Microwave-assisted rapid conversion of carbohydrates into 5-hydroxymethylfurfural by ScCl3 in ionic liquids

Article abstract of DOI:10.1016/j.carres.2013.04.003

In this study, synthesis of HMF from carbohydrates was carried out in ionic liquids 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) catalyzed by ScCl ₃ under microwave irradiation. Under the optimal reaction conditions, HMF was obtained in a high yield of 73.4% in 2 mins with the microwave power at 400 W. Compared with the conventional oil-bath heating manner, the use of microwave irradiation not only reduced reaction times from hours to minutes, but also improved HMF yield. This catalytic system could be reused several times without losing its catalytic activity. This efficient catalytic system will generate a promising application strategy for biomass transformation.

Full text of DOI:10.1016/j.carres.2013.04.003

Carbohydrate Research 375 (2013) 68–72
Contents lists available at SciVerse ScienceDirect
Carbohydrate Research
journal homepage: www.elsevier.com/locate/carres
Microwave-assisted rapid conversion of carbohydrates into
5
-hydroxymethylfurfural by ScCl in ionic liquids
3
⇑
⇑
Xuanmu Zhou, Zehui Zhang , Bing Liu, Zheng Xu, Kejian Deng
Key Laboratory of Catalysis and Materials Sciences of the State Ethnic Affairs Commission & Ministry of Education, College of Chemistry and Material Science, South-Central
University for Nationalities, Wuhan 430074, China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, synthesis of HMF from carbohydrates was carried out in ionic liquids 1-butyl-3-methylimi-
Received 13 January 2013
Received in revised form 27 February 2013
Accepted 3 April 2013
dazolium chloride ([Bmim]Cl) catalyzed by ScCl
tion conditions, HMF was obtained in a high yield of 73.4% in 2 mins with the microwave power at
00 W. Compared with the conventional oil-bath heating manner, the use of microwave irradiation
not only reduced reaction times from hours to minutes, but also improved HMF yield. This catalytic sys-
tem could be reused several times without losing its catalytic activity. This efficient catalytic system will
generate a promising application strategy for biomass transformation.
3
under microwave irradiation. Under the optimal reac-
4
Available online 11 April 2013
Keywords:
Sucrose
Scandium(III) chloride
Ó 2013 Elsevier Ltd. All rights reserved.
5
-Hydroxymethylfurfual
Ionic liquids
Microwave irradiation
1
. Introduction
With the diminishing of fossil fuel reserves, there is a growing
the industrial production of sucrose. Compared with the price of
fructose and glucose, the price of sucrose is relative lower. There-
fore, sucrose shows a great potential in the large-scale synthesis
9
interest in the development of alternative sources for energy and
chemicals. Renewable biomass, in particular carbohydrates, has
been widely regarded as a promising alternative to fossil resources
to supply fuels and chemicals.1–3 Recently, much effort has been
devoted to develop new catalytic routes for the conversion of car-
bohydrates into valuable chemicals. Among the various bio-based
chemicals, 5-hydroxymethylfurfural (HMF), the acid-catalyzed
dehydration product of C6-based carbohydrates, is considered to
of HMF. To date, there were some reports on the synthesis of
HMF from sucrose. Synthesis of HMF from carbohydrates was usu-
ally carried out in water, polar organic solvents, and biphasic
10
water–organic systems. Although a high HMF yield of HMF was
usually obtained from fructose, these systems were not very effec-
tive for the conversion of glucose into HMF. Recently, ionic liquids
(ILs) have been widely used as convenient and popular solvents for
many chemical reactions, due to their excellent properties such as
negligible vapor pressure, nonflammability, and high thermostabil-
4
5
be one of top building-block chemicals. HMF can be used as a ver-
1
1
satile and key precursor for the synthesis of pharmaceuticals, furan
based polymers, and fine chemicals.6 In recent years, synthesis of
HMF has been extensively studied. Generally, fructose is the pref-
erable feedstock for the synthesis of HMF with high efficiency and
selectivity. However, the large-scale synthesis of HMF from fruc-
tose is limited due to the high price of fructose. Therefore, it is
desirable to use the cheap and renewable glucose based carbohy-
drates for the synthesis of HMF.
ity. Synthesis of HMF from sucrose was reported to be effective in
,7
12,13
14
ILs with metallic chlorides as a catalyst.
developed an efficient method for the synthesis of HMF from su-
crose with a yield of 87% using CrCl as a catalyst and NH Br as a
co-catalyst. Although a good result was achieved, the high toxicity
of CrCl inevitably resulted in a risk of the contamination of the fi-
Recently, Wang et al.
