Oxidation of 5-hydroxymethylfurfural over supported Pt, Pd and Au catalysts

Article abstract of DOI:10.1016/j.cattod.2010.06.004

Supported Pt, Pd, and Au catalysts were evaluated in the aqueous-phase oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) at 295 K and high pH in a semibatch reactor. The intermediate reaction product 5-hydroxymethyl-2-furancarboxylic acid (HFCA) was formed in high yield over Au/C and Au/TiO₂ at 690 kPa O₂, 0.15 M HMF and 0.3 M NaOH, but did not continue to react substantially to FDCA at the specified O₂ pressure and base concentration. In contrast, the final reaction product FDCA was formed over Pt/C and Pd/C under identical conditions. The initial turnover frequency of HMF conversion was an order of magnitude greater on Au catalysts compared to either Pt or Pd. Increasing the O₂ pressure and NaOH concentration facilitated the conversion of HFCA to FDCA over the supported Au. The significant influence of base concentration on the product distribution indicates an important role of OH^- in the activation, oxidation and degradation of HMF.

Full text of DOI:10.1016/j.cattod.2010.06.004

Catalysis Today 160 (2011) 55–60
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Catalysis Today
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Oxidation of 5-hydroxymethylfurfural over supported Pt, Pd and Au catalysts
Sara E. Davisa, Levi R. Houkb, Erin C. Tamargoa, Abhaya K. Datyeb, Robert J. Davisa,∗
a Department of Chemical Engineering, University of Virginia, 102 Engineers Way, Charlottesville, VA 22904-4741, USA
b Center for Microengineered Materials and Department of Chemical & Nuclear Engineering, University of New Mexico, Albuquerque, NM 87131-0001, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 7 July 2010
Supported Pt, Pd, and Au catalysts were evaluated in the aqueous-phase oxidation of 5-
hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) at 295 K and high pH in a semibatch
reactor. The intermediate reaction product 5-hydroxymethyl-2-furancarboxylic acid (HFCA) was formed
in high yield over Au/C and Au/TiO2 at 690 kPa O2, 0.15 M HMF and 0.3 M NaOH, but did not continue
to react substantially to FDCA at the specified O2 pressure and base concentration. In contrast, the final
reaction product FDCA was formed over Pt/C and Pd/C under identical conditions. The initial turnover
frequency of HMF conversion was an order of magnitude greater on Au catalysts compared to either Pt or
Pd. Increasing the O2 pressure and NaOH concentration facilitated the conversion of HFCA to FDCA over
the supported Au. The significant influence of base concentration on the product distribution indicates
an important role of OH− in the activation, oxidation and degradation of HMF.
Keywords:
5-Hydroxymethylfurfural
Oxidation
Gold catalysis
Heterogeneous catalysis
Biomass
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
have shown polymers of FDCA to have many properties similar to
PET [8].
The rising cost, diminishing supply and environmental impact
of fossil fuels have spawned increased interest in the production
of sustainable alternative energy and chemical feedstocks. While
much attention is paid to renewable energy such as wind, solar
and geothermal, none of those renewable sources can be used to
produce organic chemicals currently derived from fossil fuels. How-
ever, carbon-containing molecules found in renewable biomass
could potentially serve as a sustainable feedstock for the chemical
industry.
The molecule 5-hydroxymethylfurfural (HMF) is one of the
many potential platform chemicals for biorenewable chemicals
production [1] because it is formed by dehydration of fructose
and glucose [2–4]. For example, a high yield biphasic method of
HMF synthesis from fructose (80% HMF selectivity at 90% fructose
conversion, from 10 to 50 wt% fructose) has been reported [5]. In
addition, glucose can be converted catalytically in ionic liquids to
HMF [6]. One reason HMF is considered to be an important plat-
form chemical is that it can be oxidized to 2,5-furandicarboxylic
acid (FDCA). A recent study commissioned by the U.S. Department
of Energy identified FDCA as one of 12 potentially useful building
blocks for value added chemicals from biomass [7]. For example,
FDCA is a possible replacement monomer for terephthalic acid
used to produce polyethylene terephthalate (PET). Fig. 1 shows the
structural similarity of terephthalic acid and FDCA. Gandini et al.
