WCl6 catalyzed cellulose degradation at 80?°C and lower in [BMIM]Cl

Article abstract of DOI:10.1016/j.carbpol.2019.02.050

Degradation of cellulose to reducing sugar is the key step for the conversion of cellulose to valuable chemicals. Cellulose was degraded by WCl₆ in 1-butyl-3-methyl imidazole chloride at 80 °C and lower. 83% and 85.5% yield of total reducing sugar was gotten at 70 and 80 °C, respectively. Compared with inorganic acid, heteropoly acid, acidic ionic liquid and other metal chlorides, WCl₆ has shown better catalytic performance for degradation of cellulose to reducing sugar. The effect of reaction temperature, reaction time, WCl₆ amount and cellulose concentration were investigated. Degradation of cellulose by WCl₆ in 1-butyl-3-methyl imidazole chloride is a zero reaction. WCl₆ also showed excellent catalytic performance for the degradation of nature cellulose and lignocellulose. Catalyst can be reused at least 5 times without decrease of reducing sugar yield. The mechanism of degradation of WCl₆ was also suggested.

Full text of DOI:10.1016/j.carbpol.2019.02.050

Carbohydrate Polymers 212 (2019) 289–296
Contents lists available at ScienceDirect
Carbohydrate Polymers
journal homepage: www.elsevier.com/locate/carbpol
WCl catalyzed cellulose degradation at 80 °C and lower in [BMIM]Cl
6
a,⁎
a
a
a
a
a,b,⁎⁎
Lilong Zhou , Shanshan Zhang , Zhengjie Li , Zhikun Zhang , Runjing Liu , Jimmy Yun
a
College of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, Shijiazhuang, Hebei Provience 050018, PR China
b
School of Chemical Engineering, The University of New South Wales, Australia, Sydney, NSW 2052, Australia
A R T I C L E I N F O
A B S T R A C T
Keywords:
Cellulose
Degradation
Degradation of cellulose to reducing sugar is the key step for the conversion of cellulose to valuable chemicals.
Cellulose was degraded by WCl6 in 1-butyl-3-methyl imidazole chloride at 80 °C and lower. 83% and 85.5% yield
of total reducing sugar was gotten at 70 and 80 °C, respectively. Compared with inorganic acid, heteropoly acid,
WCl
6
acidic ionic liquid and other metal chlorides, WCl
cellulose to reducing sugar. The effect of reaction temperature, reaction time, WCl
concentration were investigated. Degradation of cellulose by WCl in 1-butyl-3-methyl imidazole chloride is a
zero reaction. WCl also showed excellent catalytic performance for the degradation of nature cellulose and
lignocellulose. Catalyst can be reused at least 5 times without decrease of reducing sugar yield. The mechanism
of degradation of WCl was also suggested.
6
has shown better catalytic performance for degradation of
Reducing sugars
Ionic liquids
6
amount and cellulose
6
6
6
1
. Introduction
100 °C. The results showed that HCl was the best catalyst and 81% yield
of reducing sugar was got by HCl in C mimCl at 100 °C. However, HCl is
volatile, which is easy to be lost in this heating system. H SO was also
4
Degradation of biomass to chemicals is a practical way to replace
2
4
fossil resources, which face the crisis of exhausting and cause the rising
of atmospheric temperature (Hobson, 2008), for the energy and che-
micals requirements. Inexpensive, abundant, and renewable non-food
cellulose has been a cosset to chemists in recent years. However, the
complex hydrogen bonded network structures in cellulose make it hard
to be dissolved in traditional solvents (Notley, Pettersson, & Wagberg,
used for the hydrolysis of lignocellulose in ionic liquids. Dee and Bell
(2011) degraded cellulosic and hemicellulosic components of mis-
canthus by H SO in [Emim]Cl at 378 K and got 84% yield of sugar.
2 4
Sievers et al. (2009) hydrolyzed pine wood by trifluoroacetic acid in
[Bmim]Cl and got reducing sugar, furfural and 5-HMF. Although in-
organic acids showed good catalytic activity for cellulose degradation
in ionic liquids, the disadvantages cannot be ignored. The reaction
temperature was high and the inorganic acids were corrosive to the
equipments. Besides the corrosive properties of acids, reducing sugars
also have side reactions in the presence of strong acids, which produced
byproducts, humin. Acidic ionic liquids were synthesized to replace
inorganic acids for the cellulose degradation. Amarasekara and Owereh
(2009) decomposed sigmacell cellulose in acidic functional ionic liquids
and water, 62% yield of reducing sugars was gotten at 70 °C. They also
tried to degrade sigmacell cellulose in aqueous solution of acidic
functional ionic liquids (Amarasekara & Wiredu, 2011). Only about
25% yield of reducing sugars was gotten at 170 °C. Jiang, Zhu, Ma, Liu,
and Han (2011) hydrolyzed cellulose by acid functional ionic liquids in
[BMIM]Cl and got about 90% yield of reducing sugars at 80–100 °C.
Other kinds of acidic functional ionic liquids were also used for the
2004). Cellulose hydrolysis is commonly carried out in inorganic acids
aqueous solution at high temperature and high pressure, which con-
sume a lot of energy and need overpressure resistant equipments
(
Rinaldi & Schüth, 2009a, 2009b). In 2002, Rogers et al. found that
cellulose can be dissolved in [BMIM]Cl (Swatloski, Spear, Holbrey, &
Rogers, 2002). Because ionic liquids almost have no vapour pressure,
homogeneous degradation of cellulose at atmosphere pressure can be
realized.
Cellulose degradation in ionic liquids can be catalyzed by inorganic
acids, metal chlorides, acid functional ionic liquids and solid acids (Li,
Wang, & Zhao, 2008; Lima et al., 2009; Rosa, Campos-Martin, & Fierro,
2012; Su et al., 2011). Inorganic acids were firstly used as the catalysts
for cellulose degradation in ionic liquids. Li et al. (2008) hydrolyzed
lignocelluloses by different inorganic acids in different ionic liquids at
Abbreviations: HMF, 5-hydroxymethylfurfural; TRS, total reducing sugar
⁎
Corresponding author.
⁎
⁎
Corresponding author at: College of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, Shijiazhuang, Hebei Provience
050018, PR China.
E-mail addresses: lanruohe@126.com (L. Zhou), jimmy.yun@unsw.edu.au (J. Yun).
https://doi.org/10.1016/j.carbpol.2019.02.050
Received 12 December 2018; Received in revised form 27 January 2019; Accepted 14 February 2019
Available online 19 February 2019
0144-8617/ © 2019 Elsevier Ltd. All rights reserved.