3
4
3
nal product HMF and the environment.
In recent years, the use of microwave irradiation (MI) has been
widely applied in many fields such as organic chemistry, medical
Sucrose is widely present in plants with an estimated annual
8
15
production of 167 million metric tons. As the most common disac-
chemistry, and material synthesis. One of the most important
charide, sucrose is consisted of glucose and fructose units. Sugar-
cane and sugar beet are the two main sugar crops resource for
advantages in the use of MI is that MI always improves the reaction
rate, reducing the reaction time from hours down to minutes. In
some cases, the ‘microwave effect’ also improves the reaction
selectivity. It is well-known that developing more environmentally
friendly catalytic system is still remained an issue of scientific
interest and industrial significance. Some of the rare earth element
⇑
Corresponding authors. Tel./fax: +86 27 67842752.
E-mail addresses: zehuizh@mail.ustc.edu.cn (Z. Zhang), dengkj@scuec.edu.cn
(
K. Deng).
0
008-6215/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.carres.2013.04.003

X. Zhou et al. / Carbohydrate Research 375 (2013) 68–72
69
HOH C
2
O
O
Microwave irradiation
OH
HO
OH
O
O
OH
HO
HO
O
OH
OH
ScCl , [Bmim]Cl
3
Sucrose
HMF
3
Scheme 1. Conversion of sucrose into HMF catalyzed by ScCl in ILs under microwave irradiation.
based compounds are believed to be non-toxic and are widely uti-
lized for the replacement of toxic heavy metals in technological
M
T
ðmgÞ ¼ HMF concentration ðmg=mLÞ ꢁ V
1
ꢁ ðM
=the theoretical weight of HMF ðmgÞ
100%
0
=M
1
Þ
1
6
applications. Herein, synthesis of HMF has been developed in
-butyl-3-methylimidazole chloride ([Bmim]Cl) catalyzed by scan-
dium(III) chloride (ScCl ) under microwave irradiation (Scheme 1).
HMF yield ðpercentÞ ¼ M
T
1
ꢁ
3
This new catalytic system was proved to be effective in the synthe-
sis of HMF with a good yield in a few minutes.
T
where M is the mass of HMF in the reaction mixture at a given
reaction time (corresponding to the Section 2.2);
HMF concentration (mg/mL) was based on the standard calibra-
tion curves.
2
2
. Experimental section
M
0
is the total mass of the initial reaction mixture (correspond-
ing to the Section 2.2).
is the mass of sample withdrawn from the reaction mixture
corresponding to the Section 2.2).
is the volume of the sample after diluted with water for HPLC
corresponding to the Section 2.2).
.1. Materials and instruments
M
1
1
-Butyl-3-methylimidazole chloride ([Bmim]Cl) was synthe-
(
17
sized according to the known procedures. Glucose was purchased
from ABCR GmbH & Co. (Karlsruhe, Germany). Fructose was pur-
chased from Sanland-Chem International Inc. (Xiamen, China). Su-
V
1
(
The theoretical mass of HMF was calculated from the reaction
equation of the conversion of carbohydrates into HMF.
For 100 mg of glucose (or fructose) as a substrate, the theoreti-
cal mass of HMF was 70 mg.
For 100 mg of glucose (or fructose) as a substrate, the theoreti-
cal mass of HMF was 73.6 mg (Scheme 1).
crose and ScCl
3
ꢀ6H
2
O were supplied by Sinopharm Chemical
Reagent Co., Ltd (Shanghai, China). Acetonitrile (HPLC grade) was
purchased from Tedia Co. (Fairfield, USA). All other reagents and
solvents were obtained from commercial suppliers (Wuhan
Guoyao Chemical Reagent Co., Ltd).
WBFY-205 microwave reactor was purchased from Gongyi City
Yuhua Instrument Co. Ltd (Gongyi, China).
