Cellulose-derived carbon bearing -Cl and -SO3H groups as a highly selective catalyst for the hydrolysis of cellulose to glucose

Article abstract of DOI:10.1039/c4ra05520a

A solid acid catalyst (HA-CC-SO₃H) was synthesized by the sulfonation of amorphous carbon derived from the carbonization of dilute hydrochloric acid-pretreated microcrystalline cellulose. It was found that Cl^- ions are grafted onto the cellulose-derived carbon and affect the composition and structure of the carbon carrier during the carbonization process. The electrons of the aromatic carbons transfer to -Cl and -SO₃H groups, which influence their electronic state. In the cellulose hydrolysis process, the active electronic states make the -Cl groups more liable to form hydrogen bonds with cellulose, and the -SO₃H groups with stronger acidity easily break the glycosidic bonds of cellulose to produce glucose. The HA-CC-SO₃H catalyst exhibits excellent glucose selectivity (95.8%) at a moderate temperature (155 °C) under hydrothermal conditions. This journal is

Full text of DOI:10.1039/c4ra05520a

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Cellulose-derived carbon bearing –Cl and –SO3H
groups as a highly selective catalyst for the
hydrolysis of cellulose to glucose†
Cite this: RSC Adv., 2014, 4, 41212
*
Qi Pang, Liqing Wang, Hui Yang, Lishan Jia, Xinwei Pan and Chenchao Qiu
A solid acid catalyst (HA–CC–SO3H) was synthesized by the sulfonation of amorphous carbon derived from
the carbonization of dilute hydrochloric acid-pretreated microcrystalline cellulose. It was found that Clꢀ
ions are grafted onto the cellulose-derived carbon and affect the composition and structure of the
carbon carrier during the carbonization process. The electrons of the aromatic carbons transfer to –Cl
and –SO3H groups, which influence their electronic state. In the cellulose hydrolysis process, the active
electronic states make the –Cl groups more liable to form hydrogen bonds with cellulose, and the
–SO3H groups with stronger acidity easily break the glycosidic bonds of cellulose to produce glucose.
The HA–CC–SO3H catalyst exhibits excellent glucose selectivity (95.8%) at a moderate temperature
(155 ꢁC) under hydrothermal conditions.
Received 10th June 2014
Accepted 4th August 2014
DOI: 10.1039/c4ra05520a
www.rsc.org/advances
chloride.14 These catalysts have all showed excellent catalytic
activity in the hydrolysis of cellulose. The authors considered
Introduction
the –Cl groups act as cellulose-binding sites through hydrogen
bonds formed by attacking the cellulose hydroxyl protons, and
reduce the steric hindrance between the –SO3H groups and
glycosidic bonds.14 However, existing catalysts are prepared
using costly materials like sucralose and polyvinyl chloride.
Because of the limited types of chemicals containing chlorine
in carbohydrates and resin materials, the exploration of
carbon-based solid acids containing –Cl groups is restricted.
Moreover, these hydrolysis processes are oen time
consuming and usually require the pretreatment of cellulose.
Thus, we have employed hydrochloric acid as a cheap and
widely available source of chlorine to soak microcrystalline
cellulose, which is carbonized later to load Clꢀ ions onto the
carbon carrier, in order to synthesize a highly selective catalyst
for the one-step hydrolysis of cellulose.
In this paper, we synthesized a highly selective cellulose-
derived carbon containing –Cl and –SO3H groups, named the
HA–CC–SO3H catalyst, and used it to hydrolyze cellulose under
hydrothermal conditions. In the preparation process, Clꢀ ions
adsorb onto cellulose to break the interchain hydrogen bonds
in cellulose, and then gra onto its aromatic structure through
carbonization. Moreover, –Cl groups inuence the composition
and structure of the cellulose-derived carbon, and make the
HA–CC–SO3H catalyst more stable. We show the effect of the
preparation process and active groups (–Cl and –SO3H groups)
on the catalytic activity of the catalyst. Likewise, we propose the
mechanism involved in the high glucose selectivity of the
HA–CC–SO3H-catalyzed reaction.
