Sulfated graphene as an efficient solid catalyst for acid-catalyzed liquid reactions

Article abstract of DOI:10.1039/c2jm16608a

Graphene with its two-dimensional sheet of sp²-hybridized carbon is a hot topic in the fields of materials and chemistry due to its unique features. Herein, we demonstrate that sulfated graphene is an efficient solid catalyst for acid-catalyzed liquid reactions. The sulfated graphene was synthesized from a facile hydrothermal sulfonation of reduced graphene oxide with fuming sulfuric acid at 180 °C. Combined characterizations of XRD, Raman, and AFM techniques show that G-SO₃H has a sheet structure (1-4 layers). IR spectroscopy shows that G-SO₃H has a S = O bond, and the XPS technique confirms the presence of an S element in G-SO₃H. Acid-base titration indicates that the acidic concentration of sulfonic groups in the sulfated graphene is 1.2 mmol g^-1. TG curves shows that the decomposition temperature (268 °C) of the sulfonic groups on the sulfated graphene is much higher than that of conventional SO₃H-functionalized ordered mesoporous carbon (237 °C). Catalytic tests of the esterification of acetic acid with cyclohexanol, the esterification of acetic acid with 1-butanol, the Peckmann reaction of resorcinol with ethyl acetoacetate, and the hydration of propylene oxide show that sulfated graphene is much more active than the conventional solid acid catalysts of Amberlyst 15, OMC-SO₃H, SO₃H-functionalized ordered mesoporous silica (SBA-15-SO ₃H), graphene oxide, and reduced graphene oxide, which is attributed to the fact that the sulfated graphene almost has no limitation of mass transfer due to its unique sheet structure. Very importantly, the sulfated graphene has extraordinary recyclability in these reactions, which is attributed to the stable sulfonic groups on the sulfated graphene. The advantages, including high activities and good recyclability as well as simple preparation, are potentially important for industrial applications of the sulfated graphene as an efficient heterogeneous solid acid catalyst in the future. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm16608a

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PAPER
Sulfated graphene as an efficient solid catalyst for acid-catalyzed liquid
reactions
b
c
c
a
b
a
Fujian Liu, Jing Sun, Longfeng Zhu, Xiangju Meng, Chenze Qi and Feng-Shou Xiao*
Received 15th December 2011, Accepted 25th January 2012
DOI: 10.1039/c2jm16608a
2
Graphene with its two-dimensional sheet of sp -hybridized carbon is a hot topic in the fields of
materials and chemistry due to its unique features. Herein, we demonstrate that sulfated graphene is an
efficient solid catalyst for acid-catalyzed liquid reactions. The sulfated graphene was synthesized from
ꢀ
a facile hydrothermal sulfonation of reduced graphene oxide with fuming sulfuric acid at 180 C.
Combined characterizations of XRD, Raman, and AFM techniques show that G-SO
3
H has a sheet
structure (1–4 layers). IR spectroscopy shows that G-SO H has a S]O bond, and the XPS technique
3
confirms the presence of an S element in G-SO
3
H. Acid–base titration indicates that the acidic
À1
concentration of sulfonic groups in the sulfated graphene is 1.2 mmol g . TG curves shows that the
ꢀ
decomposition temperature (268 C) of the sulfonic groups on the sulfated graphene is much higher
ꢀ
than that of conventional SO H-functionalized ordered mesoporous carbon (237 C). Catalytic tests of
3
the esterification of acetic acid with cyclohexanol, the esterification of acetic acid with 1-butanol, the
Peckmann reaction of resorcinol with ethyl acetoacetate, and the hydration of propylene oxide show
that sulfated graphene is much more active than the conventional solid acid catalysts of Amberlyst 15,
3 3 3
OMC-SO H, SO H-functionalized ordered mesoporous silica (SBA-15-SO H), graphene oxide, and
reduced graphene oxide, which is attributed to the fact that the sulfated graphene almost has no
limitation of mass transfer due to its unique sheet structure. Very importantly, the sulfated graphene
has extraordinary recyclability in these reactions, which is attributed to the stable sulfonic groups on
the sulfated graphene. The advantages, including high activities and good recyclability as well as simple
preparation, are potentially important for industrial applications of the sulfated graphene as an
efficient heterogeneous solid acid catalyst in the future.