L. Zhou, et al.
Carbohydrate Polymers 212 (2019) 289–296
degradation of cellulose, such as SO
3
H, COOH and OH functionalized
Sulfuric acid was purchased from Juhua Group Corporation (Zhejiang,
China). Microcrystalline cellulose was dried in blast oven at 100 °C for
1 h before use. All other chemicals were supplied by local suppliers and
used without further purification.
−
imidazole based ionic liquids and acidic ionic liquids with HSO
4
and
−
Cl as anions. All these acidic functional ionic liquids showed good
catalytic activity (Khan et al., 2018; Parveen, Patra, & Upadhyayula,
2
2
016; Tao, Song, & Chou, 2011a, 2011b; Zhou, Liang, Ma, Wu, & Wu,
013; Zhuo et al., 2015). However, the results showed that reducing
DF-101S constant temperature magnetic stirrer was purchased from
Zhengzhou Greatwall Scientific Industrial and Trade Co., Ltd.
(Zhengzhou, China). TU-1810 Model UV-vis spectrophotometer was
obtained by Purkinje General Co. Ltd. (Beijing, China). FT-IR spectro-
meter (Nicolet NEXUS670) which was used to analyze the leftover of
the reaction was purchased from Nicolet Co. Ltd. (USA).
sugars were unstable in this system, the yield of reducing sugar de-
creased quickly with the extending reaction time and acid functional
ionic liquids are also corrosive. The complex synthesis processes of acid
functional ionic liquids also hinder their application. A series of solid
acid catalysts were also developed to avoid these advantages for cel-
lulose degradation and high yields of reducing sugar were gotten
2.2. Synthesis of ionic liquid
(
Chutinate, Watthanaphanit, Saitoe, & Damrongsakkul, 2017; Hu, Lin,
Wu, Zhou, & Liu, 2015; Peng, Lee, Wu, & Wu, 2012; Rinaldi & Schüth,
009a, 2009b; Rinaldi, Palkovits, & Schüth, 2008; Shen, Guo, Bai, Qiu,
Qi, 2018; Suganuma et al., 2008; Yuan, Guan, Peng, Zhu, & Jiang,
017; Zhang, Shan, Liu, & Sun, 2018). However, the reaction tem-
Ionic liquid [BMIM]Cl was prepared by the reported procedures
(Tao et al., 2011a, 2011b). N-methylimidazole and acetonitrile were
added in three-necked flask, and then it was heated to 80 °C. The 1-
butyl chloride was slowly dropped in N-methylimidazole by top funnel
with mole ratio of 1.1:1. The mixture was heated to reflux for 48 h at
80 °C. When the reaction was finished, the mixture was cooled to room
temperature and washed by ethyl acetate, which was twice the volume
of mixture, for three times. Half volume of mixture acetonitrile and a
little [BMIM]Cl crystal were added into it. Then the mixture was air-
proof, still and crystallized for 12 h. The white crystal was washed by
ethyl acetate for 3 times. The pure [BMIM]Cl was prepared at last.
2
&
2
peratures were higher than 100 °C, at which reducing sugars and other
products were unstable and further reacted to byproducts. To avoid the
further degradation of reducing sugar, cellulose degradation in ionic
liquids should be performed at lower temperature than 100 °C.
Metal chlorides were also used for the degradation of cellulose in
2 2
ionic liquids. Su et al. (2011) used CuCl /PdCl catalytic system for the
degradation of cellulose and got 61% yield of reducing sugars. How-
ever, the high cost of Pd is the main obstacle for the application. Our
previous research also showed that the degradation products were
[PSMIM]HSO
4
was prepared by the following procedure (Zhou
et al., 2013). Some 1,3-propanesultone was added into three-necked
flask and dissolved by 150 mL methylbenzene. N-methylimidazole was
dropped into flask in mole ratio of 1:1. Then the temperature was in-
creased to 80 °C and reacted for 2 h. Reaction product was white solid-
[PSMIM]. The product was washed by acetic ester for 3 times and dried
at 100 °C for 5 h. Some [PSMIM] was dissolved in deionized water and
the same mole sulfuric acid was dropped into the solution. When all the
sulfuric acid was added, reaction temperature was increased to 80 °C
and reacted for 6 h. The mixture was cooled to room temperature,
washed by ether for more than 3 times and dried at 100 °C for 24 h.
really unstable in the presence of CuCl
chlorides were also used for the degradation of cellulose, such as MnCl
CrCl , AlCl , LiCl, ZrCl , InCl , RuCl , ZnCl , IrCl and AuCl (Binder &
2
(Zhou et al., 2015). Other metal
2
,
3
3
4
3
3
2
3
3
Raines, 2009; De, Dutta, & Saha, 2011; Kim et al., 2011; Liu, Zhang, &
Zhao, 2013; Tao et al., 2011a, 2011b; Tao, Song, & Chou, 2012; Li et al.,
2
2
013; Wang, Yu, Zhan, & Wang, 2011; Wei, Li, Thushara, Liu, & Ren,
011; Yang, Hu, & Abu-Omar, 2012; Yuan, Xu, Cheng, & Leitch, 2011).
All these researches mainly focused on the conversion of cellulose to
HMF and furfural and were performed at temperature higher than
1
00 °C. Hence, new catalytic systems for the cellulose degradation at
low temperature are needed. In our previous study, cellulose can be
degraded to reducing sugars very quickly by WCl in [BMIM]Cl.
However, reducing sugars were further transformed to humin at 120 °C
Zhou et al., 2015). We were curious for the catalytic properties of WCl
at low temperature for the degradation of cellulose. WCl has been used
4
[PSMIM]HSO was prepared at last.
6
2.3. Typical procedure for microcrystalline cellulose degradation
(
6
0.05 g MCC was added into 1.0 g [BMIM]Cl, and the mixture was
heated to 70 °C. After the mixture became clear which means all of MCC
was dissolved into [BMIM]Cl, the temperature was increased to the
6
in halo-de-hydroxylation, dihalo-de-oxo-bisubstitution reactions and
metathesis of olefin (Firouzabadi & Shiriny, 1996; Patton & McCarthy,
target temperature, and WCl
for some time. The WCl loading amount was 0.02 g. For the test of
catalysts loading amount, The WCl loading amounts used in this cat-
alytic system were 0.01, 0.02, 0.03, 0.04 and 0.05 g, respectively. The
loading amounts of other catalysts compared with WCl , such as CrCl
CuCl , SnCl , H SO , [PSMIM]HSO , H PW12 40, H Si12 40 and
Mo12 40 used in this research were all 0.02 g. At different time in-
6
was added into this solution, then reacted
6
+
1
987), which suggested that W can react with aldehyde groups. This
property might be positive for the degradation of cellulose.
