Microwave assisted rapid conversion of fructose into 5-HMF over solid acid catalysts

Article abstract of DOI:10.1039/c5ra22979k

In this work, we investigated the dehydration of fructose into 5-hydroxymethylfurfural using solid catalysts with microwave assistance in dimethyl sulfoxide. Five kinds of solid catalyst ZrO₂, WO_x/ZrO₂, MoO_x/ZrO₂, SO₄^2-/WO_x-ZrO₂ and SO₄^2-/MoO_x-ZrO₂ were prepared and characterized by XRD, UV-DRS, FTIR, XPS, TEM, TPD, BET, and Raman and the surface acid amount was obtained by combining pyridine adsorption and UV spectrometry. The SO₄^2-/WO_x-ZrO₂ catalyst was found be the most active catalyst, and a fructose conversion of 95.80% with 83.90% 5-HMF yield was obtained at 150 °C in a relatively short reaction time of 5 min. The value of the activation energy was comparable with previous values reported in the literature, implying that SO₄^2-/WO_x-ZrO₂ was efficient for the dehydration of fructose to 5-HMF and that microwave radiation heating had a remarkable accelerating effect on the fructose conversion.

Full text of DOI:10.1039/c5ra22979k

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Microwave assisted rapid conversion of fructose into 5-HMF over
solid acid catalysts
Received 00th January 20xx,
Accepted 00th January 20xx
Jie Wang,a Ting Qu,b Minsi Liangc and Zhenbo Zhaoa
※
This work, we investigated the dehydration of fructose into 5-hydroxymethylfurfural by using solid catalysts under
microwave-assisted in dimethyl sulfoxide. Four kinds of solid catalysts such as ZrO2, WOx/ZrO2, MoOx/ZrO2, SO42-/WOx-ZrO2
and SO42-/MoOx-ZrO2 were prepared and characterized by XRD, UV-DRS, FTIR, XPS, TEM, TPD, BET, Raman and the surface
acid amount was obtained combining pyridine adsorption and UV spectrometry. The SO42-/WOx-ZrO2 catalyst was found be
the most active catalyst, and a fructose conversion of 95.80% with 83.90% of 5-HMF yield was obtained at 150°C for a
relatively short reaction time of 5 min. The value of activation energy was comparable with previous values reported in the
literature, this implies that the SO42-/WOx-ZrO2 was efficient for the dehydration of fructose to 5-HMF and microwave
irradiation heating had a remarkable accelerating effect on the fructose conversion.
DOI: 10.1039/x0xx00000x
www.rsc.org/
properties for the aqueous hydrolysis of cellobiose, it revealed that
WOx/ZrO2 catalysts, well known for their high acid surface
chemistry and the presence of acidic sites of different natures(i.e.
1. Introduction
The decrease of fossil fuel reserves and deterioration of the social
environment drive people to seek sustainable resources. As an
abundantly available, cheap, renewable and low sulfur content
resource, biomass not only can as fuels, but also can be converted
into chemical intermediates for the production of various chemicals
and liquid fuels. In the conversion of biomass, the study of
dehydration of C6-sugars into 5-hydroxymethylfurfural(5-HMF) has
received considerable attention.1-2 5-HMF has furan ring structure,
aldehyde group, hydroxyl group and conjugated diene, therefore it
can through hydrolysis, selective oxidation, hydrogenation and
esterification, etc. methods to obtain a series of derivatives, such as
2,5-diformylfuran, levulinic acid,3 2,5-furandicarboxylic acid,4 etc.,
which are widely used in the areas of drugs, fuels, and polymeric
materials.5-6
Lewis acid sites from zirconia and both Brönsted and Lewis sites
from WOx), could potentially be good candidates for the near-
boiling water phase biomass reaction.16
HO
HO
OH
OH
O
O
OH
H+
OH
-H2O
OH
OH
OH
Fructose
H+ -H2O
O
HO
O
HO
O
O
H+
-H2O
H
OH
5-HMF
Scheme 1 The reaction mechanism for 5-HMF from fructose
Early times, many types of acid catalysts such as mineral acids(such
as H2SO4, HCl, H3PO4),7 organic acids(such as formic acid and acetic
acid),8 strong acid cation exchange resins9-10 had been used for the
dehydration of fructose. According to the earlier literature,11-12 the
reaction mechanism for 5-HMF from fructose over acid catalysts as
shown Scheme 1. In addition, ionic liquids were serving as both
solvents and catalysts for the dehydration of fructose into HMF13-14
