Efficient process for the direct transformation of cellulose and carbohydrates to 5-(hydroxymenthyl)furfural with dual-core sulfonic acid ionic liquids and co-catalysts

Article abstract of DOI:10.1039/c3ra41062e

The direct transformation of cellulose and carbohydrates into 5-(hydroxymethyl)furfural (HMF) in the solvent [BMIM]Cl using dual-core sulfonic acid ionic liquids (ILs) as catalysts and metal salts as co-catalysts was investigated, aiming at a more environmentally friendly process not involving chromium. From the high throughput screening of various metal salts, a combination of [bi-C₃SO₃HMIM][CH₃SO ₃] (IL-2) and manganese chloride (MnCl₂) was found to be the most effective catalyst. HMF was directly afforded from cellulose in 66.5% yield. Thus, synthesis of HMF was successfully performed from cellulose using ILs and MnCl₂. Following the principles of green engineering, we recycled the catalyst in our system for cellulose hydrolysis and this catalyst maintained its good performance even after four runs. Furthermore, various sugars and lignocellulosic raw materials could be directly converted into HMF in reasonable yields under these conditions. The mechanism that explains the high activity of ILs in combination with MnCl₂ is also proposed.

Full text of DOI:10.1039/c3ra41062e

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Efficient process for the direct transformation of
cellulose and carbohydrates to 5-
Cite this: RSC Advances, 2013, 3,
7782
(hydroxymenthyl)furfural with dual-core sulfonic acid
ionic liquids and co-catalysts3
Jincai Shi,a Haiyan Gao,a Yongmei Xia,ab Wei Li,a Haijun Wang*a
and Changge Zhenga
The direct transformation of cellulose and carbohydrates into 5-(hydroxymethyl)furfural (HMF) in the
solvent [BMIM]Cl using dual-core sulfonic acid ionic liquids (ILs) as catalysts and metal salts as co-catalysts
was investigated, aiming at a more environmentally friendly process not involving chromium. From the
high throughput screening of various metal salts, a combination of [bi-C3SO3HMIM][CH3SO3] (IL-2) and
manganese chloride (MnCl2) was found to be the most effective catalyst. HMF was directly afforded from
cellulose in 66.5% yield. Thus, synthesis of HMF was successfully performed from cellulose using ILs and
MnCl2. Following the principles of green engineering, we recycled the catalyst in our system for cellulose
hydrolysis and this catalyst maintained its good performance even after four runs. Furthermore, various
sugars and lignocellulosic raw materials could be directly converted into HMF in reasonable yields under
these conditions. The mechanism that explains the high activity of ILs in combination with MnCl2 is also
proposed.
Received 4th March 2013,
Accepted 7th March 2013
DOI: 10.1039/c3ra41062e
www.rsc.org/advances
packing via numerous intermolecular and intramolecular
H-bonds.7 The tight H-bond network and van der Waals
Introduction
With the rapid development of society, energy demand has
increased, and is projected to grow by more than 50% by
2025.1 However, it is known that fossil fuels cannot be
considered as the world’s main source of energy for more
than another 100 years. As a result, cellulose, which is the
most abundant form of biomass, has been the target of
intensive research for conversion to other chemicals.2–4 It has
the potential to become a promising alternative to fossil
resources for the sustainable production of 5-hydroxymenthyl-
furfural (HMF), a versatile and key intermediate in the bio-fuel
chemistry and petrochemical industries. Cellulose has a highly
crystalline structure formed from linear polymer D-an-hydro-
glucopyranose units joined together in long chains by b-1,4-
glycosidic bonds. An extensive network of such polymer chains
arranged in ordered alignment via hydrogen-bonding5 and van
der Waals forces6 results in a supramolecular cellulose
structure of varying size, crystallinity, and complexity. So it is
practically difficult to dissolve cellulose in most common
organic solvents because of its stiff molecules and close chain
interactions greatly stabilize it, too, which makes it notoriously
resistant to hydrolysis.
