Efficient acid-base bifunctional catalysts for the fixation of CO 2 with epoxides under metal- and solvent-free conditions

Article abstract of DOI:10.1002/cssc.201000305

A series of acid-base bifunctional catalysts (ABBCs) that contain one or two Br?nsted acidic sites in the cationic part and a Lewis-basic site in the anionic part are used as efficient catalysts for the synthesis of cyclic carbonates by cycloaddition of CO₂ to epoxides, without the use of additional co-catalyst or co-solvent. The effects of the catalyst structures and various reaction parameters on the catalytic performance are investigated in detail. Almost complete conversion can be achieved in 1 h for propylene oxide using [{(CH₂)₃COOH}₂im]Br under mild reaction conditions (398 K and 2 MPa). Furthermore, the catalyst can be recycled over five times without substantial loss of catalytic activity. This protocol is found to be applicable to a variety of terminal epoxides, producing the corresponding cyclic carbonates in good yields and high selectivities. A synergistic effect of the acidic and the basic sites as well as suitable hydrogen-bonding strength of ABBCs are considered crucial for the reaction to proceed smoothly. The activities of the ABBCs increase remarkably with increasing carboxylic-acid chain length of the cation. This metal- and solvent-free process thus represents an environmentally friendly process for BTC-catalyzed conversion of CO₂ into value-added chemicals.

Full text of DOI:10.1002/cssc.201000305

DOI: 10.1002/cssc.201000305
Efficient Acid–Base Bifunctional Catalysts for the Fixation
of CO2 with Epoxides under Metal- and Solvent-Free
Conditions
Jian Sun, Lijun Han, Weiguo Cheng, Jinquan Wang, Xiangping Zhang, and
Suojiang Zhang*[a]
A series of acid–base bifunctional catalysts (ABBCs) that con-
tain one or two Brønsted acidic sites in the cationic part and a
Lewis-basic site in the anionic part are used as efficient cata-
lysts for the synthesis of cyclic carbonates by cycloaddition of
CO2 to epoxides, without the use of additional co-catalyst or
co-solvent. The effects of the catalyst structures and various re-
action parameters on the catalytic performance are investigat-
ed in detail. Almost complete conversion can be achieved in
1 h for propylene oxide using [{(CH2)3COOH}2im]Br under mild
reaction conditions (398 K and 2 MPa). Furthermore, the cata-
lyst can be recycled over five times without substantial loss of
catalytic activity. This protocol is found to be applicable to a
variety of terminal epoxides, producing the corresponding
cyclic carbonates in good yields and high selectivities. A syner-
gistic effect of the acidic and the basic sites as well as suitable
hydrogen-bonding strength of ABBCs are considered crucial
for the reaction to proceed smoothly. The activities of the
ABBCs increase remarkably with increasing carboxylic-acid
chain length of the cation. This metal- and solvent-free process
thus represents an environmentally friendly process for BTC-
catalyzed conversion of CO2 into value-added chemicals.
Introduction
Many efforts towards the fixation of CO2 have been reported
during the past two decades. An important incentive is the
problem of global warming, caused mainly by CO2. Another
reason for these efforts is the use of CO2 as a C1 feedstock in
organic synthesis.[1] Atom-efficient cycloaddition of CO2 to ep-
oxides (Scheme 1) is one of the most successful examples of
cant, unsatisfactory activities, harsh reaction conditions, the
water and/or air sensitivity of the metal-containing catalysts,
and/or the need for organic solvents (e.g., DMF, toluene,
CH2Cl2) are still disadvantages that need to be overcome.
Therefore, research towards a metal- and solvent-free catalyst
system that combines stability with activity under relatively
mild reaction conditions is essential from the viewpoints of
large-scale manufacturing and environmental concerns.
