Cellulosic conversion in ionic liquids (ILs): Effects of H 2O/cellulose molar ratios, temperatures, times, and different ILs on the production of monosaccharides and 5-hydroxymethylfurfural (HMF)

Article abstract of DOI:10.1016/j.cattod.2011.03.020

The effects of several critical factors including H₂O/cellulose molar ratios, dissolution temperatures (T_dis), dissolution times (t_dis), reaction temperatures (T_rxn), reaction times (t_rxn), and types of ionic liquids (IL) were investigated for cellulosic conversion. We optimized the reaction conditions as: H ₂O/cellulose = 10/1, T_dis = 120 °C, t_dis = 0.5 h, T_rxn = 120 °C, t_rxn = 3 h for 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl)-based system, and the maximum yield of 5-hydroxymethylfurfural (HMF) up to 21% could be achieved without adding extra catalysts. Another pyridinium-typed IL, [Epyr]Cl, was also used in the cellulosic conversion, and the results showed a high selectivity toward monosaccharides. The information obtained in this study will benefit many biofuel-related applications.

Full text of DOI:10.1016/j.cattod.2011.03.020

Catalysis Today 174 (2011) 65–69
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Catalysis Today
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Cellulosic conversion in ionic liquids (ILs): Effects of H O/cellulose molar ratios,
2
temperatures, times, and different ILs on the production of monosaccharides and
5
-hydroxymethylfurfural (HMF)
∗
Wei-Hang Hsu, Yin-Ying Lee, Wun-Huei Peng, Kevin C.-W. Wu
Department of Chemical Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
The effects of several critical factors including H O/cellulose molar ratios, dissolution temperatures
2
Received 26 October 2010
Received in revised form 24 February 2011
Accepted 10 March 2011
(Tdis), dissolution times (tdis), reaction temperatures (Trxn), reaction times (trxn), and types of ionic
liquids (IL) were investigated for cellulosic conversion. We optimized the reaction conditions as:
◦
◦
H2O/cellulose = 10/1, Tdis = 120 C, tdis = 0.5 h, Trxn = 120 C, trxn = 3 h for 1-ethyl-3-methylimidazolium
Available online 9 April 2011
chloride ([EMIM]Cl)-based system, and the maximum yield of 5-hydroxymethylfurfural (HMF) up to
2
1% could be achieved without adding extra catalysts. Another pyridinium-typed IL, [Epyr]Cl, was also
Keywords:
Ionic liquids
Cellulose
Biomass conversion
HMF
used in the cellulosic conversion, and the results showed a high selectivity toward monosaccharides. The
information obtained in this study will benefit many biofuel-related applications.
©
2011 Elsevier B.V. All rights reserved.
Monosaccharides
1
. Introduction
components. Furthermore, pretreatment of cellulose (e.g., ammo-
nia or steam treatments in a high-pressure process or mechanical
milling) is typically required to increase the accessible area of cel-
lulose for a reasonable rate of enzymatic hydrolysis [8]. Mineral
acids have been extensively investigated to catalyze hydrolysis at
a variety of acid concentrations and temperatures. Only a rather
The usage of fossil fuel causes serious problems of energy crisis
and global warming. To solve these problems, so far much atten-
tion has been paid on the development of renewable energies such
as solar or wind energy. Biofuels produced from biomass is one of
the potential alternatives. First-generation biofuel (i.e., biodiesel)
produced from corn and soybean oil has proved the possibil-
ity of biomass-to-biofuel; however, the use of edible agriculture
as sources will cause another problems such as food deficiency
◦
high temperature (180–230 C) has been used in order to obtain an
acceptable rate of cellulose hydrolysis. Furthermore, degradation of
the resulting glucose becomes an issue at such high temperatures
[9].
