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Hydrolysis of cellulose to glucose by solid acid catalysts
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Cite this: Green Chem., 2013, 15, 1095
Yao-Bing Huang and Yao Fu*
As the main component of lignocelluloses, cellulose is a biopolymer consisting of many glucose units
connected through β-1,4-glycosidic bonds. Breakage of the β-1,4-glycosidic bonds by acids leads to the
hydrolysis of cellulose polymers, resulting in the sugar molecule glucose or oligosaccharides. Mineral
acids, such as HCl and H2SO4, have been used in the hydrolysis of cellulose. However, they suffer from
problems of product separation, reactor corrosion, poor catalyst recyclability and the need for treatment
of waste effluent. The use of heterogeneous solid acids can solve some of these problems through the
ease of product separation and good catalyst recyclability. This review summarizes recent advances in the
Received 18th January 2013,
Accepted 28th February 2013
DOI: 10.1039/c3gc40136g
www.rsc.org/greenchem
1.
hydrolysis of cellulose by different types of solid acids, such as sulfonated carbonaceous based acids,
polymer based acids and magnetic solid acids. The acid strength, acid site density, adsorption of the substance and micropores of the solid material are all key factors for effective hydrolysis processes. Methods
used to promote reaction efficiency such as the pretreatment of cellulose to reduce its crystallinity and
the use of ionic liquids or microwave irradiation to improve the reaction rate are also discussed.
Introduction
The discovery and utilization of fossil resources has changed
the energy supplement of the whole world. Current energy
systems are mainly based on these fossil resources such as
coal, petroleum and natural gas. However, the rapid consumption of these resources, together with the resulting global
warming caused by CO2 emissions, raises sustainability issues
Department of Chemistry, Anhui Province Key Laboratory of Biomass Clean Energy,
University of Science and Technology of China, Hefei 230026, P. R. China.
E-mail: [email protected]
Yao-Bing Huang
Yao-Bing Huang has studied at
the University of Science of Technology of China (USTC) since
2004. After obtaining his Bachelor’s Degree in 2008, he continued to study green organic
synthesis under the supervision
of Prof. Qiang-Xiang Guo and
Yao Fu. Now, he works on the
development of efficient heterogeneous catalysts for organic
reactions and biomass conversion.
This journal is © The Royal Society of Chemistry 2013
regarding the existing energy systems based on fossil
resources. To solve this problem, people are forced to explore
renewable resources to substitute the fossil resources in order
to meet increasing energy demands.1–4 Until now, various
forms of energy systems have been developed as excellent
alternatives such as solar, wind, biomass, hydroelectric and
geothermal.5,6 Among these new forms of energy sources,
biomass is the only sustainable source of organic carbon on
Earth, which is considered as one part of the solution for producing fuels and chemicals.7–9
Biomass can be obtained all over the world and it generally
occurs in the form of organic materials such as grass, wood,
agricultural crops and their residues and waste. These
Dr Yao Fu obtained his PhD
degree from the Department of
Chemistry at USTC in 2005. He
then began his academic career
at the Anhui Province Key Laboratory of Biomass Clean Energy.
He is presently a professor of
organic chemistry at USTC. His
research interests cover transition-metal catalyzed organic
reactions, the synthesis of heterogeneous catalysts for green chemistry processes and the selective
Yao Fu
conversion of biomass derived
compounds into value-added chemicals and biofuels.
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materials come from biological photosynthesis from CO2,
water and sunlight, thus making them sustainable and green
feedstocks with zero carbon emission.10 The first generation of
biofuels are mainly bioethanol and biodiesel which are produced from sugars, starches and vegetable oils. However, concerns were raised that the utilization of these materials
competed with food for feedstocks.11 Moreover, their potential
availability is limited by the amount of fertile soil.11
Due to the above limitations, the second generation of biofuels has mainly concentrated on the utilization of lignocellulosic biomass. Lignocellulose can be grown in combination
with food or on barren land. Every year, sees the formation of
about 220 billion dry tons of available biomass (ca. 45 EJ of
energy content),12,13 and lignocellulose forms about 70–95%
of it.14 Lignocelluloses are composed of mainly three components, cellulose (40–50%), hemicelluloses (25–35%) and
lignin (15–20%) ( percentages of the components vary from
different biomass resources).15,16 A schematic biorefinery for
the utilization of lignocellulose is shown in Scheme 1.17
Among these biopolymers, cellulose is the most valuable one
which can be converted into glucose and subsequently fermented into bioethanol or dehydrated into platform molecules.18
Besides, cellulosic biofuels have been considered as the most
promising candidates for the replacement of current petroleum based fuels.19
The first step for cellulose utilization is to depolymerize it
into soluble oligosaccharides and glucose. However, the
natural polymer forms a robust crystal structure with high
chemical stability, thus making the depolymerization processes more difficult. The established methods for cellulose
hydrolysis are known to be catalyzed by cellulose enzymes. But
limitations of the enzymes systems are obvious in that the processes are always low efficiency and have a high enzyme
cost.20–22 At the same time, much work has been concentrated
on the hydrolysis of cellulose to glucose with mineral acids.
Sulfuric acid is known as a typical acid for this reaction.
However, the large-scale use of acid suffers from several
Scheme 1
Green Chemistry
problems such as reactor corrosion, catalyst recovery and
requires treatment of the acid residue, producing lots of
waste.23,24 From the industrial point of view, the above limitations must be taken into account when designing production
processes. In addition, several aspects also need to be of
concern such as economy, simplicity, efficiency and environmental friendliness.
The utilization of heterogeneous acids has the potential to
overcome some of the above limitations due to the ease of separation of catalysts. Significant development of this transformation has been made by using various types of catalysts with
large pore size and strong acid strength.25–28 Associated with
these solid acids, many pretreatment technologies have also
been developed to reduce the crystallinity of cellulose and
increase its surface area in order to improve the reaction
efficiency and selectivity, for example, ball-milling,29,30 solubilization/precipitation in ionic liquids,31 liquid acids/alkaline
solutions,32,33 the non-thermal atmospheric plasma method34
and so on. It is worth noting that most hydrolysis processes
with solid acids require the pretreatment of cellulose as shown
in Table 1. Apart from that, microwave irradiation induced
assistance of the hydrolysis of cellulose is an effective heating
method compared to conventional oil heating.35–37
In this review, we summarize the recent advances in the
hydrolysis of cellulose into glucose with solid acids. The review
is divided into the following parts: (1) structure of cellulose;
(2) hydrolysis of cellulose by liquid acids; (3) hydrolysis of cellulose by solid acids. The final part of this review presents an
outlook of the future of this domain.
2.
Structure of cellulose
Early exploration work on the structure of cellulose was carried
out by Anseleme Payen.38 He obtained a resistant fibrous solid
material with the formula C6H10O5 after the treatment of
various plant tissues with acids and ammonia. It was the
Utilization of lignocelluloses to produce chemicals and fuels.
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Table 1
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Different performances of solid acid catalysts in the hydrolysis of cellulose
Catalyst
Acidity
(mmol g−1)
Pretreatment
method
HNbMoO6
Zn–Ca–Fe
Amberlyst-15
CP–SO3H
Nafion NR50
Nafion SAC50
PCPs–SO3H
AC–SO3H
BC–SO3H
CSA–SO3Hb
SC–SO3H
AC–N–SO3H-250
CMK-3-SO3H
SimCn–SO3H
H3PW12O40
H3PW12O40
H5BW12O40
H3PW12O40
Micellar HPA
[MIMPSH]nH3−nPW
CsH2PW12O40
H-beta
HY zeolite
HY zeolite
Fe3O4–SBA–SO3H
Fe3O4–SBA–SO3H
MNPs@SiO2–SO3H
Ru/CMK-3
CaFe2O4
HT–OHCa
1.90
—
—
0.067
—
—
1.80
1.90
1.98
1.76
2.15
2.23
2.39
0.37
—
—
—
—
—
—
—
1.05
—
1.36
1.09
1.09
0.5
—
0.068
1.17
—
—
—
—
[BMIm]Cl
—
—
—
—
Milled/sieved
a
Milled
Milled
Milled
Milled
—
—
Milled
—
—
—
Milled
Milled
—
—
Solvent activation
—
Milled
Milled
Milled
Solvents
H2O
H2O
[BMIm]Cl/H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
[BMIm]Cl/H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O/MIBK
H2O
H2O
[C4mim]Cl/H2O
[BMIm]Cl/H2O
H2O
H2O
H2O
H2O
H2O
H2O
Assistance
method
Temp.
