Synthesis and characterization of task-specific ionic liquids possessing two Broensted acid sites

Article abstract of DOI:10.1080/00397910601131429

Task-specific ionic liquids possessing two Broensted acid sites with -COOH, HSO-4, or H2PO-4 groups have been designed, synthesized, and characterized. Under mild conditions and without any additional organic solvent, the esterification of isopropanol by chloroacetic acid could be carried out in these new task-specific ionic liquids. In comparison with most of acidic ionic liquids in current use, these ionic liquids are halogen free and more environmentally benign as media and catalysts. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910601131429

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Synthesis and Characterization of
Task‐Specific Ionic Liquids Possessing Two
Brönsted Acid Sites
Dan Liu a b , Jianzhou Gui b , Xiangqin Zhu b , Lijuan Song b & Zhaolin Sun a b
a College of Chemistry and Chemical Engineering, China University of
Petroleum (East China), Shangdong, China
b College of Petrochemical Engineering, Liaoning University of Petroleum
and Chemical Technology, Liaoning, China
Version of record first published: 10 Mar 2007.
To cite this article: Dan Liu , Jianzhou Gui , Xiangqin Zhu , Lijuan Song & Zhaolin Sun (2007): Synthesis and
Characterization of Task‐Specific Ionic Liquids Possessing Two Brönsted Acid Sites, Synthetic Communications:
An International Journal for Rapid Communication of Synthetic Organic Chemistry, 37:5, 759-765
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Synthetic Communicationsw, 37: 759–765, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910601131429
Synthesis and Characterization of
Task-Specific Ionic Liquids Possessing
¨
Two Bronsted Acid Sites
Dan Liu
College of Chemistry and Chemical Engineering,
China University of Petroleum (East China), Shangdong, China and
College of Petrochemical Engineering, Liaoning University of Petroleum
and Chemical Technology, Liaoning, China
Jianzhou Gui, Xiangqin Zhu, and Lijuan Song
College of Petrochemical Engineering, Liaoning University of Petroleum
and Chemical Technology, Liaoning, China
Zhaolin Sun
College of Chemistry and Chemical Engineering,
China University of Petroleum (East China), Shangdong, China and
College of Petrochemical Engineering, Liaoning University of Petroleum
and Chemical Technology, Liaoning, China
¨
Abstract: Task-specific ionic liquids possessing two Bronsted acid sites with –COOH,
HSO24 , or H2PO24 groups have been designed, synthesized, and characterized. Under
mild conditions and without any additional organic solvent, the esterification of isopro-
panol by chloroacetic acid could be carried out in these new task-specific ionic liquids.
In comparison with most of acidic ionic liquids in current use, these ionic liquids are
halogen free and more environmentally benign as media and catalysts.
¨
Keywords: Bronsted acid, characterization, synthesis, task-specific ionic liquids
Received May 15, 2006
Address correspondence to Zhaolin Sun, College of Petrochemical Engineering,
Liaoning University of Petroleum and Chemical Technology, Fushun 113001,
Liaoning, China. E-mail: zlsun62@hotmail.com; zlsun@lnpu.edu.cn
759

