Efficient production of 5-hydroxymethylfurfural through the dehydration of sugars with caprolactam hydrogen sulfate ([CPL]HSO4) ionic liquid catalyst in a water/proprylene glycol monomethyl ether mixed solvent

Article abstract of DOI:10.1007/s11164-014-1633-6

Efficient production of 5-hydroxymethylfurfural (HMF) through the dehydration of sugars was achieved by developing a new catalyst caprolactam hydrogen sulfate ([CPL]HSO₄) ionic liquid or using metal halide as the co-catalyst in a new water/proprylene glycol monomethyl ether solvent system. With an optimum synthesis condition, the yield of HMF derived from fructose dehydration at 105°C for 90 min is as high as 80.5 % in the presence of [CPL]HSO₄. The nature behind the attractive results was revealed by proposing the catalysis mechanism of the fructose dehydration. The yield of HMF is about 70 % using [CPL]HSO₄ as the catalyst for fructose dehydration after being reused five times. The catalyst system [CPL]HSO₄/metal halide was used for the dehydration of sucrose, and the optimum yield is as high as 64-68 %.

Full text of DOI:10.1007/s11164-014-1633-6

Res Chem Intermed
DOI 10.1007/s11164-014-1633-6
Efficient production of 5-hydroxymethylfurfural
through the dehydration of sugars with caprolactam
hydrogen sulfate ([CPL]HSO ) ionic liquid catalyst
4
in a water/proprylene glycol monomethyl ether mixed
solvent
Pingzhen Huang
•
Aijuan Gu Jinxing Wang
•
Received: 13 December 2013 / Accepted: 23 March 2014
Ó Springer Science+Business Media Dordrecht 2014
Abstract Efficient production of 5-hydroxymethylfurfural (HMF) through the
dehydration of sugars was achieved by developing a new catalyst caprolactam
hydrogen sulfate ([CPL]HSO ) ionic liquid or using metal halide as the co-catalyst
4
in a new water/proprylene glycol monomethyl ether solvent system. With an opti-
mum synthesis condition, the yield of HMF derived from fructose dehydration at
1
behind the attractive results was revealed by proposing the catalysis mechanism of
05 °C for 90 min is as high as 80.5 % in the presence of [CPL]HSO . The nature
4
the fructose dehydration. The yield of HMF is about 70 % using [CPL]HSO as the
4
catalyst for fructose dehydration after being reused five times. The catalyst system
[CPL]HSO /metal halide was used for the dehydration of sucrose, and the optimum
4
yield is as high as 64–68 %.
Keywords 5-Hydroxymethylfurfural ꢀ Dehydration ꢀ Sugar ꢀ Catalyst ꢀ
Caprolactam–sulfuric acid ionic liquid ꢀ Solvent
Introduction
Owing to its furan structure and double functional groups (hydroxyl and aldehyde
groups), 5-hydroxymethylfurfural (HMF) has been an important raw chemical and/
or intermediate for producing many materials [1, 2]. For example, through an
P. Huang ꢀ A. Gu (&)
Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, College
of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou,
Jiangsu 215123, People’s Republic of China
e-mail: ajgu@suda.edu.cn
P. Huang ꢀ J. Wang
Suzhou Xingye Material Technology Co., Ltd., Suzhou, Jiangsu 215151,
People’s Republic of China
123