(K)
Time
(h)
Glucose and
TRSa yield/%
Ref.
—
—
403
433
373
393
403
463
393
373
363
403
383
423
423
423
423
453
333
363
443
413
433
423
373
403
423
423
423
503
423
423
12
20
5
10
2
24
3
3
1
1
4
24
24
24
2
2
6
3
8
5
6
24
0.13
2
3
3
3
24
24
24
8.5
29
11
93
35
11
5.3
64
19.8
34.6
63
62.6
74.5
50.4
18
50.5
77
75.6
39.3
36
27
12
37
50
26
50
30.2
34.2
36
40.7
63
64
68
74
76
77
78
75
83
84
85
86
86
87
97
98
99
100
101
102
103
105
106
107
108
108
112
116
117
118
—
—
Microwave
Microwave
—
—
—
—
—
—
—
Microwave
—
—
—
—
Microwave
—
—
—
—
—
—
—
TRS: total reducing sugars. b Corncob was used as the starting material.
earliest “cellulose”.39 The structure of cellulose was elucidated
in 1920 by Hermann Staudinger.40 He found that cellulose was
composed of D-glucose units, linked to each other to form long
chains (Scheme 2). The structure of cellulose was made up of
many glucose moieties linked in the form of β-D-anhydroglucopyranose units (AGUs). The AGUs are linked to each other
through glycosidic bridges on the C1 and C4 carbon atoms.41
The number of repeating AGUs is defined as the degree of
polymerization (DP) of cellulose. Generally, the DP of cellulose
varies according to the original type of materials, for example,
the common range of DP for wood pulp is about 300–1700 and
is 800–10 000 for cotton or other plant fibers.41 Due to the
huge number of hydroxyl groups and linkage of β-1,4-glycosidic bonds, it is easy to form intra- and intermolecule hydrogen
bonds which make the cellulose resistant during chemical and
biological treatment and make it highly insoluble in common
solvents.42
According to the crystal features, cellulose has at least seven
forms, Iα, Iβ, II, IIII, IIIII, IVI, IVII.43,44 In nature, most cellulose
occur in the state of Iα, Iβ. They can be found in the same
material from almost every biomass source. The ratio of the
two crystalline forms depends on the source of cellulose. Generally, cellulose Iα is rich in the cell walls of algae and bacterial
cellulose while cellulose Iβ is abundant in cotton, wood and
tunicate cellulose.45 Cellulose Iα,β is thermodynamically not
the most stable form in nature.46 When cellulose I swells in
concentrated sodium hydroxide, it easily forms the thermodynamically more stable cellulose II after the removal of the swelling agent.42,43 Cellulose III can be obtained by treatment of
cellulose Iα, Iβ with the ammonia fiber explosion process. Cellulose IV can be obtained by treating cellulose III with glycerol
at 206 °C.42,43 However, cellulose III, IV are not common and
are thus ignored in our discussion. The more generally known
and discussed forms of cellulose are microcrystalline Avicel
cellulose and α-cellulose which come from pure bacterial cellulose and the treatment of wood with alkali extraction.47
3.
Scheme 2
Structure of cellulose (degree of polymerization n = DP).
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Hydrolysis of cellulose by liquid acids
The hydrolysis of cellulose is a process to break the β-1,4-glycosidic bonds of the polymer which is an essential step for the
conversion of cellulose and opens the possibility of subsequent catalytic transformations. Direct hydrolysis of lignocellulosic biomass with acids goes back to the early 19th century
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and several processes were reported to be effective in the 20th
century.48 The first technology for the acid hydrolysis of cellulose was developed by Faith in 1923.49 In the process, sulfuric
acid solution (0.5 wt%) was used to treat wood waste in bricklined percolators. After about 45 min at 170 °C, a dilute sugar
solution was obtained. The solution was transferred for fermentation after neutralization. A 50% yield of sugars was obtained
according to the theoretical yield of fermentable sugars.
After the work, many processes were reported to be effective
in the hydrolysis of cellulose. They used different kinds of
acids such as HCl, H2SO4, HF and organic acids (oxalic,
maleic, furmaric). In 1937, Bergius reported the hydrolysis of
cellulose with 40 wt% HCl (ca. 12 mol L−1) at room temperature.50 Cellulose was solubilized in concentrated acid together
with hemicelluloses, leaving insoluble lignin. The hydrolysis
products were glucose and oligosaccharides without dehydration products. Concentrated HCl (6–7 mol L−1) was also
effective in the hydrolysis of cellulose in the presence of CaCl2
and LiCl as the additives.51 The results showed that the salts
had a swelling effect by increasing the hydrolysis rate, leading
to an 85% yield of glucose at 90 °C. Additionally, concentrated
acids had an apparent swelling effect on cellulose. Cellulose
swells when the sulfuric acid concentration is above 50%.52
Camacho et al.53 studied the effect of H2SO4 concentration
(from 31%–70%, (w/v)) on the solubilization rate of microcrystalline cellulose, revealing that acid promoted the total solubilization of cellulose when the concentration was above 62%.
Although hydrolysis with concentrated acids operates at low
temperature and atmospheric pressure, these processes had
strict requirements on water content and displayed severe
corrosion.50
Hydrolysis of cellulose with dilute acid usually requires
high temperature. Madison et al.54 reported a treating process
in which wood was treated with 0.5 wt% H2SO4 in a continuous reaction. The degradation of products was minimized for
the short residence time of cellulose in the reactor. Another
successful process using an isothermal plug flow reactor was
reported by Grethlein and Thompson.55 They used 1 wt%
H2SO4 in the continuous process at 240 °C with a short residence time of 0.22 min; 50% glucose was obtained at last. A
few years later, Harris et al.56 used a two stage system containing dilute H2SO4 in the saccharification of wood. After the
hemicellulose was extracted, cellulose was transferred for
hydrolysis and a high purity of glucose was obtained.
The hydrolysis of cellulose was achieved with gaseous HCl
as reported by the Wilke group.57 The HCl stream was
adsorbed on the dried wood particles and initiated the
hydrolysis of hemicelluloses. After extraction of the hemicellulose sugars, the residue was dried and mixed with HCl gas at
45 °C, achieving 90% conversion of the available carbohydrate
content of wood to sugars. High pressure HCl systems were
also reported for the hydrolysis reaction.57,58 Apart from
gaseous HCl, anhydrous HF was employed in the hydrolysis of
cellulose. The main advantage of using anhydrous HF is its
low boiling point (19.5 °C) which facilitated the acid recovery.
The yield of sugars was about 45% at 0 °C.59 The recovery
1098 | Green Chem., 2013, 15, 1095–1111
Green Chemistry
experiment showed that only 0.05–0.1% of HF remained in the
system, opening the possibility of reusing mineral acids in the
hydrolysis of cellulose.57
Current strategies for the hydrolysis of cellulose with acids
are mainly accomplished with the acids mentioned above.
Although homogeneous-based processes are efficient, the
mineral acid systems still suffer from major problems in
product/catalyst separation, reactor corrosion, catalyst recycling and the treatment of waste effluent. For example, in the
H2SO4 based system, the acid in the reaction mixture has to be
neutralized with a mixture of CaO/CaSO4 which forms lots of
gypsum as waste.56 For the HCl system it is difficult to reuse
the liquid acid while the HF system is very toxic when utilized
in large-scale processes.57,60 In the search for greener processes for hydrolyzing cellulose, solid acids were developed to
address some of the problems and have some unique catalytic
properties that mineral acids do not possess.
4.