The oxidation of HMF has been studied over a variety of cat-
alysts in the past two decades. Early work by Verdeguer et al.
investigating the oxidation of HMF over Pt/Pb catalysts demon-
strated a need for high pH in the reaction solution [9]. They also
found that the production of FDCA occurred in two stages; the alde-
hyde side chain was first oxidized to a carboxylic acid, producing
5-hydroxymethyl-2-furancarboxylic acid (HFCA), which was fol-
lowed by the oxidation of the hydroxymethyl side chain to produce
FDCA. Interestingly, Verdeguer et al. also found that hydroxide base
was more effective in producing FDCA than carbonate base, which
favored production of HFCA.
The oxidation of HMF over Pt/Al2O3 catalysts was also inves-
tigated by Vinke et al. at lower pH (pH = 9) [10]. Although FDCA
was eventually formed, the intermediate product 5-formyl-2-
furancarboxylic acid (FCA) was observed. The intermediate product
HFCA appeared at higher pH [10]. The investigation reports that
the reaction rate is independent of pH between pH 8 and 11, and
that the rate is greater over Pd than over Pt. Transition metals
can deactivate in high concentrations of dioxygen, presumably by
over oxidization of the metal catalyst. However, Vinke et al. sug-
gest that a strong interaction between HMF and the surface of Pt
prevents over oxidation of the metal. Thus, they speculate that
the rate limiting step of HMF oxidation is the adsorption of O2 on
the catalyst.
Partenheimer and Grushin report the results of HMF oxidation
over various metal bromide catalysts (Co/Mn/Zn/Br) in an acetic
acid solution [11]. At lower temperatures (323–348 K), the main
product was reported to be 2,5-diformylfuran (DFF) in moderately
∗
Corresponding author. Tel.: +1 434 924 6284; fax: +1 434 982 2658.
E-mail address: rjd4f@virginia.edu (R.J. Davis).
0920-5861/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2010.06.004

56
S.E. Davis et al. / Catalysis Today 160 (2011) 55–60
Fig. 1. FDCA is a potential biorenewable replacement monomer for terephthalic acid in polyethylene terephthalate plastics.
high yield (57%) while at elevated temperatures (373–498 K), the
main product was FDCA in a yield of ∼60%. Similarly, Navarro et al.
reported the production of DFF from HMF using vanadyl-pyridine
complexes in both homogeneous and heterogeneous forms and in
various solvents such as toluene, trifluorotoluene and dimethyl-
sulfoxide (DMSO) solutions [12]. At the moderate temperature of
388 K, DFF was produced in >99% selectivity. It is important to note
that the experiments utilized a very high catalyst loading (mol
substrate/mol metal = 10). Evidently, the nature of the catalyst sur-
face and the composition of the solvent both play key roles in the
effectiveness of HMF oxidation. Although there have been attempts
to produce FDCA directly from fructose, success has been limited
[13–15].
Very recently, oxidation of HMF over supported gold catalysts
has been observed. Taarning et al. reported the oxidation of HMF in
methanol over Au/TiO2 at 403 K in the presence of a small amount of
base [16]. Although a majority of the ester product formed at room
temperature was derived from HFCA, increasing the temperature
to 403 K resulted in the selective formation of the ester of FDCA.
Gorbanev et al. also reported HMF oxidation in an aqueous solution
over a commercial Au/TiO2 catalyst, but performed the reaction
with 20 equiv. of base [17]. In that study, the yield of FDCA increased
significantly when the dioxygen pressure was raised from 1000 kPa
to 2000 kPa. Their optimized conditions produced a 71% yield of
FDCA with full HMF conversion.
