Effects of cations and anions of ionic liquids on the production of 5-hydroxymethylfurfural from fructose

Article abstract of DOI:10.1039/c2cc30357d

Ionic liquids (ILs) with different cations and anions were used to study the effects on 5-hydroxymethylfurfural (HMF) preparation. It was found that both aggregations of cations and hydrogen bonds of anions with fructose played important roles. Molecular dynamics simulations were also performed to explain the experimental results.

Full text of DOI:10.1039/c2cc30357d

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COMMUNICATION
Effects of cations and anions of ionic liquids on the production of
5-hydroxymethylfurfural from fructosew
Chunyan Shi,a Yuling Zhao,b Jiayu Xin,a Jinquan Wang,a Xingmei Lu,a Xiangping Zhanga
and Suojiang Zhang*a
Received 16th January 2012, Accepted 3rd March 2012
DOI: 10.1039/c2cc30357d
Ionic liquids (ILs) with different cations and anions were used to
study the effects on 5-hydroxymethylfurfural (HMF) preparation. It
was found that both aggregations of cations and hydrogen bonds of
anions with fructose played important roles. Molecular dynamics
simulations were also performed to explain the experimental results.
It was also found that not all the ILs play a positive role in the
dehydration of monosaccharide,13 some of them restrain the
HMF production even in the presence of catalysts. Due to this
problem, some researchers have to choose ILs–organic solvents
complex systems.14 Still, researchers are trying to find the most
suitable ILs for HMF preparation by using ‘‘trying approach’’,
and are far from getting a sufficient understanding on the details
of effects of cations and anions of ILs on the dehydration of
monosaccharide. It will be of great significance to find more
effective catalytic systems and suitable ILs to produce HMF.
In this work, trifluoromethanesulfonic acid (TfOH), a kind
of promising and effective catalyst, was used for the first time
for conversion of fructose to HMF. ILs used as a medium with
various alkyl chains of imidazolium cations and various kinds
of anions were selected to study the effects of cations and
anions on fructose for HMF preparation. Molecular dynamics
simulations were also performed to verify the experimental
results. We expect that the presented results may provide some
basic aids for finding suitable ILs for HMF preparation.
TfOH was selected in our experiment as the catalyst for
HMF preparation. A typical reaction protocol was performed
at 120 1C for 60 min. The catalytic performance of TfOH was
compared with those of other catalysts commonly used for
HMF preparation, including Brønsted acids (H2SO4) and
Lewis acids (CuCl2 and FeCl3). The results are shown in
Fig. 1. The highest conversion of fructose (up to 96%) and
yield of HMF (up to 88%) were obtained by using TfOH as
the catalyst. TfOH is known for its stronger acidity than the
rest of the catalysts used. The superior performance of TfOH
is due to its stronger proton donor ability and homogeneous
catalytic system. The conversion of fructose to HMF is closely
associated with both acidity and type of acid catalyst.15 The
higher catalyst acidity leads to the higher reaction activity
because the intermediate product, cyclic furan or enediol
catalyzed by dehydration of fructose, can undergo further
dehydration steps to produce HMF with the increasing acidity
in the reaction system.15 At a reaction temperature of 120 1C,
HMF yield of 80% was obtained even with a reaction time of
5 min. When the reaction temperature was 80 1C, the HMF
yield of 70% also was obtained for 5 min reaction time
(see Fig. S1, ESIw). So the TfOH catalyst not only favoured
the dehydration of fructose, but also exhibited a high reaction rate.
HMF was separated from the reaction system with continuous
With the depletion of fossil fuels, utilization of biomass has drawn
great attention for the production of fuels and chemicals because it
is abundant, renewable and CO2 neutral. One of the top building-
block chemicals obtained from biomass1 is 5-hydroxymethyl-
furfural (HMF) which is known as ‘‘important platform
compound’’2 due to its great potential application as a versatile
intermediate between biomass-based and petroleum-based
chemicals for preparing fine chemicals, pharmaceuticals, plastic
resins, liquid transportation fuels, etc.3
The dehydration of monosaccharide to produce HMF was
studied by many scientists by employing different catalytic
systems and reaction mediums. Catalysts including mineral
and organic acids,4 salts,5 zeolites6 and ion-exchange resins7
have already been widely investigated with some advantages
and drawbacks in HMF preparation, such as catalyst toxicity,
lower fructose conversion and long reaction times.
Recently, investigations on reaction mediums for HMF
preparation presented in the literature have been mostly
focussing on ionic liquids (ILs), which have received increased
attention during the past few years.8 ILs have a lot of well
known advantages when they are used as mediums in reaction
systems.9 Some ILs are exceptionally powerful solvents for
dissolving because they disrupt the intermolecular hydrogen
bonds,10 and some can benefit the selectivity and the conversion
of monosaccharide to HMF.11 ILs also resolve the problems of
HMF recovery and high reaction temperature.12
a Beijing Key Laboratory of Ionic Liquids Clean Process,
Key Laboratory of Green Process and Engineering,
State Key Laboratory of Multiphase Complex System,
Institute of Process Engineering, Chinese Academy of Sciences,
Beijing 100190, PR China. E-mail: sjzhang@home.ipe.ac.cn;
Fax: +86 82544875; Tel: +86 82544875
b Key Laboratory of Green Chemical Media and Reactions, Ministry
of Education, School of Chemistry and Environmental Science,
Henan Normal University, Xinxiang, Henan 453007, PR China
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2cc30357d
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 4103–4105 4103

