Preparation of a carbon-based solid acid catalyst by sulfonating activated carbon in a chemical reduction process

Article abstract of DOI:10.3390/molecules15107188

Sulfonated (SO₃H-bearing) activated carbon (AC-SO₃H) was synthesized by an aryl diazonium salt reduction process. The obtained material had a SO₃H density of 0.64 mmol·g^-1 and a specific surface area of 60² m2·g^-1. The catalytic properties of ACSO₃H were compared with that of two commercial solid acid catalysts, Nafion NR50 and Amberlyst-15. In a 10-h esterification reaction of acetic acid with ethanol, the acid conversion with ACSO₃H (78%) was lower than that of Amberlyst-15 (86%), which could be attributed to the fact that the SO₃H density of the sulfonated carbon was lower than that of Amberlyst-15 (4.60 mmol·g^-1). However, AC-SO₃H exhibited comparable and even much higher catalytic activities than the commercial catalysts in the esterification of aliphatic acids with longer carbon chains such as hexanoic acid and decanoic acid, which may be due to the large specific surface area and mesoporous structures of the activated carbon. The disadvantage of AC-SO₃H is the leaching of SO₃H group during the reactions.

Full text of DOI:10.3390/molecules15107188

Molecules 2010, 15, 7188-7196; doi:10.3390/molecules15107188
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Preparation of a Carbon-Based Solid Acid Catalyst by
Sulfonating Activated Carbon in a Chemical Reduction Process
1
1
1,
1
1
Xiao-Yan Liu , Miao Huang , Hai-Long Ma *, Zeng-Qiang Zhang , Jin-Ming Gao ,
2
2
2
Yu-Lei Zhu , Xiao-Jin Han and Xiang-Yun Guo
1
College of Science, Northwest A&F University, Yangling 712100, Shaanxi, China
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, CAS, Taiyuan 030001,
China
2
*
Author to whom correspondence should be addressed; E-Mails: mahl@nwsuaf.edu.cn or
mahl1979@yahoo.com.cn; Tel.: +86-29-87092226; Fax: +86-29-87092226.
Received: 14 September 2010; in revised form: 11 October 2010 / Accepted: 13 October 2010 /
Published: 18 October 2010
Abstract: Sulfonated (SO
3
H-bearing) activated carbon (AC-SO
3
H) was synthesized by an
H density of
.64 mmol·g and a specific surface area of 602 m ·g . The catalytic properties of AC-
SO H were compared with that of two commercial solid acid catalysts, Nafion NR50 and
aryl diazonium salt reduction process. The obtained material had a SO
3
-1
2
-1
0
3
Amberlyst-15. In a 10-h esterification reaction of acetic acid with ethanol, the acid
conversion with AC-SO H (78%) was lower than that of Amberlyst-15 (86%), which could
3
3
be attributed to the fact that the SO H density of the sulfonated carbon was lower than that
-1
of Amberlyst-15 (4.60 mmol·g ). However, AC-SO H exhibited comparable and even
3
much higher catalytic activities than the commercial catalysts in the esterification of
aliphatic acids with longer carbon chains such as hexanoic acid and decanoic acid, which
may be due to the large specific surface area and mesoporous structures of the activated
carbon. The disadvantage of AC-SO H is the leaching of SO H group during the reactions.
3
3
Keywords: sulfonate; activated carbon; solid acid; catalysts

