Hydroxyl-functionalized ionic liquid: a novel efficient catalyst for chemical fixation of CO2 to cyclic carbonate

Article abstract of DOI:10.1016/j.tetlet.2008.04.022

A series of hydroxyl-functionalized ionic liquids (HFILs) were synthesized and characterized. They showed efficient reactivity and reusability toward the coupling of epoxide and CO₂ without any additional co-catalyst and organic solvent. Highest activity and selectivity were achieved in the presence of 1-(2-hydroxyl-ethyl)-3-methylimidazolium bromide (HEMIMB) in comparison with other similar catalysts investigated. The relationship between the higher catalytic reactivity and OH-functional group was proposed.

Full text of DOI:10.1016/j.tetlet.2008.04.022

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 3588–3591
Hydroxyl-functionalized ionic liquid: a novel efficient catalyst
for chemical fixation of CO to cyclic carbonate
2
a,b
a,
a
c
*
Jian Sun , Suojiang Zhang , Weiguo Cheng , Junyi Ren
a
State Key Laboratory of Multi-Phase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China
b
Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China
c
Dalian Polytechnic University, Dalian 116034, PR China
Received 9 January 2008; revised 8 March 2008; accepted 3 April 2008
Available online 6 April 2008
Abstract
A series of hydroxyl-functionalized ionic liquids (HFILs) were synthesized and characterized. They showed efficient reactivity and
reusability toward the coupling of epoxide and CO without any additional co-catalyst and organic solvent. Highest activity and selec-
2
tivity were achieved in the presence of 1-(2-hydroxyl-ethyl)-3-methylimidazolium bromide (HEMIMB) in comparison with other similar
catalysts investigated. The relationship between the higher catalytic reactivity and OH-functional group was proposed.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Ionic liquid; Carbon dioxide; Chemical fixation; Cyclic carbonate
Carbon dioxide is an attractive C1 building block in
organic synthesis as it is highly functional, abundant, eco-
nomical, and nontoxic. One of the most attractive syn-
cost needed compared to single component catalyst system.
Therefore, developing water-stable, low-cost, and efficient
single component catalyst for the synthesis of cyclic car-
bonate is still highly required.
1
–4
thetic protocols utilizing CO is the coupling reaction of
2
epoxide and CO to afford the five-membered cyclic car-
Recently, functionalized ionic liquids (FILs) containing
ether or alcohol on the alkyl arms have received much
attention in the fields of organic synthesis and cataly-
2
5
–8
bonates,
which can serve as valuable intermediates for
polycarbonates, electrolytes in lithium ion batteries, or
green solvents. Numerous catalysts have been developed
3
6–40
sis.
tional advantages such as alterable polarity,
viscosity and higher solubility of inorganic salts, and
Compared to the traditional ILs, they show addi-
9
–36
36
for this transformation,
uids (ILs) which are currently regarded as versatile and
especially including ionic liq-
lower
3
7
9
–22
38,39
novel reaction media.
Although the advances are signif-
immobilization potential.
However, few literatures
9
–12,26,27
icant, most suffer from low catalyst activity,
water
and the need for
In addition, although the combina-
reported on the synthesis of cyclic carbonate using these
1
4–16,20–23,31
41
or air sensitivity of catalyst,
FILs except Zhu et al. They found that the supported
2
7,34,35
toxic co-solvent.
choline chloride/urea (CH/urea) ionic liquid showed high
activity and selectivity for the coupling reaction of epoxide
tion of IL with Lewis acid has resulted in many diverse and
flexible ‘platform’ to establish a highly effective catalytic
system, its big problems are obvious. For example: (1)
the activities of ILs are reduced distinctly in the absence
of Lewis acids; (2) anhydrous operation is necessary due
to the water sensitivity of some Lewis acid; and (3) higher
and CO without any solvent. But similar to the aforemen-
2
tioned reports, the disadvantages of this catalytic system
are the following: (1) CH alone showed unsatisfied activity
in the absence of urea, and (2) urea is a well-known thermal
and water sensitive chemical which decomposes to ammo-
nia and isocyanate above its melting point (132 °C) and
*
hydrolyzes to ammonia and CO in water. Since OH group
Corresponding author. Tel./fax: +86 10 8262 7080.
2
E-mail address: sjzhang@home.ipe.ac.cn (S. Zhang).
has the positive effect on the ring-opening of epoxy, this
0
040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.04.022

