Efficient conversion of fructose into 5-hydroxymethylfurfural over WO3/reduced graphene oxide catalysts

Article abstract of DOI:10.1039/c6ra26309g

A sustainable and efficient catalyst for converting carbohydrates to a renewable platform chemical 5-hydroxymethylfurfural (HMF) is the goal in the study of biomass recycling. Reduced graphene oxide-supported tungsten trioxide (WO₃/RGO) as an acidic catalyst was synthesized through a one-step hydrothermal method, characterized via TEM, XPS, XRD and Raman spectroscopy and applied to the conversion of fructose to HMF. The WO₃/RGO catalyst showed a highly efficient catalytic activity, and the yield of HMF could reach up to 84.2% with complete conversion of fructose. The catalyst could be reused five times with a slight decrease in activity. Further study indicated that WO₃/RGO could also catalyze the conversion of cellulose, glucose and sucrose to HMF.

Full text of DOI:10.1039/c6ra26309g

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Efficient conversion of fructose into 5-
hydroxymethylfurfural over WO3/reduced
Cite this: RSC Adv., 2017, 7, 3790
graphene oxide catalysts†
*
Huatao Han, Hongyan Zhao, Yang Liu, Zhuofei Li, Jinyi Song, Wenyi Chu
*
and Zhizhong Sun
A sustainable and efficient catalyst for converting carbohydrates to a renewable platform chemical 5-
hydroxymethylfurfural (HMF) is the goal in the study of biomass recycling. Reduced graphene oxide-
supported tungsten trioxide (WO3/RGO) as an acidic catalyst was synthesized through a one-step
hydrothermal method, characterized via TEM, XPS, XRD and Raman spectroscopy and applied to the
conversion of fructose to HMF. The WO3/RGO catalyst showed a highly efficient catalytic activity, and
the yield of HMF could reach up to 84.2% with complete conversion of fructose. The catalyst could be
reused five times with a slight decrease in activity. Further study indicated that WO3/RGO could also
catalyze the conversion of cellulose, glucose and sucrose to HMF.
Received 4th November 2016
Accepted 16th December 2016
DOI: 10.1039/c6ra26309g
www.rsc.org/advances
Graphene, as one of the most promising carbon nano-
materials, provides a template to anchor active species for
Introduction
catalysis due to its unique two-dimensional structure, strong
surface area, and superior mechanical and electrical trans-
mission performance.21–23 Therefore, graphene-based materials
have become the focal points in the growing eld of carboca-
talysis in recent years.24–26 Our previous study proved that
Pd@PdO–NDG27 and Cu NPs@RGO28 could act as an effective
catalyst in organic reactions. In addition, tungsten oxide (WO3),
as an acidic catalyst, had been used for biomass conversion.
Prasenjit Bhaumik et al.29 reported silica-supported WO3 as
a solid acid catalyst in the synthesis of furfural, directly from
lignocellulosic biomass. Yue Liu et al.30 reported that WO3
catalyzed the conversion of cellulose into propylene glycol and
ethylene glycol. Therefore, WO3/RGO, which was prepared by
tungsten trioxide loaded onto the surface of reduced graphene
oxide, could be used as an acidic catalyst with considerable
value and research prospects for conversion of biomass.
The burning of fossil fuels does great harm to the ecological
environment, and the depletion of non-renewable energy makes
humans face a great challenge. Biomass as the sole renewable
carbon source can guarantee the sustainable use of energy.1,2
Therefore, the conversion of biomass into fuels, chemicals
and materials has received broad attention.3–6 5-Hydrox-
ymethylfurfural (HMF), produced from the acid-catalyzed
dehydration of hexose, is an important platform chemical
between carbohydrate biomass and the oil industry.7–10
Although formation of HMF from abundant renewable
carbohydrates, such as glucose and fructose, has been achieved,
scientists are still aiming to research and develop a type of green
and efficient catalyst. A series of acid catalysts, such as various
mineral acids,11,12 metal Lewis acids,13–15 organic acids,16 and
acidic ionic liquids,7,17 were synthesized and used in HMF
production from fructose, glucose and other sugars. Supported
transition metal oxide composites, the same as solid acid
catalysts used in HMF formation, form sugars due to their
Brønsted or Lewis acid sites.18–20 For several acid-catalyzed
conversions of biomass, the acidity of the supported transi-
tion metal oxide plays a crucial role in the reaction. Neverthe-
less, the structural instability under severe conditions and lower
dispersability of a traditional solvent in a carrier material
remains a concern.
