Synthesis of cyclic carbonate from carbon dioxide and epoxide using amino acid ionic liquid under 1atm pressure

Article abstract of DOI:10.1071/CH11462

Herein, we report an effective synthesis of cyclic carbonates by cycloaddition of carbon dioxide to epoxide using a modified amino acid ionic liquid as catalyst under 1atm pressure. With triethylamine as co-catalyst, the catalytic activity of the l-proline based ionic liquid was greatly enhanced, and up to 97% isolated yield of cyclic carbonate was achieved at 90C under atmospheric pressure without organic solvent and metal component.

Full text of DOI:10.1071/CH11462

CSIRO PUBLISHING
Aust. J. Chem. 2012, 65, 381–386
Full Paper
http://dx.doi.org/10.1071/CH11462
Synthesis of Cyclic Carbonate From Carbon Dioxide
and Epoxide Using Amino Acid Ionic Liquid Under
1 atm Pressure
A
A
A
A
A B
,
Qing Gong, Huadong Luo, Jin Cao, Yuhan Shang, Haibo Zhang,
A
A B
,
Wenjing Wang, and Xiaohai Zhou
A
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, 430072,
P. R. China.
B
Corresponding author. Email: haibozhang1980@gmail.com (H. B. Zhang);
zxh7954@hotmail.com (X. H. Zhou)
Herein, we report an effective synthesis of cyclic carbonates by cycloaddition of carbon dioxide to epoxide using a
modified amino acid ionic liquid as catalyst under 1 atm pressure. With triethylamine as co-catalyst, the catalytic activity
of the L-proline based ionic liquid was greatly enhanced, and up to 97 % isolated yield of cyclic carbonate was achieved at
908C under atmospheric pressure without organic solvent and metal component.
Manuscript received: 6 December 2011.
Manuscript accepted: 13 February 2012.
Published online: 20 March 2012.
Introduction
O
O
Owing to the roaring concerns of environmental balance and
sustainability, the rapid accumulation of carbon dioxide (CO2)
in the atmosphere since the 1st industrial revolution has trig-
gered a universal public awareness.[1,2] Carbon dioxide, regar-
ded as the main contributor of green-house gases, is the key
factor to climate change on our planet, namely global-warming,
sea-level rise, etc. As a natural sink of carbon, carbon dioxide
exhibits inertness in most routine chemical reactions under
gentle and mild conditions, and sometimes even can be used as
the shielding gas. However, it is a promising candidate as a green
C1 building block, with merits of nonflammability, nontoxicity,
and abundance, which reveals a beautiful virgin landscape of
carbon utilization to exert our imagination and intelligence.[3–9]
The last two decades have seen a prevalence of papers on
carbon dioxide capture and transformation. Among the many
possibilities advocated, the chemical reaction of carbon dioxide
with epoxides to form five-membered cyclic carbonates has
proven to be an effective approach for carbon dioxide utilization
(Scheme 1).[10,11] It is not just for 100 % atom utilization of the
reaction, but also the product cyclic carbonate has found general
applications as a polar aprotic solvent, high-permittivity
component of electrolytes in lithium batteries and intermediate
in adhesives, paint strippers, and some cosmetics.[12–14] Numer-
ous homogeneous and heterogeneous catalyst systems have
been designed for this cycloaddition reaction.[15–20] Though
great achievements have been made, most of the previous
catalysts suffer some drawbacks which hamper the large scale
industrial applications, such as the requirement of high pressure,
metal components, and/or volatile organic solvent usage. Now-
adays, with an increasing burden of ecology equilibrium and
energy exhaustion, developing a noble catalyst that can promote
Catalyst
O
CO2
ϩ
R
O
R
1a–5a
1b–5b
Scheme 1. Synthesis of cyclic carbonate from carbon dioxide and epoxide.
the coupling reaction of carbon dioxide and epoxides at atmo-
spheric pressure and near ambient temperature is still attrac-
tive,[21–24] especially if metal-free and solventless.
Ionic liquids, born as the ‘solvent for future’, would be an
alternative candidate to catalyst design for this reaction, which
possesses the advantages of nonflammability, undetectable
vapor pressure, and peculiar solubility towards many organic
and inorganic substances. Furthermore, the spectacular solubil-
ity of carbon dioxide by physical or chemical absorption in ionic
liquids, compared with traditional molecular solvents, provides
the facility for this cycloaddition reaction.[25,26] In fact, ionic
liquids are regarded as the most efficient catalysts for this carbon
dioxide utilization.[27–30] Being an easily accessible ionic liquid
precursor, natural occurring amino acids and their correspond-
ing ionic liquids were tested for catalytic ability towards this
CO2 transformation reaction.[31–35]
Previously reported systems required harsh reaction condi-
tions and volatile organic solvents to accomplish satisfactory
results. In 2009, Endo revealed that carbon dioxide could be
chemically fixed with epoxide under 1 atm at 458C, by employ-
ing N-methyltetrahydropyrimidine (MTHP) as a ‘carbon
dioxide carrier’.[21] This strategy shows that the high activation
barrier of this coupling reaction might be overcome, by using
basic MTHP to capture and activate CO2 gas. In this present
Journal compilation Ó CSIRO 2012
www.publish.csiro.au/journals/ajc

