One-pot synthesis of a novel catalyst with strong acid sites based on carbon/silica composite

Article abstract of DOI:10.1134/S0023158413030129

A novel catalyst with strong acid sites based on carbon/silica composite has been synthesized through one-pot hydrothermal carbonization of hydroxyethylsulfonic acid, glucose and tetraethyl orthosilicate (TEOS). The novel solid acid showed an acidity of 2.1 mmol/g, much higher than that of traditional solid acids such as Nafion and Amberlyst-15 (0.8 mmol/g). The catalytic activity of the solid acid was investigated in the acetalization and dimerization of α-methylstyrene. The results showed that the novel solid acid was very efficient for both hydrophilic and hydrophobic acid-catalyzed reactions. Because of the high acidity and catalytic activity the novel solid acid based on carbon/silica composite is a promising catalyst for the processes in green chemistry.

Full text of DOI:10.1134/S0023158413030129

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ISSN 0023ꢀ1584, Kinetics and Catalysis, 2013, Vol. 54, No. 3, pp. 378–381. © Pleiades Publishing, Ltd., 2013.
Published in Russian in Kinetika i Kataliz, 2013, Vol. 54, No. 3, pp. 394–398.
OneꢀPot Synthesis of a Novel Catalyst with Strong Acid Sites Based
on Carbon/Silica Composite1
Wu Linxi, Wu Ye, and Xuezheng Liang
Institute of Applied Chemistry, Shaoxing University, Shaoxing, 312000 China
eꢀmail: liangxuezheng@usx.edu.cn
Received April 8, 2012
Abstract—A novel catalyst with strong acid sites based on carbon/silica composite has been synthesized
through oneꢀpot hydrothermal carbonization of hydroxyethylsulfonic acid, glucose and tetraethyl orthosiliꢀ
cate (TEOS). The novel solid acid showed an acidity of 2.1 mmol/g, much higher than that of traditional solid
acids such as Nafion and Amberlystꢀ15 (0.8 mmol/g). The catalytic activity of the solid acid was investigated
in the acetalization and dimerization of ꢀmethylstyrene. The results showed that the novel solid acid was
α
very efficient for both hydrophilic and hydrophobic acidꢀcatalyzed reactions. Because of the high acidity and
catalytic activity the novel solid acid based on carbon/silica composite is a promising catalyst for the proꢀ
cesses in green chemistry.
Keywords: carbon/silica composites based solid acid, hydrophilic and hydrophobic acidꢀcatalyzed reactions,
green chemistry
DOI: 10.1134/S0023158413030129
1. INTRODUCTION
fonation mixture produced a lot of waste materials. In
order to avoid the inefficient process, we suggested
earlier using hydrothermal carbonization instead of
the thermal treatment [11, 12]. The carbon based solid
acid contains a large amount of hydrophilic functional
groups, which provides good access for hydrophilic
reactants to the SO3H groups. However, the hydroꢀ
philic functional groups prevent incorporation of
hydrophobic molecules into the acid sites, and for this
reason the carbon based solid acid show little or no
activity for hydrophobic acidꢀcatalyzed reactions [13].
With an aim to develop effective catalysts for these reacꢀ
tions the carbon/silica based solid acid was synthesized.
But the new method added the step of impregnating the
Large amounts of homogeneous acids such as sulꢀ
furic acid are annually consumed as unrecyclable catꢀ
alysts. Separating homogeneous catalysts from reacꢀ
tion products involves difficulty and expense and huge
volumes of waste that need to be treated [1–7]. Since
heterogeneous catalysts are easier to separate than
homogeneous systems and they can be reused considꢀ
erable attention has been paid in recent years to possiꢀ
ble means to substitute homogeneous acids by solid
acid catalysts. Many solid acid catalysts have been
applied to replace the unrecyclable liquid acids. Comꢀ
pared to the homogeneous acid catalyst, the solid
acids are less active, unstable and expensive. Carbon sucrose to the mesopores of SBA–15 and the mesopoꢀ
based solid acid catalysts have received increasing rous molecular sieve must be used, which further
attention in recent years because of high acidity and increased the production cost. In this work a novel effiꢀ
cient procedure is described that serves to synthesize a
novel solid acid based on carbon/silica composites. The
procedure includes oneꢀpot hydrothermal carbonizaꢀ
tion of hydroxyethylsulfonic acid, glucose and tetraꢀ
ethyl orthosilicate (TEOS) (Scheme 1). The catalytic
activities of the novel solid acid based on carbon/silica
composites was investigated in the acetalization. The
results showed that the novel solid acid was very efficient
for the reactions.
good activity they provide in catalytic reactions [8–
10]. The solid acids were generally prepared through
two steps. In the first step, the carbonization of biomꢀ
ass, such as sugar was carried out to form the polycyꢀ
clic aromatic carbon sheets. Then the aromatic carbon
sheets were sulfonated to bring the acid sites. The carꢀ
bonization process was undertaken at high temperaꢀ
ture for a long time and various wastes were produced
during the process, which caused heavy pollution.
Although the surface sulfonation of carbon materials is
now a wellꢀestablished process, it is a rather severe
method because the carbon surface is fairly inactive. In
addition, the separation of sulfuric acid from the sulꢀ
2. EXPERIMENTAL
All organic reagents were commercial products of
the highest purity available (>98%) and used for the
1
The article is published in the original.
378

