidic gases with these alkaline hydrate ionic liquids.

In summary, the SILP technology provides economically attractive �ne �ltration capabili-

ties by irreversible absorption/adsorption of a broad variety of gases. Until breakthrough,

SILP materials completely removed contaminants from the gas stream, i.e. generally below

the detection limit. Due to its carbonaceous nature and in contrast to inorganic supports,

the spherical carbon support provides additional adsorption capabilities for organic and in-

organic gases, thus bene�ting the development of broadband �lter materials. Furthermore,

SILP materials loaded with ammonia can be regenerated in pressure and temperature swing

adsorption processes allowing their application in industrial, bulk adsorption such as bio-

gas puri�cation. Compared to powder supports, spherical carbon materials exhibit very low

pressure drops, ideal for continuous gas-phase sorption processes.

182


6.2. Zusammenfassung / Abstrakt

Wegen ihrer einzigartigen Materialeigenschaften wurden Polymer-basierte, sphärische Ak-

tivkohlen als neuartiges Trägermaterial für Anwendungen in Katalyse und Gasreinigung

eingesetzt. Die hergestellten Materialien wurden im Detail charakterisiert und in Modellsys-

temen getestet. Auch wurden die Vorteile sphärischer Aktivkohle im Vergleich zu konven-

tioneller Pulverkohle und anorganischen Trägermaterialien wie Alumina und Silika nach-

gewiesen. Abbildung 6.2 zeigt schematisch die wichtigsten Punkte bezüglich der in dieser

Arbeit durchgeführten Untersuchungen.

Filtration und Druckverlust sphärischer Aktivkohle

Die E�ekte des niedrigenDruckverlustes von Festbetten sphärischer Aktivkohle in Flüssigpha-

sen-Prozessen wurden in Filtrationsexperimenten und strömungsmechanischen Berechnun-

gen aufgezeigt. Der signi�kante Filtriervorteil sphärischer Aktivkohle gegenüber Pulverkoh-

le zeigt sich in größeren Filtrationsraten und entsprechend kürzeren Filtrationszeiten. In ei-

nem Laborversuch war die Filtrationszeit für 500 μm sphärische Aktivkohle um den Faktor

4,7 kürzer als bei Pulverkohle. In der Simulation eines technischen Filtrationsprozesses nahm

die Filtrationsdauer mit zunehmender Partikelgröße bis 100 μm signi�kant ab. Für größere

Partikel verbesserte sich die Filtrationszeit wegen des Ein�usses des Membranwiderstandes

nicht. Des Weiteren zeichnen sich größere Partikel in kontinuierlichen Anwendungen aus,

da die Durch�ussraten in Katalysatorfestbetten, zum Beispiel, im Idealfall quadratisch mit

der Partikelgröße zunehmen.

Sphärische Aktivkohle als neuartiger Katalysatorträger

Eine Vielzahl sphärischer Aktivkohlen mit unterschiedlicher Partikelgröße und Porenstruk-

tur wurden als Katalysatorträger verwendet. Dafür wurde der Kohlensto� mit Salpetersäu-

re oder Schwefelsäure in einem ersten Prozessschritt oxidiert. Danach wurden Edelmetalle

wie Palladium, Ruthenium und Platin mittels Ionenadsorption abgeschieden. In einer an-

schließenden Metalltransformation in Wassersto�atmosphäre wurden die katalytisch akti-

ven Spezies gebildet.

Die Behandlung mit Salpetersäure und Schwefelsäure führte überwiegend zur Bildung von

funktionellen Sauersto�gruppen auf der Kohlensto�ober�äche. Der Grad der Ober�ächen-

funktionalisierung variierte zwischen 2-11 m%, in Abhängigkeit von der verwendeten Säure,

der Säurekonzentration und der Temperatur. Mit zunehmender Säurekonzentration und zu-

nehmender Temperatur nahm der Funktionalisierungsgrad der Ober�äche generell zu. Funk-

183


Sphärische Aktivkohle

Filtrations- und Druckverlust-

untersuchungen

Oxidieren der Kohlenstoff-oberfläche

Imprägnieren mit ionischen Flüssigkeiten

Abscheiden von Edelmetallen

Anwendung in der Katalyse

Anwendung in der Gasreinigung

HNO3 vs. H2SO4:Einflüsse auf die Eigenschaften und die Funktion-alisierung der Oberfläche und auf die Porenstruktur

Pd, Ru, Pt:Einflüsse auf die Metallbeladung, -dispersion und

-verteilung

Porendiffusions-untersuchungen

Vergleich mit Pulver-

katalysatoren

Vergleich mit Pulverkohle

Testreaktionen: z.B. Zimtsäure-,

Toluol- und Zimtaldehyd-hydrierungen; Dehydrierung

eines flüssigen, organischen Wasserstoff-

trägers

Vergleich mit Pulver-

katalysatoren

Halogenide im SILP: Einflüsse auf Metallkorrosivität

Präparation von Halogenid-freien und Metall-freien SILP Materialien

Irreversible Adsorption von NH3, H2S, C6H12,

und CH2O

Reversible Adsorption von

NH3 und H2S durch

Temperaur- und Druckwechsel-

adsorption

Abbildung 6.2.: Funktionalisierung und Charakterisierung sphärischer Aktivkohle für An-

wendungen in der Katalyse und in der Gasreinigung, wie in dieser Arbeit

untersucht

184


tionelle Carbonsäuregruppen waren vor allem in Salpetersäure-behandelten Proben vorzu-

�nden. Diese Proben enthielten auch viele stärker adsorbierende Gruppen wie Carbonyle

undChinone und eine großeAnzahl an Phenolen und Ethern. Schwefelsäure-funktionalisierte

Proben bestanden vorwiegend aus Carbonylen, Chinonen und weniger stark adsorbierenden

Carbonsäureanhydriden. Bezüglich verschiedener Ober�ächencharakterisierungsmethoden

korrelierten die TPD-MS Ergebnisse gutmit den Titrationsmessungen.Weiterhin beeinträch-

tigte die Schwefelsäurebehandlung die Porenstruktur nicht. Eine zu starke Funktionalisie-

rung mit Salpetersäure, führte zur Abnahme von Porenvolumen und Ober�äche durch Ein-

bau von Heteroatomen.

Die Funktionalisierung der Kohlensto�ober�äche hatte Ein�uss aufMetalldispersion und ka-

talytische Aktivität. Die Ober�ächenfunktionalisierung mit Schwefelsäure erzielte die kon-

sistentesten Ergebnisse. Aufgrund zunehmender Anzahl an stärker adsorbierenden Ober�ä-

chengruppen nahmen Metalldispersion und katalytische Aktivität mit zunehmender Oxida-

tionstemperatur und Schwefelsäurekonzentration zu. Im Fall der Salpetersäurebehandlun-

gen war ein ähnlicher Trend nicht zu beobachten, vermutlich wegen Ein�üssen variieren-

der Metallverteilungen durch konkurrierende Adsorption von Metallionen an fremde Ad-

sorptionsstellen. Folglich kann, im Gegensatz zu anorganischen Trägermaterialien, die in-

nere Ober�äche sphärischer Aktivkohle durch Oxidation spezi�sch angepasst werden, um

die Eigenschaften des Katalysators hinsichtlich Metalldispersion, Metallverteilung, Träger-

Substrat Wechselwirkungen und katalytischer Aktivität zu beein�ussen.

Partikelgröße und Porenstruktur der sphärischen Aktivkohlen beein�ussen die katalytische

Aktivität aufgrund von Sto�transportlimitierungen im Porensystem. Mit abnehmender Par-

tikelgröße und zunehmendemAktivierungsgrad der sphärischenAktivkohle verbesserte sich

die katalytische Aktivität.

