Hydrotalcite-supported gold-nanoparticle-catalyzed highly efficient base-free aqueous oxidation of 5-hydroxymethylfurfural into 2,5- furandicarboxylic acid under atmospheric oxygen pressure

Article abstract of DOI:10.1039/c0gc00911c

Green synthesis of 2,5-furandicarboxylic acid, one of the most important chemical building blocks from biomass, via oxidation of 5-hydroxymethylfurfural has been demonstrated using hydrotalcite-supported gold nanoparticle catalyst in water at 368 K under atmospheric oxygen pressure without addition of homogeneous base. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0gc00911c

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COMMUNICATION
Hydrotalcite-supported gold-nanoparticle-catalyzed highly efficient
base-free aqueous oxidation of 5-hydroxymethylfurfural into
2,5-furandicarboxylic acid under atmospheric oxygen pressure†
Navneet Kumar Gupta,a Shun Nishimura,a Atsushi Takagakib and Kohki Ebitani*a
Received 12th December 2010, Accepted 24th January 2011
DOI: 10.1039/c0gc00911c
Green synthesis of 2,5-furandicarboxylic acid, one of the
most important chemical building blocks from biomass,
via oxidation of 5-hydroxymethylfurfural has been demon-
strated using hydrotalcite-supported gold nanoparticle cat-
alyst in water at 368 K under atmospheric oxygen pressure
without addition of homogeneous base.
derivatives7 because it has a large potential as a replacement
for terephthalic acid, a widely used component in various
polyesters, and intermediates for other polymers, fine chemi-
cals, pharmaceuticals and agrochemicals.1,3 Therefore FDCA
synthesis is considered to be a representative biorefinery process
as alternatives for chemical production from petroleum.
HMF oxidation into FDCA was examined using stoichio-
8
Renewable biomass resources have considerable potential to
serve as a sustainable supply of fuels and chemicals.1,2 One of the
most important biorefinery processes is the efficient chemical
transformation of the most abundant cellulosic biomass into
chemicals (Scheme 1).3 Nowadays, 5-hydroxymethylfurfural
(HMF), a versatile key intermediate, has been successfully
synthesized from sugars including fructose, glucose and cellulose
using efficient homogeneous biphasic system,4 metal chloride-
containingionicliquids5 and heterogeneous acid–basecatalysts.6
metric oxidant like KMnO4 and homogeneous metal salts
(Co/Mn/Br) by autooxidation under high pressure (70 bar
air),9 which is currently used for terephthalic acid production.
Heterogeneous catalysts also afforded FDCA via HMF oxida-
tion with molecular oxygen. Supported platinum catalysts were
first demonstrated with the aid of homogeneous base, resulting
in near quantitative FDCA yield.10 Direct synthesis of FDCA
from fructose has been also attempted using a solid acid and
PtBi/C in water/MIBK, affording 25% FDCA yield with 50%
selectivity.11 Although high conversion with excellent FDCA
selectivity (99%) was obtained from fructose in the presence
of Co(acac)–SiO2 bifunctional catalyst, reaction was performed
under harsh conditions (433 K, 20 bar).12 Recently, two no-
ticeable examples were reported using supported gold catalysts
for aqueous HMF oxidation.13,14 Gorbanev et al. demonstrated
that Au/TiO2 could oxidize HMF into FDCA in 71% yield
at near room temperature.13 Casanova et al. showed Au/CeO2
was more active and selective.14 However, these catalysts require
addition of homogeneous base (1–20 equiv. NaOH) and high
oxygen pressure (10–20 bar). Alternatively, Taarning et al.