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D. Liu et al.
INTRODUCTION
Ionic liquids (ILs) have recently gained recognition as environmentally benign
alternative solvents for separations, chemical synthesis, electrochemistry, and
catalysis. Generally, they are nonvolatile, nonexplosive, recyclable, easy to
handle, and thermally robust. Therefore, ILs are regarded as “green solvents.”
They have also been refered to as “designer solvents” because their chemical
and physical properties could be adjusted by a careful choice of cation/anion.
Therefore, the covalent tethering of a functional group to one or both of the
ions of an ordinary ionic liquid can imbue the resulting salt with a capacity to
interact with dissolved substrates in specific ways. These low-melting salts are
called task-specific ionic liquids (TSIL). Until recently, this area of TSIL
research has been the most active, and TSILs are finding an increasing number
of applications in synthesis, separations, catalysis, and electrochemistry.[1]
In recent years, nonchloroaluminiate acidic ionic liquids, which are air and
moisture stable, have been synthesized and used successfully in etherifica-
tion,[2] esterification,[3] Friedel–Crafts alkylation,[4] Beckmann rearrange-
ment,[5] and so on. However, most of them described currently in the
literature for catalytic and other solvent applications contain halogen atoms
such as Cl or F.[6] With regard to the environmental impact of technical
processes using these ionic liquids, their presence may cause—under certain
conditions—serious concerns.[7] Therefore, further investigation is necessary
to look for new, even greener acidic ionic liquids.
In our previous work, our group had synthesized several acidic ionic
liquids and applied them to some reactions, such as oxidation, esterification,
Beckmann rearrangement, and alkylation.[8] Here we describe the synthesis
¨
and characterization of two Bronsted acid site functionalized ionic liquids
with imidazolium. Under mild conditions and without any additional
organic solvent, the esterification of isopropanol by chloroacetic acid could
be carried out in these new task-specific ionic liquids. In comparison with
most of acidic ionic liquids in current use, these TSILs are halogen free and
more environmentally benign. To the best of our knowledge, they are
reported here for the first time. All TSILs are characterized with Fourier
1
transform infrared spectroscopy (FTIR), H NMR, 13C NMR, thermal gravi-
metric analysis (TGA), and elemental analysis.
In summary, several task-specific ionic liquids possessing two Bronsted
¨
acid sites were synthesized. Under mild conditions and without any additional
organic solvent, the esterification of isopropanol by chloroacetic acid could be
carried out in these new TSILs. Good yields were obtained, and the ester formed
a separate phase that was decanted. The ionic liquid could be reused after
removal of water under vacuum. Being acidic ionic liquids, they have other
great potential applications as catalysts and/or media in many reactions, such
as etherification, Friedel–Crafts reaction, oxidation reaction, and so on. Some
investigations about applications of these TSILs and their acidity–catalytic
activity—structure relationships are in progress at our laboratory.

Task-Specific Ionic Liquids
EXPERIMENTAL
761
All solvents and chemicals in the experiment were commercially available and
used without further purification unless otherwise stated. N-Methylimidazole
was purchased from Aldrich, and the others were all from Shanghai Chemical
Reagent Corporation Ltd. The C, N, and H elemental analyses were performed
on an Elementar Vario EL element analyzer. IR spectra (FTIR) were recorded
on a PE SpectrumTM GX FTIR spectrometer using liquid film or KBr tablet.
1H NMR (300 MHz) and 13C NMR (75 MHz) were obtained on Varian
Mercury-Plus 300BB instruments as solutions in deuterium-substituted
reagent. Chemical shifts were reported in parts per million (ppm, d). The
thermal decomposition point of ionic liquids was determined by TGA
(Perkin-Elmer TGA Pyris1 instrument, 10 K min21 heating rate under
nitrogen). Densities were measured using a U-shape vibrating-tube densimeter
(model DA-500) operating in a static mode. The rheometer used (Brookfield,
RVDV-IIIþ) allows measurements from 293 to 393 K at atmospheric pressure
and in a wide viscosity range (from 100 to 3M cP).
Isopropanol (0.04 mol), equivalent chloroacetic acid (0.04 mol), and
TSILs (0.008 mol) were added in a round-bottom flask with a reflux
condenser. The reaction mixture was stirred for 3 h with the oil bath at
608C. After the reaction, the ester could be simply decanted out from the
ionic liquid, and the ionic liquid was reused after removal of water under
vacuum (0.01 Torr) at 1308C for 1 h.
General Procedures for the Synthesis of TSILs
The TSILs were prepared by a two-step synthesis (Scheme 1) through
1-methylimidazole combined first with chloroacetic acid or 3-chloropropionic
Scheme 1. Synthesis of TSILs. Reagents and reaction conditions: (i) chloroacetic
acid (1 equivalent), anhydrous chloroform, reflux, 50 h; (ii) H2SO4 97% or H3PO4
85% (1 equivalent), CH2Cl2, 08C then reflux, 48 h; (iii) 3-chloropropionic acid
(1 equivalent), 708C, 50 h.