Introduction
To reduce the limitation of mass transfer in catalysis, various
strategies, such as the synthesis of nanosized zeolites and the
The replacement of mineral liquid acids by solid acids for the
production of fine chemicals by acid-catalyzed liquid reactions
has attracted much attention, due to the obvious advantages of
heterogeneous catalysts, including easy separation of the catalyst
from the reaction medium, reductive corrosion, and improved
preparation of hierarchical porous materials, have been
16–23
successfully pursued.
They exhibit much better catalytic
properties than conventional porous catalysts due to a significant
12–23
increase in mass transfer in the reactions.
More recently,
Ryoo et al. have fabricated single-unit-cell nanosheets of zeolites
1
–5
regenerability. Typically, solid acids are porous materials such
with extraordinary mass transfer from the unique bifunctional
24,25
1–5
6–8
9
as zeolites, mesoporous materials, ion-exchange resins, and
surfactant templates,
but their catalytic applications are
1
0,11
SO H-functionalized porous carbons.
3
Among these porous
influenced by the use of relatively high-cost templates.
solid acids, catalytically active acidic sites are mainly located in
the pores of the catalysts, therefore the mass transfer to and from
the active sites in the pores plays an important role for catalytic
It is well known that graphene, a two-dimensional sheet of
2
sp -hybridized carbon discovered by Geim and co-workers in
26–28
2
004,
has been widely used in the fields of nanoelectronic
32,33
1–11
performance,
12–15
reactions.
in particular for acid-catalyzed liquid
29–31
34–41
42
adsorption, and energy
devices,
sensors,
catalysis,
43–46
storage
due to its excellent thermal and mechanical stabili-
ties, superior electrical conductivity, very high degree of exposure
a
Department of Chemistry, Zhejiang University, Hangzhou 310028, China.
E-mail: fsxiao@zju.edu.cn; Fax: +86-431-85168590; Tel: +86-431-
5168590
of active sites on the surface, and its outstanding dispersion in
26–28,47–50
various systems.
In spite of many successful examples of
34–41
8
b
using graphene as a catalyst support in recent years,
to the
Institute of Applied Chemistry, Shaoxing University, Shaoxing, 312000,
China
best of our knowledge, there is still no report of using sulfated
graphene as an efficient solid acid catalyst.
c
Department of Chemistry, Jilin University, Changchun 130012, China
This journal is ª The Royal Society of Chemistry 2012
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Herein, we demonstrate a facile synthesis of sulfated graphene
In comparison, SBA-15-SO H and OMC-SO H were synthe-
3
3
6,52
(
G-SO H) from hydrothermal sulfonation by fuming sulfuric
3
sized as in the literature.
ꢀ
acid at a relatively high temperature (180 C). Very importantly,
G-SO
recyclability) than conventional porous solid acids such
as SO H-functionalized ordered mesoporous carbon (OMC-
SO H), Amberlyst 15, and SO H-functionalized ordered meso-
porous silica (SBA-15-SO H) in acid-catalyzed liquid reactions
3
H exhibits much better catalytic properties (activity and
Characterization
3
X-Ray diffraction (XRD) patterns were recorded on a Rigaku
D/Max-2550 using nickel-filtered Cu Ka radiation. FTIR spectra
were recorded by using a Bruker 66V FTIR spectrometer. XPS
spectra were performed on a Thermo ESCALAB 250 with Al Ka
radiation, and binding energies were calibrated using the C1s
peak at 284.9 eV. Thermogravimetric analysis (TG) was per-
formed on a Perkin-Elmer TGA7 in flowing air with a heating
3
3
3
including the esterification of acetic acid with cyclohexanol, the
esterification of acetic acid with 1-butanol, the Peckmann reac-
tion of resorcinol with ethyl acetoacetate, and the hydration of
propylene oxide.
