Tetracarboxyl-functionalized ionic liquid: Synthesis and catalytic properties

Article abstract of DOI:10.1071/CH14720

Novel tetracarboxyl-functionalized 2,2′-biimidazolium-based ionic liquids (ILs) with different anions were synthesized in two steps from readily available and sustainable starting materials including ammonium acetate, glyoxal, and halogenated propionic acid. The functionalized IL exhibited higher catalytic activity towards the cycloaddition of CO₂ to terminal epoxides. With propylene oxide as a substrate, the optimum yield of propylene carbonate reached 82.7% at an initial CO₂ pressure of 2.0MPa for 4h at 140°C. Moreover, the functionalized IL catalyst displayed a high stability and can be reused for at least five cycles without obvious loss of catalytic activity. The results provide a simple and economical way to synthesize multi-functionalized imidazolium-based ILs with versatile potential applications.

Full text of DOI:10.1071/CH14720

CSIRO PUBLISHING
Aust. J. Chem.
Full Paper
http://dx.doi.org/10.1071/CH14720
Tetracarboxyl-Functionalized Ionic Liquid: Synthesis
and Catalytic Properties
A B
,
B
A C
,
Miaona Feng, Guoying Zhao, Hongling Gao,
B C
,
and Suojiang Zhang
A
Department of Chemistry, School of Science, Tianjin University, Tianjin 300072,
China.
B
Beijing Key Laboratory of Ionic Liquids Clean Process, State Key Laboratory of
Multiphase Complex Systems, Key Laboratory of Green Process and Engineering,
Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China.
C
Corresponding authors. Email: ghl@tju.edu.cn; Sjzhang@ipe.ac.cn
Novel tetracarboxyl-functionalized 2,20-biimidazolium-based ionic liquids (ILs) with different anions were synthesized in
two steps from readily available and sustainable starting materials including ammonium acetate, glyoxal, and halogenated
propionic acid. The functionalized IL exhibited higher catalytic activity towards the cycloaddition of CO2 to terminal
epoxides. With propylene oxide as a substrate, the optimum yield of propylene carbonate reached 82.7 % at an initial CO2
pressure of 2.0 MPa for 4 h at 1408C. Moreover, the functionalized IL catalyst displayed a high stability and can be reused
for at least five cycles without obvious loss of catalytic activity. The results provide a simple and economical way to
synthesize multi-functionalized imidazolium-based ILs with versatile potential applications.
Manuscript received: 18 December 2014.
Manuscript accepted: 17 March 2015.
Published online: 30 April 2015.
media, or as an organic ligand.[10] Therefore we explored the
synthesis of tetracarboxyl-functionalized biimidazolium-based
ILs and tested its catalytic properties in this work.
Herein, tetracarboxyl-functionalized biimidazolium-based
ILs with different anions have been synthesized and their
catalytic properties tested for the synthesis of cyclic carbonates
from CO2 and epoxides. The influence of the catalyst’s structure
and reaction conditions (temperature, reaction time, pressure,
amount of catalyst) on the catalyst performance were systemat-
ically studied. A possible reaction mechanism was proposed.
Introduction
In recent years, ionic liquids (ILs) have stood out in many fields
especially in electrochemistry, adsorption, as solvents, and for
catalysis.[1] The biggest advantages of ionic liquids lie not only
in the exhibited excellent properties such as non-volatility, high
stability, ease of handling, but also in their infinite versatility,
functionality, and potential applications.[2] Ionic liquids have
tens of thousands of species in theory, but only a few have been
synthesized and studied.[3] In addition, ionic liquids are not a
panacea and specified ionic liquids only work in limited appli-
cations.[4] Therefore, more functionalized ionic liquids need to
be synthesized and explored in an efficient and economical way.