Palladium wurde bei Metallbeladungen zwischen 2-10 m% vollständig auf die oxidierten,

sphärischen Aktivkohlen abgeschieden. Das Edelmetall eines 5 m% Palladium-Katalysators

schien homogen über denQuerschnitt der Kohlekügelchen verteilt zu sein. Palladium-Metall-

dispersionen von bis zu 63% wurden erzielt. Die sphärischen Palladium-Katalysatoren waren

in der Hydrierung von Zimtsäure aktiv. Sphärische Katalysatoren mit geringerer Palladium-

beladung zeigten größere, auf die eingesetzte Menge Edelmetall bezogene, katalytische Akti-

vitäten. Ein Katalysator mit 2 m% Palladium auf 200 μm sphärischer Aktivkohle erreichte ca.

33% der e�ektiven Geschwindigkeitskonstante des Referenz-Pulverkatalysators. Die Unter-

schiede in der katalytischen Aktivität beruhen hauptsächlich auf Sto�transportlimitierungen

im Porensystem der verwendeten sphärischen Aktivkohle.

185


Ruthenium wurde nur teilweise abgeschieden. Die Ursache dafür ist ein Gleichgewicht ver-

schiedener Ladungszustände des Metalls während der elektrostatischen Ionenadsorption.

Ruthenium-Beladungen zwischen 0,31-5,54 m% wurden realisiert. Die größte katalytische

Aktivität in derHydrierung vonToluolwurde von einem sphärischen Ruthenium-Katalysator

mit einer sehr niedrigenMetalldispersion von 19% erzielt. Im Vergleich zumReferenz-Pulver-

kohle-Katalysator wurden nur 24% der TOF (turn over frequency) für die Toluolhydrierung

erreicht, hauptsächlich aufgrund von Sto�transportlimitierung. In der Hydrierreaktion von

1-Okten war der sphärische Katalysator um den Faktor 2,7, hinsichtlich der TOF, besser als

die Referenz. Für das Ruthenium-System ergaben sich erste Anzeichen, dass möglicherweise

sterisch anspruchsvolle Moleküle wie Thymol nicht von diesen sphärischen Katalysatoren

hydriert werden. Planare und lineare Moleküle konnten hingegen gut in sphärischen Kataly-

satoren umgesetzt werden. Dieser E�ekt räumlicher Limitierung muss noch im Detail unter-

sucht werden. Eine andere Ursache für die katalytische Inaktivität dieser spezi�schen Reak-

tion kann auch eine Inhibierung durch Chlor/Chlorid sein, welches vom Rutheniumchlorid-

Precursor stammt.

Bemerkenswerterweise war ein präparierter 0,5 m% Platin-Katalysator auf Basis sphärischer

Aktivkohle besonders leistungsfähig in der Dehydrierung des �üssigen, organische Wasser-

sto�trägers H18-Dibenzyltoluol, nachdem er speziell für diese technische Reaktion optimiert

wurde. Der sphärische Katalysator übertraf zwei kommerzielle Katalysatoren basierend auf

Pulverkohle und Alumina hinsichtlich katalytischer Dehydrieraktivität. Nach 120 min Reak-

tionszeit wurde ein Dehydriergrad von 94% erreicht, im Vergleich zu 78% bzw. 83% für die

beiden Referenz-Katalysatoren. Platin wurde auch vollständig bei einer Beladung von 5 m%

und zu 84% bei einer Zielbeladung von 10 m% abgeschieden. Große Metalldispersionen von

bis zu 51% wurden erzielt. Die 0,5 und 5 m% sphärischen Platin-Katalysatoren übertrafen ei-

nen kommerziellen Pulverkohle-Katalysator in der Hydrierung von Zimtaldehyd um einen

Faktor von 3,2 bzw. 1,4, bezogen auf die e�ektive Reaktionsgeschwindigkeitskonstante.

In Stabilitätstests zeigte ein sphärischer Palladium-Katalysatorweniger Palladium- und Schwe-

felleaching als ein kommerzieller Pulverkohle-Katalysator. Deshalb sind sphärische Kataly-

satoren potenziell gut geeignet für z.B. pharmazeutische Anwendungen in denen Produkt-

reinheit essentiell ist. Zusätzlich wurden sphärische Katalysatoren mehrmals erfolgreich in

Hydrierexperimenten mit Zimtsäure wiederverwendet. Auch bemerkenswert ist, dass ein

Rhodium-SILP-Katalysator auf Basis sphärischer Aktivkohle mehrmals in Slurry-Phasen Hy-

drierexperimenten von 1,5-Cyclooctadien mit anhaltend großen katalytischen Aktivitäten

eingesetzt wurde.

186


Zusammenfassend konnte das Konzept sphärischer Aktivkohlen als Katalysatorträger er-

folgreich demonstriert und in einer Vielzahl von Testreaktionen angewendet werden. Die

Vorteile sphärischer Katalysatoren in der Handhabung sind o�ensichtlich. Die Strömungsei-

genschaften und die Reinheit des Materials sind exzellent. Im Vergleich zu Katalysatoren auf

Basis von Pulverkohle waren die katalytischen Hydrieraktivitäten gut bis hervorragend mit

e�ektiven Reaktionsgeschwindigkeitskonstanten und TOFs zwischen 24-320% der Referenz.

Auch wurden sphärische 5 m% Palladium-Katalysatoren bereits im größeren 1 kg Maßstab

produziert.

Sphärische Aktivkohle als Träger für SILP Materialien

Bezüglich des zweiten, in dieser Arbeit behandelten Anwendungsgebiets wurde unbehan-

delte, sphärische Aktivkohle als Trägermaterial für ionische Flüssigkeiten verwendet.

Bei dieser „Supported Ionic Liquid Phase (SILP)“ Technologie wird ein dünner Film ionischer

Flüssigkeit auf die innere Ober�äche eines porösen Trägers aufgebracht. Wegen des sehr

geringen Dampfdruckes ionischer Flüssigkeiten, verbleiben diese dauerhaft auf dem Träger.

Ein Austrag in die Gasphase ist damit ausgeschlossen.

Für diese SILP Materialien wurden reaktive Metallsalze in der ionischen Flüssigkeit gelöst.

Damit ließ sich die absolute Kapazität der Adsorbentien signi�kant verbessern, weil ein �üs-

siger Film in seinem Volumen mehr Gas binden kann als eine Ober�äche.

In dieser Arbeit wurden SILP Materialien basierend auf sphärischer Aktivkohle mittels Nas-

simprägnierung präpariert und in der kontinuierlichen Adsorption gesundheitsschädlicher

Moleküle aus der Gasphase getestet. Die präparierten Materialien wurden nach ihrer Ad-

sorptionskapazität, ihrer Breitbandigkeit für verschiedene Gase und ihrer Anwendbarkeit

hin beurteilt. Zwei Hauptanwendungen mit unterschiedlichen SILP Entwicklungsstrategien

wurden untersucht. Filtermaterialien für Reinräume und den Personenschutz erfordern Fä-

higkeiten zur Fein�ltration durch irreversible Chemisorption verschiedener, niedrig konzen-

trierter Schadgase. Für größere Mengen an Schadgasen in industriellen Filterprozessen ist es

wichtig, die Filtermaterialien ökonomisch regenerieren zu können. Deshalb sind schwächere

Wechselwirkungen zwischen Schadgas und Adsorbenz notwendig.

