On the mechanism of selective oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid over supported Pt and Au catalysts

Article abstract of DOI:10.1039/c1gc16074e

The mechanism of selective oxidation of aqueous 5-hydroxymethylfurfural (HMF) at high pH was studied over supported Pt and Au catalysts. Results from labeling experiments conducted with ¹⁸O₂ and H ₂¹⁸O indicated that water was the source of oxygen atoms during the oxidation of HMF to 2-hydroxymethylfurancarboxylic acid (HFCA) and 2,5-furandicarboxylic acid (FDCA), presumably through direct participation of hydroxide in the catalytic cycle. Molecular oxygen was essential for the production of FDCA and played an indirect role during oxidation by removing electrons deposited into the supported metal particles. A reaction path for HMF oxidation to FDCA was proposed.

Full text of DOI:10.1039/c1gc16074e

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PAPER
On the mechanism of selective oxidation of 5-hydroxymethylfurfural to
2,5-furandicarboxylic acid over supported Pt and Au catalysts
Sara E. Davis, Bhushan N. Zope and Robert J. Davis*
Received 30th August 2011, Accepted 27th September 2011
DOI: 10.1039/c1gc16074e
The mechanism of selective oxidation of aqueous 5-hydroxymethylfurfural (HMF) at high
pH was studied over supported Pt and Au catalysts. Results from labeling experiments
conducted with 18O2 and H218O indicated that water was the source of oxygen atoms
during the oxidation of HMF to 2-hydroxymethylfurancarboxylic acid (HFCA)
and 2,5-furandicarboxylic acid (FDCA), presumably through direct participation
of hydroxide in the catalytic cycle. Molecular oxygen was essential for the production of FDCA
and played an indirect role during oxidation by removing electrons deposited into the supported
metal particles. A reaction path for HMF oxidation to FDCA was proposed.
acid (HFCA) to FDCA is seen in Fig. 1. Recent work in
1. Introduction
our laboratory has shown that supported Au is more active
Interest in the production of commodity chemicals from renew-
than supported Pt for HMF oxidation, although Au is less
able carbon sources instead of from fossil resources continues to
selective to the desirable product (FDCA) under conditions of
grow. Oxidation of alcohols provides one such route for trans-
0.15 M HMF, 0.3 M NaOH, 690 kPa O2, and 295 K.14 For
formation of biorenewable feedstocks to value-added chemicals.
gold-catalyzed oxidation of HMF, increasing the NaOH : HMF
A potential platform alcohol derived from six-carbon sugars is
ratio as well as the Au : HMF ratio and the dioxygen pressure
5-hydroxymethylfurfural (HMF).1,2 The selective oxidation of
increased the selectivity to FDCA.5,6,14 Thus, a fundamental
HMF produces 2,5-furandicarboxylic acid (FDCA), a potential
understanding of the roles of added base and molecular oxygen
replacement for the terephthalic acid monomer used in the
is important to developing an environmentally-friendly method
production of polyethyleneterephthalate (PET plastic).3 Thus,
for FDCA production from HMF. Moreover, additional studies
a variety of catalysts have been explored in the oxidation of
HMF over a broad range of conditions.4–14
Water is a very low-cost, polar and environmentally-
on the activation of molecular oxygen on Au nanoparticles will
provide new methods to control the reactivity of oxygen in
organic transformations, thereby allowing the development of
benign solvent for highly oxygenated molecules derived from
sustainable alternatives to traditional processes utilizing harmful
biomass and catalytic oxidation in water provides a sustain-
able, environmentally-friendly route for conversion of biomass-
derived feedstocks. Gold (Au), the noblest of metals,15 is an
inorganic oxidants.
Zope et al. reported a combined experimental and compu-
tational investigation of the selective oxidation of ethanol and
excellent catalyst for oxidation reactions and recently, the
glycerol over Au and Pt catalysts in liquid water.16 That study
catalytic activity of gold in liquid water has been attributed to
the metal–solvent interface.16 This work further elucidates the
provided a detailed description of the reaction path for alcohol
oxidation. In addition, Casanova et al. propose a reaction
role of water on the reactivity of metal catalysts for oxidation
scheme for the key steps in HMF oxidation.5 In the current
reactions.
work, we extend the studies of Zope et al.16 and Casanova
The selective oxidation of HMF with molecular oxygen in liq-
et al.5 to include the oxidation of HMF over supported Au
uid water is a green alternative to oxidation in organic solvents;
and Pt catalysts with labeled reagents 18O2 and H218O.
