Complete chemical hydrolysis of cellulose into fermentable sugars through ionic liquids and antisolvent pretreatments

Article abstract of DOI:10.1002/cssc.201402466

This work describes a relatively simple methodology for efficiently deconstructing cellulose into monomeric glucose, which is more easily transformed into a variety of platform molecules for the production of chemicals and fuels. The approach undertaken herein first involves the dissolution of cellulose in an ionic liquid (IL), followed by a second reconstruction step aided by an antisolvent. The regenerated cellulose exhibited strong structural and morphological changes, as revealed by XRD and SEM analyses. These changes dramatically affect the hydrolytic reactivity of cellulose with dilute mineral acids. As a consequence, the glucose yield obtained from the deconstructed-reconstructed cellulose was substantially higher than that achieved through hydrolysis of the starting cellulose. Factors that affect the hydrolysis reaction include the type of cellulose substrate, the type of IL used in pretreatment, and the type of acid used in the hydrolysis step. The best results were obtained by treating cellulose with IL and using phosphotungstic acid (0.067 molL^-1) as a catalyst at 413 K. Under these conditions, the conversion of cellulose was almost complete (> 99 %), with a glucose yield of 87% after only 5 h of reaction.

Full text of DOI:10.1002/cssc.201402466

CHEMSUSCHEM
FULL PAPERS
DOI: 10.1002/cssc.201402466
Complete Chemical Hydrolysis of Cellulose into
Fermentable Sugars through Ionic Liquids and Antisolvent
Pretreatments
[
a]
Silvia Morales-delaRosa, Jose M. Campos-Martin,* and Jose L. G. Fierro*
This work describes a relatively simple methodology for effi-
ciently deconstructing cellulose into monomeric glucose,
which is more easily transformed into a variety of platform
molecules for the production of chemicals and fuels. The ap-
proach undertaken herein first involves the dissolution of cellu-
lose in an ionic liquid (IL), followed by a second reconstruction
step aided by an antisolvent. The regenerated cellulose exhib-
ited strong structural and morphological changes, as revealed
by XRD and SEM analyses. These changes dramatically affect
the hydrolytic reactivity of cellulose with dilute mineral acids.
As a consequence, the glucose yield obtained from the decon-
structed–reconstructed cellulose was substantially higher than
that achieved through hydrolysis of the starting cellulose. Fac-
tors that affect the hydrolysis reaction include the type of cel-
lulose substrate, the type of IL used in pretreatment, and the
type of acid used in the hydrolysis step. The best results were
obtained by treating cellulose with IL and using phospho-
ꢀ
1
tungstic acid (0.067 molL ) as a catalyst at 413 K. Under these
conditions, the conversion of cellulose was almost complete
(>99%), with a glucose yield of 87% after only 5 h of reaction.
Introduction
The progressive increase in global oil consumption and the as-
sociated depletion of oil reserves has encouraged scientists to
explore alternative routes to the synthesis of fuels and chemi-
ment of lignocellulosic biomass with a combination of two or
more pretreatment processes has shown promising results,
[20]
there is room for further development, through either the
development of a new, efficient treatment process or the im-
provement of an existing process to provide better per-
formance.
[
1]
cals. A promising feedstock for commercial-scale production
of biofuels and chemicals is lignocellulosic biomass, which is
abundant and readily available. Lignocellulose, which forms
the structural framework of plants, consisting of cellulose,
hemicellulose, and lignin, is first broken down and hydrolyzed
Conventional methodologies have technological limitations
that compromise the efficiency of the separation processes,
such as insufficient selectivity or partial degeneration of the
products. Hence, the current and envisaged investigations are
focused on understanding the pathways to improve the selec-
tive separation of lignocellulose compounds to achieve feasi-
[
2]
into simple fermentable sugars. A major bottleneck is the
need to disarray lignin, which is present as a protective cover-
ing and makes cellulose and hemicellulose recalcitrant to enzy-
matic hydrolysis. A number of biomass deconstruction or pre-
treatment processes (physical, chemical, and biological) have
been used to break the structural framework of plants and to
depolymerize lignocellulose biomass. Some of these pretreat-
[21]
ble and sustainable processes.
In pioneering work, Fort et al. reported that solvent systems
based on 1-butyl-3-methylimidazolium chloride ([BMIM]Cl)/
[
3,4]
ments include treatments with dilute sulfuric acid,
aqueous
[D ]DMSO mixed in a proportion of 84/16 wt% were capable
6
[
5,6]
[7,8]
[22]
ammonia at high temperature,
lime
or organic sol-
of partially dissolving wood chips. They noted that, based
[
9,10]
[11]
vents,
microwave irradiation,
as well as treatments by oxidative delignification,
on the color intensity and viscosity of the solution mixture,
wood particles swelled and were reduced in size during disso-
lution. Similarly, Kilpelꢀinen et al. reported the complete disso-
lution of 8 wt% dried wood sawdust samples (Southern pine)
in both [BMIM]Cl and 1-allyl-3-methylimidazolium chloride
([AMIM]Cl) ionic liquids (ILs) in the temperature range of 80 to
[
12–14]
[15,16]
ball milling,
or steam explo-
[
17–19]
sion.
Examination of these cellulose deconstruction methods re-
veals that no pretreatment technology offers 100% conversion
of cellulose into fermentable C /C sugars. Some biomass is
5
6
[23]
always lost, which affects the final yield and increases the cost
of the finished fuel or chemical product. Although pretreat-
1308C after 8 h. ILs have been recognized as promising sol-
vents for the mild and rapid hydrolysis of biomass feed-
[24–28]
stocks.
