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CATALYTIC CONVERSION OF SYNGAS INTO

ETHYLENE AND HIGHER HYDROCARBONS

NISAR AHMAD

Department of Chemistry,

Hazara University, Mansehra,

Pakistan.

CATALYTIC CONVERSION OF SYNGAS INTO

ETHYLENE AND HIGHER HYDROCARBONS

NISAR AHMAD

A dissertation submitted

in partial fulfillment of the requirement for the degree

Of

Doctor of Philosophy

In

Inorganic/Analytical Chemistry

Department of Chemistry, Hazara University,

Mansehra, Pakistan.

March, 2013

Declaration ______________________________________________________

I declare that any material in this thesis, which is not my own work, has been

identified and that no material has previously been submitted and approved for

the award of a degree by this or any other university.

Signature: _______________

Author’s Name: Nisar Ahmad

1. Prof. Dr. Bakhtiar Muhammad

(Supervisor) _____________________

Department of Chemistry,

Hazara University,

Mansehra, Pakistan.

2. Prof. Dr. Syed Tajammul Hussain

(Co-Supervisor) _____________________

Director, Nanoscience and Catalysis Division,

National Centre for Physics,

Quaid-i-Azam University Campus, Islamabad, Pakistan.

Certificate _____________________________________________________________

Certified that the work contained in this thesis is carried out by Mr. Nisar Ahmad

under my supervision and that, in my opinion, it is fully adequate in scope and

quality, for the award of Degree of Doctor of Philosophy in Inorganic/Analytical

Chemistry.

Supervisor ______________________________

Prof. Dr. Bakhtiar Muhammad Department of Chemistry,

Hazara University, Mansehra, Pakistan.

Co-Supervisor ______________________________

Prof. Dr. Syed Tajammul Hussain Director, NS&CD,

National Centre for Physics, QAU Campus, Islamabad,

Pakistan.

Chairman

______________________________

Prof. Dr. Khalid Mehmood Department of Chemistry,

Hazara University, Mansehra, Pakistan.

Dedication ___________________________________________________________

This dissertation is dedicated with all my heart:

To my father,

for all the trust and encouragement he has given me for which I’m forever indebted.

To my mother

for her sincere prayers, sacrifice, motivation, encouragement and support

throughout my studies.

Page vi

Acknowledgment

All respect is for Allah who blessed and gave me sense and insight to accomplish

this scientific assignment objectively and successfully. Allah blessed me with

vision and insight to acquire knowledge without which I can never satisfy and

justify my existence in this world.

I feel highly indebted and express heartiest gratitude to my respected supervisor,

Prof. Dr. Bakhtiar Muhammad and Chairman, Department of Chemistry, Dr.

Khalid Mehmood, for their guidance, support, constant encouragement and

invaluable ideas throughout my PhD studies and research.

I am thankful to my Co-supervisor Prof. Dr. Syed Tajammul Hussain, Director

Nanoscience and Catalysis Division, National Centre for Physics, Islamabad, for

many fruitful discussions and suggestions, which provided me the impetus to

start my work. His support in execution of day to-day work and his precious

suggestions and guidance enabled me to complete my research work in time.

I would like to express my gratitude to National Centre for Physics, QAU

Campus, Islamabad for providing me lab and other research facilities.

I am also obliged to Dr. Jim A. Anderson, Surface Chemistry and Heterogeneous

Catalysis, School of Chemistry, University of Aberdeen, UK, for providing me

the opportunity to work in his Lab.

This research work would have not been possible without the financial support

from the Higher Education Commission (HEC) of Pakistan, first through the

Indigenous PhD program and then through the International Research Support

Page vii

Initiative Program. I would like to express my gratitude to HEC Pakistan for the

financial support of this work.

I am indebted to all my honorable teachers, especially Prof. Dr. Mohsin Nawaz,

Dr.Wajid Rehman, whose teaching has brought me to this stage of academic

zenith.

I have no words of thank for, Yaqoob Khan, Nisar Ali, Syed Mustanser Abbas,

Muhammad Arshad, Rafaqat Hussain, Naeem Shezad, and Asima Sadiqa at

National Centre for Physics, QAU Campus, Islamabad, Pakistan and my

friends Shujaul Mulk Khan, Muhammad Islam, Waqas Ahmad, Mumtaz Ali, Fida

Hussain, Dunia Zeb, Mehtab Khan and Fahad khan for their wonderful company

and valuable suggestions, encouragement and motivation throughout my

research work.

I extend my heartfelt gratitude to my father and mother, who flourished my

career and maintained my life. They never let me feel sad even in the most

difficult situations in our family and also to my sisters and brothers Bahram

Khan, Ayaz Ahmad, Nabi Ahmad, Sadiq Ahmad, and Jamil Ahmad, for their

good wishes. Let me say also thanks to my wife and children, Hammad,

Hammdan, Urooba, Areeba and Rizwan who provided me with millions of

smiles, despite I could not provide them enough time and sufficient care.

May Almighty Allah shower his blessings and prosperity on all those who

assisted me in any way during my research work.

Nisar Ahmad

Page viii

Abstract

A rapid growth in population and industrialization has resulted in a shortage of

natural resources with increasing human demands. With the rapidly depleting

petroleum resources, other venues such as the utilization of coal and biomass for

energy production are under intense investigation. Fischer Tropsch (FT)

technology is extensively used for the conversion of coal, natural gas and

biomass derived syngas (CO+H2) to fuel by utilizing transition metals as

catalysts. One of the main challenges in FT synthesis is the production of higher

molecular weight waxes which blocks the active sites of the catalysts, resulting in

decreased catalytic activity. The catalyst supports in FT synthesis is also very

important as it not only enhances the dispersion of active metal catalyst but also

provide active sites for hydrogenation and cracking of higher hydrocarbons. The

present study was intended to explore Montmorillonite (MMT) as a novel

support material for Co-based FT synthesis to increases the surface acidity,

hydrogenation and cracking of higher molecular weight hydrocarbons. The

lower thermal stability and lack of porosity in MMT was overcome by replacing

the sodium ion present in the interlayer of MMT clay with different metal oxides

(MOs) (M=Al and Zr) to achieve high surface area and pore volume. Along with

the modification of catalyst support, the effect of Mn and Ce promoters have also

been investigated in this study.

A series of Al and Zr-pillared montmorillonite (Al-PILC and Zr-PILC) supported

Co catalysts were fabricated by impregnation and hydrothermal methods. FT

reaction was carried out in fixed bed micro reactor at temperature 225 oC, 260 oC

Page ix

and 275 oC and pressure of 1, 5, and 10 bar. It was found that Co supported Na

montmorillonite (NaMMT) had lower CO-conversion and higher CH4-selectivity

while the Al-PILC and Zr-PILC supported Co catalysts gives higher CO-

conversion and lower CH4-selectivity. Moreover, increase in reaction

temperature from 225 oC to 275 oC resulted in higher CH4-selectivity, higher CO-

conversion and decreased in selectivity towards C5+ hydrocarbons. Increase in

pressure from 5 to 10 bar resulted in decreased CH4-selectivity of the catalyst but

increase in C5+ hydrocarbons and CO-conversion efficiency. The Addition of Mn

as promoter to the Al-PILC and Zr-PILC supported Co nanoparticles

significantly increased the selectivity of catalyst toward C2-C12 hydrocarbons as a

result of the cracking of long chain C21+ hydrocarbons. The addition of Mn also

resulted in a decreased selectivity toward CH4. On the other hand when Ce is

used as a promoter, the selectivity toward C5-C12 hydrocarbons and CH4-

increased and that of C21+ selectivity decreased. Significant enhancement in CO-

conversion and CH4-selectivity was observed at higher reaction temperatures

(>220 oC). The increase in pressure from 1 to 10 bars eventually resulted in

enhancement in C5+ hydrocarbons and decrease in CH4 and C2-C5 hydrocarbons

selectivity. All of these could be attributed to the synergistic effect of

electronically and geometrically modified sites on the catalyst surface, their

orientations and resultant intermediates concentration.

Page x

Table of Contents

Acknowledgment ........................................................................................................... vi

Abstract ……………………………………………………………………………viii

List of Figures ............................................................................................................. xviii

List of Table ..................................................................................................................xxii

List of abbreviation and symbols ........................................................................... xxiii

Chapter 1. Introduction .................................................................................................. 1

1.1 Background ......................................................................................................... 1

1.2 Fischer Tropsch Reactions ................................................................................ 3

1.3 FT Selectivity ........................................................................................................... 5

1.4 FT Catalysts.............................................................................................................. 6

1.4.1 Ru based catalysts ............................................................................................ 7

1.4.2 Ni based catalysts ............................................................................................. 7

1.4.3 Fe based catalysts ............................................................................................. 8

1.4.4 Co based catalysts ............................................................................................ 8

1.5 Effects of promoter on the activity of FT catalyst............................................. 10

1.6 Effects of support on the activity of Co-based FT catalyst .............................. 13

1.7 Objectives of Study ............................................................................................... 21

1.8 Present Work ......................................................................................................... 22

1.9 Layout of Dissertation .......................................................................................... 23

Page xi

Chapter 2. Experimental and Characterization techniques ................................... 25

2.1 Materials ................................................................................................................. 25

2.2 Preparation of Al-PILC ........................................................................................ 25

2.3 Prepartion of Co-loaded/Al-PILC ..................................................................... 25

2.4 Preparation of different Wt% Mn-20Wt% Co-loaded /Al-PILC ................... 26

2.5 Preparation of different Wt% Ce-20Wt% Co-loaded/Al-PILC ...................... 26

2.6 Preparation of Co-loaded/Zr-PILC ................................................................... 27

2.7 Preparation of different Wt% Mn-20Wt% Co-loaded/Zr-PILC .................... 27

2.8 Characterization Techniques ............................................................................... 27

2.8.1 X-Ray diffraction (XRD).......................................................................... 27

2.8.2 Scanning Electron Microscopy (SEM) ................................................... 29

2.8.3 Thermo Gravimetric Analysis (TGA) .......................................................... 30

2.8.4 X-ray Fluorescence (XRF) Spectroscopy ..................................................... 31

2.8.5 BET Surface Area Analysis ........................................................................... 32

2.8.6 Gas Chromatography-Mass Spectrometry (GC-MS) ................................ 33

2.8.7 Temperature Programmed Desorption, Reduction, Oxidation (TPDRO)

.................................................................................................................................... 34

2.9 Catalyst Testing System ....................................................................................... 34

2.9.1 Reaction conditions ........................................................................................ 35

2.9.2 Procedure ......................................................................................................... 36

Page xii

2.9.3 Mass balance calculations ............................................................................. 37

Chapter 3. Fischer Tropsch Synthesis Over Al-modified Montmorillonite

Supported Cobalt Nanocatalysts ................................................................................ 40

3.1 Introduction ........................................................................................................... 40

3.2 Experimental .......................................................................................................... 42

3.2.1 Preparation of Al-PILC .................................................................................. 42

3.2.2 Preparation of Co-loaded/Al-PILC ............................................................. 42

3.2.3 Characterization of Prepared catalysts ....................................................... 42

3.2.4 Catalyst evaluations ....................................................................................... 42

3.3 Results and Discussion ......................................................................................... 43

3.3.1 Textural and Structural properties .............................................................. 43

3.3.2 Acidic Property ............................................................................................... 47

3.3.3 SEM studies ..................................................................................................... 48

3.3.4 Reduction behavior ........................................................................................ 49

3.3.5 Thermogravimetric analysis (TGA) ............................................................. 50

3.4 FT catalytic activity test ........................................................................................ 51

3.4.1 Catalytic activity of 20%Co/NaMMT ......................................................... 51

3.4.2 Catalytic activity of Co-loaded/Al-PILC .................................................... 53

3.4.3 Effect of reaction temperature on catalytic activity of catalysts. ............. 54

3.4.4 Effect of reaction pressure on catalytic activity of catalysts. .................... 55

Page xiii

3.5 Conclusions ............................................................................................................ 56

Chapter 4. Effect of Manganese Promotion on Al-Pillared Montmorillonite

Supported Cobalt Nanoparticles for Fischer-Tropsch Synthesis ......................... 57

4.1 Introduction ........................................................................................................... 57

4.2 Experimental .......................................................................................................... 58


4.2.2. Preparation of different Wt% Mn-20Wt% Co-loaded /Al-PILC ............ 58

4.2.3 Characterization of Prepared catalysts ....................................................... 58


4.3. Results and discussion ........................................................................................ 59

4.3.1 Textural and Structural properties .............................................................. 59

4.3.2 SEM studies ..................................................................................................... 62

4.3.3 Reduction behavior ........................................................................................ 63


4.4. FT Catalytic performance ................................................................................... 66

4.4.1 Effect of Mn-addition on Co/Al-PILC catalysts ........................................ 66

4.4.2 FT Reaction Products over Mn-promoted 20 wt% Co /Al-PILC ............ 69

4.5 Conclusions ............................................................................................................ 72

Chapter 5. Fischer Tropsch Synthesis Over Cerium Promoted Al-Modified

Montmorillonite Supported cobalt Nanocatalysts ................................................. 74

5.1 Introduction ........................................................................................................... 74

Page xiv

5.2. Experimental ......................................................................................................... 75


5.2.2 Preparation of 20 wt% Co/Al-PILC ............................................................ 75

5.2.3 Preparation of different Wt% Ce-20Wt% Co-loaded/Al-PILC ............... 75

5.2.4 Characterization of Prepared catalyst ......................................................... 75



5.3.1 Chemical composition analysis .................................................................... 75

5.3.2 XRD Characterization .................................................................................... 76

5.3.3 Textural properties of Ce-promoted Co/Al-PILC .................................... 77

5.3.4 Acidic Property ............................................................................................... 78

5.3.5 TPR studies ...................................................................................................... 79

5.3.6 Thermo gravimetric analysis (TGA) ............................................................ 80

5.3.7 SEM studies ..................................................................................................... 81

5.4 Catalytic activity ................................................................................................... 82

5.4.1 Effect of Al-pillaring on the FT activity of MMT supported Co- catalysts

.................................................................................................................................... 82

5.4.2 Effect of Ce doping on FT activity of Co supported Al-PILC .................. 82

5.4.3 Effect of reaction temperature and pressure on catalytic activity of

catalysts ..................................................................................................................... 85

Page xv

5.5 Conclusions ............................................................................................................ 87

Chapter 6. Zr-Pillared Montmorillonite Supported Cobalt Nanoparticles for

Fischer-Tropsch Synthesis ........................................................................................... 89

6.1 Introduction ........................................................................................................... 89

6.2 Experimental .......................................................................................................... 91

6.2.1 Preparation of Zr-PILC .................................................................................. 91

6.2.2 Preparation of Co-loaded/Zr-PILC ............................................................. 91

6.2.3 Characterization of Prepared catalyst ......................................................... 91



6.3.1 Chemical composition ................................................................................... 92

6.3.2 XRD characterization ..................................................................................... 92

6.3.3 BET studies of Co-loaded/Zr-PILC ............................................................. 94

6.3.4 TPR studies ...................................................................................................... 96

6.3.5 NH3-TPD studies ............................................................................................ 97


6.3.7 SEM studies ..................................................................................................... 98

6.4. FT catalytic activity .............................................................................................. 99

6.4.1 Catalytic activity of 20%Co/NaMMT ......................................................... 99

6.4.2. Catalytic activity of Co-loaded/Zr-PILC ................................................. 100

Page xvi

6.4.3 Effect of Reaction temperature on catalytic activity of catalysts ........... 101

6.4.4 Effect of reaction pressure on catalytic activity of catalysts ................... 102

6.5 Conclusions .......................................................................................................... 103

Chapter 7. Effect of Manganese Promotion on Zr-Pillared montmorillonite

Supported Cobalt Nanoparticles For Fischer Tropsch Synthesis ...................... 105

7.1 Introduction ......................................................................................................... 105

7.2 Experimental ........................................................................................................ 106

7.2.1 Preparation of Zr-PILC ................................................................................ 106

7.2.2. Preparation of different Wt% Mn-20Wt% Co -loaded/Zr-PILC .......... 106

7.2.3 Characterization of Prepared catalyst ....................................................... 107

7.2.4 Catalyst evaluations ..................................................................................... 107

7.3 Results and Discussion ....................................................................................... 107

7.3.1 Chemical composition ................................................................................. 107

7.3.2 XRD characterization ................................................................................... 108

7.3.3 BET studies of Mn-Co /Zr-PILC ................................................................ 109

7.3.4 Reduction behavior ...................................................................................... 111

7.3.5 Thermogravimetric analysis (TGA) ........................................................... 112

7.3.6 SEM studies ................................................................................................... 113

7.4 FT Catalytic performance .................................................................................. 114

7.4.1 Effect of Mn-addition on Co/Zr-PILC catalysts ...................................... 114

Page xvii

7.4.2 FT Reaction Products of Different wt% Mn-Co/Zr-PILC ...................... 117

7.5 Conclusions .......................................................................................................... 118

Chapter 8. Summary and outlook ............................................................................. 120

Future Work ............................................................................................................... 123

References .. …………………………………………………………………………126

List of publication included in this dissertation ................................................... 156

List of other Publications ........................................................................................... 158

Page xviii

List of Figures

Figure 1.1: Plot of calculated selectivity Vs probability of chain growth 6

Figure 1.2: Structure of Montmorillonite 16

Figure 2.1: Illustration of diffraction of parallel X-rays with a

wavelength from atoms in a set of crystal planes separated

by a distance of d with diffraction angle θ

29

Figure 2.2: Schematic diagram of a typical SEM 30

Figure 2.3: Electronic processes in XRF 32

Figure 2.4: Schematic diagram showing the positioning of equipment

and flow of gases

35

Figure 3.1: Schematic diagram showing the positioning of equipment

and flow of gases

43

Figure 3.2: XRD pattern of the Montmorillonite (a) NaMMT and (b) Al-

PILC

45

Figure 3.3: XRD pattern of the 10, 15, 20, 25 % Co/Al-PILC 45

Figure 3.4: N2 Adsorption desorption Isotherm of Na MMT, Al-PILC

and Co/Al-MMT catalysts

47

Figure 3.5: NH3-TPD profile of samples 48

Figure 3.6: SEM Images of (a) NaMMT (b) Co/Al-PILC (c) 10%Co/Al-

PILC (d) 15 %Co/Al-PILC (e) 20 %Co/Al-PILC (f) 25

%Co/Al-PILC

49

Figure 3.7: H2 TPR profile of the prepared samples 50

Figure 3.8: TGA Curve of the samples 51

Page xix

Figure 3.9: CO-conversion and CH4-selectivity verses time on stream for

(a) 20% Co/NaMMT (b) Different wt % Co/Al-PILC

catalysts

53

Figure 3.10: Effect of reaction temperature (a) and reaction pressure (b)

on FT catalytic activity

55

Figure 4.1: XRD pattern of (a) Co/Al-PILC (b) 0.10Mn-Co/Al-PILC (c)

0.30Mn-Co/Al-PILC (d) 0.70Mn-Co/Al-PILC (e) 1.5Mn-

Co/Al-PILC (f) 3.5Mn-Co/Al-PILC

60

Figure 4.2: N2 adsorption desorption isotherm of MMT, Al-PILC,

Co/Al-PILC and Mn-promoted Co/Al-PILC catalysts

62

Figure 4.3: SEM Images of (a) 0.10Mn-Co/Al-PILC (b) 0.30Mn-Co/Al-

PILC (c) 0.70Mn-Co/Al-PILC (d) 1.50Mn-Co/Al-PILC (e)

3.50Mn-Co/Al-PILC

63

Figure 4.4: H2-TPR profile of the prepared samples 65

Figure 4.5: TGA of Co/Al-PILC and Mn-promoted Co/Al-PILC 65

Figure 4.6: (a) CO-conversion (b) CH4-selectivity Vs time on stream over

Co/Al-PILC and Mn-promoted Co/Al-PILC catalysts

66

Figure 4.7: Influence of Mn-promotion on O/P ratio with in C2-C6

fraction

72

Figure 5.1: XRD pattern of the (a) 20 wt %Co/Al-PILC (b) 0.5Ce-Co/Al-

PILC (c) 1Ce-Co/Al-PILC (d) 1.6 Ce-Co/Al-PILC

77

Figure 5.2: NH3-TPD profile of samples 79

Figure 5.3: H2-TPR profile of samples 80

Page xx

Figure 5.4: TGA curve of the samples 80

Figure 5.5: SEM image of (a) 0.5 Ce-Co/Al-PILC (b) 1.0 Ce-Co/Al-PILC

(c) 1.6 Ce-Co/Al-PILC

81

Figure 5.6: CO-conversion/selectivity Vs time on stream for different wt

% Ce 20 wt % Co/Al-PILC catalysts

82

Figure 5.7: Effect of reaction temperature (a) and reaction pressure (b)

on FT catalytic activity

87

Figure 6.1: XRD pattern of (a) NaMMT and Zr-PILC (b) Co-loaded/Zr-

PILC

94

Figure 6.2: N2-Adsorption desorption isotherm of NaMMT, Zr-PILC

and Co-loaded/Zr-PILC

95


Figure 6.4: NH3-TPD profile of the samples 97

Figure 6.5: TGA Curve of the samples 98

Figure 6.6: SEM micrograph of (a) Zr-PILC (b) 10%Co/Zr-PILC (c)

15%Co/Zr-PILC (d) 20%Co/Zr-PILC

99

Figure 6.7: (a) CH4-selectivity (b) CO-conversion Vs TOS over

Co/NaMMT and Co-loaded/Zr-PILC catalysts

101

Figure 6.8: Effect of (a) reaction temperature and (b) reaction pressure

on FT-catalytic activity

102

Figure 7.1: XRD pattern of 0.10, 0.30, 0.70, 1.5, 3.5%Mn-Co/Zr-PILC 109

Figure 7.2: N2-Adsorption desorption isotherm of (a) NaMMT (b) Zr-

PILC (c) 20%Co/Zr-PILC (d) 0.10Mn-Co/Zr-PILC (e)

