Sulfonic-acid-functionalized porous benzene phenol polymer and carbon for catalytic esterification of methanol with acetic acid

Article abstract of DOI:10.1016/j.cattod.2010.03.082

Porous benzene phenol polymer and carbon were synthesized in the presence of Pluronic P123 as template. Sulfonation of the porous structures yielded solids with sulfonic acid groups. The compositions of the solid acids were characterized using elemental analysis and thermogravimetric analysis methods. Their pore structures were investigated by physical adsorption of nitrogen. X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy were used to characterize surface functional groups. Carbonization of porous benzene phenol polymer at evaluated temperatures transferred the polymer to carbon as proven by X-ray diffraction and transmission electron microscope techniques. The catalytic properties of the solid acids were evaluated using esterification of methanol with acetic acid. Results showed that the catalytic activity for the sulfonated polymer increased with the increase in sulfonation temperature. Sulfonic acid groups on the polymer framework were observed to be more stable than those on the carbon framework.

Full text of DOI:10.1016/j.cattod.2010.03.082

Catalysis Today 166 (2011) 53–59
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Sulfonic-acid-functionalized porous benzene phenol polymer and carbon for
catalytic esterification of methanol with acetic acid
Xiaoning Tian, Li Li Zhang, Peng Bai, X.S. Zhao∗
Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 2 July 2010
Porous benzene phenol polymer and carbon were synthesized in the presence of Pluronic P123 as tem-
plate. Sulfonation of the porous structures yielded solids with sulfonic acid groups. The compositions
of the solid acids were characterized using elemental analysis and thermogravimetric analysis meth-
ods. Their pore structures were investigated by physical adsorption of nitrogen. X-ray photoelectron
spectroscopy and Fourier-transform infrared spectroscopy were used to characterize surface functional
groups. Carbonization of porous benzene phenol polymer at evaluated temperatures transferred the
polymer to carbon as proven by X-ray diffraction and transmission electron microscope techniques. The
catalytic properties of the solid acids were evaluated using esterification of methanol with acetic acid.
Results showed that the catalytic activity for the sulfonated polymer increased with the increase in sul-
fonation temperature. Sulfonic acid groups on the polymer framework were observed to be more stable
than those on the carbon framework.
Keywords:
Sulfonation
Porous polymer
Porous carbon
Esterification
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
ity. However, the activity and recyclability of both benzene phenol
polymers and correlative carbons should be further evaluated for
Liquid sulfuric acid has been widely used as a catalyst in many
chemical reaction processes, such as hydration, esterification, and
hydrolysis. However, liquid sulfuric acid has several drawbacks,
such as high toxicity, corrosion, and difficulty of separation from the
reaction mixture. Therefore, design of environmentally friendly and
efficient solid acid catalysts for replacing the liquid acid catalysts
is highly desirable. Solid sulfonic acid catalysts based on sili-
cas [1–5], solid organic/inorganic composites [6–10], and carbons
[11,12] have been reported in the literature. Generally, organic-
based solids are desirable over inorganic solids in terms of binding
sulfonic acid (–SO3H) groups to create solid sulfonic acids because
the former is in general more hydrophobic than the latter. This fea-
ture is important in catalyzing hydrocarbon conversions, especially
when water is used as a solvent.
The synthesis of mesoporous benzene phenol polymer with
block copolymer as a soft template has been described recently
[13–17]. Subsequent carbonization of the mesoporous polymer
readily leads to the formation of mesoporous carbon. In compar-
ison with the traditional hard templating method [18,19] the soft
route to prepare mesoporous carbon is simple and cost-effective.
A recent study [20] showed that sulfonated mesoporous benzene
phenol polymer displayed a good catalytic activity and recyclabil-
practical applications.
In this work, porous polymer was synthesized in the presence of
soft template Pluronic P123. The polymer was then carbonized at
evaluated temperatures to yield porous carbon. Subsequent sul-
fonation of the porous polymer and carbons yielded sulfonated
solids with surface –SO3H groups. Experimental results showed
that the sulfonated benzene phenol polymer exhibited a better
catalytic performance than the sulfonated carbon.
