Functional ionic liquids for hydrolysis of lignocellulose

Article abstract of DOI:10.1016/j.carbpol.2013.04.061

An efficient system for hydrolysis of lignocellulosic materials to prepare reducing sugar in a series of functional acidic ionic liquids with low synthetic cost and excellent dissolved and catalytic activity was established. High yield of reducing sugar was obtained with the use of 1-H-3-methylimidazolium chloride ([HMIM]Cl). The use of ionic liquid under ultrasound irradiation greatly improved the yield of total reducing sugar. The optimum reaction conditions were as follows: ratio of water/sample was 5 (w/w), ratio of IL/sample was 25 (w/w), 70 °C, 120 min and the yield of reducing sugar was up to 53.27 mg from 0.2 g of soybean straw and 50.03 mg from 0.2 g of corn straw.

Full text of DOI:10.1016/j.carbpol.2013.04.061

Accepted Manuscript
Title: Functional ionic liquids for hydrolysis of lignocellulose
Author: Xiaomei Hu Yibo Xiao Kun Niu Yang Zhao Bixian
Zhang Baozhong Hu<ce:footnote id="fn0005"><ce:note-para
id="npar0005">Xiaomei HU and Yibo XIAO contributed
equally to this work.</ce:note-para></ce:footnote>
PII:
S0144-8617(13)00425-6
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.carbpol.2013.04.061
CARP 7679
To appear in:
Received date:
Revised date:
Accepted date:
9-1-2013
16-4-2013
17-4-2013
Please cite this article as: HU, X., XIAO, Y., NIU, K., ZHAO, Y., ZHANG, B., & HU,
B., Functional ionic liquids for hydrolysis of lignocellulose, Carbohydrate Polymers
(2013), http://dx.doi.org/10.1016/j.carbpol.2013.04.061
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Functional ionic liquids for hydrolysis of lignocellulose
Xiaomei HU a*, Yibo XIAO a, Kun NIU a, Yang ZHAO a, Bixian ZHANG b, Baozhong HU a
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a School of Life and Science, Northeast Agricultural University, Harbin 150030, China; b Heilongjiang Academy of
Agricultural Sciences, Harbin 150086, China
Xiaomei HU and Yibo XIAO contributed equally to this work.
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∗ Corresponding author, E-mail address: huxiaomei1982@163.com (X. Hu).
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Highlights
An efficient system for hydrolysis of lignocellulosic materials was established.
A series of ionic liquids were prepared with a simple protocol and low cost.
[HMIM]Cl exist excellent dissolved and catalytic activity to prepare reducing sugar.
Ultrasound assisted [HMIM]Cl was proved as the most efficient system.
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Abstract
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An efficient system for hydrolysis of lignocellulosic materials to prepare reducing sugar in a series of
functional acidic ionic liquids with low synthetic cost and excellent dissolved and catalytic activity was
established. High yield of reducing sugar was obtained with the use of 1-H-3-methylimidazolium
chloride ([HMIM]Cl). The use of ionic liquid under ultrasound irradiation greatly improved the yield of
total reducing sugar. The optimum reaction conditions were as follows: ratio of water/sample was 5
(wt/wt), ratio of IL/sample was 25 (wt/wt), 70 °C, 120 min and the yield of reducing sugar was up to
53.27 mg from 0.2 g of soybean straw and 50.03 mg from 0.2 g of corn straw.
Keywords: ionic liquid, lignocellulose, reducing sugar, ultrasound
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1. Introduction
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The exhaustion of fossil fuel and the resulting severe environmental consequences from its overuse
have necessitated the search for alternative energy sources (Insu et al., 2010). Lignocellulosic materials
are the major constituent of biomass and the most abundant renewable resource. Extensive studies have
been made to convert lignocellulosic materials such as bagasse, corn stover, wheat straw, wood chips
into valuable products (Ha et al., 2011; Ma et al., 2012; Lever & Ho, 2012). The conversion of
lignocellulosic materials into ethanol is increasingly being promoted as a potentially environmentally
and economically sustainable fuel (Lever & Ho, 2012). During the process of lignocellulose-to-biofuel,
the preparation of reducing sugar is a crucial step. However, because of the compositional features of
lignocellulose and the highly crystalline polymer of d-anhydroglucopyranose units, it is hardly
processed in common solvents and pretreatment of high temperature or high pressure is required. Low
rate and high cost of cellulose enzymes and a large amount of corrosion of inorganic acid are also
limits their application for industrial production. It is necessary to develop alternative new techniques.
