Pt catalysts for efficient aerobic oxidation of glucose to glucaric acid in water

Article abstract of DOI:10.1039/c6gc00460a

Glucaric acid, a promising bio-based chemical for the production of adipic acid, is produced on a commercial Pt/C catalyst via aerobic oxidation of glucose in water. Facile oxidation of gluconic acid, an intermediate oxidation product of glucose, under optimal conditions of an initial pH of 7.2, 80 °C, 13.8 bar O₂ and a glucose/Pt molar ratio of 54, enabled a maximum glucaric acid yield of 74%, the highest yield reported to date. It is found that in an acidic solution, gluconic acid is the major product while in a highly basic solution, selectivity to glucaric acid is poor due to its C-C bond cleavage to low carbon chain carboxylic acids. High temperatures and high Pt loadings result in lower selectivity to glucaric acid. Recyclability and characterization studies reveal that the catalyst is stable after five cycles with no sign of Pt leaching into the solution.

Full text of DOI:10.1039/c6gc00460a

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Pt Catalysts for Efficient Aerobic Oxidation of Glucose to Glucaric Acid in Water
Jechan Leea, Basudeb Sahaa*, and Dionisios G. Vlachosa,b*
Received 00th January 20xx,
Accepted 00th January 20xx
Glucaric acid, a promising bio-based chemical for the production of adipic acid, is produced on a commercial Pt/C catalyst
via aerobic oxidation of glucose in water. Facile oxidation of gluconic acid, an intermediate oxidation product of glucose, at
optimal conditions of initial pH of 7.2, 80 °C, 13.8 bar O2 and a glucose/Pt molar ratio of 54, enabled a maximum glucaric
acid yield of 74%, the highest yield reported to date. It is found that in an acidic solution, gluconic acid is the major product
while in a highly basic solution, selectivity to glucaric acid is poor due to its C–C bond cleavage to low carbon chain
carboxylic acids. High temperatures and high Pt loadings result in lower selectivity to glucaric acid. Recyclability and
characterization studies reveal that the catalyst is stable after five cycles with no sign of Pt leaching into the solution.
DOI: 10.1039/x0xx00000x
www.rsc.org/
economics depend on all steps and the complexity associated with
separation and purification of the desired products in each step.
Introduction
Non-food biomass is an appealing and sustainable starting material
for renewable liquid fuels and chemicals that can minimize our
dependence on petroleum and mitigate concerns on greenhouse
gas emissions. One such chemical is adipic acid (AA), which is used
for the production of nylon 6,6, polyurethanes and adipic esters. AA
is currently produced in large scale (>2.3 million tons per year) from
petroleum-based KA oil, a mixture of cyclohexanol and cyclohexa-
none.1-3 The current process employs corrosive nitric acid for the
oxidation of KA oil, which in turn generates nitrous oxide waste, a
greenhouse gas accounting for approximately 5-8% of the global
N2O emissions.4, 5 Therefore, current focus is on the development of
sustainable manufacturing processes for AA, preferably utilizing
abundant non-food biomass in an environmentally benign way.6
Among the reported pathways for upgrading biomass
intermediates to AA, the catalytic upgrading of 5-hydroxymethyl-
furfural (HMF), a renewable platform chemical obtained from acid
catalyzed isomerization and dehydration of glucose,7-14 has mainly
been reported.2, 3 Two routes for catalytic transformation of HMF to
AA entail (1) the hydrogenation of HMF to ring-saturated 2,5-
bishydroxymethyltetrahydrofuran (BHMTHF) followed by its ring-
opening to 1,6-hexanediol (1,6-HD) and subsequent oxidation of
1,6-HD to AA,15, 16 and (2) the aerobic oxidation of HMF to 2,5-
HOOC
O
O
HDO
RO
OH
COOH
OH OH
HO
[O]
HOH2
C
O
O
O
AA
CHO
HO
FDCA
HOH2
OH OH
HMF
Glucose
C
RO
O
OH
CH2OH
[H]
HO
HDO
1,6-HD
BHMTHF
Scheme 1. Upgrading 5-hydroxymethylfurfural (HMF) to adipic acid (AA).
