Efficient hydrolysis of cellulose over a novel sucralose-derived solid acid with cellulose-binding and catalytic sites

Article abstract of DOI:10.1021/jf405712b

A new sucralose-derived solid acid catalyst (SUCRA-SO₃H), containing -Cl and -SO₃H functional groups, has been shown to be highly effective for hydrolyzing β-1,4-glucans, completely hydrolyzing cellobiose (1) into glucose (2) in 3 h and converting the microcrystalline cellulose pretreated by the ionic liquid into glucose (2) with a yield of around 55% and a selectivity of 98% within 24 h at a relatively moderate temperature (393K). The enhanced adsorption capacity that the catalyst has for glucan by virtue of the presence of chloride groups that act as cellulose-binding sites offers the possibility of resolving the existing bottleneck in heterogeneous catalysis to hydrolyze cellulose, namely, the low accessibility of cellulose to the reaction position in typical solid catalysts. The apparent activation energy for hydrolysis of cellobiose (1) with SUCRA-SO₃H was 94 kJ/mol, which was much lower than that with sulfuric acid (133 kJ/mol) and the corresponding sucrose-derived catalyst (SUCRO-SO₃H) without chlorine groups (114 kJ/mol).

Full text of DOI:10.1021/jf405712b

Article
pubs.acs.org/JAFC
Efficient Hydrolysis of Cellulose over a Novel Sucralose-Derived Solid
Acid with Cellulose-Binding and Catalytic Sites
Shuanglan Hu,† Thomas John Smith,‡ Wenyong Lou,*,†,§ and Minhua Zong§
§
†Laboratory of Applied Biocatalysis and State Key Laboratory of Pulp and Paper Engineering, South China University of
Technology, 381 Wushan Street, Guangzhou 510640, China
‡Biomedical Research Centre, Sheffield Hallam University, Owen Building, Howard Street, Sheffield, S1 1WB, U.K.
S
* Supporting Information
ABSTRACT: A new sucralose-derived solid acid catalyst (SUCRA-SO3H), containing −Cl and −SO3H functional groups, has
been shown to be highly effective for hydrolyzing β-1,4-glucans, completely hydrolyzing cellobiose (1) into glucose (2) in 3 h
and converting the microcrystalline cellulose pretreated by the ionic liquid into glucose (2) with a yield of around 55% and a
selectivity of 98% within 24 h at a relatively moderate temperature (393K). The enhanced adsorption capacity that the catalyst
has for glucan by virtue of the presence of chloride groups that act as cellulose-binding sites offers the possibility of resolving the
existing bottleneck in heterogeneous catalysis to hydrolyze cellulose, namely, the low accessibility of cellulose to the reaction
position in typical solid catalysts. The apparent activation energy for hydrolysis of cellobiose (1) with SUCRA-SO3H was 94 kJ/
mol, which was much lower than that with sulfuric acid (133 kJ/mol) and the corresponding sucrose-derived catalyst (SUCRO-
SO3H) without chlorine groups (114 kJ/mol).
KEYWORDS: cellulose-binding site, cellulose hydrolysis, chloride group, solid acid catalyst
bottleneck remains in the heterogeneous reaction system due
to the low accessibility of the catalyst active group to the
reactive position comprising the β-1,4-glucan linkage in
cellulose. Thus, designing a solid acid catalyst with a
cellulose-binding site to lower the reaction energy is desirable
in order to reduce the energy requirements of the reaction.13
Suganuma et al.14 reported the use of cellulose-derived
amorphous carbon as an efficient catalyst for hydrolyzing
cellulose. This catalyst has a high density of sulfonic acid groups
and abundant hydroxy groups, which can adsorb β-1,4-glucans
to increase substrate concentration near the catalytic active
position to favor the hydrolytic reaction. However, a high
catalyst/cellulose ratio (12:1) was needed to conduct the
reaction. Jacobs et al.15 successfully synthesized a sulfonated
silica−carbon nanocomposite via the evaporation-induced
triconstituent self-assembly of tetraethyl orthosilicate that
takes advantage of a similar adsorption effect, yielding up to
50% glucose (2) at 61% conversion of ball-milled cellulose. A
cellulase-mimetic solid catalyst, sulfonated chloromethyl
polystyrene resin, containing cellulose-binding sites (−Cl)
and catalytic sites (−SO3H), was synthesized by Shuai et al.16
This bifunctional catalyst showed excellent catalytic activity for
hydrolyzing cellulose into glucose (2) and significantly
shortened the reaction time by effectively lowering the
activation energy of the reaction.
