One-step synthesis of novel biacidic carbon via hydrothermal carbonization

Article abstract of DOI:10.1016/j.jssc.2010.05.020

The novel biacidic carbon has been synthesized via one-step hydrothermal carbonization of glucose, citric acid, and hydroxyethylsulfonic acid at 180 °C for only 4 h. The novel carbon had an acidity of 1.7 mmol/g with the carbonyl to sulfonic acid groups molar ratio of 1:3, which was confirmed by IR, XPS, TPD, SEM, and BET analyses. The catalytic activities of the carbon were investigated through esterification and oxathioketalization. The results showed that the carbon owned the comparable activities to sulfuric acid, which indicated that the carbon holds great potential for the green processes.

Full text of DOI:10.1016/j.jssc.2010.05.020

Journal of Solid State Chemistry 183 (2010) 1721–1725
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
One-step synthesis of novel biacidic carbon via hydrothermal carbonization
Huiquan Xiao, Yingxue Guo, Xuezheng Liang n, Chenze Qi n
Institute of Applied Chemistry, Shaoxing University, Shaoxing 312000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The novel biacidic carbon has been synthesized via one-step hydrothermal carbonization of glucose,
citric acid, and hydroxyethylsulfonic acid at 180 1C for only 4 h. The novel carbon had an acidity of
1.7 mmol/g with the carbonyl to sulfonic acid groups molar ratio of 1:3, which was confirmed by IR,
XPS, TPD, SEM, and BET analyses. The catalytic activities of the carbon were investigated through
esterification and oxathioketalization. The results showed that the carbon owned the comparable
activities to sulfuric acid, which indicated that the carbon holds great potential for the green processes.
& 2010 Elsevier Inc. All rights reserved.
Received 4 February 2010
Received in revised form
14 May 2010
Accepted 20 May 2010
Available online 27 May 2010
Keywords:
Novel biacidic carbon
One-step synthesis
Hydrothermal carbonization
Acid catalyst
High activity and stability
1. Introduction
applied for a certain purpose [17]. The sulfonated carbon has been
synthesized in our previous work using hydrothermal carbonization
[18]. However, sulfonation should be taken to introduce the sulfonic
acid groups. Besides the harsh reaction conditions, the separation of
the carbon material from concentrated sulfuric acid was a tedious
work. Here a novel biacidic carbon has been synthesized via one-
step hydrothermal carbonization of glucose, citric acid, and hydro-
xyethylsulfonic acid at 180 1C for only 4 h. The sulfonic acid groups
were introduced to the carbon surface during the carbonization
processes (Scheme 1). The catalytic activities of the novel carbon
were investigated via esterification and oxathioketalization. The
results showed that the carbon had the comparable activities to
sulfuric acid.
Functional carbonaceous materials [1], e.g., fullerenes [2] and
carbyne-like one-dimensional structures [3], have received more
and more attention because of their wide variety of applications,
which include adsorbents, catalysts [4], electrode materials [5],
stationary phases in liquid chromatography [6], and others. Such
carbonaceous materials are usually synthesized on harsh conditions,
for example, electric-arc discharge techniques [7], catalytical
chemical vapor deposition [8], catalytic pyrolysis of organic
compounds [9], and high-temperature (800 1C) hydrothermal con-
version from amorphous carbon [10]. Recently, much attention has
been focussed on the sulfonated carbonaceous materials for their
potential to replace the traditional homogeneous acid catalysts
[11–15]. The sulfonated carbonaceous materials were synthesized
via two steps. First, saccharides were incompletely carbonized at
high temperature (4400 1C) under inert atmosphere for long time.
It was not the environment-friendly process with low yield and
numerous harmful wastes were formed. The sulfonation was taken
to introduce the sulfonic acid groups in the second step. The
sulfonation procedure was also carried out on harsh conditions for
the inactive surface. Hydrothermal carbonization, which involved
the hydrothermal decomposition of various carbohydrates in
aqueous solutions at low temperature, has the advantages of being
very cheap, mild, and ‘‘green’’ as it involves no organic solvents,
catalysts, or surfactants [16]. However, the carbonaceous materials
obtained from the hydrothermal carbonization had little functional
groups, and a subsequent chemical or steam activation has to be
2. Experimental
All organic reagents were commercial products of the highest
purity available (498%) and used for the reactions without
further purification. Cyclohexanone, acetic acid, n-butanol,
2-mercaptoethanol, and hydroxyethylsulfonic acid were pur-
chased from Shanghai Chemicals Co. HY zeolite was purchased
from Nankai University with the n(SiO2)/n(Al2O3) of 7 and the BET
surface of 850 m2/g. Amberlyst-15 was purchased from Fluck with
an acidity of 0.8 mmol/g and a BET surface of 35 m2/g.
2.1. Synthesis of the catalyst
The solution of the 10 g glucose, 5 g citric acid, 3 g hydro-
xyethylsulfonic acid, and 80 mL deionized water was placed in a
100 mL Teflon-lined stainless steel autoclave, which was heated in an
oven at 180 1C for 4 h. The resulting products were filtered, washed
n Corresponding authors. Fax: +86 575 88345681.
E-mail addresses: xzliang@usx.edu.cn, liangxuezheng@126.com (X. Liang),
Qichenze-1@zscas.edu.cn (C. Qi).
0022-4596/$ - see front matter & 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2010.05.020

