Efficient conversion of glucose into 5-hydroxymethylfurfural catalyzed by a common Lewis acid SnCl4 in an ionic liquid

Article abstract of DOI:10.1039/b914601f

The common Lewis acid SnCl₄ could efficiently convert glucose into 5-hydroxymethylfurfural in 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMim]BF₄). New evidence indicated that the formation of the five-membered-ring chelate complex of the Sn atom and glucose may play a key role for the formation of HMF, and the mechanism for the reaction was proposed. In addition, the [EMim]BF₄/SnCl₄ system was also suitable for the conversion of fructose, sucrose, inulin, cellobiose and starch. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b914601f

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Efficient conversion of glucose into 5-hydroxymethylfurfural catalyzed by a
common Lewis acid SnCl4 in an ionic liquid†
Suqin Hu, Zhaofu Zhang, Jinliang Song, Yinxi Zhou and Buxing Han*
Received 21st July 2009, Accepted 7th September 2009
First published as an Advance Article on the web 14th September 2009
DOI: 10.1039/b914601f
The common Lewis acid SnCl4 could efficiently con-
vert glucose into 5-hydroxymethylfurfural in 1-ethyl-3-
methylimidazolium tetrafluoroborate ([EMim]BF4). New
evidence indicated that the formation of the five-membered-
ring chelate complex of the Sn atom and glucose may
play a key role for the formation of HMF, and the
mechanism for the reaction was proposed. In addition, the
[EMim]BF4/SnCl4 system was also suitable for the conver-
sion of fructose, sucrose, inulin, cellobiose and starch.
of glucose in straight-chain form and the generation of the
enediol intermediate.12 Up to now, almost no catalyst except
chromium(II) chloride (CrCl2) was effective for this reaction in
ionic liquids.13 Therefore, both catalyst and solvent are crucial
in the conversion of glucose into HMF.14 There is no doubt that
developing a cheap, non-toxic or low-toxicity, easily handled
catalytic system for this important reaction and getting insight
into the mechanism are highly desirable and challenging.
It is well known that SnCl4 is a common, cheap, easy-handling
Lewis acid, and has much lower toxicity. In this work, we found
that SnCl4 in 1-ethyl-3-methylimidazolium tetrafluoroborate
([EMim]BF4) was an efficient catalyst to produce HMF from
glucose. A detailed study indicated that the formation of the
five-membered-ring chelate complex of Sn and glucose may play
a key role for the formation of HMF. The catalytic system was
still very efficient even when the glucose concentration was as
high as 26 wt%, and could be reused for at least four times
without considerable reduction in efficiency. In addition, the
[EMim]BF4/SnCl4 system was also suitable to the conversion
of other sugars such as fructose, sucrose, cellobiose, inulin and
starch.
Biomass is an abundant and the only sustainable carbon
resource, and biomass refinery processes must be developed in
the next few decades to produce energy and materials which
will replace the ones produced from the diminishing fossil-
based resources.1 Carbohydrates represent 75% of the annual
renewable biomass.2 The most widespread monosaccharide is
glucose which is produced by photosynthesis of carbon dioxide
and water in the plants and is the monomer of cellulose. The
utilization of glucose to generate valuable compounds effectively
is a very important topic.3
5-Hydroxymethylfurfural (HMF) is a versatile compound,
which can be used produce very useful furan derivatives.4 For
example, 2,5-furandicarboxylic acid was selected as one of the
top 12 biomass-derived building blocks4a and 2,5-dimethylfuran
was considered as a promising liquid transportation fuel.4c Up
to now, different feedstocks have been used to produce HMF,
such as fructose,5 cellulose,6 and inulin.7
Glucose is a wonderful candidate as the resource of HMF
compared with fructose.2 Efficient production of HMF from
glucose is of great imprtance.8 But some hindrances limit
the development of the reactions. Firstly, due to the high
content of hydroxyl groups, glucose has low volatility and high
reactivity, and liquid-phase technologies should be used.9 The
underived glucose can only be dissolved in a few solvents, such
as water, dimethylformamide and dimethylsulfoxide (DMSO).
But when water is used as solvent the conversion of glucose is
inefficient.10 In the two latter organic solvents, the HMF yield
was not high either,11 and both of them have disadvantages
from an environmental point of view. On the other hand, the
catalyst must be used in the reaction to induce the formation
Firstly, we screened a series of metal chlorides, at 10 mol%
amount (based on glucose), in DMSO to catalyze the conversion
of glucose at 80 ◦C for 3 h (see ESI, Table S1).† Most of them did
not work, and only CrCl3·6H2O, AlCl3·6H2O and SnCl4·5H2O
(abbreviated as SnCl4 in the following) were active. In particular,
SnCl4 was the most efficient and was selected as the catalyst for
the following study.