INTRODUCTION
■
Production of energy fuels and chemical feedstock from
biomass resources has the potential to reduce CO2 emission
through the use of renewable carbon sources.1 To produce
chemicals either from a mixture of lignocelluloses or from
separated carbohydrate polymers, the breakdown of polymers
into individual sugar molecules must occur.2 Sulfuric acid is the
most widely used catalyst for converting cellulose in industry.3
However, this process is costly, and it is inefficient to separate
the catalyst from the homogeneous reaction mixtures, resulting
in a large consumption of energy and large amounts of waste
products; it also requires the use of expensive, corrosion-
resistant reactors. Furthermore, the largest drawback to using
sulfuric acid is that it also readily degrades glucose (2) at the
high temperatures required for cellulose hydrolysis.4 When the
reaction is performed using a biocatalyst, cellulose is typically
enzymatically hydrolyzed by cellulase.5 This enzyme-catalyzed
approach can be performed under moderate conditions with
high glucose (2) selectivity, but still has significant drawbacks,
including low activity, significant reaction time, costly enzymes,
and unstable enzymes.6 Thus, solid catalysts are widely used,
which show superiority due to a catalytic functional group such
as a Brønsted or Lewis acid to imitate the liquid acid-catalyzed
hydrolysis reaction.7,8
Utilization of heterogeneous catalysts is an effective approach
for biomass conversion and numerous other green chemical
processes due to the ease of separating the catalysts from the
products and the potential of the catalyst to be recyclable.9
Recent developments, such as functionalization of carbon
materials to improve substrate adsorption10 and use of ionic
liquids (ILs) for cellulose dissolution and depolymerization,11,12
are used to address problems of substrate access to the catalyst
that can occur with heterogeneous catalysts. Nonetheless, a
In this study, the chloride group was chosen as the cellulose-
binding site based on a study by Li et al.,17 who found that
hydrochloric acid/1-n-butyl-3-methylimidazolium chloride
Received: August 7, 2013
Revised: January 30, 2014
Accepted: February 10, 2014
Published: February 11, 2014
© 2014 American Chemical Society
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sulfuric acid and water (4 mL) were loaded into a 10 mL flask with a
suitable rubber plug sealed with an aluminum cap to prevent loss of
water by evaporation. After stirring (500 rpm) for 1 min the mixture
was heated for 0−48 h to the temperature specified for each reaction.
The flask was then placed into a mixture of ice and water for 10 min to
stop reaction, and the mixtures (1−1.5 mL) were transferred into a 2
mL microcentrifuge tube and separated by centrifugation (1150g, 10
min) . Aliquots (100 μL) were withdrawn from the supernatant prior
to HPLC analysis. The residual solids in the pipet, microcentrifuge
tube, and flask were all collected together by filtration. Reasonable
leaching water volume was heated to 80 °C to elute the remaining
sugars adsorbed on the surface of the catalyst. The resultant liquor
volume was concentrated down to 2 mL for subsequent HPLC
analysis. In the reactions containing sulfuric acid, the solution was
neutralized with CaCO3 powder after the reaction, the CaSO4
precipitate was removed by centrifugation, and the supernatant
solution was analyzed. All reactions were performed in duplicate,
and yields of glucose (2) had margins of error of up to 3.4%.
Saccharide Adsorption Capacity of the Prepared Catalysts.
Glucose (2) (600 mg) or cellobiose (1) (600 mg) was added into 12
mL of water and vortexed. The mixture was divided into 12 equal
portions in microtubes. Then 100 mg of the catalyst was loaded into
each microtube, and the contents were mixed well. All microtubes
were kept at room temperature and vortexed for 2 min every 5 min.
One microtube was taken and centrifuged at each of the following
reaction times: 10, 20, 30, 60, 90, and 120 min. 2 and 1 in the
supernatant were determined via high performance ion chromatograph
to calculate the adsorption capacity of the catalyst. Other catalysts
were treated in the same manner with a reaction time of for 2 h to
investigate their adsorptive properties. Error bars represent standard
deviations from three experiments.
Hydrolysate Analysis. Glucose (2) and cellobiose (1) were
determined using an HPLC system consisting of a Waters 515 HPLC
pump, a Bio-Rad organic acid column, and a Waters 2410 refractive
index detector. The column used was a 300 mm × 7.8 mm i.d., Aminex
HPX-87H (BIO-RAD Laboratories, Hercules, CA, USA). The mobile
phase consisted of 5 mmol/L sulfuric acid aqueous solution with a
flow rate of 0.5 mL/min. The column temperature was 65 °C. The
cellulose and xylan contents were calculated from glucose (2) and
xylose (3) contents multiplied by conversion factors of 0 0.90 and
0.88, respectively. The concentration of each compound in the liquid
phase was determined using calibration curves obtained by analyzing
standard solutions with known concentrations. TOC (total organic
carbon), which is mostly composed of water-soluble polysaccharide
and products from the dehydration of saccharides, was analyzed using
(C4mim·Cl) was an efficient system for hydrolyzing lignocellu-
losic biomass. These authors attributed the excellent behavior
of the system to the dispersion of most of the cellulose and
hemicellulose molecules within the ionic liquid. Amarasekara et
al.18 successfully immobilized the sulfonic acid functionalized
ionic liquid, C4mim·Cl, on a silica surface to synthesize a silica-
supported acid catalyst, showing that an effective heteroge-
neous acid catalyst can be produced by tethering an
imidazolium moiety functionalized by attachment of a sulfonic
acid group to a solid support. To better understand the
mechanism of cellulose solvation by the above-mentioned ionic
liquid, Remsing et al.19 conducted a study and found that there
was a 1:1 stoichiometry for the hydrogen-bonded interaction
between the hydroxyl protons of carbohydrate and the chloride
ions of ionic liquid. The chloride concentration in this system
was as high as 20%. To expand on this bifunctional strategy, we
have produced a unique artificial catalyst that mimics the
structure of not only the catalytic active sites found in
homogeneous acid catalysis but also the active sites of enzymes
in the biocatalytic transformation of lignocellulose. In this
present study, sucralose was chosen as a satisfactory carbona-
ceous material for the introduction of chlorine groups in
carbon-based solid acid catalysts that can be prepared
expediently by carbonization and sulfonation of various sugar
in a manner that gives a relatively stable solid acid catalyst for
hydrolyzing cellulose to glucose (2).14 The incorporation
ability of hydrophilic molecules will make it more easy for a
cellulose chain in solution to be in contact with the −SO3H
groups in the carbon material, thus giving rise to higher
catalytic performance.20 We propose that this study will enable
the future rational design of new catalytic strategies by bonding
chloride as a functional group to promote binding of cellulose
to catalysts to overcome the intrinsic bottleneck of biomass
hydrolysis, the severe mass-transfer limitation hampering
progress, and thus enable the development of methods for
processing widely available water-insoluble polymeric biomass
under heterogeneous conditions.
