Selective conversion of fructose to 5-hydroxymethylfurfural over WO3/SnO2 catalysts

Article abstract of DOI:10.1039/c7nj00725f

Biomass derived 5-hydroxymethylfurfural (HMF) has emerged as an important platform chemical for the production of value added chemicals and liquid fuels that are currently obtained from petroleum. The effective solid acid catalyst WO₃/SnO₂

Full text of DOI:10.1039/c7nj00725f

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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Selective conversion of fructose to 5-hydroxymethylfurfural over WO3/SnO2
catalysts
G. Raveendra, ab* M. Surendarb, and P.S.Sai Prasadb
aKey Laboratory of Coal Science and Technology, Taiyuan University of Technology, No 79,
West Yingze Street, Taiyuan, Shanxi, 030024, P.R. China
bCatalysis Laboratory, Inorganic and Physical Chemistry Division,
CSIRꢀIndian Institute of Chemical Technology, Hyderabadꢀ500 607, India.
Abstract
Biomass derived 5ꢀhydroxymethylfurfural (HMF) has emerged as an important platform
chemical for the production of value added chemicals and liquid fuels that are currently obtained
from petroleum. The effective solid acid catalyst WO3/SnO2 has been synthesized and applied
for the dehydration of fructose to HMF. The synthesized samples were well characterized by
Fourier transformꢀinfrared, Laser Raman, Xꢀray diffraction, BrunauerꢀEmmettꢀTeller (BET)
Surface area, TEM, BarrettꢀJoynerꢀHalenda pore size distribution, Temperature programmed
desorption of NH3 and pyridine FTꢀIR spectra. The effects of various factors such as reaction
time, catalyst weight, and temperature on the formation of HMF have been investigated. Among
those, 20 wt%WO3/SnO2 catalyst exhibited excellent catalytic activity with HMF yield (93%) at
120 °C for 2 h. The acidity measurements reveal that the increased surface acidic site density and
accessibility of the active sites made the catalysts as excellent in the targeted conversion. The
catalyst could be reused, and its activity remained unaffected over five cycles.
Keywords: Tin oxide; Dehydration; Fructose; Dimethylsulphoxide; 5ꢀhydroxymethylfurfural
*Corresponding Author; Phone: +8613643474948;
Email: Eꢀmail address: graveendra476@gmail.com
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1. Introduction
At present, the utilization of the fossil fuels has led to the environmental pollution and
rapidly decreasing the petrochemical reserves.1In search of sustainable sources for the production
of fuels and bulk chemicals, biomass is the most attractive one and it is a widely available
feedstock. Biomass consists of carbohydrates, lignin, fatty acids, lipids and proteins.2 The U.S.A
technology predicts that, the road map of biomass by the year 2020, 18% of all the manufactured
chemicals will be originated from biomass.3 The Lignocellulosic biomass could reduce CO2
emissions since the natural renewable resources provide a closed carbon cycle.4 Lignocellulosic
biomass derived HMF and furfural have emerged as an important platform chemicals.5ꢀ7
Levulinic acid is considered as one of top 12 biomass derived feed stocks by the United States
Department of Energy (DOE) for its applications as fuel additives.8 The substantial efforts have
been focused on converting biomass into HMF.9ꢀ12 Recently, Rosatella et al. also reviewed the
synthesis and application of HMF in different fields of importance.13 The conversion of fructose
to HMF is the acid catalyzed dehydration is receiving much attention.14ꢀ17 HMF is selectively
18, 19
formed from fructose by dehydration in the presence of liquid
solid acid catalysts.20ꢀ22
Acidic catalysts can be used for the production of HMF from fructose in water, aprotic solvents
and biphasic systems. The conversion of fructose to HMF has been carried out in highꢀboiling
organic solvents, such as dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), acetone and
sub critical or high temperature water.23 However, at higher reaction temperatures the yield of
HMF have low owed to side reactions such as HMF decomposition and oligomerization.24 HMF
can be converted in to value added chemicals, HMF oxidation products, 2,5ꢀfurandicarboxylic
acid (FDCA), is a viable replacement for terephthalic acid in the polyethylene terephthalate
(PET) production and adipic acid,25 hydrogenation of HMF results in 2,5 dimethylfuran (DMF)
2

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which is often used as a fuel additive,26 and etherification of HMF to form EMF. EMF is
recognized as an excellent additive for diesel fuel.27
Heterogeneous catalysts can be useful for biomass conversion allowing for high reaction
rates and high selectivity of the target products.28 Tin dioxide (SnO2) is a versatile metal oxide
because of its two unique characteristics: variation in valence state and existence of oxygen
vacancy defects.29 It is one of the most attractive functional materials for supported catalysts
.Tungsten oxide based mixed oxide catalysts are known for their role in acid catalyzed reactions
and also known for their stability.30 In the present study WO3 supported on SnO2, as a solid acid
catalysts. The catalysts characteristics are derived from different spectroscopy techniques and
correlated with observed dehydration of fructose activity. We have found that the WO3/SnO2
catalyst shows high conversion of fructose and higher HMF yield when compared to the reported
solid acid catalysts.
