Study of preparation and thermal stability of cyano-functionalized imidazolium type ionic liquids

Article abstract of DOI:10.14233/ajchem.2013.14098

Sixteen imidazolium type cyano-functionalized Br?nsted acidic ionic liquids with HSO₄^- and H₂PO₄ ^- anion were prepared adopting twostep method and were characterized by FT-IR, 1H and 13C NMR. The onse

Full text of DOI:10.14233/ajchem.2013.14098

Asian Journal of Chemistry; Vol. 25, No. 9 (2013), 4779-4782
http://dx.doi.org/10.14233/ajchem.2013.14098
Study of Preparation and Thermal Stability of
Cyano-Functionalized Imidazolium Type Ionic Liquids
1
,2,3,*
3
, H.F. LI , G.J. GOU and A.Q. WANG
3
1
W.D. LIANG
1
2
3
Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, P.R. China
Graduate School of Chinese Academy of Sciences, Beijing, P.R. China
School of Petrochemical Engineering, Lanzhou University of Technology, Lanzhou, P.R. China
*
Corresponding author: Fax: +86 931 2973741; Tel: +86 931 2973383; E-mail: davidlucas@163.com
Received: 24 May 2012; Accepted: 6 March 2013)
(
AJC-13070
–
–
Sixteen imidazolium type cyano-functionalized Brønsted acidic ionic liquids with HSO
4
and H PO
2
4
anion were prepared adopting two-
1
13
step method and were characterized by FT-IR, H and C NMR. The onset temperatures of thermal decomposition of the ionic liquids
were measured using thermogravimetric analysis. It was found that the decomposition onset temperatures of the as-synthesized ionic
liquids were in the range of 200-260 ºC. Additionally, the decomposition temperatures were highly dependent on the nature of the anions.
–
–
Thermal stability of these cyano-functionalized ionic liquids decreased in the order of HSO
4 2 4
> H PO .
Key Words: Brønsted acid, Ionic liquid, Cyano-functionalized, Thermal stability, Imidazolium.
containing cyano group. It has been reported that alkali melt
as well as alkali melt oxides, quaternary ammonium alkali,
INTRODUCTION
There are a growing awareness of research in the field of
14
which added more difficulty on result after-treatment , play
1
-3
ionic liquids due to their particular properties, such as non-
flammability, insignificant vapour pressure and wide electro-
chemical window, which make them suitable candidates for a
15
an important role in catalyzing cyanoethylation reaction .
As far as we are aware, there is a lack of data concerning
cyano-functionalized ionic liquids containing carboxyl, alcohol,
long chain alkyl on the cation. In this paper, we report the
synthesis of precursor without catalyst in order to obtain cyano-
functionalized Brønsted acidic ionic liquids (Fig. 1) and
determine the thermal stability of these ionic liquids by
thermogravimetric analysis (TGA).
wide variety of applications. Ionic liquids based on AlCl
3
appeared the earliest that are unstable and difficult for
reutilization. Therefore, non-chloroaluminiate ionic liquids,
which are air and moisture stable, have been developed and
successfully applied to synthesis, extraction, separation, etc.
With the development of ionic liquids study, which contain
exclusively alkyl functional group cannot meet specific
demands. Thus a novel class of task specific ionic liquids
EXPERIMENTAL
All chemicals are commercially available and used with-
out further purification. FT-IR spectra were recorded on a
Bruker EQUINOX FT-IR spectrometer using KBr pellets in
(TSILs) containing special functional group with cations or
4
anions have been developed . The specific functional groups
5
6
7
include hydroxyl group , carboxyl group , ester group , cyano
-1
1
13
the range of 5000-4000 cm , H and C NMR spectra were
obtained on a Mercury-400BB spectrometer at 400 MHz in
8
group , amino group , chiral group .
9
10
As the application of ionic liquids in high-temperature
systems has progressively increased, thermal stability of ionic
liquids has become a significant property. The thermal
properties of alkyl imidazolium-type ionic liquids are stable
6
DMSO-d solution. TGA was performed on a thermal analysis
system (Seiko Instruments, STA 499C), which were run in
alumina pan with a heating rate of 10 ºC/min under argon
atmosphere and the temperature was programmed from 25-
900 ºC.
11
12
up to about 300 ºC . Amarasekara et al. studied the thermal
stabilities of sulfonic acid group-functionalized ionic liquids
and reported that the decomposition temperatures of these ionic
liquids are highly dependent on the nature of the anion. Wooster
Synthesis of precursor 1-(2-cyanoethyl)imidazole (1):
In a three-necked, 250 mL round-bottomed flask equipped
with reflux condenser and magnetic stirrer, 6.808 g (0.1 mol)
of imidazole was mixed with 30 mL methanol, then 7.25 mL
13
et al. reported the long term thermal stability of ionic liquids