EXPERIMENTAL SECTION
■
Materials. Sucralose (4) (biological reagent grade) was purchased
from Shiyuanye Co., Ltd. (Shanghai, China). Sucrose was purchased
from Jinhuada Chemical Reagent Co., Ltd. (Guangzhou, China).
Microcrystalline cellulose (Avicel PH101) was obtained from FMC
(Philadelphia, PA), and its degree of polymerization (DP) was 220.
Starch (29.0% amylose content), was supplied by Huanglong Food
Industry Co., Ltd. (Jilin, China). Rice straw and hardwood obtained
locally was mechanically powdered to a particle size of 50−400 μm.
C4mim·OAc was from Lanzhou Institute of Chemical Physics
(Lanzhou, China). Cellulase from Trichoderma reesei (6 U/mg) was
bought from Sigma (St. Louis, MO, USA). All other chemicals were
obtained from commercial sources and were of the highest grade
available.
a model 5000A TOC Analyzer (Shimadu, Tokyo, Japan). YWSOC
,
which represents the yield of total amount of water-soluble organic
compounds in the reaction liquid, was calculated based on the data
from TOC analysis. Concentration curves ranging from 1 to 150 ppm
were prepared from a TOC standard solution (100 ppm) purchased
from Sigma with a resulting R2 value of 0.998.
Glucose yields were calculated as follows:
glucose yield (%) = mol of glucose (produced)
{[
]
/(theoretical yield of glucose in mol) × 100
}
Glucose selectivity was calculated on the basis of TOC results as
Catalyst Preparation and Characterization. The SUCRA-
SO3H catalyst and SUCRO-SO3H catalysts were prepared from
sucralose (4) and sucrose (5), respectively, according to a method
similar to that reported previously by Hara et al.14 Characterization of
the carbon-based solid acid catalysts was carried out by a combination
of Fourier transform infrared spectroscopy (FT-IR), X-ray photo-
electron spectroscopy (XPS), Branauer−Emmett−Teller analysis
(BET), scanning electron microscopy (SEM), temperature-pro-
grammed desorption of ammonia anaysis (NH3-TPD), and acid−
base titration.
follows:
glucose selectivity (%)
= (mol of carbon present in the form of glucose)
/(mol of total water − soluble organic carbon)
Estimation of the Activation Energy of Glycosidic Bond
Cleavage. The activation energy of cellobiose (1) hydrolysis using
the sucralose- and sucrose-derived carbon-based solid acid catalysts
was studied by comparing the reaction rate at four temperatures (373
K, 383 K, 393 K, and 403 K). Results were plotted as Arrhenius plots
in the form of rate constants at each temperature, and activation
Hydrolysis of Cellobiose and Cellulose. The standard reaction
conditions for the hydrolysis reactions were as follows: cellobiose (1)
(0.1 g) or cellulose (0.05 g), catalyst (0.1 g) or 1 mL of 4% dilute
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energies were calculated by multiplying the slope by the gas constant
R.
Reuse Experiment. The catalyst reuse experiments were
performed at 393 K. Reaction conditions were as follows: 0.1 g of
cellobiose, 0.1 g of catalyst, 4 mL of deionized water, and reaction time
3 h, reaction temperature 393 K. The solid catalysts were washed by
filtration with distilled water until sulfate ions could no longer be
detected in the washings, after which the catalyst was reused in the
next cycle of the reaction.
RESULTS AND DISCUSSION
■
Fourier transform infrared (FT-IR) spectra of the catalysts
displayed peaks at approximately 1180 cm−1 and 1030 cm−1,
which are typically attributable to the stretching modes of
SO3H groups. X-ray photoelectron spectroscopy (XPS)
revealed only one peak at 168 eV, assigned to the SO3H
groups, implying that all sulfur in the carbon catalyst is
contained in the SO3H groups. The elemental compositions of
the catalysts were measured using the XPS data in order to
determine the content of sulfonic acid group. The amount of
sulfur per gram of catalyst was 0.28 mmol for the sucralose-
derived catalyst (SUCRA-SO3H), 0.47 mmol for the sucrose-
derived catalyst (SUCRO-SO3H), and 0.67 mmol for the
sulfonated sucralose. As expected, the carbonized sucralose
contained no detectable sulfur. The specific surface area of
SUCRA-SO3H catalyst was 0.317 m2/g, which was similar to
the 0.282 m2/g measured for the SUCRO-SO3H catalyst.