and had been proven effective, but its synthesis process complex
and they had serious drawbacks in terms of separation and
recycling. In contrast, heterogeneous catalysts showed superior
behavior in terms of easy recovery and recyclability in dehydration
of sugar.15 R. Kourieh et al. studied the WOx/ZrO2 surface acidic
Several approaches had been reported for the dehydration of
carbohydrates, such as Simeonov et al. synthesized 5-HMF by
dehydration of fructose in either batch or flow chemistry
conditions,17 Jadhav et al. carried out the dehydration reaction of
sugar in superheated water in a stainless tubular reaction cell,18 De
et al. used AlCl3 as catalyst for fructose dehydration under
microwave-assisted heating conditions.19 Above all, microwave
technology had been applied by more and more researches, it had
unique heating mode which could make organic reaction speed
faster than traditional heating method and the reaction yield was
high, it had fewer by-products. Qi et al. studied the catalytic
conversion of fructose into 5-HMF by microwave heating and
compared conventional sand bath heating with microwave heating,
it revealed that the latter had a remarkable accelerating effect not
only on fructose conversion, but also on 5-HMF yield, fructose
conversion and HMF yields by microwave heating (91.7% and
a.School of Life Sciences and Chemistry
，Changchun University of Technology,
Changchun 130012, Jilin, China. E-mail addresses: zhaozhenbo@ccut.edu.cn
†Electronic Supplementary Information (ESI) available: supplementary information
available should be included here. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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70.3%, respectively) were higher than those by sand bath heating 500cm−1. The morphology of the catalytic materials were obtained
(22.1% and 13.9% respectively).10
by a transmission electron microscope(TEM: JEM-2000EX, JEOL). In-
In this work, we introduced the W, Mo atom into ZrO2 and prepared stiu high pressure Raman measurements obtained using
a
WOx/ZrO2, MoOx/ZrO2 catalysts. Moreover, we prepared highly spectrometer (focal length, 500 mm) combined with a liquid
catalytic SO42-/WOx-ZrO2 and SO42-/MoOx-ZrO2 solid acid catalysts by nitrogen-cooled CCD (Acton SP-2500 and PyLoN:100B,Princeton
wet impregnation. These catalysts were characterized via certain Instruments). A single-mode DPSS laser (power output, 50 mW) at
methods, such as X-ray diffraction (XRD), Fourier Transform infrared 532 nm used as the excitation light source. XPS analyses were
spectroscopy (FTIR), nitrogen adsorption-desorption measurement obtained on Theromo ESCALAB 250Xi instrument. The surface acid
(BET), transmission electron microscopy (TEM), X-ray photoelectron amount of samples were quantitatively measured via UV
spectroscopy (XPS), ammonia-temperature programmed desorption spectrometry with adsorbates of pyridine (Py) in cyclohexane
(NH3-TPD), and the surface acid amount has been obtained solution on a Cary 5000 UV-Vis-NIR. The detailed steps were
combining pyridine adsorption and UV spectrometry. Furthermore, described previously by Siqian Zhang et al.20 NH3-TPD were
we investigated the dehydration of fructose into 5-HMF by using performed on a Chembet Pulsar TPR/TPD. In a typical experiment
these catalysts under microwave-assisted in dimethyl sulfoxide for the TPD measurement, the sample were pretreated at 700°C for
(DMSO) in very short reaction time.
30min in helium and then cooled to 100°C. A flow of 10% NH3/He
was then passed over the pretreated materials for 60min. Following
this ammonia adsorption procedure, the reactor was purged with
helium for 60min to remove residual/physisorbed ammonia,
ammonia desorption was carried out 700°C with a heating rate of
10°C /min. Nitrogen physisorption studies were performed using
Micromeritics ASAP 2010 model static volumetric adsorption
instrument. The samples were dried in an oven at 80°C overnight
prior to degassing. Prior to adsorption experiments, the catalysts
were outgassed at 350°C for 8h. The pore size distribution was
calculated using the Barrett-Joyner-Halenda (BJH) method.
