Synthesis of immobilized Br?nsted acidic ionic liquid on silica gel as heterogeneous catalyst for esterification

Article abstract of DOI:10.1016/j.catcom.2010.10.014

The Br?nsted acidic ionic liquid 1-(propyl-3-sulfonate) vinylimidazolium hydrogen sulfate ([(CH₂)₃SO ₃HVIm]HSO₄) was immobilized on the silica gel using tetraethoxysilane (TEOS) as silica source. The properties of samples were characterized by FT-IR, elemental analysis and TG. The results showed that [(CH₂)₃SO₃HVIm]HSO₄ had been successfully immobilized onto the silica gel, and the immobilized ionic liquid catalyst (IL/silica gel) had good thermal stability. Moreover, the IL/silica gel exhibited high catalytic activity for a series of esterification and could be separated from the reaction mixture easily. It also remained satisfactory catalytic activity for the synthesis of n-Butyl acetate after 7 times recycling.

Full text of DOI:10.1016/j.catcom.2010.10.014

Catalysis Communications 12 (2011) 353–356
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Synthesis of immobilized Brønsted acidic ionic liquid on silica gel as heterogeneous
catalyst for esterification
Jinmei Miao, Hui Wan, Guofeng Guan ⁎
College of Chemistry and Chemical Engineering, 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 31 August 2010
Received in revised form 18 October 2010
Accepted 20 October 2010
Available online 28 October 2010
The Brønsted acidic ionic liquid 1-(propyl-3-sulfonate) vinylimidazolium hydrogen sulfate ([(CH ) SO HVIm]
2
3
3
HSO
samples were characterized by FT-IR, elemental analysis and TG. The results showed that [(CH
HSO had been successfully immobilized onto the silica gel, and the immobilized ionic liquid catalyst (IL/silica
4
) was immobilized on the silica gel using tetraethoxysilane (TEOS) as silica source. The properties of
)
2 3
3
SO HVIm]
4
gel) had good thermal stability. Moreover, the IL/silica gel exhibited high catalytic activity for a series of
esterification and could be separated from the reaction mixture easily. It also remained satisfactory catalytic
activity for the synthesis of n-Butyl acetate after 7 times recycling.
Keywords:
Esterification
Immobilized ionic liquid
Silica gel
© 2010 Elsevier B.V. All rights reserved.
1-Vinylimidazole
1
. Introduction
Esters ranging from aliphatic to aromatic were important organic
and catalyst recovery, and (iii) consequently high cost for the use of
relatively large amounts of ionic liquids [15,16]. Therefore, in order to
solve these problems mentioned earlier, immobilized ionic liquid
catalyst combining the advantageous characteristics of ionic liquids,
inorganic acids and solid acids had been proposed [17].
Recently, many reports about the preparation of silica-based
immobilized acidic ionic liquids catalysts had been published [18–22]
and the catalysts were normally immobilized on the amorphous silica
gel. Compared to the amorphous silica gel, the produced mesoporous
silica using TEOS as silica source possessed the excellent characteristics
such as stable mesoporous structure, high surface area, controllable
pore size and a large amount of silanol group. These advantageous
characteristics made mesoporous silica an attractive solid support in
the field of catalysis and functional materials [23–25]. However, there
were few studies about the preparation of immobilized Brønsted acidic
ionic liquid catalyst using TEOS as silica source and its application for
esterification.
compounds. They were generally used as plasticizers, solvents, per-
fumes and also as precursors for pharmaceuticals, agrochemicals and
fine chemicals [1]. Esters were usually obtained by the esterification
of alcohols with carboxylic acids [2]. Conventionally, the esterification
was carried out using inorganic liquid acids, solid acids or bio-
enzymes as catalysts [3]. However, several disadvantages of these
catalysts made this process inefficient and uneconomical. Inorganic
acids could cause the problems of equipment corrosion and severe
environmental pollution. Solid acids also suffered from several dis-
advantages such as high molecular weight/active-site ratios, extended
reaction time, low thermal stability and rapid deactivation from coking
[4,5]. And bio-enzymes were generally expensive for commercial use
and difficult to recycle without any catalytic activity loss [6]. Therefore,
great efforts towards the development of environmentally friendly
catalysts with high catalytic activity had been made [7,8].
Herein, we reported our efforts at exploring the immobilization of
In recent years, ionic liquids had attracted significant attention as
being environmental-friendly reaction media for their unique properties
of high thermal stability, negligible vapor pressure, tunable acidity and
selective dissolvability [9–14]. Though ionic liquids possessed such
promising advantages, their widespread practical application was still
hampered by several drawbacks: (i) high viscosity, which resulted in
only a minor part of ionic liquids took part in the catalyzed reaction,
Brønsted acidic ionic liquid [(CH
2 3 3 4
) SO HVIm]HSO on silica gel using
TEOS as silica source. The properties of samples were characterized
by FT-IR, elemental analysis, and TG. And the catalytic activity and
reusability performance of IL/silica gel used for esterification were
also examined.
2. Experimental
(
ii) homogeneous reaction, which was difficult for product separation
2.1. Chemical reagents
1-vinylimidazole (N98%), 3-mercaptopropyltrimethoxysilane (MPS)
⁎
Corresponding author. Tel./fax: +86 25 8358 7198.
E-mail address: guangf@njut.edu.cn (G. Guan).
(N98%), 1,3-propane-sultone (99%), 2,2-azo-bis-isobutyronitrile (AIBN,
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.10.014

