Reusable and efficient polystyrene-supported acidic ionic liquid catalyst for esterifications

Article abstract of DOI:10.1016/j.molcata.2010.09.011

Polystyrene (PS)-supported 1-(propyl-3-sulfonate) imidazolium hydrosulfate acidic ionic liquid (PS-CH₂-[SO₃H-pIM][HSO₄]) catalyst was prepared by supporting the ionic liquid onto highly cross-linked chloromethylated polystyrene (PS-CH₂Cl). FT-IR, SEM and TG-DSC were employed to characterize the structure and property of the catalyst. Results suggested that acidic ionic liquid was supported onto the surface of PS-CH ₂Cl by covalent bond. The original rough surface of PS-CH ₂Cl was covered with acidic ionic liquid, forming a compact and thin surface layer, and its size had no obvious change. Moreover, the PS-CH ₂-[SO₃H-pIM][HSO₄] catalyst showed a better thermal stability than that of PS-CH₂Cl support. It also exhibited high catalytic activity for a series of esterifications. After the catalyst was reused for 13 times in the synthesis of n-butyl acetate, the yield only decreased 7.3%. A reaction mechanism of esterification over this new catalyst was proposed as well.

Full text of DOI:10.1016/j.molcata.2010.09.011

Journal of Molecular Catalysis A: Chemical 332 (2010) 152–157
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Reusable and efficient polystyrene-supported acidic ionic liquid catalyst for
esterifications
Zhenjin Xua, Hui Wana, Jinmei Miaoa, Mingjuan Hanb, Cao Yanga, Guofeng Guana,∗
a College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, PR China
b College of Sciences, Nanjing University of Technology, Nanjing 210009, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 June 2010
Received in revised form 8 September 2010
Accepted 9 September 2010
Available online 17 September 2010
Polystyrene (PS)-supported 1-(propyl-3-sulfonate) imidazolium hydrosulfate acidic ionic liquid (PS-
CH2-[SO3H-pIM][HSO4]) catalyst was prepared by supporting the ionic liquid onto highly cross-linked
chloromethylated polystyrene (PS-CH2Cl). FT-IR, SEM and TG-DSC were employed to characterize the
structure and property of the catalyst. Results suggested that acidic ionic liquid was supported onto the
surface of PS-CH2Cl by covalent bond. The original rough surface of PS-CH2Cl was covered with acidic
ionic liquid, forming a compact and thin surface layer, and its size had no obvious change. Moreover,
the PS-CH2-[SO3H-pIM][HSO4] catalyst showed a better thermal stability than that of PS-CH2Cl support.
It also exhibited high catalytic activity for a series of esterifications. After the catalyst was reused for
13 times in the synthesis of n-butyl acetate, the yield only decreased 7.3%. A reaction mechanism of
esterification over this new catalyst was proposed as well.
Keywords:
Polystyrene
Ionic liquids
Supported catalyst
Esterification
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
ZrO2, TiO2, Al2O3 and silica materials firstly [15]. Particularly, many
researchers focused on the investigation of supporting ionic liquids
Carboxylic esters as important intermediates for the synthe-
sis of dopes, adhesives, plasticizers, flavors, and pharmaceuticals,
had attracted much attention. Typically, they were synthesized
by Fischer esterification of carboxylic acids and alcohols [1–4].
The popular catalysts in this esterification reaction were inor-
ganic liquid-phase acids including hydrochloric acid and sulfuric
acid [5,6]. Although these homogeneous mineral acids presented
high catalytic performance in this reaction, they suffered from
some problems, such as tedious purification procedure of the
product, large amount of acidic wastes, and corrosion of equip-
ments. These could cause environmental pollution and reduce the
economic benefit [7–9]. Therefore, there was an urgent need to
develop the environmentally friendly and highly efficient catalysts
for this esterification reaction. In recent years, ionic liquids as novel
catalysts had received particular interests due to their unique prop-
erties (thermal stability, negligible vapor pressure and selective
solubility). Acidic ionic liquids including the Lewis and the Brønsted
ones had proved to be effective catalysts in esterification [10–12].