2. Experiment
2.1. Material
All chemicals used were analytically grade: Fructose (99%,
Aladdin®), 5-HMF (98%, Tianjin Fuchen Chemical Reagents Factory),
n-butanol (99%, Beijing Chemical Works), zirconium (IV) propoxide
(70wt.%, solution in 1-propanol, Sigma-Aldrich), Cyclohexane and
Pyridine (99.5%, Xilong Chemical Co., Ltd.), ammonium
molybdate(99%) and ammonium tungstate (85%~90%, Sinopharm
Chemical Reagent Co., Ltd), cetyltrimethylammonium bromide
(CTAB) and DMSO (99%, Tianjin Guangfu Fine Chemicals Research
Institute), deionized water (H2O, home-made).
2.4. Procedure for catalytic degradation of fructose
The catalytic reaction for fructose dehydration to HMF was
performed in a CEM Discover SP microwave reactor. 5mL fructose
solution and catalyst were charged in a microwave tube. The tube
was placed in the microwave reactor. The microwave power was
set to 100W. Desired temperature and time were set. After the
reaction, the tube was allowed to cool at room temperature. The
conversion of fructose was analyzed with a LC98IIRI GPC with
ZORBAX NH2 column and the refractive index detector. The UV–
visible spectrum of pure HMF solution has a distinct peak at 284nm.
The yield of HMF was determined by measuring the absorbance of
HMF product solution at 284nm using the standard curve method.
Repeated measurement of the same solution showed the
percentage of error associated with this measurement was
±0.3%.21-22
2.2. Catalyst Synthesis
The synthesizing process of zirconia support was as follows. First,
2.5g CTAB dissolved in 20 ml n-butanol with stirring at 40°C for
10min to obtain a clear solution, then 0.02 mol of the zirconium (IV)
propoxide solution was added to this solution with stirring about 10
min. In another container, 0.002mol of the transition-metal source
(W, Mo) was dissolved in 10ml water, the formed solution was then
added to the above-mentioned solution under vigorous magnetic
stirring at room temperature. After stirring for 24h, the precipitate
was transferred to a Teflon lined autoclave, and crystallization was
carried out at 80°C for 24h, then filtered, washed with bi-distilled
water several times. The obtained hydroxide precipitate was dried
at 100°C for 5h and finally calcined at 550°C for 4h, the heating rate
was controlled at 2°C /min.
3. Results and discussion
As for SO42-/WOx-ZrO2 and SO42-/MoOx-ZrO2, these catalysts were
3.1. Characterization of the catalyst
prepared similar to the above-mentioned method. The only
difference was that the obtained hydroxide precipitate was dried at 3.1.1. XRD patterns and FTIR measurement
m-ZrO2
t-ZrO2
100°C for 5h and then was impregnated with 1mol/L H2SO4 for
WO -ZrO
x
2
30min(sample/ H2SO4=1g/10mL), then filtered, washed with hot
distilled water at 80 °C several times and dried at 100°C ,and then
was calcined at 550°C for 4h.
MoO -ZrO
x
2
2-
SO /WO -ZrO
4
x
2
2.3. Catalysts characterization
2-
XRD analyses were obtained on Rigaku D/MAX2500 instrument
using Cu Kα radiation. The scattering angle 2θ was varied from 10°
to 80°, with a step length of 0.02°. Fourier transform infrared (FT-IR)
spectra were recorded on KBr pellets by a PerkinElmer (Spectrum
Two) infrared spectrometer with the wavenumber from 4000 to
SO /MoO -ZrO
4
x
2
ZrO
2
10
20
30
40
2
50
60
70
80
θ /(degree)
Fig. 1. XRD patterns of different samples.