reported production of the diester (2,5-dimethylfuroate) from
HMF through oxidative esterification using Au/TiO2 catalyst in
sodium methoxide-containing methanol solution under oxygen
pressure.15 Casanova et al. demonstrated base-free oxidative
esterification of HMF into the diester using Au/CeO2 catalyst
in methanol under 10 bar oxygen in an autoclave reactor.16
Herein, we report a much more environmentally benign and
safe process for FDCA synthesis using hydrotalcite-supported
gold nanoparticle catalyst (Au/HT). Au/HT catalyst was found
to exhibit remarkable activity for FDCA synthesis via HMF
oxidation in water at 368 K under an ambient oxygen pressure
without addition of homogeneous base. Regarding product
purification, the diester is easily purified compared to FDCA
(diacid) as mentioned by Taaring et al.15 However, the oxidative
Scheme 1 2,5-Furandicarboxylic acid (FDCA) synthesis via oxidation
of 5-hydroxymethylfurfural (HMF).
2,5-Furandicarboxylic acid (FDCA) which is obtained from
HMF via oxidation, is a promising platform of biomass
aSchool of Materials Science, Japan Advanced Institute of Science and
Technology (JAIST), 1-1 Asahidai, Nomi, 923-1292, Japan.
E-mail: ebitani@jaist.ac.jp; Fax: +81-761-51-1625;
Tel: +81-761-51-1610
bDepartment of Chemical System Engineering, School of Engineering,
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656,
Japan
† Electronic supplementary information (ESI) available: Experimental
procedure, XRD, XANES, TEM, color indicator test, and a proposed
mechanism for HMF oxidation. See DOI: 10.1039/c0gc00911c
824 | Green Chem., 2011, 13, 824–827
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Table 1 HMF oxidation in water using supported gold catalysts in the presence of oxygen without addition of homogeneous basea
Yield (%)
Entry
Catalyst
HMF/metal (mol/mol)b
HMF conv. (%)
FDCA selec. (%)
HMFCA
FFCA
FDCA
1
Au/HT
Au/HT
Au/HTd
Au/HTd
Au/HTe
Au/MgO
Au/Al2O3
Au/Cf
40
40
150 (13)
200 (13)
40
40
40
40
40
0
>99
>99
>99
>99
73
>99
35
28
0
0
0
>99
81
83
72
1
21
9
4
0
0
0
0
0
1
3
4
7
13
5
1
0
0
0
>99
81
83
72
1
21
3
1
0
0
0
2c
3
11
12
22
64
65
22
6
0
0
0
4
5
6
7
8
9
Au/SiO2
HT
Blank
10g
11
0
a Reaction conditions: HMF (1 mmol), H2O (6 ml), HMF/metal = 40 (mol/mol), under O2 flow (50 ml min-1), 368 K, 7 h. 1.92 wt% Au/HT. 2 wt% Au
was used for different supports. b Values in the parentheses are reaction time (h). c Air atmosphere. d 1.03 wt% Au. e Catalyst not reduced. f Activated
carbon. g 0.25 g of HT was used.
esterification needs higher temperature than the boiling point
of methanol, such as 403 K, resulting in a requirement for a
pressure-resistant reactor.15,16 In contrast, FDCA formation in
water using Au/HT could be operated even at 368 K using a
conventional glass reactor through atmospheric oxygen flow.
HT, consisting of layered clays with HCO3 and OH- groups
-
on the surface, is known to exhibit high activity for base-
catalyzed reactions such as aldol condensation, Knoevenagel
condensation and transesterification.17 In addition, metals sup-
ported on HTs functioned as excellent catalysts for alcohol
oxidation owing to a combination of creation of active sites
of metal species and basicity of the support.18 Recently Kaneda
Fig. 1 (a) TEM image of Au/HT (Au: 1.92 wt%). (b) Au particle size
distribution.