P. Huang et al.
oxidation reaction, HMF can be transformed to 2,5-furandicarboxylic acid, which
has similar structure to terephthalic acid and thus can replace terephthalic acid to
produce polyester, used for bottles, cans, and foils, etc. [3]. In addition, HMF can
produce dimethylfuran through hydrogenation, while dimethylfuran is a good
biobased liquid fuel [4]. In addition, HMF in aqueous solution can be hydrolyzed
into levulinic acid (LA) which is an important chemical for producing fuel
additives, and polymer and resin precursors [5]. This importance and the great
potential in applications make the green synthesis of HMF a very interesting topic.
About 100 years ago, HMF was synthesized from the conversion of C sugars
6
through a catalyzed hydrolysis, while the catalyst and solvent are two crucial
aspects for the dehydration reactions of C6 sugars [6, 7]. In recent years,
imidazolium-based ionic liquids (ILs), such as 1-butyl-3-methylimidazole chloride
(
[Bmim]Cl) and 1-ethyl-3-methylimidazolium chloride (EmimCl), were used as
catalysts and/or solvents for synthesizing HMF, and a high HMF yield from sugar or
cellulose dehydration was obtained [8–13]; however, these ILs are too expensive
and toxic to be accepted by industry. Compared with imidazole, caprolactam is
relatively cheaper and less toxic; moreover, it is also a kind of amine derivative that
could be quaternized to form new ionic compound cations [14] due to the presence
of a carbonyl group on the caprolactam. On the other hand, the synthesis of
caprolactam-cation-based ILs is easily carried out through the acid–base neutral-
ization reaction. Recently, a series of caprolactam-cation-based Brønsted acid ILs
were synthesized to be used as a catalyst and reaction media [15, 16]. However,
unfortunately, no literature has reported the synthesis of HMF through the
dehydration of sugars by using caprolactam hydrogen sulfate ([CPL]HSO ) IL as
4
the catalyst. With regard to the solvents for the dehydration of sugars, to get a high
yield of HMF, the organic solvent with high-boiling point, such as dimethyl
sulfoxide (DMSO), N,N-dimethyl-formamide (DMF), or dimethylacetamide, is
often used. However, it is difficult to separate the product from the solvent due to
the high boiling point [13].
Here, we developed a new method based on a new catalyst and a new mixed solvent
system with low boiling point, with the former being [CPL]HSO or adding metal
4
chloride as co-catalyst, and the latter makes up of proprylene glycol monomethyl ether
and water (PGME/H O). Based on these, the influences of the key parameters including
2
the catalyst, the ratio of water to PGME in the mixed solvent and the reusability of
[
CPL]HSO on the conversion of sugar and the yield of HMF were systemically
4
discussed. A similar investigation was also done for the catalytic conversion of sucrose
to HMF. Some attractive results were obtained, and the naturebehindthemwas revealed
from discussing the catalysis mechanism of the dehydration.
Experimental
Materials
Sucrose was purchased from Chinasun Specialty Products. Fructose, caprolactam,
sulfuric acid, benzene, PGME, ethyl acetate, Na SO , FeCl , AlCl , LiCl, and LiBr
2
4
3
3
1
23

Efficient production of 5-hydroxymethylfurfural
were supplied from China National Medicines. Chromic chloride (CrCl ꢀ6H O) was
3
2
purchased from Shanghai Yingrui Biotechnological, ZnCl was obtained from
2
Tianjin Damao Chemical Reagent Factory. The above reagents were of analytical
grade. HMF (99.0 %) used as the control was purchased from Aladdin Chemical
Reagent Factory in Shanghai.
Caprolactam (11.32 g, 0.1 mol) and H O (10 mL) were added into a three-
2
necked flask with a thermometer and a condenser, which was stirred until a clear
solution was obtained, and then concentrated sulfuric acid (10 g, 0.1 mol) was
added dropwise into the flask in an ice bath, followed by reacting at room
temperature for 24 h. After that, H O was removed by vacuum distillation, and the
2
mixture was washed with benzene for three times, followed by drying at 70 °C in a
vacuum oven to get the [CPL]HSO .
4
Dehydration of sugars
A specific amount of sugar (fructose or sucrose), catalysts ([CPL]HSO IL and
4
metal halide), and solvents (PGME and H O) were added to the flask with magnetic
2
stirring and a condenser. The mixture was stirred and preheated to a certain
temperature in an oil bath, and then maintained at the temperature for a certain time.
After that, the sample was decanted to a volumetric flask, and H O was used as the
2
diluent, then the obtained diluent sample was analyzed by a high performance liquid
chromatography (HPLC).
Separation for HMF
Firstly, the reaction mixture was fast cooled to 50 °C in an ice bath, and then was
distilled under reduced pressure to remove the reacted solvent in the dark. Secondly,
1
acetate was collected into a round-bottom flask, into which 5 mL H O was added.
0 mL ethyl acetate was added to extract the product three times, and then ethyl
2
Thirdly, the organic phase was collected and dried with sodium sulfate, followed by
evaporation under reduced pressure to obtain a thick yellow liquid, which is HMF
1
with a purity of 97 % (HPLC). H NMR: d3.33 (d,1H), d4.50 (s, 2H), d6.61 (d, 1H),
d7.49 (d, 1H), d9.54 (s, 1H).
Analytic methods
FTIR spectra were recorded using a Nicolet (USA) FTIR spectrometer. The
specimen was prepared by coating the sample on a potassium bromide disk, and
then heating in an oven. NMR spectra were analyzed on a Unity Inova 400 NMR
spectrometery system (USA). The melting point of [CPL]HSO IL was determined
4
by a microscopic melting point instrument made in Shanghai physical optics
instrument factory, China. Mass spectra were recorded on a TSQ Quantun Ultra AM
mass spectrometer (USA).
The quantitative analyses of the products were performed on a 1200 Agilent
HPLC equipped with both UV and refractive index detectors (RID). HMF was
quantified by HPLC on a silica gel column (C-18, 250 mm 9 4.6 mm, 5-lm
123