Recently, research interest in task-specific ionic liquids has
increased strongly because these “tailor-made” materials can
be designed for specific applications in diverse areas, ranging
from synthetic and catalytic chemistry to biotechnology, elec-
trochemistry, and materials science by introducing certain
functional groups into the cation or/and anion of traditional
ionic liquids.[9–15] Among these, imidazolium salts that contain
carboxylic acidic groups have attracted much attention be-
cause they offer the new possibility of developing environmen-
tally friendly acidic catalysts. This is due to a combination of
the advantages of liquid acids and solid acids, that is, uniform
acid sites, stability in water and air, ease of separation, and re-
usability.[10–15] They have been applied to numerous fundamen-
tal organic transformations, such as Friedel–Crafts alkylation,[10f]
Scheme 1. Synthesis of cyclic carbonates from CO2 and epoxides.
CO2 fixation, producing cyclic carbonates that find use as,
amongst others, excellent aprotic polar solvents, intermediates
for the pharmaceutical and fine chemical industries, and pre-
cursors for polycarbonate materials.[1–2] Therefore, it is not sur-
prising that these kinds of reactions have been studied exten-
sively.
Numerous catalysts for the synthesis of cyclic carbonates
have been developed successfully. Among these, there have
been many reports on imidazolium, ammonium, phosphonium
and pyridinium salts, many of which may be classified as ionic
liquids, as efficient catalysts for CO2 fixation through the syn-
thesis of cyclic carbonates from epoxides.[3–8] The combination
of these salts with Lewis acidic co-catalysts has resulted in
many diverse and flexible “platforms,” establishing highly effec-
tive catalytic systems.[3–8] Although the advances are signifi-
[a] Dr. J. Sun, Dr. L. Han, Dr. W. Cheng, Dr. J. Wang, Prof. X. Zhang,
Prof. S. Zhang
State Key Laboratory of Multiphase Complex System
Institute of Process Engineering
Chinese Academy of Sciences
100190 Beijing (PR China)
Fax: (+86)10-8262-7080
E-mail: sjzhang@home.ipe.ac.cn
502
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Acid–Base Bifunctional Catalysts for the Fixation of CO2 with Epoxides
condensation,[11c] Beckmann rearrangement,[11e] esterifica-
tion,[12b,14b] and toluene carbonylation.[14a]
Table 1. Synthesis of PC (5b) catalyzed by different ABBCs.[a]
Han and co-workers made an attempt to use carboxylic acid
groups in functionalized ammonium salts (HBetX, X=Cl, Br, I)
for the synthesis of cyclic carbonates from CO2 and epoxide.[16a]
The co-existence of a hydrogen-bond donor (ÀCO2H), ammoni-
um cation, and halide anion led to a synergy effect, promoting
the reaction.[16a] However, the activity of the catalysts was still
unsatisfactory, and further effects of the carboxylic acid group
structure on the activities of these betaine-based salts were
not investigated. In 2005, a series of imidazolium chlorides
with one or two carboxylic acidic substituent, and their zwitter-
ions, were synthesized and characterized by Fei et al.[15] Subse-
quently, these types of carboxylic-acid-functionalized imidazoli-
um salts were applied to several reactions, such as the aerobic
oxidation of alcohols,[17a] oxidative desulfurization of fuels,[17b]
and palladium-catalyzed selective oxidation of styrene.[17c]
However, to the best of our knowledge these Brønsted acid–
Lewis base bifunctional imidazolium-type compounds were
not applied to the synthesis of cyclic carbonates.
Entry
Catalysts
Conversion [%]
Yield [%][b]
1
2
[(CH2CH2OH)mim]Br
[emim]Br
CH3CO2H/[emim]Br
82
60
80
91
84
78
12
89
99
30
66
10
92
60
84
53
73
81
59
78
88
83
77
11
88
98
29
65
9
91
59
83
52
70
3[c]
4
2CH3CO2H/[emim]Br
2CH3CH2OH/[emim]Br
[(CH2CO2H)2im]Br (1a)
[(CH2CO2H)2im]Cl (1b)
[{CH(CH3)CO2H}2im]Br (1c)
[{(CH2)3CO2H}2im]Br (1d)
[{(CH2)3CO2H}2im]Cl (1e)
[(CH2CO2H)mim]Br (2a)
[(CH2CO2H)mim]Cl (2b)
[{(CH2)3CO2H}mim]Br (2c)
[(CH2CO2H)bpy]Br (3a)
[{(CH2)3CO2H}bpy]Br (3b)
[(CH2CO2H)inic]Br (4a)
[{(CH2)3CO2H}inic]Br (4b)
5[d]