HDO and RO stand for hydrodeoxygenation and ring-opening. [O] and [H]
stand for oxidation and hydrogenation, respectively.
Another approach for upgrading glucose to AA invokes the
catalytic oxidation of glucose to glucaric acid followed by its
conversion to AA (Scheme 2). The U.S. Department of Energy has
identified glucaric acid as a versatile, renewable platform-chemical
for nylons, plastics and food additives.23,
Electrochemical or
24
stoichiometric oxidation of glucose to glucaric acid with NaBr,
NaOCl, mineral acids and bleaches, have been reported.25-27 These
reactions generate significant amount of toxic by-products, thus
making the process less sustainable and environmentally benign.
The homogeneous catalyzed oxidation of glucose with HNO3,
nitroxide, and NaNO2 has also been reported26, 28-31 in the range of 5
to 80 °C, yielding <50% glucaric acid. Besides usage of corrosive and
hazardous reagents, separation and recyclability of homogeneous
furandicarboxylic acid (FDCA) followed by hydrogenation and ring-
17-22
opening of FDCA (Scheme 1).10,
Both processes rely on
catalysts from the products is
a challenge. Electrochemical
upstream steps, namely (1) depolymerization and hydrolysis of
biomass to glucose and (2) isomerization and dehydration of
glucose to HMF. Thus, the yield of AA and overall process
oxidation of glucose with nitroxide has also been reported.27, 32, 33
a.Dr. Jechan Lee, Dr. Basudeb Saha, Prof. Dionisios G. Vlachos
Catalysis Center for Energy Innovation, University of Delaware, Newark,
DE 19716, USA.
Scheme 2. Schematic illustration of
a two-step process for the
b.Prof. Dionisios G. Vlachos
production of adipic acid (AA) from glucose
.
Department of Chemical and Biomolecular Engineering
University of Delaware, Newark, DE 19716, USA.
Corresponding authors: B. Saha; bsaha@udel.edu and D. G. Vlachos;
vlachos@udel.edu
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [Scheme S1, Tables S1 to S3
and Figures S1 to S3]. See DOI: 10.1039/x0xx00000x
Most studies on aerobic oxidation of glucose using
heterogeneous catalyzed oxidation have reported gluconic acid as
the major product.34-46 For example, Miedziak et al. reported 84%
gluconic acid yield in the Au/MgO catalyzed aerobic oxidation of
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glucose in basic medium at 60 °C.40 In
a
subsequent
communication, the authors investigated glucose oxidation with Catalyst characterization
Au/TiO2 catalyst under base-free conditions.45 They found
Pulse CO chemisorption was performed on an Altamira AM-200ip
a
significant effect of the catalyst preparation methods and instrument. Before CO chemisorption, the sample was treated with
calcination temperatures on catalyst activity. Au/TiO2 catalyst, He at 120 °C and then was cooled to 30 °C. Pulses of CO in a He
prepared by sol-immobilization method, demonstrated the highest carrier gas were injected at 30 °C. The signal was monitored by a
activity in glucose conversion (71%) and gluconic acid yield (67%) thermal conductivity detector (TCD). TOF values for all catalysts
for 1 h at 160 °C and 3 bar O2. Witońska et al. prepared a series of were calculated based on 20% conversion of glucose and gluconic
bimetallic catalysts by impregnating different amounts of Pd and Te acid at 80 °C and 6.2 bar O2, and the numbers of active Pt sites in
on SiO2 and Al2O3 supports for glucose oxidation with O2.36 Among each catalyst measured from CO chemisorption. Surface areas were
catalysts, Pd(5%)-Te(1%)/SiO2 was the most effective, producing calculated from N2 physisorption data, measured at –196°C on a
gluconic acid as the only product with nearly 100% yield in basic Micromeritics 3Flex Surface Characterization Analyzer, using the
medium (pH ~9) at 60 °C for 2 h. Recently, H2O2 has been used as an Brunauer-Emmett-Teller (BET) equation. Barrett-Joyner-Halenda
oxidant for glucose oxidation under base-free conditions using an (BJH) desorption was used to estimate pore volumes and pore sizes.