In this research, cellulose degradation by WCl at low temperature
in [BMIM]Cl has been studied. WCl showed good catalytic perfor-
6
6
6
6
6
3
,
mance for the decomposition of cellulose in [BMIM]Cl at 80 °C and
lower. The kinetics and mechanism of this process were also studied to
2
4
2
4
4
3
O
3
O
H
3
O
find out the reason of WCl
6
excellent catalytic activity. This system was
tervals, two samples were withdrawn, weighted and quenched im-
mediately with cold water each time, one for TRS detection (recorded
also used for the degradation of lignocelluloses. The influence factors of
cellulose degradation have also been analyzed. This finding is pro-
mising to produce stable reducing sugars from cellulose and reduce
energy cost in cellulose degradation process.
1 2
as M ) and the other for HMF detection (recorded as M ). Every reac-
tion was repeated 3 times.
2.4. TRS analysis
2. Materials and methods
Total reducing sugar was detected by DNS method. The samples
2
.1. Materials and instruments
were taken periodically, and diluted with deionized water 150 times,
added 1 mL DNS regent, then heated for 20 min in boiling water, and
cooled by cold water. The colour intensity of the mixture was measured
in a TU-1810 Model spectrophotometer at 498 nm. The concentration
of total reducing sugars was calculated based on a standard curve ob-
tained with glucose. The yield of TRS was calculated as follows:
Heteropolyacids (H
crocrystalline cellulose (MCC, DP: 215–245), N-methylimidazole (AR),
-chlorobutane (CP), CrCl ·6H O (≥99%), SnCl and WCl (99.5%)
were supplied by Sinopharm Chemical Reagent Co. Ltd. (Shanghai,
China). CuCl ·2H O (≥99%) was obtained from Guangfu Fine Chemical
3 3 4
PW12O40, H PMo12O40 and H SiW12O40), mi-
1
3
2
4
6
2
2
Y
TRS = (CTRS × M
1
× 1500)/(Mcellulose × 1.11 × M
the concentration of total reducing sugars, M is the mass of the reac-
tion solution, Mcellulose is the mass of cellulose in the reaction, and M is
1 0
/M ), where CTRS is
Research Institute (Tianjin, China). 1,3-Propanesultone (AR) was pur-
chased from Shandong Yinghuan Chemical Co. Ltd. (Shandong, China).
0
1
290

L. Zhou, et al.
Carbohydrate Polymers 212 (2019) 289–296
the mass of sample withdrawn from the reaction mixture. The standard
errors of the experiments results were controlled in 4%.
FT-IR spectra of these solid showed that its heteropoly salts which
formed from the reaction between heteropoly acids and [BMIM]Cl,
[
3 3 3
BMIM] PW12O40, [BMIM] PMo12O40, [BMIM] SiW12O40 (Fig. 6). The
byproduct of this reaction was HCl, which was could be easily removed
during the heating process. It demonstrates that heteropoly acids are
2.5. HMF analysis
2 4
not suitable for the hydrolysis of cellulose in ionic liquids. For H SO ,
although it showed high catalytic efficiency, TRS yield decreased with
the reaction time prolonging. The acidic ionic liquid, only 48% yield of
4
TRS was gotten in [PSMIM]HSO -[BMIM]Cl system, with the
prolonging of reaction time, TRS yield decreased obviously, which
suggested the low stability of sugars in this system.
Samples were dissolved in 1500 times volume of deionized water,
and centrifuged at 4000 r/h for 5 min. The colour intensity of super-
natant fluid was measured in a TU-1810 Model spectrophotometer at
84 nm with a slit width of 0.06 mm (De et al., 2011). The concentra-
tion of furans was calculated based on a standard curve obtained with
HMF. The yield of furans was calculated as follows:
2
WCl
chlorides been used. 89% yield of TRS was gotten in 1 h at 120 °C
Table 1). However, TRS yield also decreased with the prolonging of
reaction time due to the side reactions of reduce sugars (Fig. 6). 82%
yield of TRS was gotten in CrCl -[BMIM]Cl in 6 h. In our previous re-
search, CrCl is active for the conversion of cellulose to 5-HMF, which
was generated from the dehydration of glucose (Zhao, Holladay,
Brown, & Zhang, 2007). Low yields of TRS were gotten in CuCl
6
showed the best catalytic performance among the metal
Y
C
furan = (CFURANS × M
2
2 0
× 1500)/(Mcellulose × 0.778 × M /M ), where
FURANS is the concentration of furans, M is the mass of the reaction
0
(
2
solution, Mcellulose is the mass of cellulose in the reaction, and M is the
mass of sample withdrawn from the reaction mixture.
3
3
2
.6. FT-IR analysis
2
-
6
The FT-IR spectra of [BMIM]Cl, WCl -[BMIM]Cl, cellulose-
[
BMIM]Cl (32%) and SnCl
4
-[BMIM]Cl (37%). The solid residue in
-[BMIM]Cl was black and mainly humin (Zhou et al., 2015), while
-[BMIM]Cl was white, which was mainly cellulose.
It suggests that cellulose in CuCl -[BMIM]Cl was totally degraded and
further transformed to byproducts, while the catalytic activity of SnCl
was too low. Although WCl showed better catalytic performance than
CrCl , CuCl , SnCl , [PSMIM]HSO
ficiency of WCl is lower than H
It seems that WCl
[
BMIM]Cl and WCl
lowing ways. 0.02 g WCl
tion was dropped on the KBr pellets, and then the FT-IR spectrum was
collected. The FT-IR spectra of cellulose-[BMIM]Cl and WCl -cellulose-
BMIM]Cl were collected by the same way. The constitution of cellu-
6
-cellulose-[BMIM]Cl were collected by the fol-
CuCl
the residue in SnCl
2
6
was dissolved in 1.0 g [BMIM]Cl. The solu-
4
2
6
4
[
6
lose-[BMIM]Cl and WCl
6
-cellulose-[BMIM]Cl were 0.1 g cellulose in
3
2
4
4
and H
3
PMo12
O
40, the catalytic ef-
PMo12
1
.0 g [BMIM]Cl, 0.1 g cellulose and 0.02 g WCl in 1.0 g [BMIM]Cl,
6
6
2
SO
4
, H PW12
3
O40 and H
3
40
O .