However, their high costs and unendurable viscosities restricted
their industrial applications [13,14].
onto the surface of silica [16–21]. The supported ionic liquid cat-
alysts had many advantages over their unsupported counterparts,
such as separation, reusability, and the ability to provide practical
conveniences in a continuous system. In 2004, Kim and Chi [22]
developed PS-supported ionic liquid catalysts for a series of nucle-
ophilic substitution reactions. In 2006, this research group further
[23] investigated the structural modified PS-supported ionic liquids
and examined their catalytic activities in nucleophilic fluorination
and other substitution reactions. Recently, PS-supported basic ionic
liquid catalysts had been used for the hydrolysis of propylene car-
bonate [24] and the aldol-type coupling of a variety of aldehydes
with ethyl diazoacetate in water [25]. Furthermore, Sun et al. [26]
investigated the immobilization of ionic liquids which had both
acidic and basic characteristics onto PS resin and the application
in the synthesis of cyclic carbonates. All the reported PS-supported
ionic liquid catalysts mentioned above could catalyze the organic
reactions efficiently, and give high yield, high selectivity and good
reusability.
The previous reports showed that PS-supported ionic liquid cat-
alysts were potential candidates for catalyzing organic reactions
such as CO2 cycloaddition reaction [27,28], Henry reaction [29]
and acetalization reaction [30,31]. However, there were few studies
on PS-supported acidic ionic liquid catalyst for Fischer esterifica-
tions. In this paper, we focused on investigating the synthesis of
PS-supported acidic ionic liquid (PS-CH2-[SO3H-pIM][HSO4]) cat-
alyst and its catalytic performance for Fischer esterification. FT-IR,
In order to solve these problems mentioned above, chloroa-
luminate ionic liquids had been supported onto the surface of
∗
Corresponding author. Tel.: +86 25 83587198.
E-mail address: guangf@njut.edu.cn (G. Guan).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.09.011

Z. Xu et al. / Journal of Molecular Catalysis A: Chemical 332 (2010) 152–157
153
O
O
O
-
HN
HN
N
N
HN
N
SO3
S
AIBN
PS
75°C,8h
CH2Cl
H2SO4
H2O
-
Cl
HN
N
SO3H
PS-CH2Cl
SO3
HSO4
[SO3H-pHIM][HSO4]
O
S
O
O
PS
N
NH
°C
PS
H2C
H2C
N
N
SO3H
N
NH
Cl
°C
Scheme 1. Synthesis of ionic liquid.
Cl
PS-CH2-[SO3H-pIM][Cl]
PS-CH2-[HIM][Cl]
SEM and TG-DSC were employed to characterize the structure and
property of the catalyst. A reaction mechanism of esterification over
this new catalyst was proposed as well.
H2O
H2SO4
PS
H2C
N
N
HSO4
2. Experimental
SO3H
2.1. Chemical reagents
PS-CH2-[SO3H-pIM][HSO4]
Scheme 2. Synthesis of PS-CH2Cl and PS-CH2-[SO3H-pIM][HSO4] catalyst.
Divinylbenzene (DVB, mixture of isomers, 78–80% grade, Kehua
Laboratory Instruments Sales Department, China) and styrene (St,
>99%, Sinopharm Chemical Reagent Co. Ltd., China) were extracted
with 5% (w/w) hydroxyl sodium solution to remove the inhibitor
(p-hydroquinone), respectively, and then washed with deionized
water until the pH was equal to 7.0. Vinyl benzyl chloride (VBC, 95%,
mixture of m- and p-isomers, Changzhou Wujin Linchuan Chem.