However, the high cost of ILs can be a potential
drawback. Therefore, ILs should be recovered from the hydro-
lyzate efficiently through the use of a cost-effective separation
technology. Preliminary calculations show that at least 98% of
the ILs should be recovered for an economically feasible pro-
[
a] S. Morales-delaRosa, Dr. J. M. Campos-Martin, Prof. Dr. J. L. G. Fierro
Sustainable Energy and Chemistry Group (EQS)
Instituto de Catꢀlisis y Petroleoquꢁmica, CSIC
Marie Curie, 2 Cantoblanco, 28049 Madrid (Spain)
E-mail: j.m.campos@icp.csic.es
[26]
cess. Extraction appears to be challenging because ferment-
jlgfierro@icp.csic.es
Homepage: http://www.icp.csic.es/eqsgroup/
ꢁ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 10
&
1
&
ÞÞ
These are not the final page numbers!

CHEMSUSCHEM
FULL PAPERS
www.chemsuschem.org
Results and Discussion
able sugars and 1-ethyl-3-methylimidazolium chloride
[29]
([EMIM]Cl) exhibit similar solubilities in various solvents.
Thus, a different strategy has been proposed: the pretreat-
Modification of cellulose during pretreatment
ment of lignocellulosic biomass by using ILs. This methodology
can effectively remove the lignin and reduce the crystallinity of
the cellulose to permit enzymatic hydrolysis at high solid load-
ings and low enzyme concentrations; hence, it substantially ac-
celerates the rate of enzymatic hydrolysis and increases the
The dissolution of cellulose, either fibrilar or microgranular, in
the [EMIM]Cl IL, followed by precipitation in water (antisol-
vent), induced important morphological and textural changes
(Figure 1). The original microfibrous (Figure 1a) and granular
[
30,31]
yield of fermentable sugars.
Indeed, pretreatment of cellulo-
sic biomass by using ILs can
reduce the crystallinity of the
cellulose to enable chemical hy-
drolysis at very low acid concen-
trations, and thereby, increase
the yield of fermentable sugars.
This approach allows the recov-
ery of not only the precipitated
cellulose, but also the IL em-
ployed in the solubilization step.
With this idea in mind, we
previously investigated the hy-
drolysis of cellulose without sol-
Figure 1. Original and IL-pretreated celluloses from left to right: (a) Microfibrous cellulose; (b) pretreated microfi-
brous cellulose, white sample; (c) pretreated microfibrous cellulose, transparent sample; (d) microgranular cellu-
lose; and (e) pretreated microgranular cellulose, transparent sample.
[
32]
ubilization in ILs. We observed
that the crystallinity of the cellu-
lose also affected its reactivity:
well-crystallized cellulose was more resistant to acid hydrolysis
than its less-crystalline counterpart. The highest selectivity for
glucose over levulinic acid (LA) was recorded at a reaction
temperature of 1408C and a H SO concentration in the range
(Figure 1d) samples appear in powder form with no significant
morphological differences between them. In contrast, the cel-
lulose samples obtained through solubilization of microfibrilar
cellulose in the [EMIM]Cl IL followed by reconstruction upon
precipitation with water as an antisolvent (Figure 1b, c, and e)
showed morphological changes. The color of reconstructed
cellulose appears to differ somewhat, depending on whether
the water antisolvent is added at high temperature (408 K) or
after the sample cools (303 K). In the former, the color of the
reconstructed cellulose was white (Figure 1b), whereas, in the
latter, it was almost transparent (Figure 1c and e).
2
4
ꢀ
1
of 0.2 to 0.5 molL . Under these reaction conditions, only
a small concentration of LA was detected; however, the glu-
cose yield reached only 20% in 2 h. Therefore, we undertook
the present work to improve the yield of glucose by overcom-
ing the recalcitrance of microgranular or fibrous cellulose. The
approach undertaken herein includes three steps: 1) decon-
struction of cellulose by dissolution in an IL, 2) reconstruction
of the cellulose structure by precipitation with the aid of an
antisolvent (water), and 3) hydrolysis of the resulting cellulose.
The advantage of precipitating cellulose is that the IL can be
completely recovered and is therefore not present during the
hydrolysis step. Full recovery of ILs according to the methodol-
ogy envisioned herein is critically important when the technoe-
conomic feasibility of a large-scale process for fermentable
sugar production from IL-pretreated biomass is considered.
The importance of the methodology developed herein is il-
lustrated by the almost complete cellulose conversion (>99%):
Morphology of pretreated cellulose
The micro- and sub-micrometric morphology of both the origi-
nal and deconstructed-reconstructed cellulose samples was ex-
amined by SEM. The microfibrous and granular celluloses ex-
hibit differences at low magnification: the microfibrous sample
contains long cylindrical fibers with diameters of approximately
20 mm (Figure 2), whereas the microgranular sample contains
smaller fragmented particles and fibers that are shorter than
those of fibrous cellulose (Figure 3). However, when the SEM
images were recorded at a higher magnification, both samples
were observed to be composed of fibers with similar struc-
tures, although the microgranular sample contained shorter
fibers together with some amorphous particles.
8
7% glucose yield is obtained after 5 h of reaction at 413 K by
ꢀ
1
using phosphotungstic acid (0.067 molL ) as a cellulose hy-
drolysis agent for cellulose obtained by deconstruction with an
IL and subsequent reconstruction by precipitation with water.