respectively. The solid leftover was separated, washed by deionized
water and submitted for FT-IR analysis. All the FT-IR spectra were
collected on an FT-IR spectrometer (Nicolet NEXUS670) with a re-
6
does not show the outstanding catalytic perfor-
mance at high reaction temperature. We wondered if the hydrolysis of
cellulose in [BMIM]Cl could be carried out by these catalysts at lower
−
1
−1
solution of 4 cm
. Results and discussion
.1. Selection of catalysts
The TRS yields were selected the highest one among ones from
and 32 scans in the region of 4000–400 cm
.
temperature. Degradation of cellulose by WCl
and H PW12 40 was performed at 70 °C. The results were shown in
Fig. 1. WCl showed the best catalytic performance among these cata-
lysts. 63.5% yield of TRS was obtained in WCl -[BMIM]Cl in 2 h. The
6 3 2 2 4
, CrCl , CuCl , H SO
3
O
3
6
6
3
TRS yield reached 81.2% in 6 h, and then slightly decreased to 77.5%
when the reaction time reached 10 h. TRS was also detected in the
system without catalysts. 9.4% yield of TRS was obtained from cellulose
in [BMIM]Cl after 2 h heating and stirring at 70 °C. TRS yield decreased
to 8.2% when reaction time reached 10 h. The results match the pre-
vious published research (Liang, Wang, Zhou, Wu, & Wu, 2013).
different reaction times.
The common catalysts used for the hydrolysis of cellulose in
[
BMIM]Cl were investigated, such as H
2
SO
4
, CrCl
3
, WCl
6
, SnCl
4 2
, CuCl ,
H
3
PW12 40, H SiW12 40, H PMo12 40 and [PSMIM]HSO
O
3
O
3
O
4
. As can be
Compared with the blank control group, H
were seemed to show no catalytic activity at 70 °C. 22.1% yield of TRS
was obtained in CrCl -[BMIM]Cl after 2 h reaction at 70 °C, which was
much worse than WCl . All these results demonstrate that cellulose can
be hydrolyzed by WCl in [BMIM]Cl at really low temperature.
2 4 3 2
SO , H PW12O40 and CuCl
seen in Table 1 and Figures S1–S11, heteropoly acid showed better
catalytic performance than metal chlorides and acidic ionic liquid at
3
1
20 °C. Cellulose was almost completely degraded to reducing sugar in
the presence of H PW12 40 and H SiW12 40 in half an hour. While,
only 74% yield of reducing sugar was gotten in H PMo12 40-[BMIM]Cl
system. The reason may be that the acidity of H PMo12 40 was lower
than H PW12 40 and H SiW12 40. 100% yield of reducing sugars was
gotten in 8 h in the presence of H SO . No residue was found in the
solution of H SO -[BMIM]Cl system, which also suggested the com-
6
3
O
3
O
6
3
O
3
O
3
O
3
O
2
4
2
4
pletely hydrolysis of cellulose. For heteropoly acids, some solid residues
were found after reaction. These residues were washed and dried. The
Table 1
Degradation of cellulose to reducing sugars by different catalysts in [BMIM]Cl.
Entry Catalyst
Solvent
Time (h) Temperature (°C) TRS yield
%)
(
1
2
3
4
5
6
7
8
9
WCl
CrCl
CuCl
SnCl
SO
[PSMIM]HSO
6
[BMIM]Cl
[BMIM]Cl
[BMIM]Cl
[BMIM]Cl 10
[BMIM]Cl
[BMIM]Cl
[BMIM]Cl 0.5
[BMIM]Cl 0.5
[BMIM]Cl
1
6
3
120
120
120
120
120
120
120
120
120
89
82
32
37
100
48
99
99
74
3
2
4
H
2
4
8
1
4
H
3
H
3
H
3
PW12
SiW12
PMo12
O
O
4
O
40
40
4
Fig. 1. Degradation of cellulose by different catalysts at 70 °C.
291

L. Zhou, et al.
Carbohydrate Polymers 212 (2019) 289–296
6
Fig. 2. Degradation of cellulose by WCl in [BMIM]Cl at different temperatures.
3
.2. Influence of reaction temperature and time
the degraded cellulose mole amount was calculated by following
method. The degraded molar quantities of cellulose monomer, glucose,
were calculated by the following formula: N(glucose) = m(cellulose)/
180 × Yield of TRS, where m(cellulose) is the mass of cellulose, Yield of
TRS was the yield of TRS, 180 is the molar mass of monomer of cel-
Temperature is a key factor influencing the hydrolysis of cellulose,
which would influence the reaction rate, yield of byproducts, viscosity
of ionic liquids and mass transfer rate. So the reaction temperature was
evaluated at first. As seen in Fig. 2a, with the increase of reaction
temperature, the hydrolysis rate increase obviously. It takes 180 min,
6 10 6
lulose (C H O ). The degraded molar quantities of cellulose with dif-
ferent cellulose concentration (2%, 4%, 5%, 6%, 8%, 10%) in 2 h were
−
4
4
−4
−4
−4
1
10 min, less than 30 min and less than 10 min at 70, 100 and 110 °C to
1.235 × 10
2.252 × 10
,
2.239 × 10
,
2.373 × 10
,
2.244 × 10
,
−
−4
get 75% TRS yield, respectively. There may be two reasons which ac-
celerated the reaction rate. The increase of reaction temperature can
increase the energy of system to break the β-1,4-glucosidic bonds. The
increase of temperature also can decrease the viscosity of the reaction
system and speed up the mass transfer rate. The results also showed that
the increase of temperature could increase TRS yield. The highest TRS
yields gotten at 70, 80, 90, 100, 110 and 120 °C were 80.4%, 85.8%,
and 2.241 × 10 mol, respectively. The results de-
monstrate that except the first one, the same amount of TRS was pro-
duced with different cellulose concentration at the same time. So the
reaction rate based on TRS per mole WCl
6
was the same, about
. The results showed that the cellulose concentration
has no influence on the reaction rate, and the hydrolysis of cellulose is
−
1
−1
1.126 mol g
h
−
1
−1
an apparent zero order reaction (r = 1.126 mol g
3.5. Degradation of different cellulose
h
).
