Efficient conversion of glucose and cellulose to 5-hydroxymethylfurfural in DBU-based ionic liquids

Article abstract of DOI:10.1039/c3ra43934h

Development of an efficient catalytic system for the dehydration of carbohydrates to produce 5-hydroxymethylfurfural (HMF), which is a platform molecule for biomass transformation, is a very attractive topic. In this work, three DBU-based ionic liquids (ILs) were prepared and used as the solvent for the conversion of carbohydrates into HMF. It was found that all these new ILs were excellent solvents for the dehydration of glucose, cellulose, fructose, sucrose, inulin, and cellobiose to form HMF using CrCl₃· 6H₂O as the catalyst. The effects of temperature, reaction time, catalyst amount, and substrate/solvent weight ratio on the dehydration of glucose in CrCl₃·6H₂O/Bu-DBUCl were studied systematically. It was shown that the yield of HMF could reach 64% from glucose. In addition, the CrCl₃·6H₂O/Bu-DBUCl system could be easily separated from the product, and could be reused five times without considerably decreasing in activity and selectivity. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra43934h

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Efficient conversion of glucose and cellulose to
5-hydroxymethylfurfural in DBU-based ionic liquids
Cite this: RSC Adv., 2013, 3, 20085
Jinliang Song, Binbin Zhang, Jinghua Shi, Honglei Fan, Jun Ma, Yingying Yang
*
and Buxing Han
Development of an efficient catalytic system for the dehydration of carbohydrates to produce 5-
hydroxymethylfurfural (HMF), which is a platform molecule for biomass transformation, is a very
attractive topic. In this work, three DBU-based ionic liquids (ILs) were prepared and used as the solvent
for the conversion of carbohydrates into HMF. It was found that all these new ILs were excellent
solvents for the dehydration of glucose, cellulose, fructose, sucrose, inulin, and cellobiose to form HMF
using CrCl3$6H2O as the catalyst. The effects of temperature, reaction time, catalyst amount, and
substrate/solvent weight ratio on the dehydration of glucose in CrCl3$6H2O/Bu-DBUCl were studied
systematically. It was shown that the yield of HMF could reach 64% from glucose. In addition, the
CrCl3$6H2O/Bu-DBUCl system could be easily separated from the product, and could be reused five
times without considerably decreasing in activity and selectivity.
Received 26th July 2013
Accepted 16th August 2013
DOI: 10.1039/c3ra43934h
www.rsc.org/advances
Solvents have great impact on the activity and selectivity for
the dehydration of carbohydrates to form HMF. The dehydra-
Introduction
tion reaction has been carried out in water,14 traditional organic
solvents,15 multiphase systems,16 and in ionic liquids (ILs).17 ILs
have received extensive attention in recent years because of
their unusual properties, such as negligible vapor pressure,
non-ammability, high thermal and chemical stability, and
adjustable solvent power for organic and inorganic
substances.18 Due to the good solubility of carbohydrates in ILs,
transformation of carbohydrates into HMF using ILs as solvents
or catalysts is very promising, and there have been many reports
on this.19 For example, Zhao et al. reported that satisfactory
yields of HMF could be reached by the dehydration of fructose
and glucose in 1-ethyl-3-methylimidazolium chloride ([Emim]-
Cl) using chromium(II) chloride as the catalyst.20 1-H-3-Methyl
imidazolium chloride was used as both solvent and catalyst for
the conversion of fructose to HMF with satisfactory yield.21
Furthermore, efficient conversion of fructose and inulin to HMF
was studied in choline chloride (ChoCl)-based ILs.17b Qu et al.
Concern about sustainable has led to increasing interest in the
alternatives of fossil resources for chemicals and energy
supply.1 Recently, dehydration of carbohydrates (e.g. cellulose,
glucose, fructose, sucrose, inulin, and cellobiose) to produce 5-
hydroxymethylfurfural (HMF) has attracted much attention
because it has been recognized as a platform compound for the
production of ne chemicals, polymeric materials, and
biofuels.2
Although excellent yields of HMF can be achieved from the
dehydration of fructose using many methods,3 the production
of HMF from glucose and cellulose would be more attractive
because they are much more abundant. However, the prepara-
tion of HMF from glucose and cellulose is more difficult and
many efforts have been devoted to this topic. Different catalysts
have been developed for the dehydration of glucose to produce
HMF, such as chromium chlorides,4 stannous chloride,5
lanthanide chlorides,6 boric acid,7 germanium(IV) chloride,8
tantalum compounds,9 chromium(0) nanoparticles.10 But for
direct transformation of cellulose into HMF, only a few catalysts
were successful, which included CrCl2/RuCl3,11 CuCl2/CrCl2,12
Brønsted–Lewis–surfactant-combined heteropolyacid,13 and so
on. Regardless of these developments, exploration of efficient
catalytic system for the dehydration of glucose and cellulose is
still very interesting and challenging.