As previously mentioned, the deconstruction–reconstruction
pretreatments led to a dramatic change in the structure and
morphology of the starting cellulosic substrates. The morphol-
ogy of the cellulose fibers disappeared after the pretreatments,
irrespective of the cellulose source (Figures 4 and 5). At low
ꢁ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 10
&
2
&
ÞÞ
These are not the final page numbers!

CHEMSUSCHEM
FULL PAPERS
www.chemsuschem.org
Figure 2. SEM micrographs of the starting microfibrous cellulose. The top
panel depicts the same sample at higher magnification.
Figure 4. SEM micrographs of microfibrous cellulose pretreated with
[EMIM]Cl (15 min) and precipitated at high temperature (white sample). The
bottom panel is an image of the same sample at higher magnification.
magnification, the surface appears rather smooth; however, at
high magnification, some porous structures can be distin-
guished. The surface of the cellulose precipitated at lower tem-
perature (transparent) is similar at low magnification to that of
the cellulose precipitated at higher temperature; however, its
porous structure observed at high magnification is less marked
(
Figure 6).
The crystalline structures of the original and pretreated cel-
lulose samples were analyzed by XRD (Figure 7). The XRD pro-
files show a prominent cellulose peak at 238 that corresponds
to the (200) reflection; other less intense peaks appear at ap-
proximately 15 and 178, which are characteristic of (1 1¯ 0) and
(
110) reflections, respectively, and a composite of several peaks
[33,34]
that includes the (004) reflection at 348.
The intensity of
the peaks is rather high, which indicates high crystallinity of
the samples. These diffraction peaks clearly disappear after the
deconstruction–reconstruction pretreatment (Figure 7); the
only diffraction line that still appears in the transparent sam-
ples is that at 228, which indicates that the solid retains some
of its crystallinity.
Cellulose hydrolysis
After we subjected the cellulosic substrates to deconstruction–
reconstruction processes, we subsequently tested them in the
hydrolysis reaction with dilute acids to evaluate the influence
of the morphology and crystallinity changes induced by pre-
Figure 3. SEM micrographs of the starting microgranular cellulose. The
bottom panel is an image of the same sample at higher magnification.
ꢁ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 10
&
3
&
ÞÞ
These are not the final page numbers!

CHEMSUSCHEM
FULL PAPERS
www.chemsuschem.org
Figure 5. SEM micrographs of microgranular cellulose pretreated with
Figure 6. SEM micrographs of microfibrous cellulose pretreated with
[EMIM]Cl (15 min) and precipitated at low temperature (transparent sample).
The bottom panel is an image of the same sample at higher magnification.
[EMIM]Cl (15 min) and precipitated at high temperature (white sample). The
bottom panel is an image of the same sample at higher magnification.
treatments of the raw cellulose substrate. For this purpose,
very soft reaction conditions were selected to maximize the
yield of sugars with very low formation of byproducts such as
[
32]
LA. The reaction temperature was fixed at 413 K, and the
concentration of the homogeneous acid catalyst used was
ꢀ
1
0
.2 molL .
Effect of cellulose type
The glucose yield of unpretreated cellulose is very low, irre-
spective of the cellulose type used (Figure 8); simultaneously,
the amount of unreacted solid is high (Table 1). However,
deeper analysis of the results shows that the hydrolysis of the
microgranular sample is more efficient than that of the fibrous
one. This finding is consistent with XRD and SEM results that
showed greater crystallinity and longer fibers for the fibrous
sample. These characteristics make the fibrous sample more re-
calcitrant to hydrolysis because a higher order (i.e., greater
Figure 7. XRD patterns of both pretreated and unpretreated cellulose sam-
ples: (a) fibrous and (b) granular samples.
+
crystallinity) and longer fibers impede the ability of H ions to
treated sample than that for the fibrous one. These differences
are attributed to differences in the surface texture observed by
SEM, for which the microgranular IL-pretreated sample is more
porous than its fibrous counterpart (Figures 4 and 5).
reach the b-glycosidic bonds; these characteristics thus inhibit
[
35]
the hydrolysis reaction.
However, the reactivity toward hydrolysis changes when IL-
pretreated samples are used. We observe a dramatic increase
in the formation of glucose (Figure 8) and in the conversion of
solid (Table 1). This behavior is similar for the two types of cel-
lulose employed (microgranular and fibrous; Figure 8). Howev-
er, the yield of glucose is higher for the microgranular IL-pre-
Effect of precipitation procedure
The kinetics of the two celluloses (white and transparent cellu-
loses) obtained according to the deconstruction–reconstruc-
ꢁ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 10
&
4
&
ÞÞ
These are not the final page numbers!

CHEMSUSCHEM
FULL PAPERS
www.chemsuschem.org
cose yields are much higher for the two cellulose samples pre-
pared according to the deconstruction–reconstruction proce-
dure. Notably, the glucose yield at 300 min of reaction is some-
what higher for the white cellulose (76%) than for its transpar-
ent counterpart (70%). Indeed, the glucose yield of these two
samples is much higher than that of untreated cellulose (21%),
which indicates a strong influence of the cellulose morphology
on the hydrolysis kinetics. These differences in reactivity are
consistent with the morphology differences of the solids, as
determined by SEM (Figures 4 and 6), for which the surface of
the white sample clearly appears to be more porous and ac-
cessible to reactant, which is a critical factor for the hydrolysis
reaction.
Figure 8. Yields of glucose (a) and LA (b) during the hydrolysis of microfi-
brous and microgranular celluloses that were either unpretreated (empty
symbols) or pretreated (full symbols) with [EMIM]Cl in the presence of
Effect of the IL type
The next steps consisted of using different ILs for the decon-
struction of the cellulose. Three different imidazolium salts,
ꢀ
1
0.2 molL
2 4
H SO at 413 K.