Sulfonated sucralose showed a clear advantage in the amount of
sulfonyl group attachment compared to other catalysts,
indicating that both the carbonization and sulfonation
processes cause a decrease in chloride group content, negatively
affecting adsorption ability and reaction results. Here, we
evaluated the best catalyst among different materials that were
capable of hydrolyzing polysaccharide. Both SUCRO-SO3H
and SUCRA-SO3H catalysts are composed of irregular shaped
amorphous carbon particles with layer structures, and the size
of catalyst particles was between 0.4 and 1.0 mm. However,
there were no obvious mesoporous or microporous particles
observed in the SEM micrographs. The acidic properties of
SUCRO-SO3H, SUCRA-SO3H, and carbonated sucralose were
investigated by temperature-programmed desorption of ammo-
nia analysis. Both samples of SUCRO-SO3H and SUCRA-
SO3H show a distinct broad peak at around 473 K due to the
presence of abundant hydroxyl groups. The curve of
incompletely carbonated sucralose shows a little higher
intensity and desorption in the temperature range of 723−
923 K than the other two samples, due to the more severe
decomposition of the solid sample. Both SUCRO-SO3H and
SUCRA-SO3H catalysts showed the presence of medium acid
sites, as their respective desorption temperatures are around
623 K, which corresponded to the carboxyl groups.
Figure 1. Time courses for D-glucose formation from cellobiose.Yield
of glucose (2) (circular symbols) and selectivity of glucose (2) (square
symbols and triangles) during hydrolysis of cellobiose (1) over
SUCRA-SO3H (closed symbols) and SUCRO-SO3H (open symbols).
To investigate the performance of the new catalysts in
comparison to other catalytic systems, their performance was
compared with that of sulfonated sucralose, sulfonated sucrose,
Amberlyst-15, and niobic acid (Figure 2). Only trace amounts
Figure 2. Hydrolysis of cellobiose (1) catalyzed by various catalysts.
of cellobiose (1) were converted over niobic acid, presumably
due to the lack of efficient acid catalytic sites,21 while the
catalysts bearing Brønsted acid sites (Amberlyst-15 and the
sulfonated carbon materials) generally exhibited good catalytic
performance. Furthermore, the addition of halogen as cellulose
binding sites in SUCRA-SO3H significantly enhanced the
reaction activity compared to the SUCRO-SO3H catalyst,
which does not contain halogen.
This encouraging progress could be attributed to increases in
adsorption capacity. Catalysts with cellulose binding sites are
thought to be superior at adsorbing cellobiose (1) (the shortest
β-1,4-linked polysaccharide, which is often used to model the
cellulose hydrolysis reaction) and 2 (the expected product),
since the chloride group is a strong hydrogen bond acceptor
that may enter into hydrogen bonding with the hydroxyl groups
on cellulose, cellobiose (1), and glucose (2). To confirm this
conjecture, 0.1 g portions of the solid catalyst were added to
separate 2 mL aliquots of aqueous cellobiose (1) and glucose
(2) solutions (50 g/L), and the mixture was oscillated at room
temperature (298 K). As shown in Figure 3, 21.5% of the added
cellobiose (1) was absorbed by the SUCRA-SO3H catalyst
within 2 h, which was higher than the value of 18% absorbed by
the SUCRO-SO3H catalyst, and the Cl-bearing catalyst
SUCRA-SO3H required a shorter time before adsorption
equilibrium was reached compared to the Cl-free SUCRO-
SO3H catalyst. Similarly, the SUCRA-SO3H catalyst absorbed
8.5% of the added glucose (2), which was more than the 5%
absorbed by the SUCRO-SO3H catalyst and agrees with the
Initially, we tested the capacity of the catalysts to hydrolyze
β-1,4-glycosidic bonds by using cellobiose (1) as the substrate,
which is commonly used as model compound for cellulose.
Figure 1 shows time courses for D-glucose (2) formation from
cellobiose (1) by the SUCRA-SO3H catalyst. As a comparison,
the results for the SUCRO-SO3H catalyst are also included.
Conversion of cellobiose (2) reached 100% after 3 h with both
catalysts. Taking the sulfur content of the catalysts into account,
which based on the XPS results equates to the content of
sulfonic acid groups in the catalysts, the SUCRA-SO3H catalyst
(containing 0.28 mmol/g of sulfur) performs much better than
the higher sulfur content SUCRO-SO3H catalyst (containing
0.47 mmol/g of sulfur).
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group loading. Although niobic acid showed the third greatest
saccharide-binding capacity, possibly caused by swelling of the
solid acid catalyst, it exhibited poor catalysis presumably owing
to its low acidity (70% that of sulfuric acid). However,
Amberlyst-15, with its strong acid groups, performed
conversely, indicating that only when a certain amount of
acid groups and requisite electron pair acceptor are present on
the catalyst will a highly effective catalyst result.22
Catalytic activity was further investigated by examining the
effectiveness of the catalysts for cellulose and starch hydrolysis.
Compared to starches, cellulose is more difficult to hydrolyze
due to its crystalline form and the abundant hydrogen bonding
present in the cellulose molecule, and the β-glycosidic linkage
of cellobiose (1) among the various types of simple
disaccharides is one of the most recalcitrant bonds to acid
hydrolysis compared to maltose, sucrose, lactose, and
xylobiose.23 As shown in Table 1, in the case of starch, the
carbon-based catalyst SUCRA-SO3H showed a glucose (2)
yield of approximately 90%, with an equivalent amount of
fructose observed after a 6 h reaction. For hydrolysis of
unpretreated cellulose, however, only 4% glucose (2) yield was
achieved after 24 h of reaction. Nonetheless, the glucose (2)
yield from this reaction could be increased to 53% by
converting the water-soluble polysaccharide in the resulting
reaction liquid through a 48 h enzyme hydrolysis, and 48%
saccharide was detected in the final product according to the
TOC result based on mass.