In Scheme 1, reduced graphene oxide-supported tungsten
trioxide (WO3/RGO) was designed and synthesized through
School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080,
P. R. China. E-mail: wenyichu@hlju.edu.cn; sunzz@hlju.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra26309g
Scheme 1 The synthesis and application of WO3/RGO.
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a simple one-step hydrothermal method. WO3/RGO, as an acid
catalyst, was studied for conversion of sugars to HMF due to its
acid-catalytic performance. A high HMF yield (up to 84.2%) was
obtained using WO3/RGO catalyst in ionic liquids.
Experimental
Synthesis of WO3/RGO composites
Graphene oxide (GO) was synthesized by the modied
Hummer's31 method. Exfoliate graphite, as the starting mate-
rial, was oxidized by potassium permanganate and concen-
trated sulfuric acid. WO3/RGO composites were synthesized
through a one-step hydrothermal method. GO was dispersed
into deionized water and then sonicated for 0.5 hours to create
a homogeneous dispersion (1 mg mLÀ1). Then, 0.3 g Na2WO4-
$2H2O, 0.05 g sodium dodecyl sulfate (SDS) and 0.05 g NaCl
were dissolved in 20 mL GO dispersion and kept stirring for 1
hour. The pH of the dispersion was adjusted to about 1.5 using
HCl solution. Aer stirring for 3 hours, the suspension was
transferred to a 25 mL Teon-lined stainless steel autoclave and
heated to 180 ꢀC. The suspension was maintained for 24 hours
and naturally cooled to room temperature. The nal product
was washed with deionized water and ethanol, dried at 50 ꢀC for
12 hours and heated at 350 ꢀC in a muffle furnace under
Fig. 1 TEM images of (a) RGO and (b) WO3/RGO, (c) HRTEM images of
WO3/RGO, (d) XRD patterns of the GO, WO3/RGO and pure WO3
samples.
to be a 2D sheet structure with crinkles. As shown in Fig. 1(b),
the WO3 nanoparticles, dispersed on graphene, had a 12–15 nm
average diameter. The HRTEM image in Fig. 1(c) revealed lattice
fringes with a 0.342 and 0.314 nm uniform interlayer distance
corresponding to the (001) and (200) WO3 lattice planes.
X-ray diffraction (XRD) was used to characterize the sample
structure. The XRD of GO, WO3/RGO and pure WO3 are shown
in Fig. 1(d). The diffraction peaks of WO3/RGO and pure WO3
correspond well to a standard XRD spectrum (JCPDS: 33-1387),
which proved their excellent crystalline form. The disappear-
ance of the strong diffraction peak around 10.2ꢀ of graphene
oxide in the diffraction peak of WO3/RGO proved that graphene
oxide was reduced to graphene. There is no evident diffraction
peak of graphene in the WO3/RGO diffraction pattern because
the weak graphene diffraction peaks may be masked by the
WO3 peak.
The FTIR spectra of WO3 and WO3/RGO catalyst are shown in
Fig. 2(a). The prominent broad and strong absorption band at
a high frequency of 3428 cmÀ1 is ascribed to the O–H stretching
vibration. The characteristic absorption bands of GO were
observed at 1047 cmÀ1 (C–OH), 1634 cmÀ1 (C]C) and 1733
cmÀ1 (C]O). The broad absorption band of the WO3/RGO
composite at low frequencies is ascribed to the W–O–W bond
vibration. Moreover, as demonstrated in the WO3/RGO FTIR
spectrum, the GO peaks corresponding to C]O and C–OH
vibrations at 1733 cmÀ1 and 1047 cmÀ1 were not observed. The
absorption band at 1610 cmÀ1 might be attributed to the skel-
etal vibration of the RGO sheets. These results indicated that
the original GO was reduced to RGO in the process of preparing
WO3/RGO.