Hydrolysis of cellulose by solid acids
The hydrolysis of cellulose into sugars with solid acids has
received increasing attention. Solid acid catalysts have various
advantages over liquid acid catalysts: ease of product separation, recyclability, less damage to the reactor. Besides, the use
of solid acid catalysts can reduce the pollutants which will
have a minimal impact on the environment. Up till now,
numerous reviews have been published on the transformation
of cellulose with solid acids.25–28 Herein, we would like to summarize the recent advances in this subject. Several types of
solid acids are included in the following part and their catalytic performance is summarized in Table 1.
4.1
Metal oxides
Metal oxides are a type of solid catalyst with many Lewis acid
sites. Metal oxides are always prepared with high specific
surface and pore sizes which are easy for the reactants to
access and contact with the active sites inside the metal oxide
pores. These metal oxides can be used for the hydrolysis of
sucrose, cellobiose and even cellulose.
As a type of strong solid acid, mesoporous transition-metal
oxides have been prepared and used in organic chemical transformations.61 Recently, Domen et al.62 reported that mesoporous Nb–W oxide could be used as a solid catalyst for the
hydrolysis of sucrose and cellobiose. The rate of glucose production and the turnover frequency (TOF) for the hydrolysis of
sucrose were higher than those of other solid acids (i.e. Amberlyst-15, Nb2O5). The acid strength increased gradually with the
addition of W, reaching the highest reaction rate with mesoporous Nb3W7 oxide. The high catalytic performance of Nb3W7
oxide was attributed to the high surface area mesoporous
structure and strong acid sites. However, the Nb–W oxide catalyst had a lower activity for the cellobiose hydrolysis due to the
low Brønsted acid sites.
In order to solve the problem, Domen et al. also reported
a layered transition-metal oxide HNbMoO6 that exhibited
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remarkable catalytic performance for the hydrolysis of sucrose,
cellobiose, starch and cellulose (Scheme 3).63 For the hydrolysis of sucrose and cellobiose, the layered HNbMoO6 catalyst
exhibited the highest activity, producing glucose at twice the
rate of Amberlyst-15 (Fig. 1). The high activity of HNbMoO6 in
these reactions was attributed to its strong acidity, water-tolerance and intercalation ability. The XRD patterns and FT-IR
spectrum of HNbMoO6, immersed with glucose, sucrose and
cellobiose, revealed that the saccharide was successfully intercalated into the interlayer gallery. However, the total yield of
the products (glucose and cellobiose) was only 8.5% in the
hydrolysis of cellulose (Fig. 2). Thus, increasing the acid site
density and the surface area of a layered transition-metal oxide
is necessary for the full conversion of cellulose to sugars.
In addition, nanoscale metal oxide catalysts have the potential to improve the catalytic performance of the hydrolysis reaction. Fang et al.64 used nano Zn–Ca–Fe oxide as the catalyst in
the crystalline cellulose hydrolysis reaction. The catalyst exhibited good catalytic activities. The cellulose conversion and the
glucose selectivity were 42.6% and 69.2%, respectively. Nano
Zn–Ca–Fe oxide gave better performances with respect to
hydrolysis rates and glucose yields than fine particle Zn–Ca–Fe.
Besides, the paramagnetic nature of Fe oxides made it easy
to separate the nano Zn–Ca–Fe oxide from the reaction
mixture by simple magnetic filtration techniques.
4.2
Polymer based acids
Polymer based acids with Brønsted acid sites have been used
as effective solid catalysts for many organic reactions
including hydrolysis reactions.65–67 Macroreticulated styrene-
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Fig. 2 Hydrolysis of starch and cellulose with HNbMoO6 and Amberlyst-15.
(Adapted from ref. 63.)
divinylbenzene resins with sulfonic groups (–SO3H) are one of
these polymer based acids, known as Amberlyst. They are commercially available, inexpensive and stable in most solvents.
The macroporous structures of these acids allow small molecules to enter into the pores and interact with more active acid
sites.
Recent pioneering work on the hydrolysis of microcrystalline cellulose and α-cellulose with Amberlyst 15DRY resin was
reported by Schüth et al.68 Purified cellulosic substrates were
dissolved in 1-butyl-3-methylimidazolium chloride ([BMIm]Cl)
in order to increase their solubility and make the transport of
cellulose chains to the acid sites a highly demanding process.
After hydrolysis with Amberlyst 15DRY, cellulosic materials
were selectively converted to cello-oligomers or sugars. As the
reaction proceeded, the hydrolysis performances using Amberlyst 15DRY showed an induction period of about 1.5 h (Fig. 3).
However, when soluble p-toluenesulfonic acid ( p-TSA) was
used, resembling the acid sites of Amberlyst 15DRY, no induction period was given during the first 1.5 h. HPLC analysis of
the reducing sugars showed that almost no mono- and disaccharides were produced during the initial 1.5 h, while the
production of small sugars was detected for the reaction catalyzed
by p-TSA. The difference of the two systems was investigated by
Scheme 3 Hydrolysis of cellulose to glucose with HNbMoO6. (Reproduced
from ref. 63.)
Fig. 1
Hydrolysis of sucrose with different solid acids. (Adapted from ref. 63.)
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Fig. 3 (A) Total release of reducing sugars from ▲ α-cellulose (using p-TSA), ■
microcrystalline cellulose, ▼ wood (spruce), ● α-cellulose (using Amberlyst
15DRY), determined by 3,5-dinitrosalicylic acid (DNS) assay. (B) Production of
mono- and disaccharides, monitored by HPLC, for the hydrolysis of α-cellulose
catalyzed by ▲ p-TSA and by ● Amberlyst 15DRY. Reaction conditions: cellulose
(5 g) dissolved in 100 g [BMIm]Cl, Amberlyst 15DRY (1.00 g, 4.6 mmol H+) or
p-TSA (4.6 mmol), water (111 mmol), 373 K. (Reproduced from ref. 68 with
permission from Wiley-VCH.)
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Fig. 4 Hydrolysis of microcrystalline cellulose. Appearance of cellulose recovered from [BMIm]Cl by addition of water after the reaction times indicated. The
values between parentheses represent the percentage of isolated cellulose.
(Adapted from ref. 68 with permission from Wiley-VCH.)
the visual appearance of isolated cellulose suspensions and
degree of polymerization (DP) analysis. The recovered cellulose
from the ionic liquid appeared in different forms with the
reaction variation (Fig. 4). The isolated cellulose became
smaller with increasing reaction time, giving a colloidal dispersion after 5 h of reaction. The figure shows that there were
substantial changes in the cellulose structure. The degree of
polymerization (DP) analysis of cellulose at different recovered
times also demonstrated that cellulose was effectively depolymerized in the presence of Amberlyst 15DRY. It was worth
noting that cello-oligomers with a DP of around 30 were found
after 1.5 h of hydrolysis of microcrystalline cellulose and could
be precipitated in 90% yield. No fermentable sugars were
detected during the first 1.5 h which was in accordance with
the induction time observed in Fig. 4. Since the high solubility
of sugars in ionic-liquid made the sugar extraction and ionic
liquid recovery very difficult, the route gave a cello-oligomer
production method instead of total hydrolysis into fermentable sugars which was an advantage of this process. Further
extension of the reaction time led to the production of sugars
and some dehydration products. The most appealing part of
the route was that it can selectively convert cellulose or even
wood into cello-oligomers, which could be further broken
down into sugars. The hydrolysis reaction proceeded in the
cellulose → cello-oligomers → sugars → dehydration products
series, and if the process was properly terminated at different
times, cellulose fragments or sugars could be selectively
obtained for different use in biorefineries.
In order to gain more insight into the controlling factors in
hydrolysis reactions, Rinaldi and Schüth et al.69 investigated
the effect of different parameters on the hydrolysis of cellulose
in [BMIm]Cl with Amberlyst 15DRY such as acid amount, reaction temperature and impurities. They found that depolymerization is a first-order reaction with respect to catalyst
concentration. Particularly, they discovered that the induction
period depends heavily on the amount of acid used for the
reaction (Fig. 5). Increase of catalyst concentration from 0.46
to 6.9 mmol of H3O+ decreased the induction time from 1.9 h
to less than 5 min. This means that the acid concentration played
an important role in the hydrolysis reaction, and the use of a
larger amount of Amberlyst 15DRY resembling the use of a
soluble strong acid showed no induction period for the production of glucose. The activation energy for the depolymerization of cellulose is 108 kJ mol−1, which is relatively lower than
that with liquid acid (i.e. H2SO4 170 kJ mol−1).
