One-step synthesis of a novel carbon-based strong acid catalyst through hydrothermal carbonization

Article abstract of DOI:10.1007/s00706-010-0332-2

A novel carbon-based strong acid catalyst was synthesized through the one-step hydrothermal carbonization of furaldehyde and p-toluenesulfonic acid (PTSA) in aqueous solution at 180 °C for only 4 h. The novel carbonbased solid acid possessed high acidity, and the catalytic activities were investigated by esterification and oxathioketalization. The results showed that the novel catalyst demonstrated much greater activity than the traditional solid acids and was comparable to sulfuric acid. The onestep method provides an efficient procedure for the synthesis of various functionalized carbons by one-step hydrothermal carbonization. Springer-Verlag 2010.

Full text of DOI:10.1007/s00706-010-0332-2

Monatsh Chem (2010) 141:929–932
DOI 10.1007/s00706-010-0332-2
ORIGINAL PAPER
One-step synthesis of a novel carbon-based strong acid catalyst
through hydrothermal carbonization
•
•
•
Huiquan Xiao Yingxue Guo Xuezheng Liang
Chenze Qi
Received: 17 December 2009 / Accepted: 16 May 2010 / Published online: 9 June 2010
Ó Springer-Verlag 2010
Abstract A novel carbon-based strong acid catalyst was
synthesized through the one-step hydrothermal carboniza-
tion of furaldehyde and p-toluenesulfonic acid (PTSA) in
aqueous solution at 180 °C for only 4 h. The novel carbon-
based solid acid possessed high acidity, and the catalytic
activities were investigated by esterification and oxa-
thioketalization. The results showed that the novel catalyst
demonstrated much greater activity than the traditional
solid acids and was comparable to sulfuric acid. The one-
step method provides an efficient procedure for the syn-
thesis of various functionalized carbons by one-step
hydrothermal carbonization.
inexpensive as sulfuric acid. Recently, carbon-based solid
acid catalysts have increasingly attracted attention [8–10].
The materials were generally synthesized via two steps.
First, saccharides were incompletely carbonized at high
temperature ([400 °C) under inert atmosphere for a long
time. This was not an environmentally friendly process, had
low yields, and numerous harmful wastes were formed.
Then sulfonation took place to introduce the sulfonic acid
groups in the second step. The sulfonation procedure was
also carried out in harsh conditions for the inactive surface of
the carbonaceous materials. Furthermore, the separation of
the carbon materials from the concentrated sulfuric acid was
tedious work. Hydrothermal carbonization, which involved
the hydrothermal decomposition of various carbohydrates in
aqueous solutions at 180 °C, has the advantage of being
very cheap, mild, and absolutely ‘‘green’’ as it involves no
organic solvents, catalysts, or surfactants [11]. However, the
carbonaceous materials from this procedure had few func-
tional groups, and a subsequent chemical or steam activation
had to be applied for certain purposes [12]. The conventional
modification of the carbon surface involves treatment with
acids or ozone, thereby generating functionalities such as
carboxylic acids, esters, or quinines [13]. Although the
surface activation of carbon materials is now a well-estab-
lished process, it is a rather harsh method because of the
extremely low reactivity and adds another step to the syn-
thesis of functional carbon. Furthermore, corrosion of the
carbon surface often occurs during the treatment [14].
Hydrothermal carbonization was used for the preparation of
carbon-based solid acid in our previous work [15]. The
material was obtained via two steps, and sulfonation was
carried out under tedious conditions. Here a novel carbon-
based acid has been synthesized via one-step hydrothermal
carbonization of furaldehyde and p-toluenesulfonic acid
(PTSA) in aqueous solution at 180 °C for only 4 h. The
Keywords Acidity Á Catalysis Á Green chemistry Á
Carbon-based acid catalyst Á One-step synthesis
Introduction
Acid-catalyzed reactions are very important in chemical
processes. Over 15 million tons of sulfuric acid is annually
consumed as an unrecyclable catalyst, which requires costly
and inefficient separation of the catalyst from homogeneous
reaction mixtures [1–7]. Many solid acid catalysts have been
developed for the replacement of the unrecyclable homo-
geneous acids. However, a major obstacle to such progress is
the lack of a solid acid that is as active, stable, and
H. Xiao Á Y. Guo Á X. Liang (&) Á C. Qi (&)
Institute of Applied Chemistry, Shaoxing University,
Shaoxing, China
e-mail: xzliang@usx.edu.cn
C. Qi
e-mail: qichenze@usx.edu.cn
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catalytic activities of the novel carbon-based acid were
investigated via esterification and oxathioketalization. The
results showed that the novel solid acid possessed compa-
rable activities to sulfuric acid.