354
J. Miao et al. / Catalysis Communications 12 (2011) 353–356
CP grade), EO20PO70EO20 (P123, Aldrich), p-hydroquinone, TEOS, ether
and other chemicals (AR grade) were commercially available and used
without further purification.
spectrophotometer with anhydrous KBr as standard. The loading
amount of ionic liquid immobilized on the thiol-functionalized silica
gel was determined by elemental analysis (Elementar Vario ELIII,
Germany). The thermal properties were examined by a STA 409PC
thermo gravimetric analyzer with a heating rate of 10 K per min under
2
2 3 3 4
.2. Preparation of Brønsted acidic ionic liquid [(CH ) SO HVIm]HSO
a dynamic N
2
atmosphere and the temperature was ranging from
2
5 °C to 800 °C.
First, 1,3-propane-sultone (12.2 g) was slowly added to 1-
vinylimidazole (9.4 g) and p-hydroquinone (0.044 g) in a 100 mL
beaker at 0 °C. The mixture was stirred until it turned into solid
totally. Then the formed solid was washed with ether for 3 times and
dried in vacuum at 50 °C [19]. Second, the formed solid (21.6 g) was
2.5. Esterification and analysis
Taking the esterification of acetic acid with n-butanol as an ex-
ample, the typical procedure was performed as follows: IL/silica gel
(0.96 g, 8% of the total mass of n-butanol and acetic acid), acetic
acid (4.80 g), n-butanol (7.12 g) and cyclohexane (8 mL, as a water-
carrying agent) were charged successively into a 250 mL round
bottom flask with a reflux condenser, a water segregator and a
magnetic stirrer. Then the reaction was allowed to proceed at desired
temperature for 3 h. All the reactions were analyzed using a gas
chromatograph equipped with an FID detector (SP6800A, SE-54
capillary column 30 m×0.25 mm×0.3 μm). The same procedures
were carried out for the other esterification of carboxylic acids by
alcohols.
2
dissolved in 5 mL H O in a 250 mL round bottom flask, and equal-
molar sulfuric acid was slowly dropped into the flask at 0 °C. After the
dropping was finished, the mixture was heated up to 60 °C gradually
and then stirred for 12 h [20]. Finally, the formed wine-colored liquid
was washed with ether for 3 times and dried in vacuum at 50 °C
for 5 h [Scheme 1].
2
.3. Preparation of immobilized Brønsted acidic ionic liquid
First, P123 (1.58 g) was dissolved in H O (18.0 g) and HCl (5.89 g)
2
at 45 °C. After TEOS (10.40 g) was added and pre-hydrolyzed for 1.5 h,
MPS was added. The mixture was stirred at 60 °C for 6 h to form thiol-
In a recycling experiment, IL/silica gel was recovered by vacuum
filtration, and collected for catalytic reusability test without any
disposal. The procedures for recycling experiment were the same as
described earlier.
functionalized silica gel. Then, [(CH
molar faction of [(CH SO HVIm]HSO
the reaction was carried out for another 30 h to form the final product.
The whole process was operated under N atmosphere. The product
2
)
3 3 4
SO HVIm]HSO and AIBN (20%
2
)
3
3
4
) were successively added and
2
was filtered, subsequently washed with acetone, ether and ethanol,
and then refluxed in ethanol for 24 h [26] to remove the template
P123. Finally, the formed immobilized Brønsted ionic liquid was dried
in vacuum at 60 °C for 6 h [Scheme 2].
3
. Results and discussion
3.1. Effect of molar ratio of IL to TEOS on catalytic performance of
catalysts
2
.4. Characterization
In order to obtain the appropriate amount of IL, effect of molar
ratio of IL to TEOS on catalytic performance of catalysts was
investigated. The results were shown in Table 1. The more [(CH
SO HVIm]HSO was added, the more loading amount of IL and the
Fourier Transform Infrared (FT-IR) spectra of the samples in the
2 3
)
−
1
range of 400–4000 cm were carried out on a Thermo Nicolet 870
3
4
2 3 3 4
Scheme 1. Preparation of [(CH ) SO HVIm]HSO .
Scheme 2. Preparation of IL/silica gel.