A number of mono- or dual functionalized ionic liquids, for
example, with hydroxyl, thiol, amino, or carboxyl functionali-
ties have been synthesized and widely applied in the field of
chemistry over the last decade.[5] Alkylation of nitrogen-
containing compounds is the most common strategy to decorate
the cation with reactive functionalities.[6] The number of nitro-
gen atoms in the starting compounds determines the maximum
number of introduced reactive groups. However, more function-
al groups often means higher activity and lower usage for
various applications.[7] Sun et al. reported dual carboxyl func-
tionalized ionic liquids showed higher catalytic properties in the
cycloaddition of CO2 to epoxides.[8] On the other hand, diqua-
ternized 2,20-biimidazole molecules have recently been effi-
ciently synthesized, which provided an opportunity to
synthesize tetra-functionalized ionic liquids.[9] However, biimi-
dazolium-based multifunctional ILs have been relatively rarely
studied for their potential versatility, such as as a catalyst, as
Results and Discussion
Two tetracarboxyl-functionalized 2,20-biimidazolium-based
ILs with different anions have been synthesized in this study,
which is outlined in Scheme 1. 2,20-Biimidazole was first syn-
thesized in one step in high yield using cheap substrates such as
ammonium acetate and glyoxal. The 2,20-biimidazole was then
alkylated with halogenated propionic acid in the presence of
KOH. The key to successful synthesis of two tetracarboxyl-
functionalized 2,20-biimidazolium dihalide was achieved by
precisely controlling the pH during the synthesis. The structures
of the compounds were confirmed by 1H NMR spectroscopy and
electrospray ionization-mass spectrometry (ESI-MS). The tet-
racarboxyl-functionalized 2,20-biimidazolium dihalides were
then tested as acidic catalysts for the coupling reaction of CO2
and epoxide. The propylene carbonate yields were 80.8 and
82.7 % for the tetracarboxyl-functionalized 2,20-biimidazolium
ILs with Clꢀ and Brꢀ anions respectively, which is in agreement
Journal compilation Ó CSIRO 2015
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B
M. Feng et al.
O
O
O
O
H
N
X
HO
HO
N
N
OH
OH
N
CH3COONH4
HOOC
ꢂ
ꢁ
O
O
2X
ꢁ
N
N
N
H
N
TCBM-Br: X ꢀ Br
TCBM-CI: X ꢀ CI
Scheme 1. Synthesis of 1,3,10,30-tetracarboxyl-2,20-biimidazolium dihalide.
A
100
90
80
70
60
Xꢂ
O
O
R
ꢁ
N
HOOH2CH2C
N
O
O
HO
R
O
A
A
HOOCH2CH2C
ꢁ
ꢁ
N
HOOCH2CH2C
N
N
N
O
O
O
O
H
ꢂ
H
O
O
O
O
X
HOOCH2CH2C
R
Xꢂ
A
R
N
ꢁ Xꢂ
N
O
H
1.0
1.5
2.0
2.5
ꢂ
O
O
CO2
Initial CO2 pressure [MPa]
R
X
Fig. 1. Effect of initial CO2 pressure on PC yield. Reaction conditions: PO
(14.3 mmol), TCBM-Br (1.19 mol-%), 1408C, 4 h.
Scheme 2. Proposed reaction mechanism for cycloaddition of epoxide and
CO2 catalyzed by TCBM-X.
100
90
with the nucleophilicity of these anions tethered by the imida-
zolium cation.[11]
The catalytic activity of 1,3,10,30-tetracarboxyl-2,20-
biimidazolium dibromide (TCBM-Br) for the cycloaddition of
CO2 and propylene oxide (PO) was further investigated. The
effects of several important reaction parameters, such as
temperature, pressure, reaction time, and catalyst loading on
the yield of the propylene carbonates (PC) were studied by
designing a series of single factor experiments.