[
EMIM]Cl, [BMIM]Cl, and [EMIM][Ac], were used. Chloride salts
are known to be slightly acidic, whereas [EMIM][Ac] is more
inert toward cellulose hydrolysis than the chloride-based ILs
counterparts and is able to solubilize a greater amount of bio-
Table 1. Hydrolysis of original cellulose and cellulose pretreated with
ꢀ1
2 4
[EMIM]Cl upon using H SO as the acid catalyst (0.2 molL ) at 413 K for
2
h.
[36]
mass.
Acid catalyst
Cellulose type
Cellulose conversion [%]
Pretreated fibrous cellulose, which was precipitated with
water without the cellulose–IL mixture first being cooled
(white sample), was selected for this comparison. The yield of
glucose was higher for all of the IL-treated samples; however,
some differences were observed, depending on the IL used.
The glucose yield was higher for cellulose treated with the
chloride IL than for that treated with the acetate IL. A similar
effect was observed with respect to the LA yield (Figure 10).
These differences are more evident if the conversion of cellu-
lose is compared (Table 2): the conversion is clearly higher for
samples treated with chloride ILs to reach more than 95% at
5 h of reaction. This behavior is attributed to the acidity of the
chloride ILs, which results in prehydrolysis of the cellulose
during the deconstruction–reconstruction treatment. The glu-
cose yield is smaller for the cellulose treated with [EMIM][Ac];
however, in this case, the ratio of glucose/LA is higher than
that in the products generated by using the other two ILs.
Another advantage of using the [EMIM][Ac] IL is the absence
of chloride ions, which makes the process safer; avoids corro-
sion; and inhibits downstream processes, such as fermentation
of the sugars.
original
pretreated
H
H
2
SO
SO
4
4
microfibrous
microgranular
16
18
86
84
2
tion methodology described herein were studied. A white
sample was obtained when cold water was added to the high-
temperature IL–cellulose liquid phase (white sample), whereas
a transparent sample was obtained when the IL–cellulose
liquid phase was first cooled to ambient temperature before
cold water was added.
The kinetics of hydrolysis of these two cellulose samples are
displayed in Figure 9. For comparative purposes, the kinetics of
hydrolysis of the unpretreated original cellulose are also in-
cluded. Both the initial hydrolysis rates and the extent of glu-
Influence of the catalyst acid strength
Different acids, such as p-TSA, H SO , and H PW O , were
2
4
3
12 40
used as homogeneous catalysts to efficiently hydrolyze the b-
[27]
1
,4-glycosidic bonds. The number of acidic protons liberated
by these acids depends on the acid used. In the case of H SO ,
2
4
only one acidic proton should be considered because less than
% of the bisulfate ions dissociate under the conditions em-
ployed for the hydrolysis of cellulose; therefore, only the first
1
[35]
ionization of H SO appears to be relevant. Similarly, p-TSA
2
4
has only one acidic proton; however, all three protons of phos-
photungstic acid are available, which make it a strong acid. For
Figure 9. Hydrolysis of pretreated in [EMIM]Cl and unpretreated microfibrous
cellulose with H SO (0.1 molL ) at 413 K.
2 4
ꢀ
1
ꢁ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 10
&
5
&
ÞÞ
These are not the final page numbers!

CHEMSUSCHEM
FULL PAPERS
www.chemsuschem.org
Figure 11. Glucose yield from the hydrolysis of cellulose with H
2
SO
4
, p-TSA
Figure 10. Hydrolysis of microfibrous white cellulose pretreated in different
ꢀ
1
ꢀ1
ꢀ
1
(0.2 molL ), and H PW12O40 (0.067 molL ) at 413 K: (a) unpretreated cellu-
3
ILs ([EMIM]Cl, [BMIM]Cl, and [EMIM][Ac]) with H
2 4
SO (0.2 molL ) at 413 K for
lose; (b) cellulose pretreated with [EMIM]Cl.
5
h with respect to (a) glucose and (b) levulinic acid yield.
lows the order H PW O ·xH O>H SO >p-TSA. Notably,
3
12 40
2
2
4
Table 2. Cellulose hydrolysis pretreated with different ILs in the presence
H O PW exhibited excellent performance in the hydrolysis of
3 40 12
ꢀ
1
of H
2 4
SO (0.2 molL ) (413 K for 2 and 5 h)
cellulose, leading to the conversion of more than 99% of the
starting cellulose and to a glucose yield of approximately 90%
in only 5 h of reaction time.
IL
Conversion of white cellulose [%]
5 h
2
h
Another important finding was the low yield of LA (<2.5%)
in all of the experiments that involved unpretreated cellulose
(Figure 12), although small differences in LA yield were ob-
served among the acids employed. The LA yield is higher
when pretreated cellulose is used (Figure 12), most likely be-
cause of the higher concentration of glucose present in the re-
action medium. This increase occurs for all three of the acids
employed; however, the LA/glucose ratio appears somewhat
higher for H SO than those for H O PW ·xH O and p-TSA.
This observation indicates that sulfuric acid performs better
than its counterparts as a dehydrating acid. Given the glucose
yield and the LA/glucose ratio, the most suitable catalyst
among those investigated for cellulose hydrolysis is phospho-
tungstic acid.
[
[
[
BMIM]Cl
EMIM]Cl
EMIM][Ac]
78
78
36
97
99
63
this reason, we added the same concentration of acid for p-
TSA and H SO , but we used a threefold lower molar concen-
2
4
tration in the case of H PW O .
