ZnBr2-based choline chloride ionic liquid for efficient fixation of CO2to cyclic carbonate

Article abstract of DOI:10.1080/00397911.2011.562337

(Chemical Equation Presented) In this work, ZnBr₂-based choline chloride (CH) was first investigated to catalyze the synthesis of cyclic carbonates from CO₂ and epoxides under solventless conditions. It was demonstrated that ZnBr

Full text of DOI:10.1080/00397911.2011.562337

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ZnBr2-Based Choline Chloride Ionic
Liquid for Efficient Fixation of CO2 to
Cyclic Carbonate
Weiguo Cheng a , Zengzeng Fu a , Jinquan Wang a , Jian Sun a &
Suojiang Zhang a
a State Key Laboratory of MultiPhase Complex System, Institute of
Process Engineering, Chinese Academy of Science, Beijing, China
Accepted author version posted online: 02 Jan 2012.Version of
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To cite this article: Weiguo Cheng, Zengzeng Fu, Jinquan Wang, Jian Sun & Suojiang Zhang (2012):
ZnBr2-Based Choline Chloride Ionic Liquid for Efficient Fixation of CO2 to Cyclic Carbonate, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
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Synthetic Communications1, 42: 2564–2573, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2011.562337
ZnBr2-BASED CHOLINE CHLORIDE IONIC LIQUID FOR
EFFICIENT FIXATION OF CO2 TO CYCLIC CARBONATE
Weiguo Cheng, Zengzeng Fu, Jinquan Wang, Jian Sun, and
Suojiang Zhang
State Key Laboratory of MultiPhase Complex System, Institute of Process
Engineering, Chinese Academy of Science, Beijing, China
GRAPHICAL ABSTRACT
Abstract In this work, ZnBr2-based choline chloride (CH) was first investigated to catalyze
the synthesis of cyclic carbonates from CO2 and epoxides under solventless conditions. It
was demonstrated that ZnBr2-based CH was very efficient and selective. Under the opti-
mum reaction conditions, 99% yield of propylene carbonate was achieved. The catalyst
can be reused five times without loss of catalytic activity. It was proved to be very active
in a variety of terminal epoxides with CO2. In addition, a synergistic catalytic mechanism
by Zn2þ and -OH was proposed.
Keywords Carbon dioxide; choline chloride; cycloaddition; epoxide; zinc bromide
INTRODUCTION
As CO2 is nontoxic, abundant, inexpensive, nonflammable, and highly func-
tional, much attention has been paid to its use as an environmentally benign chemical
reagent.[1] One of the applications is the synthesis of cyclic carbonates via cycloaddi-
tion of epoxides and CO2,[2] which have been widely used for various purposes, such
as poplar aprotic solvents, intermediates in organic synthesis, monomers for synthesiz-
ing polycarbonate, agricultural chemicals, alkylating agents, electrolytic elements of
lithium secondary batteries, and chemical ingredients for preparing medicines.[3–6]
Moreover, cycloaddition of epoxides and CO2 are 100% atom-economical reactions.
Received December 12, 2010.
Address correspondence to Weiguo Cheng, State Key Laboratory of Multiphase Complex System,
Institute of Process Engineering, Chinese Academy of Science, 1 Beiertiao Zhongguancun, Beijing, 100190,
China. E-mail: wgcheng@home.ipe.ac.cn
2564

