glucose oxidation into gluconic acid : from batch to

Glucose Oxidation into Gluconic Acid : from Batch to Trickle Bed ReactorFrom Batch to Trickle Bed Reactor
Dissertation for the academic degree of Doctor of Science
Faculty of Chemistry and Biochemistry of Ruhr-Universität Bochum
Alessia Padovani
Bochum
December 2016
The present work was made in the period from December 2012 to December 2015 in the
Department of heterogeneous catalysis at Max Planck Institute für Kohlenforschung in
Mülheim an der Ruhr , headed by Prof. Dr. Ferdi Schuth .
Supervisor: Prof. Dr. Ferdi Schüth
Co-supervisor: Prof. Dr. Wolfgang Grünert
For my Parents
“Above all, don't fear difficult moments. The best comes from them.”
“The body does whatever it wants. I am not my body; I am my mind”.
Rita Levi-Montalcini
“Nothing in life is to be feared, it is only to be understood.”
Marie Curie
Acknowledgements
At the end of this challenging experience, I would like to thank all the people who were
involved in this PhD research work.
First of all, I am really thankful to Prof. Dr. Ferdi Schüth for the great opportunity to work
in his Group, for his supervision on this PhD work and for the academic independence and
autonomy he gave me.
Thanks to Prof. Dr. Wolfgang Grünert for the co-supervision and for the interest in my
research.
I would like to thank the HPLC department, especially Heike Hinrichs and Marie Sophie
Sterling for the many measurements, for their evaluation and useful discussion. Thanks to
Inge Springer for the ICP analysis and Silvia Palm for the EDX measurements. Big thanks
go to Bernd Spliethoff for patiently teaching me how to use the TEM and for the help in
evaluating the images.
Thanks to Wolfgang Kersten and Knut Gräfenstein from the Workshop for the great
technical support, especially in the construction and repair of the batch reactor used in this
PhD work. Thanks to the Glassblowing, especially for the trickle bed reactors.
I would like to thank also Andre Pommerin and Laila Sahraoui for the practical support in
the laboratory and for their effort in keeping the laboratories clean and functional.
Thanks to Annette Krappweis and Kirsten Kalischer for helping me in my move to
Germany and for all the support in all the organizational matters.
In the success and enjoyment of the work, my officemates played an important role.
Heartfelt thanks to Valentina Nese, Dr. Mariem Meggouh, Dr. Tobias Grewe, Jean Pascal
Schulte and Xiaohui Deng for our daily chats and leisure activities outside the Institut.
Thanks to Vale and Mariem for our friendship, to Tobi for all the laughs and also for
helping me with the design of the trickle bed reactor. Thanks to JP, for sharing his precious
fume hood with me and for the time we spent together in the laboratory.
Infinite thanks go to my parents Antonio and Cristina, for all the support during both my
academic studies and my PhD work, for their trust and for their encouragement to always
pursue and achieve my goals.
Last but not least, I would like to deeply thank my boyfriend Daniel for the endless
patience and fondness he always showed me and for all the support he gave me.
Big thanks the Max Planck Society for financial support.
Abstract
The central point of this work is the metal catalyzed liquid phase oxidation of glucose to
gluconic acid. During recent years, this reaction has indeed received much attention, since
gluconic acid is a fine chemical which finds many industrial applications, mainly as water
soluble cleansing agent and as additive for food and beverages.
In this study, the glucose oxidation is performed starting from an alkaline sugar solution.
However, no basic solution (NaOH for example) is added to the reaction mixture to
maintain the pH at a fixed value; the reaction is therefore carried out at uncontrolled pH.
The reaction is first performed in a batch reactor. Au, Pd and Pt nanoparticles immobilized
on metal oxides, resins and porous carbons are used as catalysts; among them, carbon
supported metal materials, mainly prepared according to the sol immobilization procedure
[1] , are the most used ones. After performing the glucose oxidation with varying
temperature, pressure and oxidizing agent (pure O2 or air), it can be observed that, at 70°C
and 3 bar pure O2, SX carbon supported Au(1wt%) catalyst shows the best performance.
Indeed, already after 30 minutes, glucose is almost fully converted into gluconic acid (98%
yield).
As the maximum gluconic acid formation is achieved in a very short time, the carbon
supported Au catalyst might be successfully used also in a continuous system, i.e. in a
trickle bed reactor (TBR). However, as powdered catalysts like Au(1wt%)/SX are difficult
to handle in TBRs, a “in home” carbon (IHC) in grain form is chosen as support for the
Au(1wt%) catalyst for use in the TBR. The glucose oxidation is performed with varying
liquid and gas flow rate, temperature and initial glucose concentration; the optimal reaction
conditions, which allow to achieve 81.5% yield of gluconic acid, are 20 ml/h (1.2 minutes
as average residence time) and 575 ml/min as liquid and gas flow rate, respectively, 70°C
and 5wt% starting concentration of glucose.
Both under batch conditions and in the trickle bed reactor, with carbon supported Au
catalysts, high gluconic acid yields were obtained in a very short time and without pH
control during the reaction.
1.2. Gluconic Acid ......................................................................................................... 5
1.3. Metal Catalysed Liquid Phase Glucose Oxidation to Gluconic Acid ..................... 9
1.3.1. State of the Art .................................................................................................... 9
1.3.2. Supported Metal Catalysts: Preparation Methods ............................................. 17
1.3.3. Batch and Trickle Bed Reactors ........................................................................ 20
2. Motivation and Aim ...................................................................................................... 22
3. Results and Discussion ................................................................................................. 25
3.1. Glucose Liquid Oxidation in Batch Reactor – Finding the Best Catalyst for the
Trickle Bed Reactor ......................................................................................................... 25
3.1.1. Batch Reactor - Metal Oxides as Metal Nanoparticles Supports ...................... 26
3.1.2. Batch Reactor - Resins as Metal Nanoparticles Supports ................................. 36
3.1.3. Batch Reactor - Carbon as Metal Nanoparticles Support ................................. 45
3.1.3.1. IHC-1 and IHC-2 Carbons as Metal Nanoparticles Supports .................... 47
3.1.4. Non Powdered Catalyst: from Batch to Trickle Bed Reactor ........................... 50
3.2. Glucose Liquid Oxidation in Batch Reactor – Optimal Catalyst and Reaction
Conditions ........................................................................................................................ 52
3.2.3. Mesoporous SX carbon ..................................................................................... 62
3.3. Glucose Liquid Oxidation under Oxygen Flow .................................................... 93
3.4. Glucose Liquid Oxidation in Trickle Bed Reactor ................................................ 97
3.4.1. Trickle Bed Reactor – Preliminary Tests .................................................... 103
3.4.2. Trickle Bed Reactor - Effect of the Liquid Flow Rate ................................ 104
3.4.3. Trickle Bed Reactor – Effect of the Gas Flow Rate .................................... 110
3.4.4. Trickle Bed Reactor - Effect of the Temperature ........................................ 112
3.4.5. Trickle Bed Reactor - Effect of the Initial Glucose Concentration ............. 114
3.4.6. Trickle Bed Reactor - Effect of the Reactor Diameter ................................ 117
4. Conclusions ................................................................................................................ 121
5.2.1. Synthesis of Metal Nanoparticles Supported on Commercial Resins ......... 130
5.2.2. Synthesis of Au Nanoparticles supported EGD/DVB Resin ...................... 132
5.2.3. Synthesis of Metal Nanoparticles supported on Different Carbons ............ 133
5.2.4. Evaluation of the Metal Content of the Catalysts ........................................... 138
5.3. Reaction Set-ups ................................................................................................. 141
6. Appendix .................................................................................................................... 146
7. References .................................................................................................................. 154
1
1.1. The importance of biomass conversion and catalysis
Catalysis has become a very significant and important field of chemistry, as, currently,
more than 60% of chemical syntheses and 90% of the chemical transformations in
chemical industries are using catalysts [2]
. Moreover, because of environmental issues,
catalysts will become even more important than in the past and will be one of the major
drivers of improvements in our society [3]
.
Nowadays, the development of new processes is based on driving forces which correspond
more to a market-driven strategy. Preferred are less-capital-intrusive processes and the use
of cheaper feedstocks. Society issues have also become an important modern motivation
since the end of the twentieth century. Intensive research of new catalytic materials and
more efficient processes was indeed devoted to convert by-products to useful products and
.
More efficient catalytic processes require improvements in the catalytic activity and
selectivity, which can be enhanced by tailoring catalytic materials with the desired
structure and dispersion of active sites. Different kinds of solid catalyst are available; these
include metals, oxides, carbons, etc., which can be used as bulk materials or immobilized
on a more or less catalytically active support like silica, alumina, titania, carbons, etc.
These materials may possess specific chemical properties, such as acid-base, redox,
dehydrogenating, hydrogenating or oxidizing, and physical properties like porosity, high
surface area, thermal and/or electrical conductivity, etc. The largest family of catalysts
correspond to oxides, which are used both as catalysts and supports.