80
Effect of Pressure
The effect of initial CO2 pressure on the yield of propylene
carbonates was carried out at 1408C with 14.3 mmol of PO and
1.19 mol-% catalyst loading of TCBM-Br for 4 h. The results
are shown in Fig. 1. The yield of propylene carbonates first
increased according with increasing CO2 pressure from 1 to
2 MPa and then decreased at pressures greater than 2 MPa. The
maximum yield obtained was 82.7 %, at 2 MPa. The existence
of optimum CO2 pressure can be explained from the coupling
mechanism of CO2 and epoxide,[12] as shown in Scheme 2.
After the formation of PO-IL hydrogen bonding complexes, the
Brꢀ anion attacks the less sterically hindered b-carbon atom of
the epoxide to afford an alkoxide anion. CO2 insertion of the
alkoxide anion intermediate and subsequent dehalogenation
gives PC and regenerates the TCBM-Br catalyst. As shown in
the reported reaction mechanism, the interaction between PO,
CO2, and catalyst all have an effect on the yield of the PC.
Increasing the pressure over 2 MPa resulted in decreased cata-
lyst activity.
70
0
2
4
6
Reaction time [h]
Fig. 2. Effect of reaction time on PC yield. Reaction conditions: PO
(14.3 mmol), TCBM-Br (1.19 mol-%), 1408C, 2 MPa.
Effect of Reaction Time
The effect of reaction time on the carbonate yield was examined
at 2 MPa with a catalyst loading of 1.19 mol-% at 1408C (Fig. 2).
As reaction time is increased from 1 to 4 h, the yield of the
propylene carbonate increased from 68.3 to 82.7 %. The PC
yield improved to 83.6 % when the reaction time was further
prolonged to 5 h. Therefore, the optimal reaction time was 4 h
from a practical standpoint.

Tetracarboxyl-Functionalized Ionic Liquid
C
100
100
80
60
40
20
0
90
80
70
60
50
90
100
110
120
130
140
150
160
1
2
3
4
5
Temperature [ꢃC]
Run
Fig. 3. Effect of temperature on PC yield. Reaction conditions: PO
Fig. 5. Recycling test of TCBM-Br. Reaction conditions: PO (14.3 mmol),
TCBM-Br (0.1 g), initial CO2 pressure 2 MPa, 1408C, 4 h.
(14.3 mmol), TCBM-Br (1.19 mol-%), 2 MPa, 4 h.
100
90
80
70
60
sharply when the catalyst loading is greater than 2.4 mol-%,
possibly due to additional side effects.
Reusability of the Catalyst
Stability and recyclability of TCBM-Br were examined by
recycle experiments at 1408C, 2.0 MPa for 4 h. Typically, the
catalyst was recycled by distilling out the PC product and
directly reused for the next cycle. The results are displayed in
Fig. 5. There was only a slight decrease in its catalytic activity,
after five consecutive cycles, which showed that TCBM-Br
exhibited good stability.
Coupling Carbon Dioxide and Other Epoxides
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
In order to explore the potential applications of the ILs, various
epoxides were coupled with CO2 using TCBM-Br catalyst under
the optimized reaction conditions. The results are summarized
in Table 1. It can be seen that the TCBM-Br catalyst exhibited a
high activity towards a variety of aliphatic and aromatic epox-
ides (entries 1 to 5). The glycidyl phenyl ether was the most
preferred substrate due to its good electron-withdrawing capa-
bility as the carbon atoms on the epoxide ring can be easily
attacked by the halide. Very low yield of cis-hexahydrobenzo
[1,3]-dioxolan-2-one[8] was obtained for the cyclohexene oxide
(entry 5) due to its higher hindrance originating from the two
fused rings.[13]
Catalyst loading [mol-%]
Fig. 4. Effect of catalyst loading on PC yield. Reaction conditions: PO
(14.3 mmol), 2 MPa, 1408C, 4 h.
Effect of Temperature
The influence of temperature on the PC yield is shown in Fig. 3.