In the next step, different solvents were studied. As we known,
some ionic liquids (ILs) have a strong solvent power for many
carbohydrates,15 and some wonderful catalytic results have also
been obtained in ILs for different reactions.16 Besides DMSO,
we studied the reaction catalyzed by SnCl4 at 100 ◦C for 3 h in a
series of ILs, including 1-butyl-3-methylimidazolium chloride
([BMim]Cl), 1-butyl-3-methylimidazolium tetrafluoroborate
([BMim]BF4), 1-butyl-3-methylimidazolium hexafluorophos-
phoate ([BMim]PF6), 1-butyl-3-methylimidazolium bistriflate
imide ([BMim]Tf2N), 1-butyl-3-methylimidazolium trifluo-
roacetate ([BMim]TFA), 1-butyl-3-methylimidazolium trifluo-
romethylsulfonate ([BMim]Trif), 1-butyl-3-methylimidazolium
saccharin ([BMim]Sacc), N-butylpyridinium tetrafluoroborate
([Bpyr]BF4) and [EMim]BF4. The results are given in Fig. 1a.
Fixing the cation of the ILs as [BMim]+, the best result was
Beijing National Laboratory for Molecular Sciences, Institute of
Chemistry, Chinese Academy of Sciences, Beijing, 100080, China.
E-mail: Hanbx@iccas.ac.cn; Fax: +86-10-62562821
† Electronic supplementary information (ESI) available: Materials, anal-
ysis methods, experimental procedures, more data of the experiments
(Table S1 and S2), and Fig. S1. See DOI: 10.1039/b914601f
-
obtained when the anion was BF4 . It was observed that lower
HMF yields were obtained when the anions of ILs were Cl-,
Tf2N-, TFA-, Trif-, Sacc-, all of which have coordination ability.
These anions would have stronger interactions with the Sn atom
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It is known that separation of the product and solvent requires
energy. Using ethyl acetate as the extracting solvent, the energy
cost of separation can be relatively low because of its low boiling
point.
It is very interesting and challenging to study the mechanism
of the reaction. The catalytic mechanism of the reaction was
studied and is discussed in the following. When the glucose inter-
acts with the Sn atom, there are opportunities for the formation
of complexes with the acyclic and five- or six-membered-ring
chelate structures. We added three alcohols, including ethanol,
ethylene glycol and 1,3-propanediol (molar ratio to glucose was
1 : 1), respectively, into the reaction mixture. After reaction,
the conversion of glucose was less than 5% in the presence of
ethylene glycol; but ethanol and 1,3-propanediol did not reduce
glucose conversion and HMF yield. This indicates that ethylene
glycol, that can form a five-membered-ring chelate complex
with Sn, inhibited the conversion of glucose significantly, while
ethanol or 1,3-propanediol, which can form acyclic or six-
membered-ring chelate complex with Sn (Fig. 2), did not affect
the reaction. This suggests that the five-membered-ring chelate
structure may be more stable than the acyclic and six-membered-
ring chelate structures. We thought that formation of a five-
membered-ring chelate structure with the Sn atom with the two
neighbouring hydroxyl groups in glucose may play an important
role in the catalytic reaction.
Fig. 1 a: HMF yields in various ILs and DMSO: glucose (100.0 mg,
0.56 mmol), SnCl4 (19.5 mg, 0.056 mmol), ILs or DMSO (1.0 g), 100 ◦C,
3 h; b: Yields of HMF at different initial glucose concentrations in
[EMim]BF4: SnCl4 (10 mol% based on glucose), [EMim]BF4 (1.0 g),
100 ◦C, 3 h; c: Reuse of SnCl4/[EMim]BF4: glucose (200.0 mg,
1.11 mmol), SnCl4 (39.0 mg, 0.111 mmol), [EMim]BF4 (1.0 g), 100 ◦C,
3 h.
which competed with the interaction between glucose and Sn
atom and inhibited the formation of HMF, resulting in the lower
HMF yield. The HMF yield in [BMim]PF6 was low due to the
insoluble nature of glucose in the IL. It can be seen from the
above results that when SnCl4 was used as the catalyst, a suitable
medium should have strong solvent power for glucose but low
coordination ability to Sn. Thus, fixing the anion of the ILs as
-
BF4 , the highest yield of HMF was obtained in [EMim]BF4
which was also higher than that obtained in DMSO. In the
following, [EMim]BF4 was selected as the solvent.