Currently, petroleum and natural gas represent more than 60% of the primary energy
worldwide supplied; coal remains an important source of energy mainly in Asia. The oil,
natural gas and coal consumption is expected to rise in the near future, whether used in the
field of energy production, transport, heating or as a source of chemical raw materials. By
increasing the production of fossil resources, it was possible to satisfy the significant
increase in energy demand over the past 20 years. Because of this situation and the
problem of CO2 induced global warming, several governments approved new laws aiming
at CO2 emissions reduction by promoting the use of renewable sources and biofuel. 10% of
the world energy is derived from biomass and 7% from nuclear power. For the production
of nuclear power the isotope uranium-235 ( 235
U) is employed. Unfortunately, the by-
Introduction The importance of biomass conversion and catalysis
2
product of this reaction is the very hazardous element plutonium-239 ( 239
Pu, 235
U 239
Pu).
Moreover, since the disaster following the tsunami in Japan in 2011 and the collapse of the
nuclear plant in Fukushima, the construction of nuclear plants has been strongly questioned
in many countries, although nuclear power is essentially CO2 free and does not consume
fossil resources [2]
. Although many challenges relative to the competition between uses and
the management of local natural resources, the biomass introduction into energy systems
presents some advantages, such as the reduction in greenhouse gas emission, as its
synthesis uses CO2 and water. This aspect is in agreement with the principle of green
chemistry, by which only chemical processes which are environmentally benign should be
used [4]
. In future energy scenarios, biomass, i.e. lignin, cellulose and hemicellulose, has
emerged as an important source of energy and raw chemicals in the replacement, at least
partially, of oil, natural gas and coal. [5]
. The European Union receives approximately 66%
of its renewable energy from biomass; this surpasses the total combined contribution from
hydropower, wind power, geothermal energy and solar power. There are three main
strategies for biomass valorisation, as shown in Fig. 1.1; the components from cellulose
and hemicelluloses streams are integrated within the lignin conversion framework [6]
. In
the first strategy, biomass is gasified to syngas or degraded by pyrolysis to a mixture of
small molecules, which can be used to produce chemicals using the technologies
developed for petroleum feedstocks. The second strategy consists in the extensive removal
of the functional groups present in the lignin monomers; this results in simple aromatic
compounds, such as phenol, benzene, toluene and xylene. In the third strategy, biomass is
converted directly to valuable chemicals in a one-pot process, which requires highly
selective catalysts able to eliminate functionalities and linkages. However, product
separation and purification is an important step of each process, as none of the three
strategies is expected to generate a single product in high yield. Since cellulose is the main
constituent of the most abundant renewable lignocellulosic feedstock and it is non-edible,
its transformation had attracted significant attention in recent years. Unlike other
conversion routes, like high-temperature gasification, pyrolysis and enzymatic
fermentation, for the transformation of cellulose a low-temperature and selective process is
desirable. This process should be preferably carried out in a water medium and it should be
able to produce platform molecules, which can be converted into valued chemicals and
fuels. Currently, glucose, polyols, organic acid and 5-hydroxymethylfurfural (5-HMF) are
the most promising platform molecules [2]
.
3
Analogous to the history of the petroleum refinery, with the development of catalytic
technology the biorefinery may become, too, an efficient and highly integrated system to
meet the chemical and fuel requirements of the twenty-first century.
Figure 1.1. Lignocellulosic bio refinery scheme with particular emphasis on the lignin stream
[5] . Reprinted (adapted) with permission from
[5] . Copyright (2010) American Chemical
Society.
The development of catalytic technologies is an important step towards the realization of
this system, by which, in addition to the catalytic conversion of cellulose and
hemicellulose, the lignin fraction of biomass can be transformed from a low-quality and
low-price waste product into high-quality and high-value feedstocks for bulk and specialty.
However, there are still some scientific, environmental, economic and energy challenges
for the future. The scientific challenges consist mainly in the design, preparation,
evaluation and optimization of new catalytic materials and the probing/understanding of
catalyst behaviour in terms of activity and selectivity. From an environmental point of
view, by-products should be minimized by converting them into useful products, replacing
multistep processes by direct schemes, in order to avoid the exposure to dangerous
intermediates, and by using sustainable sources of raw materials and energy supplies.
Economic challenges correspond to the use of cheaper and readily available raw materials,
Introduction The importance of biomass conversion and catalysis
4
but also in increased productivity and decreased lag-time between discovery and
commercialization and development of more selective processes and of new catalysts. The
reduction of the energy consumption remains the main energy challenge.
In the future, solar, geothermal and presumably nuclear power plants will probably be used
for generation of electricity, while biomass, oil, natural gas and coal predominantly for the
production of syngas and chemicals. Hydrogen will be used for GTL and hydroprocessing,
.
1.2. Gluconic Acid
Organic acids represent the third largest category after antibiotics and amino acids in the
global market of fermentation. The market of organic acids is dominated by citric acid due
to its application in various fields. The market of gluconic acid is comparatively smaller;
however 60000 tonnes are produced worldwide annually [7]
.
Gluconic acid (Fig. 1.2a) is a noncorrosive, non-volatile, nontoxic, mild organic acid. It is
a natural constituent in fruit juices and honey, and is used in the pickling of foods. Its inner
ester, glucono-δ-lactone (Fig. 1.2b), imparts an initially sweet taste which later becomes
partly acidic. It is used in meat and dairy products, especially in baked goods, and as
flavouring agent. Generally speaking, gluconic acid and its salts are used in the
formulation of food, but also of pharmaceutical and hygienic products. Different salts of
gluconic acid find various applications based on their properties. Gluconic acid derives
from glucose through a simple oxidation reaction. Microbial production of gluconic acid
(by the enzymes glucose oxidase and glucose dehydrogenase) is the preferred method. The
most studied fermentation process (FDA approved) involves the fungus Aspergillus Niger,
which allows to covert nearly 100% of the glucose to gluconic acid under the appropriate
conditions [7]
.
Gluconic acid production started back in 1870 when it was discovered by Hlasiwetz and
Habermann. Ten years later, Boutroux found for the first time that acetic acid bacteria are
Introduction Gluconic Acid
6
capable of producing sugar acids, and in 1922 gluconic acid was detected in Aspergillus
Niger by Molliard [7]
and Currie
et al. filed a patent employing submerged culture using Penicillium lautem, giving yields
of gluconic acid up to 90% in 48-60 h. Later, Moyer et al. used A. Niger in pilot plant
.
Different approaches are possible for the production of gluconic acid, namely,
electrochemical, biochemical and bioelectrochemical [9]
[10]
. There are several different
oxidizing agents available, but these processes appear to be more expensive and less
efficient compared to the fermentation processes. Although the conversion is a simple one-
step reaction, the chemical method is not favoured. This is the reason why fermentation,
involving fungi and bacteria, is one of the most efficient and dominant technique for
manufacturing gluconic acid. Among various microbial fermentation processes, the
method utilising the fungus A. Niger is one of the most widely used. This method is based
on the modified process developed by Blom et al. [11]
, which involves fed-batch cultivation
with intermittent glucose feeding and the use of sodium hydroxide as neutralising agent.
The pH is held at 6.0-6.5 and the temperature at about 34°C. The productivity of this
process is very high, since glucose is converted at a rate of 15 g/(Lh). Irrespective of the
use of fungi or bacteria, the importance lies on the product which is produced (sodium
gluconate or calcium gluconate, for example). As the reaction leads to an acidic product,
neutralization is required by the addition of neutralising agents; otherwise the acidity
inactivates the glucose oxidase, resulting in the arrest of gluconic acid production [7]
. In the
production of calcium gluconate and sodium gluconate, the conditions for the fermentation
processes differ in many aspects, i.e. glucose concentration (initial and final) and pH
control. The process for sodium gluconate (readily soluble in water, 39.6% at 30°C) is
highly preferred, as glucose concentrations up to 350 g/L can be used without any
problems, and the pH is controlled by the automatic addition of NaOH solution. In
contrast, in the calcium gluconate production process, pH control is achieved by calcium
carbonate slurry addition. The calcium gluconate solubility in water (4% at 30°C) is lower
than the sodium gluconate one. At high glucose concentration (>15%), supersaturation
occurs and, if it exceeds the limit, the calcium salt precipitates on the mycelia, with oxygen
transfer inhibition as a consequence [7]
.
The main product among the gluconic acid derivatives is the sodium gluconate, which has
a high sequestering power and is a good chelator at alkaline pH. Aqueous solutions of
7
sodium gluconate are resistant to oxidation and reduction at high temperatures. It is an
efficient plasticizer and a highly efficient set retarder, but it is easily biodegradable (98% at
48 h). Calcium gluconate is mainly used in the pharmaceutical industry as a source of
calcium for treating calcium deficiency [7]
.
Although the gluconic acid production is a simple oxidation process, which can be carried
out by electrochemical, biochemical or bioelectrochemical methods, production by
fermentation process involving fungi and bacteria is commercially well established.
However, development of novel and more economical processes for glucose conversion to
gluconic acid with longer shelf life would be promising [7]
. A chemical process, consisting
in the aerobic liquid phase glucose oxidation involving the use of metal catalysts, could be
a valid and alternative method. Different products can be obtained, depending on the
functional group that is oxidized (Fig. 1.3).