As is shown in Scheme 1, the first barrier in the conversion
of cellulose to HMF is the decrystallization and cleavage of
cellulose to glucose. In the past few years, many studies have
focused on this pathway, because cellulose itself is too stable
for the crystalline structure to be easily broken.8–12 However,
the depolymerization of cellulose into HMF still remains an
expensive and poor efficiency process.
So far, the depolymerization of cellulose into HMF has still
been a great challenge from either an environmental or an
efficiency point of view. Due to the increased interest and
demand for industrial applications, substantial efforts have
been devoted to the development of appropriate hydrolysis
schemes, including catalysis using mineral acids and hetero-
aThe Key Laboratory of Food Colloids and Biotechnology, Ministry of Education,
School of Chemical and Material Engineering, Jiangnan University, Wuxi, Jiangsu
bState Key Laboratory of Food Science and Technology, Jiangnan University, 214122,
China
Electronic supplementary information (ESI) available. See DOI: 10.1039/
3
Scheme 1 The chemical pathway of the direct conversion of cellulose into HMF.
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c3ra41062e
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poly acids. However, there are many disadvantages to these
processes, such as the use of mineral acids only being efficient
at too high a temperature (170–240 uC), the corrosion of
equipment, and the generation of large amounts of acidic
waste water. The inherent advantages of heteropoly acid
hydrolysis are their high catalytic activity, reusability, fewer
side reactions, strong Brønsted acidity and so on. Moreover, a
remarkably high yield of glucose (82.4%) was obtained,13 but
the disadvantage was the low selectivity of HMF. Therefore,
more efficient catalytic systems have been developed recently;
the utilization of metal ions in ionic liquids (ILs) was
demonstrated as an efficient approach for the conversion of
cellulose into HMF. Zhao’s group14 studied the conversion of
lignocellulosic biomass into furans in ionic liquids in the
presence of CrCl3 under microwave irradiation, and the yield
of HMF reached 45%–52%. In 2009, Raines et al.15 reported
that N,N-dimethylacetamide containing lithium chloride was a
privileged solvent that enabled the synthesis of HMF in a
single step; the yield of HMF obtained from cellulose was 50%.
Cho et al.16 found that a combination of CrCl2 and RuCl3 was
an effective catalyst; HMF was directly afforded from cellulose
in nearly 60% yield in [EMIM]Cl. But as these studies had all
used amounts of chromic salts, they failed to conform to the
requirement of ‘‘green chemistry’’, although a high yield of
HMF had been obtained. Recently, Kerton’s group17 reported a
novel route that produced levulinic acid (4-oxopentanoic acid,
LA) and HMF from chitosan in water under microwave
irradiation. The volume of water used and the loading of
SnCl4?5H2O could be varied to produce either LA or HMF with
good selectivity. The results of this study may lead to the
possibility of the chemical transformation of non-toxic and
cheap biopolymers and yield useful industrial applications.
Along with this research progress, sulfonic acid functiona-
lized ILs are considered as high-efficiency catalysts, especially
when combined with metal chloride salts. Tao’s group18,19
investigated the hydrolysis of cellulose using catalytic amounts
of FeCl2 and CoSO4 in ILs in the presence of 4-methyl-2-
pentanone (MIBK) at 150 uC for 300 min, and HMF yields of 34
wt% and 24 wt% were achieved, respectively. However, the
harsh conditions (high temperature and long time) and low
yield did not reflect the superiority of sulfonic acid functio-
nalized ILs.
Fig. 2 Mono- and disaccharide precursors for HMF.
C3SO3HMIM][CH3SO3], which have strong acidity (Fig. 1).
