Base-free glucose oxidation using air with supported gold catalysts

Article abstract of DOI:10.1039/c4gc00087k

We report the selective oxidation of glucose to gluconic acid under mild conditions and show that if a basic support is used then the reaction can be carried out without the addition of sacrificial base or pH control. The use of sol-immobilisation prepared catalysts supported on magnesium oxide facilitates the use of ambient air as an oxidant source. These mild conditions resulted in an excellent selectivity towards gluconic acid. Different heat treatments result in an improvement in the activity of the catalyst, these improvements are discussed in terms of XRD, DRIFTD and TEM analysis of the catalysts, despite significant particle growth and phase segregation occurring during the thermal treatments. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00087k

Green Chemistry
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Base-free glucose oxidation using air with
supported gold catalysts
Peter J. Miedziak,a Hamed Alshammari,b Simon A. Kondrat,a Tomos J. Clarke,a
Thomas E. Davies,a Moataz Morad,a David J. Morgan,a David J. Willock,a
David W. Knight,a Stuart H. Taylora and Graham J. Hutchings*a
Cite this: Green Chem., 2014, 16,
3132
We report the selective oxidation of glucose to gluconic acid under mild conditions and show that if a
basic support is used then the reaction can be carried out without the addition of sacrificial base or pH
control. The use of sol-immobilisation prepared catalysts supported on magnesium oxide facilitates the
use of ambient air as an oxidant source. These mild conditions resulted in an excellent selectivity towards
gluconic acid. Different heat treatments result in an improvement in the activity of the catalyst, these
improvements are discussed in terms of XRD, DRIFTD and TEM analysis of the catalysts, despite significant
particle growth and phase segregation occurring during the thermal treatments.
Received 17th January 2014,
Accepted 8th April 2014
DOI: 10.1039/c4gc00087k
www.rsc.org/greenchem
use of bimetallic catalysts has also been reported for this oxi-
Introduction
dation Besson et al.10 who used palladium catalysts supported
The oxidation of glucose has been the focus of considerable on carbon promoted with bismuth. The selectivity and conver-
attention in recent years. Glucose is a bio-renewable feedstock sion were close to 100% when the reaction was carried out at
and its major product of its oxidation, gluconic acid, is used in pH 9 with bubbling air as the oxidant and the pH was main-
the pharmaceutical, food, paper and concrete industries. tained at a constant level throughout the reaction by addition
Typically to achieve high conversion of glucose a sacrificial of NaOH. Although the authors demonstrated five reuses of
base is added to the reaction and the pH of the solution is the catalyst they did not rule out deactivation as the reaction is
maintained by the continual addition of a base, typically run under mass transfer control. Gallezot11 also reported the
sodium hydroxide.1–3 Furthermore, pure oxygen is generally oxidation of bismuth palladium supported on carbon, noting
used as the oxidant,4–8 either at raised pressures or by bub- that 3–10 nm particles were more active than 1–2.5 nm par-
bling oxygen through the solution. The preparation of oxygen ticles and suggesting the bismuth prevents the over-oxidation
from air, together with its purification, requires considerable of the products.
energy which adds significantly to the cost of the process. The
Wenkin et al.12 carried out an extensive study into the
removal of the base from the process and its replacement with preparation of carbon-supported bismuth-promoted palladium
a catalytic equivalent, together with the utilisation of air as a catalysts using various bismuth precursors. Organic based
source of oxygen, would represent a significant advance in the acetate precursors were found to be the most active precursors,
process for glucose oxidation adhering to the principles of however, bismuth leaching into the reaction solution was
green chemistry.