found
that
1-hydroxyethyl-3-methylimidazolium
tetra-
uoroborate could catalyze the dehydration of glucose to HMF
efficiently in DMSO.22 Other ILs, such as triisobutyl(methyl)-
phosphonium tosylate,23 dicationic ILs24 and 1-ethyl-3-methyl-
imidazolium tetrauoroborate ([Emim]BF4),5 could also be used
in the dehydration of carbohydrates to generate HMF. Although
there have been many reports about the application of ILs in
HMF production, it is still highly desirable to develop new and
easily prepared ILs as the efficient reaction media for trans-
formation of carbohydrates into HMF.
Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190, China. E-mail: hanbx@iccas.ac.cn; Fax: +86-
10-62562821; Tel: +86-10-62562821
In this work, we synthesized three new DBU-based ILs. The
main advantages of the DBU-based ILs are facile synthesis from
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commercially available and relatively inexpensive starting
materials, excellent air/water and thermal stability. More
importantly, the tertiary nitrogen existed in the cations affords
Lewis basic property to the ILs, which could improve the
dehydration of carbohydrates to produce HMF through a
hydrogen bond formed in situ between the hydroxy groups of
carbohydrate and the tertiary nitrogen atom of the ILs. Indeed,
we found that the DBU-based ILs were excellent solvents for the
dehydration of glucose and cellulose to produce HMF using
CrCl3$6H2O as the catalyst. In addition, the DBU-based IL/
CrCl3$6H2O catalytic system was also suitable to the conversion
of other sugars such as fructose, sucrose, inulin and cellobiose.
To the best of our knowledge, this is the rst application of
DBU-based ILs to produce HMF from carbohydrates.
Scheme 2 Dehydration of glucose to produce HMF in Bu-DBUCl.
Table 1 Dehydration of glucose catalyzed by various catalystsa
Entry
Catalyst
Solvent
Yieldb (%)
Ref.
1
2
3
4
5
6
7
8
None
LaCl3$6H2O
H3BO3
Bu-DBUCl
Bu-DBUCl
Bu-DBUCl
Bu-DBUCl
Bu-DBUCl
Bu-DBUCl
Bu-DBUCl
Bu-DBUCl
[Emim]BF4
[Bmim]Cl
[Emim]Cl
[Bmim]Cl
[Emim]Cl
0
0
1
ZrOCl2$8H2O
SnCl4$5H2O
Cr(NO3)3$9H2O
CrCl2
CrCl3$6H2O
SnCl4$5H2O
YbCl3 or Yb(OTf)3
H3BO3
6
27
54
63
64
61
24
41
48
70
Results and discussion
Synthesis of the ILs
The routes to synthesize the DBU-based ILs and their structures
are presented in Scheme 1, and the synthetic procedures and
the characterization are discussed in detailed in the Experi-
mental section.
9
5
6
7
8
10
11
12
13
GeCl4
CrCl2
20
a
Reaction conditions of this work: 0.1 g glucose, 10 mg catalyst, 1 g Bu-
b
Catalytic performance of different catalysts on dehydration of
DBUCl, reaction time 3 h, reaction temperature 100 ꢀC. Yields were
glucose in Bu-DBUCl
determined by HPLC.
The activity of various catalysts for the dehydration of glucose to
produce HMF (Scheme 2) was tested at 100 ꢀC in Bu-DBUCl, and
the results are summarized in Table 1. It can be seen from Table
1 that no product was detected without catalyst (entry 1). Some
Lewis acids (entries 4–8) and H3BO3 (entry 3) could catalyze the
dehydration of glucose in the IL. Among the catalysts we tested,
CrCl3$6H2O (entry 8) had the best activity for the reaction and
the yield of HMF could reach 64%, which was comparable to the
results obtained by Zhao et al.20 using catalytic system CrCl2/
[Emim]Cl. In addition, some results of typical catalytic systems
used in the production of HMF from glucose were also given in
Table 1. It can be known that the catalytic system of this work
was very efficient. Based on the results discussed above, we
chose CrCl3$6H2O as the dehydration catalyst to study effect of
reaction parameters on the reaction in Bu-DBUCl.