Our objective in this work was to combine the ILs with metal
ion catalysts and investigate the catalytic effect on the direct
conversion of cellulose to HMF. Thus, an imidazolium-based
IL ([BMIM]Cl) was used as the solvent together with various IL
and metal ion catalysts at the mild temperatures of 100 uC and
120 uC. Additionally, the investigation studied the mechan-
isms of cellulose degradation to HMF. This process consisted
of the hydrolysis of cellulose to b-glucose, isomerization of
b-glucose to fructose, and fructose dehydration to HMF.
Furthermore, metal ions mainly act as the catalyst for
promoting the rapid conversion of a-glucose to b-glucose
and the isomerization of b-glucose to fructose,14 while
cellulose hydrolysis and fructose dehydration can be simply
achieved under acidic conditions. From the viewpoint of green
chemistry, the reuse of the catalyst is investigated in our study
of cellulose hydrolysis. Finally, the reaction system was applied
to the conversion of hexoses (Fig. 2) and lignocellulosic raw
materials to HMF.
Results and discussion
Co-catalyst screening
It is known that metal salts in ILs have been demonstrated to
be an efficient catalyst system for the conversion of cellulose to
HMF.14,15 In our study, we initially screened the hydrolysis of
cellulose catalyzed with IL-1 by adding a catalytic amount of
metal salt at 120 uC for 3 h (the catalyst dosage and molar yield
are relative to the number of glucose units in the cellulose,
Herein, we investigate the conversion of cellulose to HMF
using several acidic ILs and metal salts, especially dual-core
sulfonic acid ILs, such as [bi-C3SO3HMIM][HSO4], [bi-
Fig. 1 Various sulfonic acid ILs used in the study.
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The effect of reaction time on the direct transformation of
cellulose into HMF
The reaction time is a key parameter determining the
conversion of cellulose and the yield of HMF. The results of
the investigation into the effect of time are presented in Fig. 5.
When reactions were carried out at 120 uC in [BMIM]Cl using
MnCl2 and an acidic ionic liquid as the co-catalyst, a
maximum HMF yield of 45.62% was achieved after 2 h
followed by a sharp decrease. Obviously, the hydrolysis rate at
120 uC was higher than the rate at 100 uC, when only a 34.95%
yield of HMF was obtained after 2 h. As can be seen in Fig. 5, it
would improve the yield of HMF if a high reaction temperature
and long reaction time were used as this enhances the catalytic
activity in the hydrolysis and dehydration steps, so for future
reactions we used a temperature of 120 uC, although this is
above the boiling point of water.
Fig. 3 High throughput screening of various metal catalysts for the direct
transformation of cellulose into HMF in [BMIM]Cl. Conditions: each metal
catalyst (4 mol% to cellulose), IL-1 (9 mol% to cellulose), cellulose (150 mg),
[BMIM]Cl (3 mL), H2O (0.2 g), T = 120 uC, t = 3 h.
The effects of the (co-)catalyst and H2O on the reaction
As mentioned by Shimizu et al.,20 an acid catalyst with
stronger Brønsted acidity could result in a higher relative rate
of acid-catalyzed cellulose hydrolysis, thus the effect of the
acidic ionic liquid dosage on the hydrolysis of cellulose should
be studied. As shown in Fig. 6, the conversion of cellulose to
HMF could be promoted by using acid catalysts in a reason-
able dosage range. The amount of IL-1 used was 5 mol%, 9
mol%, 18 mol% and 54 mol%, respectively. When the dosage
of IL-1 was increased from 5 mol% to 9 mol%, the yield of
HMF increased obviously, from 15.78% to 51.98% after 1 h.