382
Q. Gong et al.
Table 1. Synthesis of styrene carbonate from styrene oxide with different catalysts under 1 atm CO2 pressureA
Entry
Catalyst
Co-Catalyst
Cat/Co-Cat
molar ratio
Catalyst loading
[mol %]
YieldB
[%]
SelectivityC
[%]
1
L-Proline
Pro4,4Br
Pro4,4Cl
–
–
–
1
4.2
19
99
99
99
99
99
98
99
98
96
98
99
99
99
2
–
–
1
3
–
–
1
12
4
Et3N
Et3N
BuCl
Et3N
DBU
DMAP
Bu3N
–
–
1
0.93
32
5
KBr
1 : 1
1 : 3
1 : 1
1 : 1
1 : 1
1 : 1
–
1
6
Pro4,4Br
Pro4,4Br
Pro4,4Br
Pro4,4Br
Pro4,4Br
mPro4,4Br
Pro4,4Br
Pro4,4Br
1
5.3
77
7
1
8
1
83
9
1
79
10
11D
12
13
1
75
1
70
Et3N
Et3N
1 : 1
1 : 1
0.5
0.1
68
8.3
AReaction conditions: 1a (16 mmol), 908C, 24 h.
BIsolated yield.
CSelectivity was determined by GC.
DmPro4,4Br ¼ N,N-di-n-butyl-L-proline methyl ester bromide.
contributions provided a significant new insight into the
generally accepted Lewis-acid mechanism of this cycloaddition
reaction.[37] It was believed that the catalytic effect of ammo-
nium bromide derived from two roles of the catalyst. The
synergistic effect of bromide ion and in situ generated amine,
which came from the decomposition of ammonium bromide at
elevated temperature, facilitated the reaction with carbon
dioxide under mild conditions. In control experiments, Et3N was
proved not to be the ideal catalyst (entry 4), although with the
addition of potassium bromide, this cycloaddition reaction
afforded a moderate yield of 1b (entry 5), which was in accor-
dance with this standpoint. These results prompted us to use
three equivalents of butyl chloride (BuCl) to Pro4,4Br. In the
presence of excess BuCl, the nitrogen atom in AAIL would
probably exist as an ammonium ion, lowering its probability to
reversibly form amine species in situ. However, the yield of 1b
under these conditions was reduced to 5.3 % (entry 6).
In pioneering explorations by Jessop,[38]1,8-diazabicyclo[5.4.0]
undec-7-ene (DBU) was employed to capture and fix carbon
dioxide which inspired us to select Et3N as the co-catalyst to
improve the catalytic activity of AAIL, and the isolated yield of
1b was improved up to 77 % (entry 7). Considering the low
reactivity of 1a rising from b-carbon steric hindrance, this yield
of styrene carbonate under atmospheric pressure without metal
and organic solvent was really encouraging. In order to explore
the role of the amine base, other amines such as DBU,
4-dimethylaminopyridine (DMAP), and tri-n-butylamine
(Bu3N) were also tested with Pro4,4Br (entries 8–10). As there
was no significant difference in the catalytic activity of these
amines, the additional Et3N might be interacting with a proton in
the carboxylic moiety of the ionic liquid to facilitate the closure
of ROCOꢀ2 to yield the cyclic carbonate. We then selected
N,N-di-n-butyl-L-proline methyl ester bromide (mPro4,4Br)[39]
to catalyze this transformation reaction, and the yield of 1b was
found to be 70 % (entry 11). These results revealed that the
carboxylic acid in the AAIL might interact with the ROCO2ꢀ
through hydrogen bonding; adding an amine such Et3N could
eliminate this intermolecular force and reduce the activation
energy of this cycloaddition reaction. In the presence of
Pron,nBr
amino acid ionic liquid
(AAIL)
COOH
BrϪ
COOH
ϩ
ϩ
2RBr
N
N
H
R
R
Scheme 2. Preparation of amino acid ionic liquid from natural L-proline:
L-proline (0.10 mol), alkyl bromide (0.25 mol), anhydrous acetonitrile,
reflux, 48 h.
work, we report a novel chemical fixation of carbon dioxide
under mild conditions, with a L-proline based ionic liquid as the
main catalyst, in the presence of triethylamine (Et3N). As
expected, carbon dioxide was smoothly incorporated into the
epoxide under 1 atm pressure with satisfactory isolated yield of
cyclic carbonate, by tuning the pH of the ionic liquid.
Results and Discussion
Owing to the steric hindrance of the benzyl group, the trans-
formation of styrene oxide (1a) has been investigated less
compared with other terminal epoxides, especially at 1 atm
pressure. For this reason, 1a was selected as a model substrate to
carry out our elementary experiments in search for a highly
efficient catalyst working under mild reaction conditions with-
out metal and organic solvent, and the results are listed in
Table 1. To the best of our knowledge, the coupling reaction of
carbon dioxide with epoxide embodies three major steps: ring
opening, insertion of CO2, and finally cyclization. L-proline was
not a suitable catalyst towards effective chemical fixation of
carbon dioxide under 1 atm pressure (entry 1), which might be
ascribed to lack of a good nucleophile to open the three-
membered epoxide ring. Based on this hypothesis, we modified
natural L-proline with 1-bromobutane to yield the amino acid
ionic liquid (AAIL) Pro4,4Br (Scheme 2).[36] With bromide ion
to attack the epoxide, Pro4,4Br catalyzed the atmospheric pres-
sure carbon dioxide transformation with a reasonable yield of
styrene carbonate (1b) (entry 2). When the bromide was
replaced by chloride, the yield of cyclic carbonate was reduced
from 19 % to 12 % (entry 3), which could be ascribed to the
nucleophilicity order Br . Cl. North and co-workers’ brilliant