110

Page xxi

0.30Mn-Co/Zr-PILC (f) 0.70Mn-Co/Zr-PILC (g) 1.50 Mn-

Co/Zr-PILC and (h) 3.50 Mn-Co/Zr-PILC


Figure 7.4: TGA Curves of the samples 113

Figure 7.5: SEM micrograph of (a) 0.10%Mn-Co/Zr-PILC (b) 0.30%Mn-

Co/Zr-PILC (c) 0.70%Mn-Co/Zr-PILC (d) 1.5%Mn-Co/Zr-

PILC (e) 3.5%Mn-Co/Zr-PILC

114

Figure 7.6: (a) CO-conversion CH4-selectivity (b) CH4-selectivity Vs TOS

over Co/NaMMT and different wt %Mn-Co /Zr-PILC

catalysts

117

Page xxii

List of Table

Table 1.1: Catalytic application of PILCs in chemical synthesis 19

Table 2.1: Molar response factors for hydrocarbon products 37

Table 3.1: Textural properties of the prepared catalysts. 45

Table 3.2: Results of Different catalysts for FT synthesis 54

Table 4.1: Textural properties of the prepared catalysts 61

Table 4.2: Two steps Co3O4 reduction temperature 64

Table 4.3: Results of different catalysts for FT synthesis. 71

Table 5.1: Chemical composition (wt %) of the prepared catalysts

determined by EDX analysis

76


Table 5.3: Results of Different catalysts for FT synthesis at

reaction temperature of 220 oC

85

Table 6.1: Chemical composition of the prepared catalysts (wt %) 92


Table 6.3: Results of different catalysts for FT-synthesis 103

Table 7.1: Chemical composition of the prepared catalysts (wt %) 108


Table 7.3: Two steps Co3O4 reduction temperature 112

Table 7.4: FT reaction products over different catalysts 118

Page xxiii

List of abbreviation and symbols

Å: Angstrom

AA: Atomic absorption

Al-PILC: Aluminium pillared clay

ASF: Anderson-Schulz-Flory

BET: Brunnauer, Emmett and Teller

CFB: Circulating Fluidised Bed

CSTR: Continuous Stirred Tank Reactor

CTL: Coal-To-Liquid

EDS: Energy Dispersive Spectroscopy

EDTA: Ethylenediamine tetraacetic acid

e.V: Electron volt

EXAFS: Extended X-ray Absorption Fine Structure

FID: Flame Ionisation Detector

FT: Fischer-Tröpsch

FTS: Fischer-Tröpsch Synthesis

GC: Gas Chromatograph

gcat: Gram of catalyst

gCo: Gram of cobalt in the catalyst

MCM-41: Mobil Catalytic Material number 41

MMT : Montmorillonite

NaMMT: Sodium montmorillonite

nm: Nanometer

Page xxiv

PFR: Plug Flow Reactor

PPQ: Poropak-Q

RPM: Rounds Per Minute

SA: Specific surface Area

SBA-15: Santa Barbara 15

SEM Scanning Electron Microscopy

SMSI: Strong Metal-Support Interactions

SSITKA: Steady State Isotopic Transcient Kinetic Analysis

Syngas: Synthesis gas (CO + H2)

TCD: Thermal Conductivity Detector

TGA: Thermo-gravimetric Analysis

TOS: Time-On-Stream

TPD: Temperature Programmed Desorption

TPR: Temperature Programmed Reduction

Vol.%: Percentage by volume

WGS: Water-Gas-Shift

Wn: Weight fraction of hydrocarbon with carbon number n

Wt.%: Percentage by weight

XRD: X-ray Diffraction

XRF: X-Ray Fluorescence

Zr-PILC: Zirconium pillared clay

α: Chain growth probability

Chapter 1 Introduction

Page 1

Chapter 1. Introduction

1.1 Background

The catalytic conversion of a mixture of CO and H2 to valuable hydrocarbons in

the presence of metal catalysts via polymerization reaction can be named as

Fischer Tropsch (FT) Synthesis. This reaction further leads to the formation of a

wide range of products depending on the nature of catalysts employed and

reaction conditions [1]. The hydrogenation of CO was initially explored by

Sabatier and coauthors in 1902. CH4-production using cobalt and nickel catalysts

was observed in their findings [2]. Two patents were also awarded in 1913 and

1914 for the conversion of syngas to synthol, oxygenated derivatives, using alkali

promoted cobalt and osmium catalyst in high pressure reaction cell [3]. Later on

in 1920 alkalized iron shavings were used to prepare almost similar products to

synthol at a high temperature (400 oC) and pressure (100 atm) reported by Fischer

and Tropsch [4]. Both these authors were also able to prepare small amounts of

ethane and hydrocarbons using iron and zinc catalysts [5]. But the iron based

catalysts resulted in rapid deactivation emphasizing the requirement of research

using other catalysts as well. Cobalt and nickel catalysts were explored by Fischer

and Meyer in 1930’s [6]. Initially the studies were focused on Ni due to its

comparatively easy availability as compared to Co. But, since Ni based catalysts

showed higher yields of methane, further studies were focused on Co. The

industrial applications of FT synthesis to produce fuels from coal reserves started

in Germany in 1938, where 9 plants were established which were making fuel

using Co based catalysts [7]. Further expansion of FT Synthesis plants ceased


Page 2

around 1940 but the plants which already existed continued to operate during the

Second World War. The fear of expected fuel shortage in future, this process

remained alive. Japan was operating 3 plants based on FT synthesis in 1944.

Despite meeting considerable scientific and technical success, this process was

found to be comparatively expensive than the crude oil refining process in 1950’s.

This was further aggravated following major oil discoveries in the Middle East

which reduced the worldwide oil prices significantly. Because of the embargoes

owing to the South Africa’s apartheid policies, South Africa coal oil and Gas

Corporation (SASOL) started using coal based FT synthesis for making fuel and

chemicals in Sasolburg, South Africa. Sasol further constructed 2 more plants

which were commissioned in 1980 and 1982. Due to increase in crude oil prices

throughout the world and growing energy demands worldwide, the FT synthesis

is gaining importance once again. Major investment decisions in recent years by

big petroleum companies, such a Shell and Exxon Mobil [8, 9] to operate large

scale FT synthesis plants in Qatar in a joint venture by Sasol with Qatar

Petroleum. It is estimated that 5% of the industrial chemical productions in 2020

would be based on this technology replacing crude oil with CH4 and coal [10].

Since the coal reserves in the world are 20 times than the crude oil, FT technology

is likely to prove quite promising in the near future. The fastest growing power

in the world, China, is also focusing for exploring her rich coal reserves in order

to meet the increasing energy demands.


Page 3

1.2 Fischer Tropsch Reactions

The conversion of CO and H2 to olefins, paraffin and other higher hydrocarbons

is generally a hydrogenation reaction occurs in the following various steps. In

first step hydrogenation reaction of CO taking place produces hydrocarbon

chains as under in (Eq 1.1).

CO + 2H2CH2( ) + H2O (1.1)

HR = -164.7 kJ (227 oC, n-hexane)

Due to the formation of water in this reaction results into the WGS reaction (Eq

1.2). The WGS reaction further results in shifting of the reaction stoichiometry to

the right [11], as shown below.

CO + H2O CO2 + H2 (1.2)

HR = -39.7 kJ (227 oC)

By combining Eq (1.1) and (1.2) we get the overall reaction in Eq (1.3).

2CO + 2H2CH2( ) + CO2 (1.3)

HR = -204.4 kJ (227 oC, n-hexane)

By substituting (_CH2-) with CH4, In Eq (1.1), we will get Eq (1.4).

CO + 3H2CH4 + H2O (1.4)

HR = -214.4 kJ (227 oC)

Along with the formation of CH4, the problem of carbon deposition associated

with hydrocarbons synthesis is also a great problem in FT synthesis describe by

the Boudart reaction (Eq 1.5).


Page 4

C + CO2 (1.5)2CO

HR = -133.8 kJ (227 oC)

We can scrutinize the production of CH4 and deposited carbon from CO

hydrogenation discussed above need to be addressed necessarily.

The CO and H2 produced called synthesis gas, commonly named as syngas The

catalytic conversion of CO and H2 (syngas) over a metallic catalysts yielding long

chain hydrocarbons is known as FT synthesis [12]. Syngas can either be produced

by partial oxidation or steam reforming, accounting for more than 60% of the

total cost of the FT synthesis, a very high energy is required for the gasification

process which is highly endothermic [11, 13, 14]. The syngas produced is then fed

into a FT reactor for transforming it into a paraffin wax and its further conversion

into different chemicals like diesel, lubricants and gases by hydrocracking. FT

synthesis is a complex process, as it results in a lot of preferred products like

olefins, alcohols, paraffin etc., and some undesired byproducts like carbon,

ketones, acids, esters etc [15]. FT synthesis reaction products vary, depending

upon many factors which ultimately control the conversion of syngas and

selectivity of hydrocarbons. The material used as catalysts and its support, the

procedure adopted for its synthesis, type of reactor used and reaction conditions

are some of these factors [15]. FT synthesis involves following steps [16].

(i) Adsorption and likely dissociation of CO and H2.

(ii) Formation of olefins and paraffins as a result of surface reactions

leading to alkyl chains.


Page 5

(iii) Production of main byproducts of this process as a result of desorption

of hydrocarbon products

(iv) Production of secondary hydrocarbon byproducts like olefin re

adsorption further leading to hydrogenation or re initiation of chain

growth.

1.3 FT Selectivity

The selectivity of the FT reactions is closely related to the CH4 production. It

becomes uneconomical due to back conversion of syngas leads to severe yield

and thermal penalties. Considerable efforts are being made in order to decrease

the production of CH4 during these reactions. The economics of the gas

conversion processes is ascertained by its production price as well as capital

investment costs. Syngas production plants involves high capital investments.

The considerations of selectivity are most important while designing a Fischer

Tropsch section of a gas conversion plant. If the olefins and waxes are being

produced simultaneously, their selectivities need to be optimized by adjusting

the composition and choice of different catalysts and its supports. The choice of

catalyst is central to the FT synthesis process. A graph showing the probability of

chain growth versus FT products selectivity is shown in Fig. 1.1 [16]. From this

graph, we can observe that all other products can have a high yield while only

heavy (α→∞) or light (α→0) products show high selectivity. The conditions of

operation, temperature, feed gas composition, pressure, type of catalyst and

promoters etc significantly influences the distribution of FT product during FT

synthesis.


Page 6

Figure 1.1 Plot of calculated selectivity vs probability of chain growth [16]

1.4 FT Catalysts

The use of catalysis for syngas production has gained significant interest in the

recent past. There have been a lot of research on exploring different catalysts for

this process. The basic criteria for selection of the catalyst generally remain the

same and have been summarized as below [17].

(i) The catalyst must be capable of reforming the methane.

(ii) The catalyst should be capable of regeneration.

(iii) The catalyst should be strong.

(iv) The catalyst should be inexpensive.

(v) It should be resistant to deactivation.

The catalyst significantly improves the process economics by decreasing the

volume requirements due to high volume productivity. Most widely researched


Page 7

catalysts which have been found to be active in FT synthesis are group VIII

transition metals. Among them Ni, Co, Fe and Ru are the only active phase

catalysts which have CO hydrogenation activity for commercial purposes.

1.4.1 Ru based catalysts

Another catalytically active component using Ru Catalysts are also used for FT

reaction [18]. FT synthesis reactions are considered to be structure intensive

reactions and various research studies show that supported catalysts like Ru, Rh,

Co and Fe in nanometer range sizes display lower metal surface area specific

activity [19-22]. Ru catalyst has been found to be quite active for the FT synthesis

[23-25]. Ru is likely to provide simplest catalytic system for FT synthesis due to

the ability of Ru to achieve chain growth in their cleanest mode producing high

molecular weight hydrocarbons at high pressures and low temperature, but its

use is restricted due to its limited availability and high costs [26, 27]. Ru is also

prove to be more selective to the C5+ hydrocarbon fraction and lesser towards

CH4 [28].

1.4.2 Ni based catalysts

Significant research has been carried out which concerns with the nickel catalyst

which is commercially available for CH4 and hydrocarbon reforming. Nickel

catalysts results in production of syngas and also eliminating or restricting the

hydrocarbon and CH4 production at high temperatures, while methanation

reactions are favored thermodynamically. It can sometimes be optimized when

CH4 formation is desired [29]. Aznar et al. [30] investigated commercially

available nickel based catalysts and reported that minimal catalysts deactivation


Page 8

was observed over 45 h stream. Some of these catalysts showed high CH4

removal efficiency as well. Baker et al. [31] also reported that use of commercially

available catalyst changed the gas composition to give a rich composition of CH4

at high temperatures, while this composition was quite closer to syngas at high

temperatures.

1.4.3 Fe based catalysts

When carbon-rich source, such as coal, is to be used for syngas conversion, Fe

catalysts are preferred because of their high water-gas shift (WGS) activity for the

conversion of low H2/CO ratio [32-34]. Fe catalysts are quite cheap as compared

to other catalysts. It is advisable to employ Fe based catalysts if we require linear

olefins as end products because of lesser secondary hydrogenation of primary

formed olefins. These catalysts are also known to generate by-products like

aromatics and oxygenated compounds, which is not the case for Co based

catalysts. Fe catalysts also have sensitivity for feed gas impurities like H2S and

have proved to be more resistant to sulphur. However, due to growing

environmental concerns regarding green house effects, may restrict future use of

Fe catalysts due to their high WGS activity.

1.4.4 Co based catalysts

Cobalt based catalysts are considered to be the most active catalysts for FT

synthesis are extensively being researched because of their high selectivity for

long chain paraffins, low WGS and high activity in direct conversion of syngas

[35].


Page 9

These catalysts are found to be quite resistant towards oxidation. One of the

byproducts of FT synthesis is water which deactivates the catalysts, but Co based

materials have proved to be relatively stable against this problem. But in fact, this

water has also been hypothesized as one of the major causes of deactivation of Co

based catalysts. The FT synthesis reactions govern the catalyst selectivity and

deactivation and the water effects on the syngas conversion, which is a complex

mechanism in FT synthesis reactions [36]. Co based catalysts are preferred for FT

synthesis because of their comparatively low cost, higher CO-conversion and

their C5+ selectivity [37, 38]. Latest research encompasses oxide supports for these

catalysts and most commonly used among them are Al2O3, SiO2, and TiO2. In

order to improve the Co dispersion, some noble metals are also added to these

catalyst materials as promoters. But such noble metals add to the production cost

and a considerable research is being carried out throughout the world in order to

find a FT catalyst which is cheap, stable and gives best performance. For

achieving high surface active sites, Al2O3, SiO2 are the most frequently used

porous material to disperse Co particles on their surfaces and to a lesser degree,

TiO2 also being one of them [39-42]. Reuel et al.[43] studied the effect of these

supports on Co for its activity and selectivity in CO hydrogenation. These

authors used low Co loading in their investigations to study the support effects

as it presented a more close Co and support interaction. They observed a

descending effect on the CO hydrogenation activity in the following order:

Co/TiO2, Co/SiO2, Co/Al2O3, Co/C and Co/MgO. To find a catalyst which


Page 10

responds to the preferred properties, all aspects while designing catalysts related

to the support effects must be kept in mind.

1.5 Effects of promoter on the activity of FT catalyst

The stability and activity of catalysts can be improved by the use of certain

additives called promoters [44]. The improved olefin selectivity and catalytic

activity in term of CO hydrogenation have been reported by the use of alkali and

alkaline earth metals as a promoters [45]. In addition, alkali promoter modify the

binding energy and aptitude of reactive molecules to adsorb on the surface of

active metal [10]. Furthermore alkali metal enhance the CO dissociation and help

in the homogenous dispersion of metal particles on the surface of support [46]. A

considerable change in the CO adsorption while a minor influence on the H2

adsorption have been reported for alkali metals K, Na, Li, Cs and Rb on

supported pd catalysts [47].

The increase CO dissociation, olefin to paraffin ratio, hydrocarbon chain growth

and decreased CH4-selectivity is reported for potassium promoted Fe catalysts

[48]. The addition of K2O as promoter increases the catalytic activity and light

olefin selectivity as result of lower CH4 production and due to the limited

secondary reaction of ethylene produced [49]. The increased catalytic activity of

K2O promoted catalyst was also attributed to the increased CO adsorption on the

catalysts surface. The CH4-selectivity of FT catalysts can also be decreased by the

promoting with K2CO3 [50, 51].

Use of alkaline earth metal and their oxides (BaO, SrO, CaO and MgO) as

catalysts inhibits the secondary reactions of ethylene and propylene


Page 11

hydrogenation thereby increasing the alkene selectivity of CO hydrogenation

[52]. Wang et al. [49] reported the carbon-supported catalysts and found some

very interesting results. However, this carbon supported catalyst showed very

less activity (1.4%) for CO-conversion to hydrocarbons.

The FT reaction over Fe2O3 has been reported to increase by the addition of ZnO

and Cu compound as promoters [47, 53]. The addition of Cu to K promoted FT

catalysts only enhance the reaction rate and there is no marked effect on the

selectivity of FT products [54, 55]. The increased WGS reaction has been reported

for the Cu and K promoted Fe-based FT catalysts. Hussain et al studied the effect

of Cu and Ru oxides as a promoter for Fe–Zn–K catalysts. Their results showed

that these promoters help in the reduction steps of Fe oxides and changing it to

FeCx crystallites responsible for increased active sites densities and FT activity

[56, 57]. Thus the addition of Cu and Ru promoter only enhances the dispersion

of active metals and increases the active site densities responsible for higher FT

catalytic activity.

The promoter effect of manganese on FT catalysts has been studied in great

detail. Lower CH4 and higher selectivity of olefins have been reported for Mn

promoted Fe-based FT catalysts. The selective yield of olefins in case of Mn

promoted catalysts system was attributed to the increased CO and lowered H2

adsorption capability which in turn produces an increase in the olefin/paraffin

ratio [58]. The effect of Mn to enhance the reduction ability of Fe based FT

catalysts and their effect to suppress C2H4 and C3H6 further hydrogenation have

been reported by various researcher [49].


Page 12

Manganese oxide promoters for TiO2-supported cobalt based FT catalysts have

also been investigated for its effects on the CO and H2 adsorption properties [59].

In their investigations, they found a close association of Mn to the FT active Co

sites present at the catalysts surface. Because of this intimate association, Co

reducibility was decreased. But with increase in Mn loading (>2.15 wt%), the

active Co sites were covered with oxides of TiO2 and Mn which strongly

suppressed the hydrogen chemisorptions, therefore, lesser Mn loading is feasible

for increased Co surface availability and enhanced catalytic activity. During the

FT synthesis reactions, Mn oxide acts as an electronic promoter and maintains

close association with the Co particles. This results in lower hydrogenation rate

since comparatively larger amount of linearly bonded CO is produced because of

the withdrawal of Mn2+ electron density from the Co sites. This is also illustrated

by increased olefin formation. Consequently, the overall FT catalytic efficiency is

increased since the higher C5+ product selectivity leads to the higher metal

dispersions and lower hydrogenation activity of the catalyst. It was also noted

that MnO itself assists in WGS reaction converting CO to CO2 which affects the

performance of the catalyst.

The incorporation of manganese onto cobalt for optimized above mentioned

catalysts to rationalize the design of promoted Co catalyst for FT synthesis was

also explored in another study by applying Strong Electrostatic Adsorption (SEA)

[60]. Selective deposition of [MnO4]- anion on supported Co3O4 was used to

prepare a series of Mn/Co/TiO2 catalysts. This study showed that method of

SEA allowed to achieve selective adsorption of a Mn promoter onto the


Page 13

supported Co3O4 thereby closely associating Mn with Co which enhanced its

adsorption strength which otherwise not possible by dry impregnation. This also

demonstrated that Co catalyst showed more stability and became more active

with very small Mn additions, while higher degree of Co and Mn interactions led

to enhanced catalyst selectivity at high Mn loadings.

1.6 Effects of support on the activity of Co-based FT catalyst

It is worthwhile to notice that the modern Co catalysts for FT synthesis are more

or less quite similar to the ones prepared by Fischer and coworkers as it also

contains promoted Co particles which are supported on a metal oxide. Almost all

the Co based FT catalyst systems are based on metallic components and oxide

supports as described in the preceding paragraphs being one of them [53, 61-67].

The aim of support material in FT catalysts is its property of enhancing the

availability and exposure of active metals to the reactant in liquid as well as in

gaseous phase. Previously no considerable attention were given to the effect of

support on the activity and product selectivity of FT catalysts but later on it was

proved by different investigator that support can affect the adsorption,

morphology, selectivity and activity of FT catalysts [68-76].

Support can influence the activity of catalysts mainly by the following three types

of mechanism

(i) By the incorporation of some part to the active metal need for controlling

their activity. e,g the presence of sulfur in support material can slow the

reaction at any step needed by poisoning of catalysts.


Page 14

(ii) Some time the Support materials itself taking part in the catalytic reaction,

for example Al2O3 play its role in Pt-Al2O3 system.

(iii) By the geometrical modification and homogenous dispersion of catalysts

particles.

The catalyst selectivity strongly depends upon the nature of the supports as has

been demonstrated by Barrault and Renard [52]. The support material plays a

significant role in the catalyst activity as it makes the Co catalysts more thermally

stable and provides mechanical strength in addition to the higher Co dispersion.

The active Co sites are usually a key factor in determining the FT catalytic

activity, always depends on the selection of support oxides. The percentage of

supported Co oxides that can be reduced to cobalt metals is also considered to be

important. A strong Co support oxide interaction leads to low reducibility which

limits the accessible surfaces on Co metal sites besides favoring the Co dispersion

as found in the case of Al2O3 and TiO2.

Due to the shape-selective character of Zeolites and their high surface area,

increase in the dispersion of metal particles on a molecular scale, helps to control

the molecular weight of the products. Molecular sieves supports have also been

investigated in many studies [77], because the selectivity can be affected due to

metal interactions with basic or electron-donor sites in zeolites.

The use of Molybdenum to modify Al2O3 support material for Co-based catalysts

has also been studied by some authors [78]. They observed the Co metal surface

was partially covered by molybdenum oxide resulting into a strong decrease in

available metal surface area. The adsorption for CO on metallic sites is increased


Page 15

as a result of molybdenum oxide and metallic cobalt interaction. Co adsorbs on

the metallic sites as well as molybdenum sites within the catalyst. Owing to the

increased carbon deposition because of the C-O dissociation activity, the

molybdenum-modified catalyst deactivates quite fast.

Naturally occurring clay are also been used as a support material as well as a

catalysts in various field of catalysis [79]. Montmorillonite (MMT) is one of these

clay largely been used in petroleum industries for hydrocraking of petroleum

waxes [80, 81]. Structurally MMT is composed of 2:1 layer structure, in which the

octahedral Al2O3 sheet is sandwiched between the two tetrahedral silica sheet

describe by marshall [82].

The general formula of MMT is (Si 7.8 Al 0.2)IV(Al 3.4 Mg 0.6)VI O 20(OH)4 with a

nominal composition is Al2O3 (28.3%), SiO2 (66.7%), H2O (5%) without the

material present in their interlayer. From the formula given above it can be

concluded that there is replacement of Si4+ with Al3+ and Al3+ with Mg2+. Thus,

the net charge on MMT layer can be calculated is :[7.8 (+4)]+[0.2(+3)]+[3.4

(+3)]+[0.6 (+2)]+[20 (−2)]+[4 (−1) ]= −0.8 charge/ unit cell. Alkali metals and

alkali earth metal cations located between its layers helps to balance the net

negative charge produced in its lattice due to these substitution [82].


Page 16

Figure 1.2 Structure of Montmorillonite 82

Different methods have been used to tune the properties of MMT. Acid activation

and pillaring are the two important methods among these [82].

Acid activation is a method of treating MMT with concentrated inorganic acid at

high reaction temperature [83, 84]. In this method the replaceable cations of MMT

is replaced by H+ and Al3+ ion along with the removal of all other cations present

in MMT octahedral and tetrahedral sites which in turn results an increase in the

acidity and surface area of MMT [85, 86].