It is noted that temperature has a greater influence on the cata-
lytic activity of TCBM-Br. When the reaction temperature is
increased from 100 to 1408C, the yield of PC increased from
55.0 to 82.7 %. When the temperature is increased to 1608C, the
PC yield reached 89.2 %. However, characterization of the
products via ESI-MS demonstrated that after completion of
the reaction, the catalyst may have changed into an acid anhy-
dride. Thus, 1408C was chosen as the most suitable temperature.
Conclusions
Tetracarboxyl-functionalized 2,20-biimidazolium-based ILs
with different anions were synthesized and used as catalysts
for the cycloaddition of CO2 to epoxides without the use of a
co-catalyst and solvent. TCBM-Br exhibited high catalytic
activities towards a variety of terminal epoxides. With PO as
a substrate, PC yield of 89.2 % was obtained (1.19 mol-%
catalyst at initial CO2 pressure of 2.0 MPa for 4 h at 1608C),
and 78.7 % yield was achieved with 0.47 mol-% catalyst at
1408C. Compared with the reported carboxyl-functionalized
imidazolium-based ILs, our tetracarboxyl-functionalized ILs
showed relatively lower catalytic activities at high loading but
high activity at low catalyst loadings for the cycloaddition of CO2
to epoxide. In addition, the TCBM-Br catalyst showed good sta-
bility without obvious loss in catalytic activity after five cycles.
Effect of Catalyst Loading
The experimental result of catalyst usage demonstrated that the
tetracarboxyl-functionalized ILs TCBM-Br can efficiently cat-
alyze PC production from CO2 and PO at low catalyst loading
(0.47 mol-%). As catalyst usage increases, the PC yield first
increased slowly and reached the optimum in the range of 1.19
to 2.4 mol-% catalyst loading. There was little change in PC
yield with a 1.19 to 2.4 mol-% catalyst loading (Fig. 4).
Therefore, 1.19 mol-% of catalyst was the most suitable catalyst
loading in terms of turnover number. The PC yield decreased

D
M. Feng et al.
Table 1. Coupling of CO2 and various epoxides with TCBM-Br
Reaction conditions: epoxide (14.3 mmol), catalyst (TCBM-Br, 0.1 g),
1408C, 2 MPa, 4 h.
1,3,10,30-Tetracarboxyl-2,20-biimidazolium dichloride
(TCBM-Cl)
2,20-Biimidazole (0.4 g, 3 mmol) was added to a 100 mL
three-necked flask charged with 3-chloropropionic acid (1.3 g,
12 mmol), KOH (0.336 g, 6 mmol), and water (10 mL) at room
temperature. The reaction mixture was adjusted to a pH level of
10 to 12 with a 5 M aqueous solution of KOH and the reaction
mixture was heated slowly to reflux for 8 h. The product mixture
was acidified to pH ¼ 2-3 with hydrochloric acid (1 M), and then
concentrated under reduced pressure. The concentrated mixture
was diluted with ethanol and filtered to remove the undissolved
salt. The final filtrate was concentrated to give a brown viscous
liquid (85.1 % yield). The product was dried overnight under
vacuum before use and analysis. dH (D2O) 2.51 (t, J 6.0, 8H),
3.74 (t, J 6.0, 8H), 7.40 (s, 4H). m/z (ESI-MS, þve mode)
423.1565 [M ꢀ H]þ; calcd for C18H24N4O28þ, 424.1589.
Entry
Epoxide
Product
Yield [%]
O
O
O
O
1
82.7
O
O
O
O
CH2Cl
2
3
73.0
87.7
CH2Cl
O
O
O
Ph
O
1,3,10,30-Tetracarboxyl-2,20-biimidazolium dibromide
(TCBM-Br)
Ph
1,3,10,30-Tetracarboxyl-2,20-biimidazolium dibromide was
prepared in a similar process to TCBM-Cl. Brown viscous liquid
was obtained (88.2 % yield). dH (D2O) 2.37 (t, J 6.0, 8H), 3.62
(t, J 6.0, 8H), 7.28 (s, 4H). m/z (ESI-MS, þve mode) 423.1389
[M ꢀ H]þ; calcd for C18H24N4O28þ, 424.1589. m/z (ESI-MS,
ꢀve mode) 80.9157; calcd for Brꢀ, 80.92.