ꢀ
À1
rate of 10 C min . Atomic force micrographs (AFM) were
obtained using a NanoWizard II BioAFM (JPK Instrument AG,
Berlin, Germany) in tapping mode. Raman spectra were
Experimental
obtained with
spectrometer.
a Renishaw Raman system model 1000
Synthesis
Chemicals and reagents. Amberlyst 15 and the nonionic block
copolymer
surfactant
poly(ethyleneoxide)-poly(propylene-
Catalytic reactions
Before reaction, the catalysts were activated by 0.1 M H SO
oxide)-poly(ethyleneoxide) block copolymer (Pluronic 123,
molecular weight of about 5800) were purchased from Sigma-
2
4
Aldrich Company. Graphite, sodium nitrate (NaNO ), potas-
3
for 4 h at room temperature. Esterification of acetic acid
with cyclohexanol (EAC), esterification of acetic acid with
butanol (EAB), the Peckmann reaction of resorcinol with
ethyl acetoacetate (PRE), and hydration of propylene oxide
(HPO) were chosen as the models for acid-catalyzed liquid
reactions.
sium permanganate (KMnO
4 4
), sodium borohydride (NaBH ),
sulfuric acid (H SO ), cyclohexanol, acetic acid, resorcinol, ethyl
2
4
acetoacetate, toluene, propylene oxide, 1-butanol, phenol,
formaldehyde (37 wt%), tetraethyl orthosilicate (TEOS), fuming
sulfuric acid, and 3-mercaptopropyltrimethoxysilane (3-MPTS),
were obtained from Tianjin Guangfu Chemical Reagent Co.
EAC was performed by mixing 0.2 g of catalyst, 17.5 mL
(
305 mmol) of acetic acid, and 11.5 mL (110 mmol) of cyclo-
Synthesis of graphene oxide (GO) and reduced graphene oxide
hexanol in a three-necked round-bottomed flask equipped with
a condenser and a magnetic stirrer. After heating the mixture to
(
RGO). The GO dispersion was prepared using Hummers-
51
ꢀ
method as described previously. As a typical run, (1) 5.0 g of
100 C in an oil bath under stirring, 17.5 mL (305 mmol) of acetic
graphite power was added into a mixture of 5.0 g of NaNO and
3
acid were rapidly added, and the reaction lasted for 5 h. In this
reaction, the molar ratio of acetic acid to cyclohexanol was 2.8
and the mass ratio of catalyst to cyclohexanol was 0.018. The
product was cyclohexyl acetate with a selectivity of nearly 100%.
EAB was performed by mixing 0.01 g catalyst, 50 mmol of
acetic acid and 50 mmol of butanol in a glass flask equipped with
a condenser and a magnetic stirrer. After heating the mixture to
1
20 mL of H
2
SO
4
(98%) in a 500 mL flask. (2) After stirring for 30
was slowly added under
min in an ice bath, 30 g of KMnO
4
vigorous stirring. (3) After stirring at room temperature over 12 h,
the mixture gradually became paste-like and the color turned light
brownish. (4) After the addition of 300 mL water under stirring,
ꢀ
the mixture was heated to 98 C in a short time and kept at this
ꢀ
temperature for 24 h, giving a yellow sample. (5) 100 mL of H
2
O
2
90 C, the reaction lasted for 4 h. In this reaction, the molar ratio
(
50 wt%) was added to the mixture, stirring for 24 h at room
of acetic acid to butanol was 1.5 and the mass ratio of catalyst to
butanol was 0.0027. The product was n-butylacetate with
a selectivity of nearly 100%.
temperature. (6)After rinsingandcentrifugationwith5%HCland
deionized water several times, the GO dispersion was obtained.