Cellulose is a major source of glucose, and its common hydro-
lysis methods include enzymatic hydrolysis and acidic hydro-
lysis.1–4 Though enzymatic hydrolysis can obtain high
selectivity, it requires long reaction times and rigorous condi-
tions.5 Liquid acid hydrolysis usually operates at high temper-
atures to obtain a high conversion rate, but it oen has low
glucose selectivity due to the formation of undesirable by-
products.1 Moreover, homogeneous catalysts also have other
drawbacks such as equipment corrosion, difficult product
separation, and catalyst recycling.1,6
In recent years, several kinds of solid acid catalysts have
been used to hydrolyze cellulose, such as a Ru catalyst sup-
ported on heteropolyacids,7 the H-form zeolite,6,8 and acid-
activated montmorillonite.8,9 Among the various types of
catalysts, sulfonated solid acids have superior catalytic
activity10–12 with the sulfonic group serving as the main active
catalytic center.
Some researchers have introduced –Cl groups into solid
acid catalysts to further improve catalytic performance. For
example, Li and Pan synthesized a sulfonated chloromethyl
polystyrene resin for hydrolyzing cellulose.12 Hu et al.
employed sucralose as a chlorine precursor for solid acid
catalyst synthesis.13 Shen et al. prepared a carbon-based solid
acid upon sulfonation of cocarbonized starch and polyvinyl
Department of Chemical and Biochemical Engineering, College of Chemistry and
Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, China. E-mail:
jials@xmu.edu.cn; Fax: +86-592-2184822; Tel: +86-592-2188283
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra05520a
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Results and discussion
Characterization of the HA–CC–SO3H catalyst
The SEM images of microcrystalline cellulose and the
sulfonated carbon carrier derived from the carbonization of
hydrochloric acid-pretreated microcrystalline cellulose catalyst
(HA–CC–SO3H) are shown in Fig. 1(a and b). It is clear that the
catalyst particles are cellulose-derived carbon fragments, which
are smaller than microcrystalline cellulose in average size. It
might be because hydrochloric acid absorbs on cellulose and
facilitates hydrogen bonds cleavage during carbonization,
which affects the morphology of the cellulose-derived carbon.
As illustrated in Fig. 2, the crystallographic diffraction peaks
of microcrystalline cellulose at about 2q ¼ 34ꢁ for the (040)
Fig. 2 The powder XRD patterns of HA–CC–SO3H and microcrys-
talline cellulose.
plane, 14ꢁ for the (101) plane and 16ꢁ for the (101) plane
ꢀ
disappear in the pattern for HA–CC–SO3H.8 Instead, the XRD
pattern of HA–CC–SO3H shows a weak and wide diffraction
peak at around 24ꢁ, which is attributed to the (002) plane of the
micrographites oriented in a random way, and a new weak
diffraction peak at around 44ꢁ, which corresponds to the (100)
axis of the graphite structure.15 These diffraction peaks are
related to amorphous carbon composed of aromatic carbon
sheets oriented in a considerably random fashion.16 This result
suggests that the HA–CC–SO3H catalyst is composed of amor-
phous carbon, aromatic rings, and even polycyclic structures.
The FT-IR pattern of HA–CC–SO3H is depicted in Fig. 3. The
absorption peaks at around 3428 cmꢀ1 and 2923 cmꢀ1 are
attributed to the stretching vibrations of hydroxyl (–OH) groups8
and C–H groups,15 respectively. In addition, the band at about
1702 cmꢀ1 is due to the bending vibration of C]O bonds.14,15
These peaks are attributed to incomplete carbonization or
oxidation of concentrated sulfuric acid, implying that this
catalyst contains –OH and carboxyl (–COOH) groups.14 A strong
absorption peak at 1600 cmꢀ1 caused by aromatic C]C
stretching vibration,17 also suggests the existence of aromatic
ring structures. The peaks at around 1181 and 1039 cmꢀ1 are
assigned to O]S]O and –SO3H stretching vibrations,
respectively, suggesting –SO3H groups are introduced into
HA–CC–SO3H.13,14 Comparing HA–CC and HA–CC–SO3H spectra
with the CC spectrum, two absorption peaks appear at 832 and
880 cmꢀ1, which are assigned to C–H deformation aromatic
bonds and out-of-plane O–H deformation bonds,18 respectively.