O-H
Results and discussion
C=C
C=O
Characterization of the novel catalyst
=C-H
S-O
The sulfonic acid groups were attached to the carbonaceous
material during the carbonization process (Scheme 1). The
formation of the novel carbonaceous material involves the
dehydration of furaldehyde and PTSA as the first step.
Upon subsequent dehydration (polymerization), micro-
scopic carbon-containing spheres with sulfonic acid and
hydroxyl groups were formed. Subsequent loss of water by
these assemblies leads to further coalescence of micro-
scopic spheres (Scheme 1) [16]. Here, PTSA was used as
the functional molecule to introduce the sulfonic acid
groups to the carbonaceous material, which had an aro-
matic ring for the intermolecular reaction with furaldehyde.
The acidity of the novel sulfonic acid groups functionalized
carbonaceous material was 1.8 mmol/g, which was deter-
mined through neutralization titration. The titration was
carried out as follows: carbonaceous material (40 mg) and
2 N aqueous NaCl (4 cm3) were stirred at room tempera-
ture for 24 h. The solids were filtered off and washed with
water (4 9 2 cm3). The combined filtrate was titrated with
0.01 N NaOH using phenol red as indicator [17]. The
catalyst had much higher acidity than the sulfonated car-
bonaceous materials, which were obtained via the
sulfonation of the subsequent inactive surface of the car-
bon. The acid strength of the catalyst was determined by
thermodesorption of chemisorbed ammonia (NH3-TPD).
The result showed that the catalyst had great acid strength
in which ammonia was desorbed at 400–600 °C. The BET
surface of the material was 125 m2/g, which was much
higher than the sulfonated carbonaceous materials (2 m2/g).
Furthermore, the acidity of the carbon could be adjusted
through the molar ratio of furaldehyde and PTSA. The
sulfonic acid groups increased with the PTSA.
C-O
S=O
3500
3000
2500
2000
1500
1000
-1
Wavenumbers/cm
Fig. 1 IR spectrum of the novel carbon-based acid
of the sulfonic acid groups. Also, the FT-IR spectrum showed
that the carbon materials contain resident functionalities
including carboxylate (1,704 cm-1), =C–H (740 cm-1), C–O
groups (1,204 cm-1), –OH groups (3,400 cm-1), S–O groups
(978 cm-1), and C=C groups (1,604 cm-1).
The SEM images of the novel carbon-based acid show
that the resulting particles grow in size with the reaction
time with a diameter of 2–4 lm as depicted in Fig. 2.
Figure 2 shows the morphologies of the materials as
micrometer-sized microporous carbon spheres, and many
micro-sized carbon spheres were assembled together to
form big carbon spheres (Fig. 2a). The carbonaceous
material obtained from only furaldehyde formed carbon
spheres with smooth surfaces (Fig. 2d). The addition of
PTSA induces a change in the particle morphology: the
surface of the particles is no longer smooth, and the
microspheres indicate that they are formed out of small
aggregated particles. PTSA appears to stabilize the first
formed small droplets, thus preventing them from further
growth, as might occur in the case of pure furaldehyde.
Catalytic activities for the esterification of acetic acid
and butanol
The novel carbon-based acid (C–SO3H) was applied to cat-
alyze the esterification of acetic acid and butanol first
(Fig. 3). For comparison, the results for concentrated
sulfuric acid, zeolite (HY), Amberlyst-15, and the carbo-
naceous materials from only furaldehyde (carbon) are also
shown. HY zeolite is an inorganic solid acid that exhibits
only low activity in low acidity, whereas Amberlyst-15
exhibits high activity in the reaction. Amberlyst-15, with an
acidity of 0.8 mmol/g and a BET surface of 35 m2/g, was
purchased from Fluka. The lower acidity and BET surface
resulted in lower activity than the novel carbonaceous
The IR spectrum of the novel carbon-based acid is shown in
Fig. 1. Theabsorbanceat1,040 cm-1 confirmedthe existence
Furaldehyde
HO
O
HO3S
CHO
SO3H
HO3S
SO3H
OH
SO3H
SO3H
SO3H
HO3S
SO3H
SO3H
p-Toluenesulfonic acid (PTSA)
HO3S
Scheme 1
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One-step synthesis of a novel carbon-based strong acid catalyst
931
Fig. 2 SEM images of the carbonaceous materials
80
70
60
50
40
30
20
10
0
CH COOH + -C H OH
CH3COOC4H9 + H2O
n
3
4
9
C-SO3H
Carbon
H2SO4
Amberlyst-15
HY
1
2
3
4
5
Run
0
10
20
30
40
50
60
70
80
Fig. 4 Reusability of the carbonaceous material for esterification
Yield/%
Fig. 3 Catalytic activities for esterification. Reaction conditions:
butanol, 20 mmol; acetic acid, 24 mmol; catalyst, 50 mg, 25 °C, 7 h
oxathioketalization of cyclohexanone and mercaptoethanol.