RGO was synthesized by the chemical reduction of GO using
sodium borohydride. As a typical run, 0.5 g of the GO dispersion
was added into water (500 mL), followed by sonication for 30
min. Then, 1.2 g of sodium borohydride was added into the
PRE was performed by mixing 0.2 g of catalysts, 10 mmol of
resorcinol, and 10 mmol of ethyl acetoacetate in a glass flask
equipped with a condenser and a magnetic stirrer. After the
addition of 10 mL toluene solvent, the temperature was increased
ꢀ
ꢀ
mixture, heating at 100 C for 24 h. After repeated washing with
to 110 C in an oil bath under stirring. The reaction lasted for 2 h.
water and centrifugation, and dispersing in the water, RGO was
finally obtained.
In this reaction, the molar ratio of resorcinol to ethyl acetoace-
tate was 1.0 and the mass ratio of catalyst to ethyl acetoacetate
was 0.15. The product was 7-hydroxy-4-methylcoumarin with
a selectivity of nearly 100%.
3 3
Hydrothermal sulfonation of graphene (G-SO H). G-SO H was
synthesized from the hydrothermal sulfonation of RGO using
ꢀ
HPO was performed by mixing 0.1 g of catalyst, 50 mmol of
fuming sulfuric acid at 180 C. As a typical run, 1.0 g of RGO
propylene oxide, and 500 mmol of H O in a glass flask equipped
2
was added into 50 mL of fuming sulfuric acid. After sonication
with a condenser and a magnetic stirrer. The reaction lasted for
ꢀ
for 30 min, the mixture was transferred into an autoclave to heat
ꢀ
6 h at 27 C under stirring. The molar ratio of propylene oxide to
at 180 C for 24 h under stirring. After washing with a large
ꢀ
water was 0.1 and the mass ratio of catalyst to water was
0.011. The product was 1,2-propylene glycol with a selectivity of
nearly 100%.
amount of water and drying at 80 C for 12 h under vacuum,
G-SO
3
H was finally obtained.
5
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The products in these reactions were analyzed by gas chro-
matography (Shimazu 14C and Agilent 7890) with a flame
ionization detector (FID), and dodecane was used as an internal
standard. The column was OV-1 (30 m), the temperature region
Fig. 2 shows the Raman spectra of graphite, GO, RGO, and
À1
G-SO H. Graphite displays a prominent G peak at 1581 cm ,
3
57
corresponding to the first-order scattering of the E2g mode. In
À1
contrast, GO shows peaks at 1594 and 1363 cm , which are
2
attributed to the G band (the vibration of sp carbon atoms in
ꢀ
ꢀ
À1
was 100–220 C with a rate of 20 C min , and the temperature
ꢀ
of the FID detector was 280 C. In these reactions, the stirring
a graphitic 2D hexagonal lattice) and the D band (the vibrations
3
58,59
of sp carbon atoms of defects and disorder), respectively.
RGO and G-SO H exhibit similar Raman spectra to GO, but
rate was 800 rpm.
3
their intensity ratios of D to G bands are a little different (Fig. 2c
Results and discussion
& 2d). Compared with GO (0.89), G-SO H gives a relatively high
3
ratio of D : G (1.02), suggesting a decrease in the average size of
2
the sp carbon domains by the formation of sp carbons due to
Normally, the sulfonation of graphene is carried out with the aryl
3
53,54
diazonium salts of sulfanilic acid.
In this work, a sulfated
the incorporation of oxygen or sulfur heteroatoms during the
graphene (G-SO H) is synthesized from the hydrothermal
3
53,54
chemical treatment.
Fig. 3 shows the FT–IR spectra of RGO and G-SO
ꢀ
sulfonation of RGO by fuming sulfuric acid at 180 C.
3
H.
Compared with a conventional route, the hydrothermal sulfo-
nation is very simple and the use of aryl diazonium salts is
completely avoided, which could be very useful for industrial
applications.