1100 | Green Chem., 2013, 15, 1095–1111
Fig. 5 Depolymerization of cellulose over Amberlyst 15 DRY: Effect of the
amount of catalyst on the yield of glucose. (Adapted from ref. 69 with permission from Wiley-VCH.)
Compared to the enzymatic hydrolysis of cellulose, the
solid or liquid acid catalyzed depolymerization of cellulose
always requires higher temperature due to its higher reaction
activation energy. It was reported that the activation energy for
cellulose hydrolysis with dilute acid is 170–180 kJ mol−1 (no
matter what temperature and acid concentration are used).70
On the other hand, the activation energy for enzymatic
hydrolysis is only 3–50 kJ mol−1, resulting in the hydrolysis
reaction being conducted at the low temperature of 50 °C.71–73
An ideal cellulose-mimetic catalyst consisting of a cellulosebinding domain and a catalytic domain was developed for
cellulose hydrolysis by Pan’s research group.74 The catalyst
contained a chlorine (–Cl) group which plays the cellulosebinding role by forming hydrogen bonds, and sulfonic acid
groups (–SO3H) functioning as the hydrolytic domains. Chloromethyl polystyrene (CP) resin with –Cl groups was chosen as
the catalyst support. The –SO3H groups were introduced by
partially substituting –Cl groups with sulfanilic acid. The
proposed mechanism of cellulose hydrolysis is shown in
Scheme 4. The catalyst was firstly applied to the hydrolysis of
cellobiose. It was completely hydrolyzed to glucose at 120 °C
over 2 h, while only 8% of cellobiose was obtained with sulfuric acid under the same conditions. Adsorption experiments
showed that CP–SO3H adsorbed both glucose and cellobiose
through hydrogen binding between the chloride groups on the
catalyst and the hydroxyl groups of the sugars (Fig. 6). The preferable adsorption of cellobiose over glucose is critical for cellobiose hydrolysis to proceed which ensures the desorption of
glucose product from the catalyst. CP–SO3H showed excellent
catalytic performance for the hydrolysis of crystalline cellulose
(Avicel). The best yield of glucose from CP–SO3H was up to
93%, while almost no Avicel was hydrolyzed on using sulfuric
acid. The activation energy of cellulose hydrolysis catalyzed by
CP–SO3H was 83 kJ mol−1 at 373–413 K and is much lower
than that of sulfuric acid (170 kJ mol−1) or sulfonated active
carbon (AC–SO3H) (110 kJ mol−1).75 This was the key point as
to why the hydrolysis reaction could be conducted at lower
temperature.
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Scheme 4 Hydrolysis mechanism of a cellulase-mimetic solid catalyst. (Reproduced from ref. 74.)
Scheme 5 Schematic representation of the structure of PCPs–SO3H. (Adapted
from ref. 78 with permission from Wiley-VCH.)
Fig. 6 Adsorption curve of glucose and cellobiose onto resins in aqueous solution (a) glucose on Amberlyst-15; (b) cellobiose on Amberlyst-15; (c) glucose on
CP–SO3H; and (d) cellobiose on CP–SO3H. (Adapted from ref. 74.)
Apart from the Amberlyst-type resins, Nafion (sulfonated
tetrafluoroethylene based fluoropolymer-copolymer) was
another type of effective solid acid for the hydrolysis of cellulose. Suh et al.76 developed a cellulose pretreatment process
using [BMIm]Cl before the subsequent hydrolysis over Nafion
NR50. The pretreatment step was to decrease the crystallinity
of cellulose. The hydrolysis of regenerated cellulose (from the
pretreatment of cellulose with [BMIm]Cl) yielded 35% glucose.
Moreover, Lucht et al.77 reported the hydrolysis of cellulose by
a Nafion supported on amorphous silica (Nafion SAC 13) catalyst under mild conditions. The catalyst had the potential to
be recycled or applied in a continuous flow reactor. It showed
excellent performance in the hydrolysis of cellobiose; 100%
glucose was obtained at 130 °C over 24 h. The hydrolysis of
cellulose needed a higher temperature (190 °C); the yield of
glucose was 11% for the first time. The catalyst provided 8%
and 7% yields of glucose for the second and third reaction
runs, which suggested it to be a recyclable catalyst for the conversion of cellulose into useful chemicals in the future.
Porous coordination polymers (PCPs) are a new class of
porous solids with a large variety of pore surfaces and pore
structures. Recently, it was demonstrated by Akiyama et al.78
that PCPs decorated with sulfonic acid groups could be used
as catalysts for cellulose hydrolysis. MIL-101, composed of a
chromium oxide cluster and terephthalate ligand, was chosen
as the platform to introduce the –SO3H groups, and the BET
surface area was 1915 m2 g−1 (Scheme 5). The hydrolysis reaction was carried out at 393 K over 3 h and produced 2.6% 1.4%
This journal is © The Royal Society of Chemistry 2013
and 1.2% yields of xylose, glucose and cellobiose, respectively.
By increasing the reaction time, the yields of these sugars
increased. The merit of this new catalyst is that it prevents the
formation of side-products such as 5-hydroxymethylfurfural or
levulinic acid which are normally found in the hydrolysis of
cellulose under strong acidic conditions. However, it was hard
for cellulose to diffuse deep inside the porous solid due to the
large size of cellulose. Since cellulose is a crystal and insoluble
in water, it is assumed that only one end of the cellulose chain
diffused into the pores and had contact with the acidic sites,
leading to a clean cut of the chain. The reuse experiments of
the PCPs catalyst showed no decrease in the catalytic activity
after testing 13 times, indicating that it was a highly stable
catalyst for the hydrolysis reaction.
4.3
Sulfonated carbonaceous based acids
Among various types of solid acid for the hydrolysis of cellulose, carbonaceous solid acids have superior catalytic activities.
The good recyclability and cheap naturally occurring raw
materials of these carbonaceous acids make them better candidates for the production of biofuel precursors. The first carbonaceous acids from sulfonated D-glucose/sucrose materials
were reported by Hara et al.79 D-Glucose and sucrose were
incompletely carbonized at low temperature to form small
polycyclic aromatic carbon rings and subsequently sulfonated
with sulfuric acid to introduce sulfonic groups (–SO3H). They
used the carbonaceous acids for the transesterification of vegetable oils to biofuels. After their work, much attention was
paid to these new types of amorphous solid acid which could
be used as catalysts for the cellulose hydrolysis reaction.
Carbon-based solid acids were prepared by sulfonation of
incompletely carbonized natural polymers, such as sugars, cellulose and starch or by incomplete carbonization of sulfopolycyclic aromatic compounds in conc. H2SO4. Hara et al.75,80–82
reported the preparation method of carbon material bearing
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–SO3H groups from microcrystalline cellulose at 723 K overr
5 h under N2 flow. The black powder was then boiled in 15 wt%
H2SO4 at 353 K under N2 for 10 h; after filtration and
washing with hot water, carbon based acid was obtained.
Characterization of this solid acid showed that the carbon
material is composed of uniform functionalized graphene
sheets, bearing –SO3H, –COOH and phenolic –OH groups
which are different to conventional solid acids with single
functional groups (Scheme 6). The effective surface area of the
carbon material during hydrolysis was about 560 m2 g−1,
larger than the BET surface area after hydrolysis (only
2 m2 g−1). The H2O vapor adsorption–desorption isotherm
showed that a larger amount of water was incorporated into
the bulk of the catalyst. The incorporation ability of hydrophilic molecules made it easy for a cellulose chain in solution
to be in contact with the –SO3H groups in the carbon material,
which gave rise to high catalytic performance.