Figure 1.3. Possible products obtained from glucose oxidation.
8
If the oxidation process involves only the aldehyde group in the glucose molecule,
gluconic acid is formed; from further oxidation of gluconic acid, 2-keto gluconic acid and
5-keto gluconic acid are produced. When only the primary alcohol function is oxidized,
glucuronic acid is formed; further oxidation of glucuronic acid produces glucaric acid.
Additional side products can result from glucose isomerization, i.e. fructose, and from C-C
bond cleavage, i.e. formic acid and glycolic acid.
Introduction Metal Catalysed Liquid Phase Glucose Oxidation to Gluconic
Acid
9
1.3. Metal Catalysed Liquid Phase Glucose Oxidation to Gluconic Acid
1.3.1. State of the Art
In very recent years, the aerobic oxidation of glucose to gluconic acid has gained much
consideration due to gluconic acid´s application as food and beverage additives and in
detergents [12]
. Biochemical pathways are used in the glucose oxidation reaction; however,
these routes are cumbersome, multistep processes and expensive [13]
. In addition, the
catalysts are not recyclable.
In the last decade, metal nanoparticles (NPs) have received substantial interest due to their
unique properties, finding potential application in the catalysis of glucose oxidation. In
particular, over the last twenty years, gold nanoparticles have established an important role
after Haruta [14]
and Hutchings [15]
oxidation and ethylene hydrochlorination. Gold has shown promising behaviour in both
selectivity and resistance to deactivation, compared to Pd and Pt catalysts. Although the
employment of gold in catalysis has been widely expanded [16]
, since the beginning of its
application, the use of this metal in creating new catalytic systems was affected by the high
variation in the catalytic performance, depending on the preparation method employed and
the support used [17]
reported the use of palladium catalysts supported on active
charcoal in the oxidation of a water solution of glucose, with air at 313 K. They obtained
high gluconate yields (99.3%) in the presence of a bismuth promoted catalyst; bismuth was
deposited via a surface redox reaction on Pd/C catalysts containing 1 to 2 nm Pd particles.
Bismuth adatoms were able to prevent oxygen poisoning of the palladium surface by
acting as co-catalyst in the oxidative dehydrogenation mechanism. Via STEM-EDX, it was
shown that bismuth atoms were selectively and homogeneously dispersed on the palladium
particles. The catalyst was recycled without activity or selectivity loss and without bismuth
leaching during both the reaction and the recycling.
In 2002, the selective oxidation of D-glucose to D-gluconic acid in the presence of a
carbon supported gold catalyst, prepared by metal sol immobilization procedure, was
investigated by Biella et al. [21]
. The reaction was performed at both controlled (7-9.5) and
free pH in an aqueous solution using dioxygen as the oxidant under mild conditions (323-
373 K, pO2= 100-300 kPa). No glucose isomerization to fructose was observed during the
Introduction State of the Art
10
reaction and total selectivity to D-gluconate was reached. In comparison to commercial
palladium and platinum-derived catalysts, supported gold showed unique properties, i.e. it
was active at low pH (2.5). At a buffered higher pH (9.5), carbon supported gold and
bismuth-doped platinum-palladium catalysts showed comparable selectivity, although gold
had a higher activity. Furthermore, upon recycling, gold was found to be more stable
toward deactivation (although this also depended on the pH). Also Önal et al. [22]
studied
the activity of Au/C catalysts in the heterogeneously catalysed oxidation of D-glucose to
D-gluconic acid. They prepared a series of Au/C catalysts by the sol immobilization
method, using different reducing agents and different kinds of carbon support. The
materials with Au mean particle diameters in the range of 3-6 nm prepared on Black Pearls
and Vulcan type carbons were shown to be active in the liquid phase glucose oxidation to
gluconic acid. The best results were obtained at 50°C and pH 9.5; the reaction was
described by an oxidative dehydrogenation mechanism in the aqueous phase. From kinetic
tests, carried out excluding mass transfer limitations by intensive stirring and high
volumetric air flow rate, Önal and co-workers [22]
showed that the rate-limiting step was
the surface reaction. Rossi et al. [23]
reported that both carbon-supported and naked colloid,
i.e. in the absence of common protectors (PVA, PVP or THPC), Au nanoparticles with a
mean diameter of 3.6 nm exhibited very high activity in converting D-glucose to D-
gluconic acid [24]
. Unfortunately, the unsupported Au colloids rapidly deactivated within
several hundred seconds; this was attributed to the increasing particle size over 10 nm due
to the agglomeration of Au nanocrystallites. Therefore, in order to improve the catalytic
performance, catalyst supports, such as carbon, are needed to stabilize the structure and
activity of colloidal Au nanoparticles. In the work of M.B. Zhang et al. [25]
, the Au/C
catalysts used for the glucose oxidation was prepared following a standard wet
impregnation method; one portion of the sample was reduced by hydrogen and the other by
plasma using argon as the plasma-forming gas. The samples reduced by plasma showed
highly dispersed gold nanoparticles on carbon and a better catalytic performance than their
hydrogen-reduced counterparts. The plasma reduced the metal leaching and increased the
hydrophilicity of the samples by enhancing the amount of oxygen groups on the surface.
Especially in liquid phase oxidations, when dioxygen or air is used as the oxidant, the
industrial application of metal-supported catalysts is limited by their durability [26]
.
Furthermore, the presence of a base in the gold catalysed reactions is a serious drawback
11
monometallic gold catalysts suffer from some intrinsic defects that sometimes limit the
application of gold nanocatalysts to a great extent. There are two main limitations of these
kinds of catalysts: 1) upon heat treatment, gold NPs tend to aggregate; 2) gold NPs are
highly sensitive to moisture [28]
, often resulting in poor reproducibility of the catalytic
performances. One of the most promising approaches to overcome the problems related to
monometallic Au catalysts is the addition of a second metal to gold [29]
. Bimetallic
materials can combine the properties associated with the two constituent metals resulting in
a great enhancement in their specific physical and chemical properties, due to a synergistic
effect. According to their mixing pattern, bimetallic systems may have one of the four
structural types shown in Fig. 1.4 [30]
. Based on the chemical properties of the second
metal, gold-based bimetallic catalysts are classified into two types. The first type is Au-
BM catalysts, where BM refers to a base metal, and the second type is Au-PGM catalysts,
where PGM refers to platinum group metal [29]
. In the Au-BM bimetallic catalysts, BM is
much more susceptible to oxidation than gold. Phase segregation tends to occur upon
treatment in an oxidizing atmosphere and, as a consequence, the BM will be enriched on
the surface and may form BMOx patches or shells, decorating the gold-rich core.
Depending on the ratio of the two metals [31]
[32]
provide reactive oxygen. Since BM can participate directly in oxidation reactions by
providing reactive oxygen, only a small amount of BM is required to achieve significant
synergy [33]
. In the Au-PGM catalysts, the PGM is much more active than Au toward H2
dissociation and, at the same time, it is typically far less selective toward activation of only
one functional group in polyfunctionalized substrate molecules [34]
. When Au-PGM is used
in oxidation reactions, surface enrichment might take place, forming an Au-rich core and a
PGM-rich shell. In this case, the catalytic performance is actually dominated by the
chemical composition of the PGM -rich shell, and Au behaves more like a promoter of
PGM to prevent over-oxidation and poisoning of PGM by intermediates or products. In
.
In addition, the presence of the second metal may also limit the growth of gold
nanoparticles. This anti-sintering effect is common in Au-PGM bimetallic systems due to a
higher melting point of PGM than of gold [29]
.
12
Figure 1.4. Schematic representation of possible mixing patterns: core–shell (a), subcluster
segregated (b), mixed (c), three-shell (d). The pictures show cross sections of the clusters.
Reprinted (adapted) with permission from [30]
- Published by The Royal Society of Chemistry.
Among the gold-based bimetallic systems, AuPd catalysts are the most extensively studied.
Au is miscible with Pd in all compositions; this facilitates obtaining AuPd alloys and limits
segregation of the single metals. Venezia et al. [35]
prepared AuPd catalysts on silica using
polyvinyl pyrrolidone (PVP) as the protective agent. Au and Pd were reduced in the
presence of PVA either simultaneously or by sequential reduction, usually using NaBH4 as
the reducing agent. Polyvinyl alcohol (PVA) is the most employed stabilizer for the
generation of AuPd nanoparticles [36]
. In order to obtain bimetallic nanoparticles, the
fundamental step is the control of the reduction and the nucleation processes of the two
metals, because of their different redox potentials and the different chemical nature. To
avoid any segregation of the two metals, a proper reducing agent and/or reaction system
should be selected. Prati et al. [37]
were among the first to prepare PVA-protected AuPd
nanoparticles in a liquid phase reaction (selective oxidation of glycerol). By co-reduction
of Au and Pd, an alloy was obtained even though partially segregated palladium was
detected. In contrast, Hutchings´ group [38]
obtained pure alloys. This difference was
ascribed to the different amounts of PVA used: a higher amount of protective agent
13
probably limited the diffusion of Pd on the gold nanoparticles and segregation of Pd was
observed. The role of the protective agent for the Au precursor on the formation of an alloy
of uniform composition has been investigated [39]
. A uniform AuPd alloy could only be
obtained when the Au-PVA system was used. With unprotected Au or weakly stabilized
Au, the NPs underwent reconstruction during the deposition/reduction of Pd, not providing
efficient seeds for alloying the Pd. In these latter cases, the segregation of the two metals or
.
