High glucose yields from the hydrolysis of cellulose dissolved in ionic
liquids
Silvia Morales-delaRosa, Jose M. Campos-Martin* and Jose L. G. Fierro*
Sustainable Energy and Chemistry Group, Instituto de Catálisis y Petroleoquímica, CSIC, Marie
Curie, 2 Cantoblanco, 28049 Madrid, Spain, http://www.icp.csic.es/eqs/
Fax: (+) 34 915854760
E-mail: j.m.campos@icp.csic.es (JMCM), jlgfierro@icp.csic.es (JLGF)
ABSTRACT
Here we show a method to hydrolyze cellulose selectively to glucose and cellobiose, sugars
that can be easily fermented. Selecting an ideal ionic liquid ([EMIM]Cl), optimizing the
dissolution time/temperature (15 min., 408 K) and the reaction procedure (addition of water
during the reaction time) led to a very high selectivity to glucose + cellobiose (99.6 %).
Keywords: cellulose, hydrolysis, ionic liquid, glucose, acid.
INTRODUCTION
Among the renewable energies, biomass is an abundant and carbon-neutral energy resource
for the production of biofuels and valuable chemicals [1-3]. Shifting world market dependence
away from fossil-based energy resources to renewable alternatives, such as biomass, can be
regarded as an important contribution towards favorable climate conditions and a sustainable
economy [4].
One of the most important bottlenecks of commercializing lignocellulosic bioethanol is the
discovery of a cost-effective hydrolysis of cellulose [5, 6]. The -glycosidic linkages of the sugar
1
molecules contained in cellulose or lignocellulose are strongly protected by the tight packing of
cellulose chains in microfibers, making hydrolysis challenging. Accordingly, the hydrolysis of
cellulose requires harsh conditions such as the use of dilute sulfuric acid at high temperatures.
As cellulose is dissolved into ionic liquids (ILs) [7-10], the protons in the solution can more easily
access the -glycosidic bonds to hydrolyze the bond. It appears that the "physical" barrier can
be overcome through the formation of a cellulose solution that facilitates the acid-catalyzed
hydrolysis under mild reaction conditions and a lower catalyst loading.
Chemical deconstruction of cellulose to produce glucose in ionic liquids has resulted in only
moderate yields,[11-13] which contrasts with the high yields obtained from cellulose in
concentrated acids and other cellulose solvents.[6] This low yield is due to the small amount of
water added in the reaction in the presence of IL; a large amount of water leads to the
precipitation of unreacted cellulose. However, a large excess of water is essential to reach
higher sugar yields dissolved in ionic liquids because it favors the hydrolysis of cellulose and
retards the dehydration of glucose. This effect is described by the “Le Chatelier's Principle”. By
adding water successively during the reaction, this issue was elegantly overcome.[14]
We report a simple method to selectively hydrolyze cellulose to glucose and cellobiose that
reaches the highest yields of glucose and cellobiose can be obtained when the optimal
conditions of dissolution and hydrolysis of cellulose in ionic liquid are selected.
Experimental Section
Materials: Fibrous and microgranular cellulose was purchased from Sigma-Aldrich. Ionic liquids
manufactured by BASF (Basionics) were purchased from Sigma-Aldrich and used as received.
Cellulose (467.5 mg) was dissolved in 9.50 g of hot ionic liquid, in a temperature range between
373 and 413 K, in a Mettler-Toledo Easy Max® equipped with mechanical stirring. After the
complete dissolution of the solid, the temperature was fixed to 378 K, and 1.00 mmol of HCl
was added to the mixture using an aqueous acid solution (1.66M). This mixture remained
stirring for 180 min. Deionized water aliquots were added at the following reaction times: 10
2
min., 2000 µL; 20 min., 1000 µL; 30 min., 1500 µL; and 60 min., 2500 µL. Finally, the reaction
was stopped by adding 17.52 mL of distilled water. After the addition of water, the unreacted
cellulose precipitated and was separated by centrifugation. The isolated unreacted cellulose
was thoroughly washed with water and recovered by centrifugation. Cellulose samples were
dried at 363 K overnight. The quantity of cellulose isolated was determined by weighing.