To improve the accessibility of the catalytic sites of the
catalyst to the β-1,4-glycosidic bond in cellulose, we pretreated
cellulose with the ionic liquid C4mim·OAc and then
regenerated it with water. It is generally accepted that the
primary mechanism for dissolution of cellulose by ionic liquids
proceeds through disruption of the interstrand hydrogen-
bonding network between adjacent polymer chains, and
C4mim·OAc has been found to be an efficient ionic liquid for
reducing the crystallinity of cellulose.24 XRD analysis indicated
that the amorphous region increased greatly in the regenerated
cellulose, and thereby it is possible that the glycosidic bonds in
the substrates become more exposed to the catalysts. Three
diffraction peaks, at around 2θ of 16°, 22°, and 34°,
respectively, for untreated microcrystalline cellulose, but only
one weak peak remained at around 22° after treatment with
C4mim·OAc. The most important result of pretreatment is that
it enables maximum sugar yield and minimizes the loss of
sugars and the subsequent formation of inhibitory products.
The yield of glucose (2) increased up to 55% with a 98%
Figure 3. Adsorption curve of glucose (2) (circular symbols) and
cellobiose (1) (square symbols) onto SUCRA-SO3H catalyst (closed
symbols) and SUCRO-SO3H catalyst (open symbols) in aqueous
solution. 100% adsorption would correspond to binding of 50 mg of
carbohydrate to 100 mg of catalyst in a volume of 1 mL.
results of Suganuma et al.21 and Shuai et al.16 The ability of the
catalysts to take up large amounts of saccharides is likely caused
by the simultaneous incorporation of water and adsorption of
saccharides. It is possible that the formation of hydrogen bonds
occurs between the hydroxyl groups of cellulose and not only
the chloride groups but also the hydroxyl groups on catalyst as
minor electronegative groups. Furthermore, the stronger
electrophilicity of surface chloride groups than hydroxyl groups
on the carbon affords better adsorption capacity.
Further, the adsorption capacity of the SUCRA-SO3H
catalyst was compared with those achieved with other
heterogeneous catalysts (Figure 4). Among the range of
Figure 4. The adsorption capacity of the tested catalysts. 100%
adsorption would correspond to binding of 50 mg of carbohydrate to
100 mg of catalyst in a volume of 1 mL.
catalysts tested, the sulfonated sucralose shows the best
adsorption ability, which correlates with its highest chloride
Table 1. Hydrolytic Degradation of Polysaccharides to Glucose Using Various Catalysts
a
b
c
d
entry
catalyst
substrate
time (h)
temp (K)
YGlu (%)
YWSOC (%)
SGlu (%)
1
2
SUCRA-SO3H
SUCRA-SO3H
SUCRA-SO3H
SUCRA-SO3H
SUCRA-SO3H
SUCRO-SO3H
SUCRO-SO3H
Amberlyst-15
niobic acid
cellulose (Avicel PH101)
IL-pretreated cellulose
IL-pretreated cellulose
IL-pretreated cellulose
starch
24
24
24
24
6
393
393
413
423
383
393
393
393
393
393
4
55
46
41
92
3
48
56
63
67
96
26
49
3
8
98
73
61
47
11
97
−
3
4
5
6
cellulose (Avicel PH101)
IL-pretreated cellulose
IL-pretreated cellulose
IL-pretreated cellulose
IL-pretreated cellulose
24
24
24
24
30
7
49
−
−
8
9
−
34
−
75
10
dilute sulfuric acid
25
a
b
c
The optimum reaction time for the hydrolysis. YGlu was the yield of glucose. YWSOC was the yield of water-soluble organic compounds determined
d
via a TOC analyzer. SGlu was the selectivity of glucose on tested catalysts. − represents “not detected”.
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selectivity for regenerated cellulose using SUCRA-SO3H
catalyst at an optimum reaction time of 24 h and 393 K,
which was superior to that with SUCRO-SO3H catalyst (49%)
at an optimum reaction time of 24 h and 393 K and that with
dilute sulfuric acid (25%) at an optimum reaction time of 30 h
and 393 K (Table 1). Additionally, in comparison with other
liquid acid catalysts reported previously such as maleic acid25
and phosphoric acid,26 our prepared SUCRA-SO3H catalyst
gave a higher yield of glucose (2) in hydrolysis of cellulose.