2. Experimental
2.1. Preparation of porous benzene phenol polymer and carbons
Porous benzene phenol polymer was synthesized following
the method described by Zhao et al. [15]. Soluble phenolic resin
was synthesized by mixing 2 g of phenol and 7.0 ml of formalde-
hyde solution (38 wt%) in 50 ml of 0.1 mol/l NaOH solution at
70 ◦C for 30 min. Then, 50 ml of water with 8 g of Pluronic P123
(EO20PO70EO20) was added into the above phenolic resin solution.
After stirring at 65 ◦C for 96 h and then at 70 ◦C for 24 h, the final
solid was collected by sedimentation and dried in air. The sample
thus obtained is designed as MP. Carbonization of MP was con-
ducted in a quartz tube under pure nitrogen flow at different tem-
peratures for 8 h with a heating rate of 1 ◦C/min. The resultant mate-
rials are donated as MC(x), where x stands for the temperatures
used during carbonization, namely 350, 400, 500, 600, and 800 ◦C.
∗
Corresponding author. Tel.: +65 65164727; fax: +65 67791936.
E-mail address: chezxs@nus.edu.sg (X.S. Zhao).
0920-5861/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2010.03.082

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X. Tian et al. / Catalysis Today 166 (2011) 53–59
2.2. Sulfonation of porous benzene phenol polymer and carbons
About 0.15 g of a solid sample was combined with 50 ml of con-
centrated H2SO4 (>98%, Merck) at a given temperature. After 4 h,
the sample was cooled down to room temperature. Then, 1000 ml
of deionized water (DI water) was added to the mixture to form
a black precipitate, which was then washed repeatedly using DI
water till no sulfate ion in the filtrate was detectable. The resultant
sulfonated solid was donated as MC(x,y), where y stands for the
sulfonation temperature.
2.3. Characterization
The pore structure of porous polymer and carbon materi-
als was investigated using physical adsorption of nitrogen at
−196 ◦C on an automatic volumetric sorption analyzer (Quan-
tachrome, NOVA 1000). Prior to each measurement, a sample was
degassed at 120 ◦C for 3 h under vacuum. The specific surface
area was determined according to the Brunauer–Enmett–Teller
(BET) method in the relative pressure range of 0.05–0.2. The total
pore volume was obtained from the volume of nitrogen adsorbed
at relative pressure of 0.99. Pore size distribution (PSD) curves
were obtained using the density functional theory (DFT) method,
which allows the determination of pore size for both microp-
ores and mesopores using a single analysis method [21–23]. The
CO2 adsorption was detected using physical adsorption of CO2
at 0 ◦C on a Mircomeritics ASAP 2020 instrument. X-ray diffrac-
tion (XRD) patterns were collected on an XRD-6000 (Shimadzu,
Japan) with Cu K␣ radiation (ꢀ = 1.5418 Å). Field-emission scan-
ning electron microscope (FESEM) images were obtained from
JSM-6700F (JEOL Japan) operated at 10 kV. High-resolution trans-
mission electron microscope (HRTEM) images were obtained from
JEM 2010F (JEOL, Japan) operated at 200 kV. X-ray photoelectron
spectroscopy (XPS) spectra were recorded on an AXIS HIS 165 spec-
trometer (Kratos Analytical) with a monochromatized Al K␣ X-ray
source. Thermogravimetric analysis (TGA) was conducted on a ther-
mogravimetric analyzer TGA 2050 (Thermal Analysis Instrument,
USA) in nitrogen with a flow rate of 100 ml/min. The elemen-
tal compositions of the samples were detected using a CHNS-O
Analyzer (FLASH EA 1112 series, Thermo electron corporation).
Fourier-transform infrared (FT-IR) spectra were acquired using
the KBr technique on a Shidmazu 8400 by averaging 64 scans
in the frequency range of 4000–400 cm−1 with a resolution of
8 cm−1. Temperature-programmed desorption of ammonia (NH3-
TPD) was carried out on a Quantachrome instrument (ChemBET
Pulsar TPR/TPD).
Fig. 1. N2 adsorption/desorption isotherms of (a) MC(350), (b) MC(400), (c)
MC(500), (d) MC(600), and (e) MC(800).