6
7
8
9
10
11
12
13
14
15
16
17
[a] Reaction conditions: PO (0.2 mol), catalyst (1 mol%), T=398 K, P=
2.0 MPa, t=1 h. [b] GC yield. [c] [emim]Br (2 mmol), CH3CO2H (2 mmol).
[d] [emim]Br (2 mmol), CH3CH2OH (4 mmol).
Herein, we report the first use of four kinds of acid–base bi-
functional catalysts (Scheme 2) as catalysts for the reaction of
epoxides with CO2. More importantly, the effect of the catalyst
structure on catalytic performance was investigated in detail.
The catalysts were proved to be efficient for the chemical fixa-
tion of CO2, being environmentally friendly substitutes for
Lewis-acid/Lewis-base binary catalytic systems.
the presence of equal amount of CH3CO2H (entry 3). A better
result (91% PO conversion) is obtained in the presence of
more CH3CO2H (entry 4). When CH3CH2OH is introduced into
the catalytic system as a promoter, instead of CH3CO2H, the
obtained 2CH3CH2OH/[emim]Br catalytic system resulted in a
lower PO conversion (84%) (entry 5). The carboxylic acid group
thus appears to be more beneficial to activating the epoxide
ring than the hydroxyl group.
The structure of the cation of ABBCs has a strong effect on
catalytic activity towards the synthesis of PC. With BrÀ as an
anion, the effect of different types of cations on the reaction
has been studied under the same reaction conditions. For imi-
dazolium, the activity orders are [{(CH2)3CO2H}2im]+ >[{CH-
(CH3)CO2H}2im]+ >[(CH2CO2H)2im]+ (Table 1, entries 6, 8, and
9), and [{(CH2)3CO2H}mim]+ >[(CH2CO2H)mim]+ (entries 11 and
13). For bipyridinium- and isonicotinic-based ABBCs, the activi-
ty of cations varies in the order [{(CH2)3CO2H}bpy]+ >
[(CH2CO2H)bpy]+ and [{(CH2)3CO2H}inic]+ >[(CH2CO2H)inic]+
(entries 14–17). As reported, hydrogen bonding has a positive
effect on the ring-opening of epoxide to promote the synthe-
sis of cyclic carbonate.[17–18] However, hydrogen bonding may
also weaken the nucleophilicity of the anions by, to some
extent limiting, their flexibility. Because in ABBCs the acidity of
carboxylic acids with a short alkyl chain is stronger than the
acidity of those with a longer chain,[15] we propose that the in-
crease in the acidity of ABBCs might weaken the nucleophilici-
ty of anions and thus strengthen the ring-opening capability
of cations, causing a decrease in the yield of PC. Although di-
carboxylic acidity is slightly higher than carboxylic acidity,[15] di-
carboxylic ABBCs 1a and 1d show higher activities than car-
boxylic ABBCs 2a and 2c, respectively (Table 1, entries 6, 9,11,
and 13). This can be partially explained by the fact that the ad-
ditional ÀCO2H group in the cation of ABBCs strengthen the
ring-opening capability of the cation. Compared to imidazoli-
Scheme 2. Acid–base bifunctional catalysts used in this study.
Results and Discussion
Effect of catalysts
The reaction of propylene oxide (PO, 5b) with CO2 to produce
propylene carbonate (PC, 6b) was chosen as the model reac-
tion to test the catalytic activities of various acid–base bifunc-
tional catalysts (ABBCs), and the results are summarized in
Table 1.