Co. Ltd., China,), 2,2-azo-bis-isobutyronitrile (AIBN, Shanghai NO.4
Reagent & H.V. Chem. Co. Ltd.), polyvinyl alcohol (PVA, degree of
polymerization: 1788), dichloromethane, toluene, imidazole, 1,3-
propane sultone (99%) and other corresponding chemicals were
commercially available and used without further purification.
removal of other impurities. Finally, the product was dried under
vacuum at 50 ◦C for 24 h to obtain the cross-linked PS-CH2Cl. The
cross-link degree of PS-CH2Cl was 93.5%.
2.4. Preparation of PS-CH2-[SO3H-pIM][HSO4] catalyst
The synthetic process of PS-CH2-[SO3H-pIM][HSO4] catalyst
was shown in Scheme 2. First, PS-CH2Cl (10.5 g) was suspended
into a 250 mL three-necked flask reactor containing 100 mL anhy-
drous toluene and an excess of imidazole (10.5 g). Then the mixture
was reacted with stirring at 90 ◦C for 24 h. After the completion
of the reaction, the mixture was filtered to get the solid product
named as PS-supported imidazolium chloride (PS-CH2-[HIM][Cl]).
The obtained PS-CH2-[HIM][Cl] was washed with toluene, ethanol
and methanol, separately, and dried under vacuum at 50 ◦C for 4 h.
Second, PS-CH2-[HIM][Cl] (10.5 g) reacted with 1,3-propane sul-
tone (10.5 g) at 90 ◦C for 24 h, using anhydrous toluene as a solvent.
The product named as PS-supported 1-(propyl-3-sulfonate) imida-
zolium chloride (PS-CH2-[SO3H-pIM][Cl]) was repeatedly washed
with toluene, ethanol and acetone, respectively, and dried under
vacuum at 50 ◦C for 4 h. Finally, the prepared PS-CH2-[SO3H-
pIM][Cl] (10.5 g) was directly acidified with sulfuric acid solution
composed of sulfuric acid (14.0 g) and water (35.0 g) under stirring
condition at 60 ◦C for 24 h. The target catalyst PS-CH2-[SO3H-
pIM][HSO4] was collected by filtration and washed with large
amount of diethyl ether and ethanol, repeatedly, and dried under
vacuum at 50 ◦C for 12 h.
2.2. Preparation of ionic liquid [SO3H-pHIM][HSO4]
The synthetic process of [SO3H-pHIM][HSO4] was shown in
Scheme 1 [32]. First, imidazole was dissolved in ethanol and an
equal mole of 1,3-propane sultone was slowly added. The mix-
ture was stirred at room temperature for 24 h. After the reaction
was completed, the reaction mixture was filtered to get the white
precipitate which was washed with ethanol and dried under vac-
uum at 60 ◦C for 12 h. Second, the obtained white precipitate was
dissolved in water and an equal mole of sulfuric acid was added
slowly. Then the mixture was slowly heated to 60 ◦C for 12 h. After
that, water was removed under vacuum at 60 ◦C for 12 h, resulting
in [SO3H-pHIM][HSO4] as a viscous and colorless liquid.
2.3. Preparation of PS-CH2Cl
The PS-CH2Cl was synthesized by the polymerization of con-
ventional free radical suspension according to the previous reports
[33,34] and the synthetic process was shown in Scheme 2. First,
3.5 g PVA (1.0% wt), 10.5 g sodium chloride and 1.75 g AIBN (0.5 wt%
of total water phase) were gradually added into the glass reactor
containing 350 mL deionized water, and stirred to be a transpar-
ent solution. Second, the organic phase composed of 20 mL of
the monomer mixture (volume ratio of VBC:DVB:St = 12:4:4) and
24 mL of porogen (toluene) were mixed in the beaker and regularly
stirred until this organic phase became homogeneous. Third, the
polymerization of conventional free radical suspension was con-
ducted by adding the uniformed organic phase into the transparent
solution mentioned above at 75 ◦C for 8 h under the protection of
nitrogen atmosphere. Fourth, the synthesized PS-CH2Cl was col-
lected by means of pumping filtration and washed with hot water
and methanol for three times, respectively, to remove the disper-
sant (PVA). Then, the product was extracted with dichloromethane
for 10 h in a Soxhlet to remove the porogen (toluene) and resid-
ual monomers, and washed with ethanol for twice to ensure the
2.5. Characterization
The amount of ionic liquid supported on the PS-CH2Cl was
determined by elemental analysis (Vario EL III, Elementar, Ger-
many). The nitrogen analysis of PS-CH2-[SO3H-pIM][HSO4] (N,
5.32%) indicated that its loading amount was 1.90 mmol/g. The
content of chlorin of PS-CH2Cl (12.2 wt%) was achieved by oxy-
gen flask combustion method with AgNO3 potentiometric titration.