The obvious differences between the carbon and C–SO3H
also indicated the well-functioning surface of the novel
carbon-based acid. Also, the activity remained unchanged,
even after the catalyst had been recycled for a fourth time.
These results indicated that the novel carbonaceous material
had high activity and good stability. Since many industrially
important chemicals such as alcohols, esters, ethers, and
acetals are produced by such reactions in the presence of a
sulfuric acid catalyst, the comparable performance of the
novel carbon-based acid to sulfuric acid indicated by the
present results demonstrates that it has great potential to
replace sulfuric acid for green processes. Furthermore, the
synthetic condition assured the high hydrothermal stability
of the novel carbonaceous material, which greatly enlarges
its application areas.
material. The carbonaceous material obtained from single
furaldehyde (carbon) showed almost no activity for few
functionalities on the surface. The novel carbonaceous
material (C–SO3H) exhibited a remarkably high activity for
the formation of butyl acetate. The activity is much higher
than that of the conventional solid acids and is comparable to
sulfuric acid.
The reusability of the carbonaceous material was care-
fully investigated for the reaction. After the reaction had
reached equilibrium (7 h), the novel carbon-based acid was
simply recovered by filtration and recycled for further
reaction. It was confirmed that the activity remained
unchanged, even after the carbonaceous material had been
recycled four times (Fig. 4).
Conclusion
Catalytic activities of the oxathioketalization
of cyclohexanone and mercaptoethanol
The novel carbon-based acid catalyst was synthesized via
one-step hydrothermal carbonization of furaldehyde and
PTSA under mild conditions. The material demonstrated
Besides the esterification, Fig. 5 also shows that the
novel carbonaceous material rivals sulfuric acid in the
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H. Xiao et al.
O
The esterification of acetic acid and butanol
O
S
SH
+ H2O
+
HO
A mixture of acetic acid (24 mmol), butanol (20 mmol),
and the catalyst (50 mg) was stirred at room temperature
(25 °C). The process of the reaction was monitored by GC
analysis as below.
C-SO3H
Carbon
The oxathioketalization of cyclohexanone
and mercaptoethanol
H2SO
4
Cyclohexanone (20 mmol), mercaptoethanol (24 mmol),
and the catalyst (50 mg) were mixed. The mixture was
stirred at room temperature (25 °C) for the specified per-
iod. The process of the reaction was monitored by GC
analysis of the small aliquots that were withdrawn. On
completion, the catalyst was recovered by filtering and
washing with acetone, then dried in an oven at 80 °C for
about 1 h.
Amberlyst-15
HY
0
20
40
60
80
Yield/%
Fig. 5 Catalytic activities of oxathioketalization. Reaction condi-
tions: cyclohexanone, 20 mmol; 2-mercaptoethanol, 24 mmol;
catalyst, 50 mg, 25 °C, 2.5 h
Acknowledgments This work was supported by the Shaoxing Key
Discipline Foundation, China, Zhejiang Provincial Natural Science
Foundation no.Y4080481 and Shaoxing Science and technology
projects (2009A21056).
high acidity and showed comparable activity to sulfuric
acid for the traditional acid-catalyzed reactions, which
indicated that the novel carbonaceous material has great
potential to replace the sulfuric acid catalyst for green
chemical processes. Furthermore, the novel ‘‘copolymeri-
zation’’ methodology also provided an efficient procedure
for the synthesis of variously functioning carbonaceous
materials via one-step hydrothermal carbonization of
selected compounds for certain purposes.
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Synthesis of the catalyst
A mixture of 10 g furaldehyde, 5 g PTSA, and 80 cm3
deionized water was placed in 100-cm3 Teflon-lined
stainless steel autoclaves and heated in an oven at 180 °C
for 4 h. The resulting products were filtered, washed with
water and methanol, and dried in a vacuum oven at 100 °C
for 4 h.
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