reduced by adding 0.1 M NaBH4 [Aldrich] dropwise in 4:1 molar
ratio of NaBH4:Au. Following reduction, the sol was ruby red in
color. Five grams of carbon lampblack [Fisher Scientific] was sus-
pended in 100 cm3 of deionized H2O and sonicated for 1 h and
subsequently added to the sol. The sol–carbon slurry was stirred for
1 h prior to filtration. The filtrate was clear in color, indicating the
Au sol had deposited on the support. The catalyst was washed with
several liters of water to remove any residual chlorine and dried
overnight at 403 K in air. The catalyst was subsequently reduced
in flowing N2 and H2 gases in a 9:1 ratio at 150 cm3 min−1 for
6 h at 573 K. The Au catalyst was stored in a refrigerator and used
without any additional pretreatment. Metal weight loading was
determined by ICP analysis performed by Galbraith Laboratories,
Knoxville, TN.
Gold standard catalysts were obtained from the World Gold
Council. A 1.6 wt% Au on TiO2 catalyst (Type A, Lot. No. Au–TiO2
#02-8) and a 0.8 wt% Au on C (Type C, Sample 40D) catalyst were
used in this study. The platinum and palladium catalysts used here
were both 3 wt% metal on activated carbon, supplied by Aldrich.
Both the Pd and Pt catalysts were reduced in H2 (UHP, Messer Gas)
flowing at 150 cm3 min−1 for 6 h at 573 K and cooled under flow-
ing H2. The catalysts were refrigerated and used without further
pretreatment.
Casanova et al. also investigated HMF oxidation over Au/TiO2,
Au/CeO2, Au/C and Au/FeO2 catalysts in liquid water [18]. The
Au/TiO2 and Au/CeO2 catalysts proved to be most effective for HMF
oxidation to FDCA. At their optimized conditions of 403 K, 1000 kPa
O2 and 4:1 NaOH:HMF, >99% yield of FDCA was obtained after 8 h
over Au/CeO2. Interestingly, at lower temperatures, HFCA was the
major product at complete conversion of HMF.
To the best of our knowledge, the reactivity of Pt, Pd and Au cata-
lysts has never been directly compared under identical conditions.
Moreover, the influence of reaction conditions on the turnover
frequency of Au-catalyzed HMF oxidation has not been reported
previously. Thus, we have investigated the oxidation of HMF in
water over several supported Pt, Pd and Au metal catalysts. Water
was chosen as a green solvent to minimize environmental impact.
Sodium hydroxide was used to facilitate the reaction because an
earlier report indicated that hydroxide was more effective than car-
bonate [9]. The influence of concentration of hydroxide base on the
rate of oxidation and the effect of O2 pressure on rate and product
selectivity over supported Au were also explored.
2.2. Oxidation reactions
Theaqueous phase oxidation of5-hydroxymethylfurfural (HMF)
[Acros, ≥98% purity] was carried out in a 50 cm3 Parr Instrument
Company 4592 batch reactor equipped with a glass liner. Dioxygen
was UHP, supplied by Messer Gas.
In all reactions, 7.0 cm3 of the reactant solution (0.15 M HMF
and 0.3 M NaOH) was added to the reactor along with the appro-
priate amount of catalyst. The reactor was purged with flowing O2
and then pressurized to the desired value. A constant pressure was
maintained by a continuous O2 feed. Samples of the product solu-
tion were acquired by removing the top of the reactor, taking the
sample, flushing the reactor with O2 and repressurizing. The maxi-
mum O2 transport rate from the gas to the liquid was determined by
oxidation of sodium sulfite [20], and the HMF oxidation rate was
kept significantly below this limit when quantitative rates were
measured. The oxidation of sodium sulfite at the standard agitation
speed and 690 kPa O2 resulted in a maximum gas–liquid transfer
rate of 4 × 10−7 mol O2 s−1
.
The samples from the oxidation reactions were filtered using
PTFE 0.2 ␮m filters and diluted with deionized H2O in a 1:3 sam-
ple to water ratio. The analysis was conducted using a Waters
e2695 high performance liquid chromatograph [HPLC] at 308 K
equipped with refractive index and UV/vis detectors. The HPLC
utilized either a Waters Atlantis C18 column and deionized H2O
flowing at 1 cm3 min−1 or a Bio-Rad Aminex HPX-87H column and
5 mM H2SO4 flowing at 0.5 cm3 min−1 to perform the separation.