In 2006, mono- and bimetallic catalysts (Au, Pt, Pd and Rh) in form of supported particles
or colloidal dispersion were tested in the aerobic glucose oxidation, in water solution and
under mild conditions, by Comotti et al. [40]
. They found that the activity of bimetallic
particles was enhanced by combining Au with Pd or Pt (TOF = 924 h -1
), while the activity
of single metals under acidic conditions was low in the case of Au and Pt (TOF = 51-60 h -
1 ) and very low in the case of Rh and Pd (TOF < 2 h
-1 ). The great synergistic effect of
platinum was observed working at low pH, whereas almost no effect was present at pH 9.5.
In the presence of alkali, bimetallic colloidal particles appeared more stable towards
agglomeration than monometallic gold particles, resulting in higher conversions. H. Zhang
et al. [41]
prepared unsupported AuPt bimetallic nanoparticles (BNPs) with an Au-rich core
and a Pt-rich shell, and investigated their catalytic activity in the aerobic glucose oxidation.
The materials were prepared using simultaneous reduction with rapid injection of NaBH4,
simultaneous reduction with dropwise addition of NaBH4, and simultaneous alcohol
reduction. By the use of the first reduction method, highly active PVP-protected AuPt
BNPs of about 1.5 nm in diameter were obtained. These materials were characterized by
higher and more durable catalytic activity for aerobic oxidation compared to Au
nanoparticles (NPs) with nearly the same particle size. The higher catalytic activity of
AuPt BNPs was ascribed to two main factors; (1) the small average diameter (1.5 nm) and
(2) the presence of negatively charged Au and Pt atoms due to electron donation from the
protecting polymer (PVP) by electronic charge transfer effects to the catalytically active
sites. In contrast, AuPt BNPs prepared by dropwise NaBH4 addition and alcohol reduction
were characterized by large mean particle sizes and, therefore, they showed a low catalytic
activity.
Beside the metal sol immobilization procedure, the impregnation method has widely been
used for the preparation of AuPd on titania and carbon, in particular by Hutchings´ group
14
[42] . On carbon supports random AuPd alloys were formed, whereas for oxidic supports
.
AuPd/C catalysts prepared by incipient wetness method were evaluated in the glucose
oxidation by Hermans et al. [44]
. These materials showed superior performance compared
to the corresponding monometallic Pd/C and Au/C, and no metal leaching was observed.
The AuPd/C catalysts were characterized by high Pd:C surface ratios, by full Pd reduction,
and by small Pd particles (to which the high activity was connected). The presence of small
amounts of Au in contact with Pd was used to explain the bimetallic cooperative effect, as
the synergistic effect seems to require an interface between the two metals to form.
Beside carbon, also metal oxides have been used in the preparation of supported catalysts
for use in the glucose oxidation reaction. In 2013, Delidovich et al. [45]
studied the aerobic
glucose oxidation in the presence of Au/Al2O3 catalysts with different dispersion of
supported gold and Au/C catalysts containing highly dispersed gold nanoparticles. The aim
of the work was to determine the contribution of the mass-transfer processes to the overall
reaction kinetics in different regimes. The glucose:Au molar ratios were varied. At high
glucose:Au molar ratios, the Au/Al2O3 catalysts showed higher activity than the Au/C
catalysts, with the highest TOF reached with Au/Al2O3 materials characterized by metal
particles of 1-5 nm in size. The Au/Al2O3 catalysts were most effective, if the gold
distribution through the catalyst grains was uniform. For the Au/C materials with a non-
uniform gold nanoparticle distribution, the apparent reaction rate was affected by internal
diffusion, while the interface gas-liquid-solid oxygen transfer influenced the overall
reaction kinetics as well. At a low glucose:Au ratio, the reaction rate was limited by
oxygen dissolution in the aqueous phase. In this mass transfer regime the rate of glucose
oxidation over the carbon-supported catalysts exceeded the reaction rate over the alumina-
supported catalyst, which was attributed to a higher adhesion of the hydrophobic carbon
support to the gas–liquid interface facilitating the oxygen mass transfer towards catalytic
sites. When the reaction rate was determined by oxygen dissolution, hydrophobic materials
were the supports of choice for the aerobic glucose oxidation. In 2010, M. Rosu and A.
Schumpe [46]
focused their study on the chemical enhancement of gas absorption into
catalyst particles employed in slurry systems. They prepared a silanized palladium on
alumina catalyst; they showed that, for the glucose oxidation to gluconic acid on suspended
Pd/Al2O3 particles, the particle adhesion at the gas-liquid interface was promoted by
15
moderate hydrophobization with trichloromethylsilane (TMS). Different from Pd/C
catalysts, the hydrophobized Pd/Al2O3 catalyst was not able to adsorb surface active
contaminants. They found that the silanization had no effect on the catalyst activity, when
the reaction was studied under kinetic control. In the mass transfer controlled regime, they
observed that an enhancement of the absorption rate by the hydrophobized Pd/Al2O3
catalyst particles occurred at very low catalyst loadings. The enhanced gas absorption was
ascribed to interfacial adhesion leading to a locally higher catalyst concentration in the
film. In 2011, Wintonska et al. [47]
studied the effect of tellurium introduction on the
activity and selectivity of home-made supported palladium catalysts in the oxidation of
glucose to gluconic acid. The catalysts for which the presence of PdTe was proven showed
high activity and selectivity. The modification of the catalytic properties of PdTe /support
bimetallic systems was ascribed to the strong mutual interaction between atoms of active
Pd and Te. Wintonska et al [47]
found that bimetallic PdTe /SiO2 and PdTe /Al2O3 catalysts
containing 5wt% of Pd and 0.3-5 wt% of Te were characterized by both high activity and
selectivity towards gluconic acid.
As well as carbons and metal oxides, also polymers were successfully used as metal
nanoparticle supports. Gold nanoparticles were deposited directly onto ion-exchange resins
by reducing HAuCl4 or Au(en)2Cl3 (en = ethylenediamine) with NaBH4 or with surface
amine and ammonium groups in anion-exchange resins. The catalytic activity for the
oxidation of glucose with molecular oxygen was more strongly influenced by the nature of
the polymer supports than by the size of the Au NPs, and it was observed to increase in the
order of the basicity of the ion-exchange resins. Strongly basic anion-exchange resins, such
as quaternary ammonium salt (-N + Me3) functionalized resins, exhibited a TOF as high as
27.000 h -1
for glucose oxidation at 60°C and at pH 9.5. Organic polymers have been used
as supports to efficiently stabilize small Au nanoparticles (2-10 nm in diameter) and
clusters (below 2 nm) [48]
[49]
NPs (2.4 nm) supported on polymer gel, prepared from N,N-dimethylacrylamide (DMAA)
as the main comonomer, ethylene dimethacrylate (EDMA) as the cross-linker, and N,N-
dimethylamino-ethylmethacrylate (DMAEMA) as the functional metal-binding
comonomer, in the aerobic oxidation of alcohols. These materials showed higher catalytic
activity than Au/C for the oxidation of hydrophobic alcohols but lower activity for glucose
oxidation. Except for Au NPs supported on strongly basic anion-exchange resin [50]
, there
are not many polymer supported Au catalysts showing high catalytic activity in the glucose
16
have explored renewable polymeric materials obtained from
natural feedstocks as possible supports for gold catalysts. Cellulose appeared to be a
promising candidate, since, besides being the most abundant and easily obtained organic
compound in nature, it has three significant features: (1) chemical stability and resistance
to degradation by acids or bases, (2) oxygen-rich structure (i.e. hydroxyl groups, which are
expected to interact with the metal ion precursors and stabilize the metal NPs), and (3)
hydrophilic nature, which seems to be suitable for reactions in aqueous media. Ishida et al.
[12] have attempted to properties of the nanoparticles, i.e. shape, size and stability, on the
cellulose support by depositing Au NPs onto the cellulose directly from gold complexes by
a deposition-reduction method and by a solid grinding method. Au NPs of around 2 nm in
diameter could be deposited onto the cellulose by the solid grinding method (with
Me2Au(acac)). Surface OH groups of cellulose acted as stabilizers to keep Au particles
small, although the deposited Au amount was limited (0.23%) because of the low specific
surface area. They also evaluated the catalyst performance in the aerobic oxidation of
glucose in aqueous media, observing that small Au NPs supported on cellulose showed
.
17
1.3.2. Supported Metal Catalysts: Preparation Methods
As suggested from the literature reported in Section 1.3.1., the most often used synthetic
strategies for the preparation of metal catalysts, especially gold-based materials to be
applied to liquid phase oxidations, are the metal sol immobilization procedure and the
direct impregnation of metal salts [30]
. The activity and/or the selectivity of gold catalysts
are correlated with many parameters, such as morphology, dispersion and interaction
between gold particles and support. Due to the low melting point of gold, traditional
catalyst synthesis methods, like incipient wetness and impregnation, often fail to produce
high metal dispersion, depending on the type of support employed.