[
1]. Therefore, second-generation biofuel generated from non-
Recently, ionic liquids (ILs) have attracted a lot of attention and
have been utilized as solvents for the degradation of the lignocel-
lulosic biomass [10–14]. The importance of ionic liquid in cellulose
dissolution has been emphasized in several reviews [15–17]. Ionic
liquids are kinds of the novel green solvents. They are organic salts
with relatively low melting point. In other words, ionic liquids usu-
ally appear as crystal in normal conditions; however, they can be
molten and dissociated into two ionic parts at relatively higher tem-
peratures (usually less than 100 C). In contrast to other crystalline
salts (ex: NaCl), the attractive characteristic of ILs is that they can
transform into liquid phase.
The first report on the dissolution of cellulose in an IL was back
to 1934 [18]; however, the utilization of ILs in the lignocellulose-to-
biofuel conversion just started from 2000 due to the energy crisis.
Numerous papers have been published on the control of the vis-
cosity and polarity of ionic liquids by varying their ionic structures
edible lignocellulosic biomass has attracted more attention so
far.
Lignocellulosic (or so-called wood-based) biomass consists of
three major components: cellulose (41%), hemicellulose (28%), and
lignin (27%) [2]. Generally, cellulose and hemicellulose can be used
to produce bioethanol, and ligin offers a broad spectrum of conver-
sion (thermal cracking, fast pyrolysis, and complete gasification) to
achieve valuable chemicals and transportation fuels [3]. So far, a
great deal of effort has been put toward the degradation of cellu-
lose with enzymes [4], mineral acids [5], bases [6], and supercritical
water [7]. Enzymatic hydrolysis of cellulose is effective, but the sys-
tem is sensitive to contaminants originating from other biomass
◦
∗ Corresponding author.
E-mail address: kevinwu@ntu.edu.tw (K.C.-W. Wu).
[
16,17]. The main focus of these papers was the solubility of the
0
920-5861/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2011.03.020

6
6
W.-H. Hsu et al. / Catalysis Today 174 (2011) 65–69
synthesized ILs toward different carbohydrates such as glucose,
sucrose, amylose, cellulose, and so on. In 2002, Rogers and co-
Baker), furfural alcohol (Aldrich), levulinic acid (Aldrich) were pur-
chased with a HPLC analysis standard.
workers reported that cellulose could be dissolved in ionic liquids at
◦
1
00 C [10]. The solubility of cellulose in ionic liquids results from its
2
.2. Method
anions. It can disrupt the hydrogen bonds between polysaccharide
chains of cellulose and then to dissolve cellulose [19]. This discovery
started a new pathway to deal with cellulose in low temperature
and ambient atmosphere.
In 2007, Zhao et al. discovered CrCl2 in [EMIM]Cl (1-ethyl-3-
methylimidazolium chloride; an imidazolium type ionic liquid)
can efficiently catalyze glucose to HMF [20]. HMF is a promised
platform chemical because it can further transform to a widely
used biofuel called 2,5-dimethylfuran (DMF) [21] and other use-
ful materials [22]. Since then, many efforts have worked on the
production of HMF from cellulose or glucose in the ionic liquid sys-
◦
In a typical experiment, we first heated ILs at 90 C to
melt them for easy operation. 150 mg IL was loaded into vials
(
15 mm × 45 mm) with a small stirring bar, and vials were closed
by tops rapidly. Then the vials were heated to the dissolution tem-
peratures (Tdis) using oil bath with stirring. For the experiments
of dissolution temperatures, T were controlled as 90, 100, 120,
and 140 C. After the temperature got equilibrium, 15 mg cellulose
was rapidly added into the vials, and the vials were heated for var-
ious times (defined as dissolution time tdis). For the experiments of
dissolution times, tdis were set as 0, 0.5, 1, 3, and 6 h. After disso-
lution of cellulose in ILs, H O was rapidly added into each vial. The
amounts of H O (H O/cellulose ratio in molar) were 4.17 (2.5), 8.33
dis
◦
tems. Binder and Raines combined HCl, CrCl or CrCl , DMA/LiCl and
EMIM]Cl to convert cellulose to HMF [13]; Zhang and co-workers