the molecular interior, thus hindering the reaction. Aggregations
also prevent hydrogen at the C2 position of the imidazolium
cation contacting with –OH of fructose molecules to dehydrate
and to form HMF.13 Hydrogen at the C2 position of the
imidazolium cation, once thought to be the most important
factor of dehydration of fructose, was replaced by a methyl
group named [Bdmim]Cl. The yield of HMF (37%) with
[Bdmim]Cl was about 10% lower than that with [Bmim]Cl
(48%) in the absence of any catalyst. It was shown that the C2
hydrogen just played a partial role in dehydration of fructose
because other hydrogens on the imidazolium cation also have
activities on reactions.18 Therefore, it can be concluded that
imidazolium cationic ILs with an alkyl chain of cations shorter
than 4 are suitable for HMF preparation.
Fig. 1 Conversion of fructose and its yield to HMF by employing
TfOH as catalyst in [Bmim]Cl. Reaction conditions: 0.15 g fructose,
0.1 g TfOH, 0.1 g H2SO4, 0.1 g CuCl2 and 0.1 g FeCl3, 5 g [Bmim]Cl
and 120 1C for 60 min.
Molecular dynamics simulations were performed on
ILs–fructose reaction systems for a better understanding of
experimental results (see ESIw for simulation details). Four
typical imidazolium cationic ILs used in the experiment were
chosen, and their molecular interaction energies (see Table 1)
and radial distribution functions (see Fig. S3, ESIw) were
calculated. Radial distribution functions of ClÀ anions with
different alkyl chain around H of –OH on fructose showed
that all the first peaks appeared at the same position (1.9 A).
However, the cationic structure has a significant effect on the
interaction between ILs and fructose. The interaction energy
per unit mass shown in Table 1 represents the strength of their
interaction. It was found that absolute values of interaction
energy of ILs with fructose increased with the decrease in carbon
chains. The stronger interaction promotes the dehydration of
fructose and improves the yield of HMF. The calculation results
match with the experimental results that imidazolium cationic ILs
with longer alkyl chains for HMF preparation are not competitive.
Anions and cations of ILs both play key roles in HMF
preparation. To address both economical and practical
considerations, 1-butyl-3-methylimidazolium cationic ILs with
several kinds of anions were employed for investigating the effects
of anions. As shown in Fig. 3, the higher HMF yields were obtained
in [Bmim]HSO4 and [Bmim]Cl with and without catalysts.
However, other anions, such as BF4À, PF6À, OTfÀ and SCNÀ,
showed little trace yields even in the presence of the catalyst. It
shows that some of the anions even restrain the HMF production.
[Bmim]Cl and [Bmim]OTf were selected as representatives to
investigate which kinds of anions of ILs favour the dehydration
of fructose to HMF. Spatial distribution functions of [Bmim]Cl
and [Bmim]OTf around fructose molecules were calculated and
are shown in Fig. 4 (see ESIw for simulation details). In Fig. 4(a),
it can be observed that, in the first solvation shell of [Bmim]Cl,
the anions (red) are very close to the fructose molecules when
extraction using ethyl acetate. After HMF was separated from
the reaction system, remaining ILs and TfOH were used again
for the next reaction entry.
ILs themselves with various cations and anions also have
effects on dehydration of fructose into HMF. The structures of
various ILs are shown in Fig. S2 (ESIw). All reactions were
carried out at 120 1C. Under the experimental conditions,
fructose was fully dissolved in all ILs by visual observation. In
order to study the effects of cations in HMF preparation, the
dehydration of fructose was performed in imidazolium cationic
ILs ([Cnmim]Cl) with different length of alkyl chains (n = 0, 2,
4, 6, 8, 10). A comparison of HMF yields in different length of
alkyl chains of ILs cations with and without catalysts is shown
in Fig. 2. When the alkyl chain of ILs cations is r4, in the
absence of any TfOH catalyst, the HMF yields were still around
50%, for example [Mim]Cl (78%), [Emim]Cl (51%) and
[Bmim]Cl (48%), while in the presence of the catalyst, the
HMF yields were around 90%. Here the ILs with alkyl chains
of the cations shorter than 4 exhibited an excellent synergistic
catalysis or catalytic effects, which enhanced the activity for
dehydration of fructose. When the alkyl chain of ILs cations is
44, the yields of HMF significantly decreased with a longer
alkyl chain and very few HMF were detected without catalyst.
The reason may be that the longer alkyl groups of the imidazolium
cations lead to bigger aggregations.16 The imidazolium cationic
ILs with short alkyl chains (r4) are isotropic in their neat form
and do not form definite aggregations, but those with longer alkyl
chains gradually form well-defined aggregations.17 Aggregations
lead to nonuniform and coadjacent structure of ILs systems,
which is an obstacle for the transfer of electrons and protons in
Table 1 The molecular interaction energy of ILs with fructosea
ILs
Einterb/kJ gÀ1
[Emim]Cl
[Bmim]Cl
[Hmim]Cl
[Omim]Cl
À3.0
À2.2
À1.8
À1.7
a
Simulation system: 128 ion pairs of ionic liquids, 5 fructose molecules,
b
the mole fraction of fructose is 0.75%. Einter: the interaction energy of
Fig. 2 Effects of different alkyl chain length of ILs cations with ClÀ
anions on HMF preparation. Reaction conditions: 0.15 g fructose, 0.1 g
TfOH as catalyst, 5 g ILs and 120 1C for 60 min.
ILs with fructose.
c
4104 Chem. Commun., 2012, 48, 4103–4105
This journal is The Royal Society of Chemistry 2012