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H. Xiao et al. / Journal of Solid State Chemistry 183 (2010) 1721–1725
HO
H
O
HO
H
H
HO
HO
H
HO3S
HO3S
SO3H
HO
SO3H
COOH
SO3H
OH
HOOC
OH
H
COOH
COOH
COOH
COOH
HO
HO
SO3H
HOOC
HO3S
COOH
SO3H
Scheme 1. The synthetic route of the sulfonic acid groups functioned carbonaceous material.
with water and methanol, and dried in a vacuum oven at 100 1C for
4 h and 2.3 g biacidic carbonaceous material was obtained.
the carbonyl acid groups supplier. The acidity of the novel sulfonic
acid and carboxylic acid groups functionalized carbon was
1.7 mmol/g, which was determined through the neutralization
titration. The titration was carried out as follows: carbonaceous
material (40 mg) and 2 N aqueous NaCl (4 mL) were stirred at
room temperature for 24 h. The solids were filtered off and
washed with water (4 ꢀ 2 mL). The combined filtrate was titrated
with 0.01 N NaOH using phenol red as indicator [22]. The carbon
owned much higher acidity than that of the sulfonated carbonac-
eous materials, which were obtained via the sulfonation of the
inactive carbon. The NH3-TPD showed that the molar ratio of
SO3H/CO2H was 3:1, which was determined by the acid strength.
The SO3H/CO2H was quite different from the hydroxyethylsulfo-
nic and citric acid. However, the amount of sulfonic acid groups
could be increased by adding more hydroxyethylsulfonic acid.
The results of XPS analysis showed the S content of 7.4%, which
indicated that the S existed in the forms of sulfonic acid groups
and other groups. The S 2p XPS spectrum indicated that about 55%
S existed in the forms of sulfonic acid groups (168 eV) (Fig. 1). The
results indicated that the acidity from the sulfonic acid groups
was 1.275 mmol/g, which matched well with the molar ratio of
SO3H/CO2H. On the other hand, the O content was as high as 25%,
which indicated that there were still many oxygen-containing
groups besides the carboxylic acid groups. The BET surface of the
novel carbon was 138 m2/g (Fig. 2). The results showed that there
was some mesoporous structure in the carbon.
2.2. The esterification of acetic acid and butanol
The mixture of acetic acid (24 mmol), butanol (20 mmol), and
biacidic carbon (50 mg) was stirred at room temperature (25 1C).
The process of the reaction was monitored by GC analysis of the
small aliquots withdrawn. On completion, the catalyst was
recovered by filtering and washing with acetone, and then dried
in an oven at 80 1C for about 1 h.
2.3. The oxathioketalization of cyclohexanone and mercaptoethanol
Cyclohexanone (20 mmol), mercaptoethanol (24 mmol), and the
acid carbon (50 mg) were mixed together. The mixture was stirred
at room temperature (25 1C) for the specified period. The process of
the reaction was monitored by GC analysis as mentioned above.
3. Results and discussion
3.1. Characterization of the novel catalyst
The formation for the novel biacidic carbon involves the
dehydration of the glucose, citric acid, and hydroxyethylsulfonic
acid as the first step. During the process, glucose transformed to
various organic compounds such as furfural, 5-hydroxymethyl-
furfural, organic acids, aldehydes. These compounds could react
with citric acid and hydroxyethylsulfonic acid to introduce
carboxylic and sulfonic acid groups. Upon subsequent dehydra-
tion (polymerization), microscopic carbon-containing spheres
with sulfonic acid, carboxylic acid, and hydroxyl groups were
formed. Subsequent loss of water by these assemblies leads to
further coalescence of microscopic spheres to larger spheres
(Scheme 1) [19–21]. Here, hydroxyethylsulfonic acid was used to
introduce the sulfonic acid groups, which owned the hydroxyl
groups for intermolecular dehydration. Citric acid was quite
useful here. The carbon with low acidity was obtained when
hydrothermal carbonization was carried out in the absence of
citric acid and the sulfonic acid groups also could transform to
other groups such as sulfonate and sulfone. As a result, the
amount of sulfonic acid and carboxylic acid groups of the carbon
was quite different from the molar ratio of hydroxyethylsulfonic
acid and citric acid. No solid product formed when the single citric
acid was used as raw material. Also, the carbon with low acidity
was obtained from glucose and citric acid. Furthermore, there
were still many soluble acid materials in the filtrate, which could
be reused for the next run. The carboxylic acid groups were much
more active than sulfonic acid groups and participate in the
reactions instead of sulfonic acid groups. Citric acid also acted as
The IR spectrum of the novel carbon is shown in Fig. 3.
Compared to the carbon from the single glucose, the absorbance
at 1046 cmꢁ1 confirmed the existence of the sulfonic acid groups.
Also, FT-IR spectra showed that the carbon materials contain
resident functionalities including C¼O (1704 cmꢁ1), Ar–H
(3020 cmꢁ1), C–O (1204 cmꢁ1), which indicated the existence
of phenol, ether, quinine, etc. oxygen containing groups. The
strong absorbance at 1704 cmꢁ1 indicated the existence of
carboxylic acid groups. Therefore, both the sulfonic acid and
carboxylic acid groups existed in carbon.
The SEM images of the novel acid carbon showed that the
resulting particles grow in size with the reaction time with a
diameter of 2–5 mm as depicted in Fig. 4(a–c). Fig. 4 also shows
the morphologies of the materials, micrometer sized, microporous
carbon spheres with smooth surface, which was quite different
from the amorphous structures of the sulfonated carbonaceous
materials. The carbon spheres structure also made the recycle of
the material very simple and the filtration was enough without
suspension in the reaction mixture.
3.2. The catalytic activities for the esterification of acetic acid and
butanol
The novel acid carbon (C–SO3H) was applied to catalyze the
esterification of acetic acid and butanol first (Fig. 5). For