J. Miao et al. / Catalysis Communications 12 (2011) 353–356
355
Table 1
3.2.2. TG
a
Effect of molar ratio of IL to TEOS on catalytic performance of catalysts . CH3COOH +
CH3ðCH2 Þ OH→CH3COOðCH2 Þ CH3 + H2O:
Thermal stability of samples was investigated by thermo gravimet-
ric (TG) analysis, in which the observed weight loss was associated
with the loss of the organic components attached to the silica gel. TG
curves for silica gel, thiol-functionalized silica gel and IL/silica gel
were shown in Fig. 2, respectively. As shown in Fig. 2a and b, small
amount of weight loss within 200 °C was attributed completely to
the loss of adsorbed water molecules as well as the condensation
of silanol groups. However, the weight loss in Fig. 2c within 200 °C
decreased significantly, which might be ascribed to the increased
3
3
Entry
IL:TEOS
mol/mol)
N
(%)
Loading amout of IL
(mmol/g)b
Yield
(%)
c
(
1
2
3
4
0.6
0.8
1
1.16
1.34
2.33
3.66
0.48
0.56
1.13
2.22
80.3
83.3
88.8
99.2
1.2
a
n-Butanol (7.12 g); acetic acid (4.80 g); IL/silica gel (0.96 g); and cyclohexane
(
8.0 mL); 89 °C.
b
N% = 128
00−N% M = 28
hydrophobicity of particle surface after immobilization of [(CH
2 3-
)
Loading amount (mmol/g thiol-functionalized silica gel): 1
where M was the molecular weight of [(CH SO HVIm]HSO , 314.
× 1000,
×
SO HVIm]HSO ionic liquid. Thiol-functionalized silica gel obtained
3
4
)
2 3
3
4
c
Yield was achieved by GC analysis.
after the chemical modification with trialkoxysilane showed a higher
weight loss due to the loss of the mercaptopropyl moieties in
Fig. 2b. After immobilization of ionic liquid, the weight loss in Fig. 2c
increased again which was due to the further increased organic
components on the silica gel. As shown in Fig. 2c, the solid catalyst IL/
silica gel exhibited good thermal stability under 300 °C. The loading
amount of IL of fresh catalyst was 2.22 mmol/g, and those of cata-
lysts treated under 200 °C, 250 °C, and 300 °C were 2.07 mmol/g,
higher yield of n-butyl acetate were obtained. When the molar ratio
of IL to TEOS increased up to 1.2, the high yield of 99.2% was achieved.
It was unnecessary to further increase the amount of IL. Next, the
catalyst (Table 1, entry 4) was characterized as follows.
2
.01 mmol/g and 1.92 mmol/g, respectively. The loading amounts of
treated IL decreased slightly. The weight markedly decreased above
00 °C and organic components on the silica gel were decomposed
3
3
1
.2. Characterization
3
.2.1. FT-IR
completely around 700 °C.
2 3 3 4
First, the characteristic peaks of [(CH ) SO HVIm]HSO around
−1
−1
−1
−1
−1
642 cm , 1567 cm , 3139 cm , 1153 cm and 1046 cm could
3.3. Catalytic activities of different catalysts
be clearly observed in Fig. 1a and b, which were ascribed to CjC,
CjN, and C–H stretching vibrations of the imidazole ring [19], and
In order to evaluate the catalytic activity of IL/silica gel, the same
esterification of n-Butanol with acetic acid was carried out without
any catalyst or with [(CH ) SO HVIm]HSO (mass of the active site of
3
SjO asymmetric and symmetric stretching vibrations of the –SO H
−
1
−1
group, respectively. The bands at 1408 cm in Fig. 1a and 3437 cm
in Fig. 1b were assigned to the end-group CjC shear vibration of
(CH SO HVIm]HSO [27] and the O–H of physical adsorbed water,
respectively. Meanwhile, typical peaks of Si–O–Si and S–H of thiol-
2
3
3
4
IL/silica gel) and IL/silica gel, respectively. It could be seen in Table 2
that without catalyst, the yield just could reach 23.0% (Table 2, entry
1) which was far lower than those catalyzed by [(CH ) SO HVIm]
[
2
)
3
3
4
2
3
3
−
1
functionalized silica gel could be observed around 460 cm
2
and
566 cm in Fig. 1d. However, the peak of S–H and end-group CjC
totally disappeared in Fig. 1b, which illustrated that [(CH SO HVIm]
HSO ionic liquid was immobilized on silica gel by chemical covalent
bond via the route shown in Scheme 1.
HSO and IL/silica gel (Table 2, entries 2 and 3). This result indicated
4
−1
that the esterification was hardly occurred without catalyst. In order
2
)
3
3
to compare the catalytic activity of IL and IL/silica gel, we did kinetic
experiments to get the activation energies of two reactions. The
results showed that the activation energy of reaction catalyzed by IL/
silica gel (38.2 kJ/mol) was a little higher than that catalyzed by its
homogenous counterpart (33.9 kJ/mol). However, the heterogeneous
IL/silica gel could be easily separated by vacuum filtration and reused.
As shown in Fig. 3, catalytic performance of IL/silica gel could still
achieve 88.7% after 7 times recycling. The loading amount of IL of
4
Second, comparing the FT-IR spectra in Fig. 1b and c, we could find
that there were no significant changes. The result could explain the
satisfactory catalytic activity given by the recycled catalyst (R-IL/silica
gel).
Fig. 1. FT-IR spectra of [(CH
2
)
SO
3 3
HVIm]HSO
4
(a), IL/silica gel (b), R-IL/silica gel (c), and
Fig. 2. Thermo gravimetric curves of silica gel (a), thiol-functionalized silica gel (b) and
thiol-functionalized silica gel (d).
IL/silica gel (c).