The retention times and calibrations for observed products were
determined by injecting known concentrations.
2. Experimental methods
2.1. Catalyst preparation
A gold on carbon catalyst was prepared through the formation
of a gold sol and the subsequent deposition of the gold sol onto
a carbon support [19]. The sol was prepared by adding 0.075 g of
HAuCl4·4H2O [∼50 wt% Au, Aldrich] to 1500 cm3 of deionized water
along with 0.00375 g polyvinyl alcohol [Acros]. The colloid was

S.E. Davis et al. / Catalysis Today 160 (2011) 55–60
57
Table 1
Catalyst characterization results.
Catalyst
Metal loading (wt%)
Mean metal particle size (nm)
Surface average diameter (nm)a
Dispersion
Pt/C
Pd/C
Au/C (WGC)
Au/C (sol)
Au/TiO2
3.04b
2.9b
2.5c
3.3c
–
–
0.40
0.32
0.05g
0.11g
0.38i
0.8d
10.5e
3.0h
2.6e
18.8f
8.8h
–
0.77b
1.6d
ꢀ
ꢀ
a
Surface average diameter = (
d3/
d2).
b
c
d
e
f
From ICP analysis (Galbraith Labs, Knoxville, TN).
Inverse of dispersion (determined via H2 chemisorption).
From ICP analysis provided by the World Gold Council.
From TEM analysis provided by the World Gold Council.
From TEM analysis [23].
g
h
i
Inverse of surface average diameter, used to normalize rates for exposed Au atoms for Au/C (WGC) and Au/C (sol).
From TEM analysis, this work.
Inverse of mean particle diameter (nm), used to normalize rates for exposed Au atoms for Au/TiO2.
2.3. Chemisorption of H2
particle size distribution (Fig. 2) sample indicated that the major-
ity of particles are about 3 nm in diameter, with a small number
of particles larger than 4.0 nm. The mean particle size determined
by evaluating 792 particles over five different regions of the sam-
ple was 3.0 nm. The surface average diameter for the Au/C (sol)
sample was 8.8 nm, which corresponds to an estimated disper-
sion (fraction of metal at the surface) of 0.14 (determined as the
inverse of the surface average diameter) [22]. Although a mean par-
ticle size of 10.5 nm was provided by the WGC for their Au/C, the
Au/C (WGC) catalyst has been previously imaged by Ketchie et al.
[23]. The particle size distribution reported that work resulted in
a mean particle diameter of 12.2 nm, which compares well to that
provided by the WGC. However, the surface average diameter for
Au/C (WGC) was 18.8 nm, corresponding to a dispersion of about
0.05. Because the Au/TiO2 catalyst was not imaged independently,
the mean particle size of 2.6 nm provided by the WGC was used
to estimate a dispersion of 0.38. Chemisorption of H2 was used to
determine the dispersion for the Pt/C and Pd/C catalysts as 0.40 and
0.32, respectively.
The dispersion of Pt and Pd was determined using a Micromerit-
ics ASAP 2020 automated adsorption system. The Pt/C catalyst was
heated to 648 K at 4 K min−1 under flowing H2 (UHP, Messer Gas)
and reduced at that temperature for 1.5 h. The sample was sub-
sequently evacuated for 2 h at 648 K and then cooled to 308 K for
analysis. The Pd/C catalyst was heated to 473 K at 4 K min−1 under
flowing H2. The sample was evacuated and held for 2 h at 473 K
before being cooled down to 373 K for analysis. The analysis was
carried out at 373 K in the pressure range of 0.06–0.6 atm to avoid
formation of the ␤-phase hydride [21].
2.4. Scanning transmission electron microscopy
High-angle annular dark field (HAADF) imaging was performed
using a JEOL 2010F FASTEM field emission gun scanning trans-
mission electron microscope operated at 200 kV. The Au/C catalyst
sample was suspended in ethanol by grinding in an agate mortar
and pestle, and deposited on a holey carbon support film on Cu
TEM grids. The images were recorded and analyzed using Digital
Micrograph software.