Ionic liquids (ILs) are environmental friendly molten salts. ILs have the virtues of low melting
points, low volatility, excellent solvency and designability as their cations or anions can be introduced
into functional groups. The application of ILs in cellulose occurred in 2002. Swatloski et al. (2002)
reported the dissolution of cellulose in ILs with the cation of 1-butyl-3-methylimidazolium ([BMIM])
and the anion of Cl-, Br- or SCN- at 100 °C. Following that, ILs with the cation of
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1-alkyl-3-methylimidazolium and with the anion of chloride, acetate, or formate appeared effective in
dissolving cellulose (Luo et al., 2005; Fukaya et al., 2006; Kosan et al., 2008). Currently, Li et al. (2008)
reported the hydrolysis of ligocellulosic materials in [BMIM]Cl to prepare reducing sugar with sulfuric
acid as a catalyst. Amarasekara and Owereh (2009) reported the hydrolysis of Sigmacell cellulose in
1-(1-propylsulfonic)-3-methylimidazolium chloride. The highest yield of total reducing sugar (62%)
was obtained in 1 h of preheating at 70 °C and 30 min of heating at 70 °C after adding water. Xiao et al.
(2012) described enzymatic hydrolysis of cellulose with pretreatment by [BMIM]Cl, [BMIM]COOH or
1-ethyl-3-methylimidazolium acetate. Ma et al. (2012) reported the combined use of ultrasound
irradiation and [BMIM]Cl as a potential alternative method in preparation of glutamates using cellulose
isolated from sugarcane bagasse.
In many reports, pure cellulose was used. There are very few reports concerning to original
lignocellulosic materials. With the aim to produce the ionic liquids with a simply synthetic protocol,
low cost, good dissolved and catalytic activity for the preparation of reducing sugar from agricultural
lignocellulosic waste, a series of studies were taken. The optimal conditions were also investigated.
2. Materials and methods
2.1. Materials
All chemicals were purchased from Aladin Company (China) and used as received. The corn straw
and soybean straw were supplied by Northeast Agriculture University (China), and both of them were
pulverized by high-speed universal pulverizer and through 40 mesh sieves. KQ-200VDE ultrasound
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was used. Analysis of reducing sugar was performed on Xinshiji T6 UV-Vis Spectrophotometer. H
NMR Spectra was recorded on a Bruker Advance DRX-400 spectrometer at 400 MHz.
2.2. General procedure for the synthesis of ionic liquids
2.1.1. Synthesis of 1-H-3-methylimidazolium chloride ([HMIM]Cl)
1-methyl imidazole (20 g, 0.244 mol) and hydrochloric acid (30 g, 0.296 mol) were mixed in a
round-bottomed flask in ice-water bath and stirred for 24 h at room temperature. The resulting liquid
was washed with ether (20 ml × 3). Ether was then removed under rotary evaporation and the product
was further dried under vacuum for 10 h to give a colorless liquid. The synthesis of ionic liquids
[HMIM][HSO4], [HMIM][CH3COO] and [HMIM][H2PO4] followed a similar protocol as used for
[HMIM]Cl.