detected for all the catalysts tested. Gold catalysts have also
Early reports, that have used heterogeneous catalysts for been reported for this oxidation, Biella et al.4 proposed the use
this oxidation, focused on the use of platinum on carbon as a of gold supported on carbon prepared by the sol-immobili-
catalyst. However, it was found that these catalysts suffered sation method for this oxidation, under various pH conditions,
from marked deactivation, which was reduced when the reac- the gold catalyst was found to be more active than platinum,
tion was carried out at high initial pH; but this deactivation palladium–bismuth or platinum–palladium–bismuth trimetal-
could not be eliminated.9 The deactivation of these catalysts lic catalysts. A pH of 9.5 was reported as the optimum value,
was addressed when the pH of the solution was readjusted to however, the gold was also reported to maintain activity at pH
pH 9 during the reaction by continuous addition of base. The values other than the optimised pH. Upon recycling the gold
catalyst did display a decrease in its activity, thought to be due
to metal leaching. In a further study13 “naked” gold nanoparti-
aCardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building,
cles were formed by the colloidal method and tested for the
Park Place, Cardiff, CF10 3AT, UK. E-mail: hutch@cardiff.ac.uk
bChemistry Department, Faculty of Science, Ha’il University, P. Box 2440, 81451
Ha’il, Saudi Arabia
oxidation of glucose, the particles of gold were found to
undergo growth during the reaction preventing the reaction
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from reaching completion but the initial turnover frequencies when compared to the gold or palladium monometallic
(TOFs) of the oxidation were extremely high demonstrating the equivalents,22 we have also shown that the sol immobilisation
potential of gold as a catalyst for this oxidation. Similar nano- method can give catalysts with a narrow particle size distri-
particles of copper, silver, palladium and platinum were tested bution that are extremely active for alcohol oxidations.23 Heat
but found to be inactive. Önal et al.14 studied different sol treatment of these sol-immobilised catalysts can remove the
preparation methods and different carbon supports at pH polyvinyl alcohol (PVA) ligand but simultaneously leads to
7–9.5 and found the best results were obtained at 50 °C and metal sintering causing deactivation of the catalyst during CO
pH 9.5 using Vulcan-type carbons, Comotti et al.7 also reported oxidation.24 Recently we have reported the use of magnesium
the oxidation with H2O2 and Prüße and co-workers published oxide as a support for gold–palladium and gold platinum for
a series of papers investigating the long term stability of gold the oxidation of glycerol, we have shown that during catalysts
catalysts on various supports.15–18 Gold on alumina was pre- preparation the magnesium is predominately hydrated to form
pared by the deposition precipitation and incipient wetness Mg(OH)2. The use of these catalysts facilitated the oxidation of
methods, the relatively low loaded (0.3% Au) catalysts were glycerol without the addition of sacrificial base and at lower
shown to be active for this reaction. Further work reported temperatures than those reported when other support
0.45% Au catalysts supported on titania16 prepared by depo- materials are used.25 We now show that by combining the use
sition precipitation and sol methodologies, gave reasonable of magnesium oxide/hydroxide support and the sol immobili-
stability of the catalysts demonstrated over seventeen reuses. sation preparation method we can carry out glucose oxidation
They further demonstrated the incipient wetness prepared under green, base-free conditions, using air at ambient
gold on alumina catalysts in a continuous flow reactor17 with pressure as the source of oxygen with excellent selectivity
high catalyst stability for 110 h. Different deposition precipi- towards gluconic acid. We have shown that various heat treat-
tation (DP) methods were studied for gold on alumina18 and ments on the support and catalyst have a significant effect of
urea was reported to be the best precipitating agent for this the activity of the catalyst and explain these differences using
preparation method.
XRD and TEM analysis.
An interesting comparison between a gold catalyst, sup-
ported on carbon and an enzymatic reaction was provided by
Rossi and co-workers,19 the work demonstrated that if the
activity of the gold catalysts was calculated based on theore-
tical surface gold atoms, the rate of reaction would be of the
same order of magnitude as the enzymatic reaction. Ishida
et al.20 used the solid grinding method to prepare gold cata-
lysts which they found gave higher activity than the DP catalyst
preparation when supported on alumina or zirconia.
Bimetallic catalysts comprising combinations of gold, plati-
num, palladium and rhodium were studied by Comotti et al.8
and the gold platinum catalysts were the most active and with
the exception of gold rhodium the alloying of the metals
improved the activity. The fine-tuning of the ratio of gold to
platinum led to an optimised ratio of Au : Pt of 2 : 1 (molar)
and with this catalyst at pH 9.5 a TOF of 17 600 h−1 was
observed.
Experimental
Catalyst preparation
All catalysts were prepared after pre-treatment of the support.