3.2. Oxidation reactions
The oxidation of HMF in an aqueous solution under 690 kPa O2
at 295 K did not occur in 6 h over Pt/C or Au/C (WGC) without the
presence of base. Thus, NaOH was added to the reaction medium to
provide the basic environment. It should be noted that HMF in the
presence of NaOH in water reacted to form undesirable decomposi-
tion products such as formic acid and 2,5-bis(hydroxymethyl)furan.
Therefore, the concentration of base was optimized so that side
3. Results and discussion
3.1. Catalyst characterization
The Au metal loadings and particle sizes for the catalysts are
listed in Table 1. A typical STEM image of the Au/C (sol) and the
Fig. 2. STEM image of Au/C (sol) catalyst. The accompanying particle size distribution indicates a mean particle size of 3.0 nm.

58
S.E. Davis et al. / Catalysis Today 160 (2011) 55–60
Fig. 3. Products of HMF oxidation.
reactions were minimized while oxidation proceeded. A concentra-
tion of 0.3 M NaOH (NaOH:HMF 2:1) was chosen. The background
conversion of HMF in NaOH solution without catalyst was 10% over
0.5 h.
reaction conditions. Despite the complete conversion of HMF after
1 h, the relative concentrations of FDCA and HFCA were virtually
unchanged between 1 h and 6 h. After 6 h, additional feed solution
(0.15 M HMF, 0.3 M NaOH) was added to the reactant mixture to
yield a new HMF concentration in the reactor of 0.05 M, which was
readily oxidized to HFCA in less than 1 h. These results conclusively
demonstrated that the Au catalyst did not deactivate significantly
during HMF oxidation. Because the product selectivity was nearly
the same for all of the Au catalysts tested, the reaction path was not
affected by the carbon or titania support.
The oxidation products of HMF are depicted in Fig. 3. The product
2,5-bis(hydroxymethyl)furan (BHMF) was formed in the Canniz-
zaro reaction, which is a disproportionation reaction induced by
high concentration of the base. A typical reaction profile for HMF
oxidation over supported Pt is reported in Fig. 4. The oxidation of
HMF proceeded through HFCA and FCA to FDCA. The oxidation of
HMF over Pd and Au catalysts apparently proceeded through HFCA
because FCA was not observed in the course. In general, the prod-
uct distributions reported here are consistent with those discussed
earlier [9,10,17,18].
While the time to reach complete conversion of HMF over Pt
was much longer than for Au or Pd, the desired product (FDCA)
was obtained in majority (SFDCA = 79%). Oxidation of HMF over Pd
also yielded a majority of FDCA (SFDCA = 71%).
All catalysts were investigated for the oxidation of 0.15 M HMF
in 0.3 M NaOH at 690 kPa O2 and 295 K and a catalyst loading equiv-
alent to metal:substrate = 6.7 × 10−3. The results of these runs after
6 h (high HMF conversion) can be seen in Table 2. Although the
conversion of HMF was 100% after 1 h over all three Au catalysts,
the major product was HFCA, even after 6 h. In contrast to Pt and
Pd, Au did not readily oxidize the alcohol side chain under these
The catalyst loadings for the runs in Table 2 were selected to give
high conversions in relatively short reaction times. However, the
flux of O2 through the gas–liquid film was very near the mass trans-
fer limit. Therefore, the amount of catalyst loaded into the reactor
was lowered in order to obtain rate data unaffected by gas–liquid
transport effects. The results of those reactions are summarized in
Table 3. It should be noted that the amount of catalyst used for the
results in Table 3 was substantially lower for the Pd and Au catalysts
relative to the amount used for Table 2.