2.0 Experimental
2.1. Chemicals
All the chemicals used in this study were of analytical grade. Fructose (99%) and Dimethyl
sulphoxide obtained from Hychem Sainor and HMF were obtained from SigmaꢀAldrich.
2.2. Catalyst preparation
Hydrothermal method
In this method 1:1 ratio of Sn4+ to cetyl trimethyl ammonium bromide (CTAB), 3.7g (10 mmol)
of tin (IV) chloride, 8.14g (10 mmol) of cationic surfactant (CTAB), and 24 g of urea were
dissolved in 250 mL of distilled water in a beaker under continuous stirring. The solution was
stirred to homogeneity, and then the mixture was transferred to a 100 mL teflonꢀlined autoclave.
The solution was kept at 100 °C for 24 h. The resulting product was washed with water and
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ethanol via certification to remove any impurities, and then dried in an oven at 80 °C overnight.
The dried solid mass was ground into powder using a mortar and pestle before calcination at 400
°C for 4 h. The obtained SnO2 is used as a support and WO3 is impregnated with it by
impregnated method. In a typical procedure, required amount of ammonium metatungstate was
dissolved in water and this solution was added drop wise to the tin oxide with continuous
stirring. The resultant mixture was allowed to stand for 3 h and the excess water was evaporated
on a water bath. The dried catalyst masses were kept overnight for further drying in an air oven
o
o
at 100 C and finally calcined at 400 C for 2 h. The catalysts are denoted as x% WO3/SnO2
where x indicates the content of WO3.
2.3. Characterization of catalysts
The FTꢀIR spectra were recorded on a BioꢀRad Excalibur series spectrometer using the KBr
disc method.
The nature of the acid sites (Bronsted and Lewis) of the catalysts was determined by FTꢀ
IR spectroscopy with chemisorbed pyridine. The pyridine adsorption studies were carried out in
the diffuse reflectance infrared Fourier transform (DRIFT) mode. Prior to the pyridine adsorption
catalysts were degassed under vacuum at 200 °C for 3 h followed by suspending dry pyridine.
Then, the excess pyridine was removed by heating the sample at 120 °C for 1 h. After cooling
the sample to room temperature, FTꢀIR spectra of the pyridineꢀadsorbed samples were recorded.
Xꢀray powder diffraction patterns were recorded on a Rigaku Miniflex diffractometer
using Cu Kα radiation (1.5406oA) at 40 Kv and 30 mA and secondary graphite monochromatic.
The measurements were obtained in steps of 0.045o with count times of 0.5 s, in the 2θ range of
10–80o.
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Confocal MicroꢀRaman spectra were recorded at room temperature in the range of 200–1200
cm−1 using a Horiba JobinꢀYvon Lab Ram HR spectrometer with a 17 mW internal He–Ne
(Helium–Neon) laser source of excitation wavelength of 632.8 nm. The catalyst samples in
powder form (about 5–10 mg) were usually loosely spread onto a glass slide below the confocal
microscope for measurements.
The data on surface area, pore volume and average pore diameter were obtained by
nitrogen adsorption at liquid N2 temperature (ꢀ196 °C) using Autosorbꢀ1 (Quanta chrome, USA)
instrument using standard procedure. The powders were first outꢀgassed at 200 °C to ensure a
clean surface prior to construction of the adsorption isotherms.
The acidity of the catalysts was measured by temperature programmed desorption of
ammonia (TPD of NH3). In a typical experiment, 0.1 g of catalyst was loaded and pretreated in a
o
o
flow of He at 300 C for 1 h. After pretreatment the temperature was brought to 100 C and the
adsorption of NH3 is carried out by passing a mixture of 10% NH3 balanced He gas over the
catalyst for 1 h. The catalyst surface was flushed in He gas at the same temperature for 1 h to
flush off the physisorbed NH3. TPD of NH3 was carried within the temperature range of 100 to
o
800 °C at ramp of 10 C/min and desorption of ammonia was monitored using a thermal
conductivity detector (TCD) of a gas chromatograph.