The MgO (nanoscale corporation) was calcined at 400 °C for
5 h under static air (5 °C min−1). Au–Pd bimetallic catalysts
were prepared using the following sol-immobilisation method:
aqueous solutions of PdCl2 (Johnson Matthey, 6 mg in 1 ml)
and HAuCl4·3H2O (Johnson Matthey, 12.25 g in 1000 ml) were
freshly prepared. Polyvinyl alcohol (PVA) (1 wt% solution,
Aldrich, Mw = 10 000, 80% hydrolyzed) was added to the solu-
tion and stirred with HAuCl4·3H2O for 15 min. 0.1 M of NaBH4
(>96% Aldrich, NaBH4/Au (mol/mol = 5)) was freshly prepared
and then added to form a dark brown sol. The mixture was
stirred for 30 min and adjusted to pH 1 by the drop-wise
addition of sulphuric acid. The pre-treated MgO was added to
mixture and the slurry was then stirred for 1 h. Following this
the catalyst was recovered by filtration, washed with 2 L dis-
tilled water and dried at 110 °C overnight. The 0.5% Au–Pd/
Mg(OH)2 catalyst, denoted with the subscript z (zero treat-
ments) was calcined at 450 °C for 5 h under air (20 °C min−1),
denoted with the subscript a (air calcination) or treated at
450 °C for 5 h under N2 (20 °C min−1), denoted with the sub-
script n (nitrogen heat treatment) or subjected to both heat
treatments sequentially, denoted with the subscript b (both
treatments).
Karski et al.21 used thallium alloyed with palladium on
silica catalysts and there was an observable increase in the
selectivity when the thallium was alloyed. However, high
amounts of thallium were required to observe a significant
effect and there was significant leaching of the toxic thallium
which would be prohibitive for industrial applications.
Witońska et al.6 investigated palladium tellurium on silica cata-
lysts and found that tellurium could also be used as a promo-
ter for this reaction, again however deactivation of the catalysts
was reported after ten reuses of the catalyst. Recently Zhang
and co-workers have prepared homogeneous colloidal nano-
particles of gold–silver2 and gold–platinum–silver catalysts,
once again these catalysts have demonstrated the extremely
Glucose oxidation
high TOF’s with 20 090 h−1
.
Oxidation of glucose using air was carried out in a glass
We have previously shown that alloying gold and palladium reactor consisting of a round-bottomed flask (50 ml) fitted
can lead to a significant increase in the activity of the catalyst with a reflux condenser. Typically, a supported gold catalyst
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(0.06 g) was suspended in the aqueous glucose solution (5 ml,
0.5 M glucose) at 50 °C. The reaction mixture was stirred for
24 h at atmospheric pressure. A sample was taken from the
final reaction mixture (0.5 ml), diluted with 1 ml of distilled
water and analysed by HPLC using an Agilent 1260 infinity
fitted UV and RI detectors and a metacarb 87H column. Mass
balances were based on calibration against known standards
(Aldrich). In all experiments the mass balance was observed to
be between 96–104%.
Catalyst reuse
To reuse the catalyst a reaction was performed under standard
conditions then the catalyst was collected by centrifuging the
reaction mixture for 20 min. Acetone (30 ml) was added to the
catalyst to remove any residues of reactants and products from
the catalyst. The catalyst was recovered by decantation and
drying (120 °C, 16 h).
Fig. 1 The conversion of glucose using Au–Pd/MgO(b): reaction con-
ditions: glucose (5 ml, 0.5 M), catalyst (0.06 g), 24 h at atmospheric
pressure, selectivity 100% except at 70 °C which was 98%.
Catalyst characterisation
Elemental analysis. Microwave Plasma Atomic Emission
Spectroscopy (MP-AES) was performed using a Agilent 4100 Table 1 The oxidation of glucose under blank conditionsa
MP-AES. The Au/Pd content was analysed with two emission
Catalyst
Conversion (%)
Selectivity (%)
lines for each metal, 242 nm and 267 nm for Au with 340 and
363 nm used for Pd. The samples were introduced to the nitro-
gen plasma using a single pass spray chamber at a pressure of
120 kPa without air injection. The instrument was calibrated
with 0.5 ppm, 1 ppm, 2 ppm, 6 ppm and 10 ppm Au/Pd stan-
dards in 10% aqua regia along with a 10% aqua regia blank. To
test the reaction liquor, a sample of the reaction mixture
(0.5 ml) was made up to 5 ml with 10% aqua regia. The
samples were tested 3 times and the average result was used. A
10% aqua regia rinse solution was introduced between
samples for 15 s to ensure there was no sample contamination.