7
6.8%, 82.1%, 93.7%, and 88.7%, respectively. However, when reac-
tion temperature reached 90 °C, TRS yield decreased quickly due to the
side reactions of glucose. HMF was the main byproducts in this reaction
were produced from the dehydration of glucose (Zakrzewska, Bogel-
Łukasik, & Bogel-Łukasik, 2011). It was also the reactant for the pro-
duction of humin. The results showed that with the increase of reaction
temperature, HMF production rate and yield all raised obviously
The polymerization degree of microcrystalline cellulose used in
previous experiments was 215–245, which was much smaller than the
nature cellulose and lignocellulose. In order to test the catalytic per-
(
Fig. 1a). It demonstrates that glucose can be more easily translated to
6 6
formance of WCl for nature cellulose, WCl -[BMIM]Cl system was used
byproducts at high temperature than at low temperature. When reac-
tion was performed at temperature higher than 100 °C, not only TRS
yield decreased, but also HMF decreased with the prolonging of reac-
tion time. It suggests that TRS was stable in the system when tem-
perature was lower than 90 °C.
for the degradation of cotton (cellulose content: about 98%; poly-
merization degree: 6000–11,000), filter paper (cellulose content: 98%;
polymerization degree: 3000–6000) and poplar wood (cellulose and
hemicelluloses content: 82%; polymerization degree: about 10,000) at
90 °C. Because of large polymerization degree, cotton or filter paper was
dissolved in [BMIM]Cl and formed high viscosity colloid. After the
3.3. Influence of catalyst amount
6
addition of WCl , the colloid became lilac and transformed to solution
in 20 min, and then the lilac solution became brown slowly which
suggested the generation of coloured products, such as HMF and fur-
fural. This phenomenon suggests that the first step to degrade cellulose
may be cutting the long chains of cellulose to short chains. For poplar
wood, the dissolution process took 5 h and formed brown colloid. The
results showed that the highest TRS yield was gotten from filter paper
(88.5%). The TRS yields of cotton and polar wood were 83.1% and
82.8%, respectively. The cellulose and hemicelluloses content of polar
wood is 82%, which suggested that all the cellulose and hemicelluloses
were degraded to TRS. TRS yield of cotton was lower than filter paper,
which has the similar cellulose content, while the polymerization de-
gree of cotton was larger than that of filter paper. It demonstrates that
the low polymerization degree is positive for the degradation of cellu-
lose in ionic liquids. The cellulose hemicelluloses content of polar wood
is 82%, while TRS yields of polar wood was 82.8%. It suggests that
almost all the cellulose and hemicelluloses in polar wood were de-
graded to TRS in our experiments. These results showed that lig-
nocelluloses and other nature cellulose can be decomposed to TRS by
To test the efficiency of WCl
6
, WCl
6
amount was evaluated in the
experiments. When 0.01 g WCl
6
was added in the reaction system, it
took 7 h to reach the highest TRS yield (41.8%). With the catalyst
amount increasing to 0.02 g, 85.8% yield of TRS was gotten in 5 h. The
highest HMF yield was also achieved with 0.02 g catalysts. When the
catalyst amount further increased, both TRS and HMF yield decreased,
while the amount of humin increased obviously. It suggests that too
many catalysts would lead to the further degradation of TRS and HMF.
The results showed that the most suitable catalyst is amount 0.02 g/
(
1.0 g [BMIM]Cl, 0.05 g cellulose) (Fig. 3).
3
.4. Degradation of cellulose with different concentration of cellulose
Cellulose concentration in [BMIM]Cl is a main factors influence the
catalytic performance. The cellulose concentration was varied to test
the effect of cellulose concentration on catalytic performance. As can be
seen in Fig. 4, 100% yield of TRS was gotten when cellulose con-
centration was 2%. With the cellulose concentration increasing to 10%,
TRS yield decreased to 36.3%. To find the reasons behind these data,
6
WCl in [BMIM]Cl at temperature lower than 100 °C (Fig. 5).
292

L. Zhou, et al.
Carbohydrate Polymers 212 (2019) 289–296
Fig. 3. Reduce sugar and HMF yield of degradation of cellulose by different amount of WCl
6
at 80 °C.
Fig. 4. TRS yield from degradation of cellulose with different cellulose con-
centration at 90 °C (WCl 0.02 g; [BMIM]Cl 1 g; 2 h).
6
Fig. 6. FT-IR spectra of cellulose and residue of cellulose degradation at 120 °C
for 2 h.
residue from H
Keggin structure were shown at 1109 [vs(PeO
3
PW12
O
40-[BMIM]Cl system, the characteristic bands of
)], 955 [vs(WeO )], 894
c d
vs(WeO eW)], 794 [vs(WeO eW)], which suggested the presence of
a
d
[
3−
PW12
O
40
in residue (Ghanbari-Siahkali, Philippou, D+wyer,
&
Anderson, 2000). The vibration bands of NeH in NH
4
(2933,
−
1
−1
1
471 cm ), iminazole ring (1572, 1166 cm ) and CeH in iminazole
−
1
ring (3100 cm ) were also found in this spectrum (Xie et al., 2018). All
these vibration bands can be assigned to the bands in [BMIM] PW12
Xie et al., 2014). It suggests that H PW12 40 reacted with [BMIM]Cl.
The characteristic bands of cellulose were also emerged at 1056, 1461,
3
O
40
(
3
O
−
1
2977 and 3350 cm . It suggests that there is some cellulose in residue.
−1
The vibration band emerged at 1638 cm which can be assigned to vs
C]O) was much stronger than it in the spectrum of cellulose. It de-
monstrates that there was also humin formed in H PW12 40-[BMIM]Cl
system. The FT-IR spectra showed that the TRS was easily transformed
to humin in the presence of WCl and H PW12 40at high temperature.
PW12 40 can reacted with [BMIM]Cl and fromed [BMIM] PW12
(
3
O
6
Fig. 5. Degradation of different kinds of cellulose by WCl for 2 h at 90 °C.
6
3
O
H
3
O
3
40
O .
3
6 3 40
.6. Degradation residue of cellulose by WCl and H PW12O
To investigate the degradation process of cellulose by WCl
BMIM]Cl, the residues of cellulose were characterized by FT-IR
6
in
3.7. Recycle of the catalytic system
[
(
(
[
Fig. 6). The characteristic bands of cellulose emerged at 3350.9 [vs
It is important that catalysts can be recovered after cellulose de-
gradation and can be reused for the reaction. WCl was added in cel-
lulose ionic liquids solution and reacted for 3 h at 80 °C. After the re-
action, deionized water was added into the cellulose degradation
system, and then the solid was separated. The solid was added into
cellulose ionic liquid solution. RS yield of the first recycle was 76.5% at
OeH)], 2977 [vs(CeH)], 1640 [vs(C]O)], 1371 [vs(CeOeC)], 1162
−
1
6
vs(CeC)] and 1056.7 [vs(CeOeH)] cm
, FT-IR spectrum of solid residue was different
from the one of cellulose. The characteristic bands of cellulose
(Zhou et al., 2013). After
degradation by WCl
6
−1
(
1056–1461 cm ) were disappeared or weakened. The vibration band
−1
of OeH at 3350 cm
stitutes of residue were the polymerized products of glucose, HMF and
furfural, which was commonly called humin. For the spectrum of solid
was weakened. The results showed that con-
8
0 °C. However, after the first cycle, the time for degradation of cellu-
lose increased obviously. When reaction temperature increased to
00 °C, 76.5% yield of RS was gotten in 1 h. So beside the first cycle, the
1
293

L. Zhou, et al.
Carbohydrate Polymers 212 (2019) 289–296
Fig. 8. FT-IR spectra of [BMIM]Cl, [BMIM]Cl-WCl
BMIM]Cl-cellulose-WCl
6
, [BMIM]Cl-cellulose and
[
6
.