P. Huang et al.
particle size; Agilent), using water/methanol (80 v/20 v) solution as the mobile phase
at a flow rate of 1 mL/min, and the column temperature was maintained at 35 °C. The
absorbance value for HMF was at 284 nm. The sugar concentration was quantified on
a HPX-87C (460 9 7.5 mm) column using HPLC with RID. The mobile phase was
H O/acetonitrile (25 v/75 v) solution with a flow rate of 0.8 mL/min at 35 °C. LA and
2
formic acid (FA) were determined on a HPX-87H column by HPLC with RID using
2
.5 mmol/L H SO solution with a flow rate of 0.5 mL/min.
2 4
The yield of HMF (M), the conversion of sugar (C), and the yield of glucose and
fructose (N) from dehydration of sucrose were calculated by using Eqs. (1), (2), and
(
3), respectively:
ðMoles of 5ꢁHMF formedÞ
ðMoles of sugar in substrate)
M ¼
ꢂ 100%
ð1Þ
ð2Þ
ð3Þ
ꢀ
ꢁ
ðMoles of carbohydrates loaded in productionÞ
C ¼ 1 ꢁ
ꢂ 100%
ðMoles of sugar in substrate)
ꢀ
ꢁ
ðMoles of sugar loaded in productionÞ
ðMoles of sucrose in substrate)
N ¼
ꢂ 100%
Results and discussion
Characterization of [CPL]HSO4
The structure of [CPL]HSO was characterized using many techniques. The mass
4
spectrometry shows that the molecular ion peak of the cationic appears at
m/z = 114.09, which is identical to the calculated value of caprolactam cationic
(
m/z = 114.1). The melting point of [CPL]HSO is 26 °C, which is lower than that
4
of caprolactam (68–70 °C), suggesting that a new hydrogen bonding is formed
between the hydrogen ions and the carbonyl of caprolactam, and thus reducing the
electrostatic force of the anion and cationic of caprolactam.
Figure 1 gives the FTIR spectrum of [CPL]HSO , which shows the character-
4
istics peaks of [CPL]HSO including the stretching vibrations of O–H (H O,
4
2
-
1
?
-1
,410 cm ), N–H2 (CPL, 3,237 cm ). N–C–H (CPL, 2,943 cm ), C=O (CPL,
-1
3
1
-
,690 cm ), and N–H (CPL, 1,445 cm ).
1
-1
The structure of [CPL]HSO was further confirmed by the NMR spectra as
4
1
shown in Fig. 2. As shown in Fig. 2a, the H NMR spectrum gives multiplet
resonances in the range from 1.73 to 1.80 ppm, assigned to the protons in two
–
CH – and one –CH –, respectively. The protons in –CH – near carbonyl and –NH–
2 2 2
group shift to 2.56 and d3.34 ppm, respectively, showing triplet resonances. The
single resonance at 7.41 or 8.77 ppm reflects the proton in –NH2 – or HSO4 . The
?
-
chemical shift of each carbon in the structure of [CPL]HSO can be observed in the
4
1
3
C NMR spectrum of [CPL]HSO (Fig. 2b).
4
Based on the above data, it is concluded that caprolactam-sulfuric acid IL has
been synthesized.
1
23