Comparable experiments for the hydrolysis of cellulose with
different solid acids revealed that carbon-based acids exhibit
remarkable hydrolysis performances (Table 2). After reacting
for 3 h, 68% of cellulose was hydrolyzed to glucose (4% yield)
and soluble β-1,4-glucan (64% yield), the liquid acid H2SO4
also has a high activity for the hydrolysis of cellulose (10%
glucose and 38% β-1,4-glucan). The enhanced hydrolytic catalysis of the amorphous carbon with –SO3H, –OH and –COOH
and was further investigated via an adsorption experiment
with cellobiose, a short β-1,4-glucan of cellulose. The results
Scheme 6 Schematic structure of the carbon-based solid acid. (Reproduced
from ref. 82a.)
Table 2
Hydrolysis of crystalline cellulose by various acid catalysts (ref. 75)a
Catalyst
H2SO4
Niobic acid
H-mordenite
Nafion
Amberlyst-15
Carbon material (CH0.62O0.54S0.05)
a
showed that the strong interactions (hydrogen bonds to oxygen
atoms in glycosidic bonds) between the phenolic OH groups
and the glycosidic bonds in β-1,4-glucan led to higher catalytic
activities. Such hydrogen bonds were expected to bind cellulose to the surface of the catalysts, making it easier for the
hydrolysis to proceed. The activation energy for cellulose
hydrolysis with this carbon based acid was 110 kJ mol−1 which
was lower than that with sulfuric acid (170 kJ mol−1). The
amorphous carbon acid was recovered from the reaction
mixture and reused at least 25 times without decrease in
activity. Elemental analysis and ion chromatography revealed
that only 1% –SO3H groups leached into the solution during
the first reaction and no further leaching was detected in subsequent reactions. The excellent performance of the carbon
based catalyst opens the possibility of converting cellulose into
value-added molecules with cheap and efficient solid acids,
which has the potential for use in industrial production in the
future.
A mild process for the hydrolysis of cellulose to reducing
sugars under microwave irradiation was developed by Fu and
Yin et al.83 They used carbon sulfuric acids (BC–SO3H) derived
from bamboo, cotton and starch as catalysts. The introduction
of microwave irradiation greatly accelerated the rate of the
hydrolysis reaction. Comparison of hydrolysis results between
the two heating methods over BC–SO3H revealed that the accelerating effect of microwave irradiation on the reaction was
evident. The hydrolysis reaction operated at a lower temperature of 70–100 °C and the yield of reducing sugars was higher
than that using conventional heating. The yields of hydrolysis
products were different with the different types of carbon
acids. The correlation between the hydrolysis product yields
and adsorption values confirmed the results from Hara’s work
that the adsorption of the phenolic OH groups of BC–SO3H to
the oxygen atoms in the β-1,4-glycosidic bonds was responsible
for the high catalytic activities of these carbon-based solid
acids when hydrolyzing cellulose to glucose.
Carbonaceous solid acid (CSA) can also be applied to real
biomass substrates under microwave irradiation. The Jiang
and Mu research group84 illustrated that lignocellulosic
biomass (e.g. corncob) can be well hydrolyzed by self-derived
CSA under microwave irradiation. The CSA was prepared from
hydrolyzed corncob residue and was used for the hydrolysis of
Functional
groups
Density
(mmol g−1)
Max. acidity
H0
Surface area
(m2 g−1)
Acidic OH
Acidic OH
SO3H
SO3H
SO3H
COOH
Phenolic OH
20.4
0.4
1.4
0.9
4.8
1.9
0.4
2.0
−11
−5.6
−5.6
−11 to −13
−2.2
−8 to −11
—
—
—
90
480
<1
50
2
Yields of hydrolysis products
Glucose: 10%, β-1,4-glucan: 38%
—
—
—
Glucose: 4%, β-1,4-glucan: 64%
Conditions: catalyst 0.3 g, cellulose 25 mg, H2O 0.7 g, 3 h.
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Scheme 7 Procedure for corncob hydrolysis by CSA derived from the hydrolysis
residue. (Reproduced from ref. 83.)
corncob (Scheme 7). In this way, the whole corncob could be
utilized in the process and two products were obtained that
were sugars (from the cellulose and hemicelluloses part) and
CSA (from the unconverted solid residue). The hydrolysis reactions produced 78% yield of xylose and arabinose at 403 K.
The effective utilization of the hydrolyzed residues reduces the
cost of the catalysts and pollution.
Apart from microwave irradiation, ionic liquids were also
investigated as effective solvents for the hydrolysis of cellulose
with glucose derived sulfonated carbon acid, reported by Qi
et al. (Scheme 8).85 Cellulose has good solubility in the ionic
liquid which enables easier contact with more acid sites.
When the reaction was conducted at 110 °C, the total reducing
sugars yield was up to 72.7%. Further increase in the reaction
temperature led to an increase reaction rates but decreased the
TRS yield, due to the rapid decomposition of the formed
sugars at higher temperature.
Whereas much effort has been invested in the optimization
of the hydrolysis reaction conditions of cellulose over carbonbased solid acids, the sulfonation conditions have rarely been
considered. Recently, Zhang et al.86 reported that by changing
the sulfonation temperature of the active carbon (AC), the catalyst exhibited different catalytic activities in the hydrolysis reaction. The active carbon was treated with/without nitric acid
and sulfonated at different temperature ranging from
150–300 °C. With the increase of the sulfonation temperature,
the acid density of the sulfonated carbon increased. The
specific areas of the samples were increased when the temperature was increased from 150 to 250 °C, but decreased on
Scheme 8
Hydrolysis of cellulose with glucose derived sulfonated carbon acid.
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further increasing the temperature. The hydrolysis of ballmilled cellulose was conducted at 150 °C over these solid acids
and the AC–N–SO3H sulfonated at 250 °C gave the best performance; the cellulose conversion and glucose yield were
74.3% and 62.6%, respectively. Apart from the sulfonation conditions, the carbon sources were also evaluated. Among the
tested carbons, CMK-3, a type of ordered mesoporous material,
gave the best performance with a cellulose conversion of
94.4% and a glucose yield of 74.5% which is the highest
glucose yield achieved via catalysis by carbon based solid acids
until now. The sulfonated CMK-3 material has a higher acid
density and surface area (412 m2 g−1); both parameters are
important for cellulose conversion.
A new class of sulfonated silica/carbon nanocomposite catalyst was reported by Jacobs and Sels et al. for the hydrolysis of
ball-milled cellulose to sugars.87 The sample was prepared by
evaporation of a sucrose/TEOS/F127 solution (carbon source/
silica precursor/structure-directing amphiphilic surfactant)
and followed by carbonization in N2 to decompose F127 and
convert the sucrose into carbon residues. The resulting hybrid
materials were finally treated with sulfuric acid to obtain sulfonated silica/carbon nanocomposites. As the carbonization
temperature varied, the ratio of the Si and C in the samples
changed, denoted as SimCn-T-SO3H (m, n are the weight percentages of silica and carbon, T is the carbonization temperature).
The catalysts exhibited good performances in the hydrolysis
reaction. The Si33C66-823-SO3H catalyst resulted in the highest
glucose yield of 50% (Fig. 7). The hybrid surface structure
facilitated the adsorption of β-1,4-glucan on the solid acid, and
together with the strong accessible Brønsted acid sites
suggested that the sulfonated silica/carbon nanocomposites
Fig. 7 Hydrolytic conversion of cellulose over heterogeneous acids. Reaction
conditions: cellulose pretreated by ball-milling 0.05 g, catalyst 0.05 g, water
5 mL, reaction time 24 h, temperature 423 K. (Adapted from ref. 87.)
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have potential for the selective hydrolysis of cellulose to
glucose.
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4.4
Heteropoly acids
Heteropoly acids (HPAs) are a type of solid acid, consisting of
early transition metal–oxygen anion clusters, and they are
usually used as a recyclable acid in chemical transformations.88,89 The most common and widely used heteropoly
acids are Keggin type acids with the formula [XYxM(12−x)O40]n−
(X is the heteroatom and M and Y are addendum atoms).