356
J. Miao et al. / Catalysis Communications 12 (2011) 353–356
Table 2
Table 3
a
a
Esterification of n-Butanol and acetic acid under different conditions . CH3COOH +
CH3ðCH2 Þ OH→CH3COOðCH2 Þ CH3 + H2O:
Esterification of different alcohols and carboxylic acids in IL/silica gel . R ꢀ COOH +
IL=silica gel
R′ ꢀ OH
R ꢀ COO ꢀ R′ + H2O.
R′
3
3
Entry
Catalysts
Temperature
°C)
Time
(h)
Yield
(%)
Selectivity
(%)
Entry
R
Temperature Time Yield Selectivity
b
c
b
(%)c
(
(°C)
(h)
(%)
1
2
3
–
23.0
98.9
99.2
100
100
100
1
2
3
4
5
6
7
8
9
CH
CH
CH
CH
3
3
3
3
n-C
CH
4
H
9
89
95
94
93
93
88
71
88
90
99.2 100
99.4 100
98.6 100
99.0 100
86.3 100
92.2 100
84.1 100
93.4 100
98.5 100
IL
89
3
3
(CH
(CH
(CH
2 5
)
IL/silica gel
CH
CH
n-C
3
2
)
3
(C
2
H
5
)CHCH
CH
2
3
2
)
3
OCH
2
2
a
n-Butanol (7.12 g); acetic acid (4.80 g); IL/silica gel (0.96 g); IL (0.39 g, mass of the
CH
3
(CH
2
)
2
4
H
9
3
active site of IL/silica gel); and cyclohexane (8.0 mL).
CH
CH
CH
CH
3
3
3
3
CH
2
4 9
S-C H
b
Yield was achieved by GC analysis.
Selectivity for ester.
C
C
C
2
6
6
H
H
H
5
5
5
c
CH
2
CH
CH
2
2
a
Alcohol (96 mmol); carboxylic acid (80 mmol); IL/silica gel (8 wt.%); and cyclohexane
catalyst used 7 times was 1.17 mmol/g. It was demonstrated that the
decrease in catalytic activity was ascribed to the loss of ionic liquid.
(
8.0 mL).
b
c
Yield was achieved by GC analysis.
Selectivity for ester.
3
.4. Catalytic activity of IL/silica gel for esterification of other alcohols
and carboxylic acids
a higher catalytic performance than that of pure ionic liquid system.
Satisfactory catalytic activity 88.7% could still been kept after the
catalyst was recycled for 7 times for the synthesis of n-Butyl acetate.
Thus, the immobilized ionic liquid catalyst in this study was proved to
be a stable, efficient, reusable and potential catalyst for the synthesis
of carboxylic esters.
In order to investigate the scope and limitation of IL/silica gel as
catalyst for esterification, different alcohols and carboxylic acids as
the reactants were also tested and the results were summarized in
Table 3. It could be found that IL/silica gel could give very high
catalytic activity for esterification. Good to excellent yields ranging
from 84.1% to 99.4% with perfect selectivity for corresponding esters
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Fig. 3. Recycling of IL/silica gel for the synthesis of n-Butyl acetate.