used CrCl /CuCl as catalysts in [EMIM]Cl [14]; Han and co-workers
2
3
2
[
2
2
2
2
(5), 12.50 (7.5), 16.67 (10), 20.83 (12.5), 25.00 (15), 29.17 (17.5),
also discovered SnCl4 in [EMIM]BF4 can convert glucose to HMF
with a high yield [23]; Riisager and co-workers discussed HMF
produced from lanthanide-containing ionic liquid systems [24];
Chidambaram and Bell discovered that 12-molybdophosphoric
acid in [EMIM]Cl/acetonitril or [BMIM]Cl/acetonitril can selectively
convert glucose to HMF [25]. Although there have been many
researches focused on the addition of various kinds of catalysts
in ionic liquid systems, very few papers discussed the effects of
reaction conditions such as dissolution temperatures and times of
ILs, reaction temperatures and times, and the amounts of water on
the conversion efficiency in ionic liquids without additional cata-
lysts [26]. In fact, in the above-mentioned papers, HMF still could
be produced when using ILs only (no other additives) although the
yields were very low. This indicates that ILs in these systems serve
not only as solvent but also as catalyst. We suggest that the low
HMF yield was because the reaction conditions for HMF produc-
tion in these cases were not optimized. For example, Zhao et al. has
shown that the yield of HMF converted from fructose was greatly
affected by the reaction temperature in an [EMIM]Cl only system
and 33.33 (20) ␮L. After addition of H O, the vials were moved to
2
an oil bath which was preheated to a reaction temperature (Trxn).
For the experiments of reaction temperatures, Trxn were controlled
◦
as 80, 100, 120, and 140 C. The reaction times (trxn) were 0.5, 3,
6
, and 24 h. After reaction, an exceeded amount of Na HPO4 buffer
2
(10 mM, 2 mL, pH 7.2) was added into the vials to quench the reac-
tion, and the vials were then shaken by a shaker (VTX-30002) for at
least 1 min followed by sonication with a ultrasonic cleaner (DELTA)
for least 5 min to ensure all products dissolved in buffer. The solid
residue precipitated in the bottom of each vial was then removed
by syringe filter (Nylon 25 mm × 0.45 ␮m). The filtered liquid was
collected, and the products in the liquid were analyzed by HPLC.
HPLC analysis was performed on EC2000 equipped with RI 2000
Refractive Index Detector and a Shodex Asahipak GS-220 HQ col-
umn and a Shodex Asahipak NH2P-50 4E precolumn. During the
◦
analysis process, the temperature of the column remained at 50 C;
the mobile phase was 10 mM Na HPO4 buffer (pH 7.2) at a flow
2
rate of 0.6 mL/min. The volumn for each injection was 20 ␮L. The
HPLC peak of HMF standard, the calibration curve of HMF, and the
calculation of the HMF yield are shown in Supporting Information.
X-ray diffraction (XRD) was carried out on a Rigaku diffractometer
with Cu K␣ irradiation.
[
20]. Very recently, Binder and Raines discussed the sequence and
timing of the addition of water into the cellulosic conversion and
showed that an optimal sequence and timing strongly affected the
conversion efficiency [27].
Consequently, we believed that the optimization of reaction
conditions is a very important and fundamental work in the ini-
tial stage of cellulosic conversion using ILs as both solvent and
catalyst. Herein we studied the effects of several critical factors:
The structure of the final product was also analyzed by 1
H
and 13C NMR spectroscopy (400 MHz, CDCl ) in order to confirm
3
1
it is 5-hydroxymethylfurfural (HMF). H NMR peaks at ı (ppm):
4
(
1
.49 (s, 2 H), 6.35 (d, J = 3.4 Hz, 1 H), 7.06 (d, J = 3.4 Hz, 1 H), 9.32
s, 1 H). 13C NMR peaks at ı (ppm): 176.907, 151.315, 109.427,
09.427, 162.099, and 56.141. The original NMR spectra of HMF
H O/cellulose ratios, dissolution temperatures of ILs, dissolution
2
times of ILs, reaction times, reaction temperatures, and two kinds
of ILs (including [EMIM]Cl and [Epyr]Cl) on the production of glu-
cose and HMF directly from cellulose. We aimed to set up the most
efficient reaction conditions for cellulosic conversion in ILs without
additional catalysts.
standard, [EMIM]Cl, and our product are also shown in Supporting
Information.