It is supposed that Clꢀ ions interact with cellulose during
carbonizing, and nally affect the structure of the cellulose-
derived carbon. However the absorption peaks, which are
attributed to C–Cl bonds in the range of 500–700 cmꢀ1 are
weak.19 To further investigate these functional groups, XPS
measurements were carried out.
The XPS results for HA–CC–SO3H are shown in Fig. 4.
Fig. 4(a) gives the high-resolution Cl 2p spectrum of HA–CC–SO3H.
The strong peak obtained can be divided into two individual
Fig. 3 The FT-IR spectra of the catalysts.
Fig. 1 SEM images of microcrystalline cellulose (a), HA–CC–SO3H (b) and CC–HA–SO3H (c). Observation conditions: extractor voltage is 5.46
kV, work distance is 4 mm and magnification is 1000.
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peaks at about 199.5 eV and 201.1 eV, which are assigned to Cl the TG curve of HA–CC–SO3H shows relatively gentle slopes as
2p 3/2 and Cl 2p 1/2, respectively, suggesting that the –Cl groups the temperature increases, indicating that HA–CC–SO3H is
are covalently bonded to the carbons in HA–CC–SO3H including relatively stable and hard to break at high temperatures when
aromatic carbons.14 The S 2p spectrum of HA–CC–SO3H is compared to the CC–HA–SO3H catalyst. Taking the preparation
shown in Fig. 4(b), the S 2p 3/2 and S 2p 1/2 peaks at 168.6 and process into consideration, the differences of the two catalysts
169.8 eV, respectively are assigned to –SO3H groups, which may be caused by the order of the dilute hydrochloric acid
indicates that –SO3H groups are introduced into the catalyst soaking and carbonization process. For HA–CC–SO3H, hydro-
upon sulfonation.14,20
chloric acid absorbs on cellulose to break the interchain
hydrogen bonds, making cellulose break into pieces and facil-
itates the production of aromatic carbon through carboniza-
tion; thus this catalyst is more stable.
The mechanism of high selectivity
Table 2 shows the compositions of the catalysts. Obviously,
comparing with compositions of CC, the content of carbon
increases greatly in the carbon carrier derived from the
carbonization of hydrochloric acid-pretreated microcrystalline
cellulose (HA–CC), which indicates Clꢀ ions affect the elemental
composition of cellulose-derived carbon during the carboniza-
tion process. However, the dilute hydrochloric acid soaking
process makes –Cl groups form on this carbon carrier, leading
to a signicant decrease in the carbon content in CC–HA. Since
the catalysts introduced –SO3H groups by sulfonation, the
content of oxygen increases resulting in a decreased carbon
content in both HA–CC–SO3H and CC–HA–SO3H.15
Upon inspection of the FT-IR spectra in Fig. 3, HA–CC shows
a characteristic peak for aromatic C]C stretching vibrations at
about 1600 cmꢀ1, as well as CC, which clearly shows aromatic
rings and polycyclic aromatic hydrocarbons are formed. The
CC–HA and CC–HA–SO3H spectra show a 20 cmꢀ1 blue shi at
about 1600 cmꢀ1, which is caused by electronic effects arising
from the interaction between the –Cl groups and carbon carrier.
In addition, the CC–HA–SO3H spectrum shows two new strong
peaks at 1232 cmꢀ1 and 580 cmꢀ1 aer sulfonation, which are
caused by C–O stretching vibrations21 and characteristic double
6-membered ring vibrations,22 respectively. These suggest the
carbon carrier in CC–HA, which has a low stability is further
oxidized during the sulfonation process to get its functional
groups and structure change in CC–HA–SO3H. Therefore, the
low BET surface area may be caused by a collapse of the partial
micropores in the carbon carrier during sulfonation.