ꢀ
nitrogen atmosphere for 6 hours with a heating rate of 15 C
minÀ1 under nitrogen atmosphere. The catalyst was obtained
with an 8.7% RGO mass content in the WO3/RGO.
Changing the amount of GO resulted in 4.5% and 12.5% of
RGO obtained in the WO3/RGO. Pure WO3 was obtained under
the same condition without adding GO in the process.
Catalytic reaction procedure
In a typical experiment for the synthesis of HMF from fruc-
tose, 1 mmol fructose was added in 2.0 g of 1-butyl-3-
methylimidazolium chloride ([BMIM]Cl) in a small reaction
vessel, and 10 mg catalyst of WO3/RGO was subsequently added.
The reaction mixture was stirred and kept 120 ꢀC. Aer the
reaction, the mixture was transferred into 5 mL distilled water.
The catalyst was separated via ltration and recycled; the ltrate
was extracted with ethyl acetate (5 mL Â 3). The organic phase
was collected, dried with anhydrous sodium sulphate, and the
solvent was evaporated by rotary evaporator to obtain pure
HMF. The production and aqueous phase were analyzed via
high-pressure liquid chromatography (HPLC).9,10 The yield of
HMF and fructose conversion rate were calculated from a cali-
bration curve.
HMF was identied via 1H NMR spectroscopy. 1H NMR (400
MHz, CDCl3): dH (400 MHz, CDCl3) 9.56 (1H, s), 7.23 (1H, d, J
3.5), 6.52 (1H, d, J 3.5), 4.71 (2H, s).
Results and discussion
Characterization of WO3/RGO catalyst
The Raman spectra of WO3, GO and WO3/RGO are shown in
The morphology and microstructure of the obtained WO3/RGO Fig. 2(b). The D and G peaks of GO and WO3/RGO appear at
were characterized via transmission electron microscopy (TEM) 1352 cmÀ1 and 1573 cmÀ1. In the Raman spectrum of WO3/
and are shown in Fig. 1. Fig. 1(a) shows the TEM image of RGO RGO, sharp peaks belonging to WO3 could be observed. Raman
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Fig. 3 (a) C 1s XPS of GO, (b) the full spectra of WO3/RGO, (c) C 1s of
WO3/RGO, (d) W 4f of WO3/RGO.
35.1 eV was slightly lower than that of pure WO3;33,34 such a shi
might be attributed to the interaction between WO3 and gra-
phene (Scheme 2).
Fig. 2 (a) FT-IR spectra of WO3, GO and WO3/RGO. (b) Raman spectra
of WO3, GO and WO3/RGO.
Catalytic reactions
To evaluate the catalytic performance of WO3/RGO, various
catalysts were used under identical reaction conditions, and the
results are summarized in Table 1. Fructose was converted to
HMF in experimental conditions including 1 mmol fructose,
10 mg catalyst, 2.0 g [BMIM]Cl and 2 hours reaction time.
According to comparison (Table 1, entries 1–3), 8.7% RGO
content in the catalyst obtained the highest fructose conversion
(100%) and HMF yield (82.9%). Humins and polymers were be
formed during the reaction, and the generated HMF was further
hydrolyzed to levulinic acid (LA) and formic acid (FA),9,10 which
would lead to a decrease in the HMF selectivity (Scheme 1).