3. Results and discussion
3.1. Pore structure
The N2 adsorption–desorption isotherms of the MC samples
are shown in Fig. 1. The adsorption isotherm of sample MC(350)
appears both Type I and Type IV, suggesting a porous structure
with both micropores and mesopores. The adsorption and desorp-
tion isotherms are irreversible nearly in the whole relative pressure
region. This is a typical behavior of polymer materials because of the
swelling of the polymer in condensed nitrogen [13,14]. Polymers
possess a large amount of void space, referred to as free volume. As
nitrogen condenses in the free volume, the structure of the polymer
relaxes, allowing nitrogen access to the void space at high relative
pressures. In desorption, those nitrogen molecules having swollen
polymer cannot be desorbed, causing such a hysteresis behavior in
a wide range of relative pressure. It is also seen from Fig. 1 that sam-
ples MC(400), MC(500), MC(600) and MC(800) all exhibited a Type I
isotherm, indicating a microporous structure [27], which is consist
with the pore size distribution results (see Fig. 2b–e). The adsorp-
tion and desorption branches for these samples were irreversible
almost throughout the whole relative pressure range. This observa-
tion may be due to the swelling of carbon microparticles by nitrogen
during adsorption and/or the strong interaction between nitrogen
molecules and carbon surface [23,29–31]. As the oxygen content
measured using elemental analysis (see Table 1) for these carbon
samples were all very high, they must contain a large number of
oxygen-containing functional groups, which may provide the inter-
action with nitrogen molecules. Moreover the N2 (see Fig. 1e) and
CO2 (see Fig. S1 in Supporting Information) adsorption isotherms of
sample MC(800) revealed that the micropore network was accessi-
ble to N2 and CO2 at −196 and 0 ◦C respectively. Therefore, sample
MC(800) is not an ultramicroporous material [28].
2.4. Esterification of methanol with acetic acid
The catalytic activity of the sulfonated solids was evaluated
using the esterification reaction of methanol with acetic acid.
The reaction was carried out in a round bottom flask immersed
in a silicon oil bath at 55 ◦C. 0.4 mol of methanol, 0.04 mol of
acetic acid and 0.08 g of solid catalyst were added into the flask.
Quantitative analysis was carried out after 5 h of reaction using a
gas chromatograph (HP 6890 series GC) mass spectrometer (HP
5973 Mass selective detector) with a capillary column (HP 5MS, L
30 m, I.D. 0.25 mm, Film 0.25 ␮m). A used catalyst was collected,
washed using DI water, and dried at 100 ◦C for the next reaction
cycle.
The deactivation nature of the sulfonated solids were studied
by using a BaCl2 solution to detect H2SO4 due to the hydrolysis of
–SO3H groups [24,25] using a UV–vis spectrometer (UV-1601PC,
Shimhdzu) to detected the leaching of carbon sheets connected
with –SO3H groups [26].
Sample MP consists of small pellets (see Fig. S2a in Supporting
Information). After carbonization at 350 ◦C, the pellets particle
size of MC(350) became smaller (see Fig. S2b in Supporting
Information). As is known, thermal treatment causes shrinkage of
the polymer framework [13]. After high-temperature carbonization
treatment, more obvious size shrinkage can be found for MC(800)
(see Fig. S2c in Supporting Information). Synthesized MP pallets are

X. Tian et al. / Catalysis Today 166 (2011) 53–59
55
Table 1
Composition of samples analyzed using elemental analysis.
Sample
C (wt%)
O (wt%)
H (wt%)
S (wt%)
MC(350)
74.3
60.9
66.5
56.9
62.1
79.3
71.1
74.5
88.5
79.9
83.4
91.3
76.9
82.9
89.5
78.6
80.4
21.2
30.4
26.9
33.5
29.6
16.2
23.3
21.1
7.8
16.2
12.7
7.2
19.6
14.9
9.9
4.5
4.2
4.2
3.9
4.1
4.5
3.9
4.2
3.7
3.3
3.5
1.5
2.3
1.9
0.6
1.1
3.3
0
MC(350,40)
MC(350,40)a
MC(350,100)
MC(350,100)a
MC(400)
MC(400,40)
MC(400,40)a
MC(500)
MC(500,40)
MC(500,40)a
MC(600)
MC(600,40)
MC(600,40)a
MC(800)
4.5
2.4
5.7
4.2
0
1.7
0.2
0
0.6
0.4
0
1.2
0.3
0
MC(800,40)
18.3
15.5
2.0
0.8
MC(800,40)a
a
Catalyst after the 4th reaction cycle.