Figure 5A−C illustrates the effects of reaction time on
drop in the yield and selectivity of glucose (2) (Figure 5A). The
YWSOC with SUCRA-SO3H showed only slight increase when
reaction time was more than 24 h, suggesting that there is a
recalcitrant portion of insoluble cellulose present in the
cellulose that is not accessible to hydrolysis. This similar
trend was seen with increasing reaction time in hydrolysis of
cellulose catalyzed by SUCRO-SO3H (Figure 5B). Obviously,
the optimum reaction time for hydrolysis of cellulose using
SUCRA-SO3H or SUCRO-SO3H was 24 h. In the case of dilute
sulfuric acid, the yield and selectivity of glucose (2) reached
maximum values at reaction time of 30 h, being the optimum
reaction time for the reaction. Similarly, no significant variation
in the YWSOC was observed when reaction time was further
prolonged. Although both carbon-based catalysts and dilute
acid suffered from an undesirable side reaction, namely, the
conversion of glucose (2) into hydroxymethyl furfural (HMF),
the yield of glucose (2) showed a sharper decline with
prolonged reaction time in the case of the dilute acid catalyst, in
comparison with SUCRA-SO3H and SUCRO-SO3H. The
result suggests that the formed glucose (2) from cellulosic
biomass may be less converted into byproducts by the carbon-
based catalysts. When reaction temperature was elevated, the
formed glucose (2) was further converted into HMF with high
concentrations, thus lowering the selectivity of glucose (2)
(Table 1). We also performed the reaction using rice straw and
hardwood (Table 2) as commercially significant lignocellulosic
substrate to check the practicability of tested catalysts. When
rice straw pretreated with C4mim·OAc was hydrolyzed by the
SUCRA-SO3H catalyst, the obtained yield of glucose (2) was
much less than that of hydrolysis of pure cellulose similarly
pretreated under the same conditions. As evident from the data
summarized in Table 2, rice straw was more easily degraded by
the SUCRA-SO3H and SUCRO-SO3H catalysts than hard-
wood. SUCRA-SO3H slightly surpassed SUCRO-SO3H for
hydrolysis of both rice straw and hardwood. Hemicellulose as
an important component of ligincellulose can be hydrolyzed by
our prepared solid acid catalysts as well as by liquid acid
catalysts.27 Interestingly, both the cabon-based catalysts
preferentially hydrolyzed hemicellulose.
Figure 5. Hydrolysis of cellulose pretreated with C4mim·OAc over
various catalysts: (A) SUCRA-SO3H, (B) SUCRO-SO3H, and (C)
dilute sulfuric acid.
It has been reported that cellulose hydrolysis can be modeled
based on a pseudo-first-order reaction followed by degradation
of the generated glucose (2).28 The reaction rate constant
obeys modified Arrhenius equations, including effects of
temperature and acid concentration. The advantage of
adsorbing the substrate onto the catalyst surface is that it will
increase the opportunity for the substrate to come into contact
with the local acid moieties dispersed on the surface of the
amorphous carbon sheet, thereby increasing the rate of the
reaction associated by reducing activation energy (Figure 6).
Kinetic studies showed that the apparent activation of
cellobiose (1) hydrolysis was 94 kJ/mol at 373−403 K for
hydrolysis of cellulose pretreated by ionic liquid with
SUCRA-SO3H, SUCRO-SO3H, and dilute sulfuric acid as the
catalyst, respectively. It was found that the reaction time had
significant influence on yield of glucose (2), selectivity of
glucose (2), and yield of water-soluble organic compounds
(YWSOC) in hydrolysis of cellulose using various catalysts. The
yield and selectivity of glucose (2) and the YWSOC with SUCRA-
SO3H increased clearly with increasing reaction time up to 24
h, but a further increase in reaction time resulted in a significant
Table 2. Hydrolytic Degradation of Lignocellulose to Glucose Using SUCRA-SO3H
yield (%)
composition of regenerated lignincellulose (%)
SUCRA-SO3H
SUCRO-SO3H
a
b
c
a
b
c
raw material
cellulose
hemic-ellulose
lignin
ash
YGlu
YXyl
YWSOC
YGlu
YXyl
YWSOC
rice straw
hardwood
47.7
54.3
15.6
16.5
14.5
14.4
16.4
1.3
19.5
12.7
57.3
44.4
36.0
32.0
16.5
11.2
56.0
39.8
34.5
29.8
a
b
YGlu was the maximum yield of glucose based on the weight of cellulose in regenerated lignincellulose. YXyl was the maximum yield of xylose (3)
c
based on the weight of hemicellulose in regenerated lignincellulose. YWSOC was the yield of water-soluble organic compounds determined via a TOC
analyzer.
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Journal of Agricultural and Food Chemistry
Article
carbonization, which was similar to the observation reported by
Siril et al.29 In fact, it is known that aromatic carbon−SO3H
bonds in sulfoaromatic compounds bearing electron-with-
drawing functional groups such as halogens, COOH, and
phenol OH are more stable against loss of the sulfonyl group as
a result of the increased electron density between the carbon
and sulfur atoms. Moreover, the prepared SUCRA-SO3H
catalyst was incubated for 24 h at a relatively higher
temperature of 413 K to evaluate its thermal stability, and
still maintained more than 80% of its initial activity, showing
good stability which was comparable to that of the catalyst
Amberlyst 35.23 A possible mechanism for the hydrolysis
process over the solid acid catalysts prepared is that the
stronger affinity for β-1,4-glucan of the novel carbon-based
catalyst than the traditional solid acid catalysts may provide
better access by reactants to the −SO3H groups in the carbon
material to cleave the β-1,4-glucosidic bonds. Catalysis over the
tentative carbon catalyst with −SO3H, −COOH, −Cl, and
phenolic −OH groups thus resulted in high catalyst activity.
Figure 6. Arrhenius plot of cellobiose (1) hydrolysis over (A)
SUCRA-SO3H and (B) SUCRO-SO3H. Rate constant k was calculated
on the basis of the average rate during the initial reaction period
(<50% conversion).