HMF was hydrolysed into equimolar amounts of LA and FA. The
yield of FA was lower than LA due to the decomposition of FA
during the reaction.6 The reaction without catalyst gave a much
lower HMF yield (Table 1, entry 4). The yield of HMF with GO as
the catalyst was 67.4% (Table 1, entry 5). GO possessed catalytic
activity because it contains large amounts of carboxylic acid
functional groups. When used WO3 with no carrier materials,
the HMF yield was 62.5% (Table 1, entry 6). WO3/RGO exhibited
excellent catalytic performance because the presence of RGO
can increase the contact area between the catalyst and the
bands of 804 cmÀ1 and 688 cmÀ1 correspond to the O–W–O
stretching modes, which are the main characteristic peaks of
WO3 crystallites. The band located at 253 cmÀ1 is related to the
W–O–W bending mode for WO3. Compared with that of pure
WO3, the peak at 688 cmÀ1 was slightly lower, probably due to
the formation of C–O–W bonds between the graphene and WO3
nanoparticles.32 This chemical bond indicated that WO3 was
chemically bonded to the surface of graphene layer via C–O–W
bonds rather than physically interacting with graphene.
X-ray photoelectron spectroscopy (XPS) was used to investi-
gate the surface element composition of catalysts and the cor-
responding valence state. As shown in Fig. 3(a), the C 1s XPS
spectrum of GO was dened by four peaks, corresponding to C
atoms in several representative functional groups of C–C (284.6
eV), C–O (hydroxyl and epoxy groups, 286.6 eV), C]O (287.7 eV)
and the carbonyl C at 288.9 eV. Fig. 3(b) shows the full spectrum
of WO3/RGO catalyst. It could be clearly observed that the
sample consisted of W, O, and C elements without impurities.
The peaks of W 4f, O 1s, C 1s were very close to the XPS results
provided by the literature reported for WO3.33 According to the
C 1s XPS spectrum of WO3/RGO in Fig. 3(c), the carbonyl C at
288.9 eV had disappeared, the C]O (288 eV) and the C–O (286.1
eV) signal decreased markedly in intensity compared with C 1s
of GO. It proved that GO had been reduced to graphene
accompanied by elimination of the oxygen-containing group.
The spectrum W 4f of WO3/RGO in Fig. 3(d) shows two peaks at
35.1 eV and 38.2 eV that could be ascribed to W 4f7/2 and W 4f5/2
,
respectively. These results completely correspond to the valence
of W6+. However, the binding energy value of W 4f7/2 located at
Scheme 2 Fructose conversion into HMF and its by-products.
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Table 1 Catalytic performance of fructose to HMF over different catalystsa
Yieldb (%)
HMF[Ref]
Entry
Substrate
Catalyst
T (ꢀC)
Conversionb (%)
LA
FA
1
2
3
4
5
6
7
8
Fructose
Fructose
Fructose
Fructose
Fructose
Fructose
Fructose
Fructose
Fructose
Fructose
Fructose
Glucose
Sucrosec
Cellulosed
WO3/RGO (4.5%)
WO3/RGO (8.7%)
WO3/RGO (12.5%)
—
GO
WO3
120
120
120
120
120
120
120
80
120
140
170
140
140
180
87.2
100
67.8
84.2
74.3
22.6
67.4
62.5
41.8
2.1
5.2
3.8
0.5
3.4
1.9
9.4
—
2
—
—
0.8
1.1
8.7
0.8
3.1
1.6
<0.1
1.3
0.6
5.2
—
94.2
57.0
86.6
88.0
69.2
100
H2SO4
H3PW12O40
WO3/ZrO2
SBA-15
99.0[12]
94.0[14]
41.9[18]
75.0[11]
36.4
9
100
10
11
12
13
14
89.2
89.0
58.6
70.2
44.6
—
—
<0.1
<0.1
4.9
ZSM-5
WO3/RGO
WO3/RGO
WO3/RGO
51.2
18.8
a
Reaction conditions: substrate (1 mmol), catalyst 10 mg, [BMIM]Cl 2.0 g, 2 hours. b Determined by HPLC analysis. c Sucrose 0.5 mmol, catalyst
10 mg, [BMIM]Cl 2.0 g, 2 hours. d Cellulose 1.5 g, catalyst 100 mg, [BMIM]Cl/H2O 1 : 4 (10 mL), closed system 8 hours, the conversion (wt%) was
calculated by the weight of cellulose before and aer reaction.