Transmission electron microscopy (TEM) and highꢀresolution TEM (HRTEM) experiments were
performed on a JEOL JEMꢀ2100 electron microscope, operating at 200 kV. The powder samples
were ultrasonically dispersed in ethanol at room temperature for 30 min and transferred onto a
carbonꢀcoated copper grid by dipping.
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2.4. Catalytic activity tests
All fructose dehydration reactions were carried out in 15 mL sealed tube. This system
was heated with a thermostatically controlled oil bath. In a typical procedure, fructose (360 mg)
was dissolved in DMSO (5ml) and then the catalyst (50 mg) was added into the fructose
solution. The resulting mixture was charged into a reactor and it was placed in the oil bath at 120
◦C. After the reaction, the mixture was decanted into a volumetric flask using H2O as diluter, the
solution was shaken well and after the settling of insoluble precipitates (humins), the upper
liquid was filtered through 0.2 ꢁm Nylon syringe filters. The filtered sample was analyzed using
HPLC.
2.5. HMF analysis
All reactions were monitored by HITACHIꢀHPLC system equipped with binary 2130
pumps, a manual sampler, and 2490 refractive index detector, maintained at 50 oC. The products
were separated in sugar column (250 × 4.8 mm) ionꢀexclusion column, maintained at 60 oC using
water as mobile phase with 0.8 mL/min flow rate. The HPLC system was controlled and
processed by Inkarp software. Standard 5ꢀHMF, fructose were prepared and calibration doing.
Each product sample was diluted with a known volume of milliq water before analysis to prevent
the overloading of the column. All experiments were done in triplicates, and the average values
were reported standard deviations of triplicates were <2.0%.
3.0 Results and discussion
3.1 Catalyst characterization
As we all know, specific surface area is an important factor that affects the catalytic
activity, because catalyst with larger surface area can provide more active sites that are beneficial
for catalytic reactions. The BET specific surface area, pore volume, and mean pore diameter of
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the catalysts with different loadings of WO3 on SnO2 are shown in Table 1. The surface area is
strongly influenced by the loading of tungsten oxide. After calcination at 400 °C, the specific
surface area and pore volume decreased gradually with increasing WO3 content confirming the
31
stabilizing effect of tungsten oxide on SnO2. The gradual decrease in the surface area with
increase in WO3 content is related to the agglomerisation of WO3 on SnO2.
Table 1. Textural parameters and acidity of WO3/SnO2 catalysts
Catalyst
Surface area
(m2/g)
Pore volume
(cm3/g)
Pore
diameter(nm)
Acidity
(mmol/g)a
5%WO3/SnO2 58.4
10%WO3/SnO2 53.8
15%WO3/SnO2 50.9
20%WO3/SnO2 48.8
25%WO3/SnO2 46.9
0.124
0.117
0.090
0.078
0.070
6.89
6.20
5.84
3.95
3.93
0.80
1.12
1.36
2.12
0.59
a Acidity can be calculated from NH3ꢀTPD.
The XRD patterns of tin oxide and WO3/SnO2 catalysts are illustrated in the Fig. 1. The
characteristic diffraction peaks of pure SnO2 at 2θ angles of 26.08, 33.46, 37.42, 51.37, 54.7,
61.9 and 65.9° can be clearly assigned to the (110), (101), (200), (211) (220), (310), and (301)
planes of crystalline SnO2 respectively. A matching of the observed and standard (hkl) planes
confirmed that SnO2, has a tetragonal structure (JCPDS card no. 41–1445). Interestingly tungsten
impregnated on SnO2 samples exhibited a typical tetragonal phase of SnO2. A closer observation
reveals that the XRD patterns of the impregnated SnO2 samples are broad and shifted to higher
32
angles compared to pure SnO2. At this point, it must be emphasized that the broadening of the
diffraction peaks distinctly reflects the nano crystalline nature of the samples.33 The shift of
peaks towards higher angles can be elucidated by the ionic radii disparity of the host (Sn4+∼0.71
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Å) and guest metal ions (W6+∼0.56 Å). Hence, the doping of smaller sized W6+ ions into the
SnO2 lattice is expected to cause lattice contraction.34 However, when the WO3 loading was
gradually increased to 25wt. %, the intensity of the characteristic diffraction peaks of WO3 at 2θ
angles of 23.2, 23.8 and 24.4° increased.35 This XRD observation indicated that the WO3 was
incorporated into SnO2 and the WO3/SnO2 catalyst was formed as mixed oxides.36 Thus, the
incorporation of tungsten compounds to SnO2 material could result in an enhanced acidity of the
solid catalyst and hence increase the catalytic activity.37
*
*
*
*
@
*
*
*
(f)
(e)
(d)
(c)
(b)
(a)
10
20
30
40
50
60
70
80
2 Theta (degree)
Figure 1 XRD patterns of WO3/SnO2 catalysts with (a) SnO2 (b) 5%, (c) 10 %,(d) 15 %,(e) 20
%, (f) 25% of WO3.