Au/Al2O3 catalyst, giving 76% gluconic acid in 10 min under Before N2 physisorption, the samples were degassed under vacuum
microwave irradiation at 120 °C.46 Base-free aerobic oxidations of at 80 °C overnight.
glucose over Au/CeO2, Au/ZrO2, Au/TiO2 catalysts have also been
reported by Wang et al. The authors achieved a maximum 90% Oxidation reaction
gluconic acid yield for 2 h with Au/TiO2 at 65 °C and 2.3 bar O2 but Batch reactions for aerobic oxidation of glucose and gluconic acid
the gluconic acid yield decreased to 80% when the initial pH was were carried out in a 100 mL Parr reactor (model number 4793),
adjusted to 1.6.39 Reports on direct oxidation of glucose to glucaric equipped with
a thermocouple for measuring the actual
acid using heterogeneous catalysts are limited because the temperature of the reaction. In typical experiments, the reactor was
subsequent oxidation of gluconic acid to glucaric acid is slow and loaded with 20 mL of an aqueous solution of glucose containing 2 g
difficult.6, 47, 48
(11.1 mmol) glucose and 0.8 g catalyst. For gluconic acid oxidation,
Rennovia Inc.’s patent reported direct oxidation of glucose to the reactor was loaded with 20 mL of an aqueous solution of
glucaric acid, with yields up to 70% for 5 h, using supported mono- gluconic acid containing 2 g (10.2 mmol) gluconic acid and 0.8 g
and bimetallic Pt and Au catalysts on TiO2, ZrO2, and SiO2 in the catalyst. Subsequently, the reactor was sealed, pressurized with
temperature range of 90-112 °C and at high pressure of O2 (27 oxygen (grade 4.4), purged for three times and then pressurized to
bar).47 In another patent, they used 5 wt% Pt/C catalyst for glucose the desired pressure. The pressurized reactor was placed in a band
oxidation to produce 57% glucaric acid for 5 h at 80 °C and 5 bar heater, heated at 10 °C /min and then held at reaction
O2.6 However, these reactions were conducted in micro-liter scale in temperature. The solution was stirred at a constant rate of 800 rpm
a
96 well-reactor system which was neither reproduced nor using a magnetic stirrer. Upon reaction, the reactor was cooled in
conducted in a reasonable laboratory scale. Moreover, the effects an ice bath and then depressurized. H2SO4 and NaOH were used to
of reaction temperature, O2 pressure, reaction time and pH on adjust the initial pH of the solutions. The following equations were
glucose oxidation and glucaric acid yields remain unknown. Most used for calculations of conversion, selectivity and yield:
recently, Jin et al. have reported glucose oxidation over a series of Conversion (%) = 100 × (moles of carbon converted)/(moles of
Pt- and Cu-based mono- and bimetallic catalysts in high carbon in initial feed)
concentration of NaOH; a Pt-Cu catalyst produced a maximum yield Selectivity (%) = 100 × (moles of carbon of a product)/(moles of
of 25% glucaric acid at 45 °C.48
carbon converted)
Herein, we report the results of glucose oxidation with activated
carbon, SiO2 and Al2O3 supported Pt catalysts in water under acidic,
basic and neutral pH. The Pt/C catalyst shows the highest activity in
glucaric acid yield (74%) in water with a turnover frequency (TOF) of
879 ± 25 h-1. Controlled experiments with gluconic acid as a starting
substrate show that gluconic acid oxidation is slower than glucose.
In acidic aqueous medium, gluconic acid is the major product while
in basic solutions, several low carbon chain carboxylic acids form via
C–C bonds scission of C6 carboxylic acids, resulting in low glucaric
acid yield. The effects of temperature, PO2 and glucose/Pt ratio on
glucose conversion and yields are elucidated. The catalyst is stable
after five cycles with no sign of Pt leaching into the solution.