however, a very high pH environment is often necessary for
the reaction to proceed.13 The reaction scheme for oxidation of
HMF through the intermediate hydroxymethylfurancarboxylic
2. Experimental methods
2.1 Catalyst preparation
A supported gold catalyst (1.6 wt% Au/TiO2; Sample 137A) was
obtained from the World Gold Council (WGC). A supported Pt
catalyst (3 wt% Pt/activated carbon) was obtained from Aldrich
Department of Chemical Engineering, University of Virginia, 102
Engineers Way, Charlottesville, VA, 22904-4741, USA. E-mail: rjd4f@
virginia.edu; Fax: +1-434-982-2658; Tel: +1-434-924-6284
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Fig. 1 Overall reaction scheme and proposed mechanism for the oxidation of HMF in aqueous solution in the presence of excess base (OH-) and
either Pt or Au. Dioxygen (not shown) serves as a scavenger of electrons that are deposited into the metal particles during the catalytic cycle.
Chemical Co. The Pt catalyst was reduced in H2 (UHP, Messer
Gas) flowing at 150 cm3 min-1 for 6 h at 573 K, cooled under
flowing H2 and gently exposed to air prior to use, whereas the
Au catalyst was used as received. The catalysts were refrigerated
and used without further pretreatment.
experiment utilizing 18O species under conditions identical to
the labeling experiments. The control experiments confirmed
that the small decrease in dioxygen pressure relative to our
previous study had no significant effect on product selectivity.
In addition, suitable choice of NaOH : HMF ratio, O2 pressure
and HMF : Au ratio resulted in a favorable product distribution
to the diacid FDCA. It should be noted that although the
activity and selectivity of Pt catalysts for HMF oxidation were
different than those of Au catalysts, our previous work ruled out
a significant influence of metal particle size and catalyst support
composition.14 Therefore, we investigated different scenarios to
elucidate the similarities and/or differences between the Au
and Pt catalysts for oxidation. Three different scenarios were
used: HMF oxidation to a majority of FDCA over Pt/C using
standard conditions (0.15 M HMF, 0.3 M NaOH, 690 kPa O2,
295 K), HMF oxidation to a majority of HFCA over Au/TiO2
using standard conditions, and HMF oxidation to a majority
of FDCA over Au/TiO2 in high base conditions (0.1 M HMF,
2.0 M NaOH, 2000 kPa O2, 295 K).
The samples from the oxidation reactions were filtered using
0.2 mm PTFE filters. The product 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 a Bio-Rad Aminex HPX-87H column and 5
mM H2SO4 flowing at 0.5 cm3 min-1 to perform the separation.
Mass spectrometry was performed using a Waters Micromass
ZQ quadrupole mass spectrometer (in electronegative mode)
using a direct injection of the product mixture. Prior work in
2.2 Oxidation of HMF
The aqueous phase oxidation of HMF (Acros, ≥98% purity) was
carried out in a 50 cm3 Parr Instrument Company 4592 batch
reactor equipped with a glass liner. Dioxygen was either UHP
(Messer Gas) or 97% 18O2 (chemical purity >99.8%, Cambridge
Isotope Laboratories).
In all reactions, 5.0 cm3 of the reactant solution (aqueous
HMF and NaOH) were added to the reactor along with the
appropriate amount of catalyst. In reactions utilizing H218O,
the reactant solution was made using 97% H218O (99.8% pure,
Cambridge Isotope Laboratories). The reactor was purged with
flowing O2 or 18O2 and then pressurized to the desired value. A
constant pressure was maintained by continuously feeding O2
or 18O2.
Because the product formation over Pt and Au as a function of
reaction conditions were well described in our previous work,14
those conditions were replicated in the present study, with one
exception. The dioxygen pressure in the experiments utilizing
18O2 was lower here (345 kPa) than in previous work due to
the pressure limit of the 18O2 lecture bottle. Control experiments
without any labeled compound were conducted prior to each
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our lab indicated that exchange of oxygen between the water and
the product can occur during HPLC analysis.16 This exchange
process was avoided by direct injection of the product into the
mass spectrometer. Known compounds, in aqueous solution
with NaOH, were directly infused into the mass spectrometer to
obtain reference spectra. The parent peak for HFCA appeared
at a mass-to-charge ratio (m/z) of 141 (mass of HFCA 142 minus
1, since H+ was removed during ionization). A sodium adduct of
HFCA also appeared at m/z of 163 (mass of HFCA with one Na
atom in place of one H atom, minus one proton removed during
ionization). In the case of FDCA, a prominent peak appeared
at m/z of 177, which corresponds to the mass of FDCA with
a Na atom in place of one H atom, minus one proton removed
during ionization.