The immobilization of a pre-formed metallic sol allows a pretty good control of metal
particle size, reducing the influence of the support on metal dispersion [51]
. This method is
based on the preparation of metallic systems through the reduction of the metal precursor
in the presence of a stabilizing agent (polymer, surfactant, polar molecule, etc.), and their
subsequent immobilization on a support [52]
. The crucial point for obtaining good metal
dispersions using this technique is the immobilization step which depends on the surface
properties and morphology of the support [53]
.
For the catalyst preparation by immobilization of metal colloids, it is very important to be
able to separate the nucleation and the growth into different steps, as suggested by Lamer
et al. [54]
. The reducing and the protective agent play both an important role in the structure
formation of the final catalyst. The use of a strong reducing agent, such as NaBH4, is
needed in order to reduce the metal; however, a quick metal reduction makes the
nucleation and growth process difficult to control. It is for this reason that the use of an
appropriate protective agent is important. It can passivate the nanoparticles´ surface and
prevent them from aggregation making the process easier to control [30]
. The stabilization
.
Electrostatic stabilization is based on the mutual repulsion of electrical charges. When two
similar particles are close to each other, van der Waals forces, resulting from an
electromagnetic effect, are always attractive. The addition of a protective agent, such as
citrate or tetrakis (hydroxymethylphosphonium chloride (THPC), to the metal precursor
generates an electrical double layer of cations and anions. The adsorbed layers result in
coulombic repulsions between the particles with the stabilization of the colloid as effect.
As THPC is an electrostatic stabilizer, the positive part of THPC molecules coordinates
Introduction Supported Metal Catalysts: Preparation Methods
18
with the negatively charged metal precursor. During the formation of the metallic sol, a
huge excess of NaOH is present. THPC/NaOH acts as the reducing agent via formation of
formaldehyde, a well-known reducing agent for gold under basic conditions, following the
equation [55]
- P(CH2OH)3 + CH2O + H2O
Sodium citrate has also been widely used as electrostatic protective agent, in particular for
monometallic Au. Sodium citrate acts as a stabilizer as well as a reducing agent. During
the metal reduction, it becomes oxidized to the intermediate ketone (acetone dicarboxylic
acid), which in return is an even better reducing agent. The steric effect was investigated
by adsorption of polymers of a sufficiently high molecular weight, forming a protective
layer and keeping the nanoparticles at a distance too large to show van der Waals
interactions and, therefore, avoiding agglomeration. Among them, polyvinyl pyrrolidone
(PVP) is the most one used. Electrosteric stabilization is achieved by the adsorbed
polymers having non-negligible electrostatic charges on the metal precursors, resulting in
significant double-layer repulsion. Polyvinyl alcohol (PVA) is a typical example and the
most employed stabilizer for the generation of Au nanoparticles.
Besides for monometallic catalysts, the metal sol immobilization procedure is also used for
the preparation of gold bimetallic catalysts. In the case of bimetallic systems, the method is
based on co-reduction or consecutive reduction of the metal precursors in the presence of
the stabilizing agent, which passivates the nanoparticles´ surface and prevents them from
aggregation, and their subsequent immobilization on a support [35]
. When the second metal,
with a lower redox potential, is reduced, it can deposit on the surface of the preformed
nucleus of the first metal symmetrically with a core-shell structure. If the two metals are
.
The impregnation method consists in the direct impregnation of the support with an
aqueous solution containing the metal precursors in the absence of any protective agent,
followed by evaporation of the water. The dried material is further reduced, normally using
high temperature treatment or gas phase reduction under a H2 flow [56]
. The characteristics
of the catalysts obtained with this method strictly depend on the post-treatment conditions
and on the type of supporting material. Even though impregnation was shown to produce
less active gold catalysts than sol immobilization, the simplicity of the methodology makes
impregnation still attractive for industrial scale-up purposes.
Introduction Supported Metal Catalysts: Preparation Methods
19
The advantage of using the sol immobilization procedure lies in its applicability, regardless
of the type of support employed; moreover, it is possible to control the particle
size/distribution obtaining normally highly dispersed metal catalysts. In contrast, direct
.
20
Glucose oxidation, and generally metal catalysed oxidation reactions, is usually carried out
in batch reactors containing the solution of the organic substrate to be oxidized and a
suspension of the catalyst in powder form. Typically, these reactions are run at
atmospheric pressure under continuous stirring with air bubbling through the suspension
maintained at constant temperature in the range of 20-80°C. Oxidation reactions are carried
out with pH ranging from 2 to 13, but in many instances from 7 to 9. The pH is regulated
by the addition of dilute alkali solutions under the control of a pH regulator. The reaction
kinetics are followed by monitoring the addition of alkali solution required to maintain a
constant pH or by chromatographic analysis of the reaction medium taken at time intervals
[57] . Generally, no difficulties and obstacles are encountered when the reaction is carried
out in a batch reactor.
In contrast, performing the glucose oxidation in a trickle bed reactor might be more
challenging: indeed, in trickle bed reactors, different transport processes occur at different
time and length scales.
On a reactor scale, gas and liquid phases are introduced from the top and they flow through
the voids of the catalyst bed. Several different flow regimes may therefore exist in trickle
bed reactors, because of different levels of interphase interactions. The distribution of
liquid phase reactants in the bed depends on the quality of liquid distribution at the inlet,
overall variation in bed porosity, wetting and capillary forces. Though liquid is distributed
uniformly in the top region of the column, non-uniformities in bed porosity and uneven
wetting may cause further non-uniformities, as the liquid flows along the length of the
reactor. Packing configuration significantly influences possible channeling and
maldistribution within the reactor. The nature of voids formed between particles affects the
flow structure inside the void and hence controls the mixing, heat and mass transport rates.
It also affects the dynamic liquid holdup and the stagnant liquid holdup (corresponding to
stagnant liquid pockets) in the bed. The exchange between these two quantities often
determines the effective residence time distribution in trickle bed reactors. For exothermic
reactions, as oxidation processes, this phenomenon is important where dry out of particles
may lead to the formation of local hotspots which may result in temperature runaway.
Solvent evaporation adds further complexities in heat and mass transfer rates.
On a single catalyst particle scale, wetting of particle and intraparticle mass and heat
Introduction Batch and Trickle Bed Reactors
21
transfer play an important role in the overall rate. Though wetting of particles is a result of
global operations, particle-scale parameters also determine the degree of wetting of each
particle. Flowing liquid forms a film over the external surface of the catalyst and partial
wetting may occur at lower liquid flow rates. The wetted part of the catalyst surface gets
exposed to the liquid phase reactants and the dissolved gas phase reactants. The non-wetted
part, instead, is exposed to the gas phase reactant. In most cases, however, due to capillary
effects, catalyst particles get completely filled with the liquid phase. However, this
condition is not always true if liquid phase is evaporating or pores are larger such that
capillary effects are negligible. Partial wetting condition affects the reaction rates in
various ways, influencing mass transfer from gas and liquid phases to catalytic sites
available on and in the particle. Gas-particle mass transfer rates are significantly enhanced
due to direct access of gaseous reactants through the non-wetted surface. The analysis of
the reaction rates under partial wetting condition is extremely complex, due to solvent and
substrate condensation/evaporation and local temperature variation; often, this leads to an
increase/decrease of catalyst effectiveness and in some cases to multiplicity of conversion
and temperature. Furthermore, most of the trickle bed reactions are exothermic in nature
and careful account of intraparticle, interphase and even bed-to-wall heat transfer is crucial
in understanding the overall performance [58]
.
2. Motivation and Aim
In the chemical industry, oxidation reactions play a relevant role for the production of
many significant and crucial compounds [59]
. New “green” catalytic oxidation approaches
must meet both health and environmental standards, and at the same time aim at a
reduction of cost and time [60]
.
Homogeneous catalysis has been widely used in oxidative processes for the manufacturing
of bulk and fine chemicals [61]
. The main advantage of molecular catalysts is that they
dissolve in the reaction medium; hence all catalytic sites are available, resulting in high
reaction rates and in a reaction time reduction. However, homogeneous catalysts are rather
difficult to separate from the reaction mixtures and they may also cause corrosion to the
industrial materials with possible deposition on the reactor walls [62]
. Heterogeneous
catalysis is considered a better alternative for the synthesis of commodity materials.
Different materials, i.e. silica, carbon, clay, zeolite, metal oxide, polymers and other
materials are currently used as solid supports [63]
. In heterogeneous catalytic reactions, the
catalyst and the reactants exist in different phases; actually, the majority of heterogeneous
catalysts are solids and the reactants are usually either liquids or gases [64]
. Solids catalysts
are easier to prepare and handle, as they are stable, reusable, and easy to separate and they
can also be used as fixed beds. The catalyst is often a metal to which chemical and
structural promoters or poisons are added in order to enhance the efficiency and/or the
selectivity. Currently, heterogeneous catalysis dominates the industries of chemical
transformation and energy generation [62]
.
