Imidazolium ionic liquid-supported sulfonic acids: Efficient and recyclable catalysts for esterification of benzoic acid

Article abstract of DOI:10.1016/j.cclet.2011.09.016

Several imidazolium ionic liquid (IL)-supported sulfonic acids with different anions, [C₃SO₃Hmim]HSO₄, [C ₃SO₃Hmim]BF₄, [C₃SO ₃Hmim]PF₆, and [C₃SO₃Hmim]CF ₃SO₃, were synthesized and applied as catalysts for esterification reaction of benzoic acid. The experimental results indicate that imidazolium IL-supported sulfonic acid containing anion of HSO₄ ^- shows the best catalytic activity. Only when less [C ₃SO₃Hmim]HSO₄ (0.3 equiv.) applied, was the product obtained with high yield of 97%. Furthermore, the produced esters could be separated by decantation, and the catalyst could be reused after the removal of water.

Full text of DOI:10.1016/j.cclet.2011.09.016

Available online at www.sciencedirect.com
Chinese Chemical Letters 23 (2012) 1–4
www.elsevier.com/locate/cclet
Imidazolium ionic liquid-supported sulfonic acids: Efficient and
recyclable catalysts for esterification of benzoic acid
b
c
b
b
Yue Qin Cai a, , Guo Qiang Yu , Chuan Duo Liu , Yuan Yuan Xu , Wei Wang
*
a School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, China
b School of Chemistry and Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China
c Shanghai Key Laboratory of Chemical Biology, Institute of Pesticides & Pharmaceuticals,
East China University of Science and Technology, Shanghai 200237, China
Received 10 May 2011
Available online 9 November 2011
Abstract
Several imidazolium ionic liquid (IL)-supported sulfonic acids with different anions, [C3SO3Hmim]HSO4, [C3SO3Hmim]BF4,
[C3SO3Hmim]PF6, and [C3SO3Hmim]CF3SO3, were synthesized and applied as catalysts for esterification reaction of benzoic acid.
The experimental results indicate that imidazolium IL-supported sulfonic acid containing anion of HSO4À shows the best catalytic
activity. Only when less [C3SO3Hmim]HSO4 (0.3 equiv.) applied, was the product obtained with high yield of 97%. Furthermore,
the produced esters could be separated by decantation, and the catalyst could be reused after the removal of water.
# 2011 Yue Qin Cai. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: IL-supported sulfonic acid; Catalyst; Esterification reaction; Benzoic acid
Arylic esters are important products or intermediates in the chemical and pharmaceutical industries. Popular
catalysts for esterification reactions from undergraduate laboratories to chemical manufacturing plants are mineral
acids (sulfuric acid, hydrochloric acid, and orthophoric acid) as well as Lewis acids (boric acid and AlCl3) [1]. In
recent years, from the viewpoint of environmental consciousness, solid acids are being more widely used since they are
deemed less noxious than traditional liquid acids [2]. However, these solid acids suffer from drawbacks like restricted
accessibility of the matrix-bound acidic sites, high molecular weight/active-site ratios, and rapid deactivation from
coking [3]. On the other hand, ionic liquids (ILs) have attracted more and more interests in various chemical processes
due to their many fascinating and intriguing properties such as nonvolatile, thermally stable properties, and ease of
recyclability [4]. Recently, esterification reaction in the presence of ILs with or without an additional catalyst has been
investigated and showed good performance [5]. For example, Zhao et al. reported the esterification of aliphatic acid
only with 0.5–1.0 mol% SO3H-functionalized ionic liquids [5i]. Nevertheless, in most cases aliphatic esters synthesis
in ILs were discussed, and few reports on arylic esters synthesis in ILs were found. Xing et al. [1] synthesized task-
specific ionic liquids (TSILs) that bear an alkane sulfonic acid group in pyridinium cation, and also investigated their
catalytic activities for esterification reaction of benzoic acid. However, the reaction was accomplished by combining
1:1 molar quantity of substrate and catalyst, which hindered their industrial applications. A following work by Gui and
* Corresponding author.
E-mail address: caiyueqin@ecust.edu.cn (Y.Q. Cai).
1001-8417/$ – see front matter # 2011 Yue Qin Cai. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2011.09.016