3. Results and discussion
3
.1. Effects of H O/cellulose molar ratios
2
2
. Experimental
The conversion processes of cellulose to HMF are summarized
2
.1. Chemicals
in Fig. 1. The amount of water is a critical issue because water
acts both as a reactant (for producing monosaccharides) and an
inhibitor (for producing HMF) in the overall cellulosic conversion.
Therefore, we firstly studied the effect of the amount of water
on the production of monosaccharides and HMF. In the hydroly-
sis step of cellulose to monosaccharides, increasing the amount of
water should increase the yield of monosaccharides according to
the Le Chatelier’s principle. Our results indeed showed that the
total amounts of monosaccharides and HMF increased when the
Cellulose (powder, CA. 20 ␮m) was purchased from Sigma.
-Ethyl-3-methylimidazolium chloride ([EMIM]Cl, 98%) and 1-
1
Butyl-3-methylimidazolium ([BMIM]Cl, ꢀ95%) were supplied
by Aldrich. 1-Ethylpyridinium chloride ([Epyr]Cl, 98%) was
obtained from Acrose. Glucose (Sigma), fructose(Sigma), HMF(Alfa
Aesar), arabinose (Fluka), cellobiose (Sigma), xylose (Sigma),
sucrose (Sigma), mannose (Sigma), galactose (Sigma–Aldrich),
sorbitol (Sigma–Aldrich), mannitol (Sigma–Aldrich), 1,6-anhydro-
H O/cellulose molar ratios increased and the maximum yield of
2
␤
-glucopyranose (Alfa Aesar), formic acid (Sigma), furfural (J.T.
25% was obtained when the ratio was 10 (Fig. 2). However, it is

W.-H. Hsu et al. / Catalysis Today 174 (2011) 65–69
67
Fig. 1. The processes and critical factors for production of HMF from cellulosic conversion.
interesting to note that when the ratios were over 10 the amount of
monosaccharides still increased but the amount of HMF decreased,
resulting the similar total yields in the range of 22–26%. Because the
conversion of fructose to HMF is a dehydration process, too much
water would inhibit the production of HMF. Therefore, we decided
of HMF remained similar. We supposed that cellulose would fur-
ther decompose (the final products are still unclear at the present
stage) when the Tdis was closed to its melting point (around 150 C).
◦
Next, we examined the effect of dissolution time (tdis) on the
conversion efficiency. As shown in Fig. 3b, the lowest yield was
obtained at the condition without any dissolution (tdis = 0). When
the tdis increased to 0.5 h, the yield of monosaccharides increased
while the yield of HMF was similar, resulting an increasing total
yield (from 30% increased to 45%). The results indicated that disso-
lution of cellulose in IL indeed help depolymerization of cellulose
to generate monosaccharides. Since longer dissolution periods only
caused a slight increase of total yield, we chose 0.5 h as the optimal
tdis from the viewpoint of economy.
the optimal H O/cellulose molar ratio to be 10.
2
3.2. Effects of dissolution temperature (Tdis) and dissolution time
(
tdis
)
Remsing et al. have reported that cellulose can be dissolved into
ILs because the chloride ions in IL can go into the interspace of
,4-linked ␤-d-glucose units and disrupt intermolecular hydrogen
1
Although it seemed that the dissolution time has little effect on
the final conversion rate for the case of [EMIM]Cl, we found that the
tdis is a critical issue for other ILS. For example, when [BMIM]Cl was
used as a solvent, the yield of monosaccharides raised from 11% to
bonds between the units [19]. However, as cellulose was mixed
with ILs, the viscosity of the mixture usually increases. The highly
viscous environment leads to a low rate of molecular transport
(
i.e., chloride ions move very slow) so the efficiency of dissolu-
tion is usually very low. Although one can reduce the viscosity of
IL/cellulose mixture by adding water, it may contrariously affect
the dissolution efficiency [10]. Therefore, raising temperatures is an
alternative strategy to reduce the viscosity of IL/cellulose mixture
and can also accelerate the speed of molecular transfer, enhancing
the dissolution of cellulose. To prevent the energy waste, an opti-
mal dissolution temperature and time period is therefore a very
significant issue.