Der Imprägnierprozess während der SILP Präparationwurde durch den Einsatz von niedriger

siedenden Acetonitril oderWasser als Lösungsmittel optimiert. Bei der Beurteilung verschie-

dener reaktiver Metallsalze erschien die Ammoniakadsorption unabhängig vom Metallkati-

on, Cu2+

bzw. Zn2+, gleich gut zu sein. SILP Materialien auf der Basis von Kupfer(II)-Salzen

konnten Schwefelwassersto� mit 65 mal größerer Kapazität als Zinksalze irreversibel adsor-

187


bieren. Weiterhin wurde beobachtet, dass Halogenid-haltige SILP Materialien Aluminium

und Edelstahl, durch direkten Kontakt zwischen Metall und ionischer Flüssigkeit und eine

mögliche Dissoziation der ionischen Flüssigkeiten in korrosive Gase, korrodieren. Da solche

Materialien weder im Personenschutz noch für industrielle Adsorptionsprozesse anwendbar

sind, wurden Halogenid-freie, metallhaltige ionische Flüssigkeiten entwickelt.

Ein nicht-korrosives SILP Material mit einer Kupfersulfat-basierten Salzschmelze zeigte her-

ausragende Ammoniakkapazitäten, unabhängig von der relativen Feuchte. Mit Ammoniak-

Durchbruchzeiten von bis zu 740 min bei einem Porenfüllgrad an ionischer Flüssigkeit von

30 vol% wurden bisher in der Literatur bestimmte Durchbruchzeiten (519 min) für ein Kup-

fer(II)chlorid basiertes System um einen Faktor 1,4 übertro�en. Neben der irreversiblen Am-

moniakadsorption war eine ausreichende Adsorption von sauren und anorganischen Gasen

nach DIN EN 14387 ABEK1 bei einem Porenfüllgrad an ionischer Flüssigkeit von 20 vol%

gegeben. Für eine vollständige Breitbandigkeit dieses Filtermaterials war die Kapazität für

organische Gase allerdings nicht ausreichend. Unglücklicherweise gestaltete sich auch die

SILP Präparation schwierig, da eine kleine Menge Kupfersulfat auf der äußeren Ober�äche

der Kohlekügelchen ausfällt. Eine tröpfchenweise Imprägnierung verbesserte die SILP Qua-

lität deutlich.

Mit Kupfer(II)octylsulfat und Kupfer(II)octanoat wurden zwei alternative Kupfersalze syn-

thetisiert. Diese konnten, in ionischer Flüssigkeit gelöst, problemlos mittels Nassimprägnie-

rung im Porensystem der sphärischen Aktivkohle abgeschieden werden. Im Vergleich zum

Kupfersulfat-System waren die Ammoniakkapazitäten um den Faktor 3,2 bzw. 3,9 geringer,

weil eine geringere absolute Anzahl an Kupfer-Ionen im SILP Material vorlag. Unabhän-

gig davon wurde ein SILP Material zur Adsorption von Formaldehyd entwickelt. Es konnte

gezeigt werden, dass Formaldehyd mit sehr hygroskopischen Cholin-Salzimprägnierungen

reagiert. Neben der absoluten Zahl an reaktiven, funktionellen Gruppen, wurden die Ba-

sizität und die Hygroskopizität des Filtersystems als wichtige Größen ausgemacht. Für die

am stärksten alkalische Cholinhydroxid-Imprägnierung, zum Beispiel, war die Formaldehyd-

Kapazität, normiert auf die Anzahl funktioneller Gruppen, um den Faktor 1,7 größer als die

für die weniger alkalische Imprägnierung auf Basis einer Mischung von Cholinoctylsulfat

und Zinksulfat.

Für ausgewählte SILP Materialien wurde auch die reversible Adsorption von Ammoniak und

Schwefelwassersto� in Druck- und Temperaturwechseladsorptionen untersucht. Die SILP

Materialien wiesen signi�kante Kapazitäten für die reversible Adsorption von Ammoniak

auf. Für ein Kuper(II)octylsulfat basiertes Material, zum Beispiel, desorbierten 59% bzw. 22%

188


Ammoniak durch Druckerniedrigung von 4 auf 1 bar bzw. Temperaturerhöhung von 20 auf

160°C. Damit sind diese SILPMaterialien nicht nur für die Fein�ltration vonGasspuren geeig-

net, sondern auch für die großtechnische, reversible Adsorption von Ammoniak. Mit Schwe-

felwassersto� beladene SILP Materialien konnten aufgrund von irreversibler Chemisorption

nicht regeneriert werden. Schließlich wurden auch neuartige, Metall-freie SILP Materialien

präpariert. Diese zeigten eine bemerkenswert große Schwefelwassersto�kapazität von bis zu

80m% für eine Tetramethylammonium�uorid-Imprägnierung. Die Adsorptionwar allerdings

nicht reversibel. Wegen allgemeiner Säure-Base-Wechselwirkungen gelöster, saurer Gasen

mit diesen alkalischen ionischen Flüssigkeiten sind diese SILP Materialien möglicherweise

auch für die Adsorption anderer saurer Gase geeignet.

Zusammenfassend ermöglicht die SILP Technologie auf Basis sphärischer Aktivkohle ökono-

misch attraktive Fein�lterkapazitäten durch irreversible Absorption/Adsorption einer Band-

breite an Gasen. Bis zum Durchbruch der zu �lternden Gase entfernen die SILP Materialien

diese vollständig aus dem Gasstrom, d.h. bis unterhalb der Detektionsgrenze. Im Gegensatz

zu anorganischen Trägern, bietet die Kohlensto�ober�äche der sphärischen Aktivkohle zu-

sätzliche Adsorptionskapazitäten für organische und anorganische Gase und begünstigt da-

mit die Entwicklung von Breitband-Filtermaterialien. Des Weiteren konnten mit Ammoniak

beladene SILP Materialien durch Druck- und Temperaturwechsel regeneriert werden, was

ihren großtechnischen Einsatz zum Beispiel in der Biogasreinigung ermöglicht. Im Vergleich

zu pulverförmigen Trägermaterialien weisen die sphärischen Adsorbentien für den Einsatz

in kontinuierlichen Gasphasen-Prozessen ideale, sehr niedrige Druckverluste auf.

189


6.3. Outlook

Catalysts based on spherical carbon

Concerning catalyst material design, the biggest potential in performance optimization likely

is improving mass transport in the pore system of the spherical catalysts. Furthermore, the

preparation process needs to be optimized to allow for economical large-scale catalyst pro-

duction.

In order to investigate possible space con�nement e�ects in more detail, the distribution of

metal clusters in the pore system can be determined. The metal cluster should ideally be

located within the larger pores [218]. Also, to provide access of large reactant molecules to

catalytically active sites, spherical carbons with larger pores might need to be developed.

For scale-up investigations, the entire catalyst preparation procedure needs to be re-evaluated.

The aim is to reduce costs by e.g. reducing the number of preparation steps, the duration of

each step, the amount of solvents and the extent of drying. In order to ease the production

of larger quantities of spherical catalyst material, a wet chemical reduction process can also

be established.

Gas adsorbents based on spherical carbon

Concerning SILP preparation, optimizations regarding ionic liquid amount and composition

can further improve the performance of �lter materials for irreversible, �ne �ltration and re-

versible, bulk �ltration applications. Additionally, alternative impregnation processes need

to be investigated to produce SILP materials from aqueous impregnation solutions at an in-

dustrial scale.

For improved �ne �ltration in clean rooms and for personal protection, the next steps include

optimization of ionic liquid loading and ionic liquid composition to facilitate irreversible

broadband �lters that meet DIN EN 14387 ABEK1 requirements.

Regarding regenerative SILP materials for industrial applications, metal amine complex de-

composition temperatures can be screened to �nd reactive metal salts for energetically ef-

�cient temperature swing adsorption operations. For a regenerative hydrogen sul�de �lter,

utilizing a less reactive metal salt should be tested. Metal complexes based on transition

metals like zinc and cadmium exhibit signi�cantly lower dissociation constants than cop-

per salts [146]. Possibly, regeneration can also be realized without change in temperature

or pressure by introducing oxidizing gases during reactor �ushing. Furthermore, copper(I)

salts previously showed good regeneration performance [219].