Compared with RGO, G-SO
À1
3
H exhibits an additional band at
60,61
1
090 cm , which is associated with a S]O bond,
the successful grafting of SO H groups onto graphene. It is worth
indicating
3
À1
noting that the peak at 1720 cm for both samples is assigned to
Fig. 1 shows the XRD patterns of graphite, graphene oxide
ꢀ
a C]O bond, attributed to the lower content of unreduced
Furthermore, we evaluated the
(
GO), RGO, and G-SO
3
H. Graphite gives a strong peak at 26.5
ꢀ
60,61
oxygen atoms on the samples.
(
Fig. 1a). However, GO only shows a peak at 12.6 and the peak
ꢀ
number of acidic sites over G-SO H by using an acid–base
3
at 26.5 associated with graphite completely disappears (Fig. 1b),
indicating the successful insertion of oxygen species between the
À1
titration, which gives 1.2 mmol g . This value is comparable
À1 52
with that of OMC-SO H (1.3 mmol g ), but still much lower
3
55
graphitic layers. After the reduction of GO by sodium boro-
À1
than that of Amberlyst 15 (4.7 mmol g ).
hydride, RGO shows a disappearance of the diffraction peak at
ꢀ
Fig. 4 shows the X-ray photoelectron spectrum (XPS) of
1
2.6 associated with GO and the reappearance of the very weak
ꢀ
G-SO
3
H, which gives signals mainly associated with C1s, O1s,
H,
and broad diffraction peak ranging from 21.3 to 27.6 , centered
ꢀ
and S2p, confirming the presence of a sulfur element in G-SO
3
at 24.1 (Fig. 1c), suggesting that GO is reduced to graphene with
in good agreement with the IR results. The high resolution C1s
56
3
only a few layers. After hydrothermal sulfonation, G-SO H has
a very similar XRD pattern to RGO (Fig. 1d), suggesting their
similar graphene layers.
Fig. 2 Raman spectra of (a) graphite, (b) GO, (c) RGO, and (d)
Fig. 1 XRD patterns of (a) graphite, (b) GO, (c) RGO, and (d) G-SO
This journal is ª The Royal Society of Chemistry 2012
3
H.
3
G-SO H.
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oxygen-containing functional groups have been successfully
removed in G-SO H. Fig. 5 shows the S2p spectrum of G-SO H,
3
3
OMC-SO H, and Amberlyst 15. G-SO H displays a peak at
3
3
1
68.3 eV associated with a S–O bond (Fig. 5a), which is slightly
lower than those (168.8 and 168.9 eV) of G-SO H and
OMC-SO H. The shift to low binding energy is explained by the
electron transfer from graphene to the SO
might result in a decrease in acidic strength for the SO
in G-SO H.
3
3
26–28
3
H group,
which
3
H group
3
Fig. 6 shows an atomic force microscopic (AFM) image of
G-SO H, confirming the sheet morphology. The thickness of the
3
sample is estimated at 0.8–3.0 nm, which corresponds to 1–4
graphene sheets, in good agreement with those reported previ-
2
6–28,62
ously.
graphene is well retained after the hydrothermal sulfonation.
Fig. 7 showed the TEM images of G-SO H and OMC-SO
Clearly, the G-SO H sample appears transparent, which is
similar to that of single graphene sheets, in good agreement
with the AFM results. Additionally, the OMC-SO H sample
exhibits a highly ordered mesostructure, as published in the
These results indicate that the sheet structure of
3
3
H.
3
Fig. 3 FT–IR spectra of (a) R-GO and (b) G-SO H.
3
62
3
52
literature.
Fig. 8 shows the TG curves of G-SO H and OMC-SO H.
3
3
OMC-SO H exhibits two obvious weight losses centered at 237
3
ꢀ
and 390 C, which are assigned to the decomposition of sulfonic
groups and the sample framework. In contrast, G-SO H shows
3
a much higher temperature for the decomposition of sulfonic
ꢀ
groups and the sample framework, at 268 and 568 C, respec-
tively. These results indicate that G-SO
thermal stability than OMC-SO H, which would be potentially
important for recycling catalysts. The higher temperature for the
decomposition of sulfonic groups over G-SO H than that over
OMC-SO H is attributed to the stronger interaction of graphene
with the sulfonic group, as evidenced by S2p spectroscopy
Fig. 5); The better stability of the framework on G-SO H than
3
H has much better
3
3
3
(
3
that on OMC-SO H is attributed to the fact that the carbon of
3
OMC-SO H is amorphous while the carbon of G-SO H is
3
3
graphene.