Pillaring is the process by which the porosity of MMT can be increased by

separating its layer through the incorporation of certain pillaring agent may be

inorganic, organic, organo metallic complexes and some larger metal cations with

no marked change in its layered structure [87-89]. Ion exchange method is

normally used for incorporation of pillaring agent to the interlayer of MMT. The


Page 17

incorporated agent on heating changes to metal oxide, results an increase in the

surface area and expansion of MMT [89]. Organic compound were used for the

first time to prepare pillared clays by Barrer and Mcleod in 1955. The main

problem associated with theses pillaring agent were its lower temperature

decomposition and collapse in MMT layer structure [90]. Due to the oil crises in

1973 there was a great pressure on the oil industries to search for a viable

material which can enhance the production of gasoline range hydrocarbons from

heavy fraction of crude oil, hence a great importance were given to the uses of

pillared clays. For this purpose a thermally stable and high surface area pillared

clays were prepared by the incorporation of inorganic polyoxocations as a

pillaring agent to the interlayer of MMT [89]. Brindley and sempels [91],

Vaughan and Lussier [89], lahav et al [92] and Vaughan et al [93-95]

independently studied the effect of Al-pillaring on MMT properties. Many

different cations like hydroxy-titanium [96], hydroxyl-chromium [97], hydroxyl-

zirconium [97] and hydroxyl-aluminium [91] have been used for preparing

pillared MMTs. An extensive research has been carried on the use of Al13

polyoxocation for the synthesis of Al-PILC [98]. In these studies it was also noted

that along with the exchange of Al13 polyoxocation, there is the possibility of

other species to exchange the interlayer cation of MMT reported by Sterte and

Otterstedt [99]. For pillaring, the solution of Al which are partially hydrolyzed

having an effective charge of +0.5 per Al are mixed with a suspension of clay

followed by washing [100]. The solution of partially hydrolyzed Al were also

used for the synthesis of pillared saponites reported by schoonheydat and


Page 18

leeman [101]. For higher catalytic cracking Ce+3 and La+3 exchanged pillared clay

with a strong acidic properties, higher basal spacing, higher surface area and

thermal stability have been reported by shabtai et al [102]. Pillared clay with

acquired pillar density can be produce by controlling the cationic exchange

capacity (CEC) of the materials used for their synthesis reported by Suzuki et al

and mori and Suzuki et al [103]. Thermally stable pillared clay having a large

surface area were synthesized by McCauley [104] and sterte [105] by the

hydrothermal mixing of rare earth element and Al-polyoxocation. Burch and

Warburton [106] ohtsuka et al [107], Pereira et al [108] have reported the

synthesis of Zr-PILC by the use of zirconyle chloride piallring solution.

Pillaring of clay helps to enhance the acidic properties of naturally occurring clay.

Both lewis acidic and bronsted acidic sites are reported in PILC [109, 110].

Bronsted acidity of the clay can be attributed to the presence of hydroxyl groups

while Lewis acidity to the presence of interlayer metal oxide [111]. Acidic

properties of MMT depends on the nature of pillaring agent, MMT clay pillared

with different metal cation posses different acidic properties reported by Ming-

Yuan [112].

Beside the acidic nature of pillared clay, its porosity also has a great effect on

their catalytic properties. PILC are microporous material with wide range of pore

distribution from 5Å to 20Å depending on the starting material, synthesis

condition [113], cation exchange capacity [114], thermal treatment [115] and type

of pillaring agent [116].


Page 19

It is reported that in 1930 acid activated clay were the main catalysts used for

carking of petroleum products, but due to their lower thermal stability and lack

of porosity it was replaced by Zeolite in 1960. Later on it was reused as a cracking

catalysts in the petroleum industries after their increased thermal stability and

porosity as a result of pillaring with inorganic complexes [91, 111, 117, 118].The

increased catalytic properties of PILC were correlated to the Lewis and Bronsted

acidic sites of PILC, because the cracking reaction normally occurs at the acidic

sites of catalysts.

Beside the importance of pillared clay in cracking reaction, it also showed

promising results in the synthesis of various chemicals. A variety of chemical

reaction such as dehydration [119-121], hydroisomerization [122-124],

dehydrogenation [125, 126], aromatization [127], hydrogenation [128, 129],

esterification [130], dispropotionation [110, 131] and alkylation [132, 133] also

involves the use of PILC. The use of PILC in various chemical reaction has been

reviewed by Z.Ding et al [111] presented in table 1.1.

Table 1.1 Catalytic applications of PILCs in chemical synthesis [111]

Reaction Substrate/product PILC catalysts Important parameter

Dehydration glucose/formic acid, Al-PILC Brønsted acidity, pore dimension

Al-(or Cr-or Fe-) PILC

1-phenylethanol/3-oxa-2, 4- Ti-PILC

Strong acid sites, large micropore

diphenylpentane structure (basal spacing = 22.2 °A)

2-propanol/propene, methanol/

Cr-PILC

Brønsted acidity

hydrocarbons

Pentan-1-ol/pentenes Al/ Al-PILC Proton content

methanol/hydrocarbons Fe/ Cr-PILC Brønsted acidity

1-butanol/butene isomers Ti-PILC Surface acidity

Hydroxylation phenol/dihydroxybenzenes Ti-PILC Brønsted acidity, type of solvent

(methanol or acetone)

Disproportionation

propene/ethene, 2-butene,

Mo/Al-PILC

Brønsted acidity, Lewis acidity, Mo


Page 20

Reaction Substrate/product PILC catalysts Important parameter

1-butene active sites

toluene/xylene, benzene Cr-(or Al-, Zr-) PILC

Strong Lewis acidity, large

macroporosity

Aromatic nitration chlorobenzene/ Fe-(or Cr-or Mn-) PILC Brønsted acidity, transition metal

paranitrochlorobenzene oxide pillars

Esterification acetic acid, 2-methoxyethanol/

Al-PILC Lewis acidity

2-methoxyethanol acetate

Alkylation biphenyl, propene/mono-, di-, tri-,

Al-PILC Brønsted acidity, particle size,

tetraalkyl isomers porous structure

toluene, methanol/xylene, Al-PILC, Ga/Al-PILC Brønsted acidity

trimethylbenzeenes

benzene, propene/cumene Al (or La)/Al-PILC

Brønsted acidity and Lewis acidity

Isomerisation 1-butene/iso-butane, iso-butene

Al-PILC Brønsted acidity

Heptane/mono-branched and

Pt/Al (or Zr)-PILC, Pt/Zr,

Strength and amount of Brønsted

dibranched isomers Al-PILC acidity, type of the starting clay

Hexanes/2,2-(or 2,3-) dimethyl-

Al (or Zr)-acid Combining effects of metallic sites

butane, 2-methylpentane, etc. activated-PILC and acid sites

Fischer–Tropsch CO, H2/highly isomerised Ru/Al-PILC Brønsted acidity, graft of Ru onto

synthesis hydrocarbons (branched intercalated alumina pillars

alkanes and internal alkenes)

Phase transfer α-tosyloxyketone, NaN3/ tetramethylammonium Presence of surfactant for easy

catalysis α-azidoketone bromide-PILC access by reactants

alkyl bromide, NaN3/

alkyl azide

CH4 reform CH4,CO2/synthesis gas Ni/La/Al-PILC Basicity, mesoporosity, Ni

particle size

Hydrogenation benzene, xylene, mesitylene/ Pt (or Pd)/Al-PILC large pore size, bimetallic (Pt-Pd)

cyclohexane, C8 cycloalkane

loading for higher stability against

sulfur poisoning

benzene/cyclohexane La/Ni/Al-PILC,

Ni loading, calcination temperature,

Ni/Al-PILC pore dimension bigger than 5.8 °A

Dehydrogenation cyclohexane/benzene Cr-PILC Large gallery height

cumene/α-methylstyrene Cr-PILC Lewis acidity

Fe/Cr-PILC Lewis acidity, Fe/Cr ratio

Aromatization C3/benzene, C4/xylene Zn/Al-PILC weak acidity

Although a heavy literature is available on the use of MMT in catalysis for

different objectives but there is very limited literature on their use in FT synthesis

to increased the catalytic activity and gasoline range hydrocarbons selectivity.


Page 21

The use of MMT as a support for iron based FT catalyst and hydrocraking have

been reported [80, 81, 134, 135]. But the main problem associated with the MMT

supported FT catalysts is the production of large amount of CH4 and carbon

dioxide, which are thought to be the most unwanted products in FT synthesis

and must be reduced to the minimum possible levels. The production of large

amount of CH4 and CO2 was attributed to hindered reduction of metal oxide to

metallic state by Na ion present in MMT (NaMMT) [136]. It is reported by

different investigators that increase in acidic properties as well as texture

modification of clay can be achieved when the clay is exchanged with different

cations. The pillaring of sodium montmorillonite (NaMMT) with Al enhances the

surface area of support MMT.It also increases the pore volume and pore diameter

due to the contribution of Al to replace the Na+ present in the interlayer of MMT

[137].

1.7 Objectives of Study

The goal of this PhD research work is to come up with a practically viable

solution for the synthesis of sulfur free clean fuel from syngas derived via

gasification of natural gas, coal and biomass by the use of FT technology.

Following are the specified objectives of this investigation to achieve the desired

goal.

(i) To select the novel, cheap, environmental friendly and highly active

support material for Co-based FT catalysts showing the properties to

increase the selectivity of gasoline range hydrocarbons and decrease

the CH4-selectivity during FT synthesis.


Page 22

(ii) One of the main problems in Co-based FTS is the production of high

molecular weight hydrocarbons (waxes) responsible for lowering the

catalytic activity by blocking the active sites of catalysts. The present

study focused on the use of naturally occurring clay MMT due to their

cheapness, environmental friendly nature and their ability of cracking

the higher molecular weight hydrocarbons to lower gasoline range

hydrocarbons due to their acidic nature.

(iii) To study the effect of Al and Zr-pillaring of MMT used as a support

material for FT technology.

(iv) To investigate the effect of reaction pressure and temperature on the

catalytic activity and product selectivity of modified MMT supported

Co-based FT catalysts.

(v) To explore the promotion effect of Mn and Ce on the activity and

product selectivity of MMT supported Co catalysts.

1.8 Present Work

The present study is dedicated to discover the feasibility of NaMMT, Al-PILC

and Zr-PILC supported Co-based catalysts and the effect of Mn and Ce

promotion on their FT catalytic activity. Catalytic reaction was carried out using

fixed bed micro reactor at a temperature of 225 oC, 260 oC and 275 oC. Different

weight % Co-loaded over NaMMT, Al-PILC and Zr-PILC catalysts were

prepared for investigating FT synthesis. Along with modification of support

materials the promoter effect of Mn and Ce incorporation to these catalysts were

also studied.


Page 23

X-ray Diffraction (XRD) spectroscopy, Scanning electron microscope (SEM) X-ray

Fluorescence (XRF) spectroscopy, Thermo-gravimetric analysis (TGA),

Temperature programmed reduction, desorption and oxidation (TPDRO) and

BET surface analysis were used for the characterization of prepared samples. FT

products were analyzed using Gas Chromatography-Mass spectrometry (GC-

MS). The present research work is dedicated to:

(i) Synthesis of NaMMT, Al-PILC, and Zr-PILC supported with different

wt% Co catalysts using impregnation and hydrothermal method.

(ii) Synthesis of Mn and Ce promoted NaMMT, AL-PILC and Zr-PILC

supported Co catalysts by hydrothermal methods.

(iii) Characterization of prepared samples for ascertaining their

morphology.

(iv) Catalytic reaction using fixed bed micro reactor.

(v) Using GC-MS for the Analysis of gas samples.

1.9 Layout of Dissertation

This dissertation has been organized in 9 chapters.

Chapter 1 Outlines the background of FT synthesis focusing particularly on

Co based catalysts and presents a concise review of the catalysts

being used along with different supports, promoters for FT

synthesis, followed by the study approach and overview.

Chapter 2 Describes the overall study framework with the experimental

materials, experimental procedures for the synthesis of materials


Page 24

and short overview of various techniques used for the

characterization of prepared materials.

Chapter 3 Include the synthesis and characterization of Al-Pillared MMT

supported Co catalysts for Fischer Tropsch synthesis.

Chapter 4 Explains the effect of manganese promotion on Al-Pillared MMT

supported cobalt nanoparticles for Fischer-Tropsch Synthesis.

Chapter 5 Describes the effect of Cerium promotion on Al-Pillared MMT

Supported Cobalt Nanoparticles for Fischer-Tropsch Synthesis.

Chapter 6 Represents the synthesis and characterization of Zr-Pillared MMT

supported cobalt nanoparticles for Fischer-Tropsch synthesis.

Chapter 7 Depicts the Effect of Manganese Promotion on Zr-Pillared MMT

Supported Cobalt Nanoparticles for Fischer Tropsch.

Chapter 8 Presents the research summary and recommendations for the future

work.

Chapter 2 Experimental and characterization techniques

Page 25

Chapter 2. Experimental and Characterization techniques

This chapter includes the procedure and methodologies used for the synthesis of

catalysts samples and a short introduction to the techniques used for the

characterization of these prepared samples.

2.1 Materials

Sodium Montmorillonite (NaMMT), Aluminium chloride (AlCl3 .6H2O), cobalt

nitrate (Co(NO3)2.6H2O), Zirconium Oxychloride (ZrOCl2.8H2O), Manganese

nitrate (Mn(NO3)2.4H2O) (Sigma Aldrich), and Cerium nitrate (Ce(No3)3.6H2O)

from Merck were of analytical grade and used without any further purification.

NaMMT was selected as a starting material to prepare the AlMMT.

2.2 Preparation of Al-PILC

In the typical synthesis procedure, 0.5 M NaOH solution was slowly added to

0.25M solution of AlCl3.6H2O with OH/Al ratio of 2/1 with continuous stirring.

The above suspension was aged at room temperature for 24 h. In the second step,

above pillaring solution was added (5mmol Al/g of clay) to the suspension of 2

wt % MMT and stirred for three hour at 100oC.The resultant slurry was

centrifuged at 5000 rpm, washed several times with deionized water in order to

remove excess sodium and chlorine, dried at 100 oC for 24 h, grinded and finally

calcined at 400 oC for 5 h [138, 139].

2.3 Prepartion of Co-loaded/Al-PILC

Wet impregnation method was adopted to synthesize Co supported on Al-PILC

with different wt% of Co, represented as 10%Co/Al-PILC, 15%Co/Al-PILC,

20%Co/Al-PILC and 25%Co/Al-PILC. To prepare 5g of catalyst 4.5 g, 4.25 g,4 g


Page 26

and 3.75 g of calcined pillared clay was added to 0.1 M solution of

Co(NO3)2.6H2O (2.46 g, 3.69 g, 4.92 g and 6.15 g) respectively with constant

stirring. The resultant suspension was evaporated at 40 oC by rotary evaporator

until the dry mass is obtained, washed several time with distilled water, dried

overnight at 100 oC and finally calcined at 400 oC for 5 h.

2.4 Preparation of different Wt% Mn-20Wt% Co-loaded /Al-PILC

20wt%Co/Al-PILC and 20wt%Co/Al-PILC with 0.10, 0.30, 0.70, 1.50, 3.50 wt%

Mn designated as 0.10Mn-Co/Al-PILC, 0.30Mn-Co/Al-PILC, 0.70Mn-Co/Al-

PILC, 1.5Mn-Co/Al-PILC, 3.5Mn-Co/Al-PILC respectively, were synthesized by

hydrothermal method.

To prepare 5g of catalyst calculated amount of calcined pillared clay was added

to calculated amount of Co(NO3)2.6H2O and Mn(NO3)2.6H2O (0.1 M solution)

with constant stirring. The resultant suspension along with NH4OH (33%) was

transferred into Teflon-lined vessel of autoclave previously flushed with argon.

The pressure of the autoclave was increased from to 10 bar on heating at 160 oC

for 1h. After thermal treatment the resultant material was cooled, filtered,

washed thoroughly with deionized water to remove all the NH4OH and then

dried overnight at 100 oC. The dried material was grinded & then sieved through

a 125µm sieve and finally calcined at 400 oC for 5h.

2.5 Preparation of different Wt% Ce-20Wt% Co-loaded/Al-PILC

Ce-Co/Al-PILC with 20 wt % Co containing 0.5, 1, 1.6 wt % Ce-loadings

designated as 0.5Ce-Co/Al-PILC, 1.0Ce-Co/Al-PILC and 1.6Ce-Co/Al-PILC

respectively was prepared by wet impregnation method. The required amount of


Page 27

calcined pillared clay was dispersed into 0.1M solution of Co(NO3)2.6H2O and

Ce(NO3)3.6H2O with constant stirring. The resultant suspension was evaporated

at 40 oC through rotary evaporator, washed with distilled water, dried and

calcined at 400 oC for 5 hours.

2.6 Preparation of Co-loaded/Zr-PILC

Co nanoparticles supported on Zr-PILC with different wt% of Co, represented as

10%Co/Zr-PILC, 15%Co/Zr-PILC and 20%Co/Zr-PILC was prepared by the

same hydrothermal method described above for the synthesis of Mn promoted

Co/Al-PILC

2.7 Preparation of different Wt% Mn-20Wt% Co-loaded/Zr-PILC

20wt%Co/Zr-PILC and 0.10, 0.30, 0.70, 1.50, 3.50 wt% Mn 20wt%Co/Zr-PILC

designated as 0.10Mn-Co/Zr-PILC, 0.30Mn-Co/Zr-PILC, 0.70Mn-Co/Zr-PILC,

1.5Mn-Co/Zr-PILC,3.5Mn-Co/Zr-PILC respectively, were synthesized by the

same hydrothermal method described above for the synthesis of Mn promoted

Co/Al-PILC

2.8 Characterization Techniques

2.8.1 X-Ray diffraction (XRD)

XRD is traditionally meant for the crystalline phase identification of materials

and provides information on the unit cell dimensions, orientation and size of

crystallites, degree of crystallinity, structural make-up of the crystalline phases,

amount of amorphous content and the measurement of sample purity.

Introducing accounts for the technique in relation to catalyst examination are

available in references [140, 141]. XRD can be employed to study alloy formation

in bimetallic and multimetallic catalysts from the position of the peaks [142].


Page 28

The basic principle of XRD involves constructive interference between

monochromatic X-rays and a crystalline sample. When any material is irradiated

with monochromatic X-rays, a pattern is obtained which is characteristic of that

material. When incident rays interact with sample atoms, a diffracted ray is

produced after constructive interference, when Bragg’s law is fulfilled.

nλ =2dsinθ (2.1)

Bragg’s law relates the wavelength of x-rays to the angle of diffraction and the

lattice spacing in a crystalline compound. The diffracted X-rays are finally

detected, counted and analyzed. In the beginning white X-radiations was used

for the experiment of diffraction. Later on the use of monochromatic X-radiations

for the verification of crystal orientation was developed by W. Bragg and his son

in 1912. Figure 2.1 illustrate this situation.

All the catalyst samples were analyzed using Scintag XDS 2000 diffractometer

using nickel filtered Cu-Kα radiation (λ=1.54Ǻ) with generator setting of 40 KeV

and 20 mA. About 0.5g of the powdered sample is hard-pressed into the glass

window and rounded to a uniform packing with a spatula. After carefully

mounting the solid sample into the diffraction chamber, the sample was scanned

from the Bragg angle 16o to 70o (2θ). The peak classification was carried out

through similarity of obtained spectra with the ASTM powder diffraction data

file.

The samples crystallite sizes were projected from Scherrer equation using FWHM

(full-width at half maximum) of XRD [143].

kλB(2θ) =

Lcosθ (2.2)


Page 29

Peak width (B) and crystalline size (L) are inversely proportional to each other. K

is the Scherer constant supposed to be 0.9.

Figure 2.1 Illustration of diffraction of parallel X-rays with a wavelength from atoms in a

set of crystal planes separated by a distance of d with diffraction angle.

2.8.2 Scanning Electron Microscopy (SEM)

Electron microscopy is a powerful tool for observing surface properties like

surface structure, porosity, particle size distributions, topography, texture and

morphology of the catalyst material. SEM enables the imaging of the topography

of a solid surface by use of backscattered or secondary electrons [144]. In this

technique a fine beam of accelerated electrons are scanned across the surface of a

sample allowing an image to be created by secondary and backscattered

electrons. The secondary electrons are lower-energy electrons emitted from

inelastic scattering. These electrons are most helpful for screening topography

and morphology of samples. On the other hand, the backscattered electrons are

useful for highlighting contrasts in composition of multiphase samples.

Electron microscopy is based on the materialization equation (eq 2.3) provided by

De Broglie.


Page 30

h h 1.22(nm)

mv 2qmV V (2.3)

SEM equipped with EDX is used to determine the elemental analysis of the

material, which utilizes the emitted X-rays as a result of high energy electrons

bombardment. These emitted X-rays are characteristics of the material and used

to determined the elements present quantitative and qualititively [145]. Surface

morphology was studied with scanning electron microscopy (JEOL JSM 6490-A)

operating at 20kV with different magnification powers equipped with Energy

Dispersive X-ray Spectrometer (EDX).

Figure 2.2 Schematic diagram of a typical SEM

2.8.3 Thermo Gravimetric Analysis (TGA)

TGA is used to measure physical properties of samples with varying temperature

while the substance is applied to a controlled temperature program. TGA can be

defined as the study of the change in weight of a sample with temperature, time,

and/or atmosphere. Within this definition the application are limited to those in

which such a change in mass occurs. Vaporization, sublimation, absorption,


Page 31

desorption, oxidation, reduction, and decomposition are some examples. The

results are presented as a thermogravimetric (TG) curve, showing weight change

as a function of temperature or time [146]. TGA data can also explain the extent

of adsorption with temperature or time in case of porous material so TGA is also

index of porosity of materials whether it is increased or decreased with

temperature or time. The analysis is usually done in the presence of inert gases

such as argon or helium or in air and change in weight is calculated as a function

of rising temperature. For the phase purity and stability confirmation, TGA

spectrum was plotted with Mettler TGA/SDTA 851e.

2.8.4 X-ray Fluorescence (XRF) Spectroscopy

XRF, alternatively recognized as X-ray secondary emission spectroscopy is

widely used for chemical and elemental analysis since it is non-destructive,

accurate and fast technique. The characteristic fluorescent X-rays emitted from a

sample, when high energy x-rays are bombarded on the sample is called XRF as

shown in Figure 2.3. Primary X-ray photon creates an inner shell vacancy which

produces an electronic transition and liberates characteristic secondary X-rays

with energy. Each element possess a unique ray of a particular energy. The

concentration of each element in a particular sample is directly proportional to

the characteristic intensity of signal for that element [147]. Mosley’s equation is

used for qualitative analysis, while quantitative analysis can be done by

measuring the X-ray energy intensity.

Energy Dispersive X-ray Florescence Spectrometer (JEOL Model JSX-3202 M) was

used to get the XRF spectra of the samples. 30 kV and 0.54 mA tube voltage and


Page 32

current were applied, respectively. Scan energy range was 0-41 keV and

collimator was of 4mm diameter.

Figure 2.3 Electronic processes in XRF

2.8.5 BET Surface Area Analysis

S. Brunauer, P.H. Emmett, and E. Teller reported an article regarding the BET

theory, in 1938. This theory explains the physisorption of gaseous molecules on

the surfaces of solid to determine the surface area, which has large effect on the

activity of catalysts. From the molar volume of adsorbed gases on the surface of a

solid sample, one can determine the surface areas of catalyst.[148]. The BET

equation is given as:

__________

v (P0/P - 1)=

_________

vmc+ _____

vmc

1 1c-1 (P/P0) (2.4)

In the above equation, P is the equilibrium and P0 is the saturation pressure of

adsorbate, v is the quantity of adsorbed gas, vm is the quantity of monolayer

adsorbed gas and c is BET constant. The above equation can be plotted with φ =

P / P0 on the x-axis and 1 / v [(P0 / P) − 1] on the y-axis. The straight line of this


Page 33

plot remains only in small range. The adsorbed gas quantity vm is calculated from

the slope A and the intercept I of the plot.

BET surface areas were measured by N2-adsorption at 77.4K using Coulter SA

3100 analyzer. Before measuring the adsorption of the samples, they were

degassed using flowing nitrogen at 120 oC for 3h.