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ONEꢀPOT SYNTHESIS OF A NOVEL CATALYST WITH STRONG ACID SITES
379
reaction without further purification. GC measureꢀ matograph. GC–MS measurements were performed
ments were taken on a Shimadzu (GCꢀ14B) gas chroꢀ on an American Agilent 6890/5973N instrument.
OH
HO3S
H
H
O
HO
H
HO3S
HO
H
OH
Glucose
Carbon/
H
OH
180
°C
silicon
composition
4 h
SO3H
SO3H
SO3H
HO3S
HO
SO3H
Hydroxyethysulfonic acid
Si(OC2H5)4
TEOS
Scheme 1. The synthesis route of the novel solid acid based on carbon/silica composites.
2.1. Synthesis of the Novel Solid Acid
Based on Carbon/Silica Composite
filtering and washing with acetone, then dried in an
oven at 80 for about 1 h.
°
C
In a typical preparation, the mixture of the glucose
(2 g), TEOS (2 g), hydroxyethylsulfonic acid (2 g) and
80 mL deionized water was placed in 100 mL Teflonꢀ
lined stainless steel autoclaves, which were heated in
3. RESULT AND DISCUSSION
3.1. Characterization of the Novel Catalyst
The acidity of the novel carbon/silica composites
based solid acid was 2.1 mmol/g as measured by the
neutralization titration. The titration was carried out
as follows: carbonaceous material (40 mg) and 2 M
aqueous NaCl (4 mL) were stirred at room temperaꢀ
ture for 24 h. The solids were filtered off and washed
an oven at 180
dried in a vacuum oven at 100
°
С
for 4 h. The resulting products were
overnight. The black
°
C
solid was grinded into powder and washed with water
until no acidity was detected in the filtrate. The novel
solid acid based on carbon/silica composites was
obtained after drying at 120 С overnight in an oven.
°
with water (4
×
2 mL). The combined filtrate was
titrated with 0.01 N NaOH using phenol red as indicaꢀ
tor [14]. The catalyst showed a much higher amount of
acidity than that of the common heterogeneous acid
catalysts such as Nafion and Amberlystꢀ15
(0.8 mmol/g). The acid strength of the catalyst was
determined by thermodesorption of chemisorbed
ammonia (NH3–TPD) (Fig. 1). NH3–TPD was carꢀ
ried out in a fixedꢀbed reactor equipped with a proꢀ
grammable temperature controller, at ambient presꢀ
sure. About 1.0 g of sample was loaded into a quartz
2.2. Acetalization of Benzaldehyde and 1,2ꢀEthanediol
In a typical preparation, benzaldehyde (0.1 mol),
10 mL cyclohexane, 1,2ꢀethanediol (0.15 mol) and
the catalyst (0.05 g) were mixed together in a three
necked round bottomed flask equipped with a magꢀ
netic stirrer and a thermometer, and a DeanꢀStark
apparatus was used to remove the water continuously
from the reaction mixture. The reaction was moniꢀ
tored by GC analysis of the small aliquots for 1.5 h. On
completion, the catalyst was recovered by filtering and
tube. The sample was preheated at 150
°
C
for 3 h in
, the
flowing (dry) nitrogen. After cooling to 100
°
C
washing with acetone, then dried in an oven at 80 C
°
sample was exposed to a stream of (dry) ammonia for
1 h at a flow rate of 20.0 mL/min. Then the sample was
left in the flowing nitrogen at the same temperature for
10 h in order to purge any excess of ammonia and
physically adsorbed ammonia. Finally, the TPD operꢀ
ation was performed by heating the sample from 25 to
for about 1 h.
2.3. Dimerization of
α
ꢀMethylstyrene
In
a
typical procedure,
α
ꢀmethylstyrene
(25 mmol) and the catalyst (0.2 g) were mixed together
in a three necked round bottomed flask equipped with
a magnetic stirrer and a waterꢀcooler condenser. The
mixture was reacted over at 373 K for 4 h. Samples
were withdrawn at intervals from the reaction mixtures
and analyzed by gas chromatography using a capillary
750
°
C
at a heating rate of 10 C/min. The result
°
showed that the solid acid had strong acid sites from
which ammonia was desorbed at temperatures of 400
to 600
°
C.
In our experiments TEOS was used to improve the
column. On completion, the catalyst was recovered by BET surface area of the materials and the solid acid
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2013