Efficient production of 5-hydroxymethylfurfural
Fig. 1 The FTIR spectrum of [CPL]HSO
4
Dehydration of fructose
Effect of the catalyst
Figure 3 shows the conversion of fructose and yield of HMF using different
contents of [CPL]HSO at 105 °C for 90 min. Without [CPL]HSO , both the yield
4
4
of HMF and the conversion of fructose are near zero. This phenomenon is obviously
changed with the use of [CPL]HSO . As the content of [CPL]HSO increases, both
4
4
the conversion of fructose and HMF yield remarkably increase, and then reach the
highest HMF yield (80.5 %) and conversion of fructose (91.1 %) when the content
of [CPL]HSO is 8.6 mmol. Therefore, a higher content of [CPL]HSO is beneficial
4
4
to achieve a better catalytic effect on the dehydration of fructose, but the produced
HMF tends to decompose into acids under acidic condition, so there is an optimum
content of [CPL]HSO to get the maximum yield of HMF.
4
To explain the above statement, the catalysis effect of [CPL]HSO4 in the
dehydration of fructose needs to be revealed. The possible reaction mechanism is
shown in Scheme 1. Initially, fructose loses one water molecule with the presence
of [CPL]HSO4 IL at the hemiketal carbon, leading to the formation of the
intermediate II. As the oxygen atom in the carbonyl group has a nucleophilic effect
and can form a hydrogen bond with fructose, so fructose is activated and the
intermediate II is stabilized [17]. Secondly, the dehydration will produce the
intermediate III, which can hence make a proton transfer from C1 to C2 [18],
leading to the formation of intermediate V. Thirdly, as the proton transfers from O1
to the carbonyl group, the appearance and rearrangement of the enolic species
produce the intermediate VI. Finally, intermediate VI loses two water molecules in
the acid condition, and then HMF is produced.
123

P. Huang et al.
1 13
Fig. 2 The H NMR (a) and C NMR (b) spectra of [CPL]HSO
4
4
Fig. 3 The yields of HMF and conversions of fructose to HMF with different contents of [CPL]HSO .
Reaction conditions: fructose = 1.5 g; water: PGME = 2.5:12.5 (W/W); T = 105 °C, and t = 90 min
Effect of the solvent
Table 1 shows the fructose conversion and the product yield using different ratios of
H O to PGME. Apparently, the ratio of H O to PGME has a large influence on the
dehydration of fructose. When only water or PGME is used as the solvent, the HMF
2
2
yield is 40.0 or 54.2 %. However, interestingly, with a small addition of H O into
2
PGME, the HMF yield significantly increases, to as high as 80.5 % when the ratio is
1
1
:5. When the content of H O is the domain in the mixed solvent, for example,
2
:0.5, the HMF yield is only 68.3 % obtained from the HPLC result.
The above results are attributed to the nature of PGME and H O. Specifically, the
hydroxyl group of PGME and the aldehyde group of HMF tend to form hydrogen
2
1
23

Efficient production of 5-hydroxymethylfurfural
Scheme 1 The possible reaction mechanism of fructose dehydration with the aid of [CPL]HSO
4
Table 1 Yields of HMF and conversions of fructose using the mixed solvent with different compositions
Water:PGME (W:W)
Yield of HMF (%)
Conversion of fructose (%)
0
1
1
1
1
1
:15
:4
54.2
73.7
80.5
76.9
68.3
40.0
63.5
82.4
91.1
87.8
80.7
65.9
:5
:2
:0.5
5:0
Reaction conditions: fructose = 1.5 g; [CPL]HSO = 8.6 mmol; T = 105 °C; t = 90 min
4
bonds, so the polymerization and decomposition of HMF will be reduced. When
pure PGME is used as the solvent, fructose cannot be dissolved completely, and thus
the dehydration of fructose is relatively difficult to take place. When a small content
of H O is added into PGME, the resultant mixed solvent becomes a good solvent for
2
fructose, so the reaction system becomes clear and homogeneous, leading to high
HMF yield and conversion of fructose. However, note that HMF easily undergoes
cross-polymerization and decomposition with the presence of water [19], so a large
content of H O is not beneficial to get a high yield of HMF. Previous researches
2
show that, when aqueous systems were used as the reaction solvent, HMF was easy
123