2 | J. Name., 2012, 00, 1-3
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Fig. 1 shows the XRD patterns of different catalysts, the samples The Raman spectra of the different catalysts are shown in Fig. 3. All
showed diffraction peaks for 2θ=30.25°, 35.5°, 50.3° and 60°, it was of them possessed the typical Raman signals of t-ZrO2 at 641, 476,
estimated that zirconia essentially presented in the tetragonal 312, and 274cm-1 and only ZrO2 showed obvious bands of m-ZrO2,32
form24 and it had been generally accepted that the tetragonal phase which conformed to the ZrO2 phase detected by XRD
was more important for the formation of acidic centers.23 The bare characterization.
A big Raman band from trace amounts of
nanosized ZrO2 mainly existed in the monoclinic phase (m-ZrO2), a crystalline WO3 NPS at 833 cm-1 was present in the spectra for the
stable phase under relatively high temperature; however, once WOx WOx-ZrO2 catalyst sample, which reflected the good dispersion of
was added, ZrO2 dominantly crystallized in the tetragonal phase (t- the tungsten oxide phase on the ZrO2 support;33 the Raman band
ZrO2), a metastable structure at low temperature.24-25 In addition, from trace amounts of crystalline MoO3 NPS at 812 cm-1 was
there was no diffraction line of the crystalline WOx phase in the present in the spectra for the MoOx-ZrO2 catalyst sample,34 which
XRD patterns, implying that WOx in these samples existed in a reflected the good dispersion of the molybdenum oxide phase on
highly dispersed state on ZrO2 surface and so do other samples else. the ZrO2 support. The band at 962cm-1 was attributed to the
The absorption spectra of UV–vis DRS are illustrated in ESI Figs.1, stretching vibration of M=O(M=W, Mo).23,35 The Raman spectra of
the band-gap energy (Eg) values of the samples are included in SO42-/WOx-ZrO2 and SO42-/MoOx-ZrO2 weren’t measured, because
Table 1. The Eg of synthesized samples were lower than ZrO2, they had a strong fluorescence effect.
indicating that domain size became bigger and the transition-metal The XPS spectra of the different catalysts are shown in Fig. 4. The
source existed in ZrO2,26-27 although the existence of the transition- O1s, Mo3d, Zr3d, S2p, W4f and Zr4p peaks of the samples
metal could not be analyzed by XRD.
demonstrated binding energies of 530.4, 232.6, 181.9, 168.7, 36.1
and 30.9eV, respectively.36-37 It directly confirmed the presence of
W or Mo in the materials.
2-
4
SO /MoO -ZrO
x
2
2-
SO /WO -ZrO
2
O 1s
1120
4
x
Zr3d
1247
1056
M o3d
M o3d
Zr4p
MoO -ZrO
x
2
S2p
762
WO -ZrO
x
2
979
ZrO
W 4f
W 4f
S 2p
2
1388
1632
3438
3500
4000
3000
2500
σ
2000
1500
1000
500
-1
/cm
Fig. 2. FT-IR spectra of different samples
600
500
400
300
200
100
0
B inding energy/ev
The FT-IR spectra of the different catalysts are shown in Fig. 2. The
band at 3438cm-1 was attributed to O-H stretching vibrations of the
physically absorbed water molecules, the band at 1632cm-1 and
1388cm-1 were attributed to HOH bending of water molecules
associated with the sulfate group.28 The band at 979cm-1 was
characteristic for stretching vibrations of W=O bond.29 The band at
762cm-1 was assigned to Mo=O stretching modes.30 The sulfated
samples exhibited strong bands at 1056, 1120, 1247cm-1, which
showed bidentate sulfate ion coordinated to the metal.31 These
bands were absent in unsulfated samples. TEM images of samples
are displays in ESI Figs.2, it revealed a globular structure. The
average particles were obtained after calcination at 550°C. The size
of the particles in the MOx-ZrO2 and SO42-/MOx-ZrO2(M=W, Mo)
samples were smaller than ZrO2.