190


Aqueous impregnation solutions, e.g. containing copper sulfate, can likely be successfully

applied in SILP preparations by using oxidized spherical carbon with better wettability, for

instance. Alternatively, other impregnation techniques such as spin or spray coating need to

be investigated.

191

Appendix

I

A. Lists of chemical substances andsuppliers

Table A.1.: Chemicals for catalyst preparation

Chemical Short Purity Origin

Nitric acid solution HNO3(aq)

65% Merck

Sulfuric acid solution H2SO4(aq)50% Applichem

Palladium chloride PdCl2 99.9% Alfa Aesar

Hydrochloric acid standard

volumetric solution

HCl(aq)

1 mol l-1

Applichem

Ruthenium chloride

hydrate

RuCl3(H2O) 99.9% Merck

Dihydrogen

hexachloroplatinate

hexahydrate

H2PtCl6(6H2O) 99.9% Alfa Aesar

(Acetylacetonato)dicar-

bonylrhodium(I)

Rh(CO)2(C5H7O2) 98% Sigma Aldrich

Sodium TPPTS P(C6H4SO3Na)3

1-

Butyl-3-methylimidazolium

hexa�uorophosphate

[C4C1IM][PF6] For synthesis Merck

Hydrogen gas H2 Linde

Nitrogen gas N2 5.0 Linde

II

A. Lists of chemical substances and suppliers

Table A.2.: Chemicals for catalyst application


Palladium (5 wt%) on

activated carbon powder

(75992)

Pd/C Sigma Aldrich

trans-Cinnamic acid C9H8O2 >99% Alfa Aesar

Ethanol C2H6O Technical

Ethan-1,2-diol C2H6O2 For analysis Merck

Platinum (5 wt%) on


(205931)

Pt/C Sigma Aldrich

trans-Cinnamaldehyde C9H8O For synthesis Merck

Tetrahydrofuran C4H8O For analysis Merck

Ruthenium (5 wt%) on


(11748.06)

Ru/C Alfa Aesar

Toluene C7H8 BASF

1-Octene C8H16 For synthesis Merck

m-Cresol C7H8O For synthesis Merck

Thymol C10H14O >98% Alfa Aesar

Cyclohexane C6H12 BASF

1.5-Cyclooctadiene C8H12 >99% Sigma Aldrich

n-Heptane C7H16 For synthesis Merck

Potassium permanganate KMnO4 For analysis Merck

III


Table A.3.: Chemicals for SILP preparation and SILP application


1-Ethyl-3-methyl

imidazolium chloride

[C2C1IM]Cl For synthesis Merck

1-Ethyl-3-methyl

imidazolium bromide

[C2C1IM]Br For synthesis Merck

1-Butyl-3-methyl

imidazolium n-octylsulfate

[C4C1IM][n-C8H17OSO3]For synthesis Merck

Tetramethyl ammonium

�uoride hydrate

[(CH3)4N]F(3H2O) 98% Alfa Aesar


hydroxide solution

[(CH3)4N]OH (aq)25% Alfa Aesar


silicate solution

[(CH3)4N]OH-SiO2(aq)15-20% Sigma Aldrich

Copper chloride CuCl2 For synthesis Merck

Copper bromide CuBr2 99% Alfa Aesar

Zinc bromide ZnBr2 99.9% Alfa Aesar

Copper sulfate

pentahydrate

CuSO4(5H2O) 99% Alfa Aesar

Acetonitrile C2H3N For analysis Merck

Dichloromethane CH2Cl2 For analysis Merck

IV


Table A.4.: Chemicals for SILP preparation and SILP application


Sodium n-octylsulfate Na[n-C8H17OSO3] 99% Alfa Aesar

Copper carbonate basic CuCO3 Cu(OH)2 Acros Organics

Amberlite Ion Exchanger

IR-120

Merck

Chlorosulfuric acid HSO3Cl For synthesis Merck

1-Octanol C8H18O 99% Alfa Aesar

Octanoic acid C8H16O2 >98% Alfa Aesar

Malonic acid C3H4O4 For synthesis Merck

Acetic acid C2H4O2 For analysis Merck

Ammonia standard

volumetric solution

NH3(aq)

1 mol l-1

Applichem

Ammonia gas NH3 3.8 Linde

Formaldehyde solution,

phosphate bu�ered

CH2O(aq)4% Applichem

V

B. Carbon surface functionalization

B.1. Influences on the amount of functional surfacegroups

Table B.1.: Quantitative surface analysis of oxidized spherical carbon (content of water and

amount of other volatile components excluding water content)

Oxidationparameters

Surface-spec.acid molarity* /10-5 mol m-2

Watercontent /wt%

Volatilecomponents/ wt%

Volatilecomponents(TPD-MS) /wt%

15% HNO3 @ RT

0.4

4.6 4.9 10.0

(reproduction) 0.5 6.5 -

36% HNO3 @ RT

0.9

1.3 1.9 2.3

(reproduction) 1.1 2.8 -

15% HNO3 @

90°C

0.4 4.8 8.4 10.6

36% HNO3 @

90°C

0.9 3.5 8.0 10.7

25% H2SO4 @ RT 0.6 0 2.6 5.8

50% H2SO4 @ RT 1.2 0.4 2.2 7.7

25% H2SO4 @

90°C

0.6 0 3.0 10.0

50% H2SO4 @

90°C

1.2 0 3.1 8.5

* The surface-speci�c acid molarity describes the number of acid molecules in solution with

regard to the absolute carbon BET surface area.

VI


B.2. Influences on the chemical composition offunctional surface groups

Table B.2.: Ratio of weakly adsorbing surface oxides desorbing between (100-500°C) and

strongly adsorbing oxides desorbing between (500-1000°C)

Acid Acid con-centration /%

Oxidationtempera-ture /°C

Ratio of strongly toweakly adsorbinggroups releasing CO

Ratio of strongly toweakly adsorbinggroups releasingCO2

HNO3

15

RT 3.1 0.4

90 3.0 0.3

36

RT 9.3 1.5

90 3.3 0.3

H2SO4

25

RT 2.0 1.8

90 2.0 2.1

50

RT 1.2 3.8

90 2.9 2.4

VII


B.3. Influences on spherical carbon acidity

Acid Acid concentration / % Oxidation temperture / °C pH (PZC) / -

No oxidation 9.1

HNO3

15

RT 3.6

90 4.2

36

RT 6.4

90 4.4

H2SO4

25

RT 3.4

90 3.4

50

RT 3.3

90 3.3

Table B.3.: Point of zero charge pH values of functionalized and non-functionalized spherical

carbon

VIII


B.4. Influences on the pore structure of spherical carbon

Table B.4.: BET surface area, total pore volume, andmicropore volume of di�erently oxidized

spherical carbon and deviations from values of pure spherical carbon

AcidAcidconc./ %

Ox.temp./ °C

MP-BET /m2 g-1

ΔMP-BET /%

Vtot /cm3 g-1

Δ

Vtot /%

Vmic /cm3 g-1

ΔVmic/ %

No oxidation 3047 2.25 1.23

HNO3

15

RT 2138 -30 1.54 -31 0.79 -30

90 2570 -16 1.94 -13 0.89 -21

36

RT 2950 -3 2.21 -1 1.00 -12

90 2559 -16 1.83 -18 0.93 -18

H2SO4

25

RT 2724 -11 1.98 -12 1.00 -11

90 2793 -8 2.05 -9 1.04 -8

50

RT 2971 -2 2.33 4 1.00 -11

90 2799 1* 2.03 -3* 1.04 0*

No oxidation* 2780 2.10 1.04

* di�erent batch of spherical carbon

IX

C. Catalytic experiments

C.1. Catalytic experiments concerning carbon porestructure

Table C.1.: E�ective reaction rate constants and palladium metal dispersions of spherical 5