The reaction was conducted at different reaction conditions
using SnCl4 as the catalyst and [EMim]BF4 as the solvent, and
the optimum condition was at 100 ◦C with a reaction time
of 3 h (see ESI, Table S2).† The yields at different glucose
concentrations are presented in Fig. 1b. It can be seen that
the yield of HMF could reach 61% when the concentration
of glucose in the IL was as high as 23 wt%, indicating that the
catalytic system was very effective for the reaction. Moreover, we
tested the reusability of the SnCl4/[EMim]BF4 catalytic system.
After extraction of the product using ethyl acetate (see ESI),†
the catalyst in IL was reused directly. The results showed that
reduction in the yields of HMF was not obvious after being
reused 4 times (Fig. 1c). One of the advantages of using the
IL as the reaction medium and ethyl acetate as the extracting
solvent is that the product HMF is soluble in ethyl acetate, while
the IL is insoluble in it, which can avoid cross-contamination.
Fig. 2 Proposed processes of glucose conversion to produce HMF
catalyzed by SnCl4 in [EMim]BF4.
Glucose is usually in the pyranose form, and its conversion
to HMF must be through a straight-chain form and an
1
enediol intermediate. It can be shown from H NMR spectra
that both a-glucose and b-glucose are in pyranose form in
[EMim]BF4 (Fig. 3a). When SnCl4 interacts with glucose at room
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Fig. 4 Yielding HMF from various sugars in [EMim]BF4: fruc-
tose/sucrose/cellobiose/inulin (200.0 mg), SnCl4 (10 mol% based on
sugar monomer), IL (1.0 g), 100 ◦C, 3 h; starch (100.0 mg, 0.62 mmol
of glucose unit), SnCl4 (21.6 mg, 0.062 mmol), IL (1.0 g), 100 ◦C, 24 h.
Sucrose is a disaccharide consisting of glucose and fructose.
In reported works, HMF was produced only from the fructose
moiety and the glucose moiety was wasted.12a,18 Here, both the
glucose and fructose moiety could be converted into HMF and
most of the carbon in sucrose was utilized. When cellobiose was
used as a resource, a good result was also obtained, with 57%
HMF yield. It is worth mentioning that 47% HMF yield was
obtained when starch was used as the feedstock.
In summary, we demonstrated that SnCl4 and [EMim]BF4
was an excellent combination for the conversion of glucose into
HMF. High HMF yield was obtained even when the glucose
concentration in the IL was as high as 26 wt%. Formation
of the five-membered-ring chelate complex of Sn and glucose
may play a key role for the formation of HMF. Satisfactory
results were also obtained when fructose, sucrose, cellobiose,
inulin and starch were as the feedstocks. The efficient, cheaper,
low toxicity, and reusable catalytic system has great potential for
application.
Fig. 3 1H NMR spectra of glucose at different temperatures: 5 wt%
glucose and 1 wt% SnCl4 dissolved in the mixture of [EMim]BF4 and
d6-DMSO with weight ratio of 9 : 1.
temperature, the peaks of the hydrogen atoms in hydroxyl groups
of glucose (HO–H) disappeared (Fig. 3b), indicating that Cl atoms
interacted with these hydrogen atoms and the intermediate 2
1
was formed (Fig. 2). Then, H NMR spectra of glucose mixed
with SnCl4 were collected at 80 ◦C or 100 ◦C for 2 min, and
a new peak at 4.90 ppm was observed and its height increased
with the increase of temperature (Fig. 3d and 3f), indicating
a new interaction between glucose and SnCl4 at the higher
temperature. It is speculated that the new peak can be assigned
to intermediate 3. This suggests that heating would enhance the
interaction between the Sn atom and O atoms to convert 2 into
3. 3 could induce the formation of the intermediate complex 4,
which consisted of glucose in straight-chain form and SnCl4. 4
was further converted into b-glucose and enediol intermediate 5
(Fig. 2). The important intermediate 5 was able to yield HMF
directly and to be isomerized into fructose.17 This was supported
by the fact that fructose was indeed observed by HPLC when
the reaction was processed at 80 ◦C for 3 h (see ESI, Fig. S1).†
We believe that Cl atoms in SnCl4 interacted with hydrogen
atoms and transferred the hydrogen atoms. The Sn atom in
SnCl4 interacted with O atoms and promoted the formation of
straight-chain glucose and the enediol intermediate. The latter
intermediate was further transformed into fructose which could
be dehydrated to form HMF in the presence of SnCl4 (Fig. 2).
This was proved by the fact that 62% HMF yield was achieved
when fructose was the reactant instead of glucose.
We thank National Natural Science Foundation of
China (20533010) and Chinese Academy of Sciences
(KJCX2.YW.H16) for the financial support.
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A wonderful result was obtained when 17 wt% sucrose in
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