3
12 40
The glucose yield in the hydrolysis of untreated fibrous cel-
lulose is very low, irrespective of the acid used (Figure 11). The
glucose yield reaches 20% after 5 h of reaction only when
phosphotungstic acid is used; however, the amount of un-
reacted solid is still rather high (Table 3), yielding a solid con-
version of approximately 24–25%. The solid conversion per-
centages were similar for all three acids used.
2
4
3
40
12
2
Conversely, when the IL-pretreated cellulose was hydrolyzed
under the same experimental conditions, a dramatic increase Discussion
in the glucose yield was observed (Figure 11). The conversion
Cellulose deconstruction–reconstruction
of solid was also very high, approximately 98–99%, after only
h of reaction (Table 3). A comparison of the glucose yield
5
Cellulose is a biopolymer in which the hydroxyl groups are ori-
ented to form strong intra- and intermolecular hydrogen
bonds that are integrated into micro- and macrofibril networks.
These molecular interactions define a complex and rigid struc-
ture to form a recalcitrant substrate against chemical or enzy-
matic hydrolysis. Deconstruction of this rigid structure requires
harsh and expensive separation processes. Although numerous
profile shows that the amount of glucose produced increases
with increasing acid strength. The glucose yield produced fol-
Table 3. Cellulose hydrolysis in the presence of different homogeneous
acid catalysts at 413 K for 5 h.
[37]
Acid catalyst
Catalyst concentration
Cellulose conversion [%]
physical and chemical methods have been assayed, no per-
fect pretreatment method has been discovered because the
suitability of a given method varies with respect to the sub-
strate. However, the recent discovery that ILs are good solvents
for mild and rapid hydrolysis of biomass feedstocks has ena-
bled the efficient conversion of cellulose into fermentable
sugars.
ꢀ
1
[a]
[
molL
]
original
pretreated
H
3
PW12
O
40·xH
2
O
0.067
0.20
0.20
24
24
25
98
99
99
[
b]
p-TSA
SO
4
H
2
[a] Pretreated with [EMIM]Cl. [b] p-TSA: para-toluenesulfonic acid.
ꢁ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 10
&
6
&
ÞÞ
These are not the final page numbers!

CHEMSUSCHEM
FULL PAPERS
www.chemsuschem.org
According to reports in the literature, the interaction of cel-
lulose–[BMIM][Ac] decreases in the order of acetone>etha-
nol>water, with cellulose–[Ac] forming the smallest number
2
[45]
of hydrogen bonds in water. However, the interactions of
cellulose–cellulose increase in the reverse order (acetone<eth-
anol<water); the largest number of hydrogen bonds between
cellulose chains are observed in water. Thus, because water is
an excellent antisolvent for cellulose, we have exploited this
property to reconstruct the cellulose. As illustrated in Figure 1,
in which fibrilar or microgranular cellulose was dissolved in
[
EMIM]Cl IL and then precipitated with water, a much more
open structure was obtained compared with that displayed by
the original cellulose. The morphology of the reconstructed
cellulose offers a highly porous texture with a much greater
exposed surface area that is prone to attack by mineral or or-
ganic acids; thus making acid hydrolysis easier.
2 4
Figure 12. Yield of LA in the hydrolysis of cellulose with H SO , p-TSA
ꢀ
1
ꢀ1
3 2
(0.2 molL ), and H PW12O40·xH O (0.067 molL ) at 413 K: (a) unpretreated
cellulose; (b) cellulose pretreated with [EMIM]Cl.
Kinetics of hydrolysis
With the objective of producing fibers from cellulose, re-
searchers have devoted great efforts to the development of
solvents for the processing of lignocellulose. The use of ILs as
solvents for cellulose is undoubtedly the most important ex-
In the course of cellulose hydrolysis, breaking of the b-1,4-gly-
cosidic linkages must occur. This reaction, which is catalyzed
by strong homo- or heterogeneous acids, primarily yields glu-
cose, which is a very useful fermentable monosaccharide. Inter-
esting platform chemicals can be obtained from further con-
[
36,38]
ample of such efforts.
ILs are salts that melt at tempera-
[46,47]
tures less than 1008C. The 1-alkyl-3-methylimidazolium-based
version of reducing sugars.
In addition, various fine chemi-
ILs with good hydrogen-bond-acceptor anions can dissolve cel-
cals and potential fuels (bioethanol) can also be directly ob-
tained from acid processing of cellulose through a multistep
fractionation process. Although cellulose hydrolysis can be cat-
alyzed with either a heterogeneous catalyst or an acid, efficient
acid-catalyzed hydrolysis has emerged recently because of the
development of numerous fine and even homogeneous cata-
lysts. Herein, we focused only on the homogeneous process by
using H SO , p-TSA, and H O PW .
[
22]
lulose and even wood. The capability of ILs to dissolve cellu-
lose was exploited herein as a starting point to convert the
solubilized fraction into useful products.