Effect of catalyst amount
Effect of the amount of CrCl3$6H2O on the dehydration of
glucose to produce HMF was examined in Bu-DBUCl at 100 C
ꢀ
with a reaction time of 3 h, and the results are given in Fig. 1. It
can be known from the gure that the yield of HMF increased to
a maximum of 64% with an increase in the amount of
CrCl3$6H2O from 0 to 10 mg and then slowly decreased to 55%
with further increase in the amount of CrCl3$6H2O from 10 to
30 mg. These results indicated that lower catalyst amount
resulted in a slow reaction rate and excess catalyst could cause
the side reactions such as polymerization and rehydration of
Fig. 1 Effect of catalyst amount on the reaction. Reaction conditions: 0.1 g
Scheme 1 Synthesis of the DBU-based ILs.
20086 | RSC Adv., 2013, 3, 20085–20090
glucose, 1 g Bu-DBUCl, reaction time 3 h, reaction temperature 100 ꢀC.
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Fig. 4 Influence of substrate/solvent weight ratio on the reaction. Reaction
conditions: reaction temperature 100 ꢀC, reaction time 3 h.
Fig. 2 Effect of reaction time on the yield of HMF. Reaction conditions: 0.1 g
glucose, 1 g Bu-DBUCl, 10 mg CrCl3$6H2O, reaction temperature 100 ꢀC.
The competition of the two opposite factors resulted in the
ꢀ
highest yield at 100 C.
the products. Therefore, 10 mg of the catalyst would be an
appropriate amount at the reaction conditions.
Inuence of substrate/solvent weight ratio
Effect of reaction time
The substrate/solvent weight ratio was also important for the
overall efficiency of the reaction system (Fig. 4). The weight ratio of
glucose/CrCl3$6H2O was xed at 10 : 1. It can be seen from Fig. 4
that the HMF yield decreased gradually from 69% to 48% when
the glucose/Bu-DBUCl weight ratio was increased from 0.05 : 1 to
0.3 : 1. This may result mainly from the increasing concentration
of CrCl3$6H2O in Bu-DBUCl, which could increase the side reac-
tions such as polymerization and rehydration of products.
The dependence of HMF yield on reaction time is presented in
Fig. 2. The reaction was performed in the presence of 10 mg
CrCl3$6H2O at 100 ꢀC in Bu-DBUCl. The yield of HMF increased
sharply at beginning, and slowly from 50 min to 180 min, and a
yield of 64% could be achieved at 180 min. No increase in the
yield of HMF was observed with further prolonged reaction time.
Therefore, the reaction time of 3 h was suitable for this system.
The reusability of the catalyst system CrCl3$6H2O/Bu-DBUCl
Effect of reaction temperature
The reusability of the CrCl3$6H2O/Bu-DBUCl catalytic system
was studied in this work. Aer the reaction, the product
was extracted by diethyl ether, and the catalytic system was used
directly for the next run aer drying, as described in
the Experimental section. The results were shown in Fig. 5. It
can be known from the gure that the decrease of the
activity of the catalyst system was not considerable aer reused
ve times.
Fig. 3 shows the effect of reaction temperature on the dehy-
dration of glucose to produce HMF in Bu-DBUCl catalyzed by
CrCl3$6H2O in the temperature range of 80–120 ꢀC with a
reaction time of 3 h. The maximum yield occurred at 100 ꢀC at
the reaction conditions. In the reaction system, reaction
temperature affected the reaction in two opposite ways. Firstly,
the dehydration of glucose was accelerated by increasing
temperature, which was favorable to obtaining high HMF yield.
Secondly, the side reactions, such as polymerization and rehy-
dration of HMF, were also accelerated by rising temperature.
Fig. 5 Reusability of CrCl3$6H2O/Bu-DBUCl catalytic system. Reaction condition:
0.1 g glucose, 1 g Bu-DBUCl, 10 mg CrCl3$6H2O, reaction temperature 100 ꢀC,
reaction time 3 h.
Fig. 3 Effect of reaction temperature on the yield of HMF. Reaction conditions:
0.1 g glucose, 1 g Bu-DBUCl, 10 mg CrCl3$6H2O, reaction time 3 h.