The liquid was analyzed by HPLC using an Agilent Technologies HPLC 1200 series. The
chromatographic separations were carried out using an AMINEX HPX-87H column at 338 K,
using 0.6 mL min-1 of sulfuric acid aqueous solution (0.01 mmol L-1) as the mobile phase. The
sugars (glucose and cellobiose) and the dehydration products (5-hydroxy-methylfurfural) were
analyzed using a refractive index detector. Identification of the components was performed by
comparison of retention times with authentic samples. The quantities of the products were
quantified using internal calibration curves.
RESULTS AND DISCUSSION
We dissolved cellulose in 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) and employed
different acids to study the effect of acid strength on the hydrolysis of cellulose to sugars. The
pKa of the acids tested ranged from -14.9 to 4.8 (trifluoromethanesulfonic acid to acetic acid)
(Table 1).
The strength of the acid had a marked effect on the reactivity; acids with a negative pK a
yielded complete cellulose conversion, whereas those with a positive pK a had a very low
conversion level (Figure 1). Yields for cellobiose and glucose (C+G) increased when the pK a
decreased; however, for pKa values lower than -2.5, the yield remained constant at
approximately 95 % (Figure 1), this effect was similar for 5-(Hydroxymethyl)furfural (HMF)
formation (yield around 0.5 %).
The highest conversion to glucose was obtained using HCl as the catalyst. This result was
similar to what was previously observed for cellobiose hydrolysis.[15]
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Dissolution of cellulose is a key factor in obtaining high yields of glucose during hydrolysis.
We performed the dissolution of cellulose at different temperatures while maintaining a constant
dissolution time of 60 min. The temperature was lowered to the reaction temperature of 378 K,
and HCl was added as the acid catalyst. In all cases, the cellulose conversion was essentially
complete (Table 3). However, some differences in the selectivity to C+G were observed. When
the dissolution temperature increased, the selectivity to C+G decreased (Table 3). These data
indicated that an increase in the dissolution temperature from 378 K to 398 K increased the
production of deeply dehydrated (humins).
It was observed that [BMIM]Cl quickly dissolved cellulose as the temperature increased. For
this reason, we studied the hydrolysis reaction of cellulose dissolved in [BMIM]Cl at decreasing
temperatures as soon as the cellulose was completely dissolved. The optimized dissolution
times are compiled in Table 4. Reaction results showed that the selectivity to C+G remained
constant regardless of the temperature used in the dissolution of cellulose if the dissolution time
was optimized. In all cases, the selectivity to sugars (C+G) was very high (above 95%).
To study the effect of the type of cellulose used, we employed the following two types:
microgranular and fibrous. Microgranular cellulose required longer dissolution times
(approximately a factor of 2) in the ionic liquid than the fibrous cellulose. This phenomenon is
well-known because it is more difficult to dissolve micro-granular than fiber samples. These
dissolution times were very short in comparison to previous works.[12, 14] In consequence,
optimizing the temperature/time of cellulose dissolution step, the total operation time can be
reduced to ca. 3 h making it more attractive from the industrial point of view. The cellulose
conversion was completed for both samples, and the selectivity to sugars (C+G) were very
similar (Table 5). The concentration of sugars was 11-12 g/mL can be used directly in the
production of ethano by fermentation.
The concentration profile of the primary products in the hydrolysis of cellulose in ionic liquid
([BMIM]Cl) (Figure 2) showed that the concentration of glucose increased with time, but the
4
concentration increase was clearly reduced after 120 min. of reaction time. The concentration of
cellobiose increased during short reaction times, reached a maximum around 60 min. of
reaction, and decreased at long reaction times. This behavior was typical of an intermediate
product. HMF was not formed at lower reaction times (< 30 min.), which is coincident with low
glucose concentration. After the 30 min. period, the concentration increased with time.