* ꢀTetragonal phase of SnO2, @ꢀ monoclinicꢀ WO3.
The FTꢀIR spectra of SnO2 and WO3/SnO2 catalysts (Fig. 2) are interpreted as follows.
The broad peaks at around 625 and 547 cm−1, corresponding to different vibrational modes of O–
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Sn–O and Sn–O–Sn groups.38 The main WO3 vibrations are found in the 1700–380 cmꢀ1 IR
region. All synthesis samples present characteristic peaks at 885 and 963 cmꢀ1 associated with
υ(WꢀOꢀW), υ(WꢀO, W=O) respectively.39 The band at 1626 cm−1 is assigned to the bending
vibrations of hydrogenꢀbonded surface OH groups of the physically adsorbed water molecules.40
(f)
(e)
(d)
(c)
(b)
(a)
1600
1400
1200
1000
800
600
400
Wave number ( )
cm-1
Figure 2 FTꢀIR spectra of WO3/SnO2 catalysts (a) SnO2 (b) 5%, (c) 10 %, (d) 15 %,
(e) 20 %, (f) 25 %,
Raman spectrum is a valuable tool for the characterization of dispersed metal oxides as it
detects vibrational modes of surface and bulk structures. For the phase identification of tungsten
oxides this is a very useful technique. The Raman spectra of SnO2 and WO3/SnO2 catalysts are
shown in Fig. 3. Pure SnO2 displayed primarily three bands at around 474, 630 and 770 cm−1 are
consistent with the vibration modes of A1g, Eg and B2g, respectively, in good agreement with the
tetragonal rutile structure. The Eg (474 cm−1) corresponds to the vibration of the oxygen atom in
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the oxygen plane and the other two modes, A1g (630 cm−1) and B2g (770 cm−1) are related to the
expansion and contraction vibration mode of Sn–O bonds.41 Interestingly, the prominent A1g
band of doped SnO2 samples was shifted to higher wave numbers than that of pure SnO2, which
could be attributed to the interaction between W species and SnO2.42
Figure 3 Raman spectra of WO3/SnO2 catalysts (a) SnO2 (b) 5%, (c) 10 %, (d) 15 %,
(e) 20 %, (f) 25 %.
The band at 449 cmꢀ1 can be attributed to the presence of W–OH2 bonds.43 The Raman bands of
W–O–W are at ~590 and ~215 cmꢀ1 ) which are indicative of polymeric structure at higher
surface coverage.44 The weak band at around 1035 cmꢀ1 indicates a small fraction of tetrahedrally
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coordinated tungsten oxide is present in catalyst. According to GutiérrezꢀAlejandre et al these
surface tetrahedral wolframyl species were coordinatively unsaturated and acted as strong Lewis
acid sites, this is beneficial for high catalytic activity.45 The Raman band at 934ꢀ933 cmꢀ1 is
assigned to the symmetric W=O stretching mode of the WO4 site respectively.46 The Raman band
at 998 cmꢀ1 are consistent with a dehydrated isolated surface WO4 structure with dioxo O=W=O
coordinated species.47 The asymmetric stretching of the W–O bonds, resulting in a Raman shift
of the 750 cm–1 feature a weaker peak at 220 cmꢀ1 that we attribute to the vibrations of the
W4+=O bonds.48, 49 The major Raman bands at 269, 320, 714, and 808 cmꢀ1 are assignable to Wꢀ
O stretching mode, WꢀO bending mode and the WꢀOꢀW deformation mode respectively of
crystalline WO3. These bands were observed for the catalyst more than with 20% WO3 revealing
that the monolayer coverage was exceeded.50, 51
NH3 acts as an excellent probe molecule to examine the acidic properties of
heterogeneous solid acids. The strong basicity and small molecular size of NH3 facilitates the
easy detection of acidic sites present in the narrow pores of the solids.44 The NH3ꢀTPD profiles
of WO3/SnO2 catalysts are shown in Fig. 4. The total number of acidic sites were calculated with
the help of NH3 desorption peak areas and the obtained values are presented in Table 1. In the
case of WO3/SnO2 catalysts a small desorption peak was observed at low temperature due to
weak acidic sites. As the WO3 loading increased to 10% a desorption peak appeared at 385ꢀ480
°C region due to presence of moderate acidic sites. Further increase in loading to 15wt% resulted
in a desorption peak appearing in the 600ꢀ700 °C region due to strong acidic sites. The 20%
WO3/SnO2 catalyst showed unresolved broad peak between 350ꢀ650 °C. This peak suggests the
presence of both moderate and strong acidic sites. With further increase in loading to 25% a
small peak appeared between 600ꢀ700 °C due to strong acid sites. The maximum acidity
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observed at this tungsten concentration is in agreement with the observations on tungsten loaded
on different supports, reported earlier.52 The high acidity is speculated to be the result of strong
interaction between the support and the WO3. The diminishing acidity suggests that bulk nature
of WO3 on tin oxide. As a whole TPD results suggests the presence of wellꢀdispersed WO3
clusters on SnO2 leads to generation of maximum number moderate to strong acidic sites.