wt% palladium catalysts with di�erent particle size and carbon activation

Particle size / μm Vtot / cm3 g-1 ke� / m3 kgPd-1 s-1 DPd / %

500

0.61 0.027 30

1.18 0.034 n/a

1.19* 0.045 34

200

0.69 0.033 28

0.95 0.065 32

1.13 0.075 37

1.67 0.091 37

50

0.61 0.056 32

0.89 0.087 34

1.07 0.096 38

* spherical carbon with enhanced mesoporous pore structure

X


0 50 100 150020406080100

mod / s kgPd m-3

X /

%

Vtot = 1.19 cm3 g-1 (meso) Vtot = 1.18 cm3 g-1 Vtot = 0.61 cm3 g-1 fits according to model 0 50 100020406080100

mod / s kgPd m-3

X /

%

Vtot = 1.67 cm3 g-1 Vtot = 1.13 cm3 g-1 Vtot = 0.95 cm3 g-1 Vtot = 0.69 cm3 g-1 fits according to model

(a) (b)

0 50 100020406080100

mod / s kgPd m-3

X /

%

Vtot = 1.07 cm3 g-1 Vtot = 0.89 cm3 g-1 Vtot = 0.61 cm3 g-1 fits according to model

(c)

Figure C.1.: Hydrogenation of cinnamic acid using 500 μm (a), 200 μm (b) and 50 μm (c) spher-

ical 5 wt% palladium catalysts and di�erent carbon activations; �ts according to

model applying equation 2.9 (T = 40°C, pH2 = 30 bar, c0, cinnamic acid = 236 mol m-3,

mcatalyst = 0.5 g)

XI


C.2. Catalytic experiments concerning carbon surfacefunctionalization

0 50 100 150020406080100

15% HNO3 at RT 36% HNO3 at RT 15% HNO3 at 90°C 36% HNO3 at 90°C fits according to model mod / s kgPd m-3

X /

%

0 50 100 150 200 250 300020406080100

mod / s kgPd m-3

X /

%

25% H2SO4 at RT 50% H2SO4 at RT 25% H2SO4 at 90°C 50% H2SO4 at 90°C fits according to model

(a) (b)

Figure C.2.: Hydrogenation of cinnamic acid using catalysts with 5 wt% palladium on di�er-

ently oxidized spherical carbon: (a) nitric acid and(b) sulfuric acid oxidation; �ts

according to model applying equation 2.9 (T = 40°C, pH2 = 5 bar, c0, cinnamic acid =

236 mol m-3, mcatalyst = 0.5 g)

XII


Table C.2.: E�ective reaction rate constants of cinnamic acid hydrogenations and palladium

metal dispersions of di�erently oxidized spherical 5 wt% palladium catalysts

Acid Acid con-centration /%

Oxidationtempera-ture /°C

ke� / m3 s-1 kgPd-1 DPd / %

HNO3

15

RT 0.0158 23

90 0.0188 50

36

RT 0.0287 37

90 0.0175 55

H2SO4

25

RT 0.0083 58

90 0.0148 31

50

RT 0.0172 57

90 0.0314 63

XIII

D. List of Algorithms

2.1. Carbon gasi�cation by steam, carbon dioxide and air [2] . . . . . . . . . . . 7

2.2. Reaction pathways of palladate deposition on graphitic carbon [97] . . . . . 35

2.3. Hydrogenation of cinnamic acid . . . . . . . . . . . . . . . . . . . . . . . . . 36

2.4. Hydrogenation of cinnamaldehyde . . . . . . . . . . . . . . . . . . . . . . . 37

2.5. Hydrogenation of 1,5-cyclooctadiene . . . . . . . . . . . . . . . . . . . . . . 39

2.6. (De-)hydrogenation of Marlotherm SH . . . . . . . . . . . . . . . . . . . . . 39

2.7. Ammonia chemisorption by amine-complex formation with metal cations . 46

2.8. Hydrogen sul�de chemisorption by sul�de-complex formation with metal

cations [147] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

2.9. Carbon dioxide chemisorption in amino-functional ionic liquids [15, 165]

(Reprinted with permission from [15]. Copyright 2012 American Chemical

Society) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

4.1. Synthesis of copper(II) octylsulfate by cation exchange of the octylsulfate salt

(1) and neutralization (2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

4.2. Synthesis of copper(II) octylsulfate by sulfatization of 1-octanol (1) and neu-

tralization (2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

4.3. Synthesis of copper(II) octanoate by neutralization of octanoic acid . . . . . 76

XIV

E. List of Figures

2.1. Schematic structure of activated carbons ([22], modi�ed) . . . . . . . . . . . 8

2.2. Pore size distributions of physically activated carbons derived from coconut

shell and coal ([2], modi�ed) . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.3. Production sequence of polymer-based activated carbon particulates . . . . 10

2.4. General molecular structures of polymer resins: phenol-formaldehyde resin

(a), resorcinol-formaldehyde resin (b), polystyrene-divinylbenzene copoly-

mer (c), acrylonitril-divinylbenzene copolymer (d) . . . . . . . . . . . . . . . 12

2.5. Lightmicroscopy images of di�erently sized polystyrene-divinylbenzene based

spherical activated carbons ([5], modi�ed) . . . . . . . . . . . . . . . . . . . 14

2.6. Schematic pore system of activated carbon catalysts . . . . . . . . . . . . . . 16

2.7. Types of oxygen functional groups on activated carbon surfaces ([8], modi�ed) 25

2.8. Adsorption of metal anions to protonated carbon surface groups by electro-

static interactions; C-π sites for alternative metal adsorption ([8], modi�ed) . 30

2.9. Adsorption of carbon monoxide to metal surfaces: linear (a), bridged (b & c),

triple bond (d), dissociative bonding (e) ([104], modi�ed) . . . . . . . . . . . 32

2.10. Metal distributions in catalyst particulates: uniform (a), egg-shell (b), egg-

white (c), egg-yolk (d) ([108], modi�ed) . . . . . . . . . . . . . . . . . . . . . 33

2.11. Substrates for ruthenium catalyst screening - molecule structures of toluene,

m-cresol, thymol, and 1-octene . . . . . . . . . . . . . . . . . . . . . . . . . . 38

2.12. Typical breakthrough curve of a continuous breakthrough experiment: (1)

exemplary breakthrough point, (2) curve in�ection point, (3) saturation point 43

2.13. SILP concept ([16] - adapted by permission of The Royal Society of Chemistry) 52

2.14. Normalized breakthrough curves of n-butyl mercaptan using di�erent SILP

materials; T = 90°C, p = 1.05 bar, mSILP = 10.6 g, h�lling = 9 cm, VN2 = 40 mlN

min-1, Vn-butyl mercaptan = 0.2 ml min

-1(adapted from Ref. [11] with permission

from The Royal Society of Chemistry) . . . . . . . . . . . . . . . . . . . . . . 55

XV

E. List of Figures

2.15. Normalized ammonia breakthrough curves of SILP materials based on spher-

ical carbon with chlorocuprate ionic liquids (xCuCl2 = 0.5, αIL = 0.4) and a cop-

per chloride impregnation as reference; T = 303 K, p = 1.21 bar, rh = 85%,

h�lling = 2 cm, VN2 = 325.6 mlN min-1, VNH3 = 0.33 mlN min

-1([16] - adapted

by permission of The Royal Society of Chemistry) . . . . . . . . . . . . . . . 56