P. Huang et al.
to decompose into LA and FAs [20, 21]; consequently, the purity of HMF
synthesized using the mixed solvent (2.5:12.5, W/W) as the reaction solvent should
be tested. The HPLC result shows that the purity of HMF synthesized here is as high
as 97 %, that is, the yields of the two by-products are very low. Therefore, there is
an optimum composition of the mixed solvent, and 2.5:12.5 (W/W) is found to be
good for getting a high yield of HMF and fructose conversion. Besides the high
yield of HMF, another advantage of the water/PGME mixed solvent is that HMF is
easily separated from the reaction liquid through distilling.
Biphasic water–organic systems have been reported to be used as the reaction
media for fructose dehydration [22–25]; these results are listed in Table 2. Due to
the low boiling points of these water–organic systems, these reaction systems
usually need high temperature under high pressure and a long reaction time. For
example, it needs a 170 °C reaction temperature and a 70-min reaction time in the
water/butanol system to get a 69.2 % yield of HMF [23]. For a comparison, the
advantage in this work is using a relatively mild reaction condition to obtain a
relatively high yield of HMF.
Reusability of [CPL]HSO in the fructose conversion to HMF
4
The reusability experiments of [CPL]HSO were conducted in the water/PGME
4
solvent. Firstly, separation for HMF was carried out after the first run, then the
catalyst system was used directly in the next run, that is to say, new fructose and
water/PGME solvent were added into the flask with the catalyst system; finally, the
reaction was carried out under the same conditions. The results for five runs are
shown in Fig. 4. The yield of HMF was slightly reduced after each run, the yield
reached approximately 70 % after the catalyst was reused for five times, indicating
that the catalyst [CPL]HSO could be effectively reused in the fructose conversion
4
reaction.
Table 2 Comparison of water/PGME solvent with water–organic systems in the literature for the
dehydration of fructose to HMF
Refs.
Solvent
Catalyst
Temp Time
(
Yield
(%)
°C)
(min)
[22]
Water?4-methyl-2-
pentanone (MIBK)
1-(4-Sulfonic acid)butyl-3-
methylimidazolium hydrogen sulfate
120
180
94.6
(
IL-1)
[
23]
24]
Water?butanol
2.5 mol/L formic acid
170
80
70
69.2
87.0
[
Water?actonitrile
1-Methyl-3-(butyl-4-chlorosulfonyl)
imidazolium chlorosulfate
320
[
25]
Water?nitromethane
Water?PGME
SBA-15-PrSO
3
H
140
105
30
90
69.7
80.5
This
work
[CPL]HSO
4
Reaction conditions: fructose = 1.5 g; water: PGME = 2.5:12.5 (W/W)
1
23