Although the metal loadings were much lower for the Au cata-
lysts, the major product was still HFCA. Nevertheless, Au catalysts
were substantially more active in converting HMF than either Pt or
Pd. The turnover frequency on Au ranged from 2 to 5 s−1 whereas
Table 2
Oxidation of HMF over supported Pt, Pd and Au catalysts.a
Catalyst
Conversion (%)
SFDCA (%)
SHFCA (%)
Pt/C
Pd/C
Au/C (WGC)
Au/C (sol)
Au/TiO2
100
100
100
100
100
79
71
8
7
8
21
29
92
93
92
a
Reaction
conditions:
0.15 M
HMF
solution
in
0.3 M
NaOH,
Fig. 4. Sample reaction profile for oxidation of HMF over Pt/C. Reaction conditions:
0.15 M HMF, 0.3 M NaOH, 295 K, 690 kPa O2.
metal:HMF = 6.67 × 10−3 mol/mol, T = 295 K, P = 690 kPa O2. S refers to selectivity
after 6 h.

S.E. Davis et al. / Catalysis Today 160 (2011) 55–60
59
Table 3
Turnover frequencies and product selectivities during HMF oxidation over supported Pt, Pd and Au catalysts.a
Catalyst
Metal:HMF
TOFb (s−1
)
Conversion (%)
SFDCA (%)
SHFCA (%)
SBHMF (%)
SFCA (%)
Pt/C
Pd/C
Au/C (WGC)
Au/C (sol)
Au/TiO2
6.67 × 10−3
2.00 × 10−3
8.70 × 10−4
5.48 × 10−4
3.57 × 10−4
0.08
0.15
5.0
2.3
1.6
50
50
50
50
50
16
18
–
21
3
33
80
95
77
84
1
2
5
2
13
50
–
–
–
–
a
Reaction conditions: 0.15 M HMF solution in 0.3 M NaOH, T = 295 K, P = 690 kPa O2.
TOF is calculated from the moles of HMF consumed in 30 min, normalized by the surface metal atoms in the reactor. Conversion of HMF was typically less than 20% after
b
30 min.
Table 4
The influence of O2 pressure on reaction and selectivity of HMF oxidation over Au/TiO2 catalyst.a
P (kPa)
TOFb (s−1
)
Conversion (%)
SFDCA (%)
SHFCA (%)
SBHMF (%)
690
2000
3000
1.6
1.2
1.4
50
50
50
3
28
36
84
69
64
13
3
–
a
Reaction conditions: 0.15 M HMF and 0.3 M NaOH, T = 295 K, metal:HMF = 3.57 × 10−4
.
b
TOF is calculated from the moles of HMF consumed in 30 min, normalized by the surface metal atoms in the reactor. Conversion of HMF was typically less than 20% after
30 min.
the TOF on Pd and Pt was 0.15 and 0.08, respectively. The Pd cata-
lyst was slightly more active than the Pt catalyst for HMF oxidation,
which is consistent with the findings reported by Vinke et al. [10].
The high activity of Au catalysts relative to Pt and Pd has also been
observed for the oxidation of octanol [24]. Moreover, Ketchie et al.
reported Au to be significantly more active than Pd for glycerol oxi-
dation in basic solution [25]. Interestingly, the Au/C (sol) catalyst
did produce some FDCA under these conditions (selectivity = 21%
at 50% HMF conversion).
The effect of O2 pressure on reaction rate and product selectivity
over the Au/TiO2 catalyst was investigated and the results are sum-
marized in Table 4. The turnover frequency was independent of the
O2 pressure in the range investigated. However, as the pressure of
O2 increased from 690 kPa to 2000 kPa to 3000 kPa, the selectivity
for FDCA increased from 3% to 28% to 36% at 50% conversion, which
agrees with the trend noted by Casanova et al. between 100 kPa
and 1000 kPa over their Au/CeO2 [18]. In contrast, Gorbanev et al.
reported the selectivity for the diacid increased as the O2 pressure
increased from 1000 kPa to 2000 kPa but then remained constant
to 3000 kPa [17]. Their selectivity to the diacid over a Au catalyst at
2000 kPa O2 was 78%, which was much higher than that observed
here.