6
Fig. 7. Reducing sugar and HMF yield of WCl recycled for different times
through the interaction between W6+ and aldehyde carbonyl groups.
According to the previous studies and FT-IR spectra (Fig. 8, S14),
6
(cellulose 0.05 g, [BMIM]Cl 1 g, WCl 0.02 g).
rest cycles of cellulose degradation were performed in 1 h at 100 °C. The
catalyst was recycled for 5 times and RS yield has no obviously de-
creased. It indicates that the catalysts can be recovered after degrada-
tion and can be reused for the reaction. However, HMF yield decreased
from 8.5% to 1.8% after the first cycle. When water was added into the
6
the mechanism of degradation by WCl in [BMIM]Cl was suggested.
After cellulose being dissolved in [BMIM]Cl, the crystal of cellulose is
destroyed. Then cellulose chain is dispersed in [BMIM]Cl and [BMIM]Cl
forms hydrogen bonds with hydroxyl group in cellulose (Fort et al.,
2007). W6+ has good ability in combination with aldehyde O
(Firouzabadi & Shiriny, 1996; Jung & Murphy, 2007; Patton &
McCarthy, 1987), so in the reaction of cellulose's degradation it can
attack glycosidic bonds which exist between two glucose units in cel-
6 3 2
reaction system, WCl might be hydrolyzed to WO ·nH O. To find out if
WO
WO
3
·nH
·nH
2
O has been the catalysts for the secondary and the rest cycles,
O was added into cellulose ionic liquid solution to degrade
3
2
cellulose at 100 °C. The results showed that similar RS yield (82.1%)
was gotten with the secondary cycle. While HMF yield was only 1.3%
6
lulose more easily than other cations. When WCl is added in the re-
action system, it ionizes into W6+ and Cl . Then W attacks the O in
glycosidic bonds (aldehyde O), and glycosidic bonds are broken down.
Water existing in [BMIM]Cl combines with glycosidic bonds which
have been broken down to form TRS. Long chains of cellulose were
broken down into short chains. Then short chain of cellulose was fur-
ther degraded into TRS. As reactions continue, TRS was dehydrated to
furans. W6+ also can interact with the aldehyde carbonyl groups, so
−
6+
which demonstrates that WO
ondary and the rest cycles. The decrease of HMF yield in the secondary
and the rest cycles may be due to the generation of WO ·nH O.
However, the mechanism of cellulose in [BMIM]Cl degraded by
WO ·nH O need further research (Fig. 7).
3 2
·nH O was the real catalyst for the sec-
3
2
3
2
furans are further polymerized to humin. To find out whether WCl
6
3
.8. Mechanism of degradation of cellulose
combines with aldehyde O or O in OH, formaldehyde and methanol
were added into the reaction system, respectively. When formaldehyde
was added in reaction system, TRS yield has decreased clearly (only
To find out the mechanism of degradation of cellulose by WCl
6
in
dis-
solved in [BMIM]Cl was performed. In the spectrum of [BMIM]Cl, the
[
6 6
BMIM]Cl, FT-IR spectra of cellulose, WCl and cellulose-WCl
4
0% yield of TRS was produced at 80 °C for 3 h). However, when the
same amount of methanol was added, TRS yield only decreased a little
78% yield of TRS was produced at 80 °C for 3 h). It is due to the
competition of the aldehyde group on formaldehyde with which on
−
1
vibration band emerged at 3381 cm can be adscripted to O-H, which
demonstrated the presence of H O in [BMIM]Cl. It can explain why
cellulose can be hydrolyzed in this system. The vibration bands of
(
2
6+
cellulose combination with W
(Fig. 9).
−
1
[
BMIM]Cl were shown at 3054 cm [vs(C-H) in iminazole ring], 1565
−
1
and 1169 cm
(skeletal vibration of iminazole ring). The vibration
and CH were also detected at 2740–2993,
459 cm . After the addition of WCl , two new peaks appeared at 902
and 819.8 cm , which can be assigned to vibration bands formed from
bands of C-H in CH
2
3
4. Conclusions
−1
1
6
−
1
Compared with inorganic acid, heteropoly acids, acidic ionic liquids
and other metal chlorides, WCl showed excellent activity for the de-
6
+
the interaction between W
solved into [BMIM]Cl, the characterization vibration bands of cellulose
can be found at 1059 and 1034 cm , which can be assigned to C-O in
cellulose. No other vibration bands of cellulose was found in the spectra
of cellulose-[BMIM]Cl solution, which suggested that cellulose was well
dissolved in [BMIM]Cl. A new vibration band emerged at 445.7 cm
which can be assigned to the vibration bands formed from the inter-
action between W and C]O (Firouzabadi & Shiriny, 1996; Patton &
McCarthy, 1987). This vibration band became stronger with the in-
crease of reaction time (Figure S14). It suggested that cellulose was
degraded by WCl
carbonyl groups which formed the 1,4-glucosidic bonds. After the
system was heating for 10 min, the vibration band at 902 cm
disappeared, and the vibration bands at 819.8 and 445.7 cm became
stronger (Figure S14). It suggested that the W
with aldehyde carbonyl group became stronger under the calefaction. It
further proved the degradation of cellulose by WCl in [BMIM]Cl was
and [BMIM]Cl. After cellulose was dis-
6
gradation of cellulose in [BMIM]Cl at temperature lower than 80 °C and
atmosphere pressure. More than 80% TRS yield can be obtained in 4 h
−1
6
at 70 °C. Reducing sugars are stable with WCl at low temperature
(
≤80 °C), while too high temperature (> 90 °C) would lead to the quick
−
1
,
decrease of reducing sugar and generated humin as byproduct. The
good catalytic activity was due to the good combination ability with
aldehyde O in cellulose to break β-1,4-glucosidic bonds, which was
6
+
6
proved by FT-IR. Degradation of cellulose by WCl in [BMIM]Cl is a
zero order reaction. Lignocelluloses and other nature cellulose also can
be almost totally degraded to reducing sugar by WCl in [BMIM]Cl at
0 °C. The most suitable catalysts loading amount and reaction time at
0 °C is 0.04 g/g cellulose·2 g [BMIM]Cl and 2 h. Catalyst can be re-
6
+
6
through the combination of W
with aldehyde
6
9
8
−
1
was
−1
cycled for 5 times without the decrease of catalytic activity. This re-
search shows an efficient way for the degradation of cellulose at low
temperature.