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Table 2 Synthesis of HMF from cellulose catalyzed by CrCl3$6H2O in Bu-DBUCla
Table 3 Effect of DBU-based ILs on HMF yield from different substratesa
Yieldb (%)
Amount of
Amount of
Time Temperature Yieldb
Entry catalyst (mg) cellulose (mg) (h)
(ꢀC)
(%)
Temperature Time
HEOE-
Bu-DBUCl Oc-DBUCl DBUCl
Entry Substrate (ꢀC)
(h)
1
2
3
4
5
6
7
8
10
10
10
10
10
10
10
10
5
100
100
100
100
100
100
100
100
100
100
50
1
2
3
4
1
2
3
4
2
2
2
2
120
120
120
120
130
130
130
130
130
130
130
130
27
34
41
40
39
41
41
40
40
40
37
38
1
2
3
4
5
6
Fructose
Glucose
Sucrose
Inulin
Cellobiose 100
Cellulose 130
100
100
100
100
1.5
3
2
2
3
69
64
63
46
39
41
68
63
60
42
37
42
67
62
61
42
45
24
2
9
a
Reaction conditions: ionic liquids 1 g, substrate 0.1 g, CrCl3$6H2O
10 mg. b Yields were determined by HPLC.
10
11
12
15
5
15
150
a
Reaction conditions: 1 g Bu-DBUCl. b Yields were determined by HPLC.
maximum HMF yield were 1.5 h, 2 h, 2 h, and 3 h, respectively for
fructose, sucrose, inulin, and cellobiose. It can also be seen that
the catalytic system was very effective for transformation of
fructose, and the yield could reach 65% in 15 min.
Synthesis of HMF from cellulose in Bu-DBUCl
It is well known that glucose can be obtained by hydrolysis of
cellulose. In this work, the direct conversion of cellulose into
HMF was also studied in the CrCl3$6H2O/Bu-DBUCl catalytic
system, and the inuence of reaction time and reaction
temperatures on the yield of HMF from cellulose were examined
(entries 1–8, Table 2). The best yield was obtained in 3 h at 120
ꢀC (entry 3) or in 2 h at 130 ꢀC (entry 6), which were both 41%. At
the higher temperature the maximum yield could be obtained
in a shorter time. In addition, the amount of cellulose was also
studied (entries 6, 11 and 12) and 10 wt% cellulose based on the
IL showed the highest HMF yield.
Effect of the DBU-based ILs
The conversion of various carbohydrates in the other two ILs
synthesized in this work was also studied using CrCl3$6H2O as
the catalyst, and the results are listed in Table 3. In general, the
yields of HMF from the carbohydrates were similar. The lower
yield of HMF from cellulose in HEOE-DBUCl could be caused by
the functional group in the side chain on DBU.
Conclusions
Three DBU-based ILs, Bu-DBUCl, Oc-DBUCl, and HEOE-DBUCl
have been synthesized. The ILs can be used as efficient solvent
for the dehydration of different carbohydrates to produce HMF
in the presence of CrCl3$6H2O catalyst. In the catalytic system
CrCl3$6H2O/Bu-DBUCl, a HMF yield of 64% can be achieved
from the dehydration of glucose at 100 ꢀC with a reaction time
of 3 h. The catalytic system is easily recovered and can be reused
ve times without considerable reduction of the reaction
activity and selectivity. In addition, the catalytic system can also
catalyze the dehydration of cellulose to produce HMF efficiently
and a yield of 41% was achieved at 130 ꢀC with a reaction time of
2 h. We believe that the simple, efficient, and easily recyclable
ILs has great potential of application for producing HMF from
the dehydration of carbohydrates.
Dehydration of other carbohydrates
In the light of the good dehydration results obtained with glucose
and cellulose, the CrCl3$6H2O/Bu-DBUCl catalytic system was
also used to catalyze the dehydration of other carbohydrates, and
the results are illustrated in Fig. 6. As expected, fructose and
sucrose gave higher HMF yield of 69% and 63%, respectively.
Inulin and cellobiose only gave moderate yields, which were 46%
and 39%, respectively. Reaction times for obtaining the
Experimental section
Materials
Fructose, sucrose, inulin, glucose, cellobiose, cellulose, 1-
chlorobutane, 1-chlorooctane, 2-(2-chloroethoxy)ethanol, 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU), CrCl3$6H2O and CrCl2
were purchased from Alfa Aesar. LaCl3$6H2O, H3BO3,
Cr(NO3)3$9H2O, ZrOCl2$8H2O, SnCl4$5H2O, acetonitrile, ethyl
acetate, and diethyl ether were all analytical grade and were
provided from Beijing Chemical Reagent Company. All chem-
icals were used as received.