Figure 4 NH3ꢀTPD of WO3/SnO2 catalysts (a) 5%, (b) 10 %, (c) 15 %, (d) 20 %, (e)
25 %WO3/SnO2 catalysts
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The NH3ꢀTPD measurements provide information about the acidic strength of the
catalysts. Concurrently, the identification of Brønsted and Lewis acidic sites is rather difficult
from NH3ꢀTPD studies. Pyridine FTꢀIR was employed to characterize the acidic nature of the
supports and catalyst (Fig.5). All samples show a variety of bands in the region of 1600−1400
cm−1. The band at 1541 cm−1 is known to be due to N+ ꢀH stretching of pyridinium ions and is
used to characterize the protonic acid sites.53 The bands at around 1438, 1457, 1508, 1558 and
1577 cmꢀ1 appeared which can be assigned to coordinatively bonded pyridine with Lewis acid
sites on catalysts sample the acidity exists because of the frame work Sn4+ sites.54, 55 The bands
1488 and 1535 cm−1 are simultaneously associated with both Brønsted and Lewis acid sites.56
The existence of a high amount of Brønsted acidic sites reveals a key role in the dehydration of
fructose. At lower loadings of WO3 on tin oxide, the intensity of these bands was very low and
an increase in these bands was observed with increase in WO3 loading. These results evidently
reveal that all WO3 supported on SnO2 samples contain both Brønsted and Lewisꢀacidic sites, of
which more number of Brønsted acidic sites could be found compared to that of Lewis acidic
sites over all samples.
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(f)
(e)
(d)
(c)
(b)
(a)
1450
1500
1550
1600
1650
Wavenumber (cm-1)
Figure 5 Pyridine IR of WO3/SnO2 catalysts (a) 5%, (b) 10 %, (c) 15 %, (d) 20 %,
(e) 25 %WO3/SnO2 catalysts
The WO3/SnO2 catalysts were investigated by using TEM analysis and the images are
o
shown in Fig. 6. The black spots of WO3 is well dispersed on SnO2 in 400 and 500 C
calcinations temperature samples whereas in the case of 600 oC sample shows little
agglomerisation of WO3 on SnO2. The results clearly explain the agglomarisation of WO3 on
SnO2 at high calcinations temperature samples.
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Figure 6 TEM analysis of calcined 20wt%WO3/SnO2 catalysts (a) 400 oC (b) 500 oC,(c) 600 oC
3.2 Catalytic performance of catalysts
3.2.1 Effect of WO3 loading on SnO2
The prepared SnO2 and WO3/SnO2 catalysts were investigated for the dehydration of
fructose to HMF and the obtained results are shown in Fig. 7. Pure SnO2 gave only 40% HMF
yield and it denoted as zero percent in the graph.The blank reaction of fructose dehydration in
o
the absence of WO3/SnO2 catalysts at a reaction temperature of 120 C only 20% yield of HMF
is formed in 120 min of reaction time.With increasing WO3 loading from 5 to 20 wt.%, the
fructose conversion was gradually improved from 32.3 to 100%. The yield of HMF increased
gradually with increase in the loading of WO3 and the maximum yield was obtained on the
catalyst with 20 wt% WO3 on SnO2.