Fourier transform infrared (FT-IR) spectroscopy was carried out
on a Thermo Nicolet 870 spectrophotometer with anhydrous KBr
as standard (Nicolet, America). Thermogravimetric and differential
scanning calorimetry (TG-DSC) analysis was performed using STA
449F3 (Netzsch, Germany) in a nitrogen atmosphere between 25 ◦C
and 800 ◦C with a heating rate 10 ◦C/min. Average diameters and
surface morphologies of PS-CH2Cl and PS-CH2-[SO3H-pIM][HSO4]
were characterized by a scanning electron microscope (SEM)
QUANTA200 (FEI, Holland).

154
Z. Xu et al. / Journal of Molecular Catalysis A: Chemical 332 (2010) 152–157
O
catalyst
R'OH
H2O
RCOOH
R
C
OR'
Scheme 3. Esterifications of various acids and alcohols.
2.6. Procedure for esterification reactions
Taking the esterification of acetic acid with n-butanol as
an example, the typical procedure was performed as follows
(Scheme 3): acetic acid (4.80 g, 80 mmol), n-butanol (7.12 g,
96 mmol), cyclohexane (8 mL) and catalyst (0.95 g, 8% of the total
mass of alcohol and acid) were added into a round-bottomed flask
with a water segregator (cyclohexane as the water-carrying agent).
The resulting mixture was refluxed at 92 ◦C for 3 h with magnetic
stirring. The content of produced ester was determined by a gas
chromatograph equipped with an FID detector (SP6800A, SE-54
capillary column 30 m × 0.25 mm × 0.3 ␮m). For other esterifica-
tion reactions, the procedure was carried out accordingly.
In a recycling experiment, the catalyst was recovered by filtra-
tion and washed with acetone, and then dried under vacuum at
50 ◦C for 2 h. The procedure for recycling reaction was the same as
described above.
Fig. 1. Comparisons of FT-IR spectra of [SO3H-pHIM][HSO4] (a), PS-CH2Cl (b), PS-
CH2-[SO3H-pIM][HSO4] catalyst (c) and PS-CH2-[SO3H-pIM][HSO4] catalyst reused
for 13 times (d).
Fig. 2. The whole and magnified SEM images of PS-CH2Cl (A and B), PS-CH2-[SO3H-pIM][HSO4] catalyst (C and D) and PS-CH2-[SO3H-pIM][HSO4] catalyst reused for 13 times
(E and F).

Z. Xu et al. / Journal of Molecular Catalysis A: Chemical 332 (2010) 152–157
155
3. Results and discussion
(a)
(b)
100
0
3.1. FT-IR
( b' )
( a' )
80
To confirm the immobilization of the active component [SO3H-
pHIM][HSO4] on the support PS-CH2Cl and the stability of
PS-CH2-[SO3H-pIM][HSO4] catalyst, FT-IR spectroscopic studies
were carried out. For [SO3H-pHIM][HSO4] (Fig. 1a), the charac-
teristic peaks at 3144 cm−1, 1642 cm−1and 1564 cm−1 were due
to the C–H, C C, and C N stretching vibration of imidazole ring,
respectively [35,36], and the bands at 1215 cm−1 and 1040 cm−1
were assigned to the S O asymmetric and symmetric stretching
vibrations of –SO3H group. For PS-CH2Cl (Fig. 1b), the characteris-
tic peaks of polystyrene exhibited the C–H stretching vibration of
aromatic ring at 3022 cm−1, and C–H asymmetric and symmetric
-2
-4
-6
60
40
20
0
0
100 200 300 400 500 600 700 800 900
Temperature (°C)
stretching vibrations of methylene at 2920 cm−1 and 2850 cm−1
.