4
780 Liang et al.
Asian J. Chem.
NCCH CH2
RX
NCCH CH
2
CH CHCN
2
2
2
R
H SO or H PO
2
4
3
4
HN
N
N
N
X-
N
N
1
(2-9) a
NCCH CH
2
2
R
N
N
-
-
HSO / H PO
4
4
2
(
2-9) b/c
R=
X= Cl, Br
(2) , CH
2
COOH (3), CH
2
CH
2
OH (4), CH
2
CH
3
(5), (CH
)
2
CH
4
3
(6), (CH
2
) CH
5 3
(7), (CH
2
)
6
CH
3
(8), (CH
)
2
7
CH (9)
3
Fig. 1. Synthetic pathway to functional Brønsted acidic ionic liquids
(
0.11 mol) of acrylonitrile was dropped into the solution over
TABLE-1
SPECIFIC TEMPERATURES, REACTION TIMES
ANDYIELDS FOR COMPOUNDS (2-9)b/c
a period of 0.5 h at 20-30 ºC. The solution was heated rapidly
to 60 ºC under stirring for an additional period of 6 h. Finally,
the solvent and residual acrylonitrile was removed under
reduced pressure. A pale yellow liquid was obtained with a
yield of 97.3 % (11.783 g).
Temp. Reaction Yield
Comp.
R
Anion
(ºC)
time (h)
(%)
98
93
87
93
97
99
95
99
97
94
90
89
96
92
99
98
2
b
b
bzl
Carboxylmethyl
Hydroxyethyl
Et
2
4
3
Synthesis of intermediates (2-9)a: The synthesis of
-(2-cyanoethyl)-3- benzylimidazolium chloride ([cebzim]Cl)
2a) was carried out in a 250 mL round-bottomed flask, which
4b
5b
6b
4
1
–
6
HSO4
80
Pen
6
(
7b
8b
9b
Hex
8
was immersed in a recirculating heated oil-bath and fitted with
a reflux condenser. 3.635 g (30 mmol) 1-(2-cyanoethyl)-
imidazole was dissolved in 20 mL of acetonitrile. Then 3.5
mL (33 mmol) of freshly distilled benzyl chloride was added
with constant stirring at 40 ºC and refluxed for 6 h. Finally,
the reaction mixture was distilled to remove the solvent. The
molten salt was repeatedly washed with absolute ether and
distilled at 50 ºC to yield objective product as a yellowish oil
liquid.And the pure product was obtained by column chroma-
tography (petroleum ether:ethyl acetate = 6:1) as a light yellow
liquid. The product was dried under vacuum for 24 h (6.986
g, 94 %).
Hep
10
10
2
Oct
2c
3c
4c
5c
Benzyl
Carboxylmethyl
Hydroxyethyl
Ethyl
4
4
6
–
H PO4
2
80
6c
7c
8c
Pentyl
8
Hexyl
8
Heptyl
10
10
9c
Octyl
RESULTS AND DISCUSSION
Chloroacetic acid instead of benzyl chloride was used for
preparation of 1-(2-cyanoethyl)-3-carboxylmethyl imidazolium
chloride ([cecmim]Cl) (3a), 1-(2-cyanoethyl)-3-hydroxy-
ethylimidazolium chloride ([ceheim]Cl) (4a) as a viscous
The decomposition onset temperature data of the prepared
cyano-functionalized Brønsted acidic ionic liquids are
summarized in Table-2. They are thermally stable up to 200-
–
2
60 ºC. It is evident that all of the HSO
4
based ionic liquids
liquid at 50 ºC under N
2
atmosphere.
–
present higher thermal stability than that of H
liquids, except for 9b which shows a distinct weight loss at
5 ºC and exhibits three peaks in the DTG curves different
2 4
PO based ionic
1-(2-Cyanoethyl)-3-alkylimidazolium bromides (5a-9a)
were prepared as Gong and coworkers described .
16
4
Synthesis of objective ionic liquids (2-9b/c): 1-(2-
Cyanoethyl)-3-benzylimidazolium hydrogen sulphate
from the other ionic liquids that show one to two peaks. There-
fore, it is suggested that their decompositions are highly
dependent upon the anion associated with the ionic liquids.
Meanwhile, all of the cyano-functionalized imidazolium
hydrogen sulphates with the unsaturated side chains (bzl,
carboxylmethyl) are thermally more stable than the corres-
ponding ionic liquids with saturated side chains.
(
4
[cebzim]HSO ) (2b) was synthesized as following: 1-(2-
Cyanoethyl)-3-benzylimidazoliolium chloride (30 mmol) was
dissolved in 30 mL of absolute alcohol. Equal molar of sulfuric
acid (98 %) was added dropwise with stirring in ice bath, which
was warmed to 80 ºC with sufficient reflux for 2 h. The
produced ionic liquid was washed repeatedly with diethyl ether
and dried in a vacuum to afford [cebzim]HSO
viscous and pale-yellow oily liquid (9.093 g, 98 %).
cebzim]H PO (2c) was a viscous liquid and also
prepared with a similar procedure by using phosphoric acid
85 %) instead of sulfuric acid (98 %). The other 14 kinds of
ionic liquids (3-9b/c) were synthesized as [cebzim]HSO
and [cebzim]H PO and obtained as yellowish oily liquid
Table-1).
4
(2b) as highly
TABLE-2
DECOMPOSITION ONSET TEMPERATURE
(ºC) OF COMPOUNDS 2b~9c
[
2
4
2
3
4
5
6
7
8
9
b
c
260
200
220
202
218
203
219
210
212
205
218
202
217
204
45
(
203
4
2
4
The order of thermal stability of ionic liquids 2b-4c was
found to be 2b > 3b > 4b > 2c ≈ 3c ≈ 4c, as illustrated in Fig. 2.
(