Heteropoly acids have received much attention due to their
fascinating architectures and excellent physicochemical properties such as Brønsted acidity, high proton mobility and good
stability.90–93 They dissolve in polar solvents and release H+,
whose acidic strength is stronger than typical mineral acids
such as sulfuric acid.94 However, the Keggin type acids cannot
be used as heterogeneous catalysts in polar solvents. The substitution of protons with larger monovalent cations such as
Cs+ gives solid catalysts that are insoluble in water and other
polar solvents.95 For example, CsxH3−xPW12O40 was used as a
heterogeneous catalyst with strong acidity and large surface
area. Heteropoly acids have been widely used in catalytic
systems for biomass conversion.96 Recently, they exhibited
excellent performances in the hydrolysis of cellulose to
glucose. After extraction with organic solvents, the heteropoly
acids could be separated from the homogeneous solution and
dried for the next use.
Shimizu et al.97 reported heteropoly acid (H3PW12O40 and
HSiW12O40) and metal salt (M3(PW12O40)n, M = cation) catalysts for the hydrolysis of cellobiose and ball-milled cellulose
into glucose or sugars. Comparison experiments on the
hydrolysis of cellulose with different acids (heteropoly acids
and mineral acids) revealed that the heteropoly acids show
better hydrolysis activity effects than mineral acids. The total
reducing sugars yield decreased in the following order:
H3PW12O40 > HSiW12O40 > HClO4 > H2SO4 > H3PO4. The
hydrolysis reaction using H3PW12O40 provided TRS and
glucose in 18% and 15% yields, respectively. The conversion of
cellulose, and TRS yield correspond well to the deprotonation
enthalpy (DEP) of these Brønsted acids, such that a stronger
Brønsted acid is more favorable for the hydrolysis of β-1,4-glycosidic bonds in cellulose (Fig. 8). They also used salts of
PW12O403− (Ag+, Ca2+, Co2+, Y3+, Sn4+, Sc3+, Ru3+, Fe3+, Hf 4+,
Ga3+ and Al3+) for the hydrolysis of cellulose. The turnover frequencies (TOFs) for cellulose conversion and TRS formation
first increased with the e/r values (ratio of charge and ionic
radius) and then decreased. Maximum TOFs for both cellulose
conversion and TRS formation were achieved with moderate
Lewis acidity (e/r values) such as for Sn0.75PW12O40.
Wang et al.98 reported the optimization of the hydrolysis
reaction conditions with the H3PW12O40 catalyst such as reaction time, temperature and the amount of cellulose. Microcrystalline cellulose was selected as the starting material. A higher
glucose yield of >50% was obtained with a selectivity of >90%
at 453 K over 2 h. In their recycling experiments, the
H3PW12O40 was extracted from the solution with diethyl ether.
1104 | Green Chem., 2013, 15, 1095–1111
Fig. 8 Effect of deprotonation enthalpy of acid catalyst on (○) TRS yield and
(●) cellulose conversion, for cellulose hydrolysis at 423 K over 2 h. (Reproduced
from ref. 97.)
The total loss of catalyst after six runs was 8.8% of the starting
amount of H3PW12O40.
Apart from the heteropoly acids mentioned above, many
other heteropoly acids have also been developed to catalyze the
hydrolysis reaction. Mizuno et al.99 reported that highly negatively charged heteropoly acids, particularly H5BW12O40, have
stronger acidity than H3PW12O40 for the hydrolysis of non-pretreated crystalline cellulose into saccharides under mild
reaction conditions. The HPAs, H5BW12O40, H5AlW12O40 and
H5GaW12O40, gave different performances and afforded watersoluble saccharides such as glucose and cellobiose (Table 3).
Among these HPAs, H5BW12O40 gave the highest glucose yield
of 77%. The hydrolysis activities of these HPAs were better
than Keggin type acids and conventionally utilized H2SO4 and
HCl. It was worth mentioning that the highly negatively
charged heteropoly acid solution was a better “cellulosesolvent” with strong hydrogen-bond accepting abilities due to
the external metal–oxygen clusters from the anions. When the
crystalline cellulose was immersed in an aqueous solution of
Na5BW12O40 at room temperature for 72 h, the crystallinity of
cellulose decreased. The crystalline peaks almost disappeared
after crystalline cellulose was immersed in an aqueous solution of H5BW12O40 at 20 °C for 24 h, while not much change
was found in the crystallinity on treatment with H2SO4 and
HCl solutions. These results show that the hydrogen bond
accepting ability of H5BW12O40 leading to a decrease in the
crystallinity of cellulose is another factor in the high catalytic
performance beyond the strong acidity. H5BW12O40 was also
used for the hydrolysis of natural lignocellulosic biomass such
as rice plant and Japanese cedar and a mixture of saccharides
with high yields (>77%) was obtained.
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Table 3
99)
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Saccharification of crystalline cellulose in aqueous acidic mediaa (ref.
Concentration
[mol L−1]
Yield [%]
4.5
Entry
Acid
Anion
Proton
Glucose
Cellobiose
1
2b
3
4b
5
6
7
8
9
10
11
H3PW12O40
H3PW12O40
H4SiW12O40
H4SiW12O40
H5BW12O40
H5BW12O40
H5AlW12O40
H5GaW12O40
H6CoW12O40
H2SO4
HCl
0.70
0.60c
0.70
0.70
0.70
0.40
0.70
0.70
0.70
1.75
3.5
2.1
3.5
2.8
3.5
3.5
2.0
3.5
3.5
4.2
3.5
3.5
8
18
37
61
77
4
68
62
59
<1
4
2
2
2
5
5
<1
3
3
5
nd
nd
a
Reaction conditions: crystalline cellulose (100 mg), aqueous acidic
solution (2 mL), 60 °C. Crystalline cellulose was immersed in the
solution at room temperature for 24 h, and then the mixture was
heated at 60 °C for 48 h. Yields are based on the glucose unit in
cellulose used. b The proton concentration was adjusted using H2SO4.
c
Saturated concentration.
As is known, the hydrolysis of cellulose is always restricted
by the poor contact between a catalyst and cellulose. The
hydrolysis reactions always require higher catalyst/substrate
mass ratios, higher temperature and longer reaction times to
achieve maximum conversion, but lead to a lower selectivity to
glucose and lots of side products. Much work has focused on
developing effective approaches or technologies for the
hydrolysis of cellulose. By using heteropoly acids under microwave irradiation, reported by the Mu research group,100 cellulose was completely converted with a high selectivity to
glucose at lower temperature (80–100 °C). The best result for
glucose (75.6%) was obtained by using 88% of H3PW12O40
solution as a catalyst at 90 °C for 3 h. Furthermore, the system
could be applied to the hydrolysis of real lignocellulosic
biomass such as corncob, corn stover and bagasse; 37.2%,
43.3% and 27.8% yields of glucose were obtained, respectively.
Another appealing route was the development of micellar heteropoly acid catalyst [C16H33N(CH3)3]H2PW12O40 with a surfactant group.101 The catalyst greatly improved the solubility of
cellulose in water, and thereby promoted the hydrolysis of cellulose. Following the work, a series of HPA ionic liquids
[C4H6N2(CH2)3SO3H]nH3−nPW12O40 was prepared for the conversion of cellulose to glucose under mild conditions.102 The
catalyst was demonstrated to be effective, and the conversion
of cellulose was 55.1% with a 36% yield of glucose in a water–
MIBK (methyl isobutyl ketone) biphasic system. The catalyst
could be separated and recovered from the reaction mixture
and reused six times without appreciable loss of activity.
The substitution of H+ by larger monovalent Cs+ leads to
insoluble CsxH3−xPW12O40 which shows easier catalyst separation. Wang et al.103 reported the hydrolysis of microcrystalline
cellulose to reducing sugars and glucose. The highest selectivity for glucose was 84% with Cs2.2H0.8PW12O40, while the
highest yield (∼27%) was obtained over CsH2PW12O40. The
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recovery of the heterogeneous catalyst was easier than that of
conventional heteropoly acids.