ZnBr2-BASED CHOLINE CHLORIDE IONIC LIQUID
2565
Scheme 1. Cycloaddition of epoxide with CO2 catalyzed by ZnBr2=CH.
Numerous compounds have already been developed to catalyze these
reactions.[7] Heterogeneous catalysts, such as metal oxides[8] and zeolites,[9] suffer
from poor activity, rigorous reaction conditions, and poor stability. Although good
performance of modified polymers and resins are obtained, the reaction is conducted
at longer reaction times under comparative high pressures.[10] Homogeneous catalysts
are effective for cyclic carbonate synthesis.[11–23] However, in most cases, organic
solvents are used in the reactions over homogeneous catalysts. More important, the
procedure of preparing homogeneous catalysts is generally complicated and the cost
of catalysts is very high. Therefore, it is desirable to explore a simple, efficient, and
available catalyst system for this transformation.
In recent years, ionic liquids (ILs) have received considerable interest because of
their unique properties, such as undetectable vapor pressure, wide liquid temperature
range, special solubility for many organic or inorganic compounds, and feasibility for
designing.[24] On green chemical principles, ILs as catalysts would be composed of
biodegradable constituents and exhibit high thermal and chemical stabilities.[25] The
bifunctional catalyst based on choline chloride has found great potential in organic
synthesis.[26] Recently, bicomponent catalysts, such as CH and urea as well as hexabu-
tylguanidinium salt and zinc bromide,[27] have been reported to efficiently catalyze the
synthesis of cyclic carbonates from CO2 and epoxides. In this respect, the IL composed
of Zn2þ is probably the most attractive catalyst for CO2 coupling with epoxides.[28]
For example, Zn2þ combined with hexabutylguanidinium,[28g] phosphonium,[29] or
imidazolium[28g] has shown high catalytic activity. In addition, Zn2þ-based CH has
been investigated because of its easy preparation, moisture stability, low cost, and
biodegradability.[30] Zn2þ-based CH has been used in the processes of electrolytic
deposition,[31] regiospecific Fischer indole reaction,[32] the protection of carbonyls
reaction,[33] and Diels–Alder reaction.[34] To the best of our knowledge, there has
no report about this kind of catalyst used for fixation of CO2 to cyclic carbonate.
In this work, we show that ZnBr2-based CH can efficiently catalyze the synthesis of
cyclic carbonates from CO2 and epoxides under solventless condition, and the -OH
group of CH plays an important role in the reaction (Scheme 1). In addition, the
catalyst can be simply prepared by a combination of CH and Zn2þ
.
RESULTS AND DISCUSSION
For cycloaddition reaction of CO2 with epoxide, propylene oxide (PO) was
chosen as the standard substrate to investigate the performance of CH with different
Lewis acid catalysts. The results are summarized in Table 1. ZnBr2 or CH alone
showed very poor activity in the reaction (entries 1 and 2). The behavior of CH with

2566
W. CHENG ET AL.
different Lewis acid catalysts varied considerably. The reaction almost did not take
place over CH with Kþ or Mg2þ (entries 3–5). The yield was unsatisfactory with
CuCl2 ꢀ 2H2O=CH, FeCl3=CH, NaCl=CH, and AlCl3=CH (entries 6–9). However,
CH combined with zinc halide showed greater activity than with other metallic
halides in accordance with the stronger Lewis acidity of Zn2þ (entries 10 and 11).
The greatest yield of propylene carbonate (PC) reached 99% over ZnBr2-based CH
(entry 11). Furthermore, 92% PC yield could be obtained even with a lower catalyst
amount (entry 12). When tetramethylammonium chloride (TMAC) substituted for
CH, the corresponding catalyst system showed very poor activity, which indicated
that the -OH of CH was crucial for the reaction to proceed smoothly (entry 13).
The performance of ZnBr2-based choline chloride for synthesis of propylene
carbonate from CO2 and propylene is shown in Table 2. The results show that the
molar ratio of CH to ZnBr2 has a significant influence on catalytic activity. The yield
of PC increased from 31% to 99% with the increase of molar ratios of CH to ZnBr2
from 1 to 5 (entries 1–4), but the yield of PC decreased with the further increase of
molar ratio (entry 5). The greatest catalytic activity was obtained when the ratio of
CH to ZnBr2 was 5:1. The reaction temperature also has a remarkable effect on
the cycloaddition reaction. As the temperature increased from 60 ꢁC to 110 ꢁC, the
yield of product increased from 1% up to 99% (entries 6, 7, 8, and 4). However, with
further increase of the temperature, the yield of PC decreased from 99% to 93%
because of side reactions such as propylene oxide isomerization and polymerization
reaction (entries 4 and 9). The results suggested that increasing the reaction time
had a positive effect on the PC yield within 60 min. The yield of PC increased sharply
with increase of time from 20 min to 60 min (entries 10, 11, and 4). High pressure is
dangerous in organic synthesis; however, the reaction proceeded smoothly even under
Table 1. Synthesis of propylene carbonate from CO2 and propylene oxide
catalyzed by CH with different Lewis acid catalystsa
Entry
Catalyst
Yieldb (%)
TOFc
1
2
CH
ZnBr2
KCl=CH
Trace
1
Trace
5
3
Trace
Trace
Trace
5
Trace
Trace
Trace
25
4
KBr=CH
5
MgCl2 ꢀ 6H2O=CH
CuCl2 ꢀ 2H2O=CH
FeCl3=CH
NaCl=CH
6
7
14
21
70
8
106
166
451
494
915
6
9
AlCl3=CH
33
90
10
11
12d
13
ZnCl2=CH
ZnBr2=CH
ZnBr2=CH
ZnBr2=TMAC
99
92
1
aReaction conditions: PO (140 mmol), CH (1.4 mmol), ZnBr2 (0.28 mmol),
CO2 (1.5 MPa), 110 ꢁC, 60 min.
bIsolated yield.
cMoles of propylene carbonate produced per mole of catalyst per hour.
dZnBr2 (0.14 mmol).