2
.4. Recycling of catalyst
2
.2. Synthesis of HMF from sucrose
At the end of each reaction, the product HMF was extracted
In a typical procedure, sucrose (100 mg) was firstly added into
using ethyl acetate successively until no HMF was detected by thin
layer chromatography (TLC) method. After HMF was completely
extracted from the reaction mixture, sucrose was added into the
remaining mixture, and otherwise steps were the same as de-
scribed for the first run.
[
1
Bmim]Cl (2.0 g). Then the mixture was violently stirred at
00 °C with a magnetic stirrer until a clear solution was formed.
After the dissolution of sucrose in [Bmim]Cl, ScCl (10 mol %) was
added into the above solution. Then the reaction mixture in a
0 mL batch reaction vessel sealed with a cap was transformed into
3
1
a WBFY-205 microwave reactor, and the reaction was carried out
under a microwave irradiation at 400 W. At the desired reaction
time, samples were withdrawn from the reaction mixture (re-
corded as M ), which were then diluted with distilled water to give
1
a crude solution. Then the solution was centrifuged at 12,000 rpm
for 5 min to precipitate the insoluble materials, and the upper solu-
3
3
. Results and discussion
.1. Synthesis of HMF from fructose and glucose in [Bmim]Cl
catalyzed by ScCl
3
ꢀ6H
2
O
Synthesis of HMF was initially carried out using fructose and
glucose as starting materials (Table 1), as sucrose is consisted of
glucose and fructose units. Firstly, catalytic dehydration of fructose
into HMF was carried out in water (Table 1, entry 1), polar organic
solvents (Table 1, entries 2 and 3) and ionic liquids (Table 1, entry
4) at 110 °C with oil-bath heating. HMF was obtained in a low yield
of 2.3% when the dehydration reaction was carried out in water
(Table 1, entry 1). The bad catalytic performance in water should
be attributed to the following two reasons. On the one hand, the
solvent water would inhibit the dehydration process, as three
molecular waters are released by the dehydration of one molecular
fructose into HMF. On the other hand, it is well known that HMF is
readily redehydrated into levulinic acid and formic acid in the
tion was analyzed by a HPLC method (recorded as V
1
).
2
.3. Quantification procedure for HMF
HMF was analyzed by external standard calibration curve meth-
od, which was carried out on VARIAN ProStar 210 HPLC system.
Samples were separated by reversed-phase C18 column
200 ꢁ 4.6 mm) at 30 °C. The mobile phase was consisted of aceto-
nitrile and 0.1 wt% acetic acid aqueous solution (v/v 15:85) at
.0 mL/min, and HMF was detected at a wavelength of 280 nm.
a
(
1
The content of HMF in solution was calculated directly by interpo-
lation from calibration curves with a coefficient of 0.999, then we
can calculate the actual weight of HMF in the reaction system,
based on the volume of the samples. The yield of HMF was calcu-
lated as the ratio of actual mass of HMF to the theoretical mass of
HMF, which was based on the substrate and the reaction equation.
HMF yield was calculated according to the following equation:
1
8
presence of water.
Some polar organic solvents such as DMSO were reported to be
a good reaction medium for the synthesis of HMF from fructose.19
When the dehydration of fructose was carried out in DMSO and
3
DMF with ScCl as catalyst, HMF yields were reached in 85.4%

7
0
X. Zhou et al. / Carbohydrate Research 375 (2013) 68–72
Table 1
from fructose under the same MI reaction conditions used for glu-
cose (Table 1, entry 9).
