Alkyl sulfonate functionalized ionic liquids: Synthesis, properties, and their application in esterification

Article abstract of DOI:10.1016/S1872-2067(10)60178-7

Ionic liquids based on alkyl sulfate anions and alkyl sulfonate functionalized cations were prepared from commercially available and cheap materials and their thermal-instability properties were studied in detail. These ionic liquids were active catalysts for esterification and they gave moderate to high yields. The catalytic performance was found to be much better than that of conventional non-cation functionalized ionic liquids and close to sulfonic acid functionalized ionic liquids. The catalytic process in this reaction was investigated by electrospray ionization mass spectrometry and a plausible reaction mechanism is suggested.

Full text of DOI:10.1016/S1872-2067(10)60178-7

CHINESE JOURNAL OF CATALYSIS
Volume 32, Issue 3, 2011
Online English edition of the Chinese language journal
Cite this article as: Chin J Catal, 2011, 32: 440–445.
RESEARCH PAPER
Alkyl Sulfonate Functionalized Ionic Liquids: Synthesis,
Properties, and Their Application in Esterification
1
,2
1,*
1,*
ZHAO Yingwei , LI Zhen , XIA Chungu
1
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
Lanzhou 730000, Gansu, China
2
Graduate University of Chinese Academy of Sciences, Beijing 100049, China
Abstract: Ionic liquids based on alkyl sulfate anions and alkyl sulfonate functionalized cations were prepared from commercially available
and cheap materials and their thermal-instability properties were studied in detail. These ionic liquids were active catalysts for esterification
and they gave moderate to high yields. The catalytic performance was found to be much better than that of conventional non-cation func-
tionalized ionic liquids and close to sulfonic acid functionalized ionic liquids. The catalytic process in this reaction was investigated by
electrospray ionization mass spectrometry and a plausible reaction mechanism is suggested.
Key words: ionic liquid; alkylsulfonate; esterification
Room temperature ionic liquids (ILs) have been extensively
sulfonate functionalized ionic liquids. Esterification is one of
the most important reactions in organic synthesis for the
preparation of fine chemicals and people have carried out
reactions using various categories of ionic liquid solvents
[15–17]. The disadvantage of this strategy is the high con-
sumption of ionic liquids. For the functionalized cations, rela-
tively high catalytic efficiencies were expected for ionic liquids
1 and 2. In this paper, the application of these ionic liquids as
catalysts in esterification was also studied. For comparison,
some known ionic liquids with similar structures to ionic liq-
uids 1 and 2 (3–7) were also prepared and used in this reaction
(Scheme 1).
studied as solvents and catalysts in organic chemistry [1–3].
Diversified ionic liquids containing various cations and anions
have been synthesized over the past few decades. Recently, an
increasing amount of attempts have been made to develop
functionalized ionic liquids (FILs) by the incorporation of
functional groups as a part of the cation and/or anion [4]. In
2
002, a series of low cost and halide-free ionic liquids con-
taining alkyl sulfate anions were prepared by Holbrey et al. [5].
They were later used as solvents in many reactions [6–9] and
their physical properties were extensively studied [10–12].
However, they are not regarded as FILs because of the catalytic
inertia of the sulfate anions.
Our efforts have focused on the further functionalization of
this type of ionic liquid by the alkylation of zwitterions (ob-
tained by the reaction of 1-methylimdazole and 1,4-butane
sultone [13]) using dimethyl sulfate or diethyl sulfate. We have
previously produced the new cation-functionalized ionic liq-
uids 1-(4-methyl sulfonate)-butyl-3- methylimidazolium
methyl sulfate (1) and 1-(4-ethyl sulfonate)-butyl-3-
ethylimidazolium ethyl sulfate (2) [14]. Herein, we report
detailed studies on the synthesis and properties of these alkyl
1 Experimental
1.1 Synthesis of ionic liquids
Dimethyl sulfate, diethyl sulfate, and 1-methylimidazole
were redistilled under reduced pressure before use. 1,4-Butane
sultone and the other reagents were used as received without
further purification. The ionic liquids 1 and 2 were prepared
using 1-methylimidazole and dimethyl sulfate or diethyl sul-
Received 29 September 2010. Accepted 13 December 2010.
*
Corresponding author. Tel/Fax: +86-931-4968129; E-mail: cgxia@licp.cas.cn, zhenli@licp.cas.cn
Foundation item: Supported by the National Natural Science Fund for Distinguished Young Scholars (20625308).
Copyright © 2011, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
DOI: 10.1016/S1872-2067(10)60178-7