J. Sun et al. / Tetrahedron Letters 49 (2008) 3588–3591
3589
idea can extend to the corresponding hydroxyl-functional-
ized ionic liquids (HFILs) directly for the synthesis of cyclic
carbonate without using other additives.
bromine),
TBAB
(tetrabutylammonium
bromide),
PPh EtBr (triphenyl(ethyl)phosphonium bromide), KI
3
and KBr (entries 6 and 13–16). By the way, almost no reac-
tivity was observed in the presence of organic base 1-meth-
ylimidazole or organic Br o¨ nsted acid 2-bromidethanol or
C H OH (entries 1–3). HEMIMC (1-(2-hydroxyl-ethyl)-3-
Herein, two kinds of HFILs (Scheme 1) were synthe-
4
2
sized, and their activities on the synthesis of cyclic carbon-
ates from terminal epoxides and CO were investigated
2
2
5
4
3
without any additional Lewis acid reagent and solvent.
methylimidazolium chloride) shows lower activity than
HEMIMB (1-(2-hydroxyl-ethyl)-3-methylimidazolium bro-
mide) (entries 4–5). Interestingly, EMIMBr could also
show high activity in the presence of equal catalysis
amount of H O or C H OH (Table 1, entries 7 and 8).
Under the optimal conditions, the performance of vari-
ous catalysts was tested using propylene oxide (PO) as a
model substrate (Table 1). It is found that HFILs showed
higher reactivity (entries 4, 5, 11, and 12) in comparison
with other non-functionalized ionic liquids or solid ionic
catalysts, such as EMIMBr (1-ethyl-3-methylimidazolium
2
2
5
However, its activity is still low in the presence of non-
OH group containing chemicals such as DMC (dimethyl
carbonate) and DMF (N,N-dimethylformamide (entries 9
and 10). It was proposed that some cooperative affect from
the OH group and Lewis basic site of the ionic liquid
played an important role in accelerating the synthesis of
PC. Among the catalysts, HEMIMB was identified as the
most effective one and could be used four times with little
loss of activity (entry 21). As also shown in Table 1, the
CH/urea mixture could show higher activity than CH or
urea alone (entries 17–19), which might be due to the
appropriate effect between CH and urea by hydrogen
N
N
OH
Cl
N
N
OH
OH
Br
H C
3
H3C
HEMIMB
C H
HEMIMC
C H
4
9
2 5
OH
Br
Br
N
N
C4H9
C4H9
C2H5
C2H5
4
4
bond. When CH was substituted by HEMIMB to form
another two-reagent mixture HEMIMB/urea (molar ratio,
HETBAB
HETEAB
4
5
1
:2), almost the same value in activity was achieved com-
Scheme 1. Hydroxyl-functionalized ionic liquids (HFILs).
pared to that of HEMIMB (entries 4 and 20). In addition,
TGA experiments (Scheme 2) show that HEMIMB is more
stable than CH under heating, and that urea decomposes
obviously by increasing temperature above 150 °C. There-
fore, the low thermal stability of HEMIMB/urea and
CH/urea mixtures would limit the utilization of these cata-
lytic systems in industrial process. Based on the results, sin-
gle HEMIMB was chosen for the further investigation
because of its good performance and thermal stability. It
is important to avoid the usage of binary system for conve-
nient and economical operation of industrial synthesis of
Table 1
a
Synthesis of PC catalyzed by different catalysts
O
O
+
CO2
O
O
3
H C
H C
3
Entry
Catalysts
Conversion (%)
Selectivity (%)
1
2
3
4
5
6
7
8
9
1-Methylimidazole
2-Bromidethanol
1.5
2.0
1.8
99.2
78
83
93
92
84
99.1
99.2
99.1
99.8
99.3
99.3
99.1
99.3
99.8
99.6
99.2
99.2
99.3
99.8
99.6
99.4
98.5
—
PC from CO and PO.
2
C
2
H
5
OH
HEMIMB
HEMIMC
EMIMBr
EMIMBr/H
EMIMBr/C
b
2
O
b
2
H
5
OH
1
00
b
EMIMBr/DMC
HEMIMB
b
10
11
12
13
14
15
16
17
18
19
20
21
EMIMBr/DMF
HETBAB
HETEAB
TBAB
85
8
0
95.8
87.8
73.6
50.1
34
60
HEMIMB/urea
CH/urea
PPh
KI
KBr
CH
Urea
3
EtBr
4
0
2.0
45.1
trace
70.9
99.7
94.7
20
0
CH/urea
98.3
99.3
99.3
HEMIMB/urea
HEMIMB
c
a
Reaction conditions: PO (0.2 mol), catalyst (3.2 mmol), temperature:
25 °C, CO pressure: 2.0 MPa, reaction time: 1 h.
Equal catalysis amount (3.2 mmol).
After using four times.
0
50
100 150 200 250 300 350 400 450
1
2
Temperature (°C)
b
c
Scheme 2. TGA of HEMIM, HEMIMB/urea, and CH/urea.