The peaks at 1604 cm−1 and 1491 cm−1 were due to the C–C
skeleton vibration of aromatic ring of polystyrene [37]. These
above-mentioned peaks were also observed in PS-CH2-[SO3H-
pIM][HSO4] catalyst (Fig. 1c). Comparatively, a typical peak at
1265 cm−1 for stretching frequency of the functional group –CH2Cl
disappeared in the spectrum of PS-CH2-[SO3H-pIM][HSO4] cata-
lyst (Fig. 1c). This observation suggested that [SO3H-pHIM][HSO4]
had been supported on PS-CH2Cl surface through covalent bond. In
addition, compared with the fresh PS-CH2-[SO3H-pIM][HSO4] cat-
alyst, these above characteristic bands also appeared in the reused
PS-CH2-[SO3H-pIM][HSO4] catalyst (Fig. 1d). This indicated that PS-
CH2-[SO3H-pIM][HSO4] catalyst was very stable. However, three
new peaks appeared at 1726 cm−1, 982 cm−1 and 612 cm−1, which
were attributed to the C O, C–O stretching vibrations, and the
Fig. 3. TG-DSC patterns of PS-CH2Cl (a, aꢀ) and PS-CH2-[SO3H-pIM][HSO4] catalyst
(b, bꢀ).
small obvious exothermic peak appeared around 50 ◦C. However,
it disappeared in the DSC curve of PS-CH2Cl (Fig. 3aꢀ). When the
temperature further increased up to 300 ◦C and 350 ◦C, weight
loss obviously displayed in TG curves of both PS-CH2Cl and PS-
CH2-[SO3H-pIM][HSO4]. The reason may be that their structures
were destroyed and disordered or the chloromethyl group and
the ionic liquid separated from the surface of PS-CH2Cl. Corre-
spondingly, sharp exothermic peaks at 440 ◦C and 450 ◦C appeared,
respectively. Finally, it was observed that the residue weight of PS-
CH2-[SO3H-pIM][HSO4] and PS-CH2Cl was about 35% and 22% at
460 ◦C, respectively. These observations indicated that the immo-
bilization of ionic liquid onto the PS-CH2Cl surface could improve
the thermal stability of PS-CH2Cl.
C
O wagging vibration of n-butyl acetate, respectively. This phe-
nomenon may be caused a little n-butyl acetate adhered on the
surface of catalyst during the recycling experiments.
3.2. SEM
3.4. Catalytic activity of PS-CH2-[SO3H-pIM][HSO4] catalyst for
esterifications
SEM was performed to characterize the morphology of
PS-CH2Cl, PS-CH2-[SO3H-pIM][HSO4] catalyst and PS-CH2-[SO3H-
pIM][HSO4] catalyst reused for 13 times as shown in Fig. 2. It
was seen that uniform PS-CH2Cl was in the size of 250–300 ␮m
(Fig. 2A). After the introduction of [SO3H-pHIM][HSO4], the size
of PS-CH2Cl had no obvious change (Fig. 2C). At high magnifica-
tions, the surface of PS-CH2Cl (Fig. 2B) was rough, whereas the
surface of PS-CH2-[SO3H-pIM][HSO4] catalyst (Fig. 2D) was much
plain. This finding demonstrated that the surface of PS-CH2Cl was
bonded with [SO3H-pHIM][HSO4] ionic liquid, forming a compact
and thin surface layer. Moreover, in comparison with fresh PS-CH2-
[SO3H-pIM][HSO4] catalyst (Fig. 2C, 2D), the size of reused catalyst
(Fig. 2E, 2F) had a little change and the surface was rough slightly.