Molecules 2010, 15
7189
1
. Introduction
Sulfonated (SO
3
H-bearing) carbon materials have been reported to act as strong solid acid catalysts.
Hara’s group first prepared sulfonated carbon catalysts via the sulfonation and carbonization of
polycyclic aromatic hydrocarbons [1]. These catalysts showed excellent activity in a series of acid-
catalyzed reactions, although, they were not stable enough and the aromatic molecules leached out
above 100 ºC [2]. The problem could be overcome by selecting saccharides as carbon precursor [2-7].
The controlled carbonization and sulfonation of these materials resulted in stable carbon structures
with a high SO H group density. Carbon-based solid acid catalysts could also be obtained by
3
sulfonation of carbon nanotubes [8] or mesoporous carbon materials [9-11].
Activated carbon (AC) is widely used as a catalyst support in a variety of industrial and
environmental applications for its chemical stability, high specific surface area, and low cost [12].
Studies on the sulfonation of AC are however quite limited [13-15]. An early experimental result from
Hara’s group showed that heating AC in H SO only produced a carbon material with a very low SO H
2
4
3
-1
group density (less than 0.15 mmol·g ), a fact attributed to the chemical inertness of AC [1]. Onda et
-1
al. reported the generation of a sulfonated carbon material with a SO
3
H density of 0.44 mmol·g by
treatment of AC with concentrated H SO [13,14]. The obtained carbon material in this case was quite
2
4
stable and showed evident catalytic activity for the hydrolysis of cellulose. Recently, another paper
from Hara’s group described the preparation of a porous sulfonated carbon catalyst with high specific
surface area by carbonization and sulfonation of ZnCl
Besides the H SO treatment method, chemical reduction of aryl diazonium salts has also been
proved to be quite efficient for the functionalization of carbon materials [16]. Feng et al. reported the
preparation of carbon-based solid acid catalysts by covalent attachment of SO H radicals on the
2
-impregnated wood powders [15].
2
4
3
surface of ordered mesoporous carbon in a chemical reduction process [17,18]. However, so far there
have been no reports on sulfonation of AC using the chemical reduction method. Here we report the
functionalization of AC with SO
sulfonate using hypophosphorous acid (H
properties of the obtained material were also examined.
3
H-containing group by reacting AC with 4-benzenediazonium
3
PO ) as reducing agent. The structure and catalytic
2
2
. Results and Discussion
2.1. Material characteristics
3
Figure 1 shows the XRD patterns of AC and the sulfonated carbon material (AC-SO H). The broad
C (002) diffraction peak (2θ = 15-30º) can be attributed to the amorphous carbon structures. The weak
and broad C (101) diffraction peak (2θ = 40-50º) is due to the a axis of the graphite structure [1-6].
3
There is no noticeable difference in the XRD patterns between AC and AC-SO H, suggesting that the
chemical reduction process does not affect the microstructure of carbon materials.
The SO H density of sulfonated carbon material was determined by combining the S elemental
3
analysis and the total acid site density results. As shown in Table 1, the S contents in AC before and
-1
after sulfonation were 0.26 and 0.90 mmol·g , respectively. The total acid density of AC increased
-1
from 0.30 to 1.01 mmol·g via the sulfonation.

Molecules 2010, 15
Figure 1. XRD patterns for activated carbon (a) before and (b) after sulfonation.
7190
Table 1. The textural properties of the materials.
(a)
N
2
adsorption
P. V.
S content
Total acid density
3
SO H density
(mmol g )
Samples
-1
+
-1
-1
(mmol g )
(mmol H g )
S. A.
P. D.
AC
AC-SO
Nafion NR50
Amberlyst-15
751
602
<1
0.47
0.38
2.5
2.4
0.26
0.90
0.45
4.60
0.30
1.01
0.49
4.66
0
3
H
0.64
0.45
4.60
45
(
a)
2
-1
3
-1
BET surface area (S. A.) in m g , pore volume (P. V.) in cm g , pore diameter (P. D.) in nm.
-1
These results showed that the increment of S content (0.64 mmol·g ) was close to that of the total
-1
acid density (0.71 mmol·g ) in the sulfonation process. Therefore the SO
3
H density of sulfonated AC
is about 0.64 mmol·g . X-ray photoelectron spectroscopy (XPS) analysis was conducted to further
verify the chemical state of sulfur in AC-SO H. As shown in Figure 2, a strong S 2p peak appears at
about 168 eV, which can be denoted as SO H group [3].
-1
3
3
3
Figure 2. S 2p XPS spectrum of AC-SO H.
The thermal stability of AC-SO
Figure 3, AC showed a slight weight loss (about 5%) below 150 ºC, which was mainly due to the loss
of a small amount of adsorbed water. The weight loss process of AC-SO H below 150 ºC was similar
to that of AC. However, the thermogravimetric curve of AC-SO H at higher temperatures showed a
3 2
H was studied by TGA results (under flowing N ). As shown in
3
3