To confirm that the products of HMF oxidation did not
exchange O appreciably with liquid water, unlabeled HFCA and
FDCA were dissolved separately in H218O in the presence of
NaOH and Au/TiO2 for the duration of a reaction experiment
(6 h in the case of 0.3 M NaOH and 22 h in the case of
2.0 M NaOH). Separate control experiments were conducted
in both 0.3 M and 2.0 M concentrations of NaOH to replicate
experimental conditions used for the reactions. The mass spectra
of HFCA and FDCA in control solutions confirmed that
negligible amounts of 18O from labeled water (H218O) were
incorporated into the products at standard and high base
conditions.
Fig. 2 Mass spectra of major products from the oxidation of HMF
The presence of hydrogen peroxide formed in a sample
reaction mixture was evaluated by a colorimetric method.17
First, a 1 cm3 sample of the filtered reaction product was
immediately acidified with 1 cm3 of 0.5 M H2SO4, to which
0.1 cm3 of TiO(SO4) (15 wt% in dilute H2SO4, Aldrich) was
added. Absorbance was subsequently measured at 405 nm on
a Varian Cary 3E UV-vis spectrometer. A calibration curve of
absorption versus H2O2 concentration was prepared by diluting
a standard mixture of 30 wt% H2O2.18 The lower limit of H2O2
detection was ~0.005 mM.
(ionized in electronegative mode). Striped bars indicate experiments
conducted in 18O2 and H216O; solid bars indicate experiments conducted
in 16O2 and H218O. In all cases, HMF conversion = 100% and T = 295
K. m/z of major products without 18O: HFCA = 141; Na Adduct of
HFCA = 163; Na Adduct of FDCA = 177. (a) Catalyst: Au/TiO2;
majority product after 6 h: HFCA. Experimental conditions (both cases)
HMF : Au = 150; NaOH : HMF = 2. In 16O2, P = 690 kPa; in 18O2, P =
345 kPa. (b) Catalyst: Au/TiO2; majority product after 22 h: FDCA.
Experimental conditions (both cases) HMF : Au = 100; NaOH : HMF =
20. In 16O2, P = 2000 kPa; in 18O2, P = 345 kPa. (c) Catalyst: Pt/C;
majority product after 6 h: FDCA. Experimental conditions (both cases)
HMF : Pt = 150; NaOH : HMF = 2. In 16O2, P = 690 kPa; in 18O2, P =
345 kPa.
3. Results and discussion
The oxidation of HMF catalyzed by Au/TiO2 under standard
conditions resulted in a majority of HFCA (≥98% selectivity)
being produced after 6 h, as the oxidation of HMF to HFCA
is rapid over Au.14 Previous studies showed no increase in
selectivity to the diacid even if the reaction time was extended
to 24 h.14 To understand the role of O2 during oxidation, an
oxidation experiment over Au/TiO2 was carried out using 18O2.
The major product of oxidation, HFCA, under 18O2 pressure
in H216O showed no incorporation of 18O atoms (Fig. 2a).
A control experiment without O2 was carried out for HMF
oxidation over Au/TiO2 under standard reaction conditions.
The major product, HFCA, was still obtained in significant
selectivity (65%) with the rest of the product mixture being
composed of 2,5-bishydroxymethylfuran (BHMF). Evidently,
in the absence of O2, HMF is both oxidized (to HFCA) and
reduced (to BHMF) in the basic reaction medium via the
Cannizzaro reaction. Similarly, Gorbanev, et al. found 100%
conversion of HMF and 51% selectivity to HFCA (selectivity to
BHMF = 38%, selectivity to levulinic acid = 11%) over 18 h under
similar oxygen-free conditions.6 In the presence of O2, the rate of
oxidation of the aldehyde side chain of HMF on the Au catalyst
was significantly greater than the Cannizzaro reaction so that
nearly complete selectivity to the monoacid HFCA product was
observed. To understand the role of water, HMF oxidation was
carried out in labeled water, H218O. The analysis of the products
from reactions under 16O2 and H218O showed peaks at m/z 145
and 167, corresponding to HFCA and the Na-adduct of HFCA
with incorporation of two 18O atoms (Fig. 2a).