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100
80
60
40
20
0
100
80
60
40
20
0
HMF Yield
HMF sel
0
5
10
15
20
25
Catalyst loading (%)
Figure 7 Effect of loading of WO3 on HMF yield
(Reaction conditions: Fructose (0.36 g), dimethyl sulphoxide (5 g), Catalyst
weight (0.05g), Reaction temperature (120 oC), Reaction time (2 h))
However, the catalytic activity was not ameliorated by further increasing the WO3 loading to 25
wt.%. The observed decrease in the activity upon the increase of the WO3 loading to 25wt%
could be attributed to both the decrease in the specific surface area and the surface acidity. These
results are in good agreement with the literature, where it was found that higher number of
surface acid sites leads to a better catalytic performance in this reaction. The maximum activity
of 20 wt% WO3/SnO2 catalyst could be explained by the results obtained from XRD, Raman and
acidity measurements.
3.2.2 Effect of calcination temperature
The influence of calcination temperature on the catalyst activity is illustrated in Fig. 8.
From the results obtained, it was found that the calcination temperature affected significantly the
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catalytic activity of the catalysts. The yield of HMF decreased with increase of calcination
temperature of the catalyst beyond 400 °C. At a lower calcination temperature the tungsten
species might have been wellꢀdispersed on the support. However, further increase in
o
calcination temperature to 600 C led to a drop in the conversion to 77.3%. Therefore, the
optimal calcination temperature for the catalyst was 400 °C. At this calcination temperature, the
highest conversion of 100% was achieved and HMF yield is higher 91%. When the calcination
temperature increased the agglomeration of W species took place and it increased the
crystallization on the support. When the calcination temperature higher than 400 °C resulted in
increased crystallinity, which was clearly seen in Raman spectra and XRD patterns.57 NH3ꢀTPD
also revealed higher acidity at lower calcination temperature with moderate and strong acidic
sites. With further increase in calcination temperature the acidity of the catalysts decreased. The
activity depended on its acidity particularly at lower calcination temperatures (400 °C),
simultaneously offering a higher HMF yield.
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Fructose conv (%)
HMF Yield (%)
100
80
60
40
20
0
400
500
600
Calcination temperature (oC)
Figure 8 Influence of calcination temperature of 20wt%WO3/SnO2 catalyst on the
dehydration activity of fructose.
(Reaction conditions: Fructose (0.36 g), dimethyl sulphoxide (5 g), Catalyst weight (0.05g),
Reaction temperature (120 oC), Reaction time (2 h))
The XRD patterns indicating the effect of calcination temperature of 20 wt% WO3/SnO2
catalysts are shown in Fig. 9. With the increase of calcination temperature from 400 to 600 °C
crystalline phase of tungsten oxide was observed. The strong intensities of the diffraction peaks
indicated a crystalline nature of the catalyst. The diffraction peaks at 23.1, 23.7, 24.4, and 28.9°
can be assigned to the (002), (020), (200), (122), planes of the monoclinic WO3, respectively.58 It
has been suggested that agglomeration of WO3 species leading to WO3 microcrystallites on the
tin oxide surface occurs when the tungsten coverage exceeds its monolayer capacity. These
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observations were further confirmed by Raman spectroscopy. The XRD results suggested that
the crystalline nature increased with increase in calcination temperature.
*
*
*
*
@
*
*
*
@
(c)
(b)
(a)
10
20
30
40
50
60
70
80
2 Theta (degree)
Figure 9 XRD patterns of calcined 20wt%WO3/SnO2 catalysts (a) 400 oC (b) 500 oC (c) 600 oC.
* Tetragonal phase of SnO2, @ Monoclinic WO3
The Raman spectra of calcined 20% WO3/SnO2 catalysts are shown in Fig. 10. The
lower calcination temperature the main bands observed at 267, 322, 714, and 808 cmꢀ1 are
assignable to WꢀO stretching mode, WꢀO bending mode and the WꢀOꢀW deformation mode
50, 51
respectively of crystalline WO3.
The band at 449 cmꢀ1 can be attributed to the presence of
W–OH2 bonds.43 The Raman bands of W–O–W are at ~590 and ~215 cmꢀ1 which are indicative
of polymeric structure at higher surface coverage.44 The Raman band at 934ꢀ933 cmꢀ1 is assigned
to the symmetric W=O stretching mode of the WO4 site respectively.45 With increasing
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calcination temperature this bands disappeared. The Raman band at 998 cmꢀ1 are consistent with
a dehydrated isolated surface WO4 structure with dioxo O=W=O coordinated species.46 With
o
increase in calcination temperature from 400 to 600 C, two bands at 713 and 808 cmꢀ1 resulted
the O–W–O stretching mode.59 The band at 272 cmꢀ1 is assigned to the bending vibrations of W–
60
O–W, these bands are characteristic of monoclinic phase of WO3. These results are good
agreement with the results obtained by XRD.