Fig.
6
Dehydration of different carbohydrates. Reaction conditions: 0.1 g
substrate, 1 g Bu-DBUCl, 10 mg CrCl3$6H2O, reaction temperature 100 ꢀC.
20088 | RSC Adv., 2013, 3, 20085–20090
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Synthesis of the ILs
Analysis methods for product
To synthesize a IL, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU, The amount of HMF was analyzed by HPLC with Shimadzu
0.1 mol) and chloroalkane (0.12 mol) were added to acetoni- LC-15C pump, Shimadzu UV-Vis SPD-15C detector at 282.0 nm,
ꢀ
`
trile (50 mL) in a round-bottomed ask of 250 mL equipped and a Supelcosil LC-18 5ım column at 30 C. Before analyzed,
with a magnetic stirrer and a reux condenser. The reaction the reaction mixture was diluted to 1000 mL. Methanol/water
mixture was stirred at 75 ꢀC under N2 for 24 h. Aer the solution (50/50 v/v) was used as the mobile phase at ow rate of
reaction, the volatile substances were removed from the 0.8 mL minꢂ1
resulting solution under reduced pressure. For the IL formed
from DBU with 1-chlorobutane (Bu-DBUCl) or 1-chlorooctane
.
Acknowledgements
(Oc-DBUCl), the remaining oil was re-dissolved in acetonitrile
(25 mL) and recrystallized by adding 500 mL of diethyl ether.
The IL was ltered, washed thoroughly with diethyl ether, and
The authors thank the National Natural Science Foundation of
China. This work was supported by National Natural Science
Foundation of China (21003133, 21133009, 21021003), and
Chinese Academy of Sciences (KJCX2.YW.H30).
ꢀ
dried in vacuum at 40 C for 24 h. For the IL formed by DBU
and 2-(2-chloroethoxy)ethanol (HEOE-DBUCl), the remaining
oil was washed by diethyl ether (100 mL ꢁ 10) to removed
unreacted DBU and 2-(2-chloroethoxy)ethanol. Then, the oil
Notes and references
ꢀ
was heated at 60 C under vacuum to removed diethyl ether to
´
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´
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9.6 Hz, 2H), 2.01–2.07 (m, 2H), 1.68–1.75 (m, 6H), 1.58–1.64 (m,
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9.6 Hz, 2H), 2.01–2.07 (m, 2H), 1.68–1.75 (m, 6H), 1.61–1.64 (m,
2H), 1.27–1.31 (m, 10H), 0.85 (t, J ¼ 6.4 Hz, 3H). Elemental
analysis calcd (%) for Oc-DBUCl (300.92): C 67.85, H 11.06, N
9.31; found: C 66.43, H 10.90, N 9.49%.
HEOE-DBUCl. Light yellow oil (purity >99%); 1H NMR (D2O,
400 MHz) d (ppm) 3.71–3.78 (m, 6H), 3.62–3.64 (m, 2H), 3.51–
3.57 (m, 6H), 2.87 (d, J ¼ 10 Hz, 2H), 2.05–2.10 (m, 2H), 1.70–
1.73 (m, 6H). Elemental analysis calcd (%) for HEOE-DBUCl
(276.81): C 56.41, H 10.29, N 10.12; found: C 56.23, H 10.35, N
9.96%.
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Only the procedures to convert glucose into HMF in Bu-DBUCl
catalyzed by CrCl3$6H2O are discussed because those for other
sugars in different ILs were the same except that the materials
used were different. In a typical experiment, desired amounts of
glucose and catalyst were dissolved in Bu-DBUCl in a ask of
10 mL sealed with a glass stopper. The mixture was stirred at
100 ꢀC for a desired time. Then, the mixture was cooled to room
temperature immediately. The samples were analyzed by HPLC
to obtain the yields. Each reaction was repeated at least two
times. In the experiments to test the reusability, 1 mL of water
was added into the reaction system aer the reaction. Then the
mixture was extracted with diethyl ether to remove the HMF
produced. Aer extraction, the water in the CrCl3$6H2O/Bu-
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The CrCl3$6H2O/Bu-DBUCl system was then used directly for
the next run by adding new glucose.
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20090 | RSC Adv., 2013, 3, 20085–20090
This journal is ª The Royal Society of Chemistry 2013