Magnesium leaching into the reaction media was examined
using the Exeter analytical service by inductively coupled
plasma mass spectrometry.
Powder X-ray diffraction. Characterisation was performed
using powder X-ray diffraction (XRD) on a (θ–θ) PANalytical
X’pert Pro powder diffractometer using a Cu Kα radiation
source operating at 40 keV and 40 mA. Analysis was run using
a step size of 0.017° 2θ and a scan range of 15–85° 2θ. Refer-
ence crystal structure profiles were selected from the inorganic
crystal structure database.
No catalyst
Mg(OH)2
0.5% Au/Mg(OH)2(o)
0
1.3
7.3
0
100
100
a Reaction conditions: glucose (5 ml, 0.5 M), catalyst (0.06 g), 60 °C,
24 h at atmospheric pressure.
oxygen from atmospheric air as the oxidant gas. The results
are shown in Fig. 1, as expected increasing the temperature
leads to a steady increase in conversion. The selectivity
towards gluconic acid at the lower temperatures (40–60 °C) was
100% based on products formed with carbon mass balance
between 96–104% while at the higher temperature (70 °C) the
selectivity decreased below 100% (98%). Subsequently, we
carried out a blank reaction test to be sure that the catalyst was
having a beneficial effect on the reaction and the results are
shown in Table 1. When no catalyst is present there was no
conversion of the glucose, the addition of the support leads to
a very small amount of conversion (1.3%) and the selectivity
towards gluconic acid was 100% without the metals present.
The addition of the gold to the support and subsequent calci-
nation however led to a dramatic increase in the conversion of
the glucose to 7.3% whilst still maintaining the 100% selecti-
vity to gluconic acid. The use of a bimetallic Au–Pd/Mg(OH)2(z)
increased the conversion from 2% to 27% (Table 3) with the
same selectivity, suggesting a synergistic effect of combining
the metals, an effect we have observed and reported previously
which we consider to be due to an electronic effect.26
Transmission electron microscopy. Transmission electron
microscopy (TEM) was carried out using a Jeol 2100 with a
LaB6 filament operating at 200 kV. Samples were prepared by
dispersing the powder catalyst in ethanol and dropping the
suspension onto a lacey carbon film over a 300 mesh copper
grid.
Results and discussion
The effect of reaction temperature
The effect of support
Initial experiments were carried out over a range of tempera- Previously we have shown that titania is an effective support
tures (40–70 °C) using Au–Pd/MgO(b) (0.06 g), for 24 h with for gold–palladium catalysts27 and that during glycerol oxi-
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effect of heat treatment on sol catalysts,24 we have shown that
calcinations remove the PVA ligand from the metal particles
however there is also significant particle growth with heat
treatment. In a previous detailed study for a titania-supported
gold catalyst24 we showed that the starting colloid has a mean
particle size diameter of 3 nm, the 120 °C drying step increases
this to 3.7 nm, however, calcination at 200 °C for 3 h signifi-
cantly increases the particle size to 5.1 nm, and treatment at
400 °C led to much more significant particle growth with an
average particle size of 10.4 nm, there was also a significant
Table 2 The oxidation of glucose by gold on various supportsa
Catalyst
Conversion (%)
Selectivity (%)
0.5% Au/graphite(z)
0.5% Au/TiO2(z)
0.5% Au/Mg(OH)2(z)
0
0.8
2
0
100
100
a Reaction conditions: glucose (5 ml, 0.5 M), catalyst (0.06 g), 60 °C,
24 h at atmospheric pressure.
dation, changing the support from graphite to magnesium change in the particle size distribution from a positively skew
oxide can lead to a significant increase in the activity in the when the catalyst was dried at 120 °C to a more standard bell-
absence of base25 we therefore prepared gold catalysts sup- shaped normal distribution after the 400 °C treatment.
ported on magnesium oxide, titania and graphite. The activity
Catalyst characterisation
of these supported catalysts is shown in Table 2, the reactions
were carried out at 60 °C as this is the highest temperature
where 100% selectivity to gluconic acid was maintained.
Under these conditions the 0.5% Au/graphite(z) catalyst
showed no activity for this reaction, the titania-supported cata-
lyst displayed some conversion (0.8%) and the magnesium
oxide-supported catalyst had over double the conversion,
however this was still significantly lower than the catalytic data
reported in Table 1 and within the realms of experimental
error. The final catalyst reported in Table 1 had undergone two
heat treatments (as detailed in the Experimental section) and
it is clear that catalyst pre-treatment can significantly affect the
activity.