Vol. 25, No. 9 (2013)
Preparation and Thermal Stability of Cyano-Functionalized Imidazolium Type Ionic Liquids 4781
100
100
2
2
3
3
4
4
b
c
b
c
b
c
9c
8c
7c
6c
5c
80
60
40
20
0
8
6
4
2
0
0
0
0
0
2
2%
1
00
200
300
400
500
600
700
800
1
00
200
300
400
T (º
500
600
700
800
o
o
c
C)
T (ºC)
t/
c
Fig. 4. Thermal gravimetric analysis of (5-9)c
t/
Fig. 2. Thermal gravimetric analysis of 2b-4c
Obviously, the thermal stability of these compounds are highly
–
Most of the cyano-functionalized ionic liquids are stable
at or above 200 ºC, which is low compared to alkylimidazolium
11
based-ionic liquids (general T > 300 ºC ). The reason for it
d
dependent on the anion and decrease in the order HSO
–
4
>
. Meanwhile, all of these ionic liquids present similar
H
2
PO
4
two-step decomposition behaviours. Herein, we make the
explanation for the decomposable process of 2b as an example.
could be relative to lower symmetry of the functionalized
imidazolium. In addition, cation size increases with the
increasing of alkyl chain length, which leads to the weakening
of gravity between cation and anion with the distance widening.
Hence, ionic liquids decompose more easily in the process of
2 4
[Cebzim]H PO begins to decompose at 257 ºC and its mass
loss is 78 % when the temperature rises to 310 ºC. Ultimately,
decomposition is almost complete at 550 ºC and come to a
plateau. In addition, for the samples of 2b, 3b and 4b, which
have a thorough decomposition at nearly 615 ºC, whereas the
samples of 2c, 3c and 4c are not completely decomposed.
The TGA curves for alkylimidazolium type ionic liquids
17
heating, resulting in onset decomposition temperature drop .
Moreover, the thermal decomposition depends on the nature
of anions. Further thermal decomposition also decreases with
18
increasing hydrophilicity of anions . The decomposition step
–
–
–
4
series with HSO
4
and H PO
2
anion (R = alkyl) are shown in
was noticed to be steeper for an ionic liquid containing HSO
4
–
Figs. 3 and 4, respectively. However, the length of the alkyl
chain has not a strong influence on decomposition onset
temperature. It can be seen from Fig. 3 that decomposition
onset temperatures for the samples of 5b, 6b, 7b and 8b are
similar. The TGA measurement of 9b shows a distinct mass-
loss of ca. 19 % at ca. 200 ºC. The decomposition temperature
decreases in the order of 8b ≈ 7b ≈ 6b ≈ 5b > 9b. Interestingly,
2 4
rather than H PO and all ionic liquids (2-9)b/c show two-
stage decomposition process in the TG curves, which may be
due to the fact that their decomposition mechanisms are highly
dependent on the nature of the anion.
Conclusion
Cyano-functionalized Brønsted acidic ionic liquids were
successfully synthesized from the precursor of 1-(2-cyanoethyl)-
imidazole, which was prepared by cyanoethylation without
catalyst. The decomposition onset temperatures of ionic liquids
are in the range of 200-260 ºC in this study. The main factors
that influence the thermal stability of these ionic liquids are
attributed to the structure of the corresponding anions and the
a somewhat close thermal stability is found in the alkylimi-
–
dazolium type ionic liquids with H
2
PO
4
anion, all of which
are stable up to ca. 200 ºC, as represented in Fig. 4.
100
9
b
–
–
4 2 4
thermal stability decreases in the order HSO > H PO . The
8
b
8
1%
onset temperatures of ionic liquids are affected slightly by the
size of the alkyl chain of the cation. Most of the salts studied
exhibit apparently two-stage mass loss. In addition, all hydro-
gen sulphates containing the unsaturated side chains (R = bzl,
carboxylmethyl) are thermally more stable compared to the
corresponding ionic liquids with the saturated side chains.
Herein, it is expected that the present work may provide the
basic guidance for designing novel ionic liquids catalysts for
acid catalytic reaction.
80
60
40
20
0
7b
6
b
5
b
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