Glucose Oxidation into Gluconic Acid : from Batch to
171
Glucose Oxidation into Gluconic Acid: From 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 Born on 13.12.1988 in Verona, Italy Bochum December 2016
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
Introduction Gluconic Acid
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.
Introduction Gluconic Acid
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
Introduction State of the Art
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
Introduction State of the Art
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
Introduction State of the Art
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
Introduction State of the Art
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
Introduction State of the Art
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
Nanoparticles Supports
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.
Results and Discussion Batch Reactor - Metal Oxides as Metal
Nanoparticles Supports
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.
Nanoparticles Supports
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
Results and Discussion Batch Reactor - Metal Oxides as Metal
Nanoparticles Supports
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
Results and Discussion Batch Reactor - Metal Oxides as Metal
Nanoparticles Supports
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
Results and Discussion Batch Reactor - Metal Oxides as Metal
Nanoparticles Supports
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).
Results and Discussion Batch Reactor - Metal Oxides as Metal
Nanoparticles Supports
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).
Results and Discussion Batch Reactor - Metal Oxides as Metal
Nanoparticles Supports
+ 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
Results and Discussion Batch Reactor - Metal Oxides as Metal
Nanoparticles Supports
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
Nanoparticles Supports
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.
Results and Discussion Batch Reactor - Resins as Metal
Nanoparticles Supports
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
rpm stirring, 5wt% glucose alkaline solution (pH 13.5)). 1 run for
each test.
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.
Results and Discussion Batch Reactor - Resins as Metal
Nanoparticles Supports
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.
Results and Discussion Batch Reactor - Resins as Metal
Nanoparticles Supports
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
Results and Discussion Batch Reactor - Resins as Metal
Nanoparticles Supports
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
rpm stirring, 5wt% glucose alkaline solution (pH 13.5)). 1 run for
each test.
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
Results and Discussion Batch Reactor - Resins as Metal
Nanoparticles Supports
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