Cellulose dissolution is an industrially attractive application
of ILs because of the good solubilities of cellulose in IL sol-
vents (5–20 wt%). Complete cellulose dissolution in ILs is
highly dependent on the temperature, the type of IL, time of
dissolution, and the water content, which, in turn, should be
optimized for the specific IL–cellulose dissolution process. Nev-
ertheless, when ILs dissolve carbohydrates, the ILs are consid-
ered to effectively disrupt the intricate network of noncovalent
interactions between these polymers. Swatloski et al. suggest-
ed that the high chloride concentration and high activity of
2
4
3
40
12
In most of the previous studies, mineral acids (HCl, H SO )
2
4
and organic acids (p-TSA, oxalic, maleic, fumaric) were ob-
served to be particularly well suited to the production of glu-
cose that could undergo further consecutive reactions to pro-
duce first hydroxymethylfurfural (5-HMF) and finally levulinic
acid (LA) and formic acid (FA; Scheme 1). Acid hydrolysis of cel-
lulose is a kinetic process that is controlled primarily by 1) the
[BMIM]Cl was responsible for breaking the extensive and well-
organized hydrogen-bonding network of cellulose, and thus,
nature of the cellulose precursor, 2) the pK of the acid, 3) the
a
[
39]
promoting dissolution. ILs not only disrupt the hydrogen-
bonding interactions of crystalline cellulose in wood, but also
interact with and solvate the aromatic components of lignin
through p–p and n–p interactions, generally via the IL
acid concentration, and 4) the reaction temperature.
The acid hydrolysis of cellulose is a reaction catalyzed by
protons and also by hydroxide anions that result from water
dissociation; the protons and hydroxide ions react with cellu-
lose molecules to yield various products, such as glucose,
xylose, arabinose, and cellobiose. In addition, oligosaccharides
can be readily formed from the liquid acid-catalyzed hydrolysis
of cellulose.
[
40]
cation.
Dissolved cellulose can be modified in solution or regenerat-
ed (reprecipitated) by the addition of water; mixtures of water
with organic solvents (e.g., acetone); or protic organic sol-
[
39,41]
vents, such as ethanol, to form films and fibers.
The order-
The kinetics of cellulose hydrolysis were very fast when cel-
lulose was deconstructed in ILs. The use of H SO as a hydrolyz-
ing of regenerated cellulose is reduced relative to the initial
2
4
[
42]
state, and it is transformed into cellulose II. This transforma-
tion also results in significantly accelerated hydrolysis com-
ing agent at 413 K resulted in a solid conversion rate in the
range of 97 to 99% when cellulose deconstructed in [EMIM]Cl
and [BMIM]Cl ILs was used. In contrast, the conversion rate
was only 63% in the case of cellulose deconstructed in [EMIM]
[Ac] (Table 2). In addition, under the same reaction conditions,
hydrolysis of the deconstructed–reconstructed cellulose was
[
43,44]
pared with that of native cellulose;
an effect that is very
attractive in terms of biorefineries and has sparked interest in
the use of cellulose-dissolving ILs in lignocellulose deconstruc-
tion.
ꢁ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 10
&
7
&
ÞÞ
These are not the final page numbers!

CHEMSUSCHEM
FULL PAPERS
www.chemsuschem.org
resulting cellulose substrate exhibited a much more accessible
surface that enhanced local acid reactivity.
Several factors influenced the glucose yield, such as the type
of IL employed, the precipitation procedure, and the acid used
in hydrolysis. The best conditions were observed to be as fol-
lows: IL, [EMIM]Cl; hydrolysis reaction temperature, 413 K; and
ꢀ
1
phosphotungstic acid (0.067 molL ). Under the experimental
conditions, almost complete cellulose conversion was obtained
(
>99%) with 89% glucose yield after only 5 h of reaction.
Experimental Section
Reagents
H SO , p-TSA, H PW O , and two samples of cellulose powder
2
4
3
12 40
from cotton linters (fibrous cellulose (C6288) and microgranular
cellulose (C6413)) were purchased from Sigma–Aldrich and used
without any further purification. [EMIM]Cl, [BMIM]Cl, and [EMIM]
[Ac] ILs were purchased from Sigma–Aldrich and used without any
further purification.
Experimental techniques
The scanning electron micrographs of untreated cellulose and cel-
lulose pretreated with IL were taken with a Hitachi S-3000 N instru-
ment. The samples were treated with increasing concentrations of
ethanol to fix the structure and to dehydrate the samples. We then
proceeded to critical-point drying with a Polaron CPD7501 critical-
point drier; finally, the samples were metallized in a Balzers SCD
0
04 gold-sputter coater; they were sputter-coated with a thin layer
of gold.
XRD profiles of samples were recorded with an X’Pert Pro PANalyti-
cal diffractometer equipped with a CuKa radiation source (l=
Scheme 1. Selected products formed by acid-catalyzed reactions starting
from cellulose. Adapted from Rinaldi et al.
[
46]
0
.15418 nm) and an X’Celerator detector based on a real-time mul-
tiple strip (RTMS). The samples were ground and placed on a stain-
less-steel plate. The diffractograms were recorded in steps over
a range of Bragg angles (2q) between 4 and 908, at a scanning rate
of 0.028 per step, and at an accumulation time of 50 s. Diffracto-
grams were analyzed with the X’Pert HighScore Plus software.
The pretreatment of cellulose involved complete dissolution of the
cellulose (0.5 g) in an IL (9.5 g) at 408 K by using a continuously
stirred tank reactor (Mettler Toledo Easy Max). Cellulose was com-
pletely dissolved within 15 min. After it was dissolved, the cellulose
was precipitated by the addition of water (50 mL). The obtained
solid was washed several times with water to eliminate all remain-
ing IL.
fastest with the strongest acid. The observed trend in the glu-
cose yield was H O PW ·xH O>H SO >p-TSA (Figure 11).
3
40
12
2
2
4
Notably, the outstanding performance of the homogeneous
H O PW catalyst in the hydrolysis of cellulose, which led to
3
40
12
a conversion rate of the starting cellulose greater than 99%
with a glucose yield of approximately 90% in only 5 h of reac-
tion time.