Table 1 presents the acidic contents and catalytic data for the
esterification of acetic acid with cyclohexanol (EAC), the ester-
ification of acetic acid with 1-butanol (EAB), the Peckmann
Fig. 4 (A) XPS spectrum and (B) high-resolution C1s spectrum of G-
SO H.
3
XPS spectrum (Fig. 4b) shows peaks at 284.7, 286.3, 287.9, and
88.8 eV, which are attributed to the non-oxygenated ring
2
carbon, the carbon in the C–O bond, the carbonyl carbon, and
the carboxylated carbon, respectively. Obviously, the peak
at 284.6 eV is dominant, suggesting that most of the
3 3
Fig. 5 S2p spectra (a) G-SO H, (b) OMC-SO H, and (c) Amberlyst 15.
5
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3 3
Fig. 8 TG curves of (a) OMC-SO H and (b) G-SO H.
reaction of resorcinol with ethyl acetoacetate (PRE), and the
hydration of propylene oxide (HPO) over various catalysts.
Clearly, it is observed that G-SO H (Table 1, run 1–3) shows
3
much higher catalytic activities than those of OMC-SO H, SBA-
3
1
5-SO H, Amberlyst 15, GO, and RGO (Table 1, run 4–10) in
3
EAC. For example, G-SO
of 79.5% (Table 1, run 1), while OMC-SO
and RGO give conversions of 52.2, 58.9, 11.5 and 10.2%
Table 1, run 4, 7, 9, and 10), respectively. Interestingly, GO and
3
H gives a conversion to cyclohexanol
3
Fig. 6 (A) AFM image of G-SO H and (B) corresponding thickness
3
H, Amberlyst 15, GO
analysis.
(
RGO show very low activities, which is attributed to the absence
of acidic sites on the samples, as confirmed by acid–base titration
3
and S analysis. Considering the similar acidic content of G-SO H
to OMC-SO H and the higher acidic content of Amberlyst 15
3
than G-SO H (Table 1) as well as the weaker acidic strength of
3
G-SO H than OMC-SO H and Amberlyst 15 (Fig. 5), the higher
3
3
catalytic activities over G-SO H than OMC-SO H and Amber-
3
3
lyst 15 should be directly assigned to the difference in the sample
structure. G-SO H has a sheet structure, and almost all of the
3
sulfonic groups are exposed to the reactants, where there is no
limitation for mass transfer (Fig. 9a) due to the novel nanosheet
structure. In addition, the novel nanosheet structure is also
favorable for the high dispersion of sulfonic groups, and most of
61
the sulfonic groups are accessible to reactants. On the contrary,
OMC-SO H and Amberlyst 15 (Fig. 9b) are porous materials,
3
and the reaction is strongly dependent on the mass transfer
through the pores which exist in the samples. In addition, some
of the sulfonic groups over these catalysts might not be accessible
to reactants. To check this idea, catalytic activities over
OMC-SO
measured. The results show that there are almost the same
activities over G-SO H at a stirring rate of 800 rpm (79.5%, Table
, run 1) and at static conditions (78.9%, Table 1, run 2), in good
3 3
H and G-SO H under static conditions have been
3
1
agreement with those (94.3 and 94.8% under stirring and static
conditions in EAC, respectively) of sulfuric acid (98%), a typical
homogeneous acid catalyst. However, OMC-SO H has obvious
3
differences in catalytic activities between the stirring (52.2%,
Table 1, run 4) and static (42.1%, Table 1, run 5) conditions.