O
O
O
O
O
O
4
5
90.5
11.3
H
H
O
O
O
O
Procedure for Cycloaddition of CO2 and Epoxides
The catalytic properties of tetracarboxyl-functionalized 2,20-
biimidazolium-based ILs for the cycloaddition of CO2 and
epoxides were investigated in a 15 mL stainless steel autoclave
equipped with a magnetic stirrer. In a typical procedure,
1.19 mol-% catalyst (0.1 g), epoxide (1.0 mL), and an appro-
priate amount of biphenyl (as an internal standard for gas
chromatography analysis) were successively charged into the
reactor. The reactor was filled with a given initial CO2 pressure
and was heated to the desired temperature. After the reaction
was complete, the reactor was cooled to 08C in an ice-water bath,
and de-pressurized by slowly releasing the remaining CO2.
The products were quantitatively analyzed by a GC-SP6890
(Rainbow Chemical Instrument Co., Ltd, Shandong Lunan)
equipped with a DB-wax column. More details are shown in our
group’s previous work.[14]
Our findings provide a simple and cheap way to synthesize
multi-functionalized ionic liquids with potential applications as
catalysts, media, and organic ligands amongst others.
Experimental
Materials
CO2 was supplied by Beijing Bei Temperature Gas Factory with
a purity of 99.99 %. Ammonium acetate, glyoxal, chlor-
opropionic acid, and bromopropionic acid were obtained from
Sinopharm Chemical Reagent Co., Ltd. Other reagents and
solvents were bought from Beijing Chemical Works. All the
above reagents were used as received unless otherwise noted.
NMR spectra were recorded on a JEOL ECA-600 spectrometer.
ESI-MS analyses were performed on a Bruker micro TOF-QII
spectrometer.
Acknowledgements
This work was financially supported by the National Natural Science
Foundation of China (No. 51374193), Special Funds of the National Natural
Science Foundation of China (No. 21127011), the Special Program for
International Science and Technology Cooperation and Exchange Program
of China (No. 2014DFA61670), and International Cooperation and
Exchange of the National Natural Science Foundation of China
(No. 21210006).
Synthesis of 1,3,10,30-Tetracarboxyl-2,20-biimidazolium
dihalide (in Scheme 1)
2,20-Biimidazole
2,20-Biimidazole was synthesized according to a reported
procedure.[9] In a typical reaction, aqueous glyoxal (23.0 mL,
40 wt-%) was added dropwise to a vigorously stirred solution of
ammonium acetate (70.0 g) and H2O (13.0 mL) at 408C over a
period of 4 h. After completion of the reaction, the crude product
(4.44 g, 54.15 %) was filtered and washed several times with
water and acetone. The product was then further purified by
liquid anti-solvent crystallization and dried overnight under
vacuum before use and analysis. dH ([D6]DMSO) 7.08
(d, 4H), 12.65 (s, 2H). m/z (ESI-MS) 135.0751; calcd for
C6H7Nþ4 , 135.0609 [M]þ.
References
[1] (a) M. Armand, F. Endres, D. R. MacFarlane, H. Ohno, B. Scrosati,
Nat. Mater. 2009, 8, 621. doi:10.1038/NMAT2448
(b) D. M. D’Alessandro, B. Smit, J. R. Long, Angew. Chem. Int. Ed.
2010, 49, 6058. doi:10.1002/ANIE.201000431
(c) R. Sheldon, Chem. Commun. 2001, 2399. doi:10.1039/B107270F
(d) T. Welton, Chem. Rev. 1999, 99, 2071. doi:10.1021/CR980032T
(e) Z.-Z. Yang, Y.-N. Zhao, L.-N. He, RSC Adv. 2011, 1, 545.
doi:10.1039/C1RA00307K
[2] Y. Xie, Z. Zhang, T. Jiang, J. He, B. Han, T. Wu, K. Ding, Angew.