We studied the effects of dissolution temperatures and times
on the cellulosic conversion and aimed to find an optimal disso-
lution temperature and time. As shown in Fig. 3a, the maximum
yield (including maximum monosaccharides and HMF) was found
◦
at Tdis = 120 C. As described above, high temperatures are help-
ful for reducing the viscosity of IL. However, we found that when
◦
Tdis = 140 C the yield of monosaccharides decrease while the yield
Fig. 2. Monosaccharides and HMF yields vs. H2O/cellulose molar ratios.
Other condition were fixed as follows: [EMIM]Cl 0.17 g, cellulose 0.015 g, dissolution
Fig. 3. (a) Monosaccharides and HMF yield vs. dissolution temperature and (b)
◦
◦
Monosaccharides and HMF yield vs. dissolution time. Other conditions were fixed as
temperature 100 C, dissolution time 0.5 h, reaction temperature 120 C, reaction
time 3 h. Yields are based on HPLC analysis and are relative to the mol of glucose
unit in the cellulose.
◦
follows: cellulose 0.015 g, H2O/cellulose molar ratio 10/1, Trxn 120 C, trxn 3 h. Yields
are based on HPLC analysis and are relative to mol of glucose unit in the cellulose.

6
8
W.-H. Hsu et al. / Catalysis Today 174 (2011) 65–69
Fig. 4. XRD patterns of cellulose with (blue line) and without (red line) dissolution
◦
pretreatments. In the dissolution pretreatment, cellulose were heated at 120 C for
0
.5 h in [EMIM]Cl. (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of the article.)
3
6% as the tdis increased from 0.5 h to 3 h, respectively. The longer
dissolution period is necessary for the case of [BMIM]Cl because cel-
lulose needs more time to dissolve in the [BMIM]Cl that exhibiting
a longer alkyl chain.
To check the efficiency of cellulosic dissolution by IL pretreat-
ment, we examined the crystalline structure of cellulose before
and after cellulosic dissolution with XRD. As shown in Fig. 4, the
◦
crystalline peak of the cellulose at 2ꢀ = 22 disappeared after IL
◦
pretreatment (i.e., dissolution at 120 C for 0.5 h in [EMIM]Cl). It
Fig. 5. (a) Monosaccharides and HMF yield vs. reaction temperature and (b)
Monosaccharides and HMF yield vs. reaction time. Other conditions were controlled
proved that the crystalline structure was destroyed by chloride ion
of [EMIM]Cl.
as follows: [EMIM]Cl 0.15 g, cellulose 0.015 g, H2O/cellulose molar ratio 10/1, Tdis
◦
1
20 C, tdis 0.5 h. Yields are based on HPLC analysis and are relative to mol of glucose
unit in the cellulose.
3.3. Effects of reaction temperature (Trxn) and reaction time (trxn)
we found that the monosaccharides yield decreased while HMF
yield increased and reached at maximum at trxn = 6 h and then
decreased. It is reasonable that the increase of HMF within the
first 6 h was resulted from the decrease of monosaccharides (i.e.,
isomerization and dehydration reaction). However, HMF yield also
decreased when the trxn > 6 h. We supposed that this was due to
the decomposition of HMF to other byproducts. We examined the
possible byproducts such as formic acid and levulinic acid but could
not detect them. Several papers have also reported the decomposi-
tion of HMF to some unidentified products [22,29,30]. Conclusively,
Reaction temperature influences many factors in the IL-based
cellulosic conversion such as kinetics, selectivity, Kw, and the activ-
ity of catalysts. For example, siliceous MCM-41 showed activity
toward pyrolysis of lignocellulosic biomass in high temperature (up
◦
to 550 C) [28]. Chen and co-workers have reported that tempera-
+
−
ture influenced the Kw (i.e., the concentration of H and OH ), and
the pKw values varied at different temperatures (298 K and 373 K)
in the [EMIM]Cl/H O system [26]. Therefore, finding an optimal
2
reaction temperature and time is highly demanded.