an increase in porosity is probably due to the formation of more
micropores because of the release of small molecules (CO, CO2 and
H2O) at high carbonization temperatures [29,32–34] as revealed
by the TGA data (see Fig. S3 in Supporting Information). As can be
seen, sample MP showed a weight loss of about 52% in the tem-
perature range of 350–400 ◦C under nitrogen, which was assigned
mainly to the removal of P123 template [13]. About 7% weight loss
was observed in the temperature range of 500–800 ◦C, which was
mainly caused by the release of small molecules.
After sulfonation, though mesopore channels can still be
observed over sample MC(350,100) (see Fig. 3a), the surface area
and pore size distribution of samples MC(350,40) and MC(350,100)
decreased (see Fig. S4 in Supporting Information and Fig. 2a) due to
the damage of pore channels and shrinkage of framework during
the sulfonation process [34]. After sulfonation, the surface areas
of sample MC(400,40), MC(500,40), MC(600,40) and MC(800,40)
all decreased (see Table 2), indicating that the pore structure of
the carbon sample was partially destroyed during the sulfonation
process.
Fig. 4 shows the XRD patterns of all carbonized samples. It
should be noted that the peaks at around 37 and 44 degrees two
theta are attributed to the Al holder (see Fig. S5 in Supporting
Information). It is seen that samples MC(350), MC(400) and
MC(500) all exhibited a broad peak at around 23 degrees two theta,
indicating amorphous structures with little graphite domains. Sam-
ples MC(600) and MC(800) showed two broad peaks at around 23
and 43 degrees two theta, attributed to the (0 0 2) and (1 0 1) planes
Fig. 2. DFT pore size distribution of (a) MC(350), (b) MC(400), (c) MC(500), (d)
MC(600), and (e) MC(800).
dark red, whereas MC(350) appeared a darker color and MC(800)
showed a bright black color. This color change is mainly caused
by the carbonization process, because the sample compositions
changed during the carbonization process, which are analyzed in
details in the compositions part.
Fig. 2 shows the pore size distribution curves of the carbonized
samples. It can be seen that the pore size decreased with the
increase in carbonization temperature from 350 to 800 ◦C. At the
carbonization temperature of 400, 500, 600 and 800 ◦C, most of
the mesopores disappeared while micropores dominated. From the
textural parameters of the samples (see Table 2), it can be seen that
at low carbonization temperatures (350–400 ◦C), the surface area
and total pore volume decreased with the increase in carbonization
temperature, which is due to the shrinkage of the polymer frame-
workduring thecarbonizationprocess. MC(400)andMC(500) show
the similar surface area and total pore volume, which means no big
difference of framework shrinkage happened when carbonization
temperature increased from 400 to 500 ◦C. However, the surface
area and total pore volume increased with the further increase in
the carbonization temperature from 500 to 600 ◦C, and 800 ◦C. Such
Fig. 3. HRTEM images for (a) MC(350,100) and (b) MC(800).

56
X. Tian et al. / Catalysis Today 166 (2011) 53–59
Table 2
Pore properties obtained from N2 sorption isotherms.
a
a
Sample
SBET (m2/g)
Vb (cm3/g)
Sample
SBET (m2/g)
Vb (cm3/g)
MC(350)
MC(400)
MC(500)
MC(600)
MC(800)
421
314
287
350
514
0.26
0.16
0.17
0.20
0.26
MC(350,40)
MC(350,100)
MC(400,40)
MC(500,40)
MC(600,40)
MC(800,40)
44
64
–
–
–
0.04
0.04
–
–
–
–
–
a
BET surface area calculated from the linear part of the BET plot (P/P0 = 0.05 − 0.2).