2.16. Comparison of absorbed and desorbed amount of ammonia for the SILP ab-

sorber material coated with [C2C1IM]Cl-CuCl2 (xCuCl2 = 0.57, αIL = 0.2); p =

1.21 bar, h�lling = 2 cm, VN2 = 325.6 mlN min-1, VNH3 = 0.33 mlN min

-1([16] -

adapted by permission of The Royal Society of Chemistry) . . . . . . . . . . 57

4.1. Gas-phase test rig for continuous removal of ammonia ([16] - adapted by

permission of The Royal Society of Chemistry) . . . . . . . . . . . . . . . . . 72

5.1. Timeline of �ltration experiments applying spherical carbon materials and a

commercial powder catalyst (1 g material, 200 ml water, membrane with 1

μm particle retention, 0.5 bar excess pressure) . . . . . . . . . . . . . . . . . 80

5.2. In�uence of spherical carbon particle size on �ltration rate (a) and �ltration

time (b) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

5.3. In�uence of spherical carbon particle size on �uid �ux (Rm = 0) . . . . . . . . 83

5.4. Quantitative surface analyses of oxidized spherical carbon (amount of volatile

components excluding water content) . . . . . . . . . . . . . . . . . . . . . . 85

5.5. TPD-MS spectra of nitric acid treated spherical carbon with 15% HNO3 @RT

(a), 36% HNO3 @ RT (b), 15% HNO3 @ 90°C (c), and 36% HNO3 @ 90°C (d) . 87

5.6. TPD-MS spectra of sulfuric acid treated spherical carbon with 25% H2SO4 @

RT (a), 50% H2SO4 @ RT (b), 25% H2SO4 @ 90°C (c), and 50% H2SO4 @ 90°C (d) 88

5.7. Ratio of weakly adsorbing surface oxides desorbing between (100-500°C) and

strongly adsorbing oxides desorbing between (500-1000°C) . . . . . . . . . . 90

5.8. Potentiometric titration of functionalized and non-functionalized spherical

carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

5.9. Point of zero charge pH values of functionalized and non-functionalized spher-

ical carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

5.10. Water vapor isotherms of di�erently oxidized spherical carbon . . . . . . . . 94

5.11. Volume-speci�c QSDFT pore size distribution of di�erently oxidized spheri-

cal carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

5.12. Change in BET surface area and total pore volume of di�erently oxidized

spherical carbon from values of pure spherical carbon . . . . . . . . . . . . . 97

XVI

E. List of Figures

5.13. TEM image of a crushed 5 wt% palladium on 500 μm spherical carbon catalyst

with palladium clusters highlighted . . . . . . . . . . . . . . . . . . . . . . . 101

5.14. SEM image of a cross section of a 5wt% palladium on 500 μm spherical carbon

catalyst including the results of an EDX scan . . . . . . . . . . . . . . . . . . 101

5.15. Comparison of desired and actual ruthenium loadings . . . . . . . . . . . . . 108

5.16. Conversion of cinnamic acid using 5 wt% palladium catalysts based on spher-

ical carbon and carbon powder, respectively; �ts according tomodel applying

equation 2.9 (T = 40°C, pH2 = 30 bar, c0, cinnamic acid = 236 mol m-3, mcatalyst =

0.5 g) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

5.17. Conversion of cinnamic acid using a 5 wt% spherical carbon palladium cata-

lyst and a 5 wt% palladium catalyst prepared on the basis of crushed spherical

carbon, and commercial 5 wt% palladium on carbon powder; �ts according

to model applying equation 2.9 (T = 40°C, pH2 = 30 bar, c0, cinnamic acid = 236

mol m-3, mcatalyst = 0.5 g) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

5.18. Palladium on spherical carbon (5 wt%, 1 kg scale-up, wet) with varying par-

ticle size fractions tested in cinnamic acid hydrogenations; �ts according to

model applying equation 2.9 (T = 40°C, pH2 = 30 bar, c0, cinnamic acid = 236 mol

m-3, mcatalyst = 0.5 g) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

5.19. E�ective reaction rate constants (a), Thiele moduli (b), and e�ectiveness fac-

tors (c) of di�erent catalyst size fractions (kintr = 0.0072 s-1, De� = 1.18e-10 m

2

s-1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

5.20. E�ective reaction rate constants of cinnamic acid hydrogenations and palla-

dium metal dispersions of spherical catalysts with di�erent particle size and

total pore volume (T = 40°C, pH2= 30 bar, c0, cinnamic acid= 236molm-3, mcatalyst=

0.5 g) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

5.21. E�ective reaction rate constants of cinnamic acid hydrogenation reactions

and palladium metal dispersions of di�erently oxidized spherical carbon cat-

alysts (T = 40°C, pH2= 5 bar, c0, cinnamic acid= 236 mol m-3, mcatalyst= 0.5 g) . . . 121

5.22. Di�erently loaded 200 μm spherical catalysts with 2, 5, and 10 wt% palladium

in the hydrogenation of cinnamic acid; �ts according tomodel applying equa-

tion 2.9 (T = 40°C, pH2= 30 bar, c0, cinnamic acid= 236 mol m-3, mcatalyst= 0.5 g) . 123

5.23. Ruthenium on spherical carbon with di�erent metal loadings in the hydro-

genation of toluene (T = 150°C, pH2 = 50 bar, c0, toluene = 180 mol m-3, mcatalyst

= 0.1 g) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

XVII

E. List of Figures

5.24. Hydrogenation of 1-octene, toluene, m-cresol and thymol using 3.15 wt%

ruthenium on spherical carbon (a) and commercial 5.26 wt% ruthenium on

powdered carbon (b) (T = 150°C, pH2 = 50 bar, c0 = 180 mol m-3, mcatalyst = 0.1 g) 127

5.25. Molecule structures of 1-octene, toluene, m-cresol and thymol (from left to

right) - view along the x-axis of the molecule . . . . . . . . . . . . . . . . . 128

5.26. Dehydrogenation of H18-dibenzyltoluene using 5 wt% platinum on spherical

carbon, commercial 0.5 wt% platinum on aluminum oxide and commercial 5

wt% platinum on activated carbon (T = 310°C, p = 1 bar, nMetal / nLOHC = 0.1

mol%) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

5.27. Dehydrogenation of H18-dibenzyltoluene using two 0.5 wt% platinum on

spherical carbon catalysts with di�erent carbon activation, a 5 wt% platinum

on spherical carbon catalyst and commercial 0.5 wt% platinum on aluminum

oxide (T = 310°C, p = 1 bar, nMetal / nLOHC = 0.1 mol%) . . . . . . . . . . . . . 132


spherical carbon catalysts with di�erent surface functionalization, a 5 wt%

platinum on spherical carbon catalyst and commercial 0.5 wt% platinum on

aluminum oxide (T = 310°C, p = 1 bar, nMetal / nLOHC= 0.1 mol%) . . . . . . . 133


spherical carbon catalysts with di�erent metal transformation temperatures

and commercial 0.5 wt% platinum on aluminum oxide (T = 310°C, p = 1 bar,

nMetal / nLOHC= 0.1 mol%) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135

5.30. Hydrogenation of cinnamaldehyde using 0.5, 5 and 8.4wt% platinumon spher-

ical carbon and a commercial 5% platinum on pulverized activated carbon; �ts

according tomodel applying equation 2.9 (T = 100°C, pH2 = 25 bar, c0, cinnamaldehyde

= 189 mol m-3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136

5.31. Selectivity of cinnamaldehyde hydrogenation towards hydrocinnamic alco-

hol using 0.5, 5 and 8.4 wt% platinum on spherical carbon and a commer-

cial 5 wt% platinum on pulverized activated carbon (T = 100°C, pH2 = 25 bar,

c0, cinnamaldehyde = 189 mol m-3) . . . . . . . . . . . . . . . . . . . . . . . . . . 137