As shown in Table 1, among a series of prepared analogous
catalysts, HA–CC–SO3H exhibits a relatively above average
conversion, and its glucose selectivity can reach 95.8%.
Through comparison, we tried to nd out the reason for its high
selectivity. Cellulose-derived carbon (CC) provides almost an
equally poor performance as that found without catalyst,
therefore CC has no catalytic ability. When loaded with –Cl and
–SO3H groups separately, hydrochloric acid-treated cellulose-
derived carbon (CC–HA) and sulfonated cellulose-derived
carbon (CC–SO3H) can only increase conversion but not glucose
selectivity. In addition, when prepared by a different –Cl group
loading process, sulfonated hydrochloric acid-treated cellulose-
derived carbon (CC–HA–SO3H) obtains the highest conversion,
but its glucose selectivity is only 52.9%. Thus, we are able to
draw the conclusion that the existence of both –Cl and –SO3H
groups can be effective but not a sufficient condition for
improving glucose selectivity.
Combining SEM micrographs (see Fig. 1(b and c)) and BET
results (see Table S1 in the ESI†), HA–CC–SO3H shows smaller
particle size and a higher surface area than CC–HA–SO3H,
which suggests more active sites and better contact between the
catalyst and reactant for HA–CC–SO3H to facilitate the hydro-
lysis reaction. Moreover, its smaller average pore radius avoids
product access and makes excessive hydrolysis of cellulose less
likely. In addition, the TG results shown in Fig. 5 show clearly
that the TG curves of the two catalysts both have two weight loss
ꢁ
steps below 300 C, which are attributed to the evaporation of
imbibed water8 and gasication of oxide, respectively. However,
Fig. 4 The XPS spectra of HA–CC–SO3H and CC–HA–SO3H.
41214 | RSC Adv., 2014, 4, 41212–41218
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Table 1 The catalytic performance of the catalystsa
Selectivity (%)
Glucose
Sample
Time (h)
Temp (ꢁC)
Conversion (%)
WSSs
WSOCs
None
CC
CC–HA
CC–SO3H
HA–CC–SO3H
CC–HA–SO3H
4
4
4
4
4
4
155
155
155
155
155
155
4.8
5.6
10.4
9.9
11.3
17.4
47.9
44.6
41.3
48.5
95.8
52.9
13.3
15.6
18.4
25.1
1.9
38.8
39.8
40.3
26.4
2.3
24.7
22.4
a
Reaction conditions: catalyst 50 mg, cellulose 200 mg, distilled water 40 mL.
the catalysts and electronic states changes of the functional
groups.
For HA–CC, since a portion of –Cl groups gra within the
aromatic structure and affect the structure of the carbon carrier,
HA–CC is stable. Therefore during the sulfonation process, the
–SO3H groups are only loaded on the carbon carrier instead of
replacing the –Cl groups, and the electrons in the aromatic
structure ow to the –Cl and –SO3H groups, resulting in a new
charge balance. It is for these reasons that the Cl and S atoms
have a relatively low binding energy in the HA–CC–SO3H spec-
trum. For CC–HA–SO3H, the Clꢀ ions just form C–Cl bonds on
the outer-plane of the carbon carrier; hence, –Cl groups are
easily replaced by plenty of –SO3H groups,12 leading to the weak
Cl 2p binding energy peak and increased sulphur content.
In the cellulose hydrolysis process, the –Cl group as electron
donors attack the hydroxyl groups and hydrogen bonds on the
intermolecular chains of cellulose, tending to form hydrogen
bonds with cellulose.13,14 The active electronic states and strong
electronegativity of the –Cl groups in HA–CC–SO3H, which has
high hydrogen bond basicity to contribute to the formation of
more hydrogen bonds. These hydrogen bonds can x catalyst
particles onto cellulose to increase the number of contact sites
for –SO3H groups to easily cleave the glycosidic bonds in
cellulose. For –SO3H, more received electrons results in stronger
acidity, which accelerates glycosidic bond cleavage to obtain
more glucose. Given all of this, HA–CC–SO3H, which combines
advantages of both –Cl and –SO3H groups without doubt
performs best for the hydrolysis of cellulose. The root causes of
high glucose selectivity are the method used to prepare the
cellulose-derived carbon bearing –Cl groups and the electronic
states of the functional groups.