Compared to the other catalysts, [{(CH2)3CO2H}2im]Br 1d ex-
hibits the best activity and selectivity (Table 1<xtabr1). As a
traditional ionic liquid, 1-ethyl-3-methylimidazolium bromide
([emim]Br) by itself is not efficient for the synthesis of cyclic
carbonate (entry 2). However, its activity is greatly improved in
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S. Zhang et al.
um-based ABBCs, isonicotinic- and bipyridinium-based ABBCs
show lower activities in the synthesis of PC (entries 14–17).
The effect of the counter anions on the catalytic perfor-
mance was also evaluated. The activity order of anion is BrÀ >
ClÀ, which is consistent with the order of the nucleophilicity of
these anions (Table 1, entries 6, 7, 9–12). Perhaps due to the
strong hydrogen-bonding between ÀCO2H groups and ClÀ,
1b, 1e, and 2b show very low activities for the synthesis of PC
(entries 7, 10, and 12). Thus, a moderate hydrogen-bonding
effect to efficiently realize nucleophilicity is very important for
the ABBCs anion. Hence, we selected [{(CH2)3CO2H}2im]Br 1d as
a model catalyst for further investigations.
Figure 1. Effect of temperature on PC yield with catalyst 1d. Conditions: PO
(0.2 mol), 1d (1 mol%), P=2.0 MPa, t=1 h.
A possible mechanism for the ABBC-catalyzed coupling of
epoxides with CO2, according to previous reports,[16] is shown
in Scheme 3. This mechanism is different from that of the cor-
perature. The PC yield increases sharply with increasing tem-
perature from 340 to 403 K. However, further increase in tem-
perature causes a decrease of the yield. It is plausible that the
decrease of PC yield is a result of side reactions under higher
temperature, such as PC polymerization, PO isomerization, and
a reaction between PO and water.[4c,18] Therefore, from a practi-
cal standpoint the optimal reaction temperature is 398 K.
The effect of CO2 pressure on the PC yield in the presence
of catalyst 1d was also studied. As shown in Figure 2, the pres-
sure has a great effect on the yield of PC upon varying the CO2
Scheme 3. A tentative mechanism for cycloaddition of epoxide to CO2.
responding two-component metal–salt-complex catalyzed re-
action. The enhancement of catalytic performance is presuma-
bly attributable to a synergistic effect of the two functional
groups to the epoxide ring, that is, abduction polarization of
the oxygen atom by hydrogen-bonding of the Brønsted-acidic
center and nucleophilic activation of the carbon atom by the
Lewis-basic center (halide anion). Therefore, the epoxy ring
was opened easily. This efficient process suggests that a two-
component metal salt–ionic liquid catalytic system could be
substituted for a single hydrogen-bonding donor (e.g., ÀCO2H
or phenolic hydroxyl group) functional ionic liquid catalyst. Be-
cause it is well-accepted that synergistic catalysis of Lewis acid
and Lewis base is necessary for obtaining high yields of cyclic
carbonates, this work provides a concept for the development
of novel efficient acid–base type catalysts for the synthesis of
cyclic carbonates.
Figure 2. Effect of CO2 pressure on PC yields. Conditions: PO (0.2 mol), 1d
(1 mol%), T=398 K, t=1 h.
pressure in the range 0.5–2.0 MPa, whereas the yield changes
only slightly in the range 2.0–3.5 MPa. This can be explained
by a pressure effect on the concentrations of CO2 and epoxide
in the two phases.[4c,18] Since PO is in its liquid form under the
adopted reaction conditions, and when the reaction was car-
ried out in a low-pressure region of 0.5–2.0 MPa, higher CO2
pressure enhanced PC yield due to the higher CO2 concentra-
tion in the liquid phase of the reaction system. A significant
drawback of using CO2 as a reagent in organic synthesis are
the potential dangers associated with high-pressure opera-
tions. The experimental results above demonstrate that the re-
action can be operated smoothly at low pressure in the pres-
ence of catalyst 1d.