2.8.6 Gas Chromatography-Mass Spectrometry (GC-MS)

Gas chromatography-mass spectrometry (GC-MS) is the blend of two powerful

analytic techniques, known as gas chromatography (GC) and mass spectrometry

(MS). The basic priniple of GC is based on separation of components from a

mixture when it is heated. The gases produced in this heating are pushed to a

column with flowing inert gas helium and argon which ultimately flows into the

MS. In MS, these molecules break up into smaller, highly energetic positively

charged ions ions like fragment ions, molecular ions and fragment radical ions by

the bombardment of electrons. These positive ions are separated according to

their charge to mass ratio by applying a variable magnetic field. Relative

abundance of ions is proportional to the current generated at the collector and

the plot of relative abundances verses mass/charge ratio is termed as a mass

spectrum [149].

On-line gas chromatograph (Hewlett-Packard, Model 6890) were used to analyze

the products by the use of flame ionization and thermal conductivity detector.

Carrier gas used was Helium at a pressure of 0.68 bar. The GC condition was set

as column oven FID oven temperature was 80 oC, column flow was 2ml/min,

column pressure was 25 bar. MS was operated at 70eV scan rate was 0.5μ/sec.


Page 34

2.8.7 Temperature Programmed Desorption, Reduction, Oxidation (TPDRO)

The TPDRO combines the most commonly used techniques of temperature

programmed desorption, reduction and oxidation, using a wide range of pure

gases and mixtures. The TPDRO analysis is used in heterogeneous catalysis to

analyze the oxidation, reduction behavior of catalyst precursors. This technique

is mostly used to locate the most suitable reduction/oxidation conditions along

with identification of supported precursor phases, their interactions and to

determine the promotion effect of various components of a catalyst. In a typical

analysis, the sample is first heat treated at constant rate and then it is swept by an

inert gas (helium, argon or nitrogen). The chemisorbed gases desorbed from the

surface of the sample are recorded by the thermal conductivity detector. TPDRO

can also be used for energetic abundance of active sites, adsorption/desorption

energies and free metal surface area estimations [150]. The TPR and TPD study

for supported cobalt phase was carried out using H2-Temperature Program

Reduction Catalytic Surface Analyzer (TPDRO/1100 Series, Thermo Electron

Corporation, Italy).

2.9 Catalyst Testing System

The equipment used for these studies is shown schematically in Figure 2.4. It

consists of a catalytic reactor tube made up of stainless steel 1/2 inches wide and

2 ft long. A thermocouple rod is located on the walls of the reactor tube for

sensing the temperature which is displayed digitally. The reactor tube is

encapsulated by a tube furnace to obtain the desired temperature during the

reaction. On one side the reactor tube is attached to the gaseous cylinders and on


Page 35

the other side to the outlet tube to take the product gases away from the system

when required and also to the GC/MS apparatus for online analysis. The reactor

exhaust was connected with the on line GC equipped with Poropak Q column.

The exhaust gases were analyzed using TCD and FID (thermal conductivity

detector and flame ionization detector). The gas flow controllers and flow meters

also form the part of a system to control the flow of the gases [151].

Figure 2.4 Schematic diagrams showing the positioning of equipment and flow of gases

2.9.1 Reaction conditions

(i) Flow of Argon through the system at flow rate 120-140 ml/min for 30

min to create the inertness inside the system.

(ii) 0.5 to 1 g of each catalyst was placed into reactor tube as shown in the

Figure 2.4.

(iii) Reduction of the catalyst at 400 oC by passing H2 flow rate 120-140

ml/min for 6 h to activate the catalyst.

(iv) Flow of Argon through the system at flow rate 120-140 ml/min for 2 h

to get rid of any H2 in the system because it can react with the O2 at

such a high temperature exothermally. It can lead to a sudden increase

in temperature and thus ultimate blast of the system.

(v) Passage of the reactant gases (H2/CO (2:1) ) at 225-275 oC at flow rate

120-140 ml/min at pressure of 1 to 10 bar.


Page 36

(vi) Measurements regarding catalysts selectivity and efficiency were

repeated at least twice.

2.9.2 Procedure

Argon gas was passed through the catalyst testing system for 30 min at the flow

rate of 120-140 mls /min to create inert atmosphere inside the testing system. 0.5

to 1g of each catalyst was placed into reactor tube as shown in the Figure 2.4.

Catalyst was then reduced by the flow of hydrogen gas at a flow rate of 120-140

ml /min at 400˚C for 6 h. Once again argon is passed through the system at the

flow rate of 120-140 ml/min for 30 min to purge the assembly. Reactant gas

(H2/CO (2:1) ) mixtures were passed through the catalyst bed at a flow rate of

120-140 ml/min at 225˚C to 275˚C and pressure of 1-10 bar. The gaseous mixture

was analyzed by the on-line GC/MS. For organic fraction FID while for inorganic

gases TCD were used. For calibration purpose molar response factor of

hydrocarbons (Table 2.1) were used [152].


Page 37

Table 2.1 Molar response factors for hydrocarbon products

Carbon number Olefin Paraffin

2 1.00 1.00

3 0.70 0.74

4 0.78 0.55

5 0.47 0.47

6 0.40 0.40

7 0.35 0.35

8 0.32 0.32

9 0.28 0.28

10 0.24 0.24

11 0.21 0.21

12 0.19 0.19

13 0.18 0.18

14 0.17 0.17

15 0.15 0.15

2.9.3 Mass balance calculations

Mass balance was calculated according to the method described by jalama et al

[152] given below

𝐹𝑖𝑛 × 𝑋𝑁2𝑖𝑛 = 𝐹𝑜𝑢𝑡 × 𝑋𝑁2𝑜𝑢𝑡

(2.5)

Where: Fin = total molar flow rate [moles/min] of the reactor feed;

Fout = total molar flow rate [moles/min] of the reactor outlet gas stream;

𝑋𝑁2𝑖𝑛 = molar fraction of nitrogen in the reactor feed;

𝑋𝑁2𝑜𝑢𝑡 = molar fraction of nitrogen in the reactor outlet gas.

The rate of CO-conversion can be calculated as follows:

−𝑟𝐶𝑂 =𝐹𝐶𝑂 ,𝑖𝑛 −𝐹𝐶𝑂 ,𝑜𝑢𝑡

𝑚𝐶𝑎𝑡 (2.6)


Page 38

Where: FCO,in = molar flow rate [moles/min] of CO in the reactor feed;

FCO,in = molar flow rate [moles/min] of CO in the reactor outlet gas;

mcat. = mass [gram] of catalyst;

rCO = rate of CO-conversion [moles.min-1.gcat-1]. This rate is multiplied by

-1 in (2.6) to report positive values.

𝐹𝐶𝑂,𝑖𝑛 = 𝐹𝑖𝑛 × 𝑋𝐶𝑂,𝑖𝑛 (2.7)

𝐹𝐶𝑂,𝑜𝑢𝑡 = 𝐹𝑜𝑢𝑡 × 𝑋𝐶𝑂,𝑜𝑢𝑡 (2.8)

Where XCO,in and X CO,out are the CO molar fraction in the reactor feed and outlet

gas respectively.

After introducing Eq (2.7) and (2.8) in Eq (2.6) and after expressing

Fin as a function of Fout using Eq 2.5, the rate of CO-conversion rate was expressed

as:

−𝑟𝐶𝑂 =

𝐹𝑜𝑢𝑡 × [𝑋𝐶𝑂,𝑖𝑛 × (𝑋𝑁2 𝑜𝑢𝑡

𝑋𝑁2 𝑖𝑛

) − 𝑋𝐶𝑂,𝑜𝑢𝑡 ]

𝑚𝐶𝑎𝑡 (2.9)

In this thesis Eq (2.9) was used to calculate the rate of CO-conversion directly as

XCO,in and XN2, in were known from the pre-mixed gas cylinder and XCO, out and

XN2,out were obtained from the reactor outlet gas analysis. Fout was also calculated

from the total gas volumetric flow rate at the reactor exit assuming the ideal gas

law. In some cases the the rate of CO-conversion was further converted to some

other units, e.g. µmol.min-1.gactive metal-1, etc.

The CO-conversion was calculated as follows:

%𝐶𝑂𝐶𝑜𝑛𝑣 =

[𝑋𝐶𝑂,𝑖𝑛 − 𝑋𝐶𝑂,𝑜𝑢𝑡 × (𝑋𝑁2 𝑖𝑛

𝑋𝑁2 𝑜𝑢𝑡

) × 100

𝑋𝐶𝑂,𝑖𝑛 (2.10)


Page 39

The rate of formation of a gas product θi was calculated as follows:

𝑟𝜃𝑖 =𝐹𝑜𝑢𝑡 × 𝑋𝜃𝑖 ,𝑖𝑛

𝑚𝐶𝑎𝑡 (2.11)

Where rθi is the rate in mole.min-1.gcat -1 and Xθi the molar fraction of product θi

in the reactor outlet gas.

The carbon balance was checked as follows:

[𝑛𝐶]𝑔𝑎𝑠 .𝑝𝑟𝑜𝑑𝑢𝑐𝑡 + [𝑛𝐶]𝑙𝑖𝑞𝑢𝑖𝑑 .𝑝𝑟𝑜𝑑𝑢𝑐𝑡 + [𝑛𝐶]𝑤𝑎𝑥 .𝑝𝑟𝑜𝑑𝑢𝑐𝑡 = −𝑟𝐶𝑂 × 𝑡 × 𝑚𝐶𝑎𝑡 (2.12)

Where nC represents the total number of moles of carbon contained in a product

fraction (gas, liquid or wax) at the end of the mass balance period, t.

The error on the carbon balance was calculated as:

% 𝑒𝑟𝑟𝑜𝑟

={−𝑟𝐶𝑂 × 𝑡 × 𝑚𝐶𝑎𝑡 − [𝑛𝐶]𝑔𝑎𝑠 .𝑝𝑟𝑜𝑑𝑢𝑐𝑡 − [𝑛𝐶]𝑙𝑖𝑞𝑢𝑖𝑑 .𝑝𝑟𝑜𝑑𝑢𝑐𝑡

− [𝑛𝐶]𝑤𝑎𝑥 .𝑝𝑟𝑜𝑑𝑢𝑐𝑡 } × 100

−𝑟𝐶𝑂 × 𝑡 × 𝑚𝐶𝑎𝑡 (2.13)

The carbon balance was considered satisfactory when the % error was ≤ 5%.

The product selectivity was calculated on moles of carbon basis as follows:

𝑆𝑒𝑙 𝜃 =[𝑛𝐶]𝜃

−𝑟𝐶𝑂 × 𝑡 × 𝑚𝐶𝑎𝑡 (2.14)

where Sel (θ) represents the selectivity of product θ and [nC] θ represents the

moles of carbon contained in the product θ.

Chapter 3 Results and Discussion

Page 40

Chapter 3. Fischer Tropsch Synthesis Over Al-modified

Montmorillonite Supported Cobalt Nanocatalysts

3.1 Introduction

Due to the increasing energy demand and rapidly depleting resources of

petroleum, huge reservoir of natural gas, coal and biomass can be utilized as an

alternative to crude oil for the synthesis of sulfur, aromatics and nitrogen free

ultra clean fuel along with value added fine chemicals via Fischer-Tropsch (FT)

synthesis [16, 153].

FT synthesis reaction has attracted worldwide attention to convert syngas (CO

and H2) via gasification of coal, natural gas and biomass to liquid hydrocarbons

by the use of different catalysts [26, 137]. The most widely used catalysts for the

FT synthesis include different forms of Ni, Fe, Ru and Co metals. The use of these

catalysts is limited due to the excessive CH4 production in case of Ni and high

price of Ru, leaving only Co and Fe as feasible catalysts in FT synthesis [154]. Due

to the lower extent of deactivation, water gas shift activity and selectivity

towards linear hydrocarbons, Co is preferred over Fe-based catalysts in academic

as well as on industrial scale [42].

The catalytic activity of Co-based catalysts can be improved by the dispersion of

Co on the surface of refractory oxides such as SiO2 and Al2O3 and mesoporous

oxides like MCM-48, MCM-41 and SBA-15 which are thought to be responsible

for increase in the surface area of Co and hence its catalytic activity [42, 136, 155-

157]. The catalytic activity as well as the products selectivity of FT catalysts is also

affected by the nature of support. In FT synthesis, Zeolites have been used as a


Page 41

support to synthesize hydrocarbons of specific molecular weights and to change

the FT catalyst products distribution [158].

The application of montmorillonite (MMT), a naturally occurring layered

material has been studied extensively in the field of catalysis for the

hydrocracking of petroleum products in refineries [80]. Besides this pillared clay

supported Co-catalysts show good selectivity and catalytic activity in FT

synthesis [159]. However the presence of alkali metals especially Na in the MMT

catalyst is responsible for the production of large amount of CH4, CO2 and lower

FT catalytic activity, due the hindrance in CoO reduction. The FT catalytic

activity of MMT supported catalysts can be increased by pillaring MMT with

certain larger metal cations, among which Al show higher CO-conversion,

indicating that Al-PILC has a positive impact on the CO-conversion in FT

synthesis [137].

Up to now the application of pillared MMT catalysts is largely concerned with

the hydrocracking of petroleum products in refineries and there are very limited

reports on the application of Co supported MMT catalysts in FT synthesis. The

effect of reaction temperature, pressure and reactant gas flow is not reported yet.

We have set ourselves a task to design such a catalysts which can convert these

waxes to lower hydrocarbons while FT synthesis which can be fulfill by

supporting the Co on MMT to achieve the desired goal and to study the effect of

reaction condition such as reaction temperature and pressure.


Page 42

3.2 Experimental

3.2.1 Preparation of Al-PILC

The Al-PILC was prepared by the same procedure as described in section 2.2.

3.2.2 Preparation of Co-loaded/Al-PILC

For the preparation of Co-loaded/Al-PILC same procedure already discussed in

section 2.3 was followed.

3.2.3 Characterization of Prepared catalysts

The synthesized catalyst samples were analyzed using Scintag XDS 2000

diffractometer. BET pore volume and surface area were calculated at liquid

nitrogen temperature (78 K) by the use of Coulter SA 3100 BET instrument.

TPDRO/1100 Series, Thermo Electron Corporation, Italy was used for the

reduction study of supported cobalt phase, Thermo Electron Corporation, Italy).

Thermo gravimetric analysis (TGA) was carried out on Mettler TGA/SDTA 851e.

The chemical composition was determined by the use of XRF (JEOL Model JSX-

3202 M). The morphological characterization of catalysts was studied with

scanning electron microscopy (JEOL JSM 6490-A) equipped with Energy

Dispersive X-ray Spectrometer (EDX).

3.2.4 Catalyst evaluations

As seen in Figure.3.1 the equipment was made of a stainless steel cell with two

windows through which the spectrometer beam can pass. It was connected to a

temperature programmer to heat the inside and the windows of the cell. There

was a mass flow controller connected to three gas cylinders containing synthesis

gas, hydrogen and Argon. The Gas Chromatography was placed at the outlet of

the cell and was alimented by Helium, Air and Hydrogen shown in Figure 3.1.


Page 43

Figure 3.1 Schematic diagrams showing the positioning of equipment and flow of gases

The FT experiments were performed using a sample of catalyst lightly pressed so

as to obtain a disk of 13 mm of diameter with a defined weight, the disk was put

into the cell. To remove air from the cell, the experiment begun by flushing Ar at

50 ml/min for 20 min. Once the catalyst had been reduced at 400 °C during 1 h in

H2, the FTS reaction was carried out at 225 °C, 260 °C, 275 °C at 1, 5 and 10 bar

pressure and feed flow of synthesis gas during 2-30 h. The synthesis gas was

composed of carbon monoxide and hydrogen with a ratio of CO/H2 1:2. The

products were analyzed on-line by a gas chromatograph (Hewlett-Packard,

Model 6890) equipped with flame ionization detector. We can also Record the IR

spectra using a Perkin Elmer 1750 FTIR spectrometer connected to this system.

3.3 Results and Discussion

3.3.1 Textural and Structural properties

Al-PILC, 10, 15, 20 and 25 %Co/Al-PILC were synthesized and their XRD

patterns are given in Figure 3.2 and 3.3.

Figure.3.2a shows the XRD pattern of MMT having (001) diffraction peak with a

basal spacing of 12.06 Å and the thickness of the host layer of MMT is 9.3 Å[139].


Page 44

The gallery height was found to be 3.3 Å, which was calculated by subtracting

the host layer thickness from the basal spacing of the MMT.

Studying the XRD spectrum of Al-PILC, shown in Figure. 3.2b, the Al-addition

has shifted the peak to a comparatively lower 2 θ value, consequently resulted in

an increase of d-spacing from 12.06 to 18.7 Å and the gallery height from 3.3 to

9.4 Å. This causes the expansion of the clay inter planar distance with Al-pillar,

similar to the ionic structure of Keggin type [Al13O4(OH)24(H2O)12]7+ as reported

in reference [138]. The XRD spectra of Al-PILC gives diffraction planes at 8o,

19.7o, 35o and 61.8o which are characteristic of two dimensional layers structure

of clay [160]. However, the peak at 28o and 55o can be attributed to quartz and

hydroxide of metal silicate impurities [161].

The intensity of the (001) peak was also decreased which can be attributed to the

intercalation of Al into the MMT clay interlayer’s, no other structural changes

were observed in the XRD spectra of both the samples as rest of the peak

positions remained unchanged.

Figure 3.3 shows the XRD pattern of Al-PILC catalysts loaded with cobalt. All the

catalysts show (001) diffraction peaks and basal reflection of Al-PILC with a

small decrease in their intensity which corresponds to the slight decrease in

layered structure of MMT.

Cobalt loaded catalysts giving cubic cobalt oxides (Co3O4) diffraction at 19.5o,

32o, 37o, 40o and 60o (JCPDS 65-3103) indicates a uniform dispersion of pure

Co3O4 on pillared MMT.


Page 45

Figure 3.2 XRD patterns of the Montmorillonite (a) NaMMT and (b) Al-PILC

Figure 3.3 XRD patterns of the 10, 15, 20, 25 % Co/ Al-PILC

Table 3.1 Textural properties of the prepared catalysts

Sample BET

(m2/g)

Pore diameter

(Å)

Pore volume

(cm3/g)

Na MMT

Al-PILC

10 % Co/Al-PILC

15 %Co/Al-PILC

20 %Co/Al-PILC

25 %CO/Al-PILC

39.243

254

238

235

235

229.68

4.8

18.3

15.67

15.35

15.75

15.32

0.08

0.20

0.18

0.191

0.189

0.19


Page 46

Table 3.1 show that NaMMT having lower surface area of 39.243 m2/g while Al-

PILC having larger surface area of 254 m2/g. The larger surface area for Al-PILC

was attributed to the ability of Al to replace the Na+ present in MMT results an

increase in pore volume responsible for N2 adsorption[80]. Small decrease in BET

surface area for Co-loaded catalysts seen from the table can be attributed to the

presence of metal particle at pore opening and on the surface of the pores. The

average pore diameter and pore size calculated for all of the sample were

constant therefore the pore blocking of Co can be neglected [159]. Enhanced FT

activity can be expected from the Al-PILC supported Co catalysts due to its high

surface area and pore diameter larger than the interlayer distance of MMT

similar to the previous study [139]. The adsorption isotherm (Figure 3.4) for

NaMMT shows the non porous nature of NaMMT and the type I isotherm for

Co/Al-PILC confirms its microporous nature. After calcinations at 400oC the

same type isotherm obtained for the all the sample with slight change in pore

diameter and pore volume confirmed that the narrow slit shaped porous

structure of MMT did not change after calcinations.


Page 47

Figure 3.4 N2 Adsorption desorption Isotherm of Na MMT, Al-PILC and Co/Al-MMT

catalysts

3.3.2 Acidic Property

NH3-TPD was used to evaluate the acidic properties of the prepared samples

(Figure 3.5). it is reported that pillared clay poses both Lewis and Bronsted acidic

site [162]. The lower temperature desorption peak at 150 oC and higher

temperature desorption peak at 600 oC can be attributed to weaker Bronsted

acidic and stronger Lewis acidic site respectively. The lower temperature

desorption peak (at 150 oC, Bronsted site) of Al-PILC is due to the presence of

hydroxyl groups in the layered structure of MMT [163]. However, the high

temperature desorption (600 oC) for strong Lewis acidic site can be attributed to

the alumina pillars [164, 165].The Co loading shows little effect on the acidity of

Pillared MMT by increasing the intensity of Bronsted acid peak hence improving

the Bronsted acidity. The overall acidic property of the Co loaded sample slightly

decreases, may be as a results of Co deposition on pore opening and blockage of


Page 48

some pore responsible for NH3 desorption and hence decreases the density of

Lewis acidic sites of pillared clay similar result have already been reported for

vanadium loaded pillared clay [162].

Figure 3.5 NH3-TPD profile of Samples

3.3.3 SEM studies

To study the surface morphology of our prepared sample SEM characterization

was used. The SEM micrograph for NaMMT and CO/Al-PILC are given in

Figure 3.6 (a and b) shows the same flake like morphology which infer that the

calcinations temperature , pillaring of Al and Co loading have no effect on the

MMT layered structure.


Page 49

Figure 3.6 SEM Images of (a) NaMMT (b) Al-PILC (c)10%Co/Al-PILC (d) 15 %Co/Al-

PILC (e) 20 %Co/Al-PILC (f) 25 %Co/Al-PILC

3.3.4 Reduction behavior

The H2 -TPR profile of 10%, 15 %, 20% and 25% Co/Al-PILC are given in Figure 3.7.


Page 50

The small peak around 205 oC for the sample is due to the decomposition of

Co(No3)2 remain in the sample after calcinations, while the first and second

reduction spectra for 20% Co/NaMMT at 288 and 395 oC is due to the reduction

of Co3O4 to CoO and CoO to Coo respectively is similar to the previously

reported results [80]. The TPR profile of Al-PILC supported Co samples shows

that the two steps reduction of cobalt oxides occurs at lower temperature of 275

oC and 388 oC, respectively. The decrease in reduction temperature of Al-PILC

supported cobalt samples can be attributed to enhanced dispersion of cobalt

particles on the surface of support and high surface area of pillared MMT due to

Al pillaring.

Figure 3.7 H2 profiles of the prepared samples

3.3.5 Thermogravimetric analysis (TGA)

TGA results of 20 % Co/NaMMT and different wt % Co/Al-PILC has been

carried out (Figure 3.8). This is clearly seen from TGA data that all samples


Page 51

exhibit continuous weight lose behavior till 800oC. The weight lose upto 160oC

can be attributed to the removal of surface adsorbed water while the weight lose

between 160oC to 500oC is attributed to the interlayer water and due to the

dehydroxylation of the clay. Interestingly, as the wt % of Co increased, the

weight loses around 300 oC also increased. The continuous weight lose from

600oC till 800 oC was attributable to the removal of hydroxide groups as a result

of dehydroxylation of pillar and clay structure, causing collapse in MMT layer

structure [162, 166-169]. These results are also in agreement with previously

reported results for thermal behavior of vanadia-loaded pillared clays [166].

Figure 3.8 TGA Curve of the samples

3.4 FT catalytic activity test

3.4.1 Catalytic activity of 20%Co/NaMMT

The catalytic performance of 20 % Co/NaMMT in terms of selectivity, CO-

conversion and time on stream is presented in Figure 3.9a and b respectively.


Page 52

With increasing time on steam (TOS), the catalysts show increased CH4-

selectivity and very small selectivity for the remaining hydrocarbons.