Carbonate Synthesis Under 1 atm CO2 Pressure
383
100
100
80
60
40
20
0
80
60
40
20
0
0
4
8
12
16
20
24
2
4
6
8
10
Time [h]
Alkyl chain length [n]
Fig. 3. Effect of reaction time on the yield of styrene carbonate 1b.
Reaction conditions: 1a (16 mmol), AAIL Pro4,4Br (1 mol-%), Et3N
(1 mol-%), 1 atm, 908C.
Fig. 1. Effect of alkyl chain length of amino acid ionic liquid on the yield of
styrene carbonate 1b. Reaction conditions: 1a (16 mmol), AAIL (1 mol-%),
Et3N (1 mol-%), 1 atm, 908C, 24 h.
100
80
60
40
20
0
carbonate increased with increasing temperature, and reached
96 % at 1108C. The designed system lost most of its catalytic
activity at 708C, albeit with a reaction time of 24 h. Considering
the energy consumption and catalyst activity, 908C was chosen
as the optimal temperature for this reaction. Our experiments
also showed that excellent isolated yields of other epoxides were
also achieved at this temperature.
Dependence of the reaction time on the yield of 1b is depicted
in Fig. 3. The reaction was performed in the presence of 1 mol-%
of AAIL Pro4,4Br with Et3N at 908C under 1 atm pressure, and
was monitored for 24 h at regular intervals. The results indicate
that the coupling reaction experienced an induction period, and
then the yield of 1b increased steadily as time passed. It was
noteworthy that a reasonable 77 % isolated yield of styrene
carbonate 1b, with 99 % selectivity was achieved after 24 h. The
products other than cyclic carbonate were isomers of styrene
oxide, such as acetophenone, 2-phenylacetaldehyde and
1-phenylethane-1,2-diol. It is noteworthy that no polymeriza-
tion products were detected.
70
80
90
100
110
Temperature [°C]
Fig. 2. Effect of temperature on the yield of styrene carbonate 1b. Reaction
conditions: 1a (16 mmol), AAIL Pro4,4Br (1 mol-%), Et3N (1 mol-%),
1 atm, 24 h.
The cycloaddition of CO2 with other terminal monosubsti-
tuted epoxides was also investigated at 908C under 1 atm
pressure, and the results are summarized in Table 2. The
designed catalyst system was found to be applicable to a variety
of terminal epoxides, providing the corresponding cyclic carbo-
nates in high yields and excellent selectivity. Because of the
steric hindrance from the b-carbon, styrene oxide (1a) afforded
a relatively low yield of cyclic carbonate 1b (entry 1). Among
those substrates, epichlorohydrin (2a) and ally glycidyl ether
(3a) were shown to be the most reactive epoxide (entries 2 and
3). When the allyl group was substituted by the n-butyl group,
the yield of cyclic carbonate was reduced from 97 % to 88 %
(entry 4), and the relatively non-polar epoxide 5a was obtained
in slightly lower yield (entry 5). Due to high steric hindrance, the
efficient chemical fixation of carbon dioxide with cyclohexene
oxide still remained a challenge (entry 6), especially under 1 atm
pressure without a metal component.
1 equivalent of Et3N to Pro4,4Br, the isolated yield of 1b was
68 % with 0.5 mol-% Pro4,4Br (entry 12), and the chemical
fixation of carbon dioxide could hardly take place with further
Pro4,4Br loading reduced to 0.1 mol-% (entry 13).
The influence of alkyl chain length of the AAIL on the
catalytic efficiency was also investigated, and the results are
shown in Fig. 1. In general, a difference in the alkyl substituent
did not have a great effect on catalytic activity. This suggests
that the overall framework of the AAIL as catalyst with Et3N
was favourable for the efficient chemical transformation of CO2,
and the length of alkyl substituents in the series had only a minor
impact on the course of the reaction.
Fig. 2 shows the effect of temperature on the cycloaddition
reaction catalyzed by AAIL Pro4,4Br in the presence of Et3N, at
atmospheric pressure, in the temperature range of 70–1108C,
with a reaction time of 24 h. It was clear that temperature had a
notable effect on this transformation. The yield of styrene
A plausible catalytic cycle is shown in Scheme 3. First, the
Brꢀ anion attacks the less hindered carbon atom of the epoxide,
followed by ring opening step to form an oxy-anion species
(A); next, the interaction occurs between the oxygen anion of