To try to understand if similar effects were occurring with the
present catalyst characterisation by XRD and TEM was carried
out. XRD analysis (Fig. 2) of the catalyst prepared with no heat
treatment showed only the presence of crystalline Mg(OH)2
(ICDD 00-007-0239) and no reflections associated with the
original MgO support. This hydration of the MgO support
occurs during the catalyst synthesis and has previously been
reported in the preparation of sol-immobilised catalysts used
for glycerol oxidation.25 No reflections associated with the
cubic Au or Pd phases were observed, though this could be
associated with an overlap of the [101] Mg(OH)2 and [111]
cubic metal reflections. Catalyst treatment under air resulted
in dehydration of Mg(OH)2 back to MgO (ICDD 01-074-1225),
with an average crystallite size of 6 nm (determined from [200]
reflection). A reflection at 38.61° 2θ was observed which could
be associated with the [101] Mg(OH)2 plane. However, no other
reflections associated with Mg(OH)2 were observed, of particu-
lar note is the absence of the [011] reflection at 39.31° 2θ with
The effect of heat treatment
To elucidate the reasons for the significant improvement in
activity observed for the catalysts in Table 1 as compared with
Table 2, we carried out catalytic tests after each heat treatment
for the bimetallic catalysts and the results are shown in
Table 3. When no heat treatment is applied to the catalyst, the
conversion of glucose was 27%, calcination of the catalyst at
400 °C under static air led to almost a doubling of the activity
to 50%, heat treatment under an inert atmosphere (nitrogen)
led to a similar activity (53%), however, when the two heat
treatments are carried out sequentially the conversion is
improved further to 62%, again at this temperature, under
these conditions the selectivity towards gluconic acid is 100%
with all the catalysts tested. We have previously reported the
Table 3 The effect of catalyst heat treatmenta
Catalyst heat
treatment
Under
air
Under
N2
Conversion
(%)
Selectivity
(%)
Catalyst
0.5% Au–Pd/Mg(OH)2(z)
0.5% Au–Pd/MgO2(a)
0.5% Au–Pd/MgO2(n)
0.5% Au–Pd/MgO2(b)
X
√
X
X
X
√
√
27
50
53
62
100
100
100
100
Fig. 2 XRD patterns of the 0.5% Au–Pd/Mg(OH)2 catalyst after various
heat treatments: (a) dried, (b) dried with calcination at 450 °C, (c) dried
with heat treatment under nitrogen and (d) dried with both heat treat-
√
a Reaction conditions: glucose (5 ml, 0.5 M), catalyst (0.06 g), 60 °C, ments. Expanded region shows the position of [111] cubic metal phase
24 h at atmospheric pressure.
reflection.
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a reported relative intensity of 57.6%. This along with the 0.6°
shift in reflection position suggests that the observed reflec-
tion is more likely to be associated with a cubic metal [111]
reflection. The position of this reflection lies between the
reflections expected for pure Au (ICDD 00-004-0784) (38.19°
2θ) and pure Pd (ICDD 00-046-1043) (40.12° 2θ) and shows a
degree of alloy formation, though the reflection position is
lower than that expected for a bulk homogeneous alloy at
39.17° 2θ (ICDD 01-072-5376) suggesting the particles observa-
ble by XRD were Au rich. The calculated average crystallite size
of the phase was relatively large at 22 nm.
Treatment of the catalyst under nitrogen also resulted in
the dehydration of the Mg(OH)2 to MgO with a crystallite size
of 7 nm (Fig. 2). The presence of a broad poorly defined reflec-
tion, likely to be associated with a metal phase, was again
observed, at 32.29° 2θ. The reflection position is again sugges-
tive of an Au–Pd alloy, though with a unit cell size closer to Pd
than that expected for a bulk homogeneous alloy. From the
Scherrer equation this phase had an average crystallite size of
10 nm. Interestingly, the sequential heat treatments results in
the formation of a metal phase with similar crystallite size to
the nitrogen treatment (9 nm) but an apparent unit cell size
that is now closer to Au than after the nitrogen treatment but
still smaller than the calcined sample. The apparent trend we
observe is that calcination, i.e. heating in air, resulted in the
presence of an Au–Pd alloy that is possibly Au rich, whereas
nitrogen heat treatment a phase that was Pd rich and the com-
bined treatments in a phase that lies between the two single
treatments.