(c)
(b)
(a)
200
400
600
800
1000
1200
Raman shift (cm-1)
Figure 10 Raman spectra of calcined 20wt%WO3/SnO2 catalysts (a) 400 oC (b) 500 oC (c) 600oC
NH3ꢀTPD patterns of 20wt%WO3/SnO2 catalyst calcined at different temperatures are
o
shown in Fig. 11. The catalyst calcined at 400 C showed the desorption peak in between 400ꢀ
o
o
600 C due to strong acidic sites. With further increase in calcination temperature to 500 C a
small peak at 530 oC was observed. At a higher calcination temperature of 600 oC the sample did
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not show any desorption peak.61 The results made from the XRD, Raman and TPD of NH3
suggests that the catalyst calcined at 400 °C has high acidity due to the presence of well
dispersed WO3 on SnO2 surface. The high activity of the catalyst is related to its surface and
structural characteristics, which are directing the acidity of the catalysts. Lower yield of HMF for
the catalyst calcined at higher temperature related to decrease in acidity due to the formation of
monoclinic phase of WO3.
Figure 11 NH3ꢀTPD of 20wt%WO3/SnO2 catalyst calcined at (a) 400 oC (b) 500 oC (c) 600 oC
3.2.3 Effect of reaction temperature
The effect of reaction temperature on dehydration of fructose to HMF was studied and
the results are shown in Fig.12. The reaction temperature strongly affected the conversion. The
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HMF yield at different reaction temperatures from 80 to 140 °C was determined. The increase in
reaction temperature from 80 to100 °C, resulted in an increase in the yield of HMF. Between 100
and 120 °C higher HMF yield was observed. With further increase in temperature from 120 to
140 °C the HMF yield drastically decreased due to the formation of insoluble humins.62 The
reaction temperature 120 °C is observed to be the best suitable for obtaining the highest HMF
yield.
Fructose Conv (%)
HMF Yield (%)
100
80
60
40
20
0
80
100
120
140
Temperature (oC)
Figure 12 Influence of reaction temperature on the dehydration activity over 20wt% WO3/SnO2
catalyst, (Reaction conditions: Fructose (0.36 g), dimethyl sulphoxide (5 g), Catalyst
weight (0.05g), Reaction temperature (80 to140 oC), Reaction time (2 h))
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3.2.4 Effect of reaction time
The dehydration of fructose to HMF was studied as a function of reaction time and the
results are shown in Fig. 13. The yield of HMF increased from 25% to 93% upon increasing the
duration from 30 min to 2 h. At shorter reaction times the HMF yield was very low. After
increasing the reaction time from 30 min to 1 h the yield of HMF was 56%. With further increase
to 2 h the HMF yield was 93%. After 2.5 h of reaction time the HMF yield decreased. When the
reaction time was longer than 2 h, the yield of HMF slightly dropped owing to the formation of
undesired side products. By analyzing the reaction products it was noticed that surprisingly, no
rehydration byproducts such as levulinic acid and formic acid were formed, suggesting that the
yield of HMF decreased probably due to selfꢀpolymerization of HMF molecules.
100
Fructose conv (%)
HMF Yield (%)
80
60
40
20
0
0.5
1.0
1.5
2.0
2.5
Reaction time (h)
Figure 13 Effect of reaction time on dehydration of fructose over 20wt% WO3/SnO2
catalyst
(Reaction conditions: Fructose (0.36 g), dimethyl sulphoxide (5 g), Catalyst weight
(0.05g), Reaction temperature (120 oC) Time 0.5 ꢀ2.5h)
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The color of the reaction mixture gradually turned from pale yellow to deep brown when
the dehydration of fructose was continued for a longer reaction time. Perhaps, the color change
of the reaction mixture was associated with the decomposition of HMF into side products as
reported in the literature.63 Therefore; a reaction time of 2 h is optimum under the given
experimental conditions.