3590
J. Sun et al. / Tetrahedron Letters 49 (2008) 3588–3591
Table 2
Synthesis of other cyclic carbonate catalyzed by HEMIMB
O
a
R
b
c
Entry
Epoxide
Time
h)
Selectivity
(%)
Yield
(%)
Yield
(%)
O
(
H
O
A'
d
O
Br
A'
H
1
0.7
99
99
87
1
a
O
H
R
Br
O
O
O
O
2
3
4
1
99.8
99
83
H C
3
1b
step 3
step 1
A'
R
O
3
O
O
0.33
1
92
99
92
99
88
84
A'
step 2
H
O
Cl
O
O
O
1
c
Br
O
O
Br
R
CO2
R
H C
3
1
d
[
A'-OH]Br: hydroxyl-functionalized IL
Scheme 3. Proposed mechanism for the reaction.
5
1
5
99
99
99
99
83
80
1
e
entries 7–8) other chemicals containing OH groups could
also have the same effect on the accelerating activity of
the catalyst.
O
6
In conclusion, one type of hydroxyl-functionalized ionic
liquid was developed and it was found to be an ideal and
extremely simple catalyst for the synthesis of cyclic carbon-
ate with high selectivity in the absence of any co-catalyst
and any co-solvents. OH groups in the catalyst or reaction
media are crucial for the reaction to proceed smoothly due
to its cooperation function of ring-opening of epoxide.
Among all the catalysts investigated, HEMIMB ionic
liquid was the most effective for the cyclic carbonate syn-
thesis with a 99% conversion of PO and 99.8% selectivity
of PC under the mild conditions (125 °C, 2.0 MPa, and
1
f
a
Reaction conditions: Epoxide (0.2 mol), HEMIMB (3.2 mmol), tem-
perature: 125 °C, CO
2
pressure: 2.0 MPa.
b
GLC yield.
c
Catalyst: EMIMBr.
Temperature: 110 °C.
d
The highly efficient HEMIMB catalyst was also found
to be extended to a variety of terminal epoxides (1a–f)
under the optimal reaction conditions compared to EMI-
MBr catalyst. The results are summarized in Table 2. Aro-
matic 1e and aliphatic 1a, 1b, 1d epoxides are preferred
substrates for the reaction. All the reactions could be com-
plete in less than 1.5 h except 1f, which needs 5 h to reach
1
.0 h). The catalyst showed high thermal stability and
could be recycled with minor loss in activity and selectivity.
Moreover, the catalyst could be applicable to other termi-
nal epoxides. These characteristics make it an ideal catalyst
in terms of potential industrial application in chemical car-
bon dioxide fixation.
9
0% yield due to the higher hindrance that originated from
the two rings. The selectivity of the reactions to the cyclic
carbonates was almost >99% except 1c.
2
4,41,46
Acknowledgments
Based on the reports
and the results obtained, a
mechanism portraying the probable sequence of events is
shown in Scheme 3. It shows that the OH group and the
Lewis basic site bromine anion of HFIL coordinately
attack the different parts of epoxide firstly. The coordina-
tion of the H atom with the O atom of epoxide through
a hydrogen bond resulted in the polarization of C–O
bonds, and the halide anion made the nucleophilic attack
on the less sterically hindered b-carbon atom of the epoxide
at the same time. As a result, the ring of the epoxide was
opened easily (step 1). Then, the interaction occurred
This work was supported by the National Science Fund
of China for Distinguished Young Scholar (20625618); Na-
tional 863 programme of China (2006AA06Z317) and Na-
tional Natural Science Foundation of China (20603040 and
20436050).
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1
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6
H NMR (DMSO-d ,
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1
2
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6
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6
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2
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3
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2
2
2
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2
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2
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2
3
2
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