TEM images of the unused catalyst are shown in Fig. 4 and
associated particle size distributions, based on analysis of at
least 200 particles, are shown in Fig. 5. In agreement with pre-
vious reports on 1% Au–Pd/MgO for glycerol oxidation25 the
sample that is dried only has a narrow size distribution of
small nanoparticles (1–10 nm). As we have previously reported
for titania-supported catalysts24 the calcination treatment
(Fig. 3b and 4b) leads to a growth in the mean particle size,
although on magnesium hydroxide this is less evident than
when titania is used as the support,24 even at the slightly
higher calcination temperature of 450 °C, furthermore the par-
ticle size distribution remained positively skewed. The heat
treatment under nitrogen at 500 °C also led to a growth in the
average particle size although the effect was not as significant
as when the calcination regime was applied. Fig. 3d and 4d
show the TEM image and associated particle size distribution
after both heat treatments have been applied to the catalyst, it
is noteworthy that the double heat treated catalyst particle size
distribution is most similar in nature to the to the nitrogen
heat treated catalyst. The catalytic activity of both the nitrogen
heat treated catalyst and the calcined catalyst are very similar
suggesting that the particle size is not the single factor that is
affecting the activity, we do not know what effect the removal
of PVA ligand from the metal particles is having in this case,
however it seems apparent that the smallest metal particles are
Fig. 3 TEM images of the 0.5% Au–Pd/Mg(OH)2 catalyst after various
heat treatments: (a) dried, (b) dried with calcination at 450 °C, (c) dried
with heat treatment under nitrogen and (d) dried with both heat
treatments.
Non-alloyed catalysts
When the TEM data is considered in combination with the
XRD data is seems to suggest that a fully homogeneous alloy
catalyst is not the most active for this reaction. In fact a catalyst
which contains mostly gold or palladium with a small amount
of the other metal alloyed appears to be more active for this
reaction once a high temperature heat treatment has taken
place. To test this theory monometallic gold and palladium
catalysts were synthesised and subjected to the same heat
treatment regime as the bimetallic catalyst, the results are
shown in Table 4, it is clear that the heat treatments lead to a
significant increase in the catalytic activity of both monometal-
lic catalysts (57% for Au and 52.7% for Pd). The value for the
gold catalysts can be compared to the 7.3% conversion
reported in Table 1 where one heat treatment had been carried
out on an equivalent catalyst. The gold monometallic catalyst
was slightly more active than the palladium and under the
mild conditions employed in this work the selectivity towards
gluconic acid remained at 100% with both catalysts. Both
monometallic catalysts are slightly less active than the bi-
metallic catalyst supporting the theory that a small amount of
alloying is required for maximum activity. For comparison a
reaction was carried out with the addition of sodium hydrox-
ide, the result is also shown in Table 4, although the conver-
sion is significantly higher in the presence of base the
selectivity to gluconic acid was dropped below 100%.
Catalyst deactivation studies
not the most active for this reaction as has been previously To investigate the reactivity of the catalysts for glucose oxi-
suggested by Besson et al.10
dation the effect of reaction time on the conversion and
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Fig. 4 TEM particle size distributions of the 0.5% Au–Pd/Mg(OH)2 catalyst after various heat treatments: (a) dried, (b) dried with calcination at
450 °C, (c) dried with heat treatment under nitrogen and (d) dried with both heat treatments.
Table 4 Mono and bimetallic catalysts for the oxidation of glucosea
Catalyst
Conversion (%)
Selectivity
0.5% Au/MgO(b)
0.5% Au–Pd/MgO(b)
0.5% Pd/MgO(b)
57
100
100
100
94.8
62.0
52.7
89.0
b
0.5% Au/MgO(b)
a Reaction conditions: glucose (5 ml, 0.5 M), catalyst (0.06 g),
60 °C, 24 h at atmospheric pressure. b Reaction carried out in 0.3 M
NaOH solution.
selectivity was studied with the optimised Au–Pd/MgO(b) cata-
lyst at 50 °C as this temperature has been reported previously
in several publications,9,11,14 the results are shown in Fig. 6, it
is clear that while the catalyst has a high initial activity the rate
of reaction decreases with time and between 24 and 48 h there
is only a very small increase in the conversion. As the conver-
sion is still significantly less than 100% at the end of the reac-
tion it is possible that there is catalyst deactivation occurring.