The catalyst stability of Co/NaMMT was studied for 10 h and we initially

observed a CO-conversion of around 10% but it went on decreasing till it reached

to less than 5% in 10 h TOS. Simultaneously, increase in the CO2 and CH4-

selectivity of catalysts was observed which increased to more than 40% during

this TOS. These gases are the most unwanted products in FT synthesis and must

be reduced to the minimum possible levels. The low FT activity of the

Co/NaMMT observed in this case can be attributed to their lower surface

area,larger particle size of Co over NaMMT and hindrance of CoO reduction to

Co by NaMMT as reported in another study [170]. From this study, it was found

that first two had no significant effect on the CO-conversion but the lower FT

activity and higher selectivity toward CH4 was mainly due the incomplete

reduction of cobalt oxides. Beside this,the presence of alkali and alkaline earth

metals specially Na showed marked negative effect on activity of FT catalysts

due to the inhibition of CoO reduction [80].

The FT products obtained at a TOS of 10 h for this catalyst mainly include the C2-

C4 and lesser amount of C5-C7 hydrocarbons. From the above discussions, it can

be concluded that Co/NaMMT shows very poor catalytic activity in FT

synthesis.


Page 53

Figure 3.9 CO-conversion and CH4-selectivity verses Time on stream for (a) 20%

Co/NaMMT, (b) Different wt %Co/Al-PILC catalysts

3.4.2 Catalytic activity of Co-loaded/Al-PILC

The pillaring of NaMMT with Al enhances the surface area of support MMT

from 39.243 to 254 m2/g. It also increases the pore volume and pore diameter due

to the contribution of Al to replace the Na+ in the MMT interlayer [80] and hence

the catalytic activity of cobalt MMT catalysts. The inter layer cation plays a vital

role in the CO conversion. As compared to Co supported on NaMMT, AlMMT

supported catalysts show higher CO conversion. The CO-conversion was 26, 27,

28 and 27.2% for 10, 15, 20 and 25 % Co/Al-PILC catalysts respectively (Figure.

3.9 b) which decreases with TOS and reaches a minimum of 10.5% after 30 h of

TOS. The higher CO-conversion was seen for 20% Co/Al-PILC catalysts which

was 28 %, decreases with TOS and reaches a minimum of 11% after 30 h of TOS.

The CO-conversion in case of Al-PILC supported Co catalysts was higher than

NaMMT supported catalysts, indicating that the Al pillaring of MMT has a


Page 54

positive impact on the CO-conversion in FT synthesis is accordance with the

results obtained by Wang et al [80]. The FT products obtained over Co-

loaded/Al-PILC catalyst showed increased selectivity of C2-C12 hydrocarbons

and decreased selectivity towards higher molecular weight hydrocarbons C21 at a

TOS of 2-30 h. This decrease in C21 selectivity is due to the cracking of long chain

hydrocarbons at the acidic site of MMT [15, 171-174]. The selectivity towards C2-

C12 hydrocarbons was found to be maximum at a TOS of 8 h.

Table 3.2 Results of Different catalysts for FT synthesis

Catalysts CO

conversion

CO2

Selectivity

C1

(Wt %)

C2-C12

(Wt %)

C13-C20

(Wt %)

C21

(Wt %)

20%Co/NaMMT 10 2.3 40 12.5 8 37.4

10%Co/Al-PILC 26 2.3 27.27 18.23 11 41.4

15%Co/Al-PILC 27 2.08 26 19.2 11.7 41.1

20%Co/Al-PILC 28 1.98 26.89 20.03 12 39.0

25 %Co/Al-PILC 27.2 1.97 28.3 19 13 37.7

3.4.3 Effect of reaction temperature on catalytic activity of catalysts.

The effect of reaction temperature on catalytic activity of 20%Co/Al-PILC was

investigated (Figure 3.10 a).

The reaction carried out at a temperature of 225oC gives almost 28% CO-

conversion with CH4-selectivity of 26.89 %. When the reaction temperature is

increased to 260 and 275 oC the CO-conversion and CH4-selectivity were also

increased to 32, 35 and 37, 40 % respectively along with decrease in C5+

hydrocarbons selectivity. This effect of the catalysts can be attributed to the high


Page 55

hydrogenation activity of catalysts at high temperature similar to the

performance of standard Cobalt thorium catalysts [175]. Increase in reaction

temperature from 225 to 275 oC produces too much CH4 and therefore it is not

beneficial in Co based FT synthesis as an agreement with the results reported

earlier [176].

Figure 3.10 Effect of Reaction Temperature (a) and reaction pressure (b) on FT catalytic

activity

3.4.4 Effect of reaction pressure on catalytic activity of catalysts.

Reaction pressure largely effect the activity and selectivity of FT catalysts. The FT

reaction carried out at a pressure of 1, 5 and 10 bar showed CO-conversion of 28,

32 and 33% respectively (Figure 3.10b). Increased selectivity of CH4 and C6+

hydrocarbons, while decreased selectivity of C2-C5 hydrocarbons were achieved

by increasing the pressure of reaction. High pressure favors the production of

high molecular weight hydrocarbons as a result of chain growth probability

which normally increased with increasing pressure as reported earlier [177]. The


Page 56

increase in CO-conversion is due to the increase in partial pressure of hydrogen

and the increase selectivity of the C6+ hydrocarbons is due to the increase CO

partial pressure while the reaction rate in such type of reaction is proportional to

the hydrogen partial pressure have been previously reported [178].

3.5 Conclusions

The ability of MMT as a support for FT catalysts was successfully improved by

pillaring with Al. The inhabitation of CoO reduction and increased CH4-

selectivity of MMT in FT reaction caused by the presence of Na in MMT was

overcome by pillaring with Al. Increase in the surface area from 39.243 to 254

m2/g along with increase in pore volume and pore diameter may be explained

due to the contribution of Al to replace the Na+ in the MMT results into its pore

opening and many pores responsible to adsorbed N2 is produced [80].

The ability of Co catalysts for FT synthesis was improved by supporting it on Al-

PILC. Al-PILC supported Co catalysts showed higher CO-conversion and greater

selectivity towards C2-C12 hydrocarbons and deceases selectivity towards C20+

hydrocarbons as a result of hydrocracking reaction taking place at the acidic sites

of MMT. With increase in reaction temperature CH4-selectivity and CO-

conversion increased and the selectivity towards C5+ hydrocarbons decreased

due to the high hydrogenation activity of catalysts at high temperature. Decrease

in CH4-selectivity while increase in C5+ hydrocarbons and CO-conversion was

observed on increasing the pressure of reaction from 5 to 10 bar.


Page 57

Chapter 4. Effect of Manganese Promotion on Al-Pillared

Montmorillonite Supported Cobalt Nanoparticles for

Fischer-Tropsch Synthesis

4.1 Introduction

The application of montmorillonite (MMT), a naturally occurring layered

material has been studied extensively in the field of catalysis for the

hydrocracking of petroleum products in refineries [80]. Besides this pillared clay

supported Co-catalysts show good selectivity and catalytic activity in FT

synthesis [159]. The FT catalytic activity of MMT supported catalysts can be

increased by pillaring MMT with certain larger metal cations, among which Al

show higher CO-conversion, indicating that Al-PILC has a positive impact on the

CO-conversion in FT synthesis [137].

The addition of Mn, Ti, Zr etc as a promoter can also boost the performance and

suppress the metal support interaction of FT catalysts [179-185]. The CH4-

selectivity of Fe or Co-FT catalysts is significantly decreased by the addition of

Mn [186]. It is reported that Mn-promotion stabilizes the catalytic activity,

improves the formation of light olefins, higher hydrocarbons and decreases the

CH4-selectivity for Fe, Co-FT catalysts [59, 187-190]. Also the metal support

interaction in case of Co/TiO2-FT catalyst is suppressed by the addition of 3%wt

Mn and the selectivity of this system for lower olefins and C5+ hydrocarbons is

enhanced due the stabilization of larger Co particles [59, 60, 191]. To decrease the

CH4 and increase the hydrocarbons selectivity the promotional effect of Ce or Ru

on Co catalysts supported on pillared MMT has also been reported [159], though


Page 58

there is no report on the effect of Mn on MMT supported Co-catalysts for FT

synthesis.

In light of the above discussion, we have set ourselves a task to study the effect of

Al-pillaring and the promoter effect of Mn on Al-PILC supported cobalt

nanocatlysts for FT reaction.

4.2 Experimental


Al-PILC was prepared by the same procedure as described in section 2.2.

4.2.2. Preparation of different Wt% Mn-20Wt% Co-loaded /Al-PILC

The same procedure discussed in section 2.4 is adopted for the preparation of

different Wt% Mn-20Wt% Co-loaded /Al-PILC.

4.2.3 Characterization of Prepared catalysts

After preparation of our catalysts same characterization tools were used as

already discussed in section 3.2.3.


The catalytic activity of the samples was tested in fixed bed micro reactor (Figure.

4.1) at atmospheric pressure. The catalytic activity were carried out by different

experiments in term of reaction temperature, catalyst volume and reaction

pressure. 1.0g of Mn-promoted Co/Al-PILC catalysts was put into the middle of

stainless steel catalytic reactor tube and adjusted in center by mean of Quartz

wool. H2 stream at 400 ◦C were used for the reduction of catalysts before the FT

reaction. The syngas reaction was carried out for 30 h at 1 atm pressure and with

a nominal H2/CO ratio of 2. Varian, CP-3800 gas chromatograph was used for

analysis of reactant and products of FT reaction with help of flame ionization and


Page 59

thermal conductivity detector. Helium was used as a carrier gas in this

instrument. The GC was pre calibrated with standard hydrocarbons mixture. GC

was equipped with a packed column (CP-PoraPLOT Q fused silica PLOT, 25

m×0.53 mm, dn = 20 μm) [151]. The schematic diagram showing the positioning

of equipment and flow of gases is already shown in Fig. 2.4.

4.3. Results and discussion

4.3.1 Textural and Structural properties

The XRD pattern of MMT, Al-PILC and Co/Al-PILC (10%, 15%, 20%, 25%) are

discussed in chapter 3 and are shown in Figure 3.2 and 3.3 respectively.

20%Co/Al-PILC was used for loading various concentrations of Mn and their

XRD pattern are shown in Figure 4.1.The average crystallite size of the Co3O4

nanoparticles over Al-PILC was calculated using the scherer equation and was

found to be in the range of 22-50 nm. Due to high dispersion and lower content

of Mn on Al-PILC, all the XRD spectra showed no peaks corresponding to Mn or

Co3-xMnxO4 with the exception of 3.5Mn-Co/Al-PILC as shown in Figure 3f,

which showed a small peak for Co3-xMnxO4 and a shift to lower 2θ value as

observed in another study [60]. There was no reflection of cobalt aluminates or

silicate in the XRD pattern confirming that only highly dispersed Co3O4 phase

exists. The addition of Mn helped in uniform dispersion of Co3O4 nanoparticles

and enhanced the reduction responsible for increased FT activity.


Page 60

Figure 4.1 XRD pattern of (a) Co/Al-PILC (b) 0.10Mn-Co/Al-PILC (c) 0.30Mn-Co/Al-PILC

(d) 0.70Mn-Co/Al-PILC (e) 1.5Mn-Co/Al-PILC (f) 3.5Mn-Co/Al-PILC

Figure 4.2 shows the N2 adsorption and desorption isotherm for of Mn-promoted

Co/Al-PILC which is Type I isotherm indicative of porous structure of Al-PILC.

The narrow slit shaped porous structure of MMT was confirmed after Al

pillaring from the hysteresis loop. The Mn and Co-loading to the Al-PILC did not

change its narrow slit shape porous structure as obvious from the same isotherm

shown for all the catalysts, although there is a decrease in the BET surface area

for Co and Mn-loaded Al-PILC catalysts along with minor changes in pore

diameter and volume.

Table 4.1 shows pore diameter, BET surface area and average pore volume of our

prepared samples after calcinations at 400oC. There is no evidence of decrease in

surface area for all prepared catalysts upon calcinations at 400oC.


Page 61

The Pillaring of MMT with Al produces significant increase in the surface area

from 39.243 to 254 m2/g along with increase in pore volume and pore diameter,

which can be explained due to the contribution of Al to replace the Na+ in the

MMT causing its pore openings to adsorb N2 [80]. The loading of Co

nanoparticles and Mn to the catalysts produce minor decrease in the surface area,

pore diameter and pore volume of Al-PILC which may be attributed to the

presence of active metal particle on the surface and specifically at pore openings

but the average pore size and volume remain constant for all of the catalysts,

therefore the pore blocking of Co and Mn are negligible [159]. The total pore

diameter of our prepared catalysts Al-PILC, Co/Al-PILC, Mn-promoted Co/Al-

PILC were 18.3 Å, 15.75Å, 17.32 Å respectively which is larger than the inter

layer distance of these catalysts. The reason for this increase in pore size is the

formation of delaminated structure responsible for increase in MMT pore size, a

similar behavior reported in reference [139]. Due to the larger BET surface area

254m2/g and much larger pore size. This was surprise finding reported for the

first time that Al-PILC can play positive role as a support for the FT catalysts.


Sample BET

(m2/g)

Pore diameter

(Å)

Pore volume

(cm3/g)

MMT

Al-PILC

Co/Al-PILC

Mn-CO/Al-PILC

39.2

254.0

235

229.68

4.8

18.3

15.75

17.32

0.08

0.20

0.18

0.19


Page 62

Figure 4.2 N2 adsorption desorption isotherm of MMT, Al-PILC, Co/Al-PILC and

Mn-promoted Co/Al-PILC catalysts

4.3.2 SEM studies

SEM characterization was carried out to study the surface morphology of

prepared catalysts. Powdered specimens of the samples were used to obtain SEM

micrograph. The SEM micrograph in Figure 4.3 a,b,c and d showed uniform

distribution of Co nanoparticles on the surface of Al-PILC, pillaring with Al,

loading of Co and Mn to the MMT and calcinations at 400 oC showed no distinct

effect on the layered structure of MMT, hence the flake like morphology of MMT

remained unchanged.


Page 63

Figure 4.3 SEM Images of (a) 0.10Mn-Co/Al-PILC (b) 0.30Mn-Co/Al-PILC (c)

0.70Mn-Co/Al-PILC (d) 1.50Mn-Co/Al-PILC (e) 3.50Mn-Co/Al-PILC


Figure 4.4 shows the reduction profile of as prepared catalysts. The reduction of

cobalt oxide normally takes place in two steps.


Page 64

(i) Co3O4 CoO (4.1)

(ii) CoO Coo (4.2)

The Co/Al-PILC catalyst showed two reduction peaks at 275 oC and 388 oC for

Eq 4.1 and 4.2, respectively. A distinct change in the TPR profile is observed for

Mn-promoted Co/Al-PILC catalyst. The reduction of Co3O4 occurs at different

temperatures for different %wt of Mn as shown in Table 4.2.

Table 4.2 Two steps Co3O4 reduction temperature

Catalysts Reduction Temperature Reduction Steps

0.10Mn-Co/Al-PILC 265oC Eq (4.1)

350oC Eq (4.2)


335oC Eq (4.2)


345oC Eq (4.2)


368oC Eq (4.2)


595oC Eq (4.2)

A decrease in reduction temperature was observed for a lower Mn-loading up to

0.70% for both reactions in Eq 4.1 and 4.2 while for 1.5% a decrease in the

reduction temperature for reactions in Eq(2), because of high degree of

dispersion and stabilization of Co3O4. The highest Mn-loading (3.5%wt) hindered

the reduction of Co3O4 possibly due to the formation of Co-Mn spinal or mixed

Co-Mn oxides on the surface. The high temperature reduction peak at 595oC in

the TPR profile of 3.5% Mn was assigned to Co-Mn spinal reduction as reported

in the previous findings [60]. It can thus be inferred that the minimum loading of


Page 65

Mn enhances and promotes the Co3O4 reduction, which becomes less significant

as we increase the Mn-loading.

Figure 4.4 H2-TPR profile of the prepared samples

Figure 4.5 TGA of Co/Al-PILC and Mn-promoted Co/Al-PILC


The effect of high temperature on the decomposition of our prepared samples

were studied using thermo gravimetric analysis (TGA) shown in Figure 4.5. TGA

spectra showed that Co/Al-PILC and Mn-promoted Co/Al-PILC follow

continuous weight loss till 800oC. Weight loss at 160oC can be attributed to the


Page 66

removal of surface adsorbed water of the sample [162, 166-168], while the

continuous weight loss from 160oC to 800oC with variation at 400oC was

attributable to the removal of hydroxide groups as a result of dehydroxylation of

pillar and clay structure, causing collapse in MMT layer structure [169]. The

increased Mn-loaded samples showed greater weight loss due to the

dehydroxylation of Co and Mn species, which is in agreement with the previous

reported data for vanadia-loaded pillared clays [166].

4.4. FT Catalytic performance

4.4.1 Effect of Mn-addition on Co/Al-PILC catalysts

The catalytic performance of Co/Al-PILC and Mn-promoted Co/Al-PILC in

terms of CO-conversion, CH4-selectivity and time on stream is presented in

Figure 4.6 a and b respectively.

Figure 4.6 (a) CO-conversion (b) CH4-selectivity Vs Time on stream over Co/Al-PILC

and Mn-promoted Co/Al-PILC catalysts.


Page 67

One of the main disadvantage of the Al-PILC supported catalyst is its higher

selectivity towards CH4, which is the most undesirable product of FT synthesis

due its behavior of cracking reaction [192]. For this reason, the catalysts were

promoted with Mn and the effects of Mn-promotion are shown in Figure 4.6a, b.

Normalization method given below was used to calculate the (%) CO-conversion

[151, 187].

CO Conversion (%) =(Moles of COin) - (Moles of COout)

Moles of COin

x 100 (4.3)

The selectivity (%) towards the individual compomnent on carbon-basis is

calculated according to the same principle.

Selectivity of J product(%) =(Moles of COin) - (Moles of COout)

Moles of J productx 100 (4.4)

4.4.1.1 CO-Conversion

20 wt% Co/Al-PILC catalysts showed higher CO-conversion of up to 28%

(Figure 4.6a, b) which decreased with time on stream (TOS) and reached a

minimum of 11% after 10h of TOS. The CO-conversion increased to around 42%

for an increasing the Mn-loading of upto 1.5wt% but extra loading of Mn

(3.5wt%) resulted in a reduced CO-conversion of upto 30% which is nearly equal

to that for Co/Al-PILC catalyst. The catalytic activity and selectivity largely

affected by the reduction step in Co-based FT catalysis because metallic cobalt is

responsible for this reaction rather than cobalt oxides. The lowered catalytic

activity and higher CH4-selectivity Co/Al-PILC catalysts was due to the

incomplete reduction of Co3O4 near the reaction temperature. Thus increased


Page 68

catalytic activity and lower CH4-selectivity (Figure 4.6b) were achieved by Mn-

promotion obvious from the H2-TPR profile (Figure 4.4) that Mn-loading helped

to lower the reduction temperature of Co3O4, due to their ability of formation

small and well dispersed Co particles as compared to the un promoted Co3O4.

While the decreased activity of high Mn-loading (3.5wt%) as shown in Figure 4.6

b, was due to the hindered reducibility of Co3O4 as evident from the H2-TPR

profile (Figure 4.4). The highest Mn-loading (3.5 wt%) hindered the reduction of

Co3O4 possibly due to the formation of Co-Mn spinal or mixed Co-Mn oxides on

the surface, hence their catalytic activity is decreased.

Apart from this, the Mn-addition to the Co/Al-PILC catalyst increased the

catalyst stability, as the catalysts having larger Mn-loading showed greater TOS

stability and is in agreement with previously reported data [175].

4.4.1.2 CH4-selectivity

Figure 4.6 b shows the CH4-selectivity of the prepared catalysts with TOS. The

Co/Al-PILC catalyst showed higher CH4-selectivity of 27% which remained

constant on TOS for 30h. For the Mn-promoted Co/Al-PILC catalyst, the CH4-

selectivity decreased with increase of Mn-content along with increase in CO-

conversion for Co/Al-PILC catalyst [60]. The CH4-selectivity for 0.10Mn-Co/Al-

PILC, 0.30Mn-Co/Al-PILC, 0.70Mn-Co/Al-PILC, 1.5Mn-Co/Al-PILC, 3.5Mn-

Co/Al-PILC samples was 19.5%, 17.5%, 17.0%, 14.0% and 12.9%, respectively

which was much lower than that for Co/Al-PILC catalyst. The best results

showing the least CH4-selectivity were achieved for 3.5% Mn-loading where a

constant 12.9 % CH4-selectivity was observed for 30h of TOS. This was also

observed in another study where lower CH4, higher olefins and C5-C12


Page 69

hydrocarbons selectivity of Mn-promoted catalysts was attributed to the

presence of large number of Mn atoms on the catalysts surface [193].

Furthermore, the addition of small quantity of Mn decreased the CH4 and

increased the higher hydrocarbons selectivity due to the electronic effect of Mn

which increases the electron density of chemisorptions site of catalysts. This

depresses the hydrogen chemisorptions which normally donates electron density

to the metal chemisorptions sites, consequently decreasing the coverage of metal

surface by hydrogen which is responsible for CH4 formation. This inhibits CO

hydrogenation to CH4, giving rise to more unsaturated hydrocarbons and

enhanced hydrocarbons chain growth as reported in our previous study [194].

4.4.2 FT Reaction Products over Mn-promoted 20 wt% Co /Al-PILC

Al pillaring of MMT resulted in an increase in the surface area of MMT from

39.243 to 254 m2/g and frequently increase in its catalytic activity [80]. The

products obtained over Co/Al-PILC catalyst mainly consisted of of C2-C12

hydrocarbons and lower concentration of higher molecular weight hydrocarbons

C21 mainly due to the long chain hydrocarbons cracking at the acidic site of MMT

[15, 171-174]. Since the best results for all the prepared catalysts were obtained

for TOS of 8h, the combined data of the selectivity of the un-promoted and Mn-

promoted catalysts at this TOS is presented in Table 4.3. From this table we find

that the selectivity towards C1, C2-C12, C13-C20 and C21 hydrocarbons changed

with the addition of small amount of Mn. Acidic sites of MMT catalysts are

responsible for the cracking of higher hydrocarbons to increase C2-C12 and

decrease C21 hydrocarbons selectivity of Mn-promoted MMT catalysts. By the


Page 70

addition of Mn the selectivity of C1 dropped drastically while that of C2-C12

hydrocarbons increased significantly over all the Mn-promoted Co/Al-PILC

catalysts which was due to the limited hydrogen adsorbed on the surface of

catalysts or due the α olefins re-adsorption by a secondary reaction responsible

for chain growth. These findings are similar to the results reported by

Weckhuysen et al for Mn-promoted catalysts [59, 60]. This effect can also be

attributed to the Mn and Co interaction [59, 175, 176, 189, 195]. Furthermore, the

C13-C20 hydrocarbons remained almost same for all the catalysts while a

considerable decrease in the selectivity of C21+ long chain hydrocarbons were

achieved for Mn promoted samples. The catalysts having 3.5% Mn showed

lowest C21+ and highest C2-C12 hydrocarbons selectivity. The noticeable

improvement in selectivity of C2-C12 hydrocarbons and considerable decrease in

selectivity of C21+ hydrocarbons can be explained on the basis of cracking of long

chain hydrocarbons over MMT based catalysts as reported by Wang et al for

Co/ion-exchanged MMT catalysts [80]. In this study, the spillover of hydrogen

from Co to the acidic sites was attributed to the long distances between Co

present on the surface of MMT and acidic sites of the MMT originated from

interlayer region and sheets. This is responsible for increased selectivity of C2-C12

and decreased selectivity of C21+ hydrocarbons. These findings are in conformity

of the results presented in this study.