6
+
strongly interacted
6
294

L. Zhou, et al.
Carbohydrate Polymers 212 (2019) 289–296
6
Fig. 9. Mechanism of degradation of cellulose by WCl in [BMIM]Cl.
Acknowledgements
chlorides in imidazolium ionic liquid. Green Chemistry, 13, 1503–1506.
Li, C. Z., Wang, Q., & Zhao, Z. K. (2008). Acid in ionic liquid: An efficient system for
hydrolysis of lignocellulose. Green Chemistry, 10, 177–182.
The authors gratefully acknowledge the financial support of the
National Natural Science Foundation of China (No. 21106032) and the
Education Department, Hebei Province (No. ZD2016064; No.
3
Li, H., Zhang, Q., Liu, X., Chang, F., Hu, D., Zhang, Y., et al. (2013). InCl -ionic liquid
catalytic system for efficient and selective conversion of cellulose into 5-hydro-
xymethylfurfural. RSC Advances, 3, 3648–3654.
Liang, R., Wang, W., Zhou, L., Wu, T., & Wu, Y. (2013). The degradation of cellulose in
high temperature with BmimCl. Journal of Zhejiang Normal University (Nature
Science), 36(3), 315–320.
2017102002381).
Appendix A. Supplementary data
Lima, S., Neves, P., Antunes, M. M., Pillinger, M., Lgnatyev, N., & Valente, A. A. (2009).
Conversion of mono/di/polysaccharides into furan compounds using 1-alkyl-3-me-
thylimidazolium ionic liquids. Applied Catalysis A: General, 363, 93–99.
Liu, B., Zhang, Z., & Zhao, Z. K. (2013). Microwave-assisted catalytic conversion of cel-
lulose into 5-hydroxymethylfurfural in ionic liquids. Chemical Engineering Journal,
Supplementary material related to this article can be found, in the
online version, at https://doi.org/10.1016/j.carbpol.2019.02.050.
215–216, 517–521.
Notley, S. M., Pettersson, B., & Wagberg, L. (2004). Direct measurement of attractive van
der waals’ forces between regenerated cellulose surfaces in an aqueous environment.
Journal of the American Chemical Society, 126(43), 13930–13931.
References
Parveen, F., Patra, T., & Upadhyayula, S. (2016). Hydrolysis of microcrystalline cellulose
using functionalized Bronstedacidic ionic liquids – A comparative study. Carbohydrate
Polymers, 135, 280–284.
Patton, P. A., & McCarthy, T. J. (1987). Factors influencing the rate of tungsten hex-
achloride/tetramethyltin-catalyzed olefin metathesis polymerization: monomer ring
strain is not important. Macromolecules, 20, 778–782.
Peng, W. H., Lee, Y. Y., Wu, C., & Wu, K. C. W. (2012). Acid–base bi-functionalized, large-
pored mesoporous silica nanoparticles for cooperative catalysis of one-pot cellulose-
to-HMF conversion. Journal of Materials Chemistry, 22, 23181–23185.
Rinaldi, R., Palkovits, R., & Schüth, F. (2008). Depolymerization of cellulose using solid
catalysts in ionic liquids. Angewandte Chemie International Edition, 47, 8047–8050.
Rinaldi, R., & Schüth, F. (2009a). Acid hydrolysis of cellulose as the entry Point into
biorefinery schemes. ChemSusChem, 2, 1096–1107.
Amarasekara, A. S., & Owereh, O. S. (2009). Hydrolysis and decomposition of cellulose in
Bronsted acidic ionic liquids under mild conditions. Industrial & Engineering Chemistry
Research, 48, 10152–10155.
Amarasekara, A. S., & Wiredu, B. (2011). Degradation of cellulose in dilute aqueous so-
lutions of acidic ionic liquid 1-(1-propylsulfonic)-3-methylimidazolium chloride, and
p-toluenesulfonic acid at moderate temperatures and pressures. Industrial &
Engineering Chemistry Research, 50, 12276–12280.
Binder, J. B., & Raines, R. T. (2009). Simple chemical transformation of lignocellulosic
biomass into furans for fuels and chemicals. Journal of the American Chemical Society,
131, 1979–1985.
Chutinate, P. P., Watthanaphanit, A., Saitoe, N., & Damrongsakkul, S. (2017). Conversion
of cellulose into reducing sugar by solution plasma process (SPP). Carbohydrate
Polymers, 172, 230–236.
Dee, S., & Bell, A. T. (2011). Effects of reaction conditions on the acid-catalyzed hydro-
lysis of miscanthus dissolved in an ionic liquid. Green Chemistry, 13, 1467–1475.
De, S., Dutta, S., & Saha, B. (2011). Microwave assisted conversion of carbohydrates and
biopolymers to 5-hydroxymethylfurfural with aluminium chloride catalyst in water.
Green Chemistry, 13, 2859–2868.
Rinaldi, R., & Schüth, F. (2009b). Design of solid catalysts for the conversion of biomass.
Energy & Environmental Science, 2, 610–626.
Rosa, S. M., Campos-Martin, J. M., & Fierro, J. L. G. (2012). High glucose yields from the
hydrolysis of cellulose dissolved in ionic liquids. Chemical Engineering Journal,
181–182, 538–541.
Shen, F., Guo, T., Bai, C., Qiu, M., & Qi, X. (2018). Hydrolysis of cellulose with one-pot
Firouzabadi, H., & Shiriny, F. (1996). New applications of tungsten hexachloride (WCl
in organic synthesis halo-de-hydroxylation and dihalo-de-oxo-bisubstitution reac-
tions. Tetrahedron, 52(47), 14929–14936.
6
)
synthesized sulfonated carbonaceous solid acid. Fuel Processing Technology, 169,
2
44–247.
Sievers, C., Valenzuela-Olarte, M. B., Marzialetti, T., Musin, I., Agrawal, P. K., & Jones, C.
W. (2009). Ionic-liquid-phase hydrolysis of pine wood. Industrial & Engineering
Chemistry Research, 48, 1277–1286.
Su, Y., Brown, H. M., Li, G., Zhou, X., Amonette, J. E., Fulton, J. L., et al. (2011).
Accelerated cellulose depolymerization catalyzed by paired metal chlorides in ionic
liquid solvent. Applied Catalysis A: General, 391, 436–442.