et al. reported Au/HT acts as an efficient catalyst for oxidation
of various monoalcohols and diols to corresponding carbonyl
compound and lactones in the presence of molecular oxygen as
well as deoxygenation of epoxides into alkenes.19
base. Au/HT catalyst afforded FDCA with >99% selectivity
Au/HT catalyst was prepared by deposition-precipitation
(DP) methods20 using NH3 aqueous solution followed by
calcination at 473 K (see ESI, experimental†). The X-ray
diffraction measurement confirmed that the crystal structure of
Au/HT is identical to that of parent HT (Fig. S1 in ESI†). The
amount of loaded gold was determined to be 1.92 wt% analyzed
by inductively coupled plasma-atomic emission spectrometry
(ICP-AES). Au LIII-edge X-ray absorption near-edge structure
(XANES) study revealed that Au/HT catalyst has completely
reduced Au metal on HT (Fig. S2†). Fig. 1 shows a TEM image
and particle size distribution of Au/HT catalyst, indicating that
Au nanoparticles of 3.2 nm in average size (s = 1.2) are highly
dispersed on HT.
HMF oxidations were carried out in a Schlenk glass tube
attached with a reflux condenser under oxygen flow. In each
reaction the reactor was charged with 1 mmol of HMF in 6 ml
of water. Subsequently, an adequate amount of supported metal
catalyst was added and oxygen was introduced at a flow rate of
50 ml min-1 at 368 K with stirring under atmospheric pressure.
After the reaction, resultant solution was diluted 15–30 times
with water and the solid catalyst was filtered off before the HPLC
measurement (see ESI, experimental†).21,22
at total conversion of HMF (entry 1). Au/HT could catalyze
the reaction at high HMF/metal ratios, 150 and 200 (entries 3
and 4), reaching high turnover numbers of at least 138 (entry 4),
which was simply calculated by moles of Au. It should be noted
that high selectivity of FDCA (81%) was also achieved under
air atmosphere with total HMF conversion at 368 K (entry 2).
In contrast, unreduced Au/HT (without calcination) showed
negligible FDCA yield (1%) (entry 5). The activity of Au/HT
was much higher than those of Au/Al2O3, Au/C and Au/SiO2
(entries 7–9). HT itself could not convert HMF (entry 10). The
neutral supports of Al2O3 and C rarely showed the activity, and
acidic SiO2 was inactive.20 Au/HT also exhibited higher activity
than Au/MgO (entries 1 and 6) although MgO is more basic as
determined by color indicator test (Table S1).23 This indicates
that not only solid basicity of support but also formation of
metal active sites played important roles for the reaction. A TEM
measurement of Au/MgO (Fig. S3†) showed that Au particles
with large size (>10 nm) were aggregated on MgO, which was
responsible for low catalytic activity.
Fig.
2 shows the time course of product formation
for HMF oxidation over Au/HT. By monitoring the re-
action progress with time, we observed the formation of
5-hydroxymethyl-2-furancarboxylic acid (HMFCA) and 5-
formyl-2-furancarboxylic acid (FFCA) intermediates at the
Table 1 lists the results for HMF oxidation in water using
supported gold catalysts without addition of homogeneous
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again rapidly converted via hemiacetal intermediate into FDCA
(Scheme S1, ESI†).14 This mechanism could be supported by
the results of temperature dependence of product distribution
where decrease of HMFCA yield corresponded with increase of
FDCA yield with increase of reaction temperature (Table 2).
The Au/HT catalyst could be reused at least three times
without significant loss of activity (Fig. S4†). The catalyst was
simply reused again after washing thoroughly with water at room
temperature followed by drying in vacuo. The catalytic activity
was almost constant. HMF was completely converted for all
cases and FDCA yields were >99%, 92% and 90% for 1st, 2nd
and 3rd uses, respectively. No change of gold oxidation state,
morphology and particle size of the reused Au/HT catalyst was
observed by XANES and TEM measurements (Fig. S5 and S6†).
The possibility of leaching of the gold catalyst was also verified
by carrying out the oxidation reaction. The reaction was stopped
after 3 h of reaction and catalyst was filtered. The reaction
mixture was again reacted up to 10 h under the same reaction
condition (Fig. S7†). As a result, after the catalyst removal no
change of each product yield was observed. Furthermore, ICP
analysis gave no gold species in the filtrate solution.25 These
results indicate that gold species were not leaching out of the
support.