5.32. Conversion of cinnamic acid in four subsequent hydrogenation experiments

with catalyst recycling; �ts according to model applying equation 2.9 (T =

40°C, pH2 = 30 bar, c0, cinnamic acid = 236 mol m-3, mcatalyst = 0.5 g) . . . . . . . 144

XVIII

E. List of Figures

5.33. Conversion of 1,5-cyclooctadiene using a rhodium SILP catalyst that is recy-

cled in three additional experiments with �ts according to model applying

equation 2.9 (a) and cyclooctene selectivity (b) (T = 60°C, pH2 = 30 bar, c0 =

229 mol m-3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146

5.34. Conversion of 1,5-cyclooctadiene at temperatures between 60-90°C reusing

the rhodium SILP catalyst from previous recycling experiments (a) and Ar-

rhenius plot of reaction rate constants (b) (pH2 = 30 bar, c0 = 229 mol m-3) . . 147

5.35. Ammonia breakthroughmeasurements of spherical carbon coatedwith [C4C1-

IM][n-C8H17OSO3]-CuSO4(5H2O) 1:1.4 with (a) αIL=0.1, (b) αIL=0.2 and (c)

αIL=0.3 at di�erent relative humidity; �ts according to equation 2.22 (T=30°C,

p=1.2 bar, dabsorber=1.8 cm, h�lling=2 cm, vN2=0.02 m s-1, cNH3=1000 ppm) . . . 156

5.36. Images of spherical carbon coatedwith [C4C1IM][n-C8H17OSO3]-CuSO4(5H2O)

1:1.4 with αIL=0.2 applying di�erent impregnation approaches: standard wet

impregnation (a), incipient wetness impregnation (b) and dropwise impreg-

nation (c) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160

5.37. Hydrogen sul�de adsorption of spherical carbon coated with [(CH3)4N]F·-

(3H2O) (αIL = 0.3; T = 50°C, p = 1 bar, rh = 15%, dadsorber = 2.35 cm, h�lling = 5

cm, vair = 0.02 m s-1, cH2S = 1000-5000 ppm) . . . . . . . . . . . . . . . . . . . 172

6.1. Functionalization and characterization of spherical carbons for application

in catalysis and gas puri�cation as investigated in this thesis . . . . . . . . . 177

6.2. Funktionalisierung und Charakterisierung sphärischer Aktivkohle für An-

wendungen in der Katalyse und in der Gasreinigung, wie in dieser Arbeit

untersucht . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184

C.1. Hydrogenation of cinnamic acid using 500 μm (a), 200 μm (b) and 50 μm (c)

spherical 5 wt% palladium catalysts and di�erent carbon activations; �ts ac-

cording to model applying equation 2.9 (T = 40°C, pH2 = 30 bar, c0, cinnamic acid

= 236 mol m-3, mcatalyst = 0.5 g) . . . . . . . . . . . . . . . . . . . . . . . . . . XI

C.2. Hydrogenation of cinnamic acid using catalysts with 5 wt% palladium on

di�erently oxidized spherical carbon: (a) nitric acid and(b) sulfuric acid oxi-

dation; �ts according to model applying equation 2.9 (T = 40°C, pH2 = 5 bar,

c0, cinnamic acid = 236 mol m-3, mcatalyst = 0.5 g) . . . . . . . . . . . . . . . . . . XII

XIX

F. List of Tables

2.1. Desorption temperatures and desorbing gas species of oxygen surface groups

[86, 87] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

2.2. Selected gases and volatile organic compounds adsorbing to activated carbon

surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

2.3. Overview of the adsorption capabilities of di�erent carbon surface modi�ca-

tions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

2.4. Typical composition of biogas from an anaerobic digestion energy plant [171,

176] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

4.1. Standard hydrogenation parameters for testing catalyst performance . . . . 68

4.2. Particle size fractions and bulk densities of di�erent spherical carbon materials 73

4.3. Pore characteristics derived from nitrogen sorption data of di�erent spherical

carbon materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

5.1. Filtration experiments using spherical carbon materials and a commercial

powder catalyst (1 g material, 200 ml water, membrane with 1 μm particle

retention, 0.5 bar excess pressure) . . . . . . . . . . . . . . . . . . . . . . . . 80

5.2. Speci�cation of constant parameters for large-scale �ltration . . . . . . . . . 81

5.3. Metal dispersions of 5 wt% palladium catalysts based on spherical carbon and

carbon powder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

5.4. Palladiummetal loadings and dispersions of spherical catalysts with di�erent

particle size and pore volume . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

5.5. Palladium metal loadings and dispersions of di�erently oxidized spherical

carbon catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

5.6. Obtained metal loadings of spherical carbon catalysts loaded with di�erent

amounts of palladium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

5.7. Metal dispersions and derived cluster sizes of ruthenium on spherical car-

bon, with di�erent metal loadings and impregnation times, and commercial

ruthenium on carbon powder . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

XX

F. List of Tables

5.8. Platinum metal loadings of di�erently impregnated spherical catalysts . . . 110

5.9. Platinum metal dispersions of two spherical carbon catalysts with di�erent

total pore volume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

5.10. Metal loadings and dispersions of 0.5, 5, and 10 wt% platinum on spherical

carbon and 5 wt% platinum on pulverized activated carbon . . . . . . . . . . 112

5.11. E�ective reaction rate constants and metal dispersions of 5 wt% palladium

catalysts based on spherical carbon and carbon powder . . . . . . . . . . . . 114

5.12. E�ective reaction rate constants of cinnamic acid hydrogenations of spherical

catalysts with di�erent palladium loading . . . . . . . . . . . . . . . . . . . . 124

5.13. Turnover frequencies of toluene hydrogenations and metal dispersions of

ruthenium on spherical carbon with di�erent metal loadings . . . . . . . . . 125

5.14. Turnover frequencies in the hydrogenation of di�erent educt molecules with

3.15 wt% ruthenium on spherical carbon and commercial 5.26 wt% ruthenium

on powdered carbon catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . 127

5.15. Catalytic activity, �nal selectivity towards hydrocinnamic alcohol and metal

dispersions of 0.5, 5 and 8.4 wt% platinum on spherical carbon and 5 wt%

platinum on pulverized activated carbon . . . . . . . . . . . . . . . . . . . . 137

5.16. Palladium and sulfur leaching of two di�erent spherical catalysts and a com-

mercial powder catalyst with two di�erent durations of ultrasonic treatment 142

5.17. E�ective reaction rate constants of four subsequent hydrogenation experi-

ments with catalyst recycling . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

5.18. Ammonia and hydrogen sul�de adsorption capabilities of spherical carbon

coatedwith [C2C1IM]Br-CuBr2 1:1.3, [C2C1IM]Br-ZnBr2 1:1.3, and [C2C1IM]Br–

CuBr2-ZnBr2 1:0.65:0.65 in reference to previously tested [C2C1IM]Cl-CuCl2

1:1, all with αIL=0.2 according to DIN EN 14387 ABEK1 (T=23°C, dadsorber=5

cm, h�lling=2 cm, vair=0.1 m s-1, c=1000 ppm, rh=70%) . . . . . . . . . . . . . 150

5.19. Corrosion investigations of spherical carbon coated with [C2C1IM]Br-CuBr2

1:1.3, [C2C1IM]Br-ZnBr2 1:1.3, [C2C1IM]Br-CuBr2-ZnBr2 1:0.65:0.65, [C2C1-

IM] Cl-CuCl2 1:1, and [C2C1IM]Cl-ZnCl2 1:1.3, all with αIL=0.2 in aluminum

foil at 65-75% rh and 20°C for 7 days andwith stainless steel 1.4301 (X5CrNi18-

10) at 60-90% rh and 20°C for 14 days (+: no corrosion, -: slight corrosion, –:

corrosion, —: strong corrosion) . . . . . . . . . . . . . . . . . . . . . . . . . 152