H-form zeolites
Zeolites are microporous, aluminosilicate minerals that are
commonly used in petrochemistry. They are non-toxic and easy
to recover from solution. They have porous structure that can
accommodate a wide variety of cations, such as H+, Na+, K+,
Mg2+. These cations are loosely bonded to the zeolite surface
and can be released into solution to exhibit different catalytic
activities. H-form zeolites are widely used acid catalysts due to
their shape-selective properties in chemical reactions. The
acidity is related to the atomic ratio of Si/Al; the amount of Al
atoms is proportional to the amount of Brønsted acid sites,
the higher the ratio of Al/Si, the higher the acidity of the
catalyst.104
Onda et al.105 reported the hydrolysis of cellulose with
different types of solid acids in water. H-form zeolites with
various structures and Si/Al ratios were included. However, the
zeolite catalysts with high Si/Al ratio, such as H-beta (75) and
H-ZSM (45), showed higher activities than those with lower Si/
Al ratio such as H-beta (13) and H-mordenite (10) for glucose,
which was in contrast to the acidity density of zeolites. This
may due to the highly hydrophobic character of zeolites with
high Si/Al ratios. The glucose yield was relatively low compared
to AC–SO3H which was about 12%. It may due to the small
pore diameters of zeolites that limited the accessibility and
their weak and less acidic sites. In order to further improve the
catalytic activities of zeolites under milder conditions, Zhao
and Zhang106 proposed a new route by using an ionic liquid as
the solvent to increase the solubility of cellulose, improving
the contact between cellulose and zeolites. Besides, microwave
irradiation (MI) was used to accelerate the hydrolysis rates.
Among the tested zeolites, HY zeolite, with the lowest Si/Al
molar ratio, showed the best hydrolysis performance at 240 W
in 8 min (TRS yield was 47.5% with a glucose yield of 36.9%)
(Table 4). The higher MI power led to the formation of
5-hydroxymethylfurfural (HMF) (∼49% yield under MI power
of 400 W) through consecutive elimination of water during the
hydrolysis of cellulose. Compared with the conventional
heating manner in an oil bath, the TRS yield using microwave
irradiation heating was almost four times that obtained using
oil bath heating. The results strengthened the evidence that
proper solvents and heating methods could greatly improve
the reaction efficiency over solid acids. Finally, application of
this method to other cellulose samples with different origins
or with different degrees of polymerization (DP) also succeeded (Table 5). The higher the DP value of cellulose, the
longer the microwave heating time that was required to reach
the maximal TRS yield. When the DP value increased, the
yields of TRS and glucose dropped slightly.
Further insight into the mutual behavior of zeolites in ionic
liquids was gained by Zhang et al.107 By using XRD characterization, they found that the framework structure of a zeolite
is stable while the enlargement of cell parameters takes place
due to the dilatation effect of an ionic liquid. [BMIm]Cl
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Table 4 The results of the solid acid-catalyzed hydrolysis of Avicel cellulosea
(ref. 106)
Entry
Catalyst
Time/min
Yield of glucose
Yields of TRS
1b
2c
3
4d
5
6
7
8
9e
10
HY
HY
HY
HY
HZSM-5a
HZSM-5b
H-beta
NKC-9
HY
—
600
4
8
15
9.5
9.5
8.5
3.3
8
8
2.1
20.8
36.9
24.2
35.2
33.7
29.6
26.9
4.5
7.1
7.1
23.4
47.5
34.8
42.9
45.3
41.1
38.4
12.7
18.8
a
Conditions: cellulose 100 mg, [C4mim]Cl 2.0 g, H2O 10 ml, catalyst
10 mg, MI at 240 W. b Heated with an oil bath at 100 °C. c With MI at
400 W. d With MI at 80 W. e Heated with an oil bath at 180 °C.
Table 5 Results of HY zeolite-catalyzed hydrolysis of different cellulose
samples (ref. 106)a
Entry
Cellulose type
DP
Time/min
Glucose
yield
TRS
yields
1
2
3
4
5b
α-Cellulose
Avicel cellulose
Spruce cellulose
Sigmacell cellulose
Cellulose
100
220
275
450
—
7
8
9
9.5
1440
34.9
36.9
34.5
32.5
12.5
46.5
47.5
44.4
42.4
—
a
Conditions: cellulose 100 mg in [C4mim]Cl (2.0 g), H2O 10 mg, HY
zeolite 10 mg, heated with MI at 240 W. b Milled cellulose 45 mg,
H-form zeolite catalyst (HZSM-5 or H-beta) 50 mg, H2O 5 ml, heated
using an oil bath at 150 °C for 24 h.
entered into the HY channels and filled the channel space.
FT-IR results showed that H+ cations generated in situ from the
Brønsted acid sites of the zeolites were the key active species
for the hydrolysis of cellulose. According to the characterization, the authors proposed a reaction pathway for HY catalyzed cellulose hydrolysis in IL (Scheme 9). First, the [BMIM]+
of the IL enters into the channels of zeolite HY and reacts with
the acidic sites to replace the H+. After that, the released H+
moves to the surface of cellulose and hydrolysis occurs.
4.6
Magnetic solid acids
As for the real biomass feedstock and practical process for the
hydrolysis of cellulose to glucose, a challenge still exists with
respect to the transformation system. Solid catalysts cannot be
directly separated when solid residues are formed during the
hydrolysis process. Although cellulose can be degraded into
soluble sugars, the lignin components of the actual cellulosic
biomass cannot be converted and humins sometimes form as
solid residues, both of which are difficult to separate from the
recovered solid catalysts. Additionally, the separation of catalysts from the hydrolysis solid residue is important for
the analysis of the recovered catalyst which may be related to
the explanation of the catalytic mechanism. To address the
problem, we developed magnetic sulfonated mesoporous silica
1106 | Green Chem., 2013, 15, 1095–1111
Scheme 9 Proposed reaction pathway and recovery of HY. (Adapted from ref.
107 with permission from Elsevier). The released H+ can also catalyze the side
reaction, dehydrating glucose into HMF. The recovered catalyst could be reused
by simple calcination.
(SBA-15), which can be successfully separated from the reaction system with a permanent magnet, in our recent
work.108,109
The magnetic solid acid was prepared by a surfactant-templated sol–gel method similar to reported methods.110,111
Fe3O4 magnetic nanoparticles were dispersed in the block
copolymer P123 for the co-condensation of tetraethoxysilane
(TEOS). After assembly, 3-(mercaptopropyl)trimethoxysilane
MPTMS was added to introduce mercapto groups. H2O2 was
used to oxidize the mercapto groups into sulfonic acid groups
inside the pores of the mesoporous silica. X-Ray photoelectron
spectroscopy (XPS) analysis of the sample showed that there
was only one peak (169.01 eV) between 158–177 eV in binding
energy, demonstrating that the sulfur element in the catalyst
was completely in the form of sulfonic acid groups
(Scheme 10).
Experiments on the hydrolysis of cellobiose with Fe3O4–
SBA–SO3H showed that the magnetic solid acid gave an even
better performance than sulfuric acid. A proposed explanation
was that the channels in the magnetic solid acid contain concentrated acid sites and the uniform channels allow the reactants to easily enter and interact with these acid sites. Further
hydrolysis of amorphous cellulose ( pretreated with 1-butyl-3methylimidazolium chloride) with Fe3O4–SBA–SO3H gave a
50% yield of glucose. The protons diffused from the SBA-15
channels on the surface of cellulose undergo hydrolysis reaction. When the magnetic solid acid was used for the hydrolysis
of microcrystalline cellulose, the yield of glucose decreased to
26%. Comparative studies revealed that Fe3O4–SBA–SO3H
exhibited an even better hydrolysis performance than AC–
SO3H and Amberlyst-15, presumably due to its higher surface
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Scheme 11
108.)
Table 6
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Scheme 10
Catalyst separation from the reaction mixture. (Adapted from ref.
Hydrolysis of cellulose with MNPs@SiO2–SO3Ha
Preparation of Fe3O4 –SBA–SO3H. (Reproduced from ref. 108.)
area. Also, when corncob was used as lignocellulose biomass
for a hydrolysis reaction, the total reducing sugar (TRS) yield
was 45% (Fig. 9).