Yield (%) = 100 × (moles of carbon product)/(moles of carbon in
initial feed)
Product analysis
The product samples were filtered and diluted 11-fold in water
before HPLC analysis on a Waters, model e2695, instrument,
equipped with an Agilent Hi-Plex H column and a refractive index
(RI) detector (Waters 2414) as well as photodiode array (PDA)
detector (Waters 2998). The column oven temperature was set to
75 °C. An aqueous acidic solution of 8.5 mM H2SO4 was used as a
mobile phase at a flow rate of 0.1 mL/min. RI and PDA (at 210 nm)
detectors were used for analysis of organic acids and glucose. The
characteristic peaks for organic acids and unconverted glucose were
identified from their retention times. Further confirmation of the
identified organic acids and identification of two unknown peaks
(keto-glucose dialdehyde and keto-glucaric acid) was performed on
a HPLC-MS (Agilent 6120). Each peak was integrated, and the actual
concentrations of organic acids and glucose were calculated from
their respective pre-calibrated plots of peak areas vs.
concentrations. The product samples were also analyzed using a
Wavelength Dispersive X-ray Fluorescence (WDXRF) Spectrometer
(Rigaku Supermini200) and a pH meter (Oakton pH 700) to measure
Pt leaching and acidity of the solutions during oxidation.
Experimental Section
Materials
Pt/C (5 wt% Pt) and Pt/Al2O3 (5 wt% Pt) were purchased from
Sigma-Aldrich. Pt/SiO2 (5 wt% Pt) was purchased from Strem
Chemicals. All catalysts were used as purchased. Glucose (>99.5%
purity) was purchased from Sigma-Aldrich. Standard samples
(glucaric acid salt, gluconic acid, tartronic acid, oxalic acid, glyceric
acid salt, glycolic acid, lactic acid) for HPLC method development
and calibration were purchased from Sigma-Aldrich and used as
purchased. DI water was used for preparation of all solutions.
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faster than gluconic acid oxidation. This is attributed to faster
oxidation of the aldehyde group in glucose than of the –CH2OH in
gluconic acid and supports previous literature in which the
oxidation of gluconic acid has been reported to be difficult.48
To elucidate the difference in activity, we characterized all
catalysts by chemisorption and physisorption (Table S2). The BET
surface area of the Pt/C catalyst is more than 10 times higher than
the other catalysts, although CO uptake of Pt/SiO2 (82.4 μmol/g) is
higher than that of Pt/C (62.6 μmol/g). The pore volume of Pt/C (4.9
nm) is lower than the other two catalysts (7.5-9.3 nm). Based on the
characterization data, it seems the high activity of the Pt/C catalyst
correlates to its high surface area. Because of high activity of the
Pt/C catalyst, we used this catalyst for further investigations.
Results and Discussion
Initial reactions for aerobic oxidation of glucose with O2 were
carried out using a Pt/C catalyst at glucose to Pt molar ratio of 54.
As shown in Table S1 and Figure 1a, these reactions achieved high
conversion of glucose (~64%) within 10 minutes with gluconic acid
as the major product (~81% selectivity). Figure 1a shows
subsequent oxidation of gluconic acid to glucaric acid as the
reaction progresses. The stack plots of HPLC chromatograms
showing oxidation of gluconic acid and formation of glucaric acid
are given in the supporting information (Figure S1a). Figure S1b
indicates formation of gluconic acid at short times followed by its
consumption to glucaric acid, a signature of reactions in series
(Scheme S1). It is evident that glucose oxidation to gluconic acid is
faster than gluconic acid oxidation to glucaric acid. In 10 h, nearly
quantitative conversion of glucose is observed with glucaric and
gluconic acids selectivity of 65% and 19%, respectively. Small
fractions of other oxidation products, such as keto-glucose
dialdehyde and keto-glucaric acid, also form (Scheme S1).25, 49 The
total carbon balance ranged from 85 to 93%, suggesting small
fractions of other products undetected by HPLC. Given that glucaric
acid (pKa = 2.83) is a little stronger acid than gluconic acid (pKa =
3.39),50 the acidity of the solution is expected to increase as the
oxidation progresses. Figure 1b shows indeed a decrease of pH with
time.