Effect of other reaction parameters
Figure 1 shows the effect of reaction temperature on the PC
Figure 3 shows the dependence of the PC yield on the
amount of catalyst. An increase of the molar ratio of 1d to PO,
yield. The catalytic activity of 1d is sensitive to reaction tem-
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Acid–Base Bifunctional Catalysts for the Fixation of CO2 with Epoxides
catalyst under the optimized conditions. In each cycle, the cat-
alyst was separated from the volatile organic products and
starting materials by distillation, and reused for subsequent re-
actions. As shown in Figure 4, the yields and selectivities of the
product obtained from the second to fifth runs were similar to
those obtained with the fresh catalyst. After five re-uses, a
97% PO conversion with 99% PC selectivity could also be ob-
tained using this optimal catalyst, indicating its stable activity.
Figure 3. Effect of catalyst (1d) amount on the reaction. Conditions: PO
(0.2 mol), T=398 K, P=2.0 MPa, 1 h.
from 0.1 to 1.4 mol%, results in a sharp increase of the PC
yield, from 30 to 99%. At a low catalyst loading (1 mol%), a
98% PC yield with >99% selectivity can be obtained, indicat-
ing the high activity of catalyst. Because further increases of
the amount of catalyst increase the PC yield by only a small
margin, 1 mol% was chosen.
Figure 4. Recycling of the catalyst in the cycloaddition of CO2 (reaction con-
ditions similar to those of Table 1, entry 9).
Catalytic activity towards other terminal epoxides
To survey the substrate scope, the cycloaddition of other epox-
ides with CO2 was performed with the same amount of catalyst
1d. The results are summarized in Table 2. As shown, catalyst
1d exhibits a high activity and selectivity towards a variety of
terminal epoxides (5a–f) in mild conditions. Aliphatic 5a, 5b,
5d, and aromatic 1e epoxides
Conclusions
For the first time, a type of acid–base bifunctional catalysts
such as [{(CH2)3CO2H}2im]Br proved to be highly efficient for
are preferred substrates for the
Table 2. Synthesis of other cyclic carbonates catalyzed by 1d.[a]
reaction (entries 1, 2, 4 and 5).
These reactions could be com-
pleted in about 1 h. Notably, ep-
oxide 5 f, due to its higher hin-
drance originating from the two
rings, needs a long reaction time
to reach a satisfying conversion
(entry 6). The cyclic carbonate 6 f
obtained is exclusively the cis-
Entry
1
Substrate
Product
T [K]
t [h]
Selectivity [%]
98
Yield[b] [%]
99
373
0.7
5a
6a
2
3
373
373
1
99
93
99
91
5b
5c
5d
5e
6b
6c
6d
6e
1
isomer, as confirmed by HNMR
and 13CNMR.[7] From the results,
it can be seen that -CO2H group
has not obviously negative
effect on the selectivity of prod-
ucts, the selectivity values are
still kept at 99% except 5c
(Table 2, entry 3).
0.4
4
5
398
398
1
1
99
99
97
95
Catalyst recycling
Catalyst 1d showed an excellent
activity under mild conditions
towards the synthesis of PC.
Thereafter, a series of runs with
recycled catalyst were used to
investigate the reusability of the
6
398
15
99
85
5 f
6 f
[a] Reaction conditions: epoxide (0.2 mol), 1d (1 mol%), P=2.0 MPa. [b] GC yield.
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S. Zhang et al.
2.48 (t, J=9.6 Hz, 4H), 4.18 (t, J=14.4 Hz, 4H), 7.80 (s, 2H),
9.23 ppm (s, 1H); Anal. calcd for C11H17N2O4Cl: C 47.75, H 6.19, N
10.12, found: C 47.78, H 6.20, N 10.08.
the cycloaddition of CO2 to epoxides under metal- and sol-
vent-free conditions. The catalysts used in this study are stable,
easily synthesized, and environmentally benign catalysts that
can effectively activate epoxides through a synergistic effect of
a carboxylic acid center in the cation part of the salts and a
Lewis basic center in the anion part of the salts. The catalyst
can be reused at least five times without significant loss of its
catalytic activity. The results of this work are an example of the
applications of acid–base bifunctional catalysts as environmen-
tally benign alternatives to acids in organic synthesis and catal-
ysis.