Molecules 2010, 15
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more significant weight loss compared to that of AC, which is mainly due to the gradual desorption
and thermal decomposition process of PhSO H group.
3
3
Figure 3. Thermogravimetric profiles for (a) AC, (b) AC-SO H.
Figure 4 shows the N adsorption isotherms of the carbon materials. Both AC and AC-SO H exhibit
2
3
similar isotherms with evident hysteresis loops in the relative pressure range of about 0.4-0.9. The
surface properties of the carbon materials are summarized in Table 1. After sulfonation, the surface
2
-1
3
-1
2
-1
area and pore volume of the carbon materials decrease from 751 m ·g and 0.47 cm ·g to 602 m ·g
3
-1
and 0.38 cm ·g , suggesting that a small part of the pore space has been occupied by the grafted SO
3
H
groups. The pore diameter does not change during sulfonation (from 2.5 to 2.4 nm).
2 3
Figure 4. N adsorption isotherms of (a) AC and (b) AC-SO H.
2.2. Catalytic properties
3
The catalytic activity of AC-SO H has been investigated in the esterification of three kinds of
aliphatic acids with different carbon chain lengths (acetic, hexanoic and decanoic acid), as shown in
Figure 5. The results for two commercial solid acid catalysts such as Amberlyst-15 and protonated
Nafion NR50 are also shown for comparison. From Figure 5(a), the highest acetic acid conversion
(
3
86%) is obtained with Amberlyst-15 after 10 h of reaction. AC-SO H gives an acid conversion of

Molecules 2010, 15
8%. The conversion with Nafion is 61%. As shown in Table 1, the SO
7192
7
3
H density of Amberlyst-15
-1
-1
-1
(
4.60 mmol·g ) is much higher than that of AC-SO
3
H (0.64 mmol·g ) and Nafion (0.45 mmol·g ).
Therefore, it may be conferred that the catalytic activity for acetic acid esterification is mainly
dependent on the acid density of the catalyst.
Figure 5. Acid catalyzed esterification of several aliphatic acids with ethanol: (a) acetic
acid, (b) hexanoic acid, and (c) decanoic acid.
Figure 5(b) shows the esterification results of hexanoic acid with ethanol. The catalytic activity of
AC-SO H is comparable in this case with that of Amberlyst-15. The advantage of our carbon catalyst
3
becomes more evident in the esterification of decanoic acid, as shown in Figure 5(c). The decanoic
3
acid conversion with AC-SO H (52%) is much higher than that with Amberlyst-15 (36%) and with
Nafion (17%) after 10 h of reaction. The results in Figure 5(b) and 5(c) suggest that the catalytic
activity for esterification of large aliphatic acid molecules is not solely dependent on the acid density.
The surface properties of catalysts such as specific surface area and pore diameters are also quite
2
-1
3
important. As shown in Table 1, AC-SO H has a large surface area (602 m ·g ) and high density of
mesopores, which may be more convenient for larger aliphatic acid molecules to adsorb on the catalyst
surface and approach to the active acid sites. Therefore, our sulfonated carbon materials may have
potential applications in the acid-catalyzed reactions of large organic molecules.
To investigate the stability of SO H group on the carbon catalyst, AC-SO H was reused in a series
3
3
-
1
of acetic acid esterification reaction cycles. Before each cycle, AC-SO
3
H was washed with 1 mol·L
2
-
H
2
SO
4
, and then washed repeatedly with deionized water at least seven times until no SO
4
ions were
detected in the washed water [11].