Fig. 5 Thermogravimetric curves of HA–CC–SO3H and CC–HA–
SO3H.
Table 2 Elemental analysis results of catalysts
Sample
C%
H%
S%
CC
HA–CC
HA–CC–SO3H
CC–HA
CC–HA–SO3H
65.63
75.97
65.08
58.42
42.25
2.37
2.43
2.36
1.99
2.81
0
0
1.91
0
7.15
The XPS spectra were used to investigate the chemical
combination state of HA–CC–SO3H and CC–HA–SO3H as shown
in Fig. 4. The Cl 2p spectra for the HA–CC and CC–HA samples
exhibit nearly no difference, which both show Cl 2p 3/2 and Cl
2p 1/2 peaks at 200.2 eV and 201.8 eV, respectively (see Fig. S1 in
the ESI†). However, there is a 0.7 eV shi of the chlorine atom
binding energy to a lower energy in the HA–CC–SO3H spectrum
when compared with the HA–CC spectrum. While in the Cl
spectrum of CC–HA–SO3H, a 0.5 eV shi towards a higher
energy is observed, and the binding energy peak becomes very
weak, which suggests the surface chlorine concentration
reduces (see Table S2 in the ESI†). Moreover, the S 2p peaks in
the HA–CC–SO3H spectrum have a negative shi of 0.4 eV when
compared with those in the CC–HA–SO3H spectrum. These may
be caused by the differences in the components and structure of
Durability experiments of catalyst
As shown in Fig. 5, the thermal stability of HA–CC–SO3H was
examined by thermogravimetric analysis, and the TG curve for
the HA–CC–SO3H catalyst shows good thermal stability. The
durability of HA–CC–SO3H was examined by recycling experi-
ments over three cycles. Fig. 6 shows the conversion and glucose
selectivity keep stable aer recycling three times, indicating that
the catalyst displays good stability in catalytic activity. More-
over, as shown in Fig. 7, in the FT-IR spectra of HA–CC–SO3H,
all the main characteristic peaks are still identied clearly. This
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indicate that the HA–CC–SO3H catalyst has good catalytic
stability, and the active groups have no leaching problem. We
consider that this method of introducing –Cl groups onto a
carbon carrier may pave a way for the exploration of novel solid
acid catalysts.
Experimental
Materials
Microcrystalline cellulose was purchased from Alfa-Aesar.
Hydrochloric acid (AR) and sulfuric acid (AR) were obtained
from Sinopharm Chemical.
Synthesis of cellulose-derived carbon (CC) and sulfonated
cellulose-derived carbon (CC–SO3H). The cellulose-derived
Fig. 6 Recycling experiments for HA–CC–SO3H. Reaction condi-
tions: catalyst 50 mg, cellulose 200 mg, distilled water 40 mL, 155 ꢁC,
4 h. The catalyst and unreacted cellulose were recovered by filtration,
washed, dried at 80 ꢁC overnight, and directly used for the next carbon (CC) was prepared using a high-temperature calcination
method.16 Microcrꢁystalline cellulose was heated under a ow of
reaction upon the addition of fresh cellulose.
N2 for 1 h at 600 C to produce a black solid, which was then
ground to a black powder. CC was sulfonated by the addition of
98% sulfuric acid at 150 ꢁC for 10 h. CC–SO3H, was isolated by
ꢁ
ltration, washed repeatedly, and dried at 80 C overnight.