Therefore we carried out reuse tests on the catalysts to see if
Fig. 5 Time on line data for the oxidation of glucose using the 0.5%
Au–Pd/MgO catalyst. Reaction condition: glucose (5 ml, 0.5 M), catalyst
(0.06 g), 50 °C at atmospheric pressure.
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Fig. 6 Characterisation of AuPd/MgO catalyst after use in the base free glucose oxidation reaction at 50 °C. Upper figure shows XRD diffraction
□
pattern of catalyst with indicating reflections associated with Mg(OH)2. Lower figures show a representative TEM image of metal nanoparticles on
the used catalyst (left) and the corresponding particle size distribution.
Table 5 The effect catalyst reusea
Table 6 The addition of gluconic acid to the oxidation of glucosea
Conversion
(%)
Selectivity
(%)
Gluconic
acid
Conversion
(%)
Selectivity
(%)
Catalyst
Catalyst
Fresh
First reuse
First reuse with treatment of catalyst
Second reuse
Second reuse with treatment of catalyst
62.0
24.4
33.7
18.7
23.2
100
100
100
100
100
0.5% Au/MgO(b)
0.5% Au/MgO(b)
—
0.1 g
62.0
41.3
100
100
a Reaction conditions: glucose (5 ml, 0.5 M), catalyst (0.06 g), 60 °C,
24 h at atmospheric pressure.
a Reaction conditions: glucose (5 ml, 0.5 M), of 0.5% Au–Pd/MgO(b)
(0.06 g), 60 °C, 24 h at atmospheric pressure.
described previously. The results of this reaction are also
shown in Table 5. The conversion of the reaction after 24 h is
about 33% lower than under the standard reaction conditions
(41.3% (Table 5) as compared with 62% (Table 3)). With this
deactivation apparently caused by the product we heat treated
the used catalyst, the heat treatment was carried out at 450 °C
after the catalyst had been recovered by the method described
in the experimental in the catalyst reuse section. As shown in
Table 5, the catalyst heat treatment did lead to an increase in
conversion when compared with the untreated reused catalyst
but it did not restore the activity to that of the fresh catalyst
(Table 6).
there was deactivation of the catalyst between runs. The results
of these tests are shown in Table 5. The activity of the catalyst
roughly halved upon reuse therefore we carried out a product
inhibition test. As there is only one product in this reaction,
gluconic acid, this was added to glucose at the start of the reac-
tion. Gluconic acid (0.1 g, 5.1 × 10−4 mol, glucose : gluconic
acid ∼4.9) was added (representing the concentration that
would be present at ∼20% conversion) to the standard reaction
but otherwise the reaction was carried out as has been
3138 | Green Chem., 2014, 16, 3132–3141
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MgO by XRD, had a hydrated surface. This surface magnesium
hydroxide could potentially stabilise the AuPd nanoparticles
when the bulk MgO support hydrates to Mg(OH)2 under reac-
tion conditions.
DRIFTS of the used catalyst did also show distinct bands,
relative to the initial catalyst, that correspond to alcohol
groups, C–H and CO bands. Heating of the used catalyst in situ
under vacuum up to 450 °C resulted in only a reduction in the
broad OH band at 3400 cm−1, associated with physisorbed
water. This showed that the adsorbed organic species were
exceptionally strongly bound to the catalyst surface and sup-
ports the idea that product inhibition is the primary cause of
the observed deactivation. Identification of the adsorbate from
band assignments of the used and used heat treated samples
is complicated due to the range of structural isomers of
glucose and gluconic acid. However, the presence of the band
at 1653 cm−1 in the used and heated samples is suggestive of a
carbonyl group. The band position is at the extreme low end of
the range observed for carbonyls and is in a similar range to
the expected bending mode of physisorbed water. Yet its reten-
tion at 450 °C, strongly suggests that the band in question is
not associated with physisorbed water. The ν(OH) band
remaining in the heated samples, at 3295 cm−1, can be identi-
Fig. 7 DRIFTS analysis of AuPd/MgO catalyst (a) before reaction, (b)
after base free glucose oxidation reaction at 50 °C, (c) in situ spectra of
post reaction sample heated to 115 °C and (d) in situ spectra of post
reaction sample heated to 440 °C. In situ spectra were recorded on
samples at designated temperature under vacuum. Arrows indicate
bands associated with adsorbed organic species.