Finally, the hydrolysis procedure was tested with other ionic liquids that are good solvents for
lignocellulosic biomass, specifically cellulose,[7, 8] The solvents 1-ethyl-3-methylimidazolium
chloride ([EMIM]Cl) and 1-ethyl-3-methylimidazolium acetate ([EMIM]OAc) are able to dissolve
cellulose at 408 K in 15 min. Reaction results show that the nature of the solvent affects the
reaction performance (Table 6).
The change in cation from BMIMCl to EMIMCl gave rise to a slight increase in the glucose
yield with a reduction in other byproducts (not completely depolymerized or humins). The most
significant effect was observed when changing the anion from a chloride to an acetate. The
hydrolysis rate was reduced dramatically (cellulose conversion was reduced from 100 % to
10%). This result may be due to the ionic exchange reaction: [EMIM]OAc + HCl -> [EMIM]Cl +
AcOH, and the resulting AcOH does not have enough acidity to hydrolyze cellulose.
CONCLUSIONS
We have reported in the present work a useful method to hydrolyze cellulose selectively to
glucose and cellobiose, sugars that can be easily fermented, the sugars concentration is 11-12
g/mL. Furthermore, by using the ionic liquid 1-butyl-3-methylimidazolium chloride ([BMIM]Cl)
cellulose is quickly dissolved (15 min at 408 K). This means that the operation time can be
reduced to ca. 3 h making it attractive from the industrial point of view. Moreover, the hydrolysis
of cellulose has been substantially improved adding water along the course of the reaction.
According to this procedure, very high selectivity to glucose + cellobiose can be obtained
(99.6 %).
5
ACKNOWLEDGEMENTS
We are grateful to our research sponsors Comunidad de Madrid (Spain) (S2009/ENE-1743) and
CSIC (Spain) (201080E008 and 201180E038).
REFERENCES
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ionic liquids and its application: a mini-review, Green Chem., 8 (2006) 325.
6
[9] N. Sun, H. Rodríguez, M. Rahman, R.D. Rogers, Where are ionic liquid strategies most
suited in the pursuit of chemicals and energy from lignocellulosic biomass?, Chem.
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[10] H. Olivier-Bourbigou, L. Magna, D. Morvan, Ionic liquids and catalysis: Recent progress
from knowledge to applications, Appl. Catal., A, 373 (2010) 1-56.
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fermentable sugars from cellulose, ChemSusChem, 3 (2010) 1151-1153.
[12] R. Rinaldi, R. Palkovits, F. Schüth, Depolymerization of Cellulose Using Solid Catalysts in
Ionic Liquids, Angew. Chem. Int. Ed., 47 (2008) 8047-8050.
[13] C. Li, Q. Wang, Z.K. Zhao, Acid in ionic liquid: An efficient system for hydrolysis of
lignocellulose, Green Chem., 10 (2008) 177.
[14] J.B. Binder, R.T. Raines, Fermentable sugars by chemical hydrolysis of biomass, Proc.
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7
Table 1
Acid catalysts employed in the hydrolysis of cellulose dissolved in ionic liquids.
Formula
pKa
TFMS
-14.9
HCl
HCl
-8.0
H2SO4
H2SO4
-6.6
p-TSA
-2.5
TFAA
-0.3
H3PO4
2.1
AA
4.8
H3PO4
8
Table 2 Hydrolysis of cellulose dissolved in [BMIM]Cl catalyzed by several acids at 378 K. The
dissolution time was 60 min., and the reaction time was 180 min.
Acid
% Conv.