Page 71

Table 4.3 Results of different catalysts for FT synthesis

Catalysts CO-

conversion

CO2-

selectivity

C1

(wt%)

C2-C12

(wt%)

C13-C20

(wt%)

C21

(wt%)

Co/NaMMT

Co/Al-PILC

0.10Mn-Co/Al-PILC

0.30Mn-Co/Al-PILC

0.70Mn-Co/Al-PILC

1.5Mn-Co/Al-PILC

3.5Mn-Co/Al-PILC

10.0

28.0

32.0

39.0

40.9

42.5

30.5

2.3

2.5

2.6

2.4

2.5

2.6

2.6

40.0

27.2

19.5

17.5

17.0

14.0

12.9

12.5

18.2

38.8

31.9

39.5

44.0

63.0

8.0

11.0

9.3

12.0

13.0

11.0

4.0

37.2

41.0

29.3

36.3

26.5

28.4

17.2

CO2

selectivi

ty C1 C2-C12 C13-C20 C21

The olefin to paraffin ratio of FT reaction products are shown in Figure 4.7. It is

known that the interaction of Mn with Co decreased the availability of adsorbed

hydrogen hence the Mn-promotion increased the olefin to paraffin ratio (O/P)

[196]. The promotion of catalyst with Mn suppressed the H2 addition and

enhanced the light olefin production as reported previously [60].

The Co/Al-PILC catalyst showed that the C2-C4 hydrocarbons were almost

olefinic with lower CO-conversion. When the CO-conversion increased, the

fraction of C2-C4 paraffins also increased and resultantly, olefinic fractions

decreased due to the secondary hydrogenation of olefins to paraffins and higher

hydrocarbons which had also been reported earlier by Dinse et al [196].


Page 72

Figure 4.7 Influence of Mn-promotion on O/P ratio with in C2-C6 fraction

4.5 Conclusions

The FT catalytic activity of Co/Al-PILC and the effect of Mn-promotion on their

activity were investigated in this work. Following are the main conclusions of

this study.

(i) The Al-PILC having large surface area of 254 m2/g along with increase in

pore volume and pore diameter were valuable support for FT catalysts

(ii) Pillaring with Al, loading of Co and Mn to the MMT and calcinations at

400 oC had no distinct effect on the layered structure of MMT. The flake

like morphology of MMT remains unchanged.

(iii) The lower Mn-loading pf upto 1.5% enhanced the reduction of Co3O4,

which is the most important step in FT synthesis because of high degree of

dispersion and stabilization of Co3O4.

(iv) The catalytic activity of MMT was increased by pillaring with Al and the

promotion of catalysts with Mn decreased the CH4-selectivity and

increased the catalytic activity in order of 1.5Mn-Co/Al-PILC > 0.7Mn-


Page 73

Co/Al-PILC > 0.3Mn-Co/Al-PILC > 0.10Mn-Co/Al-PILC > 3.5Mn-Co/Al-

PILC > Co/Al-PILC, mainly due to the reduction behavior for all sets of

the catalysts.

(v) Mn-promoted Co/Al-PILC catalyst showed increased C2-C12 (63.0%) and

decreased C21 (17.2%) hydrocarbons selectivity due to the acidic sites of

MMT catalysts which s were responsible for the cracking of higher

hydrocarbons.

(vi) By the addition of Mn the selectivity of C1 dropped drastically while that

of C2-C12 hydrocarbons increased significantly over all the Mn-promoted

Co/Al-PILC catalysts.

(vii) The promotion of catalyst with Mn suppressed the H2-addition and

enhanced the light olefin production and also increased the olefin to

paraffin ratio (O/P).

Further studies on the effect of reaction conditions such as pressure, temperature

and the reactant gas flow rate are underway.


Page 74

Chapter 5. Fischer Tropsch Synthesis Over Cerium Promoted Al-

Modified Montmorillonite Supported cobalt Nanocatalysts

5.1 Introduction

Previously, the effect of MMT as a support for the iron FT catalyst have been

explored but the main problem associated with this system is the production of

unwanted products (CH4 and CO2 etc) due to the complexity of metal oxide

reduction to metallic state by MMT [136]. So there is an essential need to develop

a catalyst with minimum production of CH4 and CO2. For this purpose, it is

scrutinized that the use of MMT supported Co catalyst could result in increase of

C2-C12 hydrocarbons selectivity and decrease in selectivity of C21+ hydrocarbons

due to the cracking ability of MMT.

Furthermore, use of certain promoters with MMT supported catalyst can notably

decrease the higher selectivity of CO2 and CH4. Various metal promoters like Re,

Pt and Ru [197-199] and oxides promoters including ZrO2, CeO2 and La2O3 [200-

204] have been explored to increase catalytic activity and C5+ products selectivity

of cobalt based FT catalysts. Previously, CeO-Co/C has been reported as an

efficient catalyst resulting in the lower CH4, higher olefins and heavy

hydrocarbons selectivity in the FT synthesis [203, 205]. The partial reduction of

CeO2 results in the CO adsorption sites which are responsible for enhanced CO

dissociation [203].

Until now, little literature is available on the MMT supported cobalt catalysts for

enhanced gasoline selectivity including hydrocarbons via hydrocraking of FT

waxes. In current study the FT activity of MMT supported cobalt catalysts have


Page 75

been studied thoroughly. Furthermore, effect of different cerium incorporations

on the activity and product selectivity has been explored during FT synthesis.

5.2. Experimental


Al-PILC was prepared by the same procedure as described in section 2.2.

5.2.2 Preparation of 20 wt% Co/Al-PILC

Wet impregnation method was adopted to synthesize Al-PILC catalyst containing

20 wt % Co as described in section 2.3.

5.2.3 Preparation of different Wt% Ce-20Wt% Co-loaded/Al-PILC

The different Wt% Ce-20Wt% Co-loaded/Al-PILC were prepared by the

procedure already described in section 2.5.

5.2.4 Characterization of Prepared catalyst




Fixed bed micro reactor was used to evaluate the catalytic activity of prepared

samples as described in section 4.2.4.


5.3.1 Chemical composition analysis

The chemical composition of the naturally occurring clay NaMMT and our

prepared samples i.e., Al-PILC and Ce-promoted Co /Al-PILC was determined

through XRF and given in Table 5.1.

It is obvious from the Table 5.1 that there is an increase in Al2O3 content of the

clay from 30.1 to 40.4 which can be attributed to the complete substitution of Na


Page 76

cation present in MMT interlayer by Al2O3. However, Mg and SiO2 contents are

almost same for all the Co-loaded MMT samples depicting no obvious change in

the clay sheets composition.

Table 5.1 Chemical composition (wt %) of the prepared catalysts determined by EDX analysis

Samples SiO2 Al2O3 Na2O Fe2O3 MgO Co2O3 CeO2

NaMMT 55.6 30.3 4.41 2.3 5.3 - -

Al-PILCS 53.1 40.4 <0.001 0.9 5.1 - -

0.5Ce -Co/Al-PILC 39.9 34.0 <0.001 0.8 5.1 19.3 0.8

1Ce -Co/Al-PILC 39.3 33.9 <0.001 0.8 5.1 19.6 1.0

1.6 Ce-Co/Al-PILC 39.0 34.2 <0.001 0.8 5.1 18.5 1.7

5.3.2 XRD Characterization

The XRD pattern of NaMMT and Al-PILC were already described in chapter 3.3.

The presence of (001) diffraction peak, a basal reflection of Al-PICL is evident

from the XRD pattern of Ce-promoted Co/Al-PILC catalysts as shown in Figure

5.1. While the small decrease in the (001) diffraction peak intensity was observed

corresponding to the slight decrease in layered structure of MMT.

Just like Mn promoted Co/Al-PILC, XRD pattern of Ce-promoted Co/ Al-PILC

(Figure 5.1) shows cobalt oxides (Co3O4) diffraction peaks at 19.5o, 31.4o, 36.9o,

44.9o and 59.6o (JCPDS 65-3103) in all samples indicating a uniform dispersion of

pure Co3O4 on pillared MMT [80]. While no reflection peaks were observed for

cerium in all samples thus confirming the high dispersion and lower content of

cerium on Al-PILC surface [206].


Page 77

The average crystallite size of the Co3O4 was determined by employing the

Scherer equation and was found in the range of 30-40 nm. It is suggested that

cerium enhances the dispersion of Co on the MMT support surface and complete

reduction into metallic Co, thus increasing the FT activity. Furthermore, no

reflection planes of cobalt aluminates or silicate in the XRD pattern were found,

showing the presence of highly dispersed Co3O4. This also confirms that no

interaction of Co with MMT components exists [207, 208].

Figure 5.1 XRD pattern of the (a) 20 wt %Co/Al-PILC (b) 0.5Ce-Co/Al-PILC (c) 1.0Ce -

Co/Al-PILC (d) 1.6 Ce-Co/Al-PILC

5.3.3 Textural properties of Ce-promoted Co/Al-PILC

The textural data including pore volume, BET surface area and pore diameter are

presented in Table 5.2. Al-PILC posses high surface area as compared to NaMMT

similar to the results discussed in chapter 3.3.1. While the addition of cerium and

Co on Al-PILC resulted in decrease in pore volume and BET surface area which

can be attributed to the presence of metal particles at pore openings and on the


Page 78

surface of the pores. Since, the average pore diameter and pore size remained

constant for all samples, therefore the pore blocking of Co can be neglected [159].

While pore volume depicts the micro porous and non porous nature of Ce-

Co/Al-PILC and NaMMT, respectively.


Sample BET

(m2/g)

Pore diameter

(Å)

Pore volume

(cm3/g)

Na MMT 39.2 4.8 0.08

Al-PILC 254.0 14.6 0.20

0.5 Ce-Co/Al-PILC 229.0 12.2 0.19

1Ce-Co/Al-PILC 225.0 11.0 0.18

1.6 Ce-Co/Al-PILC 222.0 10.3 0.19

5.3.4 Acidic Property

The NH3-TPD Profile for Al-PILC and Co-loaded Al-PILC is already discussed in

section 3.3.2. The peak intensity (Figure 5.2) of low temperature desorption peak

for Bronsted acidic sites slightly increased with cerium loadings depicting the

slight increase in Bronsted acidity, while the intensity of Lewis acidic peak

showed very minor changes with increasing cerium loading. The cerium cobalt

loaded samples showed minor decrease in acidic property due to the presence of

these on the pore opening resulting in the decreased Lewis acidic sites [162].


Page 79

Figure 5.2 NH3-TPD profile of Samples

5.3.5 TPR studies

The reduction behavior of samples was studied by H2-TPR and the obtained TPR

profiles are presented in Figure 5.3. Normally, the reduction of cobalt oxide

occurs in two steps. First step involves the reduction of Co3O4 to CoO while in

second step CoO undergoes reduction to metallic cobalt (Coo) [80]. It can be seen

from Figure 5.3 that the 0.5 wt % cerium promoted Co/Al-PILC shows three

reduction peaks at 209 oC, 278 oC and 379 oC. The first peak at 209 oC can be

attributed to the decomposition of Co(No3)2 [209]. While peaks at 278 oC and 379

oC may be due to the two step reduction of Co3O4 to CoO and CoO to Coo,

respectively. A distinct change in the TPR profile is observed for higher cerium

loaded Co/Al-PILC, Where the two steps reduction occurs at 284 oC, 381 oC and

at 291 oC, 391 oC for 1 wt% and 1.6 wt% cerium promoted catalysts respectively.

The shift in reduction peak towards higher temperature confirms the hindrance

in Co3O4 reduction with cerium addition. Similar trends of TPR profile for cerium

promoted Co/SiO2 was reported by B Ernst et al [204].


Page 80

Figure 5.3 H2 -TPR profile of Samples

5.3.6 Thermo gravimetric analysis (TGA)

TGA of 20 wt % Co/Al-PILC with different cerium loadings has been carried out

(Figure 5.4). Different cerium loaded 20 wt % Co/Al-PILC showed similar

thermo gravimetric trend to those of Co-loaded Al-PILC (chapter 3).



Page 81

5.3.7 SEM studies

Figure 5.5, presents the typical SEM micrographs of prepared samples. NaMMT

and cerium promoted Co/Al-PILC exhibits flake like morphology. It is

interesting to note from micrographs that cerium addition and calcination

temperature has no marked effect on the layered structure of MMT. No apparent

morphology and distribution of cobalt and cerium particle was observed due to

densified nature of clay as well as lower magnification of the instruments. In

addition to it, high dispersion and lower concentration of cobalt and cerium

hinders the study of surface morphology for these metal particles on MMT clay

surface.

Figure 5.5 SEM image of (a) 0.5 Ce-Co/Al-PILC (b) 1.0 Ce-Co/Al-PILC (c) 1.6 Ce-

Co/Al-PILC


Page 82

5.4 Catalytic activity

5.4.1 Effect of Al-pillaring on the FT activity of MMT supported Co- catalysts

The catalytic performance of Co/NaMMT, Co/Al-PILC and Ce-promoted

Co/Al-PILC were evaluated and results are presented in Table 5.3. The detailed

explanation regarding the catalytic activity of Co/NaMMT, Co/Al-PILC are

given in section 3.4.1.

Figure 5.6 CO-conversion/selectivity verses time on stream for different wt % Ce 20 wt

%Co/Al-PILC catalysts

5.4.2 Effect of Ce doping on FT activity of Co supported Al-PILC

The incorporation of Ce to the Co-supported Al-PILC has pronounced effect on

the catalytic activity and product selectivity (Table 5.3). The CO-conversion

boosts up to 35.5 % resulting in higher selectivity of C5+ hydrocarbons over

cerium modified Co/Al-PILC. This may be due to the increased dispersion of

Co3O4 on the support surface and presence of larger pores as a result of cerium

addition.


Page 83

Moreover, addition of 1.6 wt% Ce to 20 wt % Co/Al-PILC resulted in the

increased selectivity of gasoline range hydrocarbons (C5-C12) and CH4 from 18.23

% to 31.7% and 27.7 to 30.8%, respectively, while selectivity of C21+ were

decreased to 23.1%. This behavior can be attributed to the ability of cerium to

promote the hydrocarking ability of cobalt supported Al-pillared clay and is in

accordance to the results reported for cerium promoted Co/SiO2 catalysts [204].

The lower catalytic activity was expected due to the increase in reduction

temperature of Co3O4 after the cerium incorporation. The unexpected increase in

initial activity and C5+ products selectivity were seen after the reaction which can

be attributed to the increased active sits as a result of CeO2 to CeO2-x partial

reduction [202, 204] on the surface of Co/Al-PILC after cerium incorporation. In

FT synthesis (CHx)ad species are formed as a result of hydrogenation of adsorbed

CO on the surface of catalysts which are mainly responsible for the production of

CH4 via hydrogenation or higher hydrocarbons via polymerization. Furthermore,

the hydrogenation of (CHx)ad suppressed with the incorporation of cerium due to

enhanced formation of Co-C intermediates, resulting in decreased (CHx)ad

hydrogenation. The improved FT catalytic activity may also be attributed to the

modified electronic properties of Co-atoms as a result of CeO2 to CeO2-x partial

reduction. This behavior is similar to results reported earlier for cerium

promoted Co/SiO2 catalysts [210].

The catalyst stability of Co/NaMMT was studied for reaction time of 10 h,

initially, 10% CO-conversion was observed which decreased up to 5% in 10 h

Time on stream (TOS). Simultaneously, increase in the CO2 and CH4-selectivity


Page 84

of catalysts was observed which increased to more than 40% during this TOS.

The CO-conversion was achieved up to 28% for 20 wt % Co/Al-PILC (Figure

5.6b) which decreased with TOS and reached at minimum level of 11% after 30 h

of TOS. However, the cerium modified catalysts showed higher TOS stability. 0.5

and 1 % cerium addition showed little decrease in CO-conversion after 8 h which

remained constant for 30h of TOS while 1.6% cerium catalyst showed higher

stability and constant catalytic activity for 30 h of TOS. It is obvious from our

results that cerium addition to the Co/Al-PILC catalyst increases the catalyst

stability and higher cerium loading (1.6 wt%) showed greater TOS stability and

is in agreement with previously reported results for Co/SiO2 system [211]. The

catalytic activity of different catalysts were in order of 1.6 Ce-Co/Al-PILC > 1Ce-

Co/Al-PILC > 0.5Ce-Co/Al-PILC > 20Co/Al-PILC. These results prove that, as

the cerium loading increases, the degree of dispersion of Co3O4 also increases on

the surface of Al-PILC and hence catalytic activity increases. This enhanced

catalytic activity with the increased cerium modified sample is mainly due to

high dispersion ability of cerium over the support.


Page 85

Table 5.3 Results of different catalysts for FT synthesis at reaction temperature of 220 oC

Catalysts CO

conversion

CO2

selectivity

C1

(Wt %)

C5-C12

(Wt %)

C13-C20

(Wt %)

C21+

(Wt %)

20wt%Co/NaMMT 10 2.3 40 12.5 8 37.2

20wt%Co/Al-PILC 28 2.5 27.27 18.23 11 41.3

0.5Ce-Co/Al-PILC 31 2 28 21.7 11.7 36.5

1.0Ce-Co/ Al-PILC 33 2.3 28.91 26.03 11.36 31.4

1.6 Ce-Co/ Al-PILC 33.5 2.4 30.8 31.7 12 23.1

5.4.3 Effect of reaction temperature and pressure on catalytic activity of

catalysts

The effect of reaction temperature and reaction pressure on catalytic activity of

1.6 Ce-Co/Al-PILC was also investigated and presented in Figure 5.7a and

Figure 5.7b respectively.

The reaction carried out at a temperature of 220 oC gives almost 33.5 % CO-

conversion resulting in the hydrocarbons selectivity of 30.8, 31.7, 12 and 25 % for

methane, C5-C12, C13-C20, C21+ respectively (Figure 5.7a). When the reaction

temperature is increased to 260 oC, the CO-conversion and CH4-selectivity also

increased to 35.4 and 32.7, respectively, and decrease in C5+ hydrocarbons

selectivity to 28.6 %. Furthermore, as the temperature is increased to 275 oC, the

CO-conversion was increased to 36.3% and selectivity of CH4 to 34 % while the

C5+ hydrocarbons decreased to 28%. However, no significant change in the

selectivity of C13-C20 and C21+ hydrocarbons was observed with increase in

reaction temperature. This effect of the catalysts can be attributed to the higher

hydrogenation activity of the catalysts at high temperature similar to the

performance of standard cobalt-thorium catalysts [175]. Increase in reaction


Page 86

temperature from 220 to 275 oC results in higher production of CH4 which is not

beneficial in Co-based FT synthesis as reported earlier [176]. Reaction pressure

also imparts crucial role on the activity and selectivity of FT catalysts.

The FT reaction carried out at a pressure of 1, 5 and 10 bar is shown in Figure 5.7b.

At a pressure of 1 bar, the CO-conversion was 33.5%, while selectivity of CH4

and C5–C12 hydrocarbons was 30.8 % and 31.7 %, respectively, over 1.6 Ce-

Co/Al-PILC. As the pressure is increased to 5 and 10 bar, the CO-conversion

increased to 35 and 36.5%, respectively and CH4-selectivity decreased to 28.3 and

27 %, respectively. While, the percent selectivity of C5–C12 hydrocarbons

increased from 33.5 to 34.4% as the pressure increased from 5 to 10 bar (Figure

5.7b) with a little change in the selectivity of C13-C20 and C21+ hydrocarbons was

observed with increasing reaction pressure. It can be concluded that higher

pressure favors the production of high molecular weight hydrocarbons as a

result of chain growth probability [177]. The increase in CO-conversion can be

ascribed to the increase in partial pressure of hydrogen because the reaction rate

is proportional to the partial pressure of hydrogen [178] and the increased

selectivity of the C5+ hydrocarbons is due to the increased CO partial pressure.


Page 87

Figure 5.7 Effect of Reaction Temperature (a) and reaction pressure (b) on FT catalytic

activity

5.5 Conclusions

The enhanced catalytic activity and lower CH4-selectivity was achieved over Al-

pillared MMT supported Co catalyst. Na+ in MMT interlayer responsible for

higher CH4 production was completely replaced by Al after pillaring along with

enhanced CoO reduction. The incorporation of small amount of cerium to the

Co/Al-PILC results in increase in reduction temperature. Instead of increase in

reduction temperature cerium incorporation results in high dispersion of cobalt

particles and formation of new active sites which are responsible for higher

catalytic activity and higher hydrocarbons (C5-C12) selectivity. The increased

selectivity towards CH4 and C5-C12 hydrocarbons were obtained over cerium

modified Co/Al-PILC as a result of cracking of C22+ hydrocarbons. The addition

of cerium to the Co/Al-PILC increases C2-C4 hydrocarbons of FT products due to

the decreased availability of adsorbed hydrogen. Higher reaction temperature


Page 88

(>220 oC) increases CO-conversion and CH4-selectivity and decreases C5+

hydrocarbons selectivity. Increase in reaction pressure from 1 to 10 bar favors an

increase in C5+ hydrocarbons selectivity and decrease in the CH4 fraction.


Nisar Ahmad et al., Progress in natural science material international, 23,374-381 (2013) Page 89

Chapter 6. Zr-Pillared Montmorillonite Supported Cobalt Nanoparticles

for Fischer-Tropsch Synthesis

6.1 Introduction

In Fischer-Tropsch (FT) synthesis the catalytic activity of various metals largely

depend upon the type of support used for the dispersion of metal particles, and

improved catalytic activity of cobalt-based catalysts have been achieved by the

dispersion of cobalt particles on high surface area supports. The catalytic activity

and products selectivity is directly related to the type of support used, therefore,

search for the new support in FT-catalysis is always worked out. Pillared clays

have recently been investigated as efficient support materials [80, 212]. A type of

naturally occurring clay like MMT is having the tendency to be used as a

catalysts support due to its cheapness, high surface area and environment

friendly nature. MMT supported catalysts have been used in the areas of material

science, chemistry, environment and physics [213].

However, the major problem associated with MMT is its low thermal stability

and lack of porosity [214] which can be overcome by different modifications like

heat treatment, acid treatment and pillaring. Among which pillaring is preferred

due the thermal stability of pillared framework at moderate temperature to

reserve the tetrahedral or octahedral structure of MMT [215]. Structurally MMT

consist of octahedral central metal layer sandwiched by two tetrahedral silicate

layers from the sides with 2:1 layered structure [216], sodium ions and calcium

ions present between the layers of MMT can balance the net negative charge on

the lattice produced by the substitution of Si4+ with Al3+ and Al3+ with Mg2+ due

to which, the MMT governs the property to grip guest molecules between its



layers. Thus pillaring agents such as hydroxy-metal cations have been used for

the separation of MMT lattice layers by replacing the alkali and alkaline earth

metal cations, which upon calcination result in the formation of large surface area

microporous material. For high surface area and pore volume, different metal

oxides (MOs) (M=Al, Ti, Zr, Cr) have been used to replace the sodium ion

present in the interlayer of MMT clay [217]. In the field of degradation, the use of

MMT support has been reported, which shows that some metal particles

intercalate in the interlayer’s of MMT thus exhibiting synergetic effect [216].