[HMIM]Cl: 1H-NMR (400 MHz, DMSO-d6): 3.775 (s, 3H), 7.298 (s, 1H), 7.445 (s, 1H), 8.441(s, 1H)
[HMIM][CH3COO]: 1H-NMR (400 MHz, DMSO-d6): 1.921 (s, 3H), 3.645 (s, 3H), 6.904 (s, 1H), 7.117
(s, 1H), 7.607 (s, 1H)
[HMIM][H2PO4]: 1H-NMR (400 MHz, DMSO-d6): 3.702 (s, 3H), 7.050 (s, 1H), 7.241 (s, 1H), 7.902 (s,
1H), 8.892(s, 3H)
[HMIM][HSO4]:1H-NMR (400 MHz, DMSO-d6): 3.874 (s, 3H), 7.607 (s, 1H), 7.675 (s, 1H), 8.990 (s,
1H), 9.123(s, 2H)
2.1.2. Synthesis of 1-allyl-3-methylimidazolium chloride ([AMIM]Cl)
[Amim]Cl was prepared according to a method described in a previous literature (Zhang et al.,
2005). 1-Methylimidazole (22.908 g, 0.275 mol) and ally chloride (25.245 g, 0.33 mol) were added to a
round-bottomed flask fitted with a reflux condenser for 7 h at 60 °C with stirring. The resulting liquid
was washed with ether (20 ml × 3). Ether was removed under rotary evaporation and the product was
further dried under vacuum at 80 °C for 24 h. [AMIM]Cl was obtained as a slightly viscous liquid.
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[AMIM]Cl H-NMR (400 MHz, DMSO-d6): 3.901 (s, 3H), 4.911 (d, 2H), 5.330 (m, 2H), 6.045 (m,
1H), 7.822 (d, 2H), 9.464 (s, 1H)
2.3. General procedure for preparation of reducing sugar
Soybean straw or corn straw (0.2 g), an appropriate amount of ionic liquid and water were rapidly
added into a vial. The mixture was treated with ultrasound (200 W) or heated with stirring under oil
bath at a certain temperature for a matter of minutes. The entire solids of the mixture were filtered out.
The filtrate was cooled to room temperature and neutralized by the addition of solid of sodium
hydroxide. The amount of reducing sugar was measured using the 3,4-dinitrosalicylic acid (DNS)
method (Miller, 1959). The concentration of reducing sugar was calculated by employing a standard
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curve prepared using glucose and the absorbance was measured at 540 nm using UV-vis
spectrophotometer.
3. Results and Discussion
3.1. Yield of reducing sugar with different treatments
In recent years, ultrasonic treatment is used widely. The effect of ultrasound is related to cavitation,
heating, dynamic agitation, shear stresses, and turbulence (Vilkhu et al., 2008; Jambrak et al., 2009).
Liu et al. (2008) reported that the controlled depolymerization of plant cellulose could be achieved by
employing suitable ultrasonication. In this work, different treatments were investigated including
ultrasound and oil bath or their combination. As presented in Fig.1, the yield of reducing sugar (RS)
was significantly higher when the mixture was treated with ultrasound than with oil bath. The yield of
RS was slightly lower with the use of ultrasound and oil bath than with ultrasound. The ultrasonic
cavitation might help to increase the reaction interface between cellulose and ionic liquids, enhance the
heat transfer and cause a reduction in molecular weight of cellulose. Ultrasound irritation might
decrease the IL’ viscosity, enhance the movement rate of chloride ions and then accelerate the
dissolution of soybean straw cellulose. This work indicated that the combined use of ionic liquid and
ultrasound irritation could promote cellulose (net) to the cellulose (chain) process and increase the
dissolution of cellulose and then improve the yield of RS. This can be explained from FTIR spectrum
either.
FTIR spectrum of the soybean straw is shown in Fig.2. In curve a, the peak at 1429 cm−1 can be
assigned to bending vibration of CH2. This band is strong in crystalline cellulose, and weak in
amorphous cellulose (Nelson & O’Connor, 1964). The CH2 band is weaker in curve b and weakest in
curve c, thus the crystallinity of the cellulose is lower when treated with ultrasound and much lower
when treated with IL/ultrasound than the original cellulose. In addition, the broad absorption in the
range from 3000 cm−1 to 3500 cm−1 is due to stretching vibration of inter molecular O–H forming
among cellulose molecules. The broad absorption is weaker in curve b and c compared to curve a,
which probably indicates the intermolecular hydrogen bonds of cellulose are broken greatly (Xiao et al.,
2012). Therefore, the optimal system should be IL/ultrasound that can help to disrupt the hydrogen
bonds between polysaccharide chains of cellulose, and thus benefit the preparation of reducing sugar.