The Effect of pH
To explore the effect of pH on glucose oxidation, we varied the
initial pH over a broad range. The results in Table 1 show a
significant effect of initial pH on glucose oxidation and product
selectivity. Glucose conversion in acid (pH 0.6, 2.5; entries 1-2) and
high base (pH 13.2; entry 7) is higher (>90%) than at nearly neutral
and mild basic conditions (pH 7.2 and 8.5; entries 3-4) for the same
reaction time (1 h). Previous reports demonstrated that gluconic
acid is the only product in glucose oxidation with Au and Au-based
bimetallic catalysts under base-free conditions.39, 40, 42, 45 In contrast,
we observed higher selectivity of glucaric acid (39-41%) in base-free
and mild basic conditions than in high base (9%) or acid (21-23%).
Lower initial glucose concentration at the same glucose/Pt molar
ratio achieves higher conversion (100%; entry 5), enabling higher
glucaric acid yield. A control experiment with gluconic acid as a
starting substrate (entry 6) gives high glucaric acid selectivity (73%)
than the reaction starting with glucose at the same pH (entry 4).
Small amounts of other oxidation products, such as oxalic, glyceric,
lactic, tartronic, and glycolic acids, are also detected at basic pH,
owing to the scission of C–C bonds of glucose and C6-carboxylic
acids.41, 48 Similar C–C bond cleavage of glyceric acid and sorbitol via
retro-aldol pathway has been reported in basic medium.51, 52 As
shown in Scheme 3, C–C bond cleavage of glucose, gluconic acid
and glucaric acid could form several C2 and C3 intermediates
(glycolaldehyde, glyoxylic acid, glyoxal, glyceraldehyde, glyceric
acid, tartronic semialdehyde). Subsequent oxidation of these
To understand the effect of support, we tested three Pt catalysts
supported on activated carbon, SiO2, and Al2O3 (5 wt% Pt loading).
Glucose did not react with O2 without a catalyst. Under comparable
reaction conditions, the Pt/C catalyst is the most effective in
glucaric acid yield (Figure 1c). The effectiveness of the catalysts
decreases in the order of Pt/C > Pt/SiO2 > Pt/Al2O3. The TOFs,
calculated at 20% conversion and at 80 °C and 6.2 bar O2, (Figure
1d) over Pt/C, Pt/Al2O3, and Pt/SiO2 for glucose oxidation are 879,
195, and 102 h-1, respectively, while those for gluconic acid
oxidation, starting from gluconic acid as a substrate, are 285, 42,
O
O
[O]
HO
OH
HO
OH
O
Glycolic acid
Oxalic acid
[O]
[O]
O
[O]
O
O
HO
O
OH
OGlyoxal
Glycolaldehyde
Glyoxylic acid
RA
RA
RA
O
OH
OH
OH
OH
OH
OH
OH
O
O
OH
[O]
[O]
HO
HO
HO
OH
OH
OH
OH
OH
OH
O
Glucaric acid
Glucose
Gluconic acid
RA
O
O
O
[O]
OH
O
HO
OH
Figure 1. (a) Glucose conversion and products selectivity for glucose
oxidation in water on Pt/C catalyst as a function of reaction time (ꢀ
HO
H
OH
OH
[O]
[O]
OH
Tartronic
semialdehyde
Glyceric acid
Glyceraldehyde
O
O
glucose conversion;
ꢁ glucaric acid selectivity; ꢀ gluconic acid
selectivity). (b) pH profile during oxidation. (c) Gluconic and glucaric acid
yields over three catalysts. Reaction conditions: 10 wt% glucose (2 g; 11.1
mmol), 20 mL H2O, 80 °C, 6.2 bar O2, glucose/Pt (mol/mol) = 54. (d) TOFs
for glucose and gluconic acid oxidations. TOFs were calculated at 20%
conversion of 10 wt% glucose and gluconic acid at 80 °C and 6.2 bar O2.