Interestingly, incorporation of multiple 18O atoms in the acid
product was observed during reaction with H218O. Because the
reaction products HFCA and FDCA did not exchange O with
water within the time scale of the reaction, the appearance of
18O in the products during reactions performed in H218O suggests
that aqueous-phase oxidation proceeds through a geminal diol
formed by the reaction of the aldehyde with the solvent. It is well
known that aldehydes in water and base (OH-) rapidly undergo
reversible hydration in two steps: nucleophilic addition of a
hydroxide ion to the carbonyl group, followed by proton transfer
from water to the alkoxide ion intermediate.19 The reversibility
of geminal diol formation accounts for two 18O atoms found in
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Table 1 Results from HMF oxidation over supported Au and Pt catalysts at 295 Ka
c
Catalyst
18O label
Molar ratio HMF : metal
Molar ratio NaOH : HMF
O2 P (kPa)
Time (h)
SHFCAb (%)
SFDCA (%)
Au/TiO2
Au/TiO2
Au/TiO2
Au/TiO2
Au/TiO2
Au/TiO2
Pt/C
none
18O2
150
150
150
100
100
100
150
150
150
2
2
2
20
20
20
2
2
2
345
345
690
345
345
2000
345
345
690
6
6
6
22
22
22
6
6
6
97
98
99
35
30
21
33
32
37
3
2
1
65
69
79
67
68
63
H218O
none
18O2
H218O
none
18O2
Pt/C
Pt/C
H218O
a Conversion of HMF is 100% in all cases. b Selectivity to HFCA. c Selectivity to FDCA.
the HFCA product formed during HMF oxidation (see Fig. 1,
step 1). Incorporation of multiple 18O atoms in reactions
performed with H218O is consistent with the findings of similar
studies of ethanol and glycerol oxidation.16
atoms, the Na-adduct of HFCA with the incorporation of two
18O atoms, and the Na adduct of FDCA with the incorporation
of four 18O atoms. Higher selectivity to the desired product,
FDCA, observed here with Pt/C at standard conditions of low
base and low catalyst loading confirms the noble nature of Au
compared to Pt. The results also suggest that the water solvent
also plays an important role during Pt-catalyzed oxidation,
despite the recognized ability of Pt to dissociate O2.
A proposed mechanism for the oxidation of HMF is depicted
in Fig. 1. As discussed above, molecular oxygen was not
essential for oxidation of the aldehyde side-chain of HMF to
produce HFCA. However, control experiments indicated that
base and a metal catalyst were required to produce FDCA
at the reaction temperature used here. The aldehyde side-
chain is believed to undergo rapid reversible hydration to a
geminal diol via nucleophilic addition of a hydroxide ion to
the carbonyl and subsequent proton transfer from water to the
alkoxy ion intermediate (Fig. 1, step 1). This step accounts for
the incorporation of two 18O atoms in HFCA when the reaction
is performed in H218O. The second step is the dehydrogenation of
the geminal diol intermediate, facilitated by the hydroxide ions
adsorbed on the metal surface, to produce the carboxylic acid
(Fig. 1, step 2).
An oxidation experiment performed in the absence of base
over both Au and Pt catalysts resulted in no conversion of
HMF, indicating the important role of OH- during oxidation
at 295 K. Production of the desired product, FDCA, requires
further oxidation of the alcohol side-chain of HFCA. Base
is believed to deprotonate the alcohol side-chain to form an
alkoxy intermediate, a step that may occur primarily in the
solution.16 Hydroxide ions on the catalyst surface then facilitate
the activation of the C–H bond in the alcohol side-chain
to form the aldehyde intermediate, 5-formyl-2-furancarboxylic
acid (FCA) (Fig. 1, step 3). The next two steps (Fig. 1, steps 4
and 5) oxidize the aldehyde side-chain of FCA to form FDCA.
These two steps are expected to proceed analogously to steps 1
and 2 for oxidation of HMF to HFCA. The reversible hydration
of the aldehyde group in step 4 to a geminal diol accounts for
two more 18O atoms incorporated in FDCA when the oxidation
is performed in H218O. Thus, the sequence in Fig. 1 explains the
incorporation of all 4 18O atoms in FDCA when the reaction is
performed in labeled water.
To obtain high yields of the desired product (FDCA), higher
NaOH concentration, catalyst loading, and dioxygen pressure
were required.14 Thus, reaction conditions (0.1 M HMF, 2.0 M
NaOH, 2000 kPa O2, 295 K) were utilized for the next set of
experiments, except in the case of 18O2 (P = 345 kPa, due to the
pressure limit of the lecture bottle). The product selectivity was
not significantly affected by the lower dioxygen pressure (Table
1). The reaction time was increased to 22 h to accommodate
the slow conversion of HFCA to FDCA. The major product
under these conditions was FDCA (selectivity ≥69%). Again, the
analysis of the product FDCA (Fig. 2b) showed incorporation
of 18O into the product during the experiment with 16O2 and
H218O but not with labeled gaseous oxygen, 18O2 and H216O.