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380
WU LINXI et al.
2.1 mmol/g corresponds to the sulfur content of 6.7%,
which is higher than the actually determined content.
These results indicated that there are still a few other
acidic groups such as carbonyl acid groups in the carꢀ
bonaceous material and the oxygen content was as
high as 28.8%, which also indicated that there were
still many oxygenꢀcontaining groups in the material.
The SEM images of the novel solid acid based on
carbon/silica composite show that the resulting partiꢀ
cles are formed as spheres 2–5
µ
m in diameter
(Fig. 2). It appears that carbon and silica components
are completely mixed since different phases cannot be
discriminated in the SEM images. Figure 2 also
showed the spheres connected with each other in the
carbon/silica composites structure. The element analꢀ
ysis also confirmed that the carbon and silica compoꢀ
nents were entirely mixed together and no core–shell
structure could be recognized from the TEM.
0
100 200 300 400 500 600 700 800
T,°C
Fig. 1. The NH –TPD of the novel solid acid based on the
carbon/silica composite.
3
with the BET surface of 532 m2/g was obtained. The
molar ratio of TEOS and hydroxyethylsulfonic acid
affect both the acidity and the BET surface. As the
amount of TEOS increases the catalyst shows an
increase in the BET surface area and a reduction in
acidity caused by the incorporation of additional
amounts of silicon dioxide in the solid. On the other
hand, with additional amounts of hydroxyethylsulꢀ
fonic acid the catalyst shows a higher acidity and lower
BET surface area because TEOS hydrolysis occurred
quickly without interacting with glucose. The IR
spectrum of the novel solid acid showed absorption
bands at 1040 and 980 cm–1 from the sulfonic acid
groups. FT–IR spectrum also showed that the solid
acid contains other functional groups including C=C
(1570 cm–1), C–C (1140 cm–1), C=O (1680 cm–1)
and OH (3400 cm–1).
3.2. Catalytic Activitiy for the Acetalization
The novel solid acid based on carbon/silica comꢀ
posite was applied to catalyze the acetalization of benꢀ
zaldehyde and 1,2ꢀethanediol first (Fig. 3). For comꢀ
parison, the results obtained with other catalytic sysꢀ
tems are shown. These include concentrated sulfuric
acid; zeolite (HY with an acidity of 0.8 mmol/g);
Amberlystꢀ15 (strong acid cation exchange resin from
Fluck with an acidity of 2.0 mmol/g), and the convenꢀ
tional carbon based solid acid produced through the
sulfonation of the incomplete carbonized carbon sheet
[8]. HY zeolite, an inorganic solid acid, exhibits relaꢀ
tively low activity, probably due to a moderate acidity,
whereas Amberlystꢀ15 is quite active in the reaction.
The novel solid acid based on carbon/silica composite
exhibits a remarkably high activity for the acetalizaꢀ
tion. The activity level of this catalyst is higher than
that of the conventional solid acids and is comparable
to that of sulfuric acid.
The elemental analysis of the novel solid acid based
on carbon/silica composite showed following percentꢀ
ages: C 44.6%; H 5.5%; S 6.3%; Si 14.8%; O 28.8%.
The spectrum of Xꢀray Photoelectron Spectroscopy
(XPS) analysis also indicated that the sample containꢀ
A detailed account was given on reusability of the
ing 6.7% of S exhibited a single S 2p peak attributable novel solid acid based on carbon/silica composite for
to sulfonic acid groups at 168 eV. It can be thus the reaction. After reactions, the reaction mixture was
inferred that the sulfur exists predominantly as sulꢀ cooled to room temperature; the catalyst was recovꢀ
fonic acid groups. On the other hand, an acidity of ered by filtration. The recycled activity of the catalyst
1 µm
10 µm
5 µm
(а)
(b)
(c)
Fig. 2. The SEM images of the novel solid acid based on the carbon/silica composite.
KINETICS AND CATALYSIS Vol. 54
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2013

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ONEꢀPOT SYNTHESIS OF A NOVEL CATALYST WITH STRONG ACID SITES
381
CHO
O
OH
+
HO
O
Carbon based
solid acid
HY
Novel solid acid
Carbon based
solid acid
Sulfuric acid
Amberlystꢀ15
Sulfuric acid
HY
Amberlystꢀ15
Novel solid acid
0 10 20 30 40 50 60 70 80 90
Yield, %
0
20
40
60
80 100
Yield, %
Fig. 4. Catalytic activity of the novel solid acid based on the
carbon/silica composite for the dimerization of
styrene.
Fig. 3. Catalytic activity of the novel solid acid based on
the carbon/silica composite for the acetalization.
α
ꢀmethylꢀ
was investigated through the acetalization of benzalꢀ cient for both hydrophilic and hydrophobic acidꢀcataꢀ
dehyde and 1,2ꢀethenediol. The results showed that lyzed reactions with the activities comparable to those
the catalyst activity was stable. The yields remained of sulfuric acid. The advantages of the catalyst include
unchanged even after the sample had been recycled high acidity, large surface area, low cost and reasonꢀ
five times. Furthermore, the catalytic activity of the able thermal and chemical stability, which made the
recycled filtrate was investigated and the results novel heterogeneous catalyst an appropriate candidate
showed no activity of the filtrate, which further conꢀ for the replacement of the homogeneous catalysts in
firmed that acid sites was not removed from the surface green processes.
of the solid acid. These results support the suggestion
of a high stability of the novel solid acid.
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α
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α
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α
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KINETICS AND CATALYSIS Vol. 54
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2013