Fig. 2 Time course of product formation for HMF oxidation over
Au/HT catalyst in water using atmospheric pressure of oxygen at
368 K. Reaction conditions: HMF (1 mmol), H2O (6 ml), HMF/Au =
40 (mol/mol), under O2 flow (50 ml min-1), 368 K. HMF conversion
(closed circle), HMFCA yield (open square), FFCA yield (open triangle)
and FDCA yield (closed diamond).
Fig. 2 shows that at the initial stage of reaction most HMF was
selectively transformed into HMFCA and HMFCA remained at
high yield for a while, as reported by Casanova et al.,14 indicating
that oxidation of the hydroxyl group was much slower than
the aldehyde oxidation. For FDCA synthesis from HMF it is
necessary to oxidize both the aldehyde group and the hydroxyl
group of HMF. In particular, oxidation of the hydroxyl group
considered as the rate-determining step should be improved.
Further study is necessary to develop highly active gold catalyst
on hydrotalcite for oxidation of alcohols in water.
In conclusion, we have found that gold nanoparticles sup-
ported on hydrotalcite (Au/HT) is a highly effective hetero-
geneous catalyst for selective oxidation of HMF into FDCA
using molecular oxygen in water under homogeneous base-free
condition, and the catalyst could be reusable at least three times
without significant loss of activity and selectivity. The role of
the basic support HT on the oxidation could be attributable to
formation of intermediate hemiacetals from aldehydes (HMF
and FFCA) and formation of metal alcoholate species via metal-
hydride shift from HMFCA.
initial stage of the reaction, and other oxidation products
such as the corresponding dialdehyde (furan-2,5-diformylfuran)
were absent. Both intermediates gradually converted into the
final product FDCA (Scheme 1). This tendency is in good
agreement with previous study using Au/TiO2 and Au/CeO2
under different reaction conditions.13,14
For elucidation of reaction mechanism, a radical scavenger
(2,6-di-tert-butyl-p-cresol) was added to the reaction medium,
which hardly influenced the oxidation of HMF (FDCA yield
78%). This result indicates that oxidation of HMF did not
proceed by the free radical mechanism. As shown in Fig. 2,
oxidation of aldehyde groups of HMF to form corresponding
monocarboxylic acid (HMFCA) was easily occurred. The
initial oxidation smoothly proceeded even at room temperature
(HMFCA yield 87%; HMF conversion >99%) (Table 2). This
first step oxidation was very fast via the formation of the
intermediate hemiacetal14 owing to the basicity of HT. For the
synthesis of the dicarboxylic acid (FDCA), the rate-determining
step is considered to be oxidation of hydroxyl group, that is
transformation of HMFCA to FFCA.13,14 HMFCA is converted
to FFCA by the formation of metal alcoholate species via
metal-hydride shift with the aid of basicity of HT.19,24 FFCA
This work was supported by a Grant-in-Aid for Scientific
Research (C) (No. 10005910) of the Ministry of Education,
Culture, Sports, Science and Technology (MEXT), Japan.
Table 2 Effect of reaction temperature on product distribution of
HMF oxidation in water using Au/HTa
Notes and references
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Yield (%)
Reaction temperature/K HMF conv. (%) HMFCA FFCA FDCA
303
323
353
368
>99
>99
>99
>99
87
62
20
0
1
2
2
0
7
31
76
>99
a Reaction conditions: HMF (1 mmol), H2O (6 ml), HMF/metal =
40 (mol/mol), under O2 flow (50 ml min-1), 7 h. 1.92 wt% Au/HT.
826 | Green Chem., 2011, 13, 824–827
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21 10 g L-1 of FDCA was insoluble in water (solubility is ca. 1 g L-1).22
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