substrate. Moreover, large amounts of hydroxyl groups on the As shown in the Fig. 4(b), it was clear that the highest yield
surface of RGO promoted the catalyst dispersion in the solvent. of the reaction that could be achieved was 84.2% with
H3PW12O40 and H2SO4, with strong Brønsted acid sites, could a complete conversion rate of fructose in [BMIM]Cl. In water,
participate in homogeneous catalysis. With H2SO4 as the cata- the obtained yield of HMF (16.4%) was low, whereas the fruc-
lyst, a lower yield of HMF (41.8%) was obtained with more tose conversion (38.2%) and the yield of LA was much higher
formation of LA (9.4%) and FA (5.2%) (Table 1, entry 7). With than others. Only 21% and 38% HMF yields could be obtained
H3PW12O40 as the catalyst, a relatively higher HMF yield of up to using [BMIM]Tf2N, [BMIM]BF4 as solvents, respectively. As is
99.0% was obtained (Table 1, entry 8). Unfortunately, it brought known, DMSO is a common solvent that could be used in
great trouble in the separation of production and recycling
of catalyst. Compared with WO3/RGO, WO3/ZrO2 as a catalyst
gave a higher HMP yield (94.0%) (Table 1, entry 9). With SBA-15
and ZSM-5, the typical solid-acid catalysts, the yields of HMF
were 41.9% and 75.0%, respectively (Table 1, entries 10 and 11).
Glucose, sucrose and cellulose could also be converted to
HMF (Table 1, entries 12–14) and the yields were 36.4%, 51.2%
and 18.8%, respectively. With cellulose as
a substrate,
increasing the temperature increased the cellulose conversion,
but the selectivity of HMF decreased greatly. Cellulose conver-
sions were determined by measuring the difference in the
weight of cellulose before and aer the reaction.30 The molar
quantities of cellulose were calculated based on the molecular
weight of the C6H10O5 unit.
The abovementioned results unequivocally demonstrated
that WO3/RGO was an outstanding acidic catalyst. In order
to further investigate the inuence of the catalyst in the reac-
tion, WO3/RGO was used as a catalyst to optimize the reaction
conditions including catalyst weight, solvents and tempera-
ture. First, different loadings of catalyst including 20 mg,
15 mg, 10 mg and 5 mg were screened (Fig. 4(a)). The use
of 5 mg WO3/RGO catalyst obtained HMF in only 44.3% yield
with 72.5% fructose conversion. The catalyst loading was 10 mg
to give the highest yield of HMF up to 84.2% with complete
conversion of fructose. The yields would reduce slightly
with increased catalyst weight. The solvent, as an important
inuential factor for the catalytic reaction, was also screened.
Fig. 4 Effects of catalyst weight (a) and solvent (b) on the conversion
of fructose to HMF.
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Conclusions
In conclusion, the reduced graphene oxide supported tungsten
trioxide (WO3/RGO) was prepared via a one-step hydrothermal
method and used as an active acid catalyst for fructose to HMF
conversion. A high yield of HMF up to 84.2% with a full fructose
conversion was obtained with the catalyst under the optimum
reaction conditions: 1 mmol fructose, 10 mg WO3/RGO, 2.0 g
[BMIM]Cl, 120 ꢀC for 2 hours. Moreover, it could also transform
cellulose, glucose and sucrose to HMF despite having a rela-
tively lower yield. WO3/RGO was proven to be a more suitable
catalyst for fructose conversion. The catalyst could be reused
ve times with a slight decrease in activity.
Fig. 5 Effects of reaction temperature and reaction time on the
conversion of fructose to HMF.
Acknowledgements
The authors thank the nancial support from the Research
Project of the Natural Science Foundation of Heilongjiang
Province of China (No. B201207 and No. B201208).
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