a
Dehydration of glucose and fructose into HMF under various conditions
Entry Substrate Solvents
Reaction
conditions
Time
(min)
Yields
(%)
The temperature of the reaction system under MI at 400 W for
2 min was detected to be 180 °C. Therefore, in order to exclude the
effect of reaction temperature on the yield of HMF, the dehydration
1
2
3
Fructose
Fructose
Fructose
Fructose
Fructose
Glucose
Glucose
Glucose
Fructose
Glucose
Glucose
Glucose
Water
DMSO
DMF
110 °C, Oil-heating 120
110 °C, Oil-heating 120
110 °C, Oil-heating 120
2.3
85.4
80.7
93.3
30.1
32.3
5.0
55.4
94.7
22.1
35.6
28.7
3
of glucose into HMF catalyzed by ScCl was carried out at 180 °C
using an oil-bath. As shown in Table 1 (entries 10–12), the maxi-
mum HMF yield was obtained in 35.6% at 15 min. Although the
high temperature with an oil-bath heating benefited the dehydra-
tion process, resulting in higher HMF in a short time (Table 1, en-
tries 6 vs 8), the result was inferior to that under MI (Table 1,
entries 8 vs 11). Compared the dehydration results under MI with
those using conversional oil-bath heating, the main reason should
4
[Bmim]Cl 110 °C, Oil-heating 120
[Bmim]Cl 110 °C, Oil-heating 120
[Bmim]Cl 110 °C, Oil-heating 120
[Bmim]Cl 110 °C, Oil-heating 120
[Bmim]Cl MI, 400 W
[Bmim]Cl MI, 400 W
[Bmim]Cl 180 °C, Oil-heating
[Bmim]Cl 180 °C, Oil-heating
[Bmim]Cl 180 °C, Oil-heating
b
5
6
b
7
8
9
0
1
2
2
2
10
15
20
1
1
1
2
2
be attributed to the ‘microwave effects’. In fact MI produces effi-
cient internal heating or in-core volumetric effect heating by direct
coupling of microwave energy with the molecules that may be the
solvents, reagents, or catalysts presented in the reaction mixture.
Therefore, it circumvented some conventional oil-bath heating
drawbacks such as partly overheating. In addition, it is also possi-
ble to lower activation energy or increase the pre-exponential fac-
tor in Arrhenius law due to orientation effect of polar species in an
electromagnetic field, leading to a more effective and clean chem-
ical reaction.
a
Reaction conditions: To 2.0 g of solvent were added 100 mg of fructose or
glucose and 10 mol % ScCl , then the reaction was carried out at 110 °C with oil-
3
heating or under MI at 400 W for the desired time.
The reaction was done under otherwise identical conditions in the absence of
catalyst.
b
and 80.7% for DMSO and DMF, respectively (Table 1, entries 2 and
). Although high HMF yields were achieved in polar organic sol-
3
vents, these processes suffered some serious drawbacks such as te-
dious procedures for the separation of HMF from the reaction
mixture, and the risk of the volatile organic solvents to our envi-
ronment. Recently, ILs have been extensively used as green sol-
vents in biorefinery, as ILs have a good ability to dissolve
carbohydrates and catalysts, leading the catalyst and carbohy-
drates to be contacted with each other effectively. In addition,
some ILs such as [Bmim]Cl were not miscible with lower boiling
point organic solvents such as ethyl acetate, therefore the product
HMF can be separated from the ILs system via extraction method
with ethyl acetate. As expected, HMF was obtained from fructose
in a high yield of 93.3% in [Bmim]Cl at 110 °C for 2 h in the pres-
3
.2. Microwave-assisted rapid conversion of sucrose into HMF
+
Glycosidic bonds are usually weakened by H . However, glyco-
sidic bonds were also weakened by Lewis acids though their coor-
dination with oxygen atoms in the glycosidic bonds, which was
then attacked by H O to release monosaccharides. The good re-
2
2
3
sults of the dehydration of glucose and fructose into HMF inspired
us to carry out the synthesis of HMF directly from sucrose, which
avoids the tedious hydrolysis and separation processes. The dehy-
dration of sucrose into HMF was carried out under MI at 400 W,
and the time course of HMF yield was recorded (Fig. 1). The yield
of HMF was slowly increased during the first min, and then rapidly
increased from 5.1% to 73.4% during 1 to 2.5 min. It indicated that
hydrolysis of sucrose was the first step for the one-pot conversion
of sucrose into HMF, and the hydrolysis step was mainly performed
in the first min. Further prolonged the reaction time from 2.5 to
ence of ScCl
3
(Table 1, entry 4). With the in-depth study of ILs, it
was reported that the hydrogen at C2 position in the imidazole ring
2
0
showed acidic property. HMF was obtained in a yield around 40%,
when fructose was subjected to be heated at 100 °C in 1-ethyl-3-
methylimidazole chloride ([Emim]Cl) for 3 h in the absence of cat-
2
1
alyst. Dehydration of fructose into HMF was also carried out at
10 °C in [Bmim]Cl without ScCl for 2 h, and HMF was obtained
in a yield of 30.1% (Table 1, entry 5). The results verified that
Bmim]Cl was positive for the conversion of fructose into HMF in
5
5
min, the yield of HMF was gradually decreased from 73.4% to
3.2%. The results indicated that HMF was not very stable under
1
3
our reaction conditions. After the completion of the reaction, some
humins were also observed, when the reaction mixture was diluted
[
comparison with other solvents, but the catalyst ScCl
key role in achieving a high HMF yield.