H. Xiao et al. / Journal of Solid State Chemistry 183 (2010) 1721–1725
1723
2000
1500
1000
500
0
O 1s
Binding energy (eV)
C 1s
S 2p XPS
S 2p3
1000
800
600
400
200
0
Binding energy/eV
Fig. 1. The S 2p XPS spectrum of the carbonaceous material.
36
34
32
30
28
26
Biacidic carbon
Carbon (G)
0.05
0.10
0.15
0.20
0.25
0.30
Relative pressure (p/p0)
1000
1500
2000
2500
3000
3500
Wavenumbers (cm-1)
Fig. 2. The IR spectrum of the carbonaceous material.
Fig. 3. The SEM images of the carbonaceous materials.
comparison, the results for concentrated sulfuric acid, zeolite
(HY), Amberlyst-15, carbon from glucose, carbon from glucose
and citric acid, carbon from glucose and hydroxyethylsulfonic acid
and the sulfonated carbon are also shown. HY and Amberlyst-15
were industrial products and used without any treatment. Various
carbon was synthesized from different raw materials under the
same condition of the novel acid carbon. HY zeolite is an inorganic
solid acid that exhibits only low activity, whereas Amberlyst-15
exhibits high activity for the reaction. The carbon (G) obtained via
the single glucose showed almost no activity for little
functionalities on the surface. The carbon (G+C) from glucose
and citric acid showed a litter higher activity than the carbon from
glucose and carbon (G+HE), but the activity was still very low for
low acidity. The sulfonated carbon also showed high activity for
the reaction, which was higher than Amberlyst-15. The novel acid
carbon (C–SO3H) exhibited a remarkably high activity for the
formation of butyl acetate. The activity is much higher than those
of conventional solid acids and the sulfonated carbon, which is
comparable to sulfuric acid. After the reaction had reached
equilibrium (9 h), the novel carbon was simply recovered by
filtration and recycled for further reaction. It was confirmed that
the activity remained unchanged, even after the carbon had been
recycled for a fourth time.
3.3. The catalytic activities for the oxathioketalization of
cyclohexanone and mercaptoethanol
Besides esterification, Fig. 6 also shows that the novel carbon
rivals sulfuric acid in the oxathioketalization of cyclohexanone