384
Q. Gong et al.
Table 2. Synthesis of other cyclic carbonates with Pro4,4Br and Et3N under 1 atm CO2 pressureA
Entry
Epoxide
Product
YieldB [%]
SelectivityC [%]
O
O
O
O
1
77
99
1a
1b
O
O
Cl
O
2a
2
3
4
95
97
88
94
99
98
O
Cl
2b
O
O
O
O
3a
O
O
3b
O
O
O
O
4a
O
O
4b
O
O
5a
5D
82
99
O
O
5b
O
O
O
6
Trace
–
6a
O
6b
AReaction conditions: epoxide (16 mmol), AAIL Pro4,4Br (1 mol-%), Et3N (1 mol-%), 908C, 24 h.
BIsolated yield.
CSelectivity was determined by GC.
D10 atm CO2.
O
OϪ
R
R
(A)
Br
Et3NHϩ
COOϪ
COOH
Nϩ
RЈ
O
CO2
Nϩ
RЈ
ϩ
Et3N
RЈ
RЈ
BrϪ
BrϪ
O
O
OϪ
R
O
(B)
Br
O
R
Scheme 3. Plausible catalytic cycle.

Carbonate Synthesis Under 1 atm CO2 Pressure
385
A and CO2, producing an alkylcarbonate anion (B); finally, the
cyclic carbonate is achieved by intramolecular cyclic elimina-
tion, releasing the catalyst for recycling. The additional Et3N
might interact with a proton in the carboxylic moiety of the ionic
liquid to facilitate the closure of ROCOꢀ2 to yield the cyclic
carbonate.
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cyclic carbonate 1b: Styrene oxide (1a) (1.9 g, 16 mmol), ionic
liquid Pro4,4Br (0.050 g, 0.16 mmol) and Et3N (16 mg,
0.16 mmol) were consecutively placed in 10 mL Schlenk flask,
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Acknowledgement
We gratefully acknowledge financial support by the National Natural
Science Foundation of China (no. 20972120).

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