Conclusions
A simple methodology was used to make cellulose highly reac-
tive toward acid hydrolysis. It essentially included first solubili-
zation of the cellulose in an IL, followed by the precipitation of
solubilized cellulose with water. The cellulose was separated
by filtration and the IL was almost completely recovered. The
XRD pattern of the treated cellulose indicated that it was non-
crystalline, and SEM images indicated that the fiber structure
present in its untreated counterpart disappeared.
Hydrolysis reaction
Hydrolysis reactions were performed batchwise in a magnetically
stirred 100 mL thermostated Teflon-lined steel Berghof reactor
equipped with a pressure addition funnel. In a typical run, cellulose
(0.5 g) and water (40 mL) were mixed in the reactor and the sus-
pension was heated to the reaction temperature (413 K). Then,
acid solution (10 mL) was added dropwise to the reactor; the “re-
action time” was measured from this moment. The total volume of
liquid in the reactor was 50 mL. The acid concentration in the reac-
The major cause of the improvement in acid hydrolysis per-
formance after cellulose deconstruction–reconstruction de-
scribed herein was the development of a completely different
cellulose morphology. Although some aggregation of cellulose
fibrils occurred at the end of the reconstruction process, the
ꢀ1
tor was 0.2 molL . Aliquots were periodically collected from the
reactor. In all cases, the reaction was stopped after 5 h, and the
mixture was quickly cooled. The solution was collected by filtra-
tion, centrifuged, and thoroughly washed with distilled water. The
ꢁ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 10
&
8
&
ÞÞ
These are not the final page numbers!

CHEMSUSCHEM
FULL PAPERS
www.chemsuschem.org
resulting solid was dried at 353 K overnight. The amount of solid
isolated was determined by weighing.
[13] J. Liu, R. Takada, S. Karita, T. Watanabe, Y. Honda, T. Watanabe, Bioresour.
Technol. 2010, 101, 9355–9360.
[
14] J. Xu, H. Chen, Z. Kꢂdꢂr, A. B. Thomsen, J. E. Schmidt, H. Peng, Biomass
Bioenergy 2011, 35, 3859–3864.
The liquid was analyzed by HPLC (Agilent Technologies HPLC 1200
series). The chromatographic separations were conducted in an
ꢀ1
[15] P. Kumar, D. M. Barrett, M. J. Delwiche, P. Stroeve, Ind. Eng. Chem. Res.
009, 48, 3713–3729.
AMINEX HPX-87H column at 338 K by using a 0.6 mLmin aque-
2
ꢀ
1
ous solution of sulfuric acid (0.01 molL ) as the mobile phase. The
sugars and dehydration products (5-HMF and LA) were analyzed
by using a refractive index detector and a UV/Vis detector. The
components were identified by comparing their retention times
with those of reference samples. The products were quantified
through the use of internal calibration curves.
[
[
16] J. Hilgert, N. Meine, R. Rinaldi, F. Schꢃth, Energy Environ. Sci. 2013, 6, 92.
17] I. Ballesteros, M. J. Negro, J. M. Oliva, A. CabaÇas, P. Manzanares, M. Bal-
lesteros, Appl. Biochem. Biotechnol. 2006, 130, 496–508.
[18] C. Hongzhang, L. Liying, Bioresour. Technol. 2007, 98, 666–676.
[19] G. Panagiotou, L. Olsson, Biotechnol. Bioeng. 2007, 96, 250–258.
[
[
20] V. Chaturvedi, P. Verma, Biotechnology 2013, 3, 415–431.
21] A. da Costa Lopes, K. Joao, A. R. Morais, E. Bogel-Lukasik, R. Bogel-Luka-
sik, Sustainable Chem. Processes 2013, 1, 3.
Herein, the results obtained for 5-HMF in the hydrolysis of cellulose
were not shown. We worked in the aqueous phase, in which this
compound readily decomposed to LA and FA, resulting in insignifi-
cant amounts of 5-HMF. For this reason, we only analyzed the re-
sults related to glucose and LA herein. The glucose and LA yields
were calculated by using Equations (1) and (2):
[
[
22] D. A. Fort, R. C. Remsing, R. P. Swatloski, P. Moyna, G. Moyna, R. D.
Rogers, Green Chem. 2007, 9, 63.
23] I. Kilpelꢀinen, H. Xie, A. King, M. Granstrom, S. Heikkinen, D. S. Argyro-
poulos, J. Agric. Food Chem. 2007, 55, 9142–9148.
[24] H. Zhang, J. Wu, J. Zhang, J. He, Macromolecules 2005, 38, 8272–8277.
[
25] N. Kamiya, Y. Matsushita, M. Hanaki, K. Nakashima, M. Narita, M. Goto,
H. Takahashi, Biotechnol. Lett. 2008, 30, 1037–1040.
%
%
glucose yield ¼ GCon ꢁ 100=GMx
ð1Þ
ð2Þ
[
26] J. B. Binder, R. T. Raines, Proc. Natl. Acad. Sci. USA 2010, 107, 4516–4521.
LA yield ¼ LCon ꢁ 100=LMx
[27] S. Morales-delaRosa, J. M. Campos-Martin, J. L. G. Fierro, Chem. Eng. J.
012, 181–182, 538–541.
2
[
[
28] R. Rinaldi, F. Schꢃth, ChemSusChem 2009, 2, 1096–1107.
29] C. Sievers, M. B. Valenzuela-Olarte, T. Marzialetti, I. Musin, P. K. Agrawal,
C. W. Jones, Ind. Eng. Chem. Res. 2009, 48, 1277–1286.
in which GCon is the measured glucose concentration, GMx is the
maximum concentration of glucose that can be obtained based on
the amount of cellulose fed to the reactor, LCon is the LA concentra-
tion, and LMx is the maximum concentration of LA that can be
formed from the cellulose added.