Fig. 4. XPS of different samples
3.1.3. TPD measurement and the BET surface area
2ꢀ
SO
/MoO ꢀZrO
4
x
2
2
2ꢀ
SO
/WO ꢀZrO
x
4
MoO ꢀZrO
x
2
WO ꢀZrO
x
2
ZrO
2
200
300
400
500
600
3.1.2. Raman spectra and XPS analysis
T/°C
312 274
Fig. 5. NH3-TPD profiles of different samples
641
812
476
962
The NH3-TPD profiles of the different catalysts are depicted in Fig. 5
2-
to evaluate the acidity of the catalysts. The NH3-TPD profile of SO4
833
M o O ꢀZ rO
x
2
/WOx-ZrO2 gave a wide region of stronger acid sites around 450°C
2-
and weak acid sites by the peak around 200°C. Although SO4
38 0
W O ꢀZ rO
x
2
/MoOx-ZrO2 showed a peak distributed at approximately 500°C but
it was small. Nevertheless, other catalysts showed a broad peak of
the amount max of acid sites around 200°C. It indicated that the
acid strength of SO42-/WOx-ZrO2 was much higher than others. The
calculated amounts of acid sites via the UV spectrometry
55 8
3 3 0
Z rO
2
5 3 7
2 2 0
2 00
6 1 5
6 00
12 00
10 00
8 00
4 00
R am an S hift/cm ꢀ1
Fig. 3. Raman spectra of different samples
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measurement were shown in Table 1, it was obvious that the
sulfated samples exhibited higher surface acid amount than
unsulfated samples, corresponding to the NH3-TPD results. The
highest acid amount of SO42-/WOx-ZrO2 may be due to the
enlargement of the surface area.38-40 Generally, the specific surface
areas of the sulfated samples were slightly larger than the
3.2. Catalytic activities of various catalysts on fructose
conversion and 5-HMF yield
3.2.1. The catalytic performances of different catalysts
The prepared samples were tested for fructose dehydration to 5-
HMF at 140°C with the 5wt% fructose in DMSO for 5min. The
reaction using 10wt%(catalyst amount with respect to fructose,
g/g×100) of catalyst are shown in Table 2. It was found that all of
the catalysts were active, furthermore, the addition of the
heteroatom and the impregnation with H2SO4 influenced the
catalytic activity. The yield increased in the following order: without
catalyst<ZrO2<MoOx/ZrO2<WOx/ZrO2<SO42-/MoOx-ZrO2<SO42-/WOx-
ZrO2, corresponding to the specific surface area and surface acid
2-
unsulfated samples, it may because the interaction between SO4
and sample stabilized the sample against sintering producing
thermally stable ZrO2 structure.38 Nitrogen adsorption–desorption
isotherms and the average pore size distribution curves of the
samples are shown in ESI Figs. 3–7. All the isotherms resemble the
Type IV isotherms based on IUPAC classification with a large type
H3 hysteresis loop, it indicated the presence of mesoporosity. The
hysteresis loop was associated with the capillary condensation
taking place in the mesopores signifying the preservation of the
mesoporous structure even after crystallization at elevated
temperature.41 The surface areas and pore size of samples
measured by the BET method are included in Table 1. The specific
surface area increased in the following order: ZrO2 < MoOx/ZrO2 <
SO42-/MoOx-ZrO2 < WOx/ZrO2 < SO42-/WOx-ZrO2, this result proved
that SO42-/WOx-ZrO2 may be an efficient catalyst, corresponding to
2-
amount. The activity of SO42-/ZrO2 was similar compared to SO4
/MoOx-ZrO2 and SO42-/WOx-ZrO2, but the reaction time was 20min.
The addition of the heteroatom could shorten the reaction time, it
may because the doped of heteroatom helped to stabilize the
tetragonal phase of zirconia in the catalyst and inhibited the
transformation from the metastable tetragonal phase to the
monoclinic phase at the high calcination temperature,24-25 and the
tetragonal phase was more important for the formation of acidic
centers. The SO42-/WOx-ZrO2 catalyst was found be the most active
catalyst, it was attributed to the stable tetragonal phases and the
amount of acidic sites in this catalyst. In addition, the SO42-/MoOx-
ZrO2 even had better catalytic effect, although the catalytic effect of
SO42-/MoOx-ZrO2 was poorer than SO42-/WOx-ZrO2. Therefore, this
work we mainly studied the dehydration of fructose into 5-HMF by
using SO42-/WOx-ZrO2 and SO42-/MoOx-ZrO2 under microwave-
assisted in dimethyl sulfoxide.
the following reaction results.