Total pore volume, taken from the volume of N2 adsorbed at P/P0 = 0.99.
b
of graphitic carbon, respectively. These two peaks for MC(800) was
more intense than that of sample MC(600), showing an increase in
the degree of stacking of graphene layers caused by the increase
in carbonization temperature [35]. From the peak at around 23
degrees two theta, the layer-to-layer distance (d-spacing) was esti-
mated to be about 0.37 nm [36], consistent with that obtained from
the HRTEM image shown in Fig. S6 (see Supporting Information).
The HRTEM image of MC(800) (Fig. 3b) indeed showed the presence
of flat graphene carbon sheets of graphitic nature [37]. It can be con-
cluded that graphite structure formed during the high-temperature
carbonization process.
ferred from polymer to carbon, in agree with the results of XRD
and HRTEM.
It is seen from Table 1 that MC(350,40) possessed higher sulfur
content than samples MC(400,40) and MC(500,40). This may come
from two reasons. First, with the increase in carbonization tempera-
ture, more aromatic rings connected together to form large carbon
sheets, resulting in a decrease in the density of –SO3H groups as
–SO3H groups are attached on the edges of carbon sheets [38]. Sec-
ond, with the increase in carbonization temperature, the carbon
sheets grew larger and larger, thus the carbon framework became
denser and denser, leading to a lower density of –SO3H groups
because compact carbon structures are more difficult to be sul-
fonated than polymers [11,38].
3.2. Chemical compositions
It is also seen from Table 1 that the sulfur content for sul-
fonated carbon-based samples, i.e. MC(500,40), MC(600,40) and
MC(800,40), increased with the increase in carbonization temper-
ature. The data in Table 2 show that the surface area and pore
volume increased with the increase in the carbonization tempera-
ture. This increase was caused by the release of small molecules
(CO, CO2 and H2O) during the carbonization process. From the
results of physical adsorption of nitrogen it can be seen that meso-
pores disappeared with the increase of carbonization temperature
and micropores dominated the main pore size distribution. The
increase in micropores increased the area for the attachment of
–SO3H groups. However, the micropores partially collapsed during
the sulfonation process. The collapsed aromatic carbon sheets with
attached –SO3H groups may be trapped inside the carbon particles
during the sulfonation and subsequent washing processes. This can
explain why the sulfur content increased for the sample carbonized
at higher temperatures in spite of larger aromatic carbon sheets
formed at high carbonization temperatures.
The C/H/O molar ratio of sample MC(350) was measured to
be about 6.2/4.5/1, confirming that the sample is polymer-based
material with a phenolic resin framework [13]. Samples MC(400),
MC(500), MC(600) and MC(800) showed a lower oxygen content
compared with sample MC(350) due to the removal of template
and the release of small molecules. The carbon content for sample
MC(400) was about 80%, indicating an low temperature incomplete
carbonization. Therefore MC(400) is an incomplete carbon-based
material and exhibits a transition status from polymer to carbon
during carbonization process. With the increase of carbonization
temperature, the carbon contents for samples MC(500), MC(600)
and MC(800) increased to about 90%, which is much higher than
that of MC(350), implying a carbon-based material [13]. There-
fore, it can be concluded that during the carbonization process
the polymer framework released small molecules through pyrol-
ysis to result in the formation of aromatic rings connected together
to form carbon sheets, through which the framework was trans-
The XPS spectra for samples MC(350,40), MC(350,100),
MC(400,40) and MC(800,40) are shown in Fig. 5. All samples exhib-
ited a single S2p peak at about 168.8 eV, which is assigned to
aromatic carbon atoms in connection with –SO3H groups [11,38].
The S2p spectra can be deconvoluted into two peak with binding
energies of 168.5 and 169.4 eV, which are assigned to S2p3/2 and
S2p1/2 of the sulfur in –SO3H, respectively [39,40].
The sulfur content measured using the elemental analysis
method (see Table 1) is higher than that measured using the XPS
technique (see Table S1 in Supporting Information) for sulfonated
polymer-based samples i.e. MC(350,40) and MC(350,100), indi-
cating that the surface density of –SO3H groups is higher than
that of the outer. However for sulfonated incomplete carbon-based
sample i.e. MC(400,40) and sulfonated carbon-based samples i.e.