Cellulose
% Selectivity. to
Cellobioseª
% Selectivity to
Glucoseª
% Selectivity
to HMFª
TFMS
100.0
9.4
84.6
0.5
HCl
100.0
9.6
89.0
0.6
H2SO4
100.0
11.2
81.5
0.5
p-TSA
100.0
11.1
82.3
0.5
TFAA
100.0
16.0
71.6
0.3
H3PO4
7.6
0.3
15.5
0.0
AA
7.9
0.3
6.0
0.0
ªPartially depolymerized or deeply dehydrated cellulose (humins) account for the remaining
percentages of selectivity for the values in the table
9
Table 3
Yields of sugars and dehydration products from cellulose catalyzed by HCl (1.66 M;
20 wt%) in [BMIM][Cl] at different dissolution temperatures (378, 388 and 398 K) for
the same dissolution time (1 h). The reaction was performed at 378 K for 3 h.
Dissolution
Temperature (K)
% Conv.
Cellulose
% Selectivity to
Cellobioseª
% Selectivity
to Glucoseª
% Selectivity
to HMFª
378
100.0
9.6
89.0
0.6
388
99.8
4.2
79.1
1.1
398
99.9
3.0
64.1
1.9
ªPartially depolymerized or deeply dehydrated cellulose (humins) account for the remaining
percentages of selectivity for the values in the table.
10
Table 4
Yields of sugars and dehydration products from cellulose catalyzed by HCl (1.66 M;
20 wt%) in [BMIM][Cl] at different dissolution temperatures (378, 388 and 398 K) at
an optimized dissolution time. The reactions were performed at 378 K for 3 h and
100 % cellulose conversion was reached in all cases.
Dissolution
Temperature (K)
Dissolution
Time (min.)
% Selectivity to % Selectivity to
Cellobioseª
Glucoseª
% Selectivity.
to HMFª
378
60
9.6
89.0
0.6
388
35
12.5
82.7
0.4
398
25
8.7
89.8
0.6
408
15
12.6
83.7
0.5
ªPartially depolymerized or deeply dehydrated cellulose (humins) account for the remaining
percentages of selectivity for the values in the table.
11
Table 5
Yields of sugars and dehydration products from cellulose catalyzed by HCl (1.66 M;
20 wt%) in [BMIM][Cl] using two different celluloses (fibrous and microgranular) at an
optimized dissolution time. The reactions were performed at 378 K for 3 h and 100 %
cellulose conversion was reached for all cases.
Cellulose
Dissol.
Time (min.)
% Selectivity
to Cellobioseª
% Selectivity
to Glucoseª
% Selectivity
to HMFª
Fibers
15
12.6
83.7
0.5
µ-granular
26
12.5
84.1
0.3
ªPartially depolymerized or deeply dehydrated cellulose (humins) account for the remaining
percentages of selectivity for the values in the table.
12
Table 6: Yields of sugars and dehydration products from cellulose catalyzed by HCl (1.66 M; 20
wt%) using different solvents at a dissolution temperature of 408 K for 15 min. The
reaction occurred at 378 K for 3 h.
Solvent
% Conversion
Cellulose
% Selectivity
to Cellobioseª
% Selectivity.
to Glucoseª
% Selectivity
to HMFª
[BMIM]Cl
99.8
12.6
83.7
0.5
[EMIM]Cl
100.0
12.5
87.1
0.4
[EMIM]OAc
10.1
10.1
0.0
0.0
ªPartially depolymerized or deeply dehydrated cellulose (humins) account for the remaining
percentages of selectivity for the values in the table.
13
Yield to Cellobiose + Glucose / %
100
80
60
40
20
0
-15
-10
-5
0
5
10
pKa
Figure 1 Performance of different homogeneous acid catalysts on the hydrolysis of cellulose
dissolved in [BMIM]Cl.
14
12
0.20
Cellobiose
Glucose
10
HMF
8
6
0.10
4
0.05
2
0
0
30
60
90
120
150
HMF Concentration / g/l
Sugars Concentration / g/l
0.15
0.00
180
Reaction time / min
Figure 2 Main products profile for the hydrolysis of cellulose dissolved in [BMIM][Cl] at 408 K
for 15 min. The hydrolysis temperature was 378 K and the catalyst was HCl.
15