The FDCA peak appeared at m/z equal to 185, indicating
four 18O atoms were incorporated into the Na-adduct of the
product. Although the reaction conditions significantly affected
the product distribution during HMF oxidation, the mode of
oxygen incorporation was unchanged.
A control experiment conducted in the absence of O2 but
under otherwise the same high base conditions (2.0 M NaOH)
resulted in 100% conversion of HMF with 85% yield of HFCA.
Moreover, simply dissolving HMF in 2.0 M NaOH solution (no
O2 ormetalcatalyst)gave97%conversionofHMFand28%yield
of HFCA, with the remainder of species being decomposition
products such as levulinic acid and BHMF. Apparently, the
presence of O2 was necessary to produce FDCA from HFCA in
high base conditions over Au/TiO2 catalysts. Molecular oxygen
was not needed to convert HMF to HFCA.
Platinum is known to deprotonate an alcohol to an alkoxy
intermediate under neutral or mildly alkaline conditions.20 Thus,
HMF oxidation was also studiedover aPt/Ccatalyst at standard
conditions and was ≥60% selective to FDCA after 6 h (Table
1). The mass spectra corresponding to FDCA from labeling
experiments with 18O are reported in Fig. 2c. Mass spectrometry
analysis of the products of HMF oxidation under 18O2 pressure
in H216O contained peaks at m/z values of 141, 163, and 177,
corresponding to HFCA, the Na-adduct of HFCA and the
Na adduct of FDCA, respectively, with no incorporation of
18O atoms. However, analysis of products from reactions under
16O2 pressure in H218O revealed peaks at 145, 167, and 185,
corresponding to HFCA with the incorporation of two 18O
Platinum is known to activate the geminal hydrogen atoms
associated with alcohols.20 Also, supported Pt catalyzes alcohol
oxidation in the absence of base, albeit at a very slow rate.16
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These facts explain the high selectivity to FDCA during HMF
oxidation over Pt compared to Au under identical conditions.
Significantly increasing the concentration of hydroxide ions
available, by increasing the concentration of NaOH in the
reaction medium, facilitates hydrogen abstraction reactions
(both C–H and O–H) on Au surfaces, therefore increasing the
rate of FDCA formation over Au catalysts.
to aldehyde intermediates to eventually form acid products.
Molecular oxygen is required to scavenge the electrons deposited
into the metal catalyst particles during the reaction mechanism,
thus closing the catalytic cycle.
Acknowledgements
This material is based upon work supported by the National
Science Foundation under Grant Nos. OISE 0730277 and EEC-
0813570, and by the United States Department of Energy
under Grant No. DE-FG02-95ER14549. Helpful discussions
with David Hibbitts and Professor Matthew Neurock are also
acknowledged.
Although the role of molecular oxygen in the oxidation
mechanism is not obvious, O2 is essential to produce FDCA
in significant amounts during HMF oxidation over supported
metal catalysts. The precise role of O2 has been debated in
literature, including direct participation of atomic oxygen during
21
dehydrogenation or oxidation steps or, more recently, as an
electron scavenger undergoing reduction to peroxide species
and hydroxide ions.16 Our results from isotopic labeling studies
indicate that molecular oxygen is not directly incorporated
into the products of HMF oxidation but instead hydroxide
ions from water act as a source of oxygen. Also, a test for
the presence of peroxide in the product mixture of a typical
reaction over Au/TiO2 under standard conditions revealed
0.3 mM of H2O2 in solution after 1 h. The presence of
peroxide during oxidation reactions over Au indicates that O2
is reduced during these reactions.22 Activation of O2 occurs
through formation of peroxide intermediates.23 In the next step,
the peroxide intermediates likely undergo further reduction to
form hydroxide species.16 Therefore, O2 is suggested to undergo
reduction by removing the electrons deposited into the metal
particles during the adsorption and reaction of hydroxide ions,
thereby completing the catalytic redox cycle. Although the
reduction of O2 to hydroxide would suggest that 18O species
should eventually be found in the acid products of reactions run
with 18O2 and H216O after many turnovers, no 18O was observed
(Fig. 2). This can be explained by the very low amount of 18O
that would be found in the unlabeled water after conversion of
HMF. Under our conditions and assuming 50% conversion of
HMF with 100% selectivity to FDCA, 0.0023 moles of 18OH-
would be produced from the oxygen reduction reactions. The
amount of 16O species present initially in the H2O is 0.28 moles;
thus, the molar ratio of 18O : 16O species in this example is only
0.008, which is below the sensitivity of our experiment.
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