3.2. Yields of RS under various reaction media
Various reaction media were studied to identify the optimal one for the preparation of RS. Because
[AMIM]Cl was an excellent solvent for the dissolution of cellulose as presented in many reports
(Kilpelainen et al., 2007), [AMIM]Cl was used as a reaction medium first. With the help of ultrasound,
a short yield of RS was produced in [AMIM]Cl as shown in Fig. 3 (No.1). In addition, an ionic liquid
with an acid as a catalyst was described as a good condition for the hydrolysis of lignocellulose (Li et
al., 2007; Li et al., 2008). In this work, [AMIM]Cl with an addition of H2SO4 was used as a reaction
medium and the resulting yield of RS was increased (No.2). Low yield of RS was obtained when the
reaction was carried out in an aqueous H2SO4 solution (No.3).
Theoretically, ILs with the anion of Cl-, CH3COO- have a good dissolved activity and ILs with the
-
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anion of HSO4 , HPO4 can provide acidic activity for cellulose. Then a series of proton ILs with these
anions were synthesized. In their chemical structures, there is an hydrogen in N(1) on the cation that
can be dissociated, so all the ILs can provide slight acidity for the hydrolysis of cellulose. Therefore,
either of ILs synthesized in this work can act as a solvent and a catalyst. No other inorganic acid is
needed. Moreover, high yield of RS was obtained when [HMIM]Cl was used as reaction media (Fig.3
No.4), which was due to the formation of strong hydrogen bond of Cl- compared with [HMIM]HSO4.
The strong hydrogen bond of ILs can disrupt the hydrogen bonds between polysaccharide chains of
cellulose and then dissolve cellulose (Mikkola et al., 2007). Therefore, [HMIM]Cl/ultrasound were
applied for the following reactions and the optimal conditions were studied.
3.3. Effect of reaction temperature
Generally, when the temperature is increased, the reaction rate will be improved. As the
temperature was from 40 °C to 80 °C, the hydrolysis rate of cellulose should be increased and then the
resulting yield of RS was improved as shown in Fig.4. Another reason for the increasing yield of RS
was perhaps due to ILs’ viscosity. As reported by Remsing et al. (2006), cellulose was dissolved into
ILs, because the chloride ions in IL could go into the interspaces of 1,4-linked β-D-glucose units and
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disrupt intermolecular hydrogen bonds between the units. It was supposed that when the temperature
was increased, [HMIM]Cl became less viscous, which would enhance the movement rate of chloride
ions and then accelerate the dissolution of cellulose and improve the yield of RS. As presented in Fig.4,
when the temperature was from 70 °C to 80 °C, the increasing rate of the yield of RS was slow down.
High temperature might cause the degradation of RS and consumed more energy. Therefore, 70 °C was
chosen for the following reactions.
3.4. Effect of the ratio of water/sample
Water is a reactant in the process of hydrolysis of cellulose. As the amount of water is increased, the
hydrolysis reaction will proceed towards the forward reaction. As shown in Fig.5, when the ratio of
water was increased from 0 to 5, the yield of product was increased, because the appropriate water
could improve the hydrolysis rate and lower the viscosity of the IL. However, when the ratio was more
than 5, the yield of product was decreased, probably because the dissolution of cellulose was reduced
when more water was added. The highest yield of reducing sugar was observed with the ratio of
water/sample was 5/1(wt/wt).
3.5. Effect of the ratio of IL/sample
In order to investigate the effect of the amount of ionic liquids, the reaction was carried out at 70 °C
for 120 min. According to Fig.6, as the ratio of [HMIM]Cl/sample was from 15 to 25, the hydrolysis
rate was improved and the yield of reducing sugar was increased. While when the ratio of mass was
more than 25, the yield was decreased. The overuse of IL was also a waste of resources for production.