MC(500,40), MC(600,40), MC(800,40) the sulfur content detected
using the elemental analysis method is lower than that measured
using XPS technique, implying that the inner –SO3H group density
is lower than that of the outer.
Fig. 6 shows the FT-IR spectra of sulfonated polymer-based
samples, namely samples MC(350), MC(350,100), and MC(350,40).
The peaks at around 1464 and 1623 cm−1 are attributed to C
C
stretching vibrations. The peak at around 1209 cm−1 is attributed
to the presence of Ar–OH [20]. The sulfonated samples, namely
Fig. 4. XRD patterns for (a) MC(350), (b) MC(400), (c) MC(500), (d) MC(600), and (e)
MC(800).

X. Tian et al. / Catalysis Today 166 (2011) 53–59
57
Fig. 5. S2p XPS spectra for (a) MC(350,100), (b) MC(350,40), (c) MC(400,40), and (d)
MC(800,40).
MC(350,100) and MC(350,40), displayed two new peaks at around
1032 and 610 cm−1 (see Fig. 6b–d), which are assigned to sym-
metric stretching vibrations of S O and –OH in –SO3H groups
(see Figs. S7B and S7D in Supporting Information), respectively
[20]. The S–O stretching band is seen at around 704 cm−1 while
the peak at round 630 cm−1 is responsible for the C–S stretch-
ing vibration [26,41]. The same peaks attributed to –SO3H groups
attached to aromatic carbon atoms can also be found for sample
MC(350,100) after the 4th reaction run (see Fig. S7C), indicating a
stable attachment of sulfonic acid groups. The sulfonated carbon-
based sample, MC(800,40), also showed peaks attributed to C–S
(630 cm−1), S O (1048 cm−1) [26,41], and S–O (704 cm−1) stretch-
ing vibrations (see Fig. S8). The intensity for these peaks is lower
than that of the polymer-based samples, indicating a lower density
of –SO3H groups.
Scheme 1. The formation of –SO3H group on (a) polymer framework, (b) carbon
framework with small carbon sheets, and (c) carbon framework with big carbon
sheets.
3.3. The formation of –SO3H groups
Xing et al. [20] and Okamura et al. [38] showed that –SO3H
groups are attached on those carbon atoms in aromatic rings in
both polymer and carbon frameworks. On the basis of the previous
works and our own XPS and FT-IR results, it is believed that –SO3H
groups were attached to aromatic carbon atoms in the porous poly-
mer as illustrated in Scheme 1a. During carbonization, the aromatic
rings connected together to form carbon sheets by releasing small
molecules, resulting in the attachment of the –SO3H groups to the
aromatic carbon atoms in the carbon sheets (see Scheme 1b). The
carbon sheets further grew with the increase in carbonization tem-
perature (see Scheme 1c).
Fig. 6. FT-IR spectra of (a) MC(350), (b) MC(350,100), (c) MC(350,100) after the 4th
reaction cycle, (d) MC(350,40).

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X. Tian et al. / Catalysis Today 166 (2011) 53–59
Fig. 7. Catalytic conversion of acetic acid over resultant sulfonic acid catalyst.
Fig. 8. Acetic acid conversion vs time for esterification reaction catalyzed by
MC(350,100) at 55 ◦C for the 2nd cycle.
3.4. Thermal gravimetric analysis and NH3-TPD
to the decomposition of catalysts during the catalytic reaction (see
Fig. S11).
The TGA–DrTGA curves for MC(350,100) are shown in
Fig. S9 (see Supporting Information). The main weight loss event in
the temperature range between 150 and 350 ◦C is mainly attributed
to the decomposition of functional groups, such as –OH, –CH2OH
and –SO3H [11]. The NH3-TPD profile for sample MC(350,100)
shown in Fig. S10 (see Supporting Information) demonstrated that
there is a intensive peak at about 300 ◦C, which was due to the
desorption of ammonia from sulfonic acid groups. Using elemental
analysis method allows one to estimate the density of sulfonic acid
groups [42].
Sample MC(800,40) exhibited the highest catalytic activity
among the sulfonated carbon-based materials in the first run due
to the high sulfur content. However, the conversion of acetic acid in
the second run decreased to 40%. Because the micropores of sam-
ple MC(800) partially collapsed during the sulfonation process, the
attached sulfonic acid groups might be trapped inside the sample.