◦
Fig. 5a shows the effect of reaction temperature (Trxn) on the
we suggest the optimal reaction temperature is 120 C and reaction
production of monosaccharides and HMF. As Trxn was increased to
time to be 3 h in [EMIM]Cl/H2O systems. This optimal condition can
obtain the highest total yield of 45.9% (24.9% for monosaccharides
and 21.0% for HMF).
◦
1
20 C, the total yield increased to 42.8%, about five times higher
◦
than that at 80 C (the total yield was 7.8%). We supposed that
higher Trxn would increase [H ] from H O and help the hydrolysis
of cellulose to glucose. Similar with the condition of T > 120 C,
when Trxn was increased from 120 C to 140 C, the total yield
decreased from 42.8% to 22.2% but the HMF yield kept almost
the same. We suggested that it was because of some side reac-
tions such as formation of insoluble humines and soluble polymers
+
2
◦
3.4. Effects of the types of IL
dis
◦
◦
We study the effects of different ILs including imidazolium-
typed ILs and pyridinium-typed ILs on the cellulosic conversion.
The positively charged cations such as imidazolium or pyridinium
contain alkyl substituent to make the cations hydrophobic and
the anions more nucleophilic. Imidazolium-typed ILs have been
widely studied and used for biomass dissolution [10,15,17]. On the
contrary, the efficiency of pyridinium-typed ILs on cellulosic con-
version still remains unclear. Different ILs may alter the efficiency
of cellulosic dissolution, efficiency of conversion, and selectivity
of final products; therefore, finding out an appropriate IL that
can dissolve cellulose quickly and catalyze further reactions effec-
tively has been strongly demanded. In this study, two types of
ILs ([EMIM]Cl (imidazolium type) and [Epyr]Cl (pyridinium type)),
[
22,29,30] (kinds of decomposition product of fructose) that prefer
to occur when the temperature was closed to the melting point of
monosaccharides. In contrast to monosaccharides, the HMF yield
◦
increased from 0.54% to 17% as the Trxn increased from 80 C to
1
◦
40 C, respectively. In addition, the selectively of HMF (defined
as HMF yield/monosaccharides yield) also increased from 6.9% to
9.8%, as shown in Fig. 5a. The enhanced selectivity toward HMF
3
+
−
also proved that higher [H ] and [OH ] resulted from increasing
temperature could efficiently catalyze the isomerization of glucose
and the dehydration reaction of fructose.
Next, we study the effect of reaction times trxn on the conversion
efficiency, and the results are depicted in Fig. 5b. As trxn increased,
were used. All reaction conditions are the same (i.e., H O/cellulose
2
◦
◦
molar ratio = 10, Tdis = 120 C, t = 0.5 h, Trxn = 120 C, and trxn = 3 h).
dis

W.-H. Hsu et al. / Catalysis Today 174 (2011) 65–69
69
Acknowledgement
This research was supported by National Science Council of
Taiwan (NSC 97-2113-M-002-020-MY2 and 99-2119-M-002-009).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.cattod.2011.03.020
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Epyr]Cl apparently had a better selectivity toward the produc-
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produce HMF with high selectivity and efficiency.
[
(
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. Conclusion
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[
conditions: H O/cellulose molar ratio = 10, dissolution temperature
2
◦
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(
(
Tdis) = 120 C, dissolution time (tdis) = 0.5 h, reaction temperature
Trxn) = 120 C, and reaction time (trxn) = 3 h for [EMIM]Cl-based sys-
◦
tem. The information we obtained here would be helpful for further
studies on the cellulosic conversion with the addition of solid cat-
alysts.