Table 1 The physical properties of samples
Surface
area
(m2/g)
Pore
size
Pore
Qi
Eg
Catalyst
Volume
(m2/g)
(umol/g)
(eV)
(nm)
ZrO2
48.19
139.70
64.23
159.55
82.75
14.08
7.59
4.74
8.18
5.35
0.17
0.22
0.08
0.32
0.11
29.60
5.10
3.75
3.19
3.58
2.91
WOx-ZrO2
131.42
76.36
MoOx-ZrO2
SO42-/WOx-ZrO2
SO42-/ MoOx-ZrO2
380.34
256.71
3.2.2. Effect of fructose concentration on the dehydration of
fructose
All the findings presented above were consistent with the following
100
2-
general model (Scheme 2).42-44 The SO4 was electron-withdrawing
Fructose conversion
5ꢀHMF Yield
5ꢀHMF selectivity
group, after it was introduced into material which could increased
the formation of acidic centers.
80
60
O O
M
O O
M
+H2O
-H2O
O
O
O
O
Zr
Zr
Zr
Zr
H O
H
O
O
2-
SO4
O O
S
SO42ꢀ/WOxꢀZrO2
40
O O
M
O O
M
SO42ꢀ/MoOxꢀZrO2
O
O
O
+H2O
-H2O
O
O
O
O
O
Zr
0
5
10
15
20
25
30
O
Zr
Zr
Zr
S
Zr
H O O H
H O O H
Fructose concentration/(wt%)
O
O
H
H
H
H
Fig. 6. Influence of fructose concentration on the dehydration of fructose.
(Reaction conditions: reaction temperature=140°C; catalyst amount=10wt%;
reaction time=7min).
Scheme 2 The balance of B acid and L acid (M=W, Mo)
Table 2 Influence of the species of catalysts on the dehydration of fructose
Reaction
solvent
Reaction
time/min
Reaction
temperature/°C
Fructose
conversion/%
5-HMF
yield/%
5-HMF
selectivity/%
catalyst
Ref.
DMSO
5
140
140
140
140
140
140
140
140
140
150
43.93
51.13
67.66
61.74
87.52
86.05
93.60
-
15.86
31.02
53.03
47.26
70.88
67.11
72.80
54.10
71.30
70.30
36.10
60.67
78.38
76.55
80.99
77.99
77.78
-
This work
This work
This work
This work
This work
This work
[28]
[21]
[19]
[10]
无
ZrO2
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
5
5
5
5
5
20
5
5
10
WOx-ZrO2
MoOx-ZrO2
SO42-/WOx-ZrO2
SO42-/MoOx-ZrO2
SO42-/ZrO2
TiO2
AlCl3
ion-exchange resin
-
-
acetone-water
91.70
76.67
4 | J. Name., 2012, 00, 1-3
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100
Fig. 6 shows the effect of the fructose concentration on the
dehydration of fructose. There was negligible effect on the
dehydration of fructose when the fructose concentration was
below 5wt%. The rate of fructose conversion had little effect when
the fructose concentration was above 5wt%, but the HMF yield and
the HMF selectivity decreased gradually with increasing initial
fructose concentration. This was probably because self-
polymerization of HMF or cross-polymerization between fructose
and HMF would occur easily at higher initial concentration of
fructose,10 giving rise to the formation of brown black soluble
polymers and insoluble humins.19 In addition, it was obvious that
the catalytic effect of SO42-/MoOx-ZrO2 was poorer than SO42-/WOx-
ZrO2, but the variation trend of the fructose conversion (the HMF
yield and the HMF selectivity) was similar. So, in conclusion, we
took 5wt% as the initial fructose concentration in this work.
80
60
40
20
Fructose conversion
5ꢀHMF Yield
5ꢀHMF selectivity
SO42ꢀ/WOxꢀZrO2
SO42ꢀ/MoOxꢀZrO2
0
5
10
15
20
25
30
catalyst amount /(wt%)
Fig. 7. Influence of the catalyst amount on the dehydration of fructose.
(Reaction conditions: fructose concentration=5wt%; reaction time=5min; reaction
temperature=140°C).