Among the heterogeneously catalysed oxidation reactions of potential industrial interest,
the metal catalysed liquid-phase oxidation of glucose to gluconic acid has recently gained
much consideration, due to the wide gluconic acid applicability [12]
. Although the gluconic
acid production by fermentation process is commercially well established, the development
of novel, more economical and one-step processes for the glucose conversion to gluconic
acid might be a valid and successful alternative [7]
.
The aim of this PhD research work is to find the optimal catalyst and reaction parameters
in order to successfully perform the metal catalysed liquid phase glucose oxidation under
batch and continuous conditions, i.e in a trickle bed reactor (TBR); furthermore, high
gluconic acid yields should be achieved in a short time and without pH control.
Motivation and Aim
23
The selective catalytic oxidation of glucose with molecular oxygen is an environmentally
benign process for the production of gluconic acid, widely used in the food, detergent and
pharmaceutical industries [24]
. In this work, pure molecular oxygen will therefore be used
as oxidizing agent to perform the reaction; however, some experiments will be carried out
with air, in order to verify if this is a valid and more convenient alternative to the use of
pure oxygen. Different solid catalysts will be evaluated in the reaction. The procedures
based on supported palladium and platinum materials have been intensively investigated in
the past years [26]
; however, since these catalysts often suffer from drawbacks of low
catalyst durability and relatively low selectivity, many studies have recently been devoted
to employ gold as catalyst to achieve selective aerobic oxidation of glucose under mild
conditions; moreover, superior selectivity, high catalytic activity and long term stability
were observed [21]
.
Contrary to the majority of the studies on glucose oxidation, in the present work no
alkaline solution will be added to the reaction mixture to maintain the pH at a fixed value
during the reaction performed in the batch reactor; the glucose oxidation will be indeed
performed starting directly from an alkaline sugar solution. This is due to the intended use
of the trickle bed reactor; in this kind of reactor, pH adjustments during the reaction are
indeed difficult.
Since performing the reaction in a trickle bed reactor might be particularly challenging,
due to the different transport processes occurring at different time and length scales, the
glucose oxidation will be first carried out in a batch reactor. In this system, the reaction
will be performed under mild conditions, under variation of the reaction temperature (25-
90°C) and oxygen pressure (1-4 bar). Different commercial and self-prepared supported
Au, Pd and Pt materials will be evaluated in the glucose oxidation in order to find the
catalyst and the reaction conditions by which maximum glucose conversion into gluconic
acid is achieved in the shortest time.
This catalyst will be later used to perform the reaction in continuous mode instead of under
batch conditions, which is the typical mode of operation described in the literature. As the
trickle bed reactor (TBR) is the most industrially used reactor to treat continuously three-
phase systems, a lab-scale TBR set-up will be assembled. Here, the glucose oxidation will
be carried out under variation of several reaction parameters, such as liquid and gas flow
Motivation and Aim
24
rate, temperature and glucose solution concentration, with the aim of obtaining the highest
gluconic acid yields in the shortest time.
Results and Discussion Glucose Liquid Oxidation in Batch Reactor –
Finding the Best Catalyst for the Trickle Bed Reactor
25
3. Results and Discussion
3.1. Glucose Liquid Oxidation in Batch Reactor – Finding the Best Catalyst
for the Trickle Bed Reactor
In the batch reactor, the reaction was carried out at 70°C, 3 bar O2 and under mechanical
stirring; high stirring rate (1000 rpm) was applied in order to exclude external mass
transfer limitations. Oxygen, due to its low solubility, is the deficit compound in the
glucose oxidation. Therefore, a sufficient concentration of dissolved oxygen has to be
ensured during the course of the reaction [69]
. The use of a closed vessel under oxygen
pressure might increase oxygen dissolution, with the additional effect of speeding up the
reaction [21]
.
The glucose oxidation was performed starting from an alkaline (pH 13.5) sugar solution
(5wt% glucose alkaline solution) with glucose:metal ratio = 1000. No basic solution
(NaOH for example) was added to the reaction mixture to maintain the pH at a fixed value;
the glucose oxidation was therefore carried out at uncontrolled pH. Alkaline conditions are
apparently necessary to increase the reaction rate and to avoid drastic catalyst deactivation;
conversely, however, such conditions are also responsible for side reactions that reduce
gluconate productivity [21]
.
The following tests performed in the batch reactor were aimed to find the catalyst by which
maximum gluconic acid yield is achieved in the shortest time, and which might therefore
be used in a trickle bed reactor. In order to be used in a trickle bed reactor, the catalyst
should be in non-powder form; generally, powder catalysts are difficult to handle in TBRs,
mainly because of low bed porosity and difficulties in flow distribution. Gold
nanoparticles, as well as palladium and platinum, supported on metal oxides, resins and
carbons, were employed as catalysts. The metal oxides, the resins and the carbon used as
supports consist of pellets, spheres and grains, respectively; for this reason, catalysts
supported on these kinds of materials might be packed as a bed through which gas and
liquid can flow.
Furthermore, by using different materials as supports, also the effect of the support on the
catalytic activity of the dispersed metal nanoparticles for the reaction in the batch reactor
could be investigated.
Nanoparticles Supports
3.1.1. Batch Reactor - Metal Oxides as Metal Nanoparticles Supports
The glucose oxidation was performed using commercial gold supported on metal oxides.
Au(1wt%)/ ZnO, Au(1wt%)/Al2O3 and Au(1wt%)/TiO2 were evaluated in the reaction at
70°C, 3 bar O2, 1000 rpm stirring and for 7h. The conversion and gluconic acid yield
profiles obtained are reported in Fig. 3.1. After 420 minutes, the highest glucose
conversion (89.3%) was achieved with Au(1wt%)/ZnO. With Au(1wt%)/Al2O3 and
Au(1wt%)/TiO2, 85.4% and 80.4% of initial glucose was converted. However, the amount
of gluconic acid detected in the three reaction mixtures was very low. After 7 h of reaction,
13.7% of gluconic acid was achieved using Au(1wt%)/ZnO, while with Au(1wt%)/Al2O3
and Au(1wt%)/TiO2 the amount of gluconic acid detected in the corresponding reaction
mixture was 7.5% and 9.5% respectively.
The catalytic performance of alumina supported gold in the glucose oxidation was already
studied by Baatz et al. in 2007 [70]
[71]
. For the preparation of gold catalysts, they used the
deposition-precipitation methods, with NaOH (DP NaOH) or urea (DP urea) as
precipitation agents, as described by Haruta [17]
and Dekkers [72]
. The catalysts prepared by
DP urea showed a strong dependence of specific activity on the gold content. Very high
activity was observed at very low gold content. Increasing the gold content led to a
decrease in the catalyst activity [71]
. Baatz et al. [70]
prepared gold catalysts also by the
incipient wetness method. They found that these catalysts had an activity-gold content
relationship similar to the one observed for the materials prepared by the DP urea method.
In addition, the alumina-supported catalysts, prepared either by DP urea or incipient
wetness, showed 100% selectivity towards D-gluconate. Although the preparation method
of the commercial gold supported on metal oxides used in this PhD work is unknown, it
can be assumed that they were synthetized by deposition-precipitation (either with NaOH
or urea) or by incipient wetness impregnation. These are indeed the most frequently used
preparation methods for metal oxide supported materials. The most active catalysts
prepared by Baatz et al. [70]
[71]
by DP urea and incipient wetness impregnation had gold
contents of 0.1 wt% and 0.3 wt%, respectively, and both showed extremely small gold
particles between 1.2 and 3 nm [71]
. In this PhD work, the commercial Au/Al2O3, as well as
Au/ZnO and Au/ TiO2, had a higher gold content (1wt%) with an average gold crystallite
size of ~ 2-3 nm. However, it should be also taken into account that the Au/Al2O3, Au/ZnO
and Au/ TiO2 were used in the form of extrudates, and that they were evaluated in the
Results and Discussion Batch Reactor - Metal Oxides as Metal
glucose oxidation without previously being crushed to powder. Therefore, mass transfer
limitations might have been significant for the reaction in the batch reactor, with the effect
of further decreasing the catalytic performance of these materials.
0 50 100 150 200 250 300 350 400 0
20
40
60
80
100
Au(1wt%)
3
Conversion
Figure 3.1. Conversion and gluconic acid yield profiles for the glucose oxidation in the batch
reactor performed with commercial gold (1%) supported on ZnO, Al2O3 and TiO2 as
catalysts (70°C, 3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)). 1 run
for each test.
The product distribution of the glucose oxidation performed with Au(1wt%)/Al2O3,
Au(1wt%)/ZnO and Au(1wt%)/TiO2, corresponding to the end of the reaction (7h), is
shown in Fig. 3.2. Glucuronic acid was the glucose oxidation side-product detected in
major amounts in all the reaction mixtures corresponding to the three catalysts. The highest
amount of glucuronic acid (36.4%) was formed when the reaction was performed with
Au/Al2O3. With Au/ZnO and Au/ TiO2, the amount of gluconic acid detected in the
corresponding reaction mixtures was 28.0% and 28.3% respectively. Beside these acids,
also other products were found in the three reaction mixtures.