This indicated that there was little ionic liquid falling off the surface
of PS-CH2Cl.
To evaluate the PS-CH2-[SO3H-pIM][HSO4] catalyst described
here, the catalytic activities of various catalysts for the esterification
reaction of n-butyl alcohol and acetic acid were examined under the
same reaction conditions (Table 1).
Obviously, the yields of n-butyl acetate over PS-CH2-[SO3H-
pIM][HSO4] (Table 1, sample 4) and acidic ionic liquid (Table 1,
sample 3) were much higher than that over the support (Table 1,
sample 2) and no catalyst (Table 1, sample 1). This meant that the
reaction hardly carried out without catalyst and the support almost
had no influence on the yield. The pure ionic liquid was considered
as the active component and the PS-CH2-[SO3H-pIM][HSO4] cata-
lyst had obvious catalytic activity. The TON of [SO3H-pHIM][HSO4]
(Table 1, sample 3) and PS-CH2-[SO3H-pIM][HSO4] (Table 1, sam-
ple 4) were almost the same. However, the heterogeneous catalyst
facilitated the separation of the product.
3.3. TG-DSC
Table 1
TG-DSC analysis was carried out to investigate the thermal
stability of PS-CH2Cl and PS-CH2-[SO3H-pIM][HSO4] catalyst, as
shown in Fig. 3.
Activities of various catalysts for the esterification of n-butyl alcohol and acetic acida.
Samples
Catalysts
Active sites
(mmol)
Yields (%)
TONb
Comparatively, in the range of 25–200 ◦C, there was more
weight loss over PS-CH2-[SO3H-pIM][HSO4] catalyst (Fig. 3b) than
that over PS-CH2Cl (Fig. 3a). The weight loss was mainly ascribed to
the physical adsorbed water. The hydrophilic structure of ionic liq-
uid [SO3H-pHIM][HSO4] facilitated its adsorption of more water.
Another aspect, some residual imidazole might be adhered to
the surface of PS-CH2Cl by electrostatic adsorption. These two
reasons could cause the difference in the weight loss. Corre-
spondingly, for PS-CH2-[SO3H-pIM][HSO4] catalyst (Fig. 3bꢀ), a
1
2
3
4
Blank
0
0
22.6
35.8
98.2
99.1
–
–
43.4
43.8
PS-CH2Clc
[SO3H-pHIM][HSO4]
PS-CH2-[SO3H-pIM][HSO4]
0.0018
0.0018
a
Reaction conditions: nn-butyl alcohol:nacetic acid = 1.2:1, (wt)% catalyst = 8%, cyclo-
hexane (8 mL), 92 ◦C, 3 h.
b
Turnover number as mole of product/mole of acidic ionic liquid in catalyst.
Prepared via PS-CH2Cl acidified with the same concentration of sulfuric acid
c
solution in the preparation of PS-CH2-[SO3H-pIM][HSO4] catalyst.

156
Z. Xu et al. / Journal of Molecular Catalysis A: Chemical 332 (2010) 152–157
Scheme 4. Mechanism of esterification catalyzed over PS-CH2-[SO3H-pIM][HSO4] catalyst.
100
80
60
40
20
0
In order to further investigate the application of PS-CH2-[SO3H-
pIM][HSO4] catalyst for various systems of esterification, different
systems were carried out and the results were displayed in Table 2.
It was concluded that yields of various esters were all higher
than 86.8%, even up to 99.4%. It indicated that the PS-CH2-[SO3H-
pIM][HSO4] catalyst possessed good catalytic activity for a series of
esterification reactions of different alcohols and acids.