Efficient production of 5-hydroxymethylfurfural
4 3
Fig. 4 The yield of HMF in the reusability of the [CPL]HSO /CrCl catalyst system. Reaction
conditions: water: PGME = 2.5:12.5 (W/W); [CPL]HSO = 8.6 mmol; T = 105 °C; t = 90 min
4
Dehydration of sucrose
It is known that sucrose can release single units of fructose and glucose after
hydrolysis, and the acid catalyst with stronger Brønsted acidity is good for
increasing the hydrolysis rate [26], so HMF can be synthesized through the
isomerization of glucose into fructose and the dehydration of fructose.
To evaluate the effect of [CPL]HSO4 on synthesizing HMF through the
transformation of sucrose, [CPL]HSO /FeCl systems with different loadings of
4
3
[CPL]HSO were used, and the yields of HMF in sucrose dehydration are depicted
4
in Fig. 5. It can be seen that the yield of HMF is nearly zero without the acid IL,
which increases to 40.4 % when 4.3 mmol [CPL]HSO is used; further increasing
4
the loading of [CPL]HSO , the yield of HMF continuously increases to about
4
6
On the other hand, if only FeCl is used, even its content is as high as 10 mol %, the
6.0 % and then almost levels off when the loading of [CPL]HSO is 17.2 mmol.
4
3
yield of HMF is nearly zero, so [CPL]HSO is the decisive catalyst, and can
4
effectively catalyze the formation of HMF derived from the hydrolysis of sucrose
and the dehydration of the monosaccharide. To the best of our knowledge, no
literature has reported the synthesis of HMF from sucrose by using IL as the catalyst
?
-
except for Tong’s group [18], who used [NMM] [CH SO ] as the catalyst, and the
3
3
mixed DMF/LiBr as the solvent; however, the yield of HMF was 47.5 %.
Obviously, the new catalyst and the solvent used in this paper have advantages in
synthesizing HMF.
On the other hand, we evaluated [CPL]HSO with other common metal halides
4
as the catalyst system in the dehydration of sucrose into HMF, and the results are
listed in Table 3. We found that the 44.0 % yield of HMF is obtained at 105 °C for
1
20 min when [CPL]HSO is used as the catalyst. For a comparison, it can be seen
4
that the presence of 10 mol % metal halide will lead to a higher yield of HMF. It
may be that cations have high affinity for oxygen atoms, and thus can coordinate
123

P. Huang et al.
4 3
Fig. 5 The yields of HMF in the dehydration of sucrose using the [CPL]HSO /FeCl catalyst system
with different loadings of [CPL]HSO . Reaction conditions: sucrose = 5 g; FeCl
4
3
= 0.23 g (10 mol %);
water: PGME = 10:20 (W/W); T = 105 °C; t = 120 min
Table 3 The effect of catalyst for the dehydration of sucrose to HMF
Catalyst
Temp. (°C)
Time (min)
Yield (%)
[CPL]HSO
[CPL]HSO
[CPL]HSO
[CPL]HSO
[CPL]HSO
[CPL]HSO
4
4
4
4
4
4
105
105
105
105
105
105
120
120
120
120
120
120
44.0
68.2
56.3
64.3
45.6
47.3
/CrCl
3
ꢀ6H
2
O
/ZnCl
2
/AlCl
/LiCl
/LiBr
3
4
Reaction conditions: sucrose = 5 g; water: PGME = 10:20 (W/W); [CPL]HSO = 17.2 mmol; Mole
percent of metal halides is 10 mol % based on sucrose
with the monosaccharide (glucose and fructose) in the reaction mixture [27]; at the
-
same time, Cl shows a positive effect in the conversion as a base and a nucleophile
[
28, 29] in the isomerization of glucose into fructose and the dehydration of
fructose. Note that CrCl and AlCl have a similar but larger effect.
3
3
Compared with the dehydration of fructose, the synthesis of HMF from sucrose
dehydration is relatively difficult. However, the price of sucrose is much lower than
that of fructose, so the synthesis of HMF based on sucrose is still a promising
strategy.
Conclusions
By developing [CPL]HSO IL or [CPL]HSO IL/metal halide as the catalyst and
4
4
water/PGME as the mixed solvent, an efficient production of HMF through the
1
23