3
played a
Secondly, catalytic conversion of glucose into HMF was also car-
ried out in [Bmim]Cl at 110 °C with ScCl as catalyst, and a moder-
3
ate yield of 32.3% was obtained (Table 1, entry 6). It indicated that
the efficiency of glucose dehydration into HMF was much lower
than that of fructose dehydration. It is generally accepted that
two successive steps are required during the conversion of glucose
into HMF, namely the isomerization of glucose into fructose and
the followed dehydration of fructose into HMF. As high HMF yield
was obtained from fructose, the relative low HMF yield from glu-
cose should be attributed to the low selectivity of the isomeriza-
tion of glucose into fructose. As a control experiment, the
dehydration of glucose was also carried out at 110 °C in the ab-
sence of SeCl
yield of 5% (Table 1, entry 7). The results clearly indicated that
SeCl played a crucial role in the dehydration of glucose into HMF.
3
for 120 min, and HMF was only obtained in a low
3
Recently, the use of microwave irradiation (MI) in chemical
reaction has attracted great interest by chemists. When the dehy-
dration of glucose was carried out under MI at 400 W, HMF yield
was largely increased from 32.3% to 55.4% in 2 min (Table 1, entries
Figure 1. Time course of HMF yield during the dehydration of sucrose into HMF.
Reaction conditions: To 2.0 g of [Bmim]Cl were added 100 mg of sucrose and
8
vs 6). In parallel, a high HMF yield in 94.7% was also obtained
3
10 mol % ScCl , then the reaction was carried out under MI at 400 W.

X. Zhou et al. / Carbohydrate Research 375 (2013) 68–72
71
Table 2
The results of microwave-assisted catalytic conversion of sucrose into HMF under different conditions
Entry
1a
Conditions
Solvent
Time (min)
HMF yields (%)
Ref.
MI 240 W, 10 mol % ScCl
MI 400 W, 10 mol % ScCl
MI 640 W, 10 mol % ScCl
MI 400 W, 5 mol % ScCl
MI 400 W, 2.5 mol % ScCl
3
3
3
[Bmim]Cl
[Bmim]Cl
[Bmim]Cl
[Bmim]Cl
[Bmim]Cl
Choline chloride
[Bmim]Cl
[Bmim]Cl
8
67.7
73.4
61.3
70.3
71.5
43
This work
This work
This work
This work
This work
Ref. 27
Ref. 28
Ref. 29
Ref. 30
Ref. 31
a
2
3
4
5
2.5
1.5
6
9
60
5
30
180
180
5
a
a
3
a
3
6
7
8
9
CrCl
CrCl
2
, 100 °C
3
, MI, 100 °C
76
GeCl
SnCl
FeCl
AlCl
4
, 120 °C
, 100 °C
55.4
65.0
38.0
42.5
4
[EMIM][BF
DMF
DMSO
4
]
1
1
0
1
3
4
–Et NBr, 90 °C
3
, MI, 140 °C
Ref. 32
a
3
Reaction conditions: To 2.0 g of [Bmim]Cl were added 100 mg of sucrose and a set amount of ScCl , then the reaction was carried out under MI at a set power for the
desired time.
with water. The humins were likely formed through the polymer-
ization and cross-polymerization of HMF.24
Taking the above results into accounts, the combination effect
3
of [Bmim]Cl, ScCl , and MI generates the magic power for catalytic
conversion of sucrose into HMF. Firstly, complete dissolution of su-
crose in [Bmim]Cl makes the sucrose chains accessible to be con-
tacted with catalyst for the chemical transformation. Secondly,
[
Bmim]Cl has excellent dielectric properties for the transformation
of microwave into heat, that was the mentioned ‘microwave ef-
fects’. More importantly, ScCl
3
would form a series of complex ions
ꢂ
2ꢂ
such as [ScCl
4
]
and [ScCl
5
]
in [Bmim]Cl in a manner similar to
2
5
26
3
LnCl ,
due to the high coordination ability of element Sc. These
complexes played an important role in the isomerization of glucose
into fructose, followed by dehydration to produce HMF.