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Y.Q. Cai et al. / Chinese Chemical Letters 23 (2012) 1–4
N
N+
SO H
-
N
N+
SO H
3
-
3
BF
HSO
4
4
[C SO Hmim]HSO
[C SO Hmim]BF
3 3 4
3
3
4
N
N+
SO H
-
N
N+
SO H
3
-
3
CF SO
PF
6
3
3
[C SO Hmim]PF
[C SO Hmim]CF SO
3 3 3 3
3
3
6
Scheme 1.
co-workers [5g] showed that esterification of aromatic acids was more difficult compared with that of aliphatic acids
and longer reaction time (16 h) was utilized [5g] using SO3H-functionalized ionic liquid as a catalyst. Therefore, we
focused our attention on the application of a highly active catalyst, which was expected to utilize only as a catalyst with
less amount for esterification in less reaction time. For this purpose, we now wish to report on a novel and appealing
strategy for the application of several TSILs bearing an alkane sulfonic acid group in imidazolium cation with different
anions, [C3SO3Hmim]HSO4, [C3SO3Hmim]BF4, [C3SO3Hmim]PF6, and [C3SO3Hmim]CF3SO3. By which the
esterification reactions of benzoic acid with alcohols have been studied from aspects of activity, recyclability,
separation work-up, and substrate availability.
The imidazolium IL-supported sulfonic acids, [C3SO3Hmim]HSO4, [C3SO3Hmim]BF4, [C3SO3Hmim]PF6, and
[C3SO3Hmim]CF3SO3 could easily be prepared according to Ref. [6] (Scheme 1).
For the initial studies on the catalytic activity of the imidazolium IL-supported sulfonic acids, the esterification of
benzoic acid with ethanol was chosen as a model reaction, which was carried out in the presence of
[C3SO3Hmim]HSO4 without other additional solvents. The reaction conditions were optimized in terms of
temperature, catalyst concentration, reaction time, and ratio of benzoic to ethanol. As we expected, the results in Table
1 indicated that the reaction proceeded smoothly with excellent yields (97–98%) for esterification with less amount of
[C3SO3Hmim]HSO4, compared to [PSPy]HSO4 reported by Xing et al. (Table 1, entry 8), which could be decreased by
about three folds from 1 molar to 0.3 molar quantities of benzoic acid without the activity loss (Table 1, entries 1, 3
Table 1
The esterification reaction of benzoic acid with ethanol catalyzed by TSILs.a
Entry
Catalyst
Acid/ethanol/IL ratio
Time (h)
Temperature (8C)
Yieldb (%)
1
2
[C3SO3Hmim]HSO4
[C3SO3Hmim]HSO4
[C3SO3Hmim]HSO4
[C3SO3Hmim]HSO4
[C3SO3Hmim]HSO4
[C3SO3Hmim]HSO4
[C3SO3Hmim]HSO4
[PSPy]HSO4
1:3:1
3.0
3.0
3.0
3.0
2.5
2.5
2.5
3
100
100
100
100
100
95
98
96
97
84
97
97
84
92
86
80
83
81
85
96
96
95
95
1:4:0.5
1:3:0.5
1:2:0.5
1:3:0.3
1:3:0.3
1:3:0.25
1:2:1
3
4
5
6
7
8c
95
85
9
[C3SO3Hmim]HSO4
[C3SO3Hmim]HSO4
[C3SO3Hmim]BF4
1:3:0.3
1:3:0.3
1:3:0.3
1:3:0.3
1:3:0.3
1:3:0.3
1:3:0.3
1:3:0.3
1:3:0.3
2.5
2.0
2.5
2.5
2.5
2.5
2.5
2.5
2.5
85
10
11
12
13
14d
15d
16d
17d
95
95
[C3SO3Hmim]PF6
95
[C3SO3Hmim]CF3SO3
[C3SO3Hmim]HSO4 (2nd run)
[C3SO3Hmim]HSO4 (3nd run)
[C3SO3Hmim]HSO4 (4nd run)
[C3SO3Hmim]HSO4 (5nd run)
95
95
95
95
95
a
Benzoic acid (5 mmol).
b
c
d
Isolated yield based on benzoic acid.
Report by Xing et al. [1].
Reuse of the catalyst.