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D. Liu et al.
acid to form zwitterionic salts, followed by addition of concentrated sulphuric
acid (97%) or o-phosphoric acid (85%). The typical synthetic procedure of
TSILs is described with IL2 as example.
To a solution of chloroacetic acid (1.0 mmol) in anhydrous chloroform
(80 mL) at room temperature, 1-methylimidazole (1.0 mmol) was added.
The resulting mixture was stirred for ca. 50 h at reflux. After the reaction,
the solid produced was filtrated and washed with anhydrous ethanol to
remove any unreacted starting materials. Then it was dried in vacuum, and
zwitterionic salt ST1 was obtained.
The IL2 (Scheme 1) were obtained by a dropwise addition of 1 equivalent
of concentrated sulphuric acid (97%) to a cooled solution of the corresponding
ST1 (1 equivalent) in anhydrous methylene chloride. The mixture was
stirred for ca. 48 h at reflux. The HCl, formed in the reaction as a by-
product, was distilled out of the condenser under a stream of dry nitrogen
and was dissolved in deionized water at 08C. The acid aqueous solution was
monitored by titration with NaOH. When the CH2Cl2 was evaporated with a
rotary evaporator, the ionic liquid was dried under vacuum (1022 Torr) at
708C for 6 h.
The spectral data of TSILs are as follows.
Data
ST1: A white solid with yield of 78%. 1H NMR (300 MHz, DMSO-d6, TMS);
3.89 (s, 3H), 5.21 (s, 2H), 7.78 (s, 2H), 9.33 (s, 1H). 13C NMR (75 MHz,
DMSO-d6, TMS); d 36.55, 50.39, 123.81, 124.35, 138.33, 168.6. IR
(cm21): 3158, 2580, 2490, 1734, 1572, 1364, 1168, 639, 776, 681.
C6H9O2N2Cl (176.52) calcd.: C, 40.80; H, 5.09; N, 15.86. Found: C, 40.86;
H, 5.39; N, 16.00. The thermal decomposition point of IL1 533.2K.
IL2: A yellow viscous liquid with yield of 97.5%; density: 1.377 g/ml
1
(298 K); viscosity: 1000.6 mPa s (353 K). H NMR (300 MHz, DMSO-d6,
TMS); 3.87 (s, 3H), 5.11 (s, 2H), 7.70 (s, 2H), 9.10 (s, 1H). 13C NMR
(75 MHz, DMSO-d6, TMS); d 36.52, 50.26, 123.89, 124.38, 138.35, 168.89.
IR (cm21): 3158, 2509, 1741, 1572, 1364, 1168, 1176, 1050, 852, 639, 588.
C6H10O6N2S (237.13) calcd.: C, 34.96; H, 4.85; N, 13.58. Found: C, 34.03;
H, 4.90; N, 13.62. The thermal decomposition point of IL2 is 578.5 K.
IL3: A yellow viscous liquid with yield of 96.0%; density: 1.340 g/ml
1
(298 K); viscosity: 9896.9 mPa s(353 K). H NMR (300 MHz, D2O, TMS);
3.22 (s, 3H), 4.44 (s, 2H), 6.81 (t, 2H), 8.10 (s, 1H). 13C NMR (75 MHz,
D2O, TMS); d 35.74, 49.55, 123.17, 124.11, 136.90, 169.54. IR (cm21):
3158, 2488, 1734, 1572, 1364, 1212, 1191, 1168, 995, 639, 621.
C6H11O6N2P (237.04) calcd.: C, 34.98; H, 5.34; N, 13.59. Found: C, 34.88;
H, 5.39; N, 13.60. The thermal decomposition point of IL3 is 572.5 K.