[(CH2CO2H)mim]Br (2a): yield: 97%; a white solid; mp: 181–1828C,
1
Tdec 2808C; H NMR (400 MHz, [D6]DMSO): d=3.93 (s, 3H), 5.21 (s,
2H), 7.77 (s, 1H), 7.78 (s, 1H), 9.22 ppm (s, 1H); Anal. calcd for
C6H9N2O2Br: C 32.73, H 4.09, N 12.73, found: C 32.74, H 4.10, N
12.74.
[(CH2CO2H)mim]Cl (2b): yield: 98.3%; a white solid; mp: 203–
1
2048C, Tdec 2508C; H NMR (400 MHz, [D6]DMSO): d=3.94 (s, 3H),
5.23 (s, 2H), 7.78 (s, 1H), 7.80 (s, 1H), 9.31 ppm (s, 1H); Anal. calcd
for C6H9N2O2Cl: C 40.81, H 5.14, N 15.86, found: C 40.78, H 5.20, N
15.86.
[{(CH2)3CO2H}mim]Br (2c): yield: 97%; a yellow liquid; Tdec 2708C;
1H NMR (400 MHz, [D6]DMSO): d=2.06 (m, 2H), 2.31 (t, J=10.1 Hz,
2H), 3.94 (s, 3H), 4.27 (t, J=9.6 Hz, 2H,), 7.82 (s, 1H), 7.88 (s, 1H),
9.37 ppm (s, 1H); Anal. calcd for C8H13N2O2Br: C 38.71, H 5.24, N
11.29, found: C 38.74, H 5.26, N 11.34.
Experimental Section
TGA was performed by using a TGA-50 (Shimadzu) in a nitrogen at-
mosphere between 298 and 873 K at a heating rate of 10 KminÀ1
.
DSC was carried out by using a TGA-50 (Shimadzu) in a nitrogen
atmosphere between 298 and 873 K at heating rate of
[(CH2CO2H)bpy]Br (3a): yield: 85%; a white solid; mp: 199–2008C,
Tdec 2208C; H NMR (400 MHz, [D6]DMSO): d=5.70 (s, 2H), 8.25 (s,
2H), 8.75 (s, 2H), 9.00 (s, 2H), 9.26 ppm (s, 2H); Anal. calcd for
C12H11N2O2Br: C 49.15, H 3.76, N 9.56, found: C 48.90, H 3.86, N
9.36.
1
a
10 KminÀ1. Elemental analysis was carried out by using a Vario EL
(Elementar Analysensysteme GmbH). 1H NMR was detected at
room temperature on a Bruker 400 MHz NMR spectrometer using
[D6]DMSO as solvent. 13C NMR was detected at room temperature
on Bruker 400 MHz NMR spectrometer using CDCl3 as solvent.
[{(CH2)3CO2H}bpy]Br (3b): yield: 80%; a yellow solid; mp: 199–
1
2008C, Tdec 2308C; H NMR (400 MHz, [D6]DMSO): d=2.09–2.25 (m,
2H), 2.40 (t, J=14.6 Hz, 2H), 4.74 (t, J=16.9 Hz, 2H), 8.32 (s, 2H),
8.72 (s, 2H), 9.03 (s, 2H), 9.34 ppm (s, 2H); Anal. calcd for
C14H15N2O2Br: C 52.01, H 4.64, N 8.67, found: C 50.23, H 4.58, N
8.41.
The ABBCs were prepared according to procedures reported earli-
er.[15] A typical synthesis route to [(CH2CO2H)2im]Br (1a) is as fol-
lows:
a
mixture of trimethylsilylimidazole (0.010 mol) and
BrCH2CO2CH3 (0.020 mol) was refluxed in toluene (20 mL) at 298 K
for 24 h under an inert atmosphere of dry nitrogen. The reaction
mixture was washed with diethyl ether (3ꢁ30 mL) and dried under
vacuum for 24 h to give product 1. Thereafter, a mixture of 1
(0.010 mol) and 16.7% HCl (10 mL) was refluxed for 2 h. The sol-
vent was removed under reduced pressure and the remaining
solid was washed with acetone and diethyl ether to give the prod-
uct 1a as a white powder. It should be noted that the addition of
methyl bromoacetate to N-methylimidazole or trimethylsilylimida-
zole to afford 1a and 2a could be completed at room temperature
(298 K). In comparison with N-methylimidazole, trimethylsilyl-imida-
zole, and iso-nicotine, reactions of bipyrid did not afford the de-
sired ÀCO2H group bis-substituted bipyridinium-based ABBCs using
the same method. Therefore, compounds 3a and 3b were also
tested for the synthesis of cyclic carbonates.