[
[
[
30] Y. Wang, M. Radosevich, D. Hayes, N. Labbꢄ, Biotechnol. Bioeng. 2011,
1
08, 1042–1048.
31] R. Rinaldi, P. Engel, J. Buchs, A. C. Spiess, F. Schuth, ChemSusChem 2010,
, 1151–1153.
32] S. Morales-delaRosa, J. M. Campos-Martin, J. L. G. Fierro, Cellulose 2014,
1, 2397–2407.
3
Acknowledgements
2
[33] Y. Nishiyama, G. Johnson, A. French, Cellulose 2012, 19, 319–336.
[34] A. French, M. Santiago Cintrꢅn, Cellulose 2013, 20, 583–588.
[35] C.-H. Lin, A. H. Conner, C. G. Hill, J. Appl. Polym. Sci. 1992, 45, 1811–
This work was supported by the Comunidad de Madrid (Spain)
through grant S2009/ENE-1743 and CSIC (Spain) through grants
1
822.
36] N. Sun, H. Rodrꢆguez, M. Rahman, R. D. Rogers, Chem. Commun. 2011,
7, 1405–1421.
201080E008 and 201180E038.
[
4
[
[
37] E. C. Bensah, M. Mensah, Int. J. Chem. Eng. 2013, 2013, 719607.
38] A. Pinkert, K. N. Marsh, S. Pang, M. P. Staiger, Chem. Rev. 2009, 109,
6712–6728.
Keywords: biomass
liquids · synthesis design
· carbohydrates · hydrolysis · ionic
[
39] R. P. Swatloski, S. K. Spear, J. D. Holbrey, R. D. Rogers, J. Am. Chem. Soc.
2
002, 124, 4974–4975.
[1] G. W. Huber, B. E. Dale, Sci. Am. 2009, 301, 52–59.
[
[
40] A. Brandt, J. Grꢀsvik, J. P. Hallett, T. Welton, Green Chem. 2013, 15, 550.
41] M. G. Freire, A. R. R. Teles, R. A. S. Ferreira, L. D. Carlos, J. A. Lopes-da-
Silva, J. A. P. Coutinho, Green Chem. 2011, 13, 3173–3180.
[
2] S. Morales-delaRosa, J. M. Campos-Martin in Catalytic Processes and Cat-
alyst Development in Biorefining (Ed.: K. W. Waldron), Woodhead, Oxford,
2
014, pp. 152–198.
3] P. Alvira, E. Tomas-Pejo, M. Ballesteros, M. J. Negro, Bioresour. Technol.
010, 101, 4851–4861.
[
[
42] G. Cheng, P. Varanasi, C. Li, H. Liu, Y. B. Melnichenko, B. A. Simmons,
M. S. Kent, S. Singh, Biomacromolecules 2011, 12, 933–941.
43] H. Zhao, C. L. Jones, G. A. Baker, S. Xia, O. Olubajo, V. N. Person, J. Bio-
technol. 2009, 139, 47–54.
[
2
[
[
[
4] A. T. W. M. Hendriks, G. Zeeman, Bioresour. Technol. 2009, 100, 10–18.
5] T. H. Kim, F. Taylor, K. B. Hicks, Bioresour. Technol. 2008, 99, 5694–5702.
6] F. Teymouri, L. Laureano-Perez, H. Alizadeh, B. E. Dale, Bioresour. Technol.
[
[
[
44] X.-f. Tian, Z. Fang, D. Jiang, X.-y. Sun, Biotechnol. Biofuels 2011, 4, 53.
45] K. M. Gupta, Z. Hu, J. Jiang, RSC Adv. 2013, 3, 12794.
46] R. Rinaldi, N. Meine, J. vom Stein, R. Palkovits, F. Schꢃth, ChemSusChem
2005, 96, 2014–2018.
[7] S. Kim, M. T. Holtzapple, Bioresour. Technol. 2005, 96, 1994–2006.
[8] B. C. Saha, M. A. Cotta, Biomass Bioenergy 2008, 32, 971–977.
[9] F. Sun, H. Chen, Bioresour. Technol. 2008, 99, 5474–5479.
2
010, 3, 266–276.
47] S. Van de Vyver, J. Geboers, P. A. Jacobs, B. F. Sels, ChemCatChem 2011,
, 82–94.
[
3
[
[
10] A. Geng, F. Xin, J.-y. Ip, Bioresour. Technol. 2012, 104, 715–721.
11] Y. Nakamura, M. Daidai, F. Kobayash, Water Sci. Technol. 2004, 50, 167–
1
72.
12] H. Ma, W.-W. Liu, X. Chen, Y.-J. Wu, Z.-L. Yu, Bioresour. Technol. 2009,
00, 1279–1284.
[
Received: May 26, 2014
1
Published online on && &&, 0000
ꢁ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 10
&
9
&
ÞÞ
These are not the final page numbers!

FULL PAPERS
S. Morales-delaRosa,
Back to basics: A relatively simple
J. M. Campos-Martin,* J. L. G. Fierro*
methodology for efficiently deconstruct-
ing cellulose into monomeric glucose,
which is easier to transform into a varie-
ty of platform molecules for the produc-
tion of chemicals and fuels, is described.
&& – &&
Complete Chemical Hydrolysis of
Cellulose into Fermentable Sugars
through Ionic Liquids and Antisolvent
Pretreatments
ꢁ
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 10
&10
&
ÞÞ
These are not the final page numbers!