In order to save the production cost, the appropriate ratio of [HMIM]Cl/sample (wt/wt) should be 25/1.
3.6. Effect of reaction time
Reaction time has an effect on the industrial production of RS. High yield of RS and short reaction
time are always the optimal conditions. As presented in Fig.7, Longer reaction time would lead to the
low yield of the product. Corn straw hydrolysis proceeded significantly faster than RS degradation, but
it is important to find an optimal time to maximize the yield of RS (Li, et al., 2008). It was supposed
that the hydrolysis of cellulose and degradation of monosaccarides into other derivates coexist. In order
to improve the efficiency of production and save the energy, the appropriate reaction time should be
120 min.
4. Conclusion
In conclusion, an efficient and economical approach to prepare reducing sugar in ionic liquids
under mild conditions with low consumption of energy for the cellulose-ethanol production from
agricultural waste was established. Ultrasound assisted [HMIM]Cl was proved as the most efficient
system. The optimum reaction conditions were as follows: ratio of water/sample was 5 (wt/wt), ratio of
IL/sample was 25 (wt/wt), reaction temperature was 70 °C with ultrasound for 120 min and the yield of
RS was up to 53.27 mg and 50.03 mg from 0.2 g of soybean straw and corn straw respectively. In this
process, soybean straw and corn straw are “green materials”, ionic liquids are “green solvents”.
[HMIM]Cl has the lowest synthetic cost compared with ILs that have been successfully reported. This
simple and environmentally acceptable method has a great potential for the preparation of bioethanol
for industrial production.
Acknowledgements
This work was financially supported by the Scientific Research Foundation for the Returned Overseas
Chinese Scholars of Heilongjiang Provincial Education Bureau (1251H01), Heilongjiang postdoctoral
foundation (LBH-Z10240) and National Science Foundation for Fostering Talents in Basic Research of
the National Natural Science Foundation of China (J1210069).
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Figure Captions
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Fig.1. Yields of reducing sugar with different treatments. Reaction conditions: 1. treatment with
ultrasound for 180 min; 2. treatment with oil bath for 180 min; 3. treatment with ultrasound for 60 min,
and then oil bath for 120 min. Other conditions were fixed as follows: soybean straw (0.2 g),
[HMIM]Cl (5 g), H2O/soybean straw 15/1 (wt/wt), reaction temperature (70 °C).
Fig.2. FT-IR spectra of (a) untreated soybean straw; (b) soybean straw treated with ultrasound for 120
min; (c) soybean straw treated with [HMIM]Cl/ultrasound for 120 min. After treated with (b) or (c), the
sample was washed with water (5 ml × 2) and then dried for 24 h.
Fig.3. Yields of reducing sugar under various reaction systems. Reaction conditions: 1. [AMIM]Cl (5 g)
and water (2 ml); 2. [AMIM]Cl (5 g) and H2SO4 (1 M, 2 ml); 3. H2SO4 (1 M, 7 ml); 4. [HMIM]Cl (5.00
g) and water (2 ml); 5. [HMIM][HSO4] (5 g) and water (2 ml); 6. [HMIM][CH3COO] (5 g) and water
(2 ml); 7.[HMIM][HPO4] (5 g) and water (2 ml). Other conditions were fixed as follows: soybean
straw (0.2 g), H2O/soybean straw 15/1 (wt/wt), reaction temperature (70 °C), ultrasound for 120 min.
Fig.4. Yields of reducing sugar under various reaction temperatures. The conditions were fixed as
follows: straw (0.2 g), [HMIM]Cl (5 g) and water (3 ml), ultrasound for 120 min.
Fig.5. Yields of reducing sugar under different ratio of water/sample (wt/wt). The conditions were fixed
as follows: straw (0.2 g), [HMIM]Cl (5 g), reaction temperature (70 °C), ultrasound for 120 min.
