1112
Published on the web October 5, 2010
A Novel and Green Method for the Synthesis of Ionic Liquids
Using the Corresponding Acidic Ionic Liquid Precursors and Dialkyl Carbonate
Wenjun Xiao,1 Xiaoxing Wang, Qin Chen, Tinghua Wu,* Ying Wu, Lizong Dai,* and Chunshan Song
,2
3
1
1
1
2
3
1
Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, P. R. China
Key Laboratory of Fire Retardant Materials of Fujian Province, College of Materials,
2
Xiamen University, Xiamen 361005, P. R. China
Institute of EMS Energy, Pennsylvania State University, University Park, PA 16802, USA
3
(
Received June 17, 2010; CL-100566; E-mail: thwu@zjnu.cn, lzdai@xmu.edu.cn)
X- (R CH ) CO
A novel and green method for the synthesis of dialkylimi-
2 2 2 3
+
-
X-
H X
dazolium ionic liquids (ILs) has been developed. It was through
a one-step alkylation of dialkyl carbonate with the correspond-
ing acidic ionic liquid precursors (AILPs) prepared from the
neutralization of imidazole derivatives with acids.
N
N
N
N
R1
N
N
R1
H
R1
CH R
2 2
-
1 1 4 2
X = acid group; R = C ~C alkyl; R = H, alkyl or aryl
Scheme 1. The procedure for the synthesis of dialkylimidazo-
lium ionic liquids through the corresponding AILPs with dialkyl
carbonate.
Ionic liquids (ILs) or room-temperature molten salts have
recently attracted increasing attention due to their potential
applications as green solvents, reusable catalysts, separation and
Table 1. AILPs synthesized with imidazole derivatives and
1
a
extraction media, electrolytes, heat-transfer fluids, and others.
acids
Generally, 1-alkyl-3-methylimidazolium halides, one type of
common ILs, are synthesized by the reaction of 1-methylimida-
zole with alkyl halide.2 However, for the volatile halogeno-
alkanes (bromomethane, chloromethane, chloroethane, 1-chloro-
propane, etc.), the low boiling points often lead to preparations
requiring either a sealed tube or an elaborate apparatus as
described in the synthesis of 1-ethyl-3-methylimidazolium
Entry
Im
AC
AILPs
Y/%
1
2
3
4
5
6
7
8
9
MIm
MIm
MIm
MIm
MIm
MIm
MIm
MIm
EIm
HCl
HBr
HI
HMImCl
HMImBr
HMImI
>99.0
>99.0
>99.0
>99.0
99.0
99.0
99.0
99.0
>99.0
>99.0
HNO3
HBF4
CH3CO2H
H2C2O4
HPF6
HCl
HMImNO3
HMImBF4
HMImCH3CO2
HMImC2O4
HMImPF6
HEImCl
1
,3
chloride (EMImCl). Therefore, imidazolium ILs with longer
chain substituents (e.g., BMImCl, 1-butyl-3-methylimidazolium
chloride) are more prefered because of the easier operation and
more facile synthesis. However, in some special fields, such as
4
5
10
BIm
HCl
HBImCl
biopolymers processing and IL electrolytes, the imidazolium
ILs with shorter chain substituents are more desired for better
results. Regarding the synthesis of non-halide anion imidazo-
lium ILs, most reported procedures include at least one anion
aIm: imidazole derivatives, AC: acids, Y is the yield of
product, MIm: 1-methylimidazole, HMIm: 1H-3-methylimi-
dazolium, EIm: 1-ethylimidazole, HEIm: 1H-3-ethylimidazo-
lium, BIm: 1-butylimidazole, HBIm: 1H-3-butylimidazolium.
1
,2,6
2
exchange step.
The anion exchange must employ Ag salts,
which would increase the cost. Another way is via the reaction
of acids with 1-alkyl-3-methylimidazolium halides.5 Unfortu-
nately, the by-products, HX (X = Cl, Br, and I), are toxic and it
is difficult to obtain pure non-halide anion imidazolium ILs by
anion exchange.
,7
available imidazole derivatives with inorganic acids to form
1
2
acidic ionic liquid precursors (AILPs) followed by their
alkylation with dialkyl carbonate. The dialkylimidazolium IL
1
3
Some progress has been made to synthesize imidazolium
ILs with shorter chain substituents. For example, 1,3-dimeth-
ylimidazolium methyl sulfate (DMIm[CH3SO4]),8 a low-cost
reaction medium, was synthesized by 1-methylimidazole and
dimethyl sulfate. But the methylating agent dimethyl sulfate is
toxic. ILs with shorter-chain-substituent-C2-symmetric-imidazo-
lium cations were also successfully synthesized.9 But the
synthetic process is rigorous and anion exchange must be
employed. Recently, dimethyl carbonate (DMC) has been
applied in some organic syntheses as a green methylation
yields were calculated by NMR.
Table 1 shows the results of neutralization of three different
imidazole derivatives with eight different acids providing ten
different salts. These AILPs can be easily obtained by the
reaction between imidazole derivatives and acids. This neutral-
ization method is quite convenient and effective for synthesizing
excellent ILs as well as designing suitable models for quater-
nized onium-type salts.
Table 2 shows the results of reactions between AILPs and
dialkyl carbonate. No catalyst was used. The cations of these ILs
can be designed by different dialkyl carbonate (Table 2) and
different starting imidazole derivatives (Table 1); the anions can
also be designed by different acids (Table 1). Simultaneously,
no by-products, such as carbamates or 1,3-dimethylimidazoli-
um-2-carboxylate were detected. This may be because we started
with AILPs that can be considered as tertiary ammonium salts as
1
0
agent. For example, 1,3-dimethylimidazolium-2-carboxylate
and 1-ethyl-3-methylimidazolium-2-carboxylate can be synthe-
sized by the reaction of 1-methylimidazole/imidazole and
11
DMC.
The synthetic process is illustrated in Scheme 1. Various ILs
reported in this paper were prepared by reaction of commercially
Chem. Lett. 2010, 39, 11121113
© 2010 The Chemical Society of Japan