SUCRA-SO3H catalyst (Figure 6A), which is significantly lower
than values for microcrystalline cellulose-derived carbon
catalyst (110 kJ/mol),21 SUCRO-SO3H catalyst (114 kJ/mol)
(Figure 6B), and sulfuric acid (133 kJ/mol).16
Figure 7 shows the operational stability of the resulting solid
catalysts by conducting sequential hydrolytic reactions with the
ASSOCIATED CONTENT
■
S
* Supporting Information
Chlorine and sulfur content of the four catalysts (SUCRO-
SO3H, SUCRA-SO3H, carbonized sucralose, and sulfonated
sucralose) (Table S1). BET parameters of SUCRO-SO3H and
SUCRA-SO3H (Table S2). Detail equation parameters of lnk-
1/T curve (Table S3). The pH of the reaction system after
removal of the catalyst for each run (Table S4). FT-IR spectra
and S2p curves from XPS analysis of SUCRO-SO3H catalyst
and SUCRA-SO3H catalyst (Figures S1 and S2). XRD analysis
of cellulose substrate before and after pretreatment using ionic
liquid (Figure S3). Nitrogen adsorption/desorption isotherms
and scanning electron microscopy images of SUCRO-SO3H
and SUCRA-SO3H (Figures S4 and S5). The NH3-TPD
profiles of SUCRO-SO3H catalyst, SUCRA-SO3H catalyst, and
carbonized sucralose (Figure S6). Glucose (2) and xylose (3)
yields of hydrolysis of IL-pretreated straw catalyzed by SUCRA-
SO3H and SUCRO-SO3H with time (Figure S7). This material
is available free of charge via the Internet at http://pubs.acs.org.
Figure 7. Reusability of SUCRA-SO3H and sulfonated sucralose in
hydrolysis of cellobiose. The relative activity for both catalysts tested
prior to use is defined as 100%, and activity is shown after each cycle.
same sample of catalyst using cellobiose (1) as substrate at 393
K for 3 h reaction time per cycle. The SUCRA-SO3H catalyst
showed no significant loss in activity after successive reuse of six
batches, but an obvious loss in activity was observed after seven
batches, and the SUCRA-SO3H catalyst remained around 56%
of the original activity after being repeatedly used for 10
batches, which was attributable to the partial leach of SO3H
groups from the catalyst with the increase of reuse batch. We
subsequently detected the pH of the reaction system after the
removal of the SUCRA-SO3H catalyst by filtration during the
course of each run, including those at the beginning and the
end. The pH of the reaction system showed only a slight fall
after successive reuse of six batches, but manifested a relatively
obvious decrease after seven batches, particular at the end of
the tenth batch, implying the partial leach of SO3H groups into
the reaction system to form free acids through possible self-
hydrolysis of the sulfonated catalysts, which was also confirmed
by analyzing the S content of the catalyst (data not shown).
This could well explain the observation that the SUCRA-SO3H
catalyst indicated relative instability after seven batches of reuse.
Moreover, as can be seen in Figure 7, the stability of the
SUCRA-SO3H catalyst prepared by carbonization and sulfona-
tion was clearly superior to that of the sulfonated sucralose,
possibly due to the increase of the COOH groups and the
rigidity of the carbon skeleton structure in SUCRA-SO3H by
AUTHOR INFORMATION
Corresponding Author
■
*Tel: +86-20-22236669. Fax: +86-20-22236669. E-mail:
wylou@scut.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We wish to thank the National Science Found for Excellent
Young Scholars (21222606), the State Key Program of
National Natural Science Foundation of China (21336002),
the National Key Basic Research Program of China
(2013CB733500), and the Fundamental Research Funds for
SCUT (2013ZG0003) for partially funding this work.
REFERENCES
■
(1) Chu, S.; Majumdar, A. Opportunities and challenges for a
sustainable energy future. Nature 2012, 488, 294−303.
(2) Dhepe, P. L.; Fukuoka, A. Cellulose conversion under
heterogeneous catalysis. ChemSusChem 2008, 1, 969−975.
(3) Geboers, J. A.; Vyver, S.; Van de; Ooms, R.; Beeck, B. Op de;
Jacobs, P. A.; Sels, B. F. Chemocatalytic conversion of cellulose:
1910
dx.doi.org/10.1021/jf405712b | J. Agric. Food Chem. 2014, 62, 1905−1911

Journal of Agricultural and Food Chemistry
Article
opportunities, advances and pitfalls. Catal. Sci. Technol. 2011, 1, 714−
726.
on cellulose crystalline structure using 1-ethyl-3-methylimidazolium
acetate. J. Phys. Chem. B 2012, 116 (33), 10049−10054.
(25) Mosier, N. S.; Ladisch, C. M.; Ladisch, M. R. Characterization of
acid catalytic domains for cellulose hydrolysis and glucose degradation.
Biotechnol. Bioeng. 2002, 79, 610−618.
(26) Palkovits, R.; Tajvidi, K.; Procelewska, J.; Rinaldi, R.; Ruppert,
A. Hydrogenolysis of cellulose combining mineral acids and
hydrogenation catalysts. Green Chem. 2010, 12, 972−978.
(27) Kim, Y.; Hendrickson, R.; Mosier, N.; Ladisch, M. R. Reaction
mechanisms and kinetics of xylo-oligosaccharide hydrolysis by
dicarboxylic acids. AIChE J. 2012, 59 (1), 188−199.
(28) Vyver, S.; Van de; Geboers, J.; Jacobs, P. A.; Sels, B. F. Recent
advances in the catalytic conversion of cellulose. ChemCatChem 2011,
3, 82−94.
(29) Siril, P. F.; Cross, H. E.; Brown, D. R. New polystyrene sulfonic
acid resin catalysts with enhanced acidic and catalytic properties. J.
Mol. Catal. A: Chem. 2008, 279, 63−68.