Task-Specific Ionic Liquids
763
IL4: A yellow viscous liquid with yield of 98.0%; density: 1.115 g/ml
1
(298 K); viscosity: 1175.4 mPa s (353 K). H NMR (300 MHz, DMSO-d6,
TMS); 2.84 (t, 2H), 3.82 (s, 3H), 4.31 (s, 2H), 7.71 (s, 1H), 7.79 (s, 1H),
9.33 (s, 1H). 13C NMR (75 MHz, DMSO-d6, TMS); d 34.92, 36.36, 45.53,
123.02, 124.05, 137.60, 172.45. IR (cm21): 3158, 2586, 1743, 1572, 1364,
1168, 639. C7H11O2N2Cl (190.53) calcd.: C, 44.13; H, 5.77; N, 14.70.
Found: C, 44.23; H, 5.59; N, 14.88. The thermal decomposition point of
IL4 is 516.4 K.
IL5: A yellow viscous liquid with yield of 96.0%; density: 1.195 g/ml
1
(298 K); viscosity: 9677.1 mPa s (353 K). H NMR (300 MHz, D2O, TMS);
2.28 (t, 2H), 3.16 (s, 3H), 3.74 (t, 2H), 6.70 (s, 1H), 6.79 (s, 1H), 8.01
(s, 1H). 13C NMR (75 MHz, D2O, TMS); d 33.84, 35.65, 44.71, 122.21,
123.59, 136.29, 173.87. IR (cm21): 3158, 3098, 2602, 2486, 1749, 1572,
1364, 1287, 1168, 1069, 1006, 885, 850, 639, 614, 454. C7H12O6N2S
(252.14) calcd.: C, 38.19; H, 5.45; N, 12.12. Found: C, 38.23; H, 5.49; N,
12.37. The thermal decomposition point of IL5 is 524.3 K.
IL6: A yellow viscous liquid with yield of 97.0%; density: 1.233 g/ml
1
(298 K); viscosity: 9539.6 mPa s (353 K). H NMR (300 MHz, D2O, TMS);
2.47 (t, 2H), 3.36 (s, 3H), 3.94 (t, 2H), 6.90 (s, 1H), 6.97 (s, 1H), 8.22
(s, 1H). 13C NMR (75 MHz, D2O, TMS); d 33.89, 35.64, 44.60, 122.20,
123.52, 136.30, 174.05. IR (cm21): 3158, 1726, 1572, 1364, 1167, 1109,
997, 639, 621. C7H13O6N2P (252.05) calcd.: C, 38.21; H, 5.91; N, 12.72.
Found: C, 38.43; H, 5.79; N, 12.58. The thermal decomposition point of
IL6 is 506.1 K.
The Esterification of Isopropanol by Chloroacetic Acid
The esterification reactions of chloroacetic acid with alcohols are very useful
in the medicinal industry. Recently, a few examples of synthesis of esters in
ILs with or without an additional catalyst have been reported and demon-
strated good catalytic performance.[9] However, most cases dealt with
aliphatic esters and arylic esters, and little information about chloroacetate
synthesis in ILs was found. Our efforts were directed toward investigating
the formation of an important commodity ester, isopropyl chloroacetate,
from isopropanol and chloroacetic acid using different TSILs as the
solvent/catalyst in a batch-type process. The results of experiments are sum-
marized in Table 1. In all cases, good and moderate yields were obtained.
Because the TSILs reported here are miscible with water but immiscible
with esters, the esterifications proceeded smoothly to completion even
without simultaneous removal of the produced water, even though esterifica-
tion is a reversible reaction. The data in entries 1–4 (Table 1) indicate that the
yield of esters in IL2 and IL5 is considerably higher than that in IL3 and IL6.

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D. Liu et al.
Table 1. Results of esterification of isopropanol by chloroacetic acid
under different TSILs
IL/substrate
(Mol/Mol)
T
Time
(h)
Yields of
esters (%)a
Entry
IL
(8C)
1
IL2
IL3
IL5
IL6
IL2
1:5
1:5
1:5
1:5
1:5
60
60
60
60
60
3
3
3
3
3
94.5
75.2
87.8
80.6
94.6
2
3
4
5b
aYields of esters are based on crude product.
bFive times reused.
It is suggested that the TSIL’s anion has a significant impact on its catalytic
activity. This phenomenon can be explained in terms of the different
Brønsted acidities of the anions. [HSO4]2 has a higher Brønsted acidity
than [H2PO4]2, so the esterification has a better conversion in IL2 and
IL5 than in the others. Furthermore, the fundamental studies on the acidities
of the Brønsted acidic ionic liquids and their acidity–catalytic activity–
structure relationships are now under way.
After reaction, the product could be conveniently decanted out from IL2.
After treatment under vacuum (0.01 Torr) at 1308C for 1 h, the reactor
containing the used IL2 was charged with isopropanol and chloroacetic
acid. It is to be noted that IL2 can indeed be reused several times without
visible loss of activity (Table 1, entry 5). This indicated that IL2 as catalyst
for the esterification was recyclable.
ACKNOWLEDGMENT
We are grateful for the financial support from the National Natural Science
Foundation of China (No. 20476042) and the Educational Department of
Liaoning Province (No. 05L234).
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