HO
OH
OH
Tartronic acid
Scheme 3. Proposed reaction pathways for glucose oxidation in basic
condition. RA stands for retro-aldol. Blue color represents the
observed C2 and C3 carboxylic acids.
and 13 h-1, respectively. For all catalysts tested, glucose oxidation is
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53
intermediates forms tartronic, oxalic and glycolic acids.52,
than an alcoholic group, and as a result, open chain glucose and
Tartronic acid is the major product following gluconic acid in basic gluconic acid adsorb with similar strengths whereas the glucaric
media, consistent with data starting from pure gluconic acid. A acid adsorbs more strongly. As a result, glucaric acid may block sites
separate experiment using glucaric acid as a starting substrate and slow the reaction down as it progresses. Another more
reveals that glucaric acid in mild base (pH 8.5) at 80 °C and 6.2 bar important factor in dictating the oxidation rate, especially at short
O2 degrades to tartronic and oxalic acids over the Pt/C catalyst times, has a kinetic origin. The transformation of an alcohol group –
(Table S3). This suggests that low carbon carboxylic acids in basic CH2OH to an acid requires first dehydrogenation to –CHO followed
solutions (entries 6-7) mainly arise from C–C bond cleavage of by oxidation to –COOH.56-58 We have previously shown via
glucaric acid. In acidic solutions, carbon loss is higher, probably due combination of isotopic labeling surface science experiments,
to the formation of other oxidation products in acidic medium or density functional theory, and microkinetic modeling that C–H bond
acid catalyzed oligomerization of the carboxylic acids into species activation in polyols, such as ethylene glycol, is a kinetically relevant
that are not detected via HPLC and HPLC-MS. Clearly, the pH has a step.59-61 Upon dehydrogenation, the chemistry becomes facile. For
profound effect on product selectivity, with nearly neutral and mild example, glycolaldehyde is much more reactive than ethylene
basic pH values leading to higher selectivity to glucaric acid.
glycol.59-61 These fundamental findings are consistent with the
It has been observed that glucose oxidation to glucaric acid general knowledge that aldehydes are much more reactive than
entails rapid conversion of the aldehyde group of glucose and much alcohols. We propose that the oxidation of gluconic acid is slow due
slower oxidation of the –CH2OH group of gluconic acid, limiting the to the (oxidative) dehydrogenation of –CH2OH to –CHO being slow;
overall process efficiency. This finding is reminiscent of the this dehydrogenation step, which is important for activating the
oxidation of HMF to FDCA,17, 18, 54 and the reasons for reactivity molecule, is unnecessary in oxidation of glucose to gluconic acid.
differences remain elusive. In order to rationalize the observed We believe high pressure of O2 facilitates oxidation of gluconic acid
phenomenon, we have estimated the free energy of adsorption of to glucaric acid in our study. This explanation is in line with previous
various compounds (Figure S2).55 Due to the multifunctional nature work where the formation of glucaric acid is reported at high
of reactants and products, these species interact via multiple pressure of O2.47
groups with the catalyst. The C=O group interacts more strongly
Table 1. Glucose and gluconic acid oxidation on Pt/C as a function of initial pH.
Selectivity (%)
Carbon
balance
(%)b
82
Initial
pH
Final
pH
Conv.
(%)
En
try
1c
Glucaric
acid
21
Gluconic
acid
58
Tartronic
Oxalic
Glyceric
Glycolic
Lactic
Othersa
acid
0
acid
0
acid
0
acid
0
acid
0
0.6
2.5
7.2
8.5
0.24
1.99
2.1
94
90
82
81
2
2
1
1
2c
23
59
0
0
0
0
0
84
3c
41
48
0
0
0
0
0
90
4c
2.2
39
49
0
0
0
0
0
89
5d
99
8.5
2.7
100
41
58
0
0
0
0
0
0
6e
7c
8.5
4.6
9.4
43
73
9
-
14
12
4
0
5
0
1
0
2
0
0
99
99
13.2
100
59
20
b
aOthers include keto-glucose dialdehyde and keto-glucaric acid. Carbon balance is the sum of carbon moles of products detected by HPLC and HPLC-
c
d
MS and carbon moles of unconverted glucose. Substrate is 10 wt% glucose (2 g (11.1 mmol) in 20 mL H2O). Substrate is 1 wt% glucose (0.2 g (1.1
mmol) in 20 mL H2O). e Substrate is 10 wt% gluconic acid (2 g (10.2 mmol) in 20 mL H2O) . Initial pH of 0.6 and 2.5 in entries 1 and 2 were adjusted with
22 and 0.1 mmol of H2SO4. Initial pH of 8.5 and 13.2 in entries 4, 5, and 7 were adjusted with 0.1 and 22 mmol of NaOH. Initial pH 8.5 of entry 6 was
adjusted with 0.15 mmol of NaOH. Reacti on conditions: 80 °C, 6.2 bar O2, 1 h, glucose/Pt (mol/mol) = 54.