Fig.6 Yields of reducing sugar under different ratio of [HMIM]Cl/sample (wt/wt).The conditions were
fixed as follows: straw (0.2 g), water (1 ml), reaction temperature (70 °C) , ultrasound for 120 min.
Fig.7. Yields of reducing sugar under various reaction times. The conditions were fixed as follows:
straw (0.2 g), [HMIM]Cl (5 g) and water (1 ml), reaction temperature (70 °C).
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Fig.1. Yields of total reducing sugar with different treatments. Reaction conditions: 1. treatment with ultrasound
for 180 min; 2.treatment with oil bath for 180 min; 3. treatment with ultrasound for 60 min, and then oil bath for
120 min. Other conditions were fixed as follows: soybean straw (0.2 g), [HMIM]Cl (5 g), H2O/soybean straw 15/1
(wt/wt), reaction temperature (70 °C).
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357
358
359
360
361
362
363
3500
3000
2500
2000
1500
1000
500
Wavenumber cm-1
364
365
366
367
Fig.2. FT-IR spectra of (a) untreated soybean straw; (b) soybean straw treated with ultrasound for120 min; (c)
soybean straw treated with [HMIM]Cl /ultrasound for 120 min. After treated with (b) or (c), the sample was
washed with water (5 ml ×2) and then dried for 24 h.
368
369
370
371
372
373
374
375
376
377
378
379
380
381
Page 8 of 13

382
383
384
385
386
387
388
389
390
391
392
393
394
395
50
40
30
20
10
0
1
2
3
4
5
6
7
396
397
398
399
400
401
Fig.3. Yields of total reducing sugar with various reaction systems. Reaction conditions: 1. [AMIM]Cl (5 g) and
water (2 ml); 2. [AMIM]Cl (5 g) and H2SO4 (1 M, 2 ml); 3. H2SO4 (1 M, 7 ml); 4. [HMIM]Cl (5 g) and water (2
ml); 5. [HMIM][HSO4] (5 g) and water (2 ml); 6. [HMIM][CH3COO] (5 g) and water (2 ml); 7. [HMIM][HPO4]
(5 g) and water (2 ml). Other conditions were fixed as follows: soybean straw (0.2 g), H2O/soybean straw 15/1
(wt/wt), reaction temperature (70 °C), ultrasound for 120 min.
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
Page 9 of 13

422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
60
50
40
30
20
10
0
soybean straw
corn straw
40
50
60
T/
70
80
℃
437
438
439
Fig.4. Yields of reducing sugar with various reaction temperatures. The conditions were fixed as follows: straw
(0.2 g), [HMIM]Cl (5 g) and water (3 ml), ultrasound for 120 min.
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
Page 10 of 13

459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
60
55
50
45
40
35
30
soybean straw
corn straw
0
5
10
15
20
water/sample(wt/wt)
475
476
477
Fig.5. Yields of reducing sugar under different ratio of water/sample (wt/wt). The conditions were fixed as follows:
straw (0.2 g), [HMIM]Cl (5 g), reaction temperature (70 °C), ultrasound for 120 min.
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
Page 11 of 13

495
496
497
498
499
500
501
502
503
504
505
506
507
soybean straw
corn straw
60
55
50
45
40
35
30
10
20
30
40
50
IL/sample(wt/wt)
508
509
510
Fig.6. Yields of reducing sugar under different ratio of [HMIM]Cl/sample (wt/wt).The conditions were fixed as
follows: straw (0.2 g), water (1 ml), reaction temperature (70 °C) , ultrasound for 120 min.
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
Page 12 of 13

530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
60
55
50
45
40
35
30
soybean straw
corn straw
100
150
200
250
Time/min
546
547
548
Fig.7. Yields of reducing sugar under various reaction times. The conditions were fixed as follows: straw (0.2 g),
[HMIM]Cl (5 g) and water (1 ml), reaction temperature (70 °C).
Page 13 of 13