100
80
100
80
60
60
1.6
1.6
1.2
0.8
0.4
Y=69.31*Xꢀ19.01
R2=0.995
Y=68.31*Xꢀ18.79
2
R
=0.996
1.2
0.8
0.4
0.0
130
140
150
160
°
°
°
°
C
C
C
C
40
20
130
140
150
160
°
°
°
°
C
C
C
C
40
20
0.275 0.280 0.285 0.290 0.295 0.300
1000/(T*R)
0.275
0.280
0.285
0.290
0.295
0.300
1000/(T*R)
0
2
4
6
8
0
2
4
6
8
Reaction time /(min)
Reaction time /(min)
(a)
(b)
Fig. 8. Influence of reaction time and temperature on fructose conversion and Arrhenius linearity fitting curve
(a:SO42-/WOx-ZrO2,b:SO42-/MoOx-ZrO2. Reaction conditions: catalys amount=10wt%; fructose concentration=5wt%)
3.2.3. Influence of the catalyst amount on the dehydration of because the side reaction in the dehydration of fructose also
fructose accelerated under over-using of catalyst. Given the above
Fig. 7 shows the effect of the catalyst amount with respect to discussion, it was evident that 10wt% of catalyst provided the best
fructose conversion, 5-HMF yield and 5-HMF selectivity. The result for the model reaction.
amount of catalyst used were 5 wt%, 10 wt%, 20 wt% and 30 wt% 3.2.4. Influence of different reaction time and temperature on the
dehydration of fructose and Kinetics Analysis.
The Fig. 8 shows the influence of different reaction time and
(catalyst amount with respect to fructose, g/g×100) respectively. In
the absence of catalyst, only 15.86% yield of 5-HMF yield were
attained at 140°C for a reaction time of 5min in DMSO, nevertheless
the 5-HMF yield reached to 61.06% (SO42-/WOx-ZrO2) and 54.21%
(SO42-/MoOx-ZrO2) when 5wt% catalysts were added. It firmly
indicated that SO42-/WOx-ZrO2 and SO42-/MoOx-ZrO2 showed
excellent catalytic performance for the dehydration of fructose to
5-HMF. When the catalyst dosage increased from 5wt% to 10wt%,
the 5-HMF yield still increased at 140°C for a reaction time of 5min,
it was because that the catalytic sites increased with the increasing
amount of catalyst, boosting the dehydration reaction. However,
when we kept increasing the amount of catalyst, there was a
smooth in fructose conversion and 5-HMF yield, this was probably
temperature on fructose conversion. It was obvious that the
reaction temperature and time had a significant effect on the
fructose conversion. While the reaction temperature increased, the
fructose conversion increased. As the reaction proceeded, the
fructose conversion also increased, when prolonging the reaction
time to 5 min, fructose conversion was almost a constant at 160°C.
When temperature was 140°C, fructose conversion was almost a
constant for 7min, it showed that increased reaction temperature
could shorten the reaction time. Many works reported a reaction
order of one for the dehydration of fructose to 5-HMF. We
performed
a
kinetic analysis of the dehydration of fructose
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Table 3 Comparison of Activation Energies and Pre-Exponential Factors for Acid-Catalyzed Fructose Dehydration to 5-HMF
Ea(KJ/mol)
A(min-1)
Reaction solvent
Catalyst
Ref
68.31
69.31
99.20
96.00
60.40
103.40
80.00
160.60
1.45×108
1.80×108
2.10×108
1.79×108
4.80×106
8.70×1011
3.10×108
-
DMSO
SO42-/WOx-ZrO2
SO42-/MoOx-ZrO2
[BMIM]OH
This work
This work
[45]
DMSO
DMSO
Subcritical water
70:30(w/w) acetone/DMSO
70:30(w/w) acetone/water
Supercritical methanol
Aqueous solution
ZnSO4
[46]
ion-exchange resin
ion-exchange resin
H2SO4
[47]
[10]
[48]
HCl
[49]
catalyzed by SO42-/WOx-ZrO2 and SO42-/MoOx-ZrO2 in DMSO. Values showed the fructose conversion of 94.97% with 82.55% 5-HMF
of ln(1− ) were plotted against reaction time(t) to obtain first-order selectivity in 1min reaction time at 160°C. In addition, with the
x
kinetic constants. With those constants, an Arrhenius plot was extending of reaction time, the 5-HMF yield and selectivity were
generated, as shown in Fig. 8. This model had been employed in improved consistently. When the reaction time was extended up to
most of the kinetic studies and was found to give reasonable levels 7min and longer at 150°C, the 5-HMF yield decreased. But when the
temperature was above 160°C, the 5-HMF selectivity was decreased
of agreement with experimental data.45-49
The activation energies
consistently. The variation trend of the HMF yield and selectivity for
SO42-/MoOx-ZrO2 were similar to SO42-/WOx-ZrO2, it showed the
fructose conversion of 95.33% with 81.64% 5-HMF selectivity in
3min reaction time at 160°C. In conclusion, it was generally noted
that higher temperatures (>140°C) and longer reaction times
(>5min) resulted in a loss of yield expectedly on account of by-
product formation and/or decomposition of HMF.