However, when the amount of IL-1was increased from 9 mol%
to 18 mol%, the yield of HMF after 1 h decreased to 48.01%,
and when the dosage of IL-1 reached 54 mol%, only 15.3% of
HMF was obtained after 1 h and the yield decreased sharply. It
also showed that time become more critical when the dosage
of IL-1 increased: (1) with increasing catalytic dosage, the
reaction rate was accelerated; a maximum HMF yield of
22.19% was achieved with 5 mol% IL-1 after 2 h, but it only
took about 1 h to reach the highest level (51.98%) with 9 mol%
IL-1, and 0.5 h (50.09%) for 54 mol%, (2) when the amount of
acidic ionic liquid was in excess (54 mol%), the yield of HMF
declined markedly to only 4.93% after 2 h. In summary, the
HMF yield decreased as the dosage of IL-1 increased (above 9
mol%), and longer reaction times lead to a decrease in HMF
yield due to an increase in the dehydration rate of the HMF
monomer.
which is the same throughout). Fig. 3 shows the results of
altering the metal catalyst by using CuCl2, CuSO4, FeCl3,
MnSO4, MnCl2 and Co(NO3)2, which each showed differing
yields of HMF from 29.34% to 43.3%, and demonstrates that
the presence of a metal salt can improve the yield of HMF
significantly compared with the reaction without a catalyst.
This is because although ILs can effectively transform
cellulose into polysaccharide, disaccharide, and hydrolyze
these into glucose, they cannot convert glucose into HMF.
However, the metal salts have an excellent ability to hydrolyze
glucose into HMF. So, when the two kinds of catalyst are used
in concert to catalyze cellulose degradation, they obtain a
higher yield of HMF. Compared with other metal salts, the
catalytic effect of MnCl2 and CuCl2 is remarkable. The yield of
HMF was increased by 27.28% and 25.16%, respectively. In our
present system, the reason for the promotional effect of the
manganese salt may be due to the Mn2+ coordination
interaction. In addition, the Mn2+ could play an important
role in promoting the rapid conversion of a-glucose to
b-glucose and the isomerization of b-glucose to fructose,
which improves the yield of HMF. In terms of conversion
pathways, the direct transformation of cellulose into HMF
involves four reactions: (I) the hydrolysis of polymeric
microcrystalline cellulose into glucose (the saccharification
process), (II) the rapid conversion of a-glucose to b-glucose,
(III) the isomerization from the aldose-type sugar (opening the
ring of b-glucose) to the 1,2-enediol intermediate which can
finally convert into fructose via a ring closure, that is, allowing
five-membered ring formation, and (IV) the dehydration of
fructose to generate the final product, HMF (Scheme 1). In this
way, we propose a mechanism in which MnCl2 in IL-1 forms
complexes of [MnCl2(HSO4)n]n2 and we suggest that these play
a role in proton transfer, and facilitating the mutarotation of
a-glucose (Fig. 4(a)). The proposed mechanism of the MnCl2
and IL-1 promoted conversion of b-glucose into HMF and
other products is shown in Fig. 4(b).
Shimizu et al.20 found that in the conversion of fructose to
HMF under acidic conditions, the complete removal of water
decreased the HMF yield while mild evacuation could improve
the HMF yield. Considering that water dosage may affect the
conversion of cellulose to HMF, in this study several special
experiments were designed in order to investigate the
influence of water on the conversion of cellulose to HMF.
The experiment was carried out at 120 uC for 1 h and 2 h. As
shown in Fig. 7, the reaction time had a relatively minor
influence on the yield of HMF, except when the dosage of H2O
was 0.1 g. In our study, the optimal dosage of H2O was found
to be 0.1 g, when we obtained the maximum yield of HMF of
59.89% after 1 h. When the dosage of H2O was 0.3 g, the yield
of HMF was only 14.41% after 1 h and 15.13% after 2 h. The
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Fig. 4 (a) Proposed mechanism of MnCl2 and IL promoted conversion of cellulose into b-glucose. (b) Proposed mechanism of MnCl2 and IL promoted conversion of
b-glucose into HMF and the other products.
results confirm that the presence of a small amount of water in
the reaction system does promote the reaction, but more water
had a negative effect on the conversion of cellulose to HMF
and results in the decomposition of HMF to levulinic acid,
which is consistent with a previous report by Hu et al.21 In
addition, when the water content continued to increase, the
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