between ILs and fructose molecules or the acid radical leads to
high reaction activity. This research not only provides a
feasible approach to explain the interaction in the structure
and molecular level in HMF preparation in ILs, but also
provides some basic aids to find suitable ILs for HMF
preparation for future research.
This work was supported by a National Basic Research Program
of China (973 Program) (No. 2009CB219901), Key Program of
National Natural Science Foundation of China (No. 21036007),
General Program Youth of National Natural Science Foundation
of China (No. 21006112) and General Program of National
Natural Science Foundation of China (No. 20976174).
Fig. 3 Effects of different anions of ILs with [Bmim]+ cations on
HMF preparation. Reaction condition: 0.15 g fructose, 0.1 g TfOH as
catalyst, 5 g ILs and 120 1C for 60 min.
Notes and references
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À
The effects of different cations of ILs with HSO4 anions on
HMF preparation are shown in Fig. S4 (ESIw). It can be seen that
the yield of HMF in [Bmim]HSO4 is higher than that in [Bmim]Cl
and in other ILs with the same cation. The high acidity of an IL
is responsible for its high catalytic activity in dehydration of
fructose.20 The decrease in the alkyl chain length ([Omim]HSO4,
[Hmim]HSO4, [Bmim]HSO4, [Emim]HSO4) in the cation of ILs
causes an increased HMF yield due to the aggregation effect.20
In conclusion, the yield of HMF was strongly affected by
aggregations of cations and the hydrogen bonds between
anions and fructose. The imidazolium cationic ILs with the
alkyl chain of the cations shorter than 4 are suitable for HMF
preparation. The anion of ILs formed strong hydrogen bonds
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 4103–4105 4105