3.5. Recycling experiments
The reusability of PS-CH2-[SO3H-pIM][HSO4] catalyst was eval-
uated in the esterification reaction of n-butyl alcohol and acetic
acid. As shown in Fig. 4, the reused PS-CH2-[SO3H-pIM][HSO4] cat-
alyst still showed high activity for the yield of n-butyl acetate, and
only decreased 7.3% after reusing for 13 times. As indicated by the
FT-IR and SEM characterizations, the size and surface of reused cat-
alyst changed slightly. Moreover, FT-IR analysis showed that there
was a little n-butyl acetate adhering on the surface of reused cat-
alyst, which might cover the active site of the catalyst. The slight
decrease of catalytic activity in the recycles may be caused by this
reason. All these results indicated that PS-CH2-[SO3H-pIM][HSO4]
catalyst was efficient and reusable.
1
2
3
4
5
6
7
8
9
10 11 12 13
Recycling times
Fig. 4. Recycling of PS-CH2-[SO3H-pIM][HSO4] catalyst for the synthesis of n-butyl
acetate.
tent with the reported one. As displayed in Scheme 4, acid was first
protonated to generate a carbocation. Then, the O atom of Rꢀ–OH
made a nucleophilic attack as a result of producing the middle prod-
uct 5 (as shown in Step 2). The protonated hydroxyl in product 6
had been formed when H+ of product 5 transferred, and then one
molecule water took off the backbone of product 6. Finally, this
catalyst was recovered when the bond of hydrogen and oxygen of
product 7 had been broken.
3.6. Mechanism of esterification
It was well known that the classical esterification mechanism
usually had two key steps, one was the protonation of acid, and
the other was the nucleophilic attack of the protonated acid by the
alcohol [38–40]. In the present work, the catalytic mechanism of
esterification over PS-CH2-[SO3H-pIM][HSO4] catalyst was consis-
4. Conclusions
Table 2
Catalytic activity of PS-CH2-[SO3H-pIM][HSO4] catalyst for different esterificationsa.
The PS-supported acidic ionic liquid catalyst PS-CH2-[SO3H-
pIM][HSO4] was prepared and applied in the synthesis of carboxylic
ester. Acidic ionic liquid was successfully supported onto the PS-
CH2Cl surface by covalent bond, and the original rough surface
of PS-CH2Cl was covered with acidic ionic liquid, forming a com-
pact and thin surface layer, and its size had no obvious change.
Moreover, the PS-CH2-[SO3H-pIM][HSO4] catalyst showed a bet-
ter thermal stability than that of PS-CH2Cl, and it also exhibited
high catalytic activity in a series of esterification reactions. The
PS-CH2-[SO3H-pIM][HSO4] catalyst could be recovered by sim-
ple filtration, and the yield only decreased 7.3% for the synthesis
of n-butyl acetate after reusing for 13 times. In conclusion, the
Samples Rb
Rꢀb
Yields (%) Selectivity (%)
1
2
3
4
5
6
7
8
CH3
CH3
CH3
CH3
CH3
CH3CH2
CH3CH2
n-C4H9
n-CH3(CH2)5
i-CH3(CH2)3CH(C2H5)CH2 98.1
C6H5CH2
CH3(CH2)3OCH2CH2
n-C4H9
C6H5CH2
99.1
99.3
100
100
100
100
100
100
100
100
99.3
97.5
99.4
86.8
87.9
CH3(CH2)2 n-C4H9
a
Reaction conditions: nalcohol:nacid = 1.2:1, (wt)% catalyst = 8%, cyclohexane (8 mL),
92 ◦C, 3 h.
b
R and Rꢀ represented different groups in acid or alcohol shown in Scheme 3.

Z. Xu et al. / Journal of Molecular Catalysis A: Chemical 332 (2010) 152–157
157
PS-CH2-[SO3H-pIM][HSO4] catalyst prepared in this paper was an
excellent recyclable catalyst for the synthesis of carboxylic esters
and showed potential application in industry.
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