The effect of reaction temperature, O2 pressure and Pt reaction temperature to 90 °C results in decreasing glucaric acid
concentration selectivity to 62% while glucose conversion remains nearly
We further investigated the effect of O2 pressure, reaction quantitative (>99%) (entry 6). This yield loss of glucaric acid at 90
°C
temperature, and glucose/Pt molar ratio on the oxidation of may be due to the formation of undetectable products as indicated
glucose. The results are summarized in Table 2. The glucose by a concomitant reduction of overall carbon balance.
conversion increases from 83 to >99% with an increase in O2
The dependence of glucose oxidation on the glucose/Pt molar
pressure from 3.1 to 13.8 bar at 80 °C at a glucose to Pt molar ratio ratio in the range of 27 to 116 at 80 °C and 13.8 bar O2 is shown in
of 54 for 3 h. Higher oxidant concentration also resulted in faster Table 2 (entries 3, 7-8). The results show an increase in gluconic
oxidation of the intermediate gluconic acid to glucaric acid, acid selectivity from 6 to 21% as the glucose/Pt ratio increased from
increasing glucaric acid selectivity from 28 to 58% at the expense of 27 to 54, while the glucaric acid selectivity (58-60%) and glucose
gluconic acid selectivity (entries 1-3). A loss of up to 18% carbon conversion (>99%) remained almost unchanged. Upon further
balance is observed at higher O2 pressure (>6.2 bar) due to decreasing the Pt loading (glucose/Pt molar ratio to 116), only 38%
undetected products (vide supra). Such carbon loss was also glucaric acid selectivity was achieved at 87% glucose conversion and
reported during glucose oxidation at 45 °C and ambient O2 a significant amount of gluconic acid (42%) remained mono-
pressure.48
oxidized. Our results indicate that lower Pt loading leads to
Temperature variation in the range of 70 to 90 °C at fixed O2 incomplete oxidation of the intermediate gluconic acid, owing to
pressure of 13.8 bar and glucose to Pt molar ratio of 54 shows an slower oxidation. The overall carbon balance also improved at
increase in glucaric acid selectivity from 69 to 74% at nearly lower Pt loading because of minimization of undetectable products.
quantitative conversion of glucose when the reaction temperature
increased from 70 to 80 °C (entries 4-5, Table 2). Further increase of
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Table 2. Effect of reaction conditions on glucose oxidation in water
on the Pt/C catalyst.
Carbon
O2
(bar
)
Selectivity (%)
T
(°C)
Glucose
/Pt
Conv
(%)
balanc
e
(%)b
Entry
Glucaric
acid
Gluconic
acid
Others
a
1
2
3
4
5
6
7
8
80
80
80
70
80
90
80
80
3.1
54
54
54
54
54
54
27
116
83
89
28
53
58
69
74
62
61
38
65
30
21
10
6
2
1
3
5
3
2
3
1
96
86
82
85
83
66
69
83
6.2
13.8
13.8
13.8
13.8
13.8
13.8
>99
94
Figure 2. (a) Recycling studies of the Pt/C catalyst for glucose oxidation
in water. Reaction conditions: 10 wt% glucose (2 g; 11.1 mmol), 80 °C,
99
13.8 bar O2, 10 h, glucose/Pt (mol/mol) = 54, initial pH 7.2. (b)
Comparison of CO uptake and BET surface area of the fresh and
recovered Pt/C catalyst after the 5th cycle (gray bars: CO uptake; white
patterned bars: BET surface area). Pore volume and size are summarized
in Figure S3.