Characterisation by DRIFTS, XRD and TEM microscopy of fied as being associated with a carboxylic acid group. It is
the Au–Pd/MgO(b) catalyst after a 24 h reaction at 50 °C was therefore probable that the adsorbed species was gluconic
performed to further investigate catalyst deactivation. XRD acid. Clearly product inhibition represents a key area that
analysis (Fig. 6) revealed that the MgO support completely con- needs to be addressed in future studies. Perhaps the use of
verted to Mg(OH)2 under the aqueous reaction conditions, polar solvents could be beneficial but this negates the green
which could be expected to result in significant agglomeration chemistry approach of using a solvent-free reaction system
of the supported nanoparticles. TEM analysis (Fig. 6) did show however, our observations do show that there may be limit-
a modest increase in particle size from a mean of 5.8 nm to ations to the application of aqueous catalysed reactions.
6.5 nm. This small increase in particle size could impact on
In addition to the previously mentioned post reaction
activity, although it being a significant factor is highly improb- characterisation, MP-AES analysis was carried out on the reac-
able. What is surprising is the minimal change in metal par- tion mixture after the 24 h reaction had been completed, to
ticle size on restructuring of the support. Interestingly DRIFTS determine if metal leaching was occurring during the reaction.
of the initial and used catalysts shows, not only the presence Analysis of both gold and palladium revealed that neither
of adsorbed organic species, but also the “surface” structure of metal was present in the post reaction sample within the
the catalyst itself (Fig. 7 and Table 7). The spectra of both the detection limits of the equipment (standards made between
initial and used catalysts show strong hydroxyl groups associ- 0.6 and 10 ppm). However, we observed that Mg was present
ated with magnesium hydroxide, demonstrating that even the (2890 ppm), showing that the Mg(OH)2 support was partially
initial Au–Pd/MgO(b) catalyst, shown to be comprised of bulk soluble in the reaction medium. Previous use of MgO as a
Table 7 IR bands associated with AuPd/MgO(b) before and after glucose oxidation reaction
Mg(OH)2
MgCO3
Fresh catalyst
Used catalyst
Used at 115 °C
Used at 450 °C
Band assignment
3698
—
—
—
—
3763, 3714, 3701, 3620
3361
—
3699, 3646
3400
2930, 2870, 2845, 2740
3699, 3646
3360
2930, 2870, 2845, 2740
3697, 3646
3295
2930, 2870, 2845, 2740
ν(Mg–OH)
ν(OH)
ν(CH)
—
—
1647
—
1454
—
1635
—
1396, 1360
1232
1130
δ(H2O)
ν(CO)
ν3(CO3)
δ(CH)–(OCH)
δ(CH)
1653
1653
—
1450
1405, 1336
1230
1126
1420, 1330
1126
ν(CO)
—
—
1080
870
1070
830
ν1(CO3)
ν4(CO3)
787
784
785
625
—
660
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Green Chemistry
solid base for base free glycerol oxidation resulted in 77 ppm acquired at the Research Complex at Harwell (UK Catalysis
of Mg2+ being leached into solution, though the reaction time Hub) by Catherine Brooks and Wilm Jones.
was far shorter at 4 h. In view of the extent of Mg2+ that is
leached in the current experiments we have to consider the
effect of the homogeneous Mg(OH)2 acting as a sacrificial
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the reaction, and a ratio of 0.16 : 1 at 70% conversion. This
suggests that while the catalyst itself is unstable, the dissolved
Mg(OH)2 does not provide significant amounts of sacrificial
homogeneous base to influence the reaction. To test this we
carried out a reaction with a stable catalyst which we have pre-
viously reported,22 with and without the addition of sodium
hydroxide at the same ratio (i.e. [OH−] : glucose = 0.048 : 1).
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Though we can determine the efficacy of the work in demon-
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under mild conditions, the instability of the catalyst support
shows that MgO is clearly not fully viable for this application.
Further work on other more stable basic supports is now
required.
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