3
.3. Microwave-assisted dehydration of sucrose into HMF under
various conditions
3
Figure 2. Recycle experiments of ScCl /[Bmim]Cl catalytic system.
The effect of MI power on the conversion of sucrose into HMF
was further investigated, and the MI power was set at 240, 400,
and 640 W, respectively (Table 2, entries 1–3). When the reaction
was carried out at 240 W, it required a longer reaction time in
with an oil-bath heating (Table 2, entries 2, 7, 11 vs 6, and 8–10).
It was also observed that ionic liquids were superior to organic sol-
vents for the dehydration of sucrose into HMF (Table 2, entries 1–9
8
min to obtain a maximum HMF yield of 67.7% in comparison
with the reaction performed at 400 W (Table 2, entries 1 vs 2). Fur-
ther increased MI power to 640 W, albeit reduced the reaction time
to 1.5 min, the maximum yield of HMF yield was decreased to
vs 10 and 11). As demonstrated in Table 2, ScCl showed a satisfied
3
catalytic activity as other Lewis acid.
6
1.3% (Table 2, entry 3). Therefore, 400 W was selected as an
appropriate MI power to obtain the highest HMF yield of 73.4%.
The effect of catalyst loading on the conversion of sucrose into
HMF was also studied. It was found that the catalyst loading had
no obvious effect on the maximum HMF yield, and the maximum
yields of HMF were 73.4%, 70.3%, and 71.5% for 10, 5, and
3.4. Catalyst reusability experiment
Finally, the reusability of this novel catalytic system was evalu-
ated. The dehydration of sucrose into HMF at 400 W with 10 mol %
ScCl was used as a model reaction. After the completion of the first
3
2
.5 mol % ScCl
3
, respectively, (Table 2, entries 2, 4, and 5). But the
run, the product HMF was extracted by ethyl acetate successively
reaction time for obtaining the maximum HMF yield was de-
creased with an increase of catalyst loading. The more the catalyst
loading was, the higher the dehydration rate was. A higher dehy-
dration rate with an increase in catalyst loading can be attributed
to an increase in the availability and number of catalytically active
sites.
until no HMF was detected by thin layer chromatography (TLC)
method. After HMF was completely extracted, sucrose was then
added into the remaining mixture system. The reaction was carried
out under the same conditions. These steps were repeated six
times, and the reusability results are shown in Figure 2. It was
found that there was a slight decrease trend in the HMF yield. As
shown in Figure 2, HMF yield was 73.4% in the first run, and it
was reduced to 66.8% in the sixth run. In our reuse experiment,
the product HMF was extracted by organic solvents from the reac-
tion mixture. Then the fresh sucrose was added into the mixture
for the second run. Thus one important reason was that trace
amount of ScCl3 was extracted from the reaction system, leading
to a slight decrease in HMF yield. It might also be caused by the
combination of byproducts such as humins with the catalyst, lead-
Our reaction system for the synthesis of HMF from sucrose was
compared with other reaction systems catalyzed by other Lewis
2
7–32
acid.
As demonstrated in Table 2, it can be seen that our reac-
tion system was comparable with some reaction systems, and it
was better than some reaction system. The efficiency for the dehy-
dration of sucrose into HMF seems depended on the Lewis acid cat-
alysts, the reaction solvent, the reaction temperature, and the
heating manner. It was clearly noted that the reaction time for
obtaining a maximum HMF yield under MI was shorter than that
3
3
ing to decease in the catalytic activity.

7
2
X. Zhou et al. / Carbohydrate Research 375 (2013) 68–72
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. Conclusion
2
1
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3
as a catalyst under MI.
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3
alytic conversion of sucrose into HMF. This catalytic system could
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Acknowledgments
1
809–1818.
The Project was supported by the National Natural Science
Foundation of China (No. 21203252) and the Special Fund for Basic
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