Y.Q. Cai et al. / Chinese Chemical Letters 23 (2012) 1–4
3
Table 2
The esterification reaction of benzoic acid with alcohols catalyzed by [C3SO3Hmim]HSO4.a
b
Entry
Alcohols
Time (h)
Yield (%)
1
2
3
4
5
Methanol
2.0
2.5
3.5
4.0
4.0
98
95
83
85
86
Butanol
Cyclohexanol
Benzenemethanol
Lauryl alcohol
a
The molar ratios of benzoic acid/alcohol/[C3SO3Hmim]HSO4/are 1:3:0.3, the temperature 95 8C.
Isolated yield based on benzoic acid.
b
and 5). Additionally, the optimal yield to the desired product was obtained when benzoic acid reacted with 3 equiv. of
ethanol at 95 8C for 2.5 h (Table 1, entry 6). Further inspection revealed that the anion influenced the esterification
dramatically, [C3SO3Hmim]HSO4 exhibited relatively better activity over [C3SO3Hmim]BF4, [C3SO3Hmim]PF6, and
[C3SO3Hmim]CF3SO3 under the same catalytic conditions (Table 1, entries 6, 11–13). The optimal condition was
achieved in the presence of [C3SO3Hmim]HSO4 (Table 1, entry 6), whereas lower yields of the products were
observed when [C3SO3Hmim]BF4, [C3SO3Hmim]PF6, and [C3SO3Hmim]CF3SO3 were used. The higher yields of
esters in [C3SO3Hmim]HSO4 are probably owing to higher Brønsted acidity of [HSO4]À as well as best immisciblity
of [C3SO3Hmim]HSO4 with the produced ester, which facilitates the shifting of the esterification reaction equilibrium
to the product side [1].
The recovery and reuse of catalysts are important from economic consideration and environmental concerns. Along
this issue, the stability and recyclability of [C3SO3Hmim]HSO4 were examined in the reaction of benzoic acid with
ethanol under the optimized reaction conditions. It was found that the activity and stability of [C3SO3Hmim]HSO4
maintained very well (yields: 95–98%) even after five recycles (Table 1, entries 6, 14–17).
To explore the generality of the esterification reaction catalyzed by [C3SO3Hmim]HSO4, a variety of alcohols were
selected to react with benzoic acid. As summarized in Table 2, [C3SO3Hmim]HSO4 showed good catalytic activities in
esterification of other alcohols like methanol, butanol, cyclohexanol, benzenemethanol, and lauryl alcohol with good
to excellent yields (83–98%). The results also showed that the steric bulk of alcohols affected the reactions.
Esterification of methanol and butanol (Table 2, entries 1 and 2) was easier compared with that of cyclohexanol,
benzenemethanol, as well as lauryl alcohol (Table 2, entries 3, 4, and 5) and higher yields were obtained.
In summary, we have developed an efficient, simple and recyclable method for esterification of benzoic acid with
alcohols catalyzed by imidazolium IL-supported sulfonic acids. Among the catalysis system with different anions,
[C3SO3Hmim]HSO4, [C3SO3Hmim]BF4, [C3SO3Hmim]PF6, and [C3SO3Hmim]CF3SO3, [C3SO3Hmim]HSO4
showed the highest activity and stability to catalyze the esterification reactions of benzoic acid. Compared to the
reported pyridinium IL-supported sulfonic acids, high yields of the esters were afforded with 0.3 equiv. of IL-
supported sulfonic acids. Moreover, the stability of [C3SO3Hmim]HSO4 facilitated high yields of the product without
the activity loss even after five recycling uses.
Acknowledgments
We acknowledge the National Natural Science Foundation of China (No. 20676033), China Postdoctoral Science
Foundation (No. 20070410169), and Shanghai Leading Academic Discipline Project (No. B507) for financial support.
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