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H. Xiao et al. / Journal of Solid State Chemistry 183 (2010) 1721–1725
Fig. 4. The catalytic activities for esterification.
CH3COOH + n-C4H9OH
CH3COOC4H9 + H2O
catalyst, the comparable performance of the novel acid
carbonaceous material as sulfuric acid indicated by the present
results demonstrates that the acid carbon has great potential
to replace the sulfuric acid catalyst for the green processes.
Furthermore, the synthesis condition assured the high
hydrothermal stability of the novel carbon material. The acidity
remained even after the acid carbon was treated with boiling
water for more than 15 h, which means that the novel carbon
could be used in some special areas.
Sulfonated Carbon
Carbon (G+HE)
Carbon (G+C)
C-SO3H
Carbon (G)
H2SO4
4. Conclusion
Amberlyst-15
HY
The novel biacidic carbon was synthesized via one-step hydro-
thermal carbonization of glucose, citrate acid, and hydroxyethylsul-
fonic acid under mild condition. The carbon had high acidity with
both sulfonic and carboxylic acid groups and showed comparable
activities to sulfuric acid for esterification and oxathioketalization.
Operational simplicity, low cost of the catalyst used, high stability,
high activity, and reusability are the key features of the novel acid
carbon, which indicated that carbon holds great potential for green
processes. Furthermore, the novel ‘‘copolymerization’’ methodology
also provided an efficient procedure for the synthesis of various
functioned carbonaceous materials via one-step hydrothermal
carbonization of selected compounds for a certain purpose.
0
10
20
30
40
Yield/%
50
60
70
80
Reaction conditions:butanol: 20 mmol; acetic acid: 24 mmol; catalyst: 50
mg, R.T. (25 °C), 6h. G:glucose; C:citric acid; HE:hydroxyethylsulfonic acid
Fig. 5. The catalytic activities for oxathioketalization.
O
O
SH
+
+ H2O
HO
S
Sulfonated Carbon
Carbon(G+HE)
Carbon(G+C)
C-SO3H
Acknowledgments
This work was supported by the Shaoxing Key Discipline
Foundation, China, Zhejiang Provincial Natural Science Founda-
tion No. Y4080481 and Shaoxing Science and technology projects
(2009A21056).
Carbon (G)
H2SO4
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