It should be noted that the amount of catalyst and concentration
of base used in Table 4 were much less than those in Ref. [17]. There-
fore, we reproduced the conditions used in Ref. [17] and compare
our results in to those published previously in Table 5. The product
selectivity on both the AuTiO2 and the Au/C catalyst was similar
to that reported by Gorbanev et al. [17]. The selectivity to diacid
(FDCA) was enhanced by substantially increasing the base con-
centration. Evidently, the aldehyde side chain is readily oxidized
over Au at lower concentrations of base, but higher concentrations
(20 equiv.) were needed to oxidize the alcohol side chain. Likewise,
Casanova et al. report higher selectivity to FDCA when a 4:1 ratio
of NaOH:HMF was used, instead of a 2:1 ratio [18]. This finding
is consistent with previous work on glycerol oxidation over sup-
ported Au catalysts in which high base concentration is thought to
deprotonate the alcohol and facilitates its conversion to aldehyde
[23]. Casanova et al. also observe the need for high base concen-
tration to produce FDCA from HMF. Moreover, they report that the
sequential reaction of intermediate HFCA to FDCA is greatly acceler-
ated by increasing the reaction temperature to values greater than
373 K and emphasize that the slow reaction of HFCA to FDCA limits
the overall rate [18].
While high pH is necessary to oxidize the alcohol side group of
HMF or HFCA, high pH can also lead to degradation of products. A
reaction utilizing a lower metal:HMF ratio (used for runs in Table 4)
and a high NaOH concentration (used for runs in Table 5) was con-
ducted over Au/TiO2 to elucidate the role of base concentration
on product distribution. Although the selectivity to diacid in this
experiment (56% conversion of HMF, 8% selectivity to FDCA) was
lower than that reported in Table 4, the major product was BHMF
at high base concentration. Continued reaction to ∼100% conver-
sion resulted in the formation of formic acid and other degradation
products. Evidently, high base concentrations can be detrimental to
selectivity because of undesirable side reactions, even at 295 K. To
avoid degradation of HMF in concentrated base solution, a high Au
catalyst loading (such as that used for the experiments reported in
Table 5) is needed to rapidly oxidize HMF to HFCA, an intermediate
product that is stable in base.
It should be noted that no evidence for leaching of Au into solu-
tion was observed. A sample of the reaction mixture at the end
of the reaction catalyzed by Au/C (sol) was filtered with a PTFE
0.2 ␮m filter to remove solid catalyst, and tested for trace Au by ICP
analysis performed by Galbraith Laboratories, Knoxville, TN. The
Table 5
Selectivity of HMF oxidation on Au catalysts at high base concentration.a
Catalyst
Reaction time = 6 hb
Reaction time = 22 hb
SFDCA
SHFCA
SFDCA
SHFCA
Au/C (sol)
Au/TiO2
Au/TiO2
31
32
50
69
68
50
72
80
78
28
20
19
c
a
Reaction conditions: 0.1 M HMF in 2.0 M NaOH, metal:HMF = 8.0 × 10−3, P = 2000 kPa O2.
Conversion of HMF was 100%.
Data from Gorbanev et al. [17].
b
c

60
S.E. Davis et al. / Catalysis Today 160 (2011) 55–60
ICP analysis determined the Au concentration in the liquid to be
<0.2 ppm.
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This work presents a comparison of the rate and product distri-
bution for HMF oxidation over supported metal catalysts, Pt/C, Pd/C,
Au/C and Au/TiO2. The rate of oxidation of HMF over Au catalysts
was an order of magnitude greater under the standard conditions of
295 K, 690 kPa O2, 0.15 M HMF and 0.3 M NaOH. However, the rapid
conversion of HMF over the Au catalysts was to the intermediate
product HFCA, formed by oxidation of the aldehyde side chain of
HMF. Under identical conditions, Pt and Pd were able to effectively
oxidize the HFCA to FDCA, which indicates Pt and Pd can activate the
alcohol side chain of HFCA whereas Au cannot. For a gold sample to
catalyze the formation of FDCA from HFCA, high pressures of O2 and
high concentrations of base were required, although the effect of
base appeared to be more important than the O2 pressure. Indeed,
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This material is based upon work supported by the National Sci-
ence Foundation under Grant Nos. OISE 0730277 and EEC-0813570.