ZHAO Yingwei et al. / Chinese Journal of Catalysis, 2011, 32: 440–445
O
S
O
S
O
S
O
S
N
N
MeO
O
EtO
O
N
N
N
N
N
N
MeO
O
O
O
MeO
O
O
O
O
O
S
S
OMe
OEt
O
O
4
1
2
3
O
O
S
O
S
N
N
F3C
S
O
N
N
HO
O
N
N
HO
O
O
O
O
O
O
S
S
OH
OH
O
O
5
7
6
Scheme 1. The ionic liquids discussed in this article.
fate under solvent-free conditions [14] (Scheme 2). The
preparation and purification of ionic liquids 3–7 and
methanesulfonate methyl ester were achieved according to the
reported methods [5,18–21].
washed with saturated aqueous NaHCO
3
and dried over anhy-
drous Na SO . Finally, the ether was removed at 30 °C under
2
4
vacuum and the yield of ester was calculated. For the synthesis
of dimethyl fumarate, the white solid product was isolated and
washed with cold saturated aqueous NaHCO after the reac-
3
1
.2 Characterization of ionic liquids
tion. Finally it was dried at 70 °C for 4 h in an oven. All the
products were qualitatively determined by an HP 6890/5973
GC-MS.
NMR spectra were recorded on an INOVA-400 spectrome-
ter. Thermal decomposition profiles were collected by ther-
mogravimetric analysis (TGA, Diamond TG/DTA/DSC, 10
1.4 Reactions of ionic liquids with phenylacetic acid
°
C/min heating rate under nitrogen). The pH values were re-
corded using a DELTA 320 pH meter. Electrospray ionization
mass spectrometry (ESI-MS) analysis was performed using a
Waters micromass ZQ alliance spectrometer (capillary 3.6
kV, cone 10 V, source temperature 100 °C, desolvation tem-
perature 120 °C, data format MCA, flow velocity 10 ȝl/min).
The ionic liquid (5 mmol) and phenylacetic acid (15 mmol)
were charged into a 25 ml round bottom flask and heated with
stirring at 80 °C for 4 h. Cold anhydrous ether was then added.
The obtained viscous liquid was washed with anhydrous ether
(15 ml × 5) to remove the phenylacetic acid methyl ester
formed in the reaction and to remove unreacted phenylacetic
acid. The ether was then distilled under vacuum for 4 h. Finally
the obtained sample was dissolved in acetonitrile before
analysis by ESI-MS.
1
.3 Typical esterification procedure
Carboxylic acid and methanol were added to the ionic liq-
uid and then the mixture was heated with stirring at 80 °C. For
the low boiling point esters, after the reaction the distillation
equipment was set up and the desired ester as well as unreacted
methanol was distilled at 100 °C. The mixture obtained from
the distillation was dried over anhydrous Na SO . The yield
1.5 Determination of the Hammett acidity function of the
ionic liquids
The ionic liquids and the indicator 4-nitroaniline were dis-
solved in dry CH Cl at concentrations of 5 mmol/L and 0.15
2
4
was obtained from an Agilent 6820 GC according to the ratio
of the chromatographic peak area of the product and an internal
standard. For the high boiling point esters, water and unreacted
methanol were evaporated under vacuum. The ether was then
added and the ionic liquid was isolated. The ether layer was
2
2
mmol/L, respectively. When dimethyl yellow was used as an
indicator its concentration was 0.085 mmol/L. The solutions
were shaken vigorously and then left to stand for 6 h. Their
UV-Vis spectra were recorded on an HP 8453 UV-Vis spec-
trophotometer.
O
MeO
S
OMe
2
Results and discussion
O
O
S
1
2
O
N
N
O
N
N
2
.1 Synthesis of ionic liquids
SO3
For the preparation of these alkyl sulfonate functionalized
O
zwitterion
EtO
S
OEt
ionic liquids, it should be noted that commercially available
dimethyl sulfate and diethyl sulfate must be redistilled before
use because trace sulfuric acid will lead to the formation of
O
Scheme 2. Synthesis of ionic liquids 1 and 2.