ZnBr2-BASED CHOLINE CHLORIDE IONIC LIQUID
2567
Table 2. Synthesis of propylene carbonate from CO2 and propylene oxide catalyzed
by ZnBr2-based choline chloridea
Yield (%)c
b
Entry
CH=ZnBr2
Time (min)
Temp. (ꢁC)
Pressure (MPa)
1
2
1:1
3:1
4:1
5:1
6:1
1:1
1:1
1:1
1:1
5:1
5:1
5:1
5:1
5:1
5:1
5:1
5:1
5:1
60
60
60
60
60
60
60
60
60
20
40
80
60
60
60
60
60
60
110
110
110
110
110
60
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
2.5
1.5
1.5
1.5
1.5
1.5
31
86
91
99
94
1
3
4
5
6
7
80
28
66
95
3
8
100
120
110
110
110
110
110
110
110
110
110
9
10
11
12
13
14d
15e
16f
17g
18h
88
97
97
99
99
98
99
99
aReaction conditions: PO (140 mmol) and CH (1.4 mmol).
bCH=ZnBr2: molar ratio of CH to ZnBr2.
cIsolated yield.
dThe first run of the catalyst.
eThe second run of the catalyst.
fThe third run of the catalyst.
gThe fourth run of the catalyst.
hThe fifth run of the catalyst.
pressure as low as 1.5 MPa (entry 4). This can be explained by effects on the concen-
trations of PO and opening of the oxirane ring.[7,10] At the lower CO2 pressure, the
concentration of PO in the liquid phase was high, which increased the reaction rate.[10]
At the same time, lower CO2 pressure increased the polarity of the reaction medium,
resulting in the acceleration of opening the oxirane ring by nucleophilic attack of the
catalyst.[7] Therefore, 5:1, 110 ꢁC, 60 min, and 1.5 MPa are the appropriate molar
ratio of CH to ZnBr2, temperature, reaction time, and pressure, respectively. To
investigate the recycle and reuse of ZnBr2-based CH, the PC and remaining PO were
distilled from the product mixture after the reaction, and the remaining catalyst was
further used for the next run directly. The catalyst can be repeatedly used five times,
and its performance for the fifth run is almost the same as that for the first run (entries
14–18). Because of the good performance of ZnBr2-based choline chloride and
simple catalyst system, this system might be used in the industrial process for the
manufacture of propylene carbonate.
To survey the applicability of various epoxides to this process, we examined the
cycloaddition reactions of other terminal epoxides with CO2 catalyzed by ZnBr2-
based CH. The results are summarized in Table 3. The ZnBr2-based CH catalyst
was found to be applicable to a variety of terminal epoxides. The catalyst system

2568
W. CHENG ET AL.
Table 3. Coupling of CO2 and various epoxides catalyzed by ZnBr2-based choline chloridea
Entry
Epoxides
Products
Yield (%)b
Selectivity (%)
1
100
99
2
100
99
3
99
98
95
98
86
20
4c
5d
aReaction condition: epoxide (140 mmol), CH (1.4 mmol), ZnBr2 (0.28 mmol), CO2
(1.5 MPa), 110 ꢁC, 60 min.
bIsolated yield.
c12 h.
d24 h.
provided the corresponding cyclic carbonates in 20–99% yields with excellent selec-
tivity. Propylene oxide (1a), ethylene oxide (1b), epichlorohydrin (1c), and styrene
oxide (1d) are good substrates to give cyclic carbonates in excellent yields. However,
1,2-cyclohexene oxide (1e) exhibited relatively poor activity, due to the greater hin-
drance of rings.
It has been suggested that the chlorine ion could open the epoxy ring when the
ring was activated by formation of hydrogen bonding.[27,35] In our catalyst system,
ZnBr2 and CH showed a synergetic effect on promoting the reaction. The tentative
mechanism is suggested and shown schematically in Scheme 2. The bromine anion
opens the epoxy ring, which is synergistically activated by Zn2þ and -OH of CH
to give the intermediate. Then, the intermediate reacts with CO2 to form the corre-
sponding cyclic carbonate and regenerate the catalyst.

ZnBr2-BASED CHOLINE CHLORIDE IONIC LIQUID
2569
Scheme 2. Proposed mechanism for the coupling of epoxides with CO2 catalyzed by ZnBr2-based CH.
CONCLUSION
A novel catalyst system, ZnBr2-based CH, exhibits excellent catalytic activity
for the synthesis of cyclic carbonates from epxides and CO2. The reaction of the syn-
thesis of PC is carried out under mild condition without any cosolvent. The catalyst
system can be reused five times without dropping in yield and selectivity under opti-
mum conditions. In addition, the catalyst system is proven to be excellently active in a
variety of terminal epoxides with CO2, and good yields of cyclic carbonates can be
obtained. Furthermore, the catalyst system is simple, economical, and environmen-
tally benign. These characteristics make it an ideal greener catalyst system in terms
of potential industrial application in chemical fixation of CO2.
EXPERIMENTAL
Choline chloride, epichlorohydrin, epoxyethane, ZnBr2, metal chloride, and
metal bromide were analytical-reagent grade and obtained from Beijing Chemical
Plant. Other epoxides were purchased from Alfa Aesar, and CO2 with 99.9% purity
was purchased from Beijing Bei temperature gas factory.