3.2.5 Effect of catalyst weight
The effect of catalyst weight on fructose conversion to HMF was investigated and the
results are shown in Fig. 14. The catalyst weight 25 mg the yield of HMF is 45%, further
increase in catalysts weight 50 mg the yield of HMF increases to 93% (120 °C for 2 h,). The
increase of the catalyst weight means that there are more available active sites for this reaction.64
When the catalyst weight increases from 50 to 100 mg the conversion of fructose and the HMF
yield decreases. More catalyst amount resulted in the continued decreasing of HMF yield, due to
the polymerization of fructose, intermediate and HMF degradation under acid conditions65.The
excess catalyst might have promoted the polymerization between HMF molecules. Hence, an
appropriate amount of catalyst is crucial for attaining higher HMF yield, which is 50 mg.
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100
80
60
40
20
0
Fructose conv(%)
HMF Yield (%)
25
50
75
100
Catalyst weight (mg)
Figure 14 Effect of catalyst weight on HMF yield over 20wt%WO3/SnO2 catalyst (Conditions:
Fructose (0.36 g), dimethyl sulphoxide (5 g), Catalyst weight (25 to 100 mg), temperature (120
oC))
3.2.6 Reusability of the catalyst
The reusability of solid catalysts is one of their main advantages over liquid homogeneous
catalysts, and it is very important aspect of any industrial process. The stability of catalysts was
tested by recycling the WO3/SnO2 catalyst by for five times and the reaction conditions remained
the same as in Fig.15. After each reaction, the catalyst was separated from the reaction mixture
by centrifugation and washed with water to remove any adsorbed reaction ingredients from the
catalyst surface. The catalyst was reused for five catalytic cycles. After the fifth cycle, the loss in
catalyst activity was marginal. The catalyst retained its activity during reuse. This is noteworthy
that the catalyst exhibited reusability without any preꢀtreatment. This indicates that high activity
of the present catalyst for dehydration of fructose.
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100
80
60
40
20
0
1
2
3
4
5
Number of cycles
Figure 15 Reusability studies on 20wt % WO3/SnO2 catalyst during
Dehydration of fructose
3.2.7 Comparison of WO3/SnO2 catalyst with reported solid acid catalysts
Catalytic activity of WO3/SnO2 catalyst is compared with other reported solid acid
catalysts and comparative results are presented in Table 2. The conversion of fructose to HMF in
DMSO solvents at 120 °C for 2 h.66 The commercial HꢀY zeolite catalyst gave 76% yield of
HMF and 100% fructose conversion, and ion exchange resins such as Nafion and Amberlystꢀ15
gave same HMF yields 75 and 76%. Heteropolyacid based iron exchanged tungstophosphoric
acid catalyst gave 49% yield of HMF and 100% conversion of fructose. From these results
suggests that at same reaction temperature and time the yield of HMF was decreased from top to
bottom. The 20%WO3/SnO2 catalyst is a superior catalyst for the production of HMF under our
reaction conditions, which gave the higher yield of HMF yield (93%).
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Table 2. Comparison of WO3/SnO2 catalyst activity with reported solid acid catalysts
Catalyst
Temperature
Time
Fructose conv HMF Yield
Ref
(%)
(%)
20%WO3/SnO2
120 °C
2h
100
93
Present
work
HꢀY zeolite
Nafion
120 °C
120 °C
120 °C
120 °C
2h
2h
2h
2h
100
100
100
100
76
75
76
49
65
65
65
65
Amberlyst ꢀ15
FePW12O40
4.0 Conclusions
In summary, SnO2 was synthesized by hydrothermal method. The WO3/SnO2 catalyst
with different WO3 loadings was synthesized and evaluated in the dehydration of fructose to
HMF. The catalyst showed good catalytic performances in the dehydration of fructose to HMF
under mild conditions, e.g., 100% fructose conversion can be obtained in 2 h and HMF yield can
o
reach 93% at 120 C. Nonetheless, excessive amounts of catalyst and high temperature resulted
in slight decrease in HMF yield due to the further condensation of HMF into byproducts. When
the calcination temperature exceeds 400 °C the catalyst shows monoclinic phase of WO3 is
formed. The monoclinic phase of WO3 is inactive giving lower yield of HMF. XRD also reveals
that at higher calcination temperature monoclinic phase of WO3 is formed.NH3ꢀTPD studies on
20wt%WO3/SnO2 reveal the condition for achieving suitable acidic sites as the active centres and
the unique surface textural properties. When the catalyst loading of WO3 is beyond 20% the
dispersion decreases due to the appearance of crystalline WO3. There was no significant loss in
activity after five cycles. The catalyst was simply reusable and offers an environmentally
friendly process for the synthesis of HMF.
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Acknowledgements
The authors GR and MS thanks to Council of Scientific & Industrial Research (CSIR), New
Delhi, India.
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