MMT supported catalysts have been widely investigated for the hydrocraking of

petroleum waxes and also in the field of FT-synthesis [80]. However MMT

supported cobalt and iron catalysts for FT-synthesis have been reported to

produce too much CH4 and CO2, the most undesirable products of FT-synthesis

due to the presence of Na+ in the interlayer of MMT responsible for the inhibition

of metal oxide reduction [137]. This lower catalytic activity and higher CH4 and

CO2 selectivity can be overcome by pillaring MMT with Al+3 which enhance the

surface area of support MMT. It also increases the pore volume and pore

diameter due to the contribution of Al+3 to replace the Na+ in the MMT interlayer

and enhance the reduction of MO to metallic particle [137]. To the best of our

knowledge there is no report on the Zr-pillared MMT (Zr-PILC) supported Co-

nano particle catalysts for FT-synthesis. In this chapter we have studied FT

reaction over cobalt nano particle supported on Zr-PILC for the first time.



6.2 Experimental

6.2.1 Preparation of Zr-PILC

In the typical synthesis procedure, 0.5 M NaOH solution was slowly added to

0.25 M solution of ZrOCl2.8H2O with OH/Zr ratio of 2/1 with continuous

stirring. The above suspension was aged at room temperature for 24 h. In the

second step, above pillaring solution was added (5mmol Zr/g of clay) to the

suspension of 2 wt % MMT and stirred for three hour at 100 oC. The resultant

slurry was centrifuged at 5000 rpm, washed several times with deionized water

in order to remove excess sodium and chlorine, dried at 100 oC for 24 h, grinded

and finally calcined at 400 oC for 5 h.

6.2.2 Preparation of Co-loaded/Zr-PILC

Co nanoparticles supported on Zr-PILC with different wt% of Co, represented as

10%Co/Zr-PILC, 15%Co/Zr-PILC and 20%Co/Zr-PILC was prepared by the

same hydrothermal method described in section 2.4 for the synthesis of Mn

promoted Co/Al-PILC





The catalytic activity was conducted in a fixed bed micro reactor as already

described in section 4.2.4 with increasing pressure from 1-10 bars.




6.3.1 Chemical composition

XRF were used for the determination of chemical composition of NaMMT, Zr-

PILC, Co-loaded/Zr-PILC, are given in Table 6.1.

Due to the complete substitution of Na+ present in MMT interlayer by ZrO2,

there is an increase in the content of ZrO2 from 0 to 36.4 for Zr-PILC, while the

content of MgO, Al2O3 and SiO2 changed slightly for the all the Zr-PILC and Co-

loaded PILC samples confirming that there is no marked change in the

composition of naturally occurring clay sheets. The contents of ZrO2 were

slightly decreased for Co-loaded catalysts.

Table 6.1 Chemical composition of the prepared catalysts (wt %)

Samples SiO2 Al2O3 ZrO2 Na2O Fe2O3 MgO Co2O3

NaMMT 57.01 29.50 0.00 4.65 2.23 6.34 -

Zr-PILC 36.10 20.13 36.40 <0.001 0.89 6.10 -

10%Co/Zr-PILC 35.27 20.09 27.77 <0.001 0.86 5.57 9.88

15%Co/Zr-PILC 34.02 19.97 25.49 <0.001 0.89 4.45 15.14

20%Co/Zr-PILC 32.20 19.20 23.67 <0.001 0.87 4.11 18.84

6.3.2 XRD characterization

Similar to the results reported for Al-PILC in chapter 3, after pillaring of NaMMT

with Zr, the replacement of smaller cation (Na) present between two dimensional

NaMMT interlayer structure causes change in the gallery height and d001 basal

plane [139]. The XRD pattern for NaMMT in Figure. 6.1a shows an intense peak

at 2θ value (7o) with (001) basal reflection, basal spacing of 1.18 nm and a host



layer thickness of 0.93 nm. Gallery height of 0.25 nm was calculated by

subtracting the host layer thickness from the obtained basal spacing of the

NaMMT which is almost similar to the previously reported value [139]. For Zr-

PILC (Figure. 6.1a) the (001) basal reflection is observed comparably at lower 2θ

value (4o) with increased d-spacing of 1.73 nm and the gallery height of 0.8 nm,

which reflects an expansion in the layered structure of Zr-PILC as a consequence

of pillaring [138]. The rest of the peak at 2θ of 19.7o and 35o showing

characteristic two dimensional layers structure of MMT, remained unchanged

[160]. However the peak at 28o is due to the quartz and hydroxide of metal

silicate impurities [161]. The decrease in intensity of (001) peak for Zr-PILC is

due to the intercalation of Zr into the pillared clay (PILC) and slight decrease in

layered structure of PILC.

The XRD pattern of Co-loaded/Zr-PILC, catalysts, as shown in Figure. 6.1b gives

the (001) diffraction peaks and basal reflection of Zr-PICL with no separate peak

for Zr, confirm a complete transfer of Zr to the interlayer of NaMMT.

Some broad cobalt oxides (Co3O4) peaks at 19.5o, 31.4o, 36.9o, 44.9o and 59.6o

(JCPDS 65-3103) in all samples indicate a uniform dispersion of pure Co3O4 nano

particles on PILC [80]. There is no reflection of cobalt aluminates or silicate in the

XRD pattern showing that only highly dispersed Co3O4 nano particles exist and

there is no interaction of Co with PILC components [207, 208]. Scherrer equation

was used to calculate the average crystallite size of the Co3O4 nano particle and

was found in the range of 16-28 nm.



Figure 6.1 XRD patterns of (a) NaMMT and Zr-PILC (b) Co-loaded/Zr-PILC

6.3.3 BET studies of Co-loaded/Zr-PILC

Table 6.2 and Figure. 6.2 show the textural properties and adsorption desorption

isotherms for Zr-PILC and Co/Zr-PILC respectively. The increased surface area

of Zr-PILC (261 m2/g) as compared to NaMMT (39.243 m2/g) indicate that Zr

has successfully replaced Na+ present in the silicate layer of NaMMT (Table 2).

The larger surface area is ascribed to the higher N2-adsorption as a result of

increased pore volume and formation of new pores because of Zr-pillaring [214].

However, decreased surface area and pore volume for Co-loaded Zr-PILC can be

attributed to the presence of metal particles at pore openings and on the surface

of the pores. The average pore diameter and pore size calculated for all of the

sample were nearly constant, therefore the pore blocking of Co can be neglected

[159]. The type-I isotherm (Figure. 6.2) along with type H4-loop according to

IUPAC classification indicate that the narrow slit shaped porous structure of



pillared clay is preserved after calcinations at 400 oC [80] while the smaller pore

size and pore volume for NaMMT proves nonporous nature of NaMMT.


Sample BET

(m2/g)

Pore diameter

(Å)

Pore volume

(cm3/g)

NaMMT 39.24 4.80 0.08

Zr-PILC 261.00 26.41 0.18

10%Co/Zr-PILC 239.01 24.00 0.17

15%Co/Zr-PILC 238.21 20.20 0.15

20%Co/Zr-PILC 236.03 18.32 0.13

Figure 6.2 N2-Adsorption desorption isotherm of NaMMT, Zr-PILC and Co-loaded/Zr-

PILC.



Figure 6.3 H2-TPR profile of Samples

6.3.4 TPR studies

The reduction behavior of prepared samples is presented in Figure. 6.3.

Generally, reduction of cobalt oxide undergoes from Co3+ to Co2+ and Co2+ to Coo

in first and second step respectively [80, 218-221]. From the TPR profile it is

obvious that 20%Co/NaMMT showed three peaks, the first small peak at 210 oC

can be credited to the decomposition of Co(No3)2 [209] while peaks at 278 oC and

379 oC are due to the two step Co-reduction. The reduction temperature for Co-

loaded/Zr-PILC was lower than that of Co/NaMMT (Figure. 6.3). This decrease

in Co reduction temperature can be ascribed to the enhanced dispersion of cobalt

nanoparticles on the surface of Zr-PILC evident from their SEM micrograph and

decreased metal support interaction as a result of Zr-pillaring. From the TPR-

profile it is obvious that Co-loaded/Zr-PILC undergo complete reduction as

compared to the Co/NaMMT. The incomplete reduction of Co/NaMMT and its



higher CH4-selectivity was attributed to the presence of Na+ present in interlayer

of MMT which is successfully replaced by Zr after pillaring [137].

6.3.5 NH3-TPD studies

NH3-TPD was used to study the acidic properties of our prepared sample. The

NH3-TPD analysis of NaMMT showed the non acidic nature of NaMMT (Figure

6.4). After pillaring with Zr, the acidity of MMT were increased and certain

amount of stronger acidic site (Lewis acidic sites) at desorption peak near

temperature maximum of 650oC while the lower temperature desorption peak

for weaker acidic site (bronsted acidic sites) were observed due to the

contribution of the acidic zirconia to replace the Na ion of MMT. There was no

marked difference in the acidity of Co-loaded Zr-PILC. All the Co-loaded

samples showed similar acidic nature to those of Zr-PILC. Due to this increased

acidic nature of Zr-PILC, higher hydrocracking ability of the prepared samples in

FT reaction was noticed.

Figure 6.4 NH3-TPD profile of the samples




Thermogravimetric analysis (TGA) was performed to study the decomposition

pattern of our prepared samples (Figure. 6.5). All of our prepared samples

follows three step decomposition pattern with greater and more continues

weight lose for Co-loaded/Zr-PILC as compared to the Co/NaMMT. Weight lose

in first step at 150 oC can be assorted to the evoportaion of physically adsorbed

water while a small weight lose between 200-600 oC is due to the dehydroxylation

and interlayer water removal. In third step comparatively greater weight loss at

700 oC can be attributed to the dehydroxylation and collapse in clay structures

[162, 166-169].


6.3.7 SEM studies

Scanning electron microscopy was used to investigate the surface morphology

and microstructure of our prepared samples. The SEM micrograph for Zr-PILC

(Figure. 6.6) showed that after calcinations at 400 oC the flake like structure of the



pillared clay which was needed for enhanced dispersion of cobalt nanoparticles

was reserved and the well dispersed and uniformly distributed Co nanoparticles

can be seen on the surface of Zr-PILC.

Figure 6.6 SEM micrograph of (a) Zr-PILC (b) 10%Co/Zr-PILC (c) 15%Co/Zr-PILC

(d) 20%Co/Zr-PILC

6.4. FT catalytic activity

6.4.1 Catalytic activity of 20%Co/NaMMT

The FT Catalytic activity of 20%Co/NaMMT have already been described in

chapter 3.



6.4.2. Catalytic activity of Co-loaded/Zr-PILC

After pillaring, Zr successfully replaced Na of MMT and resulted in an increase

of its pore volume, surface area, and pore diameter (Table 6.2). The inter layer

cation play a vital role in the CO-conversion as is obvious from the results that

20%Co/NAMMT catalyst showed much lower CO-conversion (10%) which

reaches a minimum of 5% with increase in TOS. On the other hand Co-

loaded/Zr-PILC showed higher CO-conversion of 44.5%, 47% and 53% for 10, 15

and 20 wt % Co/Zr-PILC respectively (Figure. 6.7a), which remained almost

same for 30 h of TOS. Beside the CO-conversion, 20%Co/NaMMT showed very

higher CH4-selectivity (chapter 3) which was promisingly decreased over

different wt% Co/Zr-PILC. The FT-products obtained over Co-loaded/Zr-PILC

showed much lower CH4-selectivity of 15.24%, 16.8% and 18% for 10%Co/Zr-

PILC, 15%Co/Zr-PILC and 20%Co/Zr-PILC, respectively at a TOS of 30 h

(Figure. 6.7b). The increased CO-conversion and decreased CH4-selectivity of

Co/Zr-PILC can be assorted to the homogenous and uniform distribution of Co

nanoparticles on the surface of Zr-PILC as confirmed by SEM micrograph

(Figure. 6.6) and secondly due to the ability of Zr to replace the Na of MMT

hence decreasing the reduction temperature and assuring complete reduction of

Co-oxide near the reaction temperature (Fig 6.3). From the increased catalytic

activity and lower CH4-selectivity of Co-loaded/Zr-PILC as compared to

Co/NaMMT catalysts it was confirmed that Zr-pillaring has positive impact on

the MMT supported Co catalysts similar to the effect of Al-pillaring reported by

Wang et al [80]. The FT-products obtained over Co-loaded/Zr-PILC catalysts



showed increased selectivity of C2-C12 hydrocarbons and decreased selectivity

towards higher molecular weight hydrocarbons (C21) at a TOS of 2-30 h (Table

6.3). This decrease in C21-selectivity is due to the cracking of long chain

hydrocarbons at the acidic site of MMT [15, 171-174].

Figure 6.7 (a) CH4-selectivity (b) CO-conversion Vs TOS over Co/NaMMT and Co-

loaded/Zr-PILC catalysts

6.4.3 Effect of Reaction temperature on catalytic activity of catalysts

To study the effect of reaction temperature on the catalytic activity of our

prepared catalysts, 20%Co/Zr-PILC was selected due to its higher catalytic

activity among all the samples. The catalytic activity of the selected catalyst was

tested at temperatures of 225 oC, 260 oC and 275 oC. At high reaction temperature

of 260 oC and 275 oC along with increased CO-conversion of 57% and 60%, CH4-

selectivity were also increased to 20% and 25%, respectively while gradual

decrease in C5+ hydrocarbons was noted (Figure. 6.8a). As already described in

chapter 3, this behavior of catalyst can be attributed to the higher hydrogenation



activity of catalyst at high temperature similar to the performance of standard

cobalt-thorium catalysts [175] and due to higher CH4-selectivity at high reaction

temperature, it is not beneficial in Co-based FT-synthesis was similar to the

reported results [176].

Figure 6.8 Effect of (a) reaction temperature and (b) reaction pressure on FT-catalytic

activity

6.4.4 Effect of reaction pressure on catalytic activity of catalysts

Reaction pressure also imparts crucial role on the activity and selectivity of FT-

catalysts ( Figure 6.8b).

For 20%Co/Zr-PILC, at a pressure of 1 bar, the CO-conversion was 53%, while

selectivity of CH4 and C5–C12 hydrocarbons was 18% and 45.2%, respectively. As

the pressure is increased to 5 and 10 bar, the CO-conversion increased to 55%

and 58.5%, while CH4-selectivity decreased to 16.8% and 15%, respectively. The

percent selectivity of C5–C12 hydrocarbons increased to 47.3% and 48% (Figure.

6.8b) with a little change in the selectivity of C13-C20 and C21+ hydrocarbons. It can

be concluded that higher pressure favors the production of high molecular



weight hydrocarbons as a result of chain growth probability [177]. The increase

in CO-conversion can be ascribed to the increase in hydrogen partial pressure,

because the reaction rate is proportional to the partial pressure of hydrogen [178]

and the increased selectivity of the C5+ hydrocarbons is due to the increased CO

partial pressure.

Table 6.3 Results of different catalysts for FT-synthesis

Catalysts CO-

conversion

CO2-

selectivity

C1

(wt%)

C2-C12

(wt%)

C13-C20

(wt%)

C21

(wt%)

20%Co/NaMMT 10.0 2.3 40.0 12.5 8.0 37.2

10%Co/Zr-PILC 44.5 2.1 15.2 44.2 16.5 22.0

15%Co/Zr-PILC 47.0 2.1 16.8 44.7 17.0 19.4

20%Co/Zr-PILC 53.0 2.3 18.0 45.2 17.4 17.1

6.5 Conclusions

One of the main challenge to use MMT as a support for FT-catalysts was its

lower catalytic activity and higher CH4-selectivity due to the presence of Na in

the MMT interlayer which has been completely replaced by Zr after pillaring.

Due to the increased surface area of Zr-PILC as compared to the NaMMT and

uniform and homogenous dispersion of Co nanoparticles on its surface resulted

in an increase in catalytic activity. FT reaction over Co-loaded/Zr-PILC showed

higher CO-conversion, increased selectivity towards C2-C12 hydrocarbons and

decreased selectivity of C21+ hydrocarbons as a result of hydrocraking reaction

occurred at the acidic site of Zr-PILC.



The higher CO-conversion, increased CH4-selectivity and lower C5+ hydrocabons

selectivity were obtained at higher temperature as a result of higher

hydrogenation activity of catalysts. Increase in reaction pressure from 1 to 10 bar

favored an increase in C5+ hydrocarbons and decrease in the CH4 fractions.


Nisar Ahmad et al., Submitted (2013) Page 105

Chapter 7. Effect of Manganese Promotion on Zr-Pillared

montmorillonite Supported Cobalt Nanoparticles For

Fischer Tropsch Synthesis

7.1 Introduction

To increase the overall conversion and selectivity of liquid hydrocarbons in FT

synthesis, considerable attention has been paid towards the use of bifunctional

catalysts including transition metals over acidic supports such as ZSM-5, ZSM-22,

mordenite, SAPO-31, SAPO-11, SAPO-41etc for the hydrocracking of higher

molecular weight hydrocarbon waxes [222, 223]. Due to the almost similar

properties of zeolite and MMT clay, it has been widely used for hydrocraking of

petroleum products in refineries and also in the field of FT synthesis [80]. The

effect of Zr-pillaring for increased CO-conversion and decreased selectivity of

unwanted products (CH4 and CO2 etc) in Co-based FT synthesis have been

reported in previous chapters. After Zr-Pillaring the activity of catalysts was

increased to a larger extent but still there was a need to produce such a catalysts

which can increase the catalytic activity and its time on steam stability further.

Keeping these objectives in mind we have tried to study the effect of certain

transition metal promoter on the activity and products selectivity of Co/Zr-PILC

catalysts because various metal promoters like Re, Pt and Ru [197-199] and

oxides promoters including ZrO2, CeO2 and La2O3 [200-204] have been explored

to increase catalytic activity and C5+ products selectivity of cobalt based FT

catalysts. Among the transition metals promoters Ru [159] and Mn [52, 224, 225]

have been reported to decrease the CH4 selectivity and increase the catalysts

stability, CO-conversion and olefins formation. In our previous finding we have



reported the increased catalytic activity for Mn promoted Ru/SiO2, Ru/TiO2 and

Ru/Al2O3 catalysts attributed to the geometrical and electronically modification

of catalysts surface as a result of Mn promotion.

In this chapter we will scrutinize the effect of Mn promoter for the increased

catalytic activity and time on stream stability of the Co/Zr-PILC catalysts.

7.2 Experimental

7.2.1 Preparation of Zr-PILC

Zr-PILC was pepared by the same method described in section 6.2.2.

7.2.2. Preparation of different Wt% Mn-20Wt% Co -loaded/Zr-PILC

20wt%Co/Zr-PILC and 20wt%Co/Zr-PILC with 0.10, 0.30, 0.70, 1.50, 3.50 wt%

Mn designated as 0.10Mn-Co/Zr-PILC, 0.30Mn-Co/Zr-PILC, 0.70Mn-Co/Zr-

PILC, 1.5Mn-Co/Zr-PILC, 3.5Mn-Co/Zr-PILC respectively, were synthesized by

hydrothermal method.

To prepare 5g of catalyst calculated amount of calcined pillared clay was added

to calculated amount of Co(NO3)2.6H2O and Mn(NO3)2.6H2O (0.1 M solution)

with constant stirring. The resultant suspension along with NH4OH (33%) was

transferred into Teflon-lined vessel of autoclave previously flushed with argon.

The pressure of the autoclave was increased from to 10 bar on heating at 160 oC

for 1h. After thermal treatment the resultant material was cooled, filtered,

washed thoroughly with deionized water to remove all the NH4OH and then

dried overnight at 100 oC. The dried material was grinded & then sieved through

a 125 µm sieve and finally calcined at 400 oC for 5h.




After the preparation of samples, same characterization tools discussed in section

3.2.3 were used.


The catalytic activity was conducted in a fixed bed micro reactor as already

described in section 4.2.4 at atmospheric pressure.


7.3.1 Chemical composition

XRF were used to determine the chemical composition of samples and are given

in table 7.1.

The increase content of ZrO2 from 0 to 36.4 for Zr-PILC showed the complete

substitution of Na+ present in MMT interlayer by ZrO2.All the Mn promoted

Co/Zr-PILC catalysts showed little change in the content of MgO, Al2O3 and

SiO2 confirming that there is no marked change in the composition of naturally

occurring clay sheets.



Table 7.1 Chemical composition of the prepared catalysts (wt %)

Samples SiO2 Al2O3 ZrO2 Na2O Fe2O3 MgO Co2O3 MnO

NaMMT 57.01 29.50 0.00 4.65 2.23 6.34 - -

Zr-PILC 36.10 20.13 36.40 <0.001 0.89 6.10 - -

20%Co/Zr-PILC 32.20 19.20 23.67 <0.001 0.87 4.11 19.84 -

0.10Mn-Co/Zr-PILC 32.17 19.19 23.58 <0.001 0.86 4.21 19.80 0.13

0.30Mn-Co/Zr-PILC 32.18 19.21 23.54 <0.001 0.82 4.12 19.82 0.29

0.70Mn-Co/Zr-PILC 32.00 19.17 23.46 <0.001 0.74 4.13 19.75 0.73

1.5Mn-Co/Zr-PILC 31.23 19.12 23.47 <0.001 0.73 4.13 19.67 1.47

3.5Mn-Co/Zr-PILC 30.32 19.00 22.34 <0.001 0.73 4.15 19.21 3.61

7.3.2 XRD characterization

Figure (7.1) shows the XRD pattern of Mn promoted Co/Zr-PILC. The detailed

discussion regarding the effect of Zr-pilaring on the XRD pattern of NaMMT

have been given in chapter 6. All the Mn promoted cobalt catalysts (Figure 7.1)

show (001) diffraction peaks for Zr-PILC and cubic cobalt oxides (Co3O4)

diffraction at 19.5o, 31.4o, 36.9o, 44.9o , 59.9o and 62o (JCPDS 65-3103). From the

XRD pattern it was confirmed that there was no evidence of Co-aluminates or

silicates and nor the formation of any mixed metal oxide of Mn, due to the lower

content and high dispersion of Mn, hence improved the dispersion of Co

nanoparticles on the surface of Zr-PILC. Average crystallite size of the Co3O4

nanoparticles were in the range of 18-30 nm calculated using Scherer equation.



Figure 7.1 XRD pattern of 0.10, 0.30. 0.70, 1.5, 3.5%Mn-Co/Zr-PILC

7.3.3 BET studies of Mn-Co /Zr-PILC

Table 7.2 and Figure 7.2 show the textural properties and desorption, adsorption

isotherms for NaMMT, Zr-PILC, Co/Zr-PILC and Mn promoted Co/Zr-PILC.

The textural properties of NaMMT, Zr-PILC and Co/Zr-PILC have already been

described in chapter 6 in detail. After the incorporation of Mn as a promoter

there is no significant change observed in the textural properties of the samples

except a minor change in the pore volume and surface area of the samples, can

be attributed to the presence of some Mn particles on the pores opening of Zr-

PILC responsible for N2 adsorption. All other properties of the MMT remained

reserved.




Sample BET

(m2/g)

Pore Diameter

(Ao)

Pore Volume

(cm3/g)

NaMMT 39.24 4.80 0.08

Zr-PILC 261.00 26.41 0.18

20%Co/Zr-PILC 236.03 18.32 0.13

0.10Mn-Co/Zr-PILC 234.00 18.29 0.13

0.30Mn-Co/Zr-PILC 235.10 18.02 0.12

0.70Mn-Co/Zr-PILC 230.21 18.23 0.13

1.50Mn-Co/Zr-PILC 228.65 17.96 0.11

3.50Mn-Co/Zr-PILC 226.78 17.98 0.10

Figure 7.2 N2-Adsorption desorption isotherm of (a) NaMMT (b) Zr-PILC (c)

20%Co/Zr-PILC (d) 0.10Mn-Co/Zr-PILC (e) 0.30Mn-Co/Zr-PILC (f)

0.70Mn-Co/Zr-PILC (g) 1.50 Mn-Co/Zr-PILC and (h) 3.50 Mn-Co/Zr-PILC.