Molecules 2010, 15
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3
Figure 6. Variation of the SO H density on the carbon catalysts in a series of 2-h acetic
acid esterification reaction cycles.
Figure 6 shows the variation of SO H density in the reaction cycles. The fresh catalyst has a SO H
3
3
-1
density of 0.64 mmol·g . The value decreases evidently in the beginning four reaction cycles and then
-1
remains stable at 0.42 mmol·g in the following three cycles. The results indicate that about 34%
-1
SO
is preserved. It has been pointed out elsewhere that the leaching of SO
for sulfonated catalysts [7]. More efforts should be made in the future to enhance the stability of
SO H groups.
3
H group (0.22 mmol·g ) of the fresh catalyst will leach out during the reactions, while 64% SO
3
H
3
H groups is a common problem
3
3. Experimental Section
3.1. Materials and Reagents
Activated carbon was purchased from the Taiyuan Xinhua Activated Carbon Company (Taiyuan,
Shanxi Province, P.R. China). Commercial solid acid catalysts, Amberlyst-15 and protonated Nafion
NR50, were purchased from Alfa Aesar (Tianjin, China). All other reagents were in reagent grade and
used without any further treatment.
3
.2. Catalyst preparation
The sulfonated carbon material was obtained by the reaction of 4-benzene-diazonium sulfonate with
3 2
activated carbon using hypophosphorous acid (H PO ) as the reducing agent [17,18]. 4-Benzene-
diazonium sulfonate was synthesized by diazotization of 4-aminobenzenesulfonic acid [17]. In a
typical process, 4-aminobenzenesulfonic acid (13.0 g) was dispersed in 1M HCl (75 mL) in a three-
2
necked flask. An ice-water bath was used to control the temperature at 0-5 ºC. Then NaNO (83 mL, 1
M) was added dropwise into the flask. The mixture was stirred for 50 min. The obtained white
precipitate was filtered and washed with cold water.
To functionalize AC, distilled water (100 mL), ethanol (100 mL), diazonium salt (12.0 g) and AC
3 2
(2.0 g) were added in a three-necked flask. Then 50 wt % H PO (100 mL) was added and the

Molecules 2010, 15
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temperature was controlled at 0-5 ºC. After stirring for 30 min, another portion of 50 wt % H
3
PO
2
(100
mL) was added and the mixture was stirred for another 30 min. The sulfonated AC is filtered and
washed with distilled water at least seven times and then with acetone, and dried at 80 ºC.
3.3. Catalyst characterization
The sulfur content of carbon materials and commercial catalysts (Amberlyst-15 and Nafion NR50)
was determined by a Sulfur Analyzer (KZDL-4) [19]. The total acid site density of the samples was
obtained using acid-base back titration [7]. Typically, the sample (0.1 g) was mixed with NaOH
aqueous solution (50 mL, 0.004 M) while stirring for 1 h at room temperature. Then excess NaOH was
neutralized with HCl (0.02 M). The obtained carbon material was characterized by XRD (Rigaku
D/MAX-2400), XPS (Thermo Scientific K-Alpha), N adsorption analysis (Micromeritics ASAP300),
2
and TGA (Netzsch STA 449C).
3.4. Catalytic reactions
The catalytic properties of the sulfonated carbon material were examined through the esterification
of a series of aliphatic acids (acetic acid, hexanoic acid, and decanoic acid) with ethanol at 70 ºC.
Typically, aliphatic acid (0.1 mol), ethanol (1.0 mol), and the carbon catalyst (or the commercial
catalysts) (0.2 g) were used in the reactions. The liquid reaction mixtures were analyzed by gas
chromatography (Agilent GC 7890).
4
. Conclusions
-1
3
A porous carbon-based solid acid catalyst possessing a SO H density of 0.64 mmol·g and a
2
-1
specific surface area of 602 m ·g was synthesized by sulfonating activated carbon in an aryl
diazonium salt reduction process. In the esterification reaction of acetic acid with ethanol, AC-SO
showed inferior catalytic activity to the commercial catalyst, Amberlyst-15, because the former’s
3
H
-1
3 3
SO H density is much lower than the latter’s (4.60 mmol·g ). However, AC-SO H possessed evident
advantages in the esterification of decanoic acid, which was probably due to the high specific surface
area and a large number of mesopores of the activated carbon. Our study results suggest that AC-SO H
3
may have potential applications in the acid-catalyzed reactions of large organic molecules. The main
3 3
disadvantage of AC-SO H is the leaching of SO H groups during the reactions.
Acknowledgements
This work was financially supported by Chinese Universities Scientific Fund (QN2009050) and
Opening Fund of State Key Laboratory of Coal Conversion (10-11-905).
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Sample Availability: Samples of the AC-SO H material are available from the authors.
©
2010 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution license
http://creativecommons.org/licenses/by/3.0/).
(