(4) Mosier, N.; Sarikaya, S. A.; Ladisch, C. M.; Ladisch, M. R.
Characterization of dicarboxylic acids for cellulose hydrolysis.
Biotechnol. Prog. 2001, 17 (3), 474−480.
(5) Berlin, A.; Maximenko, V.; Gilkes, N.; Saddler, J. Optimization of
enzyme complexes for lignocellulose hydrolysis. Biotechnol. Bioeng.
2007, 97, 287−296.
(6) Zhang, Z.; Donaldson, A. A.; Ma, X. Advancements and future
directions in enzyme technology for biomass conversion. Biotechnol.
Adv. 2012, 30, 913−919.
(7) Shimizu, K.; Satsuma, A. Toward a rational control of solid acid
catalysis for green synthesis and biomass conversion. Energy Environ.
Sci. 2011, 4, 3140−3153.
(8) Kobayashi, H.; Ohta, H.; Fukuoka, A. Conversion of
lignocellulose into renewable chemicals by heterogeneous catalysis.
Catal. Sci. Technol. 2012, 2, 869−883.
(9) Takagaki, A.; Tagusagawa, C.; Domen, K. Glucose production
from saccharides using layered transition metal oxide and exfoliated
nanosheets as a water-tolerant solid acid catalyst. Chem. Commun.
2008, 42, 5363−5365.
(10) Onda, A.; Ochi, T.; Yanagisawa, K. Hydrolysis of cellulose
selectively into glucose over sulfonated activated-carbon catalyst under
hydrothermal conditions. Top. Catal. 2009, 52, 801−807.
(11) Muhammad, N.; Man, Z.; Khalil, M. A. B. Ionic liquid-a future
solvent for the enhanced uses of wood biomass. Eur. J. Wood Prod.
2011, 70, 125−133.
(12) Ignatyev, I. A.; Van Doorslaer, C.; Mertens, P. G. N.;
Binnemans, K.; De Vos, D. E. Reductive splitting of cellulose in the
ionic liquid 1-butyl-3-methylimidazolium chloride. ChemSusChem
2010, 3, 91−96.
(13) Bansal, P.; Vowell, B. J.; Hall, M.; Realff, M. J.; Lee, J. H.;
Bommarius, A. S. Elucidation of cellulose accessibility, hydrolysability
and reactivity as the major limitations in the enzymatic hydrolysis of
cellulose. Bioresour. Technol. 2012, 107, 243−250.
(14) Suganuma, S.; Nakajima, K.; Kitano, M.; Yamaguchi, D.; Kato,
H.; Hayashi, S.; Hara, M. Synthesis and acid catalysis of cellulose-
derived carbon-based solid acid. Solid State Sci. 2010, 12, 1029−1034.
(15) Van de Vyver, S.; Peng, L.; Geboers, J.; Schepers, H.; de Clippel,
F.; Gommes, C. J.; Goderis, B.; Jacobs, P. A.; Sels, B. F. Sulfonated
silica/carbon nanocomposites as novel catalysts for hydrolysis of
cellulose to glucose. Green Chem. 2010, 12, 1560−1563.
(16) Shuai, L.; Pan, X. J. Hydrolysis of cellulose by cellulase-mimetic
solid catalyst. Energy Environ. Sci. 2012, 5, 6889.
(17) Li, C. Z.; Wang, Q.; Zhao, Z. B. Acid in ionic liquid: An efficient
system for hydrolysis of lignocellulose. Green Chem. 2008, 10, 177−
182.
(18) Amarasekara, A. S.; Owereh, O. S. Synthesis of a sulfonic acid
functionalized acidic ionic liquid modified silica catalyst and
applications in the hydrolysis of cellulose. Catal. Commun. 2010, 11
(13), 1072−1075.
(19) Remsing, R. C.; Swatloski, R. P.; Rogers, R. D.; Moyna, G.
Mechanism of cellulose dissolution in the ionic liquid 1-n-butyl-3-
methylimidazolium chloride: a 13C and 35/37Cl NMR relaxation
study on model systems. Chem. Commun. 2006, 1271−1273.
(20) Huang, Y. B.; Fu, Y. Hydrolysis of cellulose to glucose by solid
acid catalysts. Green Chem. 2013, 15, 1095−1101.
(21) Suganuma, S.; Nakajima, K.; Kitano, M.; Yamaguchi, D.; Kato,
H.; Hayashi, S.; Hara, M. Hydrolysis of cellulose by amorphous carbon
bearing SO3H, COOH, and OH groups. J. Am. Chem. Soc. 2008, 130,
12787−12793.
(22) Kitano, M.; Yamaguchi, D.; Suganuma, S.; Nakajima, K.; Kato,
H.; Hayashi, S.; Hara, M. Adsorption-enhanced hydrolysis of β-1,4-
glucan on graphene-based. Langmuir 2009, 25 (9), 5068−5075.
(23) Kim, Y. R.; Hendrickson, N. M.; Ladisch, M. R. Plug-flow
reactor for continuous hydrolysis of glucans and xylans from
pretreated corn fiber. Energy Fuels 2005, 19 (5), 2189−2200.
(24) Cheng, G.; Varanasi, P.; Arora, R.; Stavila, V.; Simmons, B. A.;
Kent, M. S.; Singh, S. Impact of ionic liquid pretreatment conditions
1911
dx.doi.org/10.1021/jf405712b | J. Agric. Food Chem. 2014, 62, 1905−1911