[(CH2CO2H)inic]Br (4a): yield: 97%; a white solid; mp: 190–1918C,
1
Tdec 1958C; H NMR (400 MHz, [D6]DMSO): d=5.67 (s, 2H), 8.46 (s,
2H), 9.20 ppm (s, 2H); Anal. calcd for C8H8NO4Br: C 36.64, H 3.05, N
5.35, found: C 37.21, H 3.11, N 5.47.
[{(CH2)3CO2H}inic]Br (4b): yield: 93%; a white solid; mp: 159–
1
1608C, Tdec 2008C; H NMR (400 MHz, [D6]DMSO): d=2.04–2.09 (m,
2H), 2.32 (t, J=12.6 Hz, 2H), 4.15 (t, J=14.2 Hz, 2H), 7.90 (s, 2H),
9.33 ppm (s, 2H); Anal. calcd for C10H12NO4Br: C 41.38, H 4.14, N
4.83, found: C 41.41, H 4.13, N 4.92.
All the coupling reactions were conducted in a 100 mL stainless-
steel reactor equipped with a magnetic stirrer and automatic tem-
perature control system. Typically, in the reactor, an appropriate
volume of CO2 (ca. 1.0 MPa) was added to a mixture of PO
(14.0 mL, ca. 0.2 mol), and catalyst (ca. 2.0 mmol) at room tempera-
ture. The temperature was then raised to 398 K while more CO2
was added from a reservoir tank to maintain a constant pressure
(2.0 MPa). After the reaction had proceeded for 1.0 h, the reactor
was cooled to 273 K in an ice–water bath, and the remaining CO2
was removed slowly. After the volatile organic products and start-
ing materials were removed from the catalyst by distillation, the
products were analyzed by using an Agilent 6890/5973B GC-MS
equipped with a FID detector and a DB-wax column, using aceto-
phenone as the internal standard. The product was purified by dis-
tillation or silica gel column chromatography if necessary.
[(CH2CO2H)2im]Br (1a): yield: 97%; a white solid; mp: 194–1958C,
1
Tdec 2408C; H NMR (400 MHz, [D6]DMSO): d=5.21 (s, 4H), 7.76 (s,
2H), 9.13 ppm (s, 1H); Anal. calcd for C7H9N2O4Br: C 31.72, H 3.42;
N 10.57, found: C 32.02, H 3.56, N 10.74.
[(CH2CO2H)2im]Cl (1b): yield: 95%; a white solid; mp: 259–2608C,
1
Tdec 2608C; H NMR (400 MHz, [D6]DMSO): d=5.26 (s, 4H), 7.79 (s,
2H), 9.24 ppm (s, 1H); Anal. calcd for C7H9N2O4Cl: C 38.10, H 4.10,
N 12.70, found: C 38.25, H 4.12, N 12.85.
[{CH(CH3)CO2H}2im]Br (1c): yield: 95%; a white solid; mp: 178–
1798C, Tdec 2508C; 1H NMR (400 MHz, [D6]DMSO): d=1.78 (d J=
7.6 Hz, 6H), 5.45 (t, J=22.0 Hz, 2H), 7.93 (s, 2H), 9.41 ppm (s, 1H);
Anal. calcd for C9H13N2O4Br: C 36.86, H 4.44, N 9.56, found: C 37.01,
H 4.54, N 9.81.