>99
>99
87
2
6
42
Reaction time: 3 hours for entries 1-3 and 7-8 and 10 hours for entries 4-6. Other reaction
conditions: 10 wt% glucose (2 g ;11.1 mmol) in entries 1-6, 5 wt% glucose (1 g; 5.55 mmol) in
entry in 7 and 20 wt% glucose (4 g; 22.2 mmol) in entry 8, 20 mL H2O, initial pH 7.2. aOthers
include keto-glucose dialdehyde and keto-glucaric acid. bCarbon balance was calculated
from the sum of carbon moles of products detected by HPLC and HPLC-MS and carbon
moles of unconverted glucose.
deactivation of the catalyst, in terms of glucaric acid yield, was
observed (Figure 2a). Characterization of the recovered catalyst
after the 5th cycle (Figure 2b and Figure S3) indicates the reused
catalyst has similar traits as the fresh catalyst. WDXRF of product
solutions did not show signs of Pt, indicating no Pt leaching during
oxidation.
Comparison of various studies
Our results are compared with the limited number of literature
reports in Table 3. Rennovia Inc. studied this reaction in a very small
scale (0.25 mL). The yield of glucaric acid using different mono- and
bimetallic catalysts was less than 34%. The loading of a non-noble
metal alloyed with Pt on SiO2 support had a slight effect on yield. A
Pt/C catalyst at glucose/Pt molar ratio similar to our conditions
produced 57% glucaric acid. In another patent, a maximum 70%
glucaric acid yield was reported using TiO2 supported Au-Pt
bimetallic catalysts at more extreme conditions (a higher
Conclusions
In conclusion, we report aerobic oxidation of glucose to glucaric
acid using activated carbon, SiO2 and Al2O3 supported Pt catalysts in
water. The Pt/C catalyst is the most effective of these in glucose
conversion and glucaric acid yield. The reactions in base-free and
mild basic conditions give best yield to glucaric acid as compared to
acidic and highly basic (pH 13.2) media. C–C bond scission of
glucaric acid in basic pH results in low carbon chain carboxylic acids.
temperature of 112 °C, O2 pressure of 27 bar and glucose/metal =
A maximum 74% glucaric acid yield at initial pH of 7.2, 80 °C, 13.2 bar
42) than ours. A recent study reported only 25% glucaric acid yield
using a bimetallic Pt-Cu catalyst in a concentrated basic solution for
24 h.48 In comparison, our yield is higher at milder conditions.
O2 and glucose/Pt molar ratio of 54 was achieved, which is the
highest reported and occurs at milder conditions than previous
work. Higher temperatures and higher Pt loadings cause
degradation of glucaric acid, resulting in lower yields. Importantly,
the Pt/C catalyst is stable after five consecutive catalytic cycles.
Table 3. Comparison of glucaric acid yields from glucose oxidation
over different monometallic and bimetallic catalysts.
Acknowledgements
Glucaric
O
T
(°C)
Reaction
time (h)
Glucose/metal
(mol/mol)
acid
yield
(%)
This work was supported as part of the Catalysis Center for
Energy Innovation, an Energy Frontier Research Center funded by
the U.S. Department of Energy, Office of Science, Office of Basic
Energy Sciences under Award Number DE-SC0001004. The authors
acknowledge the Advanced Materials Characterization Laboratory
at the University of Delaware for providing HPLC-MS and WDXRF
facilities and Jeffrey Everhart for help with analysis of reaction
products. Geun Ho Gu is acknowledged for performing the
adsorption calculations.
2
Catalysts
Base
Ref.
(bar)
Pt/SiO
80
80
80
5.2
5.2
5.2
5
5
5
80
80
80
No
No
No
34
38
34
2
Mn Pt /SiO
2
1
19
Co Pt /SiO
2
6
1
20
Fe Pt /SiO
80
80
5.2
5.2
27
5
5
5
80
54
42
No
No
No
28
57
70
1
19
2
Pt/C
Au Pt /TiO
112
47
48
1
1
2
Pt/TiO2
45
45
45
1
1
1
24
24
24
N/A
N/A
N/A
NaOH
NaOH
NaOH
1
Pt Cu /TiO
25
40
1
3
2
Notes and references
PtPd/TiO2
Pt/C
62
This
work
80
13.8
10
54
No
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