Fort, D. A., Remsing, R. C., Swatloski, R. P., Moyna, P., Moyna, G., & Rogers, R. D. (2007).
Can ionic liquids dissolve wood? Processing and analysis of lignocellulosic materials
with 1-n-butyl-3-methylimidazolium chloride. Green Chemistry, 9, 63–69.
Ghanbari-Siahkali, A., Philippou, A., Dwyer, J., & Anderson, M. W. (2000). The acidity
and catalytic activity of heteropoly acid on MCM-41 investigated by MAS NMR, FTIR
and catalytic tests. Applied Catalysis A: General, 192, 57–69.
Suganuma, S., Nakajima, K., Kitano, M., Yamaguchi, D., Kato, H., Hayashi, S., et al.
Hobson, A. (2008). Energy, environment, and climate. American Journal of Physics,
3
(2008). Hydrolysis of cellulose by amorphous carbon bearing SO H, COOH, and OH
76(12), 1174–1175.
groups. Journal of the American Chemical Society, 130, 12787–12793.
Hu, L., Lin, L., Wu, Z., Zhou, S., & Liu, S. (2015). Chemocatalytic hydrolysis of cellulose
Swatloski, R. P., Spear, S. K., Holbrey, J. D., & Rogers, R. D. (2002). Dissolution of cel-
lulose with ionic liquids. Journal of the American Chemical Society, 124, 4974–4975.
Tao, F., Song, H., & Chou, L. (2012). Efficient conversion of cellulose into furans catalyzed
by metal ions in ionic liquids. Journal of Molecular Catalysis A: Chemical, 357, 11–18.
into glucose over solid acid catalysts. Applied Catalysis B: Environmental, 174–175,
2
25–243.
Jiang, F., Zhu, Q., Ma, D., Liu, X., & Han, X. (2011). Direct conversion and NMR ob-
servation of cellulose to glucose and 5-hydroxymethylfurfural (HMF) catalyzed by the
acidic ionic liquids. Journal of Molecular Catalysis A: Chemical, 334, 8–12.
Jung, M. E., & Murphy, J. M. (2007). Functional group selectivity in reactions of epoxides
with tungsten hexachloride. Tetrahedron Letters, 48, 8388–8391.
Khan, A. S., Man, Z., Bustam, M. A., Nasrullah, A., Ullah, A., Sarwono, A., et al. (2018).
Efficient conversion of lignocellulosic biomass to levulinic acid using acidic ionic
liquids. Carbohydrate Polymers, 181, 208–214.
3
Tao, F., Song, H., & Chou, L. (2011a). Hydrolysis of cellulose in SO H-functionalized ionic
liquids. Bioresource Technology, 102, 9000–9006.
Tao, F., Song, H., Yang, J., & Chou, L. (2011b). Catalytic hydrolysis of cellulose into
furans in MnCl -ionic liquid system. Carbohydrate Polymers, 85, 363–368.
2
Wang, P., Yu, H., Zhan, S., & Wang, S. (2011). Catalytic hydrolysis of lignocellulosic
biomass into 5-hydroxymethylfurfural in ionic liquid. Bioresource Technology, 102,
4
179–4183.
Kim, B., Jeong, J., Lee, D., Kim, S., Yoon, H. J., Lee, Y. S., et al. (2011). Direct trans-
formation of cellulose into 5-hydroxymethyl-2-furfural using a combination of metal
Wei, Z., Li, Y., Thushara, D., Liu, Y., & Ren, Q. (2011). Novel dehydration of
295

L. Zhou, et al.
Carbohydrate Polymers 212 (2019) 289–296
carbohydrates to 5-hydroxymethylfurfural catalyzed by Ir and Au chlorides in ionic
Polymers, 346, 2019–2023.
liquids. Journal of the Taiwan Institute of Chemical Engineers, 42, 363–370.
Xie, W., Zhou, D., Ren, Y., Tang, S., Kuang, M., & Du, S. (2018). 1-Butyl-3-methylimi-
dazolium chloride pretreatment of cotton stalk and structure characterization.
Renewable Energy, 125, 668–674.
Zakrzewska, M. E., Bogel-Łukasik, E., & Bogel-Łukasik, R. (2011). Ionic liquid-mediated
formation of 5-hydroxymethylfurfurals: A promising biomass-derived building block.
Chemical Reviews, 111, 397–417.
Zhang, S., Shan, D., Liu, X., & Sun, M. (2018). Cellulose-degrading strains: Their screening
and application to corn straw in low-temperature environments. Polish Journal of
Environmental Studies, 27(5), 2349–2355.
Xie, X., Han, J., Wang, H., Zhu, X., Liu, X., Niu, Y., et al. (2014). Selective conversion of
3
microcrystalline cellulose into hexitols over a Ru/[Bmim] PW12O40 catalyst under
mild conditions. Catalysis Today, 233, 70–76.
Yang, Y., Hu, C., & Abu-Omar, M. M. (2012). Conversion of carbohydrates and lig-
Zhao, H., Holladay, J. E., Brown, H. Z., & Zhang, C. (2007). Metal chlorides inionic liquid
solvents convert sugars to 5-hydroxymethylfurfural. Science, 316, 1557–1600.
Zhuo, K., Du, Q., Bai, G., Wang, C., Chen, Y., & Wang, J. (2015). Hydrolysis of cellulose
catalyzed by novel acidic ionic. Carbohydrate Polymers, 115, 49–53.
Zhou, L., He, Y., Ma, Z., Liang, R., Wu, T., & Wu, Y. (2015). One-step degradation of
cellulose to 5-hydroxymethylfurfural in ionic liquid under mild conditions.
Carbohydrate Polymers, 117, 694–700.
3 2
nocellulosic biomass into 5-hydroxymethylfurfural using AlCl ·6H O catalyst in a
biphasic solvent system. Green Chemistry, 14, 509–513.
Yuan, B., Guan, J., Peng, J., Zhu, G., & Jiang, J. (2017). Green hydrolysis of corncob
cellulose into 5-hydroxymethylfurfural using hydrophobic imidazole ionic liquids
with a recyclable, magnetic metalloporphyrin catalyst. Chemical Engineering Journal,
3
30, 109–119.
Zhou, L., Liang, R., Ma, Z., Wu, T., & Wu, Y. (2013). Conversion of cellulose to HMF in
ionic liquid catalyzed by bifunctional ionic liquids. Bioresource Technology, 129,
450–455.
Yuan, Z., Xu, C. C., Cheng, S., & Leitch, M. (2011). Catalytic conversion of glucose to 5-
hydroxymethyl furfural using inexpensive co-catalysts and solvents. Carbohydrate
296