ZHAO Yingwei et al. / Chinese Journal of Catalysis, 2011, 32: 440–445
1
-(4-sulfonic acid)-butyl-3-methylimidazolium hydrogen sul-
1
00
fate (7) [20] in the reaction. We found that heating is required
to promote the reaction of the zwitterion with dimethyl sulfate
or diethyl sulfate indicating that the zwitterion is a relatively
weak nucleophile and fairly inert to electrophilic reagents. The
synthesis of the alkylimidazolium methyl sulfates 3 and 4
requires no heat but does require an ice-bath as the preparation
reactions are highly exothermic [5]. We also previously found
that relatively weak alkylating agents such as iodomethane did
not react with the zwitterion while alkylimidazole was easily
alkylated [22]. On the other hand, the zwitterion obtained from
the reactions of pyridine, tributylphosphine, or triethylamine
with 1,4-butane sultone could react with dimethyl sulfate or
diethyl sulfate under heating resulting in the formation of the
corresponding sulfonate functionalized ionic liquids.
Zwitterion
1
2
8
0
0
6
40
2
0
0
0
100 200 300 400 500 600 700 800
Temperature (qC)
Fig. 1. TG curves for the two ionic liquids and the zwitterion.
alized ionic liquids 3, 4, and 5, and close to that of the sulfonic
acid functionalized ionic liquids 6 and 7 and sulfuric acid at a
low catalyst loading (Table 1, entries 1–16). For the esterifica-
tion of benzoic acid, the yield of ester obtained by ionic liquid 1
is even better than that obtained by sulfuric acid (Table 1,
entries 17 and 18). Moderate to high yields were still obtained
when the molar ratio of methanol/carboxyl group was reduced
to 1:1 (Table 1, entries 20–26). In these cases, the increase of
catalyst loading to 10 or 20 mol% (relative to the carboxyl
group) was needed to create a biphasic catalytic system, which
promoted the equilibrium process in favor of the products side.
The ethyl sulfonate functionalized ionic liquid 2 also exhibited
comparable performance in the esterification of acetic acid and
ethanol to that obtained with ionic liquid 7 (Table 1, entries 22
and 23).
2
.2 TGA results
The thermal instability of the alkylimidazolium methyl sul-
fate ionic liquid 3 has been recognized previously [5]. To
characterize the thermal degradation of the sulfonate func-
tionalized ionic liquids 1 and 2, TGA were performed and the
results are shown in Fig. 1. For comparison, the TGA trace of
the zwitterion was also investigated. A multi-step decomposi-
tion process was found for alkylimidazolium alkyl sulfate ionic
liquids and this has been reported previously [5]. In our work,
the two sulfonate-functionalized ionic liquids showed a similar
thermal decomposition process consisting of gradual and con-
tinuous weight loss between 100 and 330 °C. This was fol-
lowed by a major weight loss event around 330–380 °C. In the
first step, ionic liquid 1 showed a mass loss above 30% and
ionic liquid 2 lost about 40%. These losses are in accordance
with the weight proportion of (CH ) SO in ionic liquid 1 and
As the ionic liquid and the ester were immiscible, the ionic
liquid was easily separated by simple decantation after the
excess alcohol was distilled off, it could be reused directly
without any treatment. Almost no decrease in ester yield was
obtained after the ionic liquid 1 was reused for four times
(Table 1, entry 10).
3
2
4
(
C H ) SO in ionic liquid 2 (36.6% and 41.4%, respectively).
2 5 2 4
The TGA curve of the zwitterion provides more evidence for
the release of dimethyl sulfate or ethyl sulfate from the ionic
liquids. Figure 1 shows that at temperatures above 330 °C the
TGA curves of both ionic liquids 1 and 2 gradually converge
with that of the zwitterion.
To investigate the role of the sulfonate-functionalized ionic
liquids in the esterification process, we first determined their
Hammett acidity functions in CH
2
Cl using 4-nitroaniline
2
(
pK(I) = 0.99) and dimethyl yellow (pK(I) = 3.3) as indica-
aq
aq
2
.3 Catalytic activity during esterification
tors. However, no absorbance decrease was found for the in-
dicators. This clearly indicates that the sulfonate-functional-
ized ionic liquids 1 and 2 exhibit nearly no Brönsted acidity
[23].
The esterification of a carboxylic acid with an alcohol is
typically catalyzed by a Brönsted acid or a Lewis acid. We
have already shown that the catalytic performance of a Brön-
sted acidic ionic liquid is strongly dependent on its Brönsted
acidity [23]. To our surprise, without Brönsted or Lewis acidic
functional groups in their structures, ionic liquids 1 and 2
showed catalytic activity for the esterification of carboxylic
acids.
In further experiments we found that when a large amount of
ethylsulfonate functionalized ionic liquid 2 was used in the
methyl esterification of phenylacetic acid, an ethyl ester was
present (Scheme 3, top). On the other hand, when the methyl-
sulfonate-functionalized ionic liquid 1 was used in the ethyl
esterification of pheylacetic acid, a methyl ester was formed
(Scheme 3, bottom). This phenomenon led us to consider that
the Brönsted acid center might be formed by the reaction of the
sulfonate functionalized ionic liquid and the carboxylic acid
The results listed in Table 1 suggest that for all the carbox-
ylic acid substrates the catalytic performances of 1 and 2 are
much better than that of the conventional non-cation function-