The catalyst separation is presented in Scheme 11. The
used Fe3O4–SBA–SO3H can be easily separated from the reaction mixture by using a permanent magnet. The recycled catalyst was washed with 1 M H2SO4 and water/ethanol, and dried
at 80 °C for the next use without deactivation. TEM, XRD and
BET analysis of the used catalyst revealed that the mesoporous
structure is retained and the Fe3O4–SBA–SO3H is robust in the
hydrothermal conditions.
Another appealing work based on a magnetic solid acid was
reported by Ebitani et al.112 The catalyst was prepared by using
CoFe2O4 as the magnetic core and embedded with silica, followed by oxidation of thiol groups to –SO3H groups. It was a
highly active catalyst for the hydrolysis of disaccharides and
polysaccharides. The use of nanoparticles was expected to
overcome the difficulty of the solid–solid reaction and they
were well dispersed in water solution. By introducing magnetic
parts in the catalyst, it can be easily separated from solution.
The hydrolysis of cellulose was conducted at 423 K in water
and the glucose yield was 7%; the TRS yield was 30% which
was similar to that using Amberlyst-15. But the TON number
Fig. 9 Hydrolysis of macrocrystalline cellulose by different solid acids (1.5 g
solid acid catalyst, 1.5 g cellulose, 15 ml H2O, at 150 °C for 3 h). (Reproduced
from ref. 108.)
This journal is © The Royal Society of Chemistry 2013
Catalyst
Acid amount/
mmol g−1
Yield of
TRS
Yield of
glucose
TON
MNPs@SiO2–SO3H
Amberlyst-15
0.5
4.8
30.2
29.3
7.0
6.2
3.8
0.4
a
Conditions: cellulose 0.15 g, catalyst 0.15 g, H2O 1.5 ml, 423 K.
using the MNPs@SiO2–SO3H catalyst was much higher than
that using Amberlyst-15 (3.8 vs. 0.4) (Table 6).
4.7
Supported metal catalysts
Supported metal catalysts have been widely used in biomass
conversion due to their excellent hydrogenation activities.
Numerous works have focused on the conversion of cellulose
into sugar alcohols with supported metal catalysts in the presence of acids.113–115 Cellulose was hydrolyzed to sugars over
acid and then hydrogenated into the sugar alcohols by supported metal catalysts under H2 pressure. However, the
hydrolysis of cellulose to glucose is rarely catalyzed solely by
supported metal catalysts. Recently, Fukuoka et al.116 successfully converted cellulose to glucose by using a supported Ru
catalyst. Various supported materials, including mesoporous
carbon materials (CMKs), carbon black (XC-72), activated
carbon (AC) and C60 were tested and the CMK supported Ru
catalyst (Ru/CMK-3) showed the best performance (the glucose
yield was 34% and the oligosaccharide yield 5%). The loading
amount of Ru on the catalyst had an apparent effect on the
product distribution. When the Ru loading ranged from 2% to
10%, the glucose yield increased from 28% to 34% while the
oligosaccharides decreased from 22% to 5%. The yield of total
reducing sugars was ca. 40% regardless of the Ru loading.
Besides, CMK-3 itself catalyzed the hydrolysis of cellulose in
water at 503 K and gave a 21% yield of glucose and 22% yield
of oligosaccharides (total 43%). The above results show that
CMK-3 is able to convert cellulose into oligosaccharides and
sugars, and Ru plays a very important role in the conversion of
oligosaccharides into glucose. Further experiments on the
hydrolysis of cellobiose with CMK-3 and Ru/CMK-3 confirmed
the suspicion. The hydrolysis of cellobiose with Ru/CMK-3
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Scheme 12
Hydrolysis of cellulose by Ru/CMK-3.
under the optimized conditions yielded 25% glucose, while
the CMK-3 support gave a similar performance to the blank
test (6.5% and 7.9% yield of glucose), which meant that the
support had no activity in the hydrolysis and the Ru species
played the role of hydrolyzing the β-1,4-glycosidic bonds
(Scheme 12). Later work showed that the active Ru species was
RuO2·H2O which was proposed to desorb the hydrated water to
give a Lewis acid site that can depolymerize cellulose to
glucose.117 Another hypothesis was that the Ru species worked
as a Brønsted acid by the heterolysis of water molecules on Ru
which had a low pKa. The new method for the hydrolysis of cellulose without acids has a bright future for the development of
new metal catalysts in the hydrolysis field.
4.8
Others
Recently, many new catalysts have emerged as powerful tools
for the hydrolysis of cellulose to sugars. Zhang and Fang118
reported the effective use of ferrate CaFe2O4 as a heterogeneous catalyst for cellulose hydrolysis. In their work, an
ionic liquid was used for pretreatment to reduce the crystallinity. The reaction was carried out at 423 K over 24 h. The best
yield of glucose was achieved with fresh CaFe2O4 catalyst on
hydrolyzing the pretreated cellulose; the glucose yield was 37%
and the selectivity was 74%. The catalyst could be reused
several times without much drop in the activity.
Hydrotalcite [Mg4Al2(OH)12CO3·4H2O], activated by saturated Ca(OH)2, was used as an inorganic catalyst for the
hydrolysis of cellulose, as reported by Fang et al.119 The reaction was carried out at 423 K over 24 h with ball-milled cellulose; the conversion and glucose selectivity were 46.6% and
85.3%, respectively. The catalyst could be reused 4 times and
the catalytic activity remained. Compared with carbon-based
solid acids, the activated hydrotalcite catalyst is more stable
and can be separated more easily from the reaction mixture.
5.
Green Chemistry
above were aimed at overcoming these limitations. Solid acids
have many advantages over conventional liquid acids, in separation, recycling and being environmental benign. However, the
mass transfer limitations between the insoluble polymers and
heterogeneous catalysts are bottlenecks for the hydrolysis
system. Conceptually, solid acids having higher specific
surface areas, pore sizes/volume and strong acid sites are
prone to exhibit better performances in the reaction. The
number of Brønsted acid sites, affinity with the substrates and
catalyst stability are all key factors in the hydrolysis reaction.
Among the mentioned solid catalysts, carbon-based solid acids
bearing –SO3H, –COOH and –OH groups and cellulosemimetic catalysts have strong affinity to the cellulose chain
and make the acid sites closer to the β-1,4-glycosidic bonds.
Magnetic solid acids are promising materials for the reaction
due to their easier separation and recyclability. Heteropoly
acids have been proven to be active catalysts for cellulose conversion, but their water solubility limits their large-scale application. Incorporation of HPAs into supports or the use of
insoluble salts of HPAs may solve the problem and require
further investigation.
Recent advances in the introduction of ionic liquids, microwave irradiation, supercritical water and nanoparticles to the
cellulose hydrolysis system may allow the reaction to proceed
more easily under milder conditions. Besides, cellulose pretreatment can reduce the degree of polymerization of cellulose
and increase the surface area which would significantly
improve the reaction efficiency and reduce the operation cost.
All these technologies help to overcome the recalcitrance of
cellulose and depolymerize it to a certain extent. Pretreatment
is also the first step in the conversion of lignocellulosic
biomass to chemicals, thus more convenient and efficient pretreatment methods, reaction media and advanced heating
methods should be considered in the hydrolysis of cellulose.
Although cellulose hydrolysis has been developed with
solid acids toward the greener synthesis of sugars, the reaction
efficiency and selectivity still need to be improved. The development of highly acidic catalysts with nanometre size, good
substrate affinity and thermal stability is a challenge towards
practical systems. The production of fuels and chemicals from
biomass requires multidisciplinary collaboration of researchers from enzymatic, homogeneous, and heterogeneous catalysis and the bio-/chemical engineering research areas.
Economic, environmental, and sustainable aspects should all
be considered when designing new catalytic systems.
We
thank
the
973
Program
(2012CB215305,
2012CB215306), NSFC (21172209) and CAS (KJCX2-EW-J02) for
financial support.
Summary and outlook
The hydrolysis of cellulose to glucose is a critical step for the
production of cellulosic bioethanol and many other valueadded molecules. So far, the hydrolysis process cannot be
amplified to industrial scale production due to its low
economy and efficiency. The catalytic systems mentioned
1108 | Green Chem., 2013, 15, 1095–1111
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