Fig. 10 shows the catalytic kinetic curves of G-SO H, OMC-
3
3 3
SO H, and Amberlyst 15 in the EAC. Typically, G-SO H has
a three-step profile. The first stage is the first 1 h and the reaction
is performed at high rate; the second stage in the time range of
Fig. 7 TEM images of (A) G-SO
3 3
H and (B) OMC-SO H.
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Table 1 Acidic contents and catalytic activities for esterification of acetic acid with cyclohexanol (EAC), esterification of acetic acid with 1-butanol
(EAB), the Peckmann reaction of resorcinol with ethyl acetoacetate (PRE), and hydration of propylene oxide (HPO) over various catalysts
EAC conv.
(%)
EAB conv.
(%)
PRE conv.
(%)
HPO yield.
(%)
À1
a
Run
Catalysts
Acid sites/mmol g
1
2
3
4
5
6
7
8
9
G-SO
G-SO
G-SO
OMC-SO
OMC-SO
OMC-SO
3
3
3
H
H
H
1.2
1.2
1.13
1.3
1.3
0.89
4.7
1.26
—
79.5
78.9
78.3
52.2
42.1
47.1
58.9
40.5
11.5
10.2
89.1
89.3
87.2
75.1
61.5
68.6
83.8
69.2
21.9
18.2
82.1
81.7
80.8
66.1
58.3
61.3
75.2
61.5
12.3
11.1
66.8
66.4
64.9
42.3
34.6
39.4
50.3
30.9
—
b
c
3
3
3
H
H
H
b
c
Amberlyst-15
SBA-15-SO
GO
RGO
3
H
1
0
—
—
a
b
Measured by acid–base titration. The reactions performed at static conditions. Recycled 5 times.
c
Fig. 10 Dependence of catalytic activities on the time for the esterifi-
cation of acetic acid with cyclohexanol over (a) G-SO H, (b) Amberlyst
3
1
3
5, and (c) OMC-SO H.
1
, 3, and 5 h of 67.9, 77.4, and 79.5%, respectively. In contrast,
Fig. 9 Photographs of the esterification of acetic acid with cyclohexanol
OMC-SO H and Amberlyst 15 have different kinetic curves.
3
3
(EAC) over (a) G-SO H and (b) Amberlyst 15.
Even if the reaction time is 5 h, it still does not reach equilibrium
for OMC-SO H and Amberlyst 15 catalysts.
3
1
–3 h, and the reaction rate gradually decreases; after 3 h, the
reaction almost reaches equilibrium and the conversion is basi-
cally kept constant. For example, G-SO H gives conversions at
Furthermore, the recyclability of G-SO
3
H and OMC-SO
3
H
has been investigated. After being recycled 5 times, G-SO
3
H still
3
shows a high conversion (78.3%, Table 1, run 3), indicating that
5
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there is almost no activity loss (0.8% loss for the activity).
there is almost no limitation of mass transfer for the catalytic
reactions. This feature may be potentially important for indus-
However, by the same number of recycles, OMC-SO H has
3
a relatively high activity loss (9.7%, Table 1, run 6). The acid–
trial applications of G-SO H as an efficient catalyst in the future.
3
3
base titration shows that recycling of the OMC-SO H catalyst
leads to a partial loss of acidic sites (19.1% loss of total), while
À1
Acknowledgements
there is a relatively low loss of acidic sites (1.13 mmol g , 5.8%
loss) for G-SO
sulfonic groups on G-SO
to the difference in the thermal stability of the sulfonic groups, as
shown in Fig. 7. The stable sulfonic groups on G-SO H could be
3
H by the same treatment. The more stable
This work is supported by the State Basic Research Project of
China (2009CB623507) and the National Natural Science
Foundation of China (20973079 and U1162201).
3
H than OMC-SO H are closely related
3
3
greatly important for practical applications of G-SO H as an
3
efficient heterogeneous acidic catalyst in the future.
Notes and references
Moreover, the comparison of catalytic data over various
catalysts is extended to EAB, PRE, and HPO. These reactions
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3
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3
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3
1
5-Pr-SO
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3
3
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