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Published on the web June 5, 2010
1-Methylimidazolium Chlorosulfate ([HMIm]SO3Cl):
A Novel Ionic Liquid with Dual Brønsted-Lewis Acidity
Quanxi Bao, Kun Qiao,* Daisuke Tomida, and Chiaki Yokoyama
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University,
2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577
(Received April 1, 2010; CL-100314; E-mail: kunqiao@tagen.tohoku.ac.jp)
A novel acidic ionic liquid, 1-methylimidazolium chloro-
water bath. Then the reaction mixture was stirred for 12 h at
room temperature. After another 2 h of reaction at 80 °C, excess
SOCl2 was removed from the reaction mixture via vacuum
distillation. The remaining yellow liquid was washed with
diethyl ether three times, and dried under vacuum at room
temperature. The structure of [HMIm]SO3Cl was confirmed by
NMR and HRMS analyses (see Supporting Information).11
Usually, the acidity of Brønsted acid ionic liquids can be
quantitatively characterized by Hammett acidity function based
UV-visible spectroscopy.6 Our previous study has shown that
this method can also be qualitatively applied to Lewis acid ionic
liquids.7 In the current study, the acidity of [HMIm]SO3Cl was
investigated by UV-visible spectroscopy using p-nitroaniline as
an indicator and methanol as the solvent. For comparsion, the
Brønsted acid ionic liquid [HMIm]HSO4, which was used as the
precursor of [HMIm]SO3Cl, and Lewis acidic ionic liquid 3-
methyl-1-(4-chlorosulfonylbutyl)imidazolium trifluoromethane-
sulfonate ([MSCBI]CF3SO3) were also analyzed. The results are
summarized in Figure 1.
The Brønsted acid [HMIm]HSO4 showed a typical increase
of acidity, in terms of H0, with an increase of its concentration
in methanol. The peaks at 270-280 nm gradually intensified for
the Lewis acidic [MSCBI]CF3SO3. These peaks indicate the
presence of the Lewis acidic SO2Cl in the molecular structure
of [MSCBI]CF3SO3. [HMIm]SO3Cl exhibited features of both
Brønsted and Lewis acid ionic liquids, which clearly demon-
strated that it had dual Brønsted-Lewis acidity.
sulfate ([HMIm]SO3Cl), with dual Brønsted-Lewis acidity was
synthesized. This compound provided better catalytic perform-
ance in esterification and dehydration of fructose to 5-hydroxy-
methylfurfural (HMF) than acidic ionic liquids with only
Brønsted or Lewis acidity.
Rececently, interest has increased in the use of ionic liquids
as novel solvents or catalysts in synthetic chemistry.1 Acidic
ionic liquids are an important subset of functional ionic liquids.
They can be used as efficient reaction media or catalysts for a
variety of reactions that are usually catalyzed by conventional
acidic catalysts such as AlCl3, H2SO4, HNO3, or HCl.2 In
comparison with these conventional catalysts, acidic ionic
liquids usually improve catalytic performance, enhance reaction
rates, and provide higher yields and easier product separation/
catalyst recycling.
The many different acidic ionic liquids that have been
synthesized can be classified as either Brønsted or Lewis acids,
according to the acidic groups they possess. Few studies have
reported preparation of acidic ionic liquids with both Brønsted
and Lewis acid sites. In contrast, some solid acids such as
dealuminated HY zeolite or mesoporous Al-Zr-TUD-1 can have
both Brønsted and Lewis acid sites, and a synergistic effect
between these sites has been proposed.3
Combination of Brønsted and Lewis acidity within ionic
liquids can impart new properties. For example, it is well
established that Lewis acidic chloroaluminate ionic liquids can
become superacidic in the presence of protons. This is one of the
few examples of an acidic ionic liquids with Brønsted-Lewis
dual acidity. Recently it was revealed that incorporation of an
acidic ionic liquid with a Lewis acid co-catalyst can efficiently
increase the catalytic performance for certain reactions.4 Thus,
synthesis of acidic ionic liquids with both Brønsted and Lewis
acid sites is important for expanding both the scope and
application of acidic ionic liquids.
1-Methylimidazolium bromide ([HMIm]Br) is an analogous
ionic liquid to [HMIm]HSO4. Some previous studies suggested
that the nitrogenic proton in [HMIm]Br is not labile, and that
[HMIm]Br cannot be viewed as a Brønsted acid.8 However, the
UV-visible spectra of [HMIm]HSO4 and [HMIm]SO3Cl indi-
cate that both the nitrogenic proton and the hydrogen sulfate
proton contribute to the Brønsted acidity of [HMIm]HSO4. The
respective contributions of these protons to the acidity H0 can
¹
be determined by considering the change in H0 after HSO4 is
¹
converted to SO3Cl . Due to the absence of the hydrogen sulfate
We previously reported synthesis of SO2Cl-based Lewis
acid ionic liquids.4b As a continuation of this, we herein report a
novel ionic liquid with dual Brønsted-Lewis acidity, 1-methyl-
imidazolium chlorosulfate ([HMIm]SO3Cl). This ionic liquied
can be easily synthesized by reaction of 1-methylimidazolium
hydrogen sulfate ([HMIm]HSO4),5 a common Brønsted acid
ionic liquid, with SOCl2. Our primary results showed that
[HMIm]SO3Cl provides better catalytic performance in ester-
ification and dehydration of fructose to HMF than both its
Brønsted acid precursor and the SO2Cl-based Lewis acid ionic
liquid reported previously.4b
proton, the Brønsted acidity of [HMIm]SO3Cl was slightly less
than [HMIm]HSO4 when dissolved in methanol at the same
concentration.
Esterification is one of the most frequently employed probe
reactions for examing the performance of acidic ionic liquids.9
We investigated the catalytic activity of [HMIm]SO3Cl for
esterification, and compared it to control experiments using
[HMIm]HSO4 and [MSCBI]CF3SO3. [HMIm]SO3Cl gave high-
er product yields than [HMIm]HSO4 and [MSCBI]CF3SO3
(Table 1). This can be attributed to the presence of both
Brønsted and Lewis acid sites in [HMIm]SO3Cl, which
provides many active sites for the reaction. It should be noted
that the activity of [HMIm]HSO4 was very low for the
To prepare [HMIm]SO3Cl, 5 g of SOCl2 was added
dropwise to 10 g of [HMIm]HSO4 in a 50 mL flask in an ice
Chem. Lett. 2010, 39, 728-729
© 2010 The Chemical Society of Japan