The reduction profile of our prepared catalysts is shown in Figure 7.3. As already

described in chapter 4 that reduction of Co-oxide occurs in two steps.

(i) Co3O4 CoO (4.1)

(ii) CoO Coo (4.2)

The reduction behavior of Zr-PILC and Co/Zr-PILC was given in chapter 6. In

this chapter we were interested to study the effect of Mn promotion on the

reduction behavior of Co/Zr-PILC. A distinct change in the TPR profile is

observed for Mn-promoted Co/Zr-PILC catalyst. The reduction of Co3O4 occurs

at different temperatures for different wt% of Mn as shown in Table 7.3.

After the addition of Mn a decrease in reduction temperature was observed for a

lower Mn-loading up to 1.5% for both reactions in Eq 4.1 and 4.2 as compared to

the unpromoted Co/Zr-PILC because of high degree of dispersion and

stabilization of Co3O4 by Mn promotion while for 3.5% a increase in the

reduction temperature for reactions in eq 4.2 can be assigned to the hindered

reduction of Co3O4 possibly due to the formation of Co-Mn spinal or mixed Co-

Mn oxides on the surface. It can thus be inferred that the Mn promotion enhances

and promotes the Co3O4 reduction, which becomes less significant on increasing

the Mn %age.



Figure 7.3 H2-TPR profile of Samples

Table 7.3 Two steps Co3O4 reduction temperature

Catalysts Reduction Temperature Reduction Steps

20%Co/Zr-PILC 263 oC Eq (4.1)

326 oC Eq (4.2)

0.10Mn-Co/Zr-PILC 260oC Eq (4.1)

319oC Eq (4.2)


310oC Eq (4.2)


305oC Eq (4.2)


300oC Eq (4.2)


386oC Eq (4.2)


Different weight % Mn-loaded Co/Zr-PILC follows almost similar TGA

decomposition pattern to that of Co/Zr-PILC (Figure. 7.4) already discussed in



chapter 6. As the weight % of Mn increases the % decomposition also going to

increase with inflation point at 300 oC which were attributed to the surface

adsorbed water and removal of hydroxide group as a results of dehydroxylation

of pillar and collapse in layered structure of MMT [162, 166-169].

Figure 7.4 TGA Curves of the samples

7.3.6 SEM studies

The dispersion of nanoparticles on the surface of support and morphology of

surface play an important role in controlling the activity and selectivity of

specific catalysts, thus to see the surface morphology and microstructure of

samples, SEM technique was used. SEM micrograph in Fig. 7.5 c and d shows the

uniform and homogenous dispersion of Co nanoparticles on the surface Zr-PILC.

Mn promotion help to enhance the dispersion of Co nanoparticles on the surface

of supports which was also evident from the reduction behavior of Mn promoted

samples.



Figure 7.5 SEM micrograph of (a) 0.10%Mn-Co/Zr-PILC (b) 0.30%Mn-Co/Zr-PILC (c)

0.70%Mn-Co/Zr-PILC (d) 1.5%Mn-Co/Zr-PILC (e) 3.5%Mn-Co/Zr-PILC

7.4 FT Catalytic performance

7.4.1 Effect of Mn-addition on Co/Zr-PILC catalysts

After the addition of Mn the catalytic activity of Co/Zr-PILC catalysts were

studied and presented in Figure 7.6 a,b. As described in previous chapters,

efforts were done to decrease CH4-selectivity and increase the CO-conversion of



MMT supported catalysts. The catalytic activity and CH4-selectivity of Mn

promoted Co/Zr-PILC are described below one by one.

7.4.1.1 CO-conversion

The CO-conversion of Co/Zr-PILC was successfully enhanced by the addition of

Mn (Fig 7.7a). It was noted that after the addition of 0.10, 0.30, 0.70, 1.5 and 3.5 %

Mn, the CO-conversion increased to 56, 59, 60, 62 and 58 % respectively. For

increased FT reaction metallic Co are desirable instead of oxides of Co as

reported [80]. That’s why, the hydrocarbons selectivity and FT catalytic activity

was totally reliant upon the Co3O4 reduction to metallic Co. Thus increased

catalytic activity (Figure 7.6) were achieved by Mn-promotion obvious from the

H2-TPR profile (Figure 7.3) that Mn-loading helped to lower the reduction

temperature of Co3O4 , as a result of small and highly dispersed Co particles

formation as compared to the un promoted Co3O4. Apart from the increased

catalytic activity, all the Mn promoted samples showed increased time on stream

stability which was similar to the results reported in reference [175]. Higher than

1.5 % Mn results in somehow lowering in CO-conversion may be due to the

formation of some mixed Mn and Co-oxides need to be confirmed by some other

techniques like XPS.

7.4.1.2 CH4-selectivity

The production of CH4 in case of MMT supported catalysts was successfully

addressed by pillaring MMT with Zr (chapter 6). The effect of Mn promotion on

Co/Zr-PILC for CH4-selectivity were studied and presented in Figure 7.6 b.



As per reported literature the increased CH4-selectivity in case of Co-based FT

synthesis can be ascribed to the incomplete reduction of Co-oxide [80]. Thus by

insuring the complete reduction of Co-oxides during FT reaction, CH4-selectivity

can be lowered. In this study it was noted that small amount of Mn incorporation

enhances the reduction of Co-oxides near the reaction temperature obvious from

the TPR profile (Figure 7.3) and thus CH4-selectivity of Co/Zr-PILC in FT

synthesis was successfully decreased. As compared to the unprompted Co/Zr-

PILC, Mn promoted samples gives lowered CH4-selectivity of 17%, 14%, 13%,

12% and 11% for 0.10Mn-Co/Zr-PILC, 0.30Mn-Co/Zr-PILC, 0.70Mn-Co/Zr-

PILC, 1.5Mn-Co/Zr-PILC, 3.5Mn-Co/Zr-PILC respectively. Beside the reduction

ability, Mn promotion also insured the homogenous dispersion of Co

nanoparticles on the surface of support and the addition of small quantity of Mn

decreased the CH4 and increased the higher hydrocarbons selectivity due to the

electronic effect of Mn which increases the electron density of chemisorptions site

of catalysts. This depresses the hydrogen chemisorptions which normally

donates electron density to the metal chemisorptions sites, consequently

decreasing the coverage of metal surface by hydrogen which is responsible for

CH4 formation. This inhibits CO hydrogenation to CH4, giving rise to more

unsaturated hydrocarbons and enhanced hydrocarbons chain growth as reported

in our previous study [60].



Figure 7.6 (a) CO-conversion CH4-selectivity (b) CH4-selectivity Vs TOS over

Co/NaMMT and different wt %Mn-Co /Zr-PILC catalysts

7.4.2 FT Reaction Products of Different wt% Mn-Co/Zr-PILC

As compared to the Co/NaMMT catalysts the increased catalytic activity of

Co/Zr-PILC has been explained in chapter 6. After the addition of Mn the

selectivity of CH4 and C21+ hydrocarbons decreased while that of C2-C12

hydrocarbons increased (Table 7.4). The increased selectivity of C2-C12

hydrocarbons were attributed to the limited hydrogen adsorbed on the surface of

catalysts or due the olefins re-adsorption by a secondary reaction responsible for

chain growth. These findings are in agreement with the results reported by

Weckhuysen et al for Mn-promoted catalysts [59, 60]. While decreased selectivity

of C21+ hydrocarbons were assigned to the cracking of long chain hydrocarbons

at the acidic site of Zr-PILC [15, 171-174]. The noticeable improvement in

selectivity of C2-C12 hydrocarbons and considerable decrease in selectivity of C21+

hydrocarbons can also be co relates to the cracking of long chain hydrocarbons

over MMT based catalysts as reported by Wang et al for Co/ion-exchanged



MMT catalysts [80] due to the spillover of hydrogen from Co to the acidic sites of

Zr-PILC. The FT products obtained over Mn promoted Co/Zr-PILC catalyst

showed increased olefin to paraffin O/P ratio as a results of lower availability of

H2 due to the promotion effect of Mn. The increased O/P ratio of our catalysts

was also supported by the similar results reported in references [60, 196].

Table 7.4 FT reaction products over different catalysts

Catalysts CO-

conversion

CO2-

selectivity

C1

(wt%)

C2-C12

(wt%)

C13-C20

(wt%)

C21

(wt%)

20%Co/NaMMT 10.0 2.3 40.0 12.5 8.0 37.2

20%Co/Zr-PILC 53.0 2.3 18.0 45.2 17.4 17.1

0.10Mn-Co/Zr-PILC 56.0 2.1 17 47.3 16 17.3

0.30Mn-Co/Zr-PILC 59.0 2.3 14 49.5 16.1 16.8

0.70Mn-Co/Zr-PILC 60.0 2.4 13 51.5 16 15.8

1.50Mn-Co/Zr-PILC 62.0 2.5 12 55.3 15.5 14.7

3.50Mn-Co/Zr-PILC 58.0 2.4 11 57.1 16.3 13

7.5 Conclusions

The FT catalytic activity of Zr-PILC supported catalysts were successfully

increased by the promotion of Mn. The improved homogenous dispersion of Co

nanoparticle on the surface of Zr-PILC was achieved after the incorporation of

Mn. In FT reaction the most important step was the reduction of Co-oxide, as the

FT reaction taking place at the surface of metallic Co rather than Co-oxide, thus

after the addition of Mn the reduction of Co-oxide were achieved at lower

temperature as compared to the unpromoted catalysts. As a result of this

reduction behavior, the Mn promoted samples completely reduced at reaction

temperature and gave higher catalytic activity in term of increased CO-



conversion, lowered CH4-selectivity and increased time on stream (TOS) stability.

Moreover Mn promoted catalysts showed increased C2-C12 hydrocarbons and

decreased C21+ hydrocarbons selectivity along with increased olefin to paraffin

ratio due to the cracking of higher hydrocarbons on the surface acidic sites of Zr-

PILC as well as due to the decreased surface adsorbed hydrogen.

Chapter 8 Summary and outlook

Page 120

Chapter 8. Summary and outlook

One of the main challenges in FT synthesis is the formation of higher molecular

weight waxes which blocks the active site of the catalysts resulting in decreased

catalytic activity. The present study has resulted in developing a new support

material which appreciably enhances the activity and products selectivity of

gasoline range hydrocarbons via the cracking of higher molecular weight

hydrocarbons (Waxes) formed over the catalysts. Montmorillonite(MMT), the

novel support material was invetigated for Co-based FT synthesis due to their

acidic properties, cheapness, high surface area and environment friendly nature.

The lower thermal stability and lack of porosity in NaMMT was overcome by

incorporating different metal oxides (MOs) (M=Al and Zr,) as a pillar to replace

the sodium ion present in the interlayer of MMT clay. The effect of increasing

reaction pressure and temperature along with effect of Mn and Ce as a promoter

were also studied. Following are the main conclusions from this study.

(i) The activity of NaMMT supported Co-based catalysts were studied for

FT reaction. Theses catalysts showed very lower CO-conversion and

higher CH4-selectivity of 10% and 40% respectively which were

attributed to the lower surface area 39.243 m2/g of NaMMT, small

gallery height 0.33nm of MMT interlayer and also due to the hindrance

in Co3O4 reduction when supported on NaMMT.The incomplete

reduction of Co3O4 were assigned to the presence of Na+ and other alkali

metal present in the interlayer of MMT. As already known that FT

reaction occurred at the surface of metallic Co rather than Co3O4 thats


Page 121

why NaMMT supported Co-catalysts produces too much CH4 and lower

CO-conversion.

(ii) The catalytic activity of NaMMT supported Co catalysts were

successfully improved by pillaring with Al and Zr. The effect of larger

metal cation Al and Zr to replace Na of MMT results an increase in

surface area to 254m2/g and 261m2/g respectively, increases in gallery

height of MMT interlayer to 0.94 and 1.01nm for Al-PILC and Zr-PILC

along with complete reduction of Co3O4 near the reaction temperature.

FT reaction over Co-loaded/Al-PILC and Co-loaded/Zr-PILC showed

higher CO-conversion of 28% and 53%, increased selectivity of 18.23 and

45% towards C2-C12 hydrocarbons and decreased selectivity of CH4

27.27% and 18% and 37% and 17.1% for C21+ hydrocarbons. This

increased catalytic activity were achieved due to the higher surface area,

complete reduction of Co3O4 near the reaction temperature and due to

increased acidic sites (Lewis and bronsted ) apparent from TPD analysis

responsible for hydrocraking reaction occurred at the acidic site of Al-

PILC and Zr-PILC.

(iii) The effect of reaction pressure and temperature on the catalytic

properties of Co/Al-PILC and Co/Zr-PILC were investigated and it was

found that with increase in reaction temperature CH4-selectivity and

CO-conversion increased and the selectivity towards C5+hydrocarbons

decreased due to the high hydrogenation activity of catalysts at high

temperature. The FT reaction carried out at a pressure of 1, 5 and 10 bar


Page 122

showed increase in CO conversion. With increase in pressure C5+

hydrocarbons selectivity were increased while that C2-C5 hydrocarbons

and CH4 were decreased. High pressure favors the production of high

molecular weight hydrocarbons as a result of chain growth probability

which normally increased with increasing pressure .The increase in CO-

conversion is due to the increase in partial pressure of hydrogen and the

increase selectivity of the C5+ hydrocarbons is due to the increase CO

partial pressure while the reaction rate in such type of reaction is

proportional to the hydrogen partial pressure.

(iv) The effect of Mn and Ce-promotion on high surface area Al-PILC and

Zr-PILC supported Co nanoparticles prepared by hydrothermal method

have been investigated. Mn promotion showed very positive effect on

the activity and product selectivity of Co/Al-PILC and Co/Zr-PILC

catalyst. About 42% and 58% CO-conversion were achieved over Mn

promoted samples. The increased CO-conversion were achieved due to

effect of Mn to enhance the dispersion of Co3O4 on the surface of

support and decrease the temperature need for the reduction of Co3O4

apparent from their TPR profile. By insuring the complete reduction of

Co3O4 near the reaction temperature, the selectivity towards CH4 were

decreased because In FT synthesis CH4 produced as a result of

incomplete reduction Co3O4. The promotion of catalyst with Mn

suppressed the H2-addition and enhanced the light olefin production

and also increased the olefin to paraffin ratio (O/P).


Page 123

(v) The incorporation of small amount of cerium to the Co/Al-PILC results

in increase in reduction temperature. Instead of increase in reduction

temperature cerium incorporation results in high dispersion of cobalt

particles and formation of new active sites which are responsible for

higher catalytic activity and higher hydrocarbons (C5-C12) selectivity.

The increased selectivity towards CH4 and C5-C12 hydrocarbons were

obtained over cerium promoted Co/Al-PILC as a result of cracking of

C22+ hydrocarbons. The addition of cerium to the Co/Al-PILC increases

C2-C4 hydrocarbons of FT products due to decreased availability of

adsorbed hydrogen.

Future Work

FT technology for the production of clean synthetic fuel has excellent future

prospects, because of the rapidly decreasing crude oil resources, but the main

problem associated with this technology is the production of highly active and

selective catalysts for gasoline range hydrocarbons. A variety of catalysts mainly

consist of active metal such as Co, Ni and Fe have widely been used, the effect of

some transition metal and some nobel metals like Ru, Rh, Pt, Th and Au along

with alkali and alkaline earth metal as a promoter have also been investigated.

One of the main problems in FT synthesis is the production of CH4, the most

unwanted product of this reaction and the formation of higher molecular weight

hydrocarbons (Waxes) responsible for the blockage of active sites of catalysts

used. In this study these problems were successfully addressed by the use of

novel, cheap and environment friendly support montmorillonite due to their


Page 124

almost similar acidic nature to those of zeolite cause hydrocraking of FT waxes

and increases the selectivity of gasoline range hydrocarbons.

Future investigations, however, could be conducted in the following

directions:

The catalytic activity and product selectivity of FT catalysts were

improved by supporting it on Al and Zr pillred clay. Further

studies need to be carried by pillaring it with other larger metal

cation .

The effect of Fe, TiO2 ,CrO2, and some mixed transition metal oxide

pillaring can also be studied.

Further work can be undertaken for evaluating the effects of acid

activation of clay before pillaring

Higher catalytic activity and product selectivity were achieved by

promoting the catalysts with Mn, further work can be undertaken

to study the effect of Pt, Ru and Rh used as promoter.

The detail study regarding the size of PILC interlayer and pillaring

cation and its effect on FT catalytic activity can be undertaken.

The effect of various wt% of intercalating cation on the textural

properties of catalysts can be undertaken.

Investigation to calculate the total acidic sites on the surface of PILC

could prove to be very useful for increasing the catalytic properties

of these samples by further enhancement in acidic properties.

References

Page 125

References

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List of Publications

Page 156

List of publication included in this dissertation

1. N. Ahmad, S. T. Hussain, B. Muhammad, T. Mahmood, Z. Ali, N. Ali, S.

M. Abbas, R. Hussain, S.M. Aslam., Effect of the Reaction Conditions on

Al-Pillared Montmorillonite Supported Cobalt-Based Catalysts for

Fischer Tropsch Synthesis. Digest journal of Nanomaterials and

Biostructures. 8 (1),347-357,(2013).

2. Nisar Ahmad, Syed Tajammul Hussain, Bakhtiar, Muhammad, Nisar

Ali, Syed Mustansar Abbas, Zr Pillared Montmorillonite Supported

Cobalt Nanoparticles for Fischer-Tropsch Synthesis progress in natural

science ,material international 23, 374-381 (2013).

3. Nisar Ahmad, Syed Tajammul Hussain, Bakhtiar Muhammad, Pillared

Montmorillonite Supported Cerium Modified Cobalt catalysts for

Fischer Tropsch Synthesis, . Under review in progress in natural science ,

material international.

4. Nisar Ahmad, Syed Tajammul Hussain, Bakhtiar Muhammad, Effect of

Manganese Promotion on Al-Pillared Montmorillonite Supported Cobalt

Nanoparticles for Fischer Tropsch Synthesis, Buliton of Korean Chemical

Society 34,(10), 3005-3012, (2013).

5. Nisar Ahmad, Syed Tajammul Hussain, Bakhtiar, Muhammad, Nisar

Ali, Syed Mustansar Abbas, Effect of Manganese Promotion on Zr-

Pillared Montmorillonite Supported Cobalt Nanoparticles for Fischer

Tropsch Synthesis. journal of Applied catalysis under review.


Page 157

6. Nisar ahmad,Syed Tajammul hussain,Bakhtiar Muhammad., Nano-

structured Ceramic Oxide Catalyst For Syngas Conversion to light

Olefins, Current Catlysis,1(1),24-31,(2012)

7. Nisar Ahmad, Syed Tajammul Hussain, Bakhtiar Muhammad, Nisar Ali,

Syed Mustansar Abbas, Influence of Gold Promoter on Fischer Tropsch

Synthesis over Co/Al2O3 Catalysts, IEE conference Proceedings, January

15 – 19, (2013)


Page 158

List of other Publications

8. Naeem Shahzad, Syed Tajammul Hussain, Nisar Ahmed ,Use of pure

and sulphur doped TiO2 nanoparticles for high temperature catalytic

destruction of H2S gas. Chalcogenide Letters, 10 (1) 19-26, (2013).

9. Syed Tajammul Hussain, Syed Mustanser Abbas, Masood Ahmad Khan,

Munib-ur-Rehaman, N.Ahmad, Y-doped titania-CNTs composite as an

electrochemical biosensor for Lysine. Journal of Chemical society Pakistan

35,(6),(2013).

10. Syed Mustansar Abbas, Syed Tajammul Hussain, Saqib Ali, Nisar

Ahmad, Nisar Ali, Modification of carbon nanotubes by CuO-doped

NiO nanocomposite for use as an anode material for lithium-ion

batteries Journal of Solid State chemistry 202, 43-50 (2013).


Ahmad, Nisar Ali, Structure and Electrochemical Performance of

ZnO/CNT Composite as Anode Material for Lithium-ion Batteries.

Journal Material Science 48, 5429-5436 (2013).


Ahmad, Faisal Abbas, One-pot synthesis of monodispersed nanospheres

of CuO and carbon nanotubes composite as anode material for lithium-

ion batteries. Journal Alloys & Compounds 574, 221-226 (2013)


Ahmad, Nisar Ali, Saghir Abbas, A Facile Synthesis of Carbon

nanotubes Anchored with Mesoporous CO3O4 Nanoparticles as Anode


Page 159

Material for Lithium-ion Batteries, Journal Electrochemica Acta 105, 481-

488 (2013).


Ahmad, Nisar Ali, Facile synthesis of carbon nanotubes supported NiO

nanocomposites and their high performance as lithium-ion battery

anode. Material Letters 107, 158-161 (2013).

15. N. Ali, Z. Ali, R. Akram, S. Aslam, M. J. M. N. Chaudhry, M. A. Iqbal, N.

Ahmad, Study of sb28.47Sn11.22S60. 32 compound as thin film for

photovoltaic applications. Chalcogenide Letters. 9(8) 329,(2012).

16. Nisar Ali, Arshad Hussain, S. T. Hussain, M. A. Iqbal, Mutabar Shah,

Ishrat Rahim, Nisar Ahmad, Z. Ali, Kyle Hutching, David Lane.,

Physical Properties of the Absorber Layer Sn2Sb2S5 thin Films for

Photovoltaics, Journal of Current nano sciences, 9,149-152 (2013).

17. Nisar Ali, S. T. Hussain, M. A. Iqbal, N. Ahmad, Yaqoob Khan, David

Lane., Combinatorial study of SnSbS thin films by X-Ray Diffraction and

Photoconductivity, Journal of Global Energy Issues,1, 1-8 (2013)

18. N. Ali, S. T. Hussain, M. A. Iqbal, N. Ahmad, Optoelectronic properties

of the evaporated antimony tin sulphide thin films for solar cell

applications Journal of Elixir Renewable Energy Engg. 55, 13129-13132

(2013).

19. Nisar Ali, S.T. Hussain, Yaqoob Khan, Nisar Ahmad, M.A. Iqbal,

Mustansar Abbas, Effect of air annealing on the band gap and optical


Page 160

properties of SnSbS thin films for solar cell application journal of Material

letters,100,148-151 (2013).

20. N. Ali, S. T. Hussain, S. M. Abbas, N. Ahmad, Sputtering of SnSb

metallic thin films and its sulphurization for solar cell application.

Journal of Thermal Energy and Power Engineering, 2,86-88 (2013).

21. Nisar Ali, Altaf Hussain, S.T. Hussain, Nisar Ahmad, Syed Abbas, Z.

Ali , Study of deep inelastic collision in the heavy ion reaction of 14.0

(Mev/U) 132Xe + 238U, International Journal of Nuclear Energy Science and

Technology (2013).

22. Z. Ali, M.N. Chaudhry, S.T. Hussain, S.A. Batool, S.M. Abbas, N.

Ahmad, N. Ali, N.A.Niaz, Copper and cobalt co-modified nitrogen doped

titania nano photocatalysts for degradation of erichrome black-T, Digest journal

of Nanomaterials and Biostructures. 8,1271-1280 (2013).

catalytic conversion of syngas into …prr.hec.gov.pk/jspui/bitstream/123456789/8980/1/phd...i have...

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