Synthesis of a sulfonated carbon carrier derived from the
carbonization of hydrochloric acid-pretreated microcrystalline
cellulose (HA–CC–SO3H). The carbon carrier derived from the
carbonization of hydrochloric acid-pretreated microcrystalline
cellulose (HA–CC) was prepared according to a calcination
method but with a slight modication. Microcrystalline cellu-
lose was dipped in 10% HCl and stirred for 30 minutes before
calcination. Using ltration, the HCl-pretreated microcrystal-
line cellulose was obtained. The conditions of calcination were
according to synthesis of the cellulose-derived carbon (CC). The
ꢁ
HA–CC was then sulfonated using 98% sulfuric acid at 150 C
Fig. 7 FT-IR spectra of the HA–CC–SO3H catalyst: fresh (a), 1st run
(b), 2nd run (c), 3rd run (d).
for 10 h to produce the HA–CC–SO3H catalyst.23 The obtained
catalyst was washed using deionized water until the liquid aer
washing is neutral. The catalyst was then washed with ethanol
ꢁ
and dried in a drying oven at 80 C overnight.
illustrates the chemically bonded –Cl and –SO3H groups on
HA–CC–SO3H do not have a leaching problem that happens to
most carbon based-solid catalysts. Thus, the catalyst has
excellent stability and good repeatability.
Synthesis of sulfonated hydrochloric acid-treated cellulose-
derived carbon (CC–HA–SO3H). The hydrochloric acid-treated
cellulose-derived carbon (CC–HA) was prepared by impregna-
tion method. The oven-dried CC and 10% HCl were added into a
beaker and stirred at 80 ꢁC for 10 hours in an oil bath. Aer
cooling, the suspension was ltered and washed repeatedly with
distilled water, and dried at 80 ꢁC overnight. The CC–HA–SO3H
Conclusions
A cellulose-derived solid acid catalyst bearing –Cl and –SO3H
catalyst was obtained by sulfonating CC–HA, then ltered and
washed by deionized water and ethanol, dried at 80 ꢁC
overnight.
groups named HA–CC–SO3H was synthesized. We employed the
catalyst in the hydrolysis of cellulose and it exhibits strong
potential for increasing glucose selectivity. Large amounts of
–Cl formed on cellulose-derived carbon during the carboniza-
tion of diluted hydrochloric acid-impregnated microcrystalline
cellulose, and they affected the content of aromatic carbon in
Reactor and experiment conditions
the carbon carrier. This aromatic structure not only affects the The hydrolysis of cellulose was carried out in a thermal water
electronic states of the –Cl and –SO3H groups, but also kettle. Microcrystalline cellulose (200 mg), catalyst (50 mg), and
enhances the stability of the catalyst. The active electronic states water (40 mL) were introduced into the reactor, and heated at
and strong electronegativity make the –Cl groups easily form 155 ꢁC for 4 h. The reaction mixture was separated by ltration
hydrogen bonds with cellulose; thus, the –SO3H groups with aer cooling. The lter residue was dried in drying oven at
ꢁ
strong acidity have sufficient contact with cellulose to accelerate 80 C.
the cleavage of its glycosidic bonds for high glucose selectivity.
The conversion of cellulose was calculated by the weight
In addition, the durability experiments and characterization difference of dried cellulose before and aer the reaction:
41216 | RSC Adv., 2014, 4, 41212–41218
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Trister3000 adsorption instrument. The total pore volume and
average pore radius were estimated from the N2 desorption
curves using BJH analysis.
Conversion (%) ¼
(1 ꢀ weight of unreacted cellulose/weight of cellulose) ꢂ 100
The yield of glucose was calculated as follows:
Acknowledgements
Glucose yield (%) ¼
This research is supported by the general program of the
National Natural Science Foundation of China (Grant no.
21176203). The authors are grateful to the Analysis and Testing
Centre of Xiamen University for the analysis and observation
work in this study.
(weight of glucose ꢂ 0.9/weight of cellulose) ꢂ 100
where, 0.9 ¼ molecular weight of C6H10O5/molecular weight of
C6H12O6, and C6H10O5: cellulose; C6H12O6: glucose.24
The selectivities were calculated as follows:
Glucose selectivity (%) ¼
Notes and references
(yield of glucose/conversion of cellulose) ꢂ 100
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41218 | RSC Adv., 2014, 4, 41212–41218
This journal is © The Royal Society of Chemistry 2014