Fructose
85.4%
Figure 3.2. Product distribution after 420 minutes for the glucose oxidation performed in the
batch reactor with Au(1wt%)/ZnO, Au(1wt%)/Al2O3 and Au(1wt%)/TiO2 as catalysts (70°C,
3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)).
The presence of fructose was observed during the glucose oxidation, indicating the
isomerization of glucose. It is known that by treating monosaccharides with concentrated
alkaline solutions, the sugars are destroyed [73]
and that alkaline media with lower pH
induce an isomerisation reaction of glucose to fructose, resulting in an equilibrium mixture
of the two sugars [73]
. Unreacted glucose was found in all three reaction mixtures, with the
highest amount in the case of Au(1wt%)/TiO2 (19.6%). Other oxidation side-products, like
2- and 5-keto gluconic acid and decomposition products like formic, glycolic and acetic
acid were also detected in minor amounts (<5.0%).
Since the highest amount of gluconic acid was achieved with Au(1wt%)/ZnO, this material
was used to perform further tests with variation of reaction conditions, such as the reaction
temperature and the pH of the initial glucose solution. The results are reported in Fig. 3.3.
When the reaction was performed at 70°C starting from a neutral glucose solution,
gluconic acid was formed in 41.6% yield. With respect to the reaction carried out at the
same temperature from an alkaline sugar solution (89.3% conversion and 13.7% gluconic
acid yield), the conversion was lower (67.5%) but the gluconic acid yield obtained was
higher.
si o n (
pH 13.5, RT
Figure 3.3. Conversion and gluconic acid yield values after 420 minutes for the glucose
oxidation performed in the batch reactor with Au(1wt%)/ZnO as catalyst with variation of
reaction temperature and pH (3 bar O2, 1000 rpm stirring, 5wt% glucose solution (pH 13.5)).
1 run for each test.
Starting from an alkaline glucose solution, the glucose oxidation was also carried out at
room temperature (RT). In this case, the highest conversion and gluconic acid yield were
obtained, with 86.5% gluconic acid detected in the corresponding reaction mixture. Minor
amounts of fructose and 5-Keto gluconic acid were also found. A reaction temperature of
70°C might be too high to perform the glucose oxidation with gold supported on metal
30
oxides. This is also in agreement with the above mentioned work of Baatz et al. [70]
, who
tested the performance of all catalysts in a thermostat glass reactor at the lower temperature
of 40°C. Also Mirescu et al. [73]
obtained the best results with 0.45 wt% Au supported on
titania at a reaction temperature between 40-60°C and a pH value of 9. When, in this PhD
work, the glucose oxidation was performed at 70°C, starting from a high alkalinity glucose
solution (pH = 13.5) with the commercial gold on metal oxide catalysts, the final solution
had a brown/caramel colour and a caramel odour. The high temperature induced a
degradation of glucose with fragmentation of the molecule, resulting in short chain
carboxylic acids, aldehydes, etc.; this phenomenon is known as caramelization [74]
. As the
process occurs, browning of the sugar is observed and volatile chemicals are released,
producing the characteristic caramel colour and odour. The caramelization consists of
different type of reaction, such as dehydration and fragmentation reactions, unsaturated
polymer formation, isomerization of aldoses and ketoses and condensation reactions. The
process is temperature dependent and different sugars have their specific point, at which
.
However, caramelization reactions are also sensitive to the chemical environment. The
reaction rate or the temperature at which the reaction occurs may be altered by controlling
the pH of the sugar solution. In general, the caramelization rate is lowest around pH 7 and
accelerated under both acidic (especially pH < 3) and basic (especially pH > 9) conditions.
When performing the reaction at 70°C with respect to RT, both in alkaline and neutral pH,
the caramelization process might be the main reason for the lower gluconic acid yield.
Since the caramelization rate is higher at pH > 9, the process occurs in greater extent when
the reaction is performed starting from an alkaline solution than from a neutral one. Indeed,
41.6% gluconic acid is detected in the reaction mixture at neutral pH, while only 13.7%
gluconic acid yield is formed starting from an alkaline solution. The lower conversion
observed performing the reaction at neutral pH with respect to basic pH at 70°C might
instead be due to the pH itself. The reaction rate increases with increasing pH; indeed, in
alkaline solution, the deactivation of the catalyst, due to gluconic acid blocking the active
centres on the catalyst surface, is prevented. Considering instead the reactions performed
starting from an alkaline glucose solution, the lower conversion observed at 70°C might be
related to the effect of the temperature on the solution pH. The pH of a solution decreases
with increasing temperature; this could lead to a small extent of catalyst deactivation by
gluconic acid poisoning. However, this phenomenon remains more significant in neutral
solution than at lower, but still alkaline, pH. Indeed, the difference in conversion observed
at 70°C and RT in alkaline solution is lower than the difference in conversion observed at
alkaline and neutral pH, at 70°C. The higher yield of gluconic acid obtained at RT with
respect to 70°C, at basic pH, might still be explained by the absence of caramelization
process.
According to the obtained results, in order to achieve a significant amount of gluconic acid
using metal oxides supported gold, the glucose oxidation should be carried out with
Au(1wt%)/ZnO at room temperature starting from an alkaline glucose solution.
0 50 100 150 200 250 300 350 400 0
20
40
60
80
100
Fructose
0 50 100 150 200 250 300 350 400 0
20
40
60
80
100
2-Keto gluconic a.
Figure 3.4. Conversion and products yields profiles for the glucose oxidation performed in
the batch reactor with commercial Pd(5wt%)/Al2O3 and Pt(5wt%)/Al2O3 as catalysts (70°C, 3
bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)). 1 run for each test.
Beside gold, also palladium and platinum were tested on a metal oxide support.
Commercial Pd(5wt%)/Al2O3 and Pt(5wt%)/Al2O3 were used as catalysts to carry out the
glucose oxidation in the batch reactor. Fig. 3.4 shows the conversion and yield profiles for
the products detected in the highest amounts. With Pd, higher conversion and gluconic acid
yield was achieved with respect to Pt. At the end of the reaction (7h), 98.0% and 79.0%
conversion was reached with Pd and Pt respectively. Simultaneously, the gluconic acid
32
yield obtained with Pd (51.0%) was twice higher than the amount detected in the reaction
mixture resulting from use of the Pt catalyst (~23.0%).
Similar amounts of fructose were detected with both metals, while a higher quantity of
unreacted glucose was found in the reaction mixture corresponding to Pt. With both Pd and
Pt, 7.0-8.0% of 2-Keto gluconic acid was formed, and also smaller amounts (<3.0%) of
glucaric, glycolic and 5-Keto gluconic acid were detected in the two reaction mixtures.
The conversion profiles obtained for Au(1wt%)/ZnO, Au(1wt%)/Al2O3 and
Au(1wt%)/TiO2 reported in Fig. 3.1 are all characterised by a plateau reached within 30
minutes of reaction. A possible explanation might be a product poisoning of the catalyst. In
order to verify this hypothesis, possible products were individually added to the starting
glucose solution (mmol added product:mmol glucose = 1:4); Au(1wt%)/ZnO was used as
catalyst. The aim was to observe the effect of these additions on the initial reaction rates.
Since the oxidation of glucose to glucuronic acid is in competition with the oxidation of
glucose to gluconic acid (Fig. 1.3), glucuronic and gluconic acid were considered possible
sources of catalyst poisoning. Furthermore, from further oxidation of glucuronic and/or
gluconic acid, glucaric acid is obtained; therefore, glucaric acid was also added to the
initial glucose solution in order to study its effect on the reaction rate. The influence of
glycolic acid, as possible degradation product, was also investigated. From the results
reported in Fig. 3.5, it is clear that the addition of gluconic acid, the target product of the
glucose oxidation, did not have any significant effect on the reaction rate. Indeed, after 5
minutes, around 78.0% conversion was reached with or without gluconic acid addition. In
contrast, both glucuronic acid and glucaric acid addition to the initial glucose solution
resulted in a conversion decrease. After 5 minutes, 57.7% and 68.5% conversion was
obtained with glucuronic acid and glucaric acid addition, respectively. An interesting effect
on the initial rate was observed in the case of glycolic acid addition; indeed, higher
conversion was obtained with respect to the reaction performed without any product
addition (89.4% and 77.6%, respectively).
20
40
60
80
100
Time (min)
+ Glycolic a.
Figure 3.5. Effect of the individual addition of possible products to the starting glucose
solution on the initial reaction rate. Au(1wt%)/ZnO used as catalysts (70°C, 3 bar O2, 1000
rpm stirring, 5wt% glucose alkaline solution (pH 13.5)). 1 run for each test.