ZHAO Yingwei et al. / Chinese Journal of Catalysis, 2011, 32: 440–445
Table 1 Esterification of a carboxylic acid with methanol in the presence of sulfonate functionalized ionic liquids and other related ionic liquids
Entry
Carboxylic acid
n-butyric acid
n-butyric acid
n-butyric acid
n-butyric acid
n-butyric acid
n-butyric acid
n-butyric acid
n-butyric acid
n-butyric acid
phenylacetic acid
phenylacetic acid
phenylacetic acid
phenylacetic acid
phenylacetic acid
phenylacetic acid
phenylacetic acid
benzoic acid
Catalyst
Catalyst loading (mol%)
Acid:alcohol
1:4
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
1
3
4
5
6
7
0.5
0.5
0.5
0.5
0.5
0.5
0.5
1
2
2
2
2
2
2
2
2
2
4
4
4
4
4
4
4
4
4
4
2
4
4
4
4
4
76
4
1:4
1:4
3
1:4
56
92
93
84
93
96
1:4
1:4
H
2
SO
1
4
1:4
1:4
6
1
1:4
a
1
0
1
1
1
1:4
85/84
1
2
1
1:4
78
23
34
56
82
85
68
53
86
1
1
1
1
1
1
1
1
2
2
2
3
4
5
6
7
8
9
0
1
3
1
1:4
4
1
1:4
5
1
1:4
6
1
1:4
H
2
SO
1
4
1
1:4
1
1:4
benzoic acid
6
1
1:4
benzoic acid
1
10
10
10
20
20
20
20
1:2
b
n-butyric acid
phenylacetic acid
acetic acid
1
1:1
89/84
1
1:1
86
91
93
c
2
2
3
2
1:1
c
2
acetic acid
7
1:1
d
2
4
fumaric acid
1
1:2
90
d
2
2
5
6
malonic acid
1
1:2
64
d
succinic acid
1
20
1:2
4
70
a
b
c
d
Reaction conditions: 80 °C. The fifth run (2nd run 87%, 3rd run 84%, 4th run 84%). The second run. Esterification with ethanol. Yield of diester.
O
N
N
EtO
S
O
O
O
S
OEt
O
2
O
O
O
+
+
MeOH
o
OH
80 C, 2 h
OMe
OEt
O
N
MeO
S
O
O
O
N
S
OMe
O
O
O
1
O
+
EtOH
+
o
OH
80 C, 2 h
OEt
OMe
Scheme 3. Formation of by-products in the esterification catalyzed by the sulfonate-functionalized ionic liquids.
substrate. We then mixed ionic liquid 1 with phenylacetic acid
lytic species. However, the catalytic efficiency of ionic liquid
5 with the anion of the hydrogen sulfate but without
cation-functionalization during esterification was much lower
than that of ionic liquids 1 and 6 (Table 1, entries 4 and 14).
Ionic liquids 3 and 4 with the methyl sulfate anion also exhib-
ited low catalytic activity and these were even worse than that
of ionic liquid 5 (Table 1, entries 2, 3, 12, and 13) although
their anion could also be transformed into hydrogen sulfate in
the presence of a carboxylic acid. This was shown by
ESI-MS. A reasonable explanation is that the sulfonic acid
group played a crucial role despite its low concentration.
and heated without solvent. ESI-MS and H-NMR analysis of
the ionic liquid phase and a GC-MS analysis of the organic
phase indicated that the phenylacetic acid was methylated to
afford the corresponding methyl ester. For ionic liquid 1, the
methyl sulfate anion (m/z = 110.8, į(CH SO ) = 3.14) trans-
3
4
formed to a hydrogen sulfate (m/z = 97.0) completely in 4 h at
0 °C but only a small amount of the methyl sulfonate group
in the cation (m/z = 233.2, į(C H N (CH ) SO CH ) = 3.37)
8
4
6
2
2 4
3
3
transformed to sulfonate acid group (m/z = 219.2). This result
led us to consider whether hydrogen sulfate was the real cata-

ZHAO Yingwei et al. / Chinese Journal of Catalysis, 2011, 32: 440–445
Table 2 pH of the ionic liquids at room temperature
with excellent yields. We found that these ionic liquids were
a
Entry
Ionic liquid
pH
thermally unstable and decomposed easily to zwitterions at
high temperature and they were also easily hydrolyzed. Al-
though these ionic liquids have no acidic group they were
efficient catalysts for esterification. Control experiments and
ESI-MS studies showed that the hydrolyzation of the methyl
sulfonate group in the cation of ionic liquid 1 to form a strong
Brönsted acid center played a crucial role in promoting the
esterification process.
b
1
2
3
4
5
6
1
2
3
5
6
2.72 (2.51)
3.14
7.50
2.11
1.89
CH
3
SO
3
CH
3
4.44
a
pH values were determined at 16 °C with 0.01 mol/L ionic liquid in
water. After the ionic liquids were dissolved in water the solutions were
allowed to stand for 0.5 h and then the pH was determined.
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1
988: 1925
We describe the synthesis of a series of ionic liquids con-
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3
4
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