Chem. Int. Ed. 2007, 46, 7255. doi:10.1002/ANIE.200701467

Tetracarboxyl-Functionalized Ionic Liquid
E
[3] G. A. Baker, S. N. Baker, S. Pandey, F. V. Bright, Analyst 2005, 130,
800. doi:10.1039/B500865B
[9] J. C. Xiao, J. M. Shreeve, J. Org. Chem. 2005, 70, 3072. doi:10.1021/
JO0501083
[4] Y. L. Gu, Q. H. Zhang, Z. Y. Duan, J. Zhang, S. G. Zhang, Y. Q. Deng,
J. Org. Chem. 2005, 70, 7376. doi:10.1021/JO050802I
[5] (a) K. S. Kim, D. Demberelnyamba, H. Lee, Langmuir 2004, 20, 556.
doi:10.1021/LA0355848
[10] (a) Z. Fei, T. J. Geldbach, R. Scopelliti, P. J. Dyson, Inorg. Chem.
2006, 45, 6331. doi:10.1021/IC060297N
(b) W. Shen, L.-M. Wang, H. Tian, J. Tang, J.-J. Yu, J. Fluor. Chem.
2009, 130, 522. doi:10.1016/J.JFLUCHEM.2009.02.014
[11] J. Sun, L. Wang, S. Zhang, Z. Li, X. Zhang, W. Dai, R. Mori, J. Mol.
Catal. A:Chem. 2006, 256, 295. doi:10.1016/J.MOLCATA.2006.05.004
[12] (a) T. Takahashi, T. Watahiki, S. Kitazume, H. Yasuda, T. Sakakura,
Chem. Commun. 2006, 1664. doi:10.1039/B517140G
(b) X.-L. Meng, Y. Nie, J. Sun, W.-G. Cheng, J.-Q. Wang, H.-Y. He,
S.-J. Zhang, Green Chem. 2014, 16, 2771. doi:10.1039/C3GC42331J
(c) J. Sun, S. Zhang, W. Cheng, J. Ren, Tetrahedron Lett. 2008, 49,
3588. doi:10.1016/J.TETLET.2008.04.022
(d) L.-F. Xiao, D.-W. Lv, D. Su, W. Wu, H.-F. Li, J. Clean. Prod. 2014,
67, 285. doi:10.1016/J.JCLEPRO.2013.12.031
(b) L. Han, H.-J. Choi, S.-J. Choi, B. Liu, D.-W. Park, Green Chem.
2011, 13, 1023. doi:10.1039/C0GC00612B
[6] H. Xue, Y. Gao, B. Twamley, J. M. Shreeve, Chem. Mater. 2005, 17,
191. doi:10.1021/CM048864X
[7] W. Wu, X. Sheng, Y. Qin, L. Qiao, Y. Miao, X. Wang, F. Wang, J.
Polym. Sci., Part A: Polym. Chem. 2014, 52, 2346. doi:10.1002/POLA.
27247
[13] Y. Zhou, S. Hu, X. Ma, S. Liang, T. Jiang, B. Han, J. Mol. Catal. A:
Chem. 2008, 284, 52. doi:10.1016/J.MOLCATA.2008.01.010
[14] X.-L. Meng, Y. Nie, J. Sun, W.-G. Cheng, J.-Q. Wang, H.-Y. He,
S.-J. Zhang, Green Chem. 2014, 16, 2771.
[8] J. Sun, L.-J. Han, W.-G. Cheng, J.-Q. Wang, X.-P. Zhang, S.-J. Zhang,
ChemSusChem 2011, 4, 502. doi:10.1002/CSSC.201000305