The addition of possible products influenced also the yields of the main glucose oxidation
products. The higher conversion obtained after adding glycolic acid to the initial glucose
solution is due to the higher gluconic acid amount produced (Fig. 3.6a). After 300 minutes,
while only 14.3% gluconic acid is formed when the reaction is performed without any
product addition, 63.2% gluconic acid is detected in the reaction mixture corresponding to
glucose+glycolic acid as initial solution. 32.1% and 22.5% is the gluconic acid yield
obtained with glucuronic and glucaric acid addition, respectively. Contrary to what is
observed for gluconic acid, lower glucuronic acid amounts were produced after adding
glucaric (12.1%), gluconic (11.8%) and glycolic (2.5%) acid (Fig. 3.6b). When the reaction
was performed without any product addition, 28.8% glucuronic acid was formed. When no
product was added, the amounts of glucaric, glycolic. 5- and 2-keto gluconic acid were
generally lower (<5%) than gluconic and glucuronic acid. The highest glucaric acid yield
was detected when glucuronic acid was added to the initial glucose solution (4.5%);
without any product addition, basically no glucaric acid was produced (0.4%) (Fig. 3.6c).
+ Glycolic a. : 0%
+ Gluconic a. : 0%
Figure 3.6. Effect of the individual addition of possible products to the starting glucose
solution on the product yields after 300 minutes (70°C, 3 bar O2, 1000 rpm stirring, 5wt%
glucose alkaline solution (pH 13.5)).
1.3% and 0.7% yield were obtained by addition of glycolic and gluconic acid, respectively.
The glycolic acid yield increased by addition of gluconic acid (3.7%) and slightly
35
decreased when the reaction was performed starting from a glucose+glucaric acid (2.7%)
and glucose+glucuronic acid (2.5%) solution (Fig. 3.6d). By addition of glucaric acid, the
amount of 5-keto gluconic acid was higher (3.9%) than the one obtained without any
product addition (2.3%) (Fig. 3.6e). In contrast, the addition of glucuronic acid had the
effect of decreasing the formation of 5-keto gluconic acid (1.6%). The addition of possible
products did not have any effect on the 2-keto gluconic acid yield.
According to the results reported in Fig. 3.5, the catalyst poisoning by glucuronic and
glucaric acid might be the reason for the inhibition of the catalytic activity, which
corresponds to a plateau in the conversion profile. Although the glucuronic and glucaric
acid addition to the initial glucose solution resulted in lower glucose conversion, higher
gluconic acid yields were obtained. The highest conversion and gluconic acid amount
produced was observed by adding glycolic acid to the starting sugar solution.
Results and Discussion Batch Reactor - Resins as Metal
Metal nanoparticles supported on resins, both commercial and “home-prepared”, were
evaluated in the glucose oxidation performed in the batch reactor. Equilibrium reactions
taking place within resins can be conveniently shifted to the right if the products have a
low compatibility with the resin. Using hydrophobic polymer matrices as metal
nanoparticle supports could be a strategy to favour the expulsion of the glucose oxidation
products, mostly polar, from the resin. Furthermore, the application of supported polymers
in catalytic oxidation has gained much attention because of their inertness, nontoxicity,
non-volatility, and recyclability [76]
Commercial porous resins Amberlyst A35 and A70, sulfonated styrene/divinylbenzene
(PS-DVB) copolymers (Fig. 3.7), were both used as supports for Pt, Au and Ru
nanoparticles (5wt% metal loading). Although both resins belong to the macroreticular
type (DVB > 4%), they differ in the cross-linker content (DVB) which is 20% and 8% for
the A35 and A70 resin, respectively. According to ICP-analysis, the actual metal content
matched the theoretical one (Section 5.2.4).
Figure 3.7. Example of vinyl monomer polymerization: co-polymerization of styrene and
divinylbenzene to a polystyrene resin and further sulfonation.
0 50 100 150 200 250 300 350 400 0
20
40
60
80
Conversion
Figure 3.8. Conversion and gluconic acid yield profiles for the glucose oxidation performed in
the batch reactor with Pt(5wt%)/A35 and Pt(5wt%)/A70 as catalysts (70°C, 3 bar O2, 1000
The resin supported catalysts were evaluated in the reaction at 70°C, 3 bar O2, 1000 rpm
stirring and for 7h The conversion and gluconic acid yield profiles obtained for Pt(5wt%)
are shown in Fig. 3.8; with Au and Ru similar trends were observed. In general, the
conversion reached at the end of the reaction (7h) was around 60-70% while the gluconic
acid yield was close to zero. However, when Pt(5wt%)/A35 was used as catalyst in the
glucose oxidation, slightly higher conversion was obtained. Although the reaction was
carried out for 7 h, already after 5 minutes a plateau around 65.0% with A35 and 62.0%
with A70was observed in the conversion profile. The reason for the very low gluconic acid
formation observed with all the resin supported catalysts might be found in the product
distributions corresponding to the respective reaction mixtures. As reported in Fig. 3.9,
fructose and gluconic acid were detected in major amounts. For all catalysts, a significant
quantity of glucose did not react. Only a negligible amount of gluconic acid (2%) was
formed when the glucose oxidation was performed with Pt(5wt%)/A70.
in
Fructose
A35 A70
Figure 3.9. Product distribution after 420 minutes for the glucose oxidation performed in the
batch reactor with Pt, Au and Ru nanoparticles (5wt% metal loading) supported on A35 and
A70 resins (70°C, 3 bar O2, 1000 rpm stirring, 5wt% glucose alkaline solution (pH 13.5)).
Other side-products detected in all the reaction mixtures in similar amounts (<5.0%) were
glucuronic, formic, glycolic, 2-Keto and 5-Keto gluconic acid. With Pt, Au and Ru
supported on A70 and A35 resins the carbon balance does not close. The reason might be
found in the chromatograms of the reaction mixtures corresponding to the resin supported
catalysts. Indeed, in all of them, the presence of many peaks, some of which even
overlapped, was observed. Beside the difficult quantification of the known peaks, many
other peaks could not be assigned to the known molecules and quantified. Thus, the acidic
resins supports induce many side reactions, which render them overall unsuitable for
glucose oxidation. Furthermore, with all catalysts supported on the commercial resins, the
final solution had a brown/caramel colour and a caramel odour. As in the case of gold
supported on metal oxides (Section 3.1.1), this was a sign of the degradation of glucose
with fragmentation of the molecule, resulting in short chain carboxylic acids, aldehydes,
etc. (caramelization) [74]
oxidation products. The rapid deactivation of the metal nanoparticles supported on resins,
which results in a plateau in the conversion profile, might instead be due to products
poisoning, as it was observed for the Au/metal oxides catalysts.
39
TEM images of Pt(5wt%)/A70and Pt(5wt%)/A35 samples are shown in Fig. 3.10. In both
cases, the diameter of the metal nanoparticles diameter was around 30-40 nm. It is
reasonable to assume that the low gluconic acid production with Pt, Au and Ru supported
on A70 and A35 resins was due to large dimensions of the metal nanoparticles.
Figure 3.10. TEM images of Pt(5wt%)/A70 and of Pt(5wt%)/A35.
The A70 and A35 supported Pt, Au and Ru materials were prepared by reducing the metal
nanoparticles with gaseous hydrogen. In order to investigate, if different metal reduction
methods had any effect on the catalytic performance, the same materials were prepared,
but, instead of H2, they were reduced with a freshly prepared NaBH4 solution. As an
example, Fig. 3.11 shows the product distributions for the glucose oxidation performed
with Ru(5wt%)/A70 prepared with the H2 reduction method (Ru(5wt%)/A70-H2) and via
NaBH4 solution as reduction method (Ru(5wt%)/A70-NaBH4). The amounts of un-
converted glucose and of oxidation products detected in the reaction mixtures
corresponding to Ru(5wt%)/A70-H2 and to Ru(5wt%)/A70-NaBH4 were very similar. Only
40
the yield of fructose was higher in the case of Ru(5wt%)/A70-NaBH4. This suggested that
the type of reduction method used in the preparation of resin supported materials did not
have a significant influence on the catalytic performance.
0
20
40
60
80
in u te
8.6
Figure 3.11. Product distributions after 420 minutes for the glucose oxidation performed in
the batch reactor with Ru(5wt%)/A70-H2 and Ru(5wt%)/A70-NaBH4 (70°C, 3 bar O2, 1000
Different metal distribution would be expected to be generated by the two different
reduction protocols, as shown for Au nanoparticles by Calore et al. in 2012 [77]
. According
to their work, there are mainly two reasons: 1) the different nature of the reducing agent,
and 2) the difference in the expansion of the polymer matrix between the semi-dried resin
reduced by gaseous H2 and the fully swollen resin reduced by aqueous NaBH4 solution.
The penetration of small hydrogen molecules into the interior of the resin beads is allowed
by the eventual residual water content in partially dried resins. However, the collapsed
polymer structure inhibits the mobility of the metal ions, helping to maintain the
homogeneity of the metal nanocluster distribution. According to Calore et al. [77]
, the
presence of at least small residual water amounts is highly important for the reduction by
molecular hydrogen. In fact, metal redistribution during the reduction with gaseous H2
might probably effectively be blocked by the partial wetness of the polymer. When the
reduction is carried out with aqueous NaBH4 solution, the water-saturated environment of
the swollen polymer matrix promotes the fast penetration of Na + into the resin beads,
allowi

glucose oxidation into gluconic acid : from batch to

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