[{(CH2)3CO2H}2im]Br (1d): yield: 94%; a white solid; mp: 110–1118C,
Tdec 3008C; 1H NMR (400 MHz, [D6]DMSO): d=2.02–2.05 (m, 4H),
2.47 (t, J=8 Hz, 4H), 4.17 (t, J=11.2 Hz, 4H), 7.78 (s, 2H), 9.20 ppm
(s, 1H); Anal. calcd for C11H17N2O4Br: C 41.14, H 5.34, N 8.72, found:
C 40.89, H 5.33, N 8.88.
Spectral characteristics of the products (cyclic carbonates 6a–f) in
Table 2 are as follows:
1
1, 3-dioxolan-2-one (6a): H NMR (400 MHz, CDCl3): d=4.2 ppm (t,
J=10 Hz, 4H); 13C NMR (400 MHz, CDCl3): d=63.3, 155 ppm (C=
O).
4-methyl-1, 3-dioxolan-2-one (6b): 1H NMR (400 MHz, CDCl3): d=
1.49 (d, J=6.0 Hz, 3H), 4.05 (t, J=8.8 Hz, 1H), 4.60 (t, J=8.0 Hz,
1H), 4.86–4.94 ppm (m, 1H); 13C NMR (400 MHz, CDCl3): d=18.95,
70.46, 73.51, 154.95 ppm (C=O).
[{(CH2)3CO2H}2im]Cl (1e): yield: 96%; a white solid; mp: 172–1738C,
Tdec 2508C; 1H NMR (400 MHz, [D6]DMSO): d=2.08–2.13 (m, 4H),
506
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ChemSusChem 2011, 4, 502 – 507

Acid–Base Bifunctional Catalysts for the Fixation of CO2 with Epoxides
1
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4-chloromethyl-1,3-dioxolan-2-one (6c): H NMR (400 MHz, CDCl3):
d=1.490 (d, J=6.0 Hz, 3H), 4.05 (t, J=8.4 Hz, 1H), 4.60 (t, J=
8.0 Hz, 1H), 4.86–4.94 ppm (m, 1H); 13C NMR (400 MHz, CDCl3): d=
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4-butyl-1,3-dioxolan-2-one (6d): H NMR (400 MHz, CDCl3): d=0.96
(t, J=7.2 Hz, 3H); 1.33–1.38 (m, 2H), 1.29–1.31 (m, 2H), 1.53 (m,
2H), 4.16 (d, J=8.0 Hz, 2H), 4.19 ppm (t, J=6.2 Hz, 1H); 13C NMR
(400 MHz, CDCl3): d=14.00, 23.12, 26.19, 36.23, 68.04, 75.61,
155.07 ppm (C=O).
4-phenyl-1, 3-dioxolan-2-one (6e): 1H NMR (400 MHz, CDCl3): d=
4.34 (t, J=8.4 Hz, 1H), 4.80 (t, J=8.4 Hz, 1H), 5.68 (t, 1H, J=
8.0 Hz), 7.35–7.44 ppm (m, 5H); 13C NMR (400 MHz, CDCl3): d=
71.10, 77.92, 125.81, 129.12, 129.63, 135.70, 154.81 ppm (C=O).
4, 5-tetramethylene-1, 3-dioxolan-2-one (6 f): 1H NMR (400 MHz,
CDCl3): d=1.80–1.86 (m, 2H), 1.97–2.05 (m, 2H), 2.28 (d, J=4.8 Hz,
4H), 5.06–5.11 ppm (m, 2H); 13C NMR (400 MHz, CDCl3): d=19.00,
26.61, 75.65, 155.27 ppm (C=O).
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This work was supported by the National Basic Research Pro-
gram of China (2009CB219901), National Key Technology R&D
Program (2008BAF33B04), Knowledge Innovation Program of the
Chinese Academy of Sciences (KGCX2-YW-321), National Science
Foundation of China (21006117), National Science Fund of China
for Distinguished Young Scholars (20625618), and the Science
and Technology Project of Beijing (Y090081135).
Keywords: acidity
· catalysis · carbon dioxide fixation ·
cycloaddition · epoxidation
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Published online on January 27, 2011
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