and pre-exponential factors for acid-catalyzed fructose dehydration
to 5-HMF obtained by different authors under different conditions
are summarized in Table 3. The obtained activation energies and
pre-exponential factors from this work were similar, it may depend
on the same solvents and heating methods used,47 on the other
hand, considering the experimental error/standard deviation, we
could know the SO42-/MoOx-ZrO2 and SO42-/WOx-ZrO2 exhibited
similar catalytic activities. Compared to the previous values
reported in the literature, it showed that the present process was
more efficient than previous works as evident from the shorter
reaction times.
4. Conclusions
Through this study, the ZrO2, WOx/ZrO2, MoOx/ZrO2, SO42-/WOx-
ZrO2 and SO42-/MoOx-ZrO2 solid catalysts were prepared and
characterized by XRD, UV-DRS, FTIR, XPS, TEM, TPD, BET, Raman
and the surface acid amount was obtained combining pyridine
adsorption and UV spectrometry. These catalysts were employed in
dehydration of fructose to 5-HMF under microwave-assisted in
DMSO, the SO42-/WOx-ZrO2 catalyst was found be the most active
catalyst, the fructose conversion of 95.80% with 83.90% of 5-HMF
yield was obtained at 150°C for a relatively short reaction time of
The Fig. 9 shows the influence of different reaction time and
temperature on the 5-HMF yield and 5-HMF selectivity. The 5-HMF
yield and selectivity increased with reaction temperature, the
temperature played a positive role on the dehydration of fructose.
When the temperature increased up to 160°C, the yield of 5-HMF
was slightly decreased, but the selectivity of 5-HMF was
significantly declined. When the catalyst was SO42-/WOx-ZrO2, it
80
60
80
60
40
40
90
90
80
80
130
°C
C
C
C
130°
140°
150°
160°
C
C
C
C
70
60
140
°
20
20
70
60
150
°
160°
0
2
4
6
8
0
2
4
6
8
Reaction time /(min)
Reaction time/(min)
0
2
4
6
8
0
2
4
6
8
Reaction time /(min)
Reaction time /(min)
(a)
(b)
Fig. 9. Influence of reaction time and temperature on 5-HMF yield and 5-HMF selectivity
(a:SO42-/WOx-ZrO2,b:SO42-/MoOx-ZrO2.Reaction conditions: catalyst amount=10wt%; fructose concentration=5wt%)
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5min. The catalyst amount and fructose concentration were further 20. S. Q. Zhang, Z. B. Zhao and Y. H. Ao, Appl. Catal. A:Gen., 2015,
optimized. The value of activation energy was comparable with
496, 32-39.
2-
previous values reported in the literature, this implied that the SO4
/WOx-ZrO2 and SO42-/MoOx-ZrO2 were efficient for the dehydration
21. S. Dutta, S. De, A. K. Patra, M. Sasidharan, A. Bhaumik and B.
Saha, Appl. Catal. A:Gen., 2011, 409-410, 133-139.
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Carbohydr. Res., 2010, 345, 1846-1850.
of fructose to 5-HMF under microwave-assisted in DMSO.
23. W. D. Sun, Z. B. Zhao, C. Guo, X. K. Ye and Y. Wu, Ind. Eng.
Chem. Res., 2000, 39, 3717-3725.
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