Efficient production of 5-hydroxymethylfurfural
dehydration of fructose or sucrose is achieved. With the suitable parameters at
1
05 °C for only 90 or 120 min, the yield of HMF is as high as 80.5 or 64–68 %,
respectively. When metal halide is used as a co-catalyst, a high HMF yield from the
dehydration of sucrose can be obtained. In addition, the separation of HMF is easy
to carry out owing to the relatively low boiling point of the water/PGME solvent.
Acknowledgments The authors thank the Priority Academic Program Development of Jiangsu Higher
Education Institutions (PAPD) and Suzhou Applied Basic Research Program (SYG201141) for
financially supporting this project.
References
1
2
3
4
5
. A.A. Rosatella, S.P. Simeonov, R.F.M. Frade, C.M. Afonso, Green Chem. 13, 754 (2011)
. G. Tian, X. Tong, Y. Wang, Y. Yan, S. Xue, Res. Chem. Intermed. 39, 3255 (2013)
. C. Triebl, V. Nikolakis, M. Ierapetritou, Comput. Chem. Eng. 52, 26 (2013)
. F.K. Kazi, A.D. Patel, J.C. Serrano-Ruiz, J.A. Dumesic, R.P. Anex, Chem. Eng. J. 169, 329 (2011)
. D.B. Bevilaqua, M.K.D. Rambo, T.M. Rizzetti, A.L. Cardoso, A.F. Martins, J. Clean. Prod. 47, 96
(2013)
6. B. Karimi, H.M. Mirzaei, RSC Adv. 3, 20655 (2013)
7
. A. Corma, S. Iborra, A. Velty, T. St a˚ hlberg, M.G. Sørensen, A. Riisager, Chem. Rev. 107, 2411
(2007)
8
. C. Moreau, A. Finiels, L.J. Vanoye, Mol. Catal. A 253, 165 (2006)
. B. Liu, Z.H. Zhang, Z.B.K. Zhao, Chem. Eng. J. 215–216, 517 (2013)
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
0. S.H. Xiao, B. Liu, Y.M. Wang, Z.F. Fang, Z.H. Zhang, Bioresour. Technol. 151, 361 (2014)
1. Z.F. Fang, B. Liu, J.J. Luo, Y.S. Ren, Z.H. Zhang, Biomass Bioenerg. 60, 171 (2014)
2. Z. Hu, B. Liu, Z.H. Zhang, L.Q. Chen, Ind. Crops Prod. 50, 264 (2013)
3. M.E. Zakrzewska, E. Bogl-Łukasik, R. Bogel-Łukasik, Chem. Rev. 111, 397 (2011)
4. J. Yang, Q. Zhang, L. Zhu, S. Zhang, J. Li, X. Zhang, Y. Deng, Chem. Mater. 19, 2544 (2007)
5. Z. Du, Z. Li, S. Guo, J. Zhang, L. Zhu, Y. Deng, J. Phys. Chem. B 109, 19542 (2005)
6. S. Guo, Z. Du, S. Zhang, D. Li, Z. Li, Y. Deng, Green Chem. 8, 296 (2006)
7. B.M. Kabyemela, T. Adschiri, R.M. Malaluan, K. Arai, Ind. Eng. Chem. Res. 38, 2888 (1999)
8. X. Tong, Y. Ma, Y. Li, Appl. Catal. A 385, 1 (2010)
9. B.F.M. Kuster, Starch 42, 314 (1990)
0. D. Sto sˇ si c´ , S. Bennici, V. Raki c´ , A. Auroux, Catal. Today 192, 160 (2012)
1. X.H. Qi, M. Watanabe, T.M. Aida, R.L. Smith Jr., Catal. Commun. 10, 1771 (2009)
2. F. Tao, H. Song, L. Chou, RSC Adv. 1, 672 (2011)
3. N. Jiang, R. Huang, W. Qi, R. Su, Z. He, Bioenerg. Res. 5, 380 (2012)
4. T. Okano, K. Qiao, Q. Bao, D. Tomida, H. Hagiwara, C. Yokoyama, Appl. Catal. A 451, 1 (2013)
5. Z.-D. Ding, J.-C. Shi, J–.J. Xiao, W.-X. Gu, C.-G. Zheng, H.-J. Wang, Carbohydr. Polym. 90, 792
(2012)
26. G. Yong, Y.G. Zhang, J.Y. Ying, Angew. Chem. 120, 9485 (2008)
27. Z. Zhang, B. Liu, Z. Zhao, Carbohyd. Polym. 88, 891 (2012)
28. J.B. Binder, R.T. Raines, J. Am. Chem. Soc. 131, 1979 (2009)
29. T. Stahlberg, M.G. Sorensen, A. Riisager, Green Chem. 12, 321 (2010)
123