The significant drop of conversion in the second run should thus
be due to the leaching of collapsed carbon sheets, which attached
with sulfonic acid groups. This was detected using the UV method
the results are shown in Fig. S11. Increasing carbonization temper-
ature enhanced the formation of compact carbon structure, which
is difficult to be sulfonated [11,38].
3.5. Catalytic properties
For sample MC(600,40), however, no obvious decrease in
catalytic activity was observed after the second reaction run, indi-
cating the attached –SO3H groups are stable. The main deactivation
reason forsample MC(600,40) is thus due tothe hydrolysis of –SO3H
groups. While MC(600,40) possessed a higher sulfur content than
MC(500,40) did, the activity of MC(600,40) is lower than that of
MC(500,40). Therefore, it can be assumed that the hydrolysis of
–SO3H groups in MC(600,40) is easier than that in MC(500,40).
The initial catalytic activity in esterification of methanol with
acetic acid catalyzed by MC(350,100) was investigated. An appar-
ent reaction order was determined by varying the concentration
of one reactant while keeping the other one in excess. The results
are given in Table S2. The concentrations of both acetic acid and
methanol had positive effect on the reaction rate with increas-
ing reactant concentration. Using a power law approximation, the
apparent reaction orders were determined to be 0.8 for methanol
and 0.7 for acetic acid, respectively.
Fig. 7 shows the catalytic activity of the sulfonated materials
after different reaction runs. It is seen that catalyst MC(350,100)
showed the highest conversion in the first reaction run. After the
fourth reaction run, the sulfur content of MC(350,100) decreased
from 5.7 to 4.2 wt%, indicating a stable attachment of –SO3H groups.
Fig. 8 shows the conversion of acetic acid versus time catalyzed by
MC(350,100) at 55 ◦C in the second reaction run. The activity and
recyclability for MC(350,100) are better than those of sulfonated
polypyrrole spheres [26], indicating easier and more stable attach-
ment of sulfonic acid groups on the benzene phenol rings than that
on pyrrole rings. A nafion resin catalyst has been evaluated under
the same reaction conditions. The conversion of acetic acid for the
first reaction run was about 98%, which is higher than that of the
sulfonated catalysts. However, significant swelling phenomenon
was observed for the nafion resin catalyst. The swollen nafion resin
was crashed into invisible small parts during stirring. Therefore, the
recyclability for the sulfonated polymer is better than that of the
nafion resin.
4. Conclusions
It is also seen from Fig. 7 that the catalytic activity for all sul-
fonated catalysts decreased with the increase in reaction times.
This may be due to two reasons. One is the hydrolysis of –SO3H
groups [24,25]. When BaCl2 solution was added to the filtrate after
catalytic reaction, white precipitate (BaSO4) was found. The other
one is the leaching of aromatic carbon sheets attached with –SO3H
groups, similar to the leaching of polycyclic aromatic hydrocarbons
[26,43], which was detected using the UV technique [26]. For the fil-
trate after catalytic reaction, a UV peak at about 250 nm was found
for both polymer and carbon-based catalysts. This peak is attributed
Sulfonic-acid-functionalized porous polymer and carbon were
successfully prepared. Characterization results showed that the
aromatic rings in the polymer bonded together to form carbon
sheets during the carbonization process. The size of the carbon
sheets increased with the increase in carbonization temperature.
As a result, the polymer framework transferred to carbon. Graphite
carbon structure was observed over the carbon samples prepared at
high carbonization temperatures. large carbon sheets and compact
carbon structure were found to be not favorable for the attachment

X. Tian et al. / Catalysis Today 166 (2011) 53–59
59
of –SO3H groups during sulfonation. The density and stability of
attached –SO3H groups on the polymer framework were observed
to be higher than those on the carbon framework. Therefore, the
sulfonated porous polymer catalytically performed better than
the sulfonated porous carbons in the reaction of esterification of
methanol with acetic acid.
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Acknowledgement
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This work was financially supported by MoE AcRF Tier 1 funding
(Grant Number: R279000224112).
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.cattod.2010.03.082.
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