2570
W. CHENG ET AL.
Coupling Reaction
All reactions were carried out in a 100-mL stainless-steel autoclave equipped
with a magnetic stirrer and automatic temperature control system. For a typical reac-
tion, the reactor was charged with the appropriate amounts of catalyst, epoxide, and
1.5 MPa CO2. The reaction mixture was heated to 110 ꢁC while maintaining the
desired CO2 pressure and reacted for 1 h. After the reaction was completed, the auto-
clave was cooled to 0 ꢁC in an ice-water bath. The products were isolated and ana-
lyzed by gas chromatography (Agilent 6890=5973B) equipped with a RTX-5
capillary column (30 m ꢂ 0.25 mm) and a flame ionization detector (FID).
NMR Data for Cyclic Carbonates
The products were also characterized by 1H NMR and 13C NMR spectra, which
were recorded on a Varian Mercury Plus 400 spectrometer. Chemical shifts were given
as d values referenced in parts per million (ppm) from tetramethylsilane (TMS) as an
internal standard. The structures of the isolated products were also characterized by
high-resolution mass spectroscopy (HRMS) as previously reported.[36]
4-Methyl-1,3-dioxolan-2-one (2a). 1H NMR (CDCl3, TMS, 400 MHz): 1.49
(d, J ¼ 6.0 Hz, 3 H), 4.05 (t, J ¼ 8.8 Hz, 1H), 4.60 (t, J ¼ 8.0 Hz, 1H), 4.86–4.94 (m,
1H); 13C NMR (CDCl3, TMS, 400 MHz): 18.95, 70.46, 73.51, 154.95 (C O). HRMS
=
calcd. for C4H6O3: 102.0316; found: 102.0316.
1,3-Dioxolan-2-one (2b). 1H NMR (CDCl3, TMS, 400 MHz): 4.2 (t, J ¼
13
10 Hz, 4H); C NMR (CDCl3, TMS, 400 MHz): 63.3, 155 (C O). HRMS calcd.
for C3H4O3: 88.0160; found: 88.0160.
=
4-Chloromethyl-1,3-dioxolan-2-one(2c). 1H
NMR
(CDCl3,
TMS,
400 MHz): 1.490 (d, J ¼ 6.0 Hz, 3H), 4.05 (t, J ¼ 8.4 Hz, 1H), 4.60 (t, J ¼ 8.0 Hz,
1H), 4.86–4.94 (m, 1H); 13C NMR (CDCl3, TMS, 400 MHz): 18.95, 70.46, 73.51,
=
154.95 (C O). HRMS calcd. for C4H5ClO3: 135.9927; found: 135.9927.
4-Phenyl-1,3-dioxolan-2-one(2d). 1H NMR (CDCl3, TMS, 400 MHz): 4.34
(t, J ¼ 8.4 Hz, 1H), 4.80 (t, J ¼ 8.4 Hz, 1H), 5.68 (t, J ¼ 8.0 Hz, 1H), 7.35–7.44 (m,
5 H); 13C NMR (CDCl3, TMS, 400 MHz): 71.10, 77.92, 125.81, 129.12, 129.63,
=
135.70, 154.81 (C O). HRMS calcd. for C9H8O3: 216.1150; found: 216.1150.
4,5-Tetramethylene-1,3-dioxolan-2-one (2e). 1H NMR (CDCl3, TMS,
400 MHz): 1.80–1.86 (m, 2H), 1.97–2.05 (m, 2H), 2.28 (d, J ¼ 4.8 Hz, 4H),
5.06–5.11 (m, 2H); 13C NMR (CDCl3, TMS, 400 MHz): 19.00, 26.61, 75.65,
=
155.27 (C O). HRMS calcd. for C7H10O3: 142.0630; found: 142.0630.
ACKNOWLEDGMENTS
We gratefully acknowledge the National Natural Sciences Foundation of
China (20876162 and 20625618), National Basic Research Program of China (973
Program, 2009CB219901), and Key Projects in the National Science and Technology
Pillar Program in the Eleventh Five-Year Plan Period (2008BAF33B04).

ZnBr2-BASED CHOLINE CHLORIDE IONIC LIQUID
2571
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