XXI

F. List of Tables

5.20. Pore characteristics derived from nitrogen sorption experiments of PBSAC

#1 coated with the halide-free ionic liquid melt [C4C1IM][n-C8H17OSO3]–

CuSO4(5H2O) 1:1.4 and of PBSAC #2 coated with the halide ionic liquid melt

[C2C1IM]Cl-CuCl2 1:1.3, both with αIL=0.2, in reference to the pure 500 μm

PBSAC support materials PBSAC #1 and PBSAC #2 with varying pore structure 154

5.21. Corrosion investigations of spherical carbon coated with [C4C1IM][n-C8H17-

OSO3]-CuSO4(5H2O) 1:1.4 with αIL=0.2 in aluminum foil at 65-75% rh and

20°C for 7 days and with stainless steel 1.4301 (X5CrNi18-10) at 60-90% rh

and 20°C for 14 days (+: no corrosion, -: slight corrosion, –: corrosion, —:

strong corrosion) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155

5.22. Ammonia absorption characteristics of spherical carbon coatedwith [C4C1IM]-

[n-C8H17OSO3]-CuSO4(5H2O) 1:1.4 at di�erent ionic liquid loadings and rel-

ative humidity (T=30°C, p=1.2 bar, dabsorber=1.8 cm, h�lling=2 cm, vN2=0.02 m

s-1, cNH3=1000 ppm) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

5.23. Broadband capabilities of spherical carbon coated with [C4C1IM][n-C8H17-

OSO3]-CuSO4(5H2O) 1:1.4 with αIL=0.2 in reference to pure spherical car-

bon according to DIN EN 14387 ABEK1 (T=23°C, dadsorber=5 cm, h�lling=2 cm,

vair=0.1 m s-1, c=1000 ppm, rh=70%) . . . . . . . . . . . . . . . . . . . . . . . 159

5.24. Ammonia breakthroughmeasurements of spherical carbon coatedwith [C4C1-

IM][n-C8H17OSO3]-Cu[n-C8H17OSO3]2 1:1.3, [C4C1IM][n-C8H17OSO3]-Cu-

[n-C7H15COO]2 1:1.3, and [C4C1IM][n-C8H17OSO3]-CuSO4(5H2O) 1:1.4, all

with α=0.3 (T=23°C, dadsorber=5 cm, h�lling=2 cm, vair=0.1 m s-1, cNH3=1000

ppm, rh=70%) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162

5.25. Formaldehyde adsorption capabilities of pure spherical carbon and spheri-

cal carbon coated with [C2C1IM]Cl-CuCl2 1:1.3 (αIL=0.2), [Choline]Cl-H2O

1:16 (αIL=0.3), [Choline]OH-H2O 1:16 (αIL=0.3), and [Choline]Cl-ZnCl2 1:1

(αIL=0.2); T=30°C, dadsorber=1.8 cm, h�lling=2 cm, vN2=0.02 m s-1, c=600 ppm,

rh=55% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164

5.26. Formaldehyde adsorption capabilities of nitrogen surface functionalized spher-

ical carbon and spherical carbon coated with [Choline]Cl-ZnCl2 1:1 (αIL=0.2),

[Choline][n-C8H17OSO3]-ZnSO4(7H2O) 1:1.3 (αIL=0.2) and [Choline]OH-H2O

1:16 (αIL=0.3); according to NIOSH conditions [151] (T=25°C, h�lling=2 cm,

vair=0.13 m s-1, rh=25%, c=500 ppm, cbreakthrough=1 ppm); basicity ratings: ++:

strongly alkaline system, +: alkaline system, -: more acidic system . . . . . . 166

XXII

F. List of Tables

5.27. Ammonia adsorption and desorption capabilities of spherical carbon coated

with [C2C1IM]Br-CuBr2 1:1.3 (αIL=0.2), [C4C1IM][n-C8H17OSO3]-Cu[n-C8H17-

OSO3]2 1:1.3 (αIL=0.3) and [C4C1IM][n-C8H17OSO3]-Cu[n-C7H15COO]2 1:1.3

(αIL=0.3); adsorption conditions: T=20°C, p=4 bar, dadsorber=1.5 cm, h�lling=5

cm, vair= 0.02 m s-1, c=1000 ppm until beginning saturation, then c=5000 ppm

until 90% saturation, rh=15%; pressure swing desorption conditions: T=20°C,

p=1 bar, vair=0.02 m s-1; temperature swing desorption conditions: T=91-

160°C, p=1 bar, vair=0.02 m s-1

. . . . . . . . . . . . . . . . . . . . . . . . . . 168

5.28. Hydrogen sul�de adsorption and desorption capabilities of spherical carbon

coatedwith [C4C1IM][n-C8H17OSO3] (αIL=0.3), [C4C1IM][n-C8H17OSO3]-CuSO4–

Cu(NH3)4SO4 1:0.7:0.7 (αIL=0.2) and [C4C1IM][n-C8H17OSO3]-Cu[n-C8H17OSO3]2

1:1.3 (αIL=0.3); adsorption conditions: T=20°C, p=4 bar, dadsorber=1.5 cm, h�lling=5

cm, vair=0.02 m s-1, c=1000 ppm until beginning saturation, then c=5000 ppm

until 90% saturation, rh=15%; pressure swing desorption conditions: T=20°C,

p=1 bar, vair=0.02 m s-1; temperature swing desorption conditions: T=64-

160°C, p=1 bar, vair=0.02 m s-1

. . . . . . . . . . . . . . . . . . . . . . . . . . 170

5.29. Hydrogen sul�de adsorption and desorption capabilities of spherical carbon

coated with [(CH3)4N]F(3H2O) (αIL=0.3) and [(CH3)4N]OH-SiO2 1:1 (αIL =

0.3); adsorption conditions: T = 20°C, p = 4 bar, dadsorber = 1.5 cm, h�lling =

5 cm, vair = 0.02 m s-1, c = 1000 ppm until beginning saturation, then c = 5000

ppm until 90% saturation, rh = 15%; pressure swing desorption conditions: T

= 20°C, p = 1 bar, vair = 0.02 m s-1; temperature swing desorption conditions:

T = -45-160°C, p = 1 bar, vair = 0.02 m s-1

. . . . . . . . . . . . . . . . . . . . 173

5.30. Olfactory neutrality and availability of selected hydrate ionic liquids . . . . 174

A.1. Chemicals for catalyst preparation . . . . . . . . . . . . . . . . . . . . . . . . II

A.2. Chemicals for catalyst application . . . . . . . . . . . . . . . . . . . . . . . . III

A.3. Chemicals for SILP preparation and SILP application . . . . . . . . . . . . . IV

A.4. Chemicals for SILP preparation and SILP application . . . . . . . . . . . . . V

B.1. Quantitative surface analysis of oxidized spherical carbon (content of water

and amount of other volatile components excluding water content) . . . . . VI

B.2. Ratio of weakly adsorbing surface oxides desorbing between (100-500°C) and

strongly adsorbing oxides desorbing between (500-1000°C) . . . . . . . . . . VII

B.3. Point of zero charge pH values of functionalized and non-functionalized spher-

ical carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VIII

XXIII

F. List of Tables

B.4. BET surface area, total pore volume, and micropore volume of di�erently

oxidized spherical carbon and deviations from values of pure spherical carbon IX

C.1. E�ective reaction rate constants and palladiummetal dispersions of spherical

5 wt% palladium catalysts with di�erent particle size and carbon activation . X

C.2. E�ective reaction rate constants of cinnamic acid hydrogenations and pal-

ladium metal dispersions of di�erently oxidized spherical 5 wt% palladium

catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XIII

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