Zn-based ionic liquids as highly efficient catalysts for chemical fixation of carbon dioxide to epoxides

Article abstract of DOI:10.1039/c4ra14111c

The novel Zn-based task-specific ionic liquids (Zn-TSILs) catalysts were developed for the coupling of carbon dioxide and epoxides to form cyclic carbonates under mild reaction conditions without using additional organic solvents and cocatalysts. Due to the synergistic effects of the cation and anion in this catalytic system, excellent yields and selectivities to cyclic carbonates were achieved with high TOF values up to 794 h^-1. Among the catalysts investigated, OH-containing Zn-TSILs showed better activity than COOH-containing Zn-TSILs, and [(CH₂CH₂OH)Bim]ZnBr₃ was found to be the best. Additionally, the influences of CO₂ pressure and catalyst concentration were also investigated over [(CH₂CH₂OH)Bim]ZnBr₃. In addition, the rate constants as well as the activation energies for the cycloaddition reaction catalyzed by Zn-TSIL and TSIL were comparatively determined. The activation energy was calculated to be 34.1 kJ mol^-1 for bare TSIL catalyst, whereas the Zn-TSIL reduced the activation energy value by 14.7 kJ mol^-1. Moreover, the Zn-TSIL was easily recyclable without significant loss of activity, representing the exceptionally promising candidate for the effective fixation of CO₂ to epoxides.

Full text of DOI:10.1039/c4ra14111c

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Zn-based ionic liquids as highly efficient catalysts
for chemical fixation of carbon dioxide to
epoxides†
Cite this: RSC Adv., 2015, 5, 14277
b
b
b
c
ab
*
Mengshuai Liu, Fangxiao Wang, Lei Shi, Lin Liang and Jianmin Sun
The novel Zn-based task-specific ionic liquids (Zn-TSILs) catalysts were developed for the coupling of
carbon dioxide and epoxides to form cyclic carbonates under mild reaction conditions without using
additional organic solvents and cocatalysts. Due to the synergistic effects of the cation and anion in this
catalytic system, excellent yields and selectivities to cyclic carbonates were achieved with high TOF
values up to 794 hꢀ1. Among the catalysts investigated, OH-containing Zn-TSILs showed better activity
than COOH-containing Zn-TSILs, and [(CH2CH2OH)Bim]ZnBr3 was found to be the best. Additionally, the
influences of CO2 pressure and catalyst concentration were also investigated over [(CH2CH2OH)Bim]-
ZnBr3. In addition, the rate constants as well as the activation energies for the cycloaddition reaction
catalyzed by Zn-TSIL and TSIL were comparatively determined. The activation energy was calculated to
be 34.1 kJ molꢀ1 for bare TSIL catalyst, whereas the Zn-TSIL reduced the activation energy value by 14.7
kJ molꢀ1. Moreover, the Zn-TSIL was easily recyclable without significant loss of activity, representing
the exceptionally promising candidate for the effective fixation of CO2 to epoxides.
Received 8th November 2014
Accepted 6th January 2015
DOI: 10.1039/c4ra14111c
www.rsc.org/advances
inertness.9 Therefore, catalysts are needed to reduce the acti-
vation energy of any reaction in which CO2 is involved.
1. Introduction
Accordingly, a wide range of catalysts have been explored for the
synthesis of cyclic carbonates, including metal oxides,10,11 alkali
metal salts,12,13 quaternary ammonium salts,14 phosphonium
salts,15 Schiff bases16–18 and transition metal complexes.19
However, in many cases, additives and/or cocatalysts as well as
organic solvents are needed. In addition, unsatisfactory activity,
harsh reaction conditions and inexhaustive separation of
products from the catalysts are still drawbacks that need to be
overcome. Hence, the design of a novel, solvent/cocatalyst-free
catalyst with enhanced activity still remains to be developed
toward effective conversion of CO2.20
More recently, ionic liquids (ILs) have attracted considerable
interest and CO2 cycloaddition to epoxides has been effectively
carried out in ILs, which possess unique characteristics, such as
thermal and chemical stability, negligible vapor pressure,
selective solubility towards organic and inorganic materials,
and high reusability of the catalysts.21 In 2001, Peng and Deng22
rst reported the quantitative conversion of propylene oxide
Carbon dioxide is has been in the spotlight as a renewable C1
feedstock because it is ubiquitous and readily available, either
from decomposition of organic matter or articial production
by human activities. Conversion of CO2 into industrially useful
chemicals has dual signicance in view of both environmental
protection and sustainable chemistry.1 One of the most prom-
ising strategies in this area is the synthesis of cyclic carbonates
via the cycloaddition of epoxides and CO2 (Scheme 1), especially
the synthesis of propylene carbonate (PC) and ethylene
carbonate (EC). The formation of cyclic carbonates is green for
100% atom efficiency and cyclic carbonates have found appli-
cations not only as polar aprotic solvents, electrolytes for
lithium-ion batteries, and as intermediates in the manufacture
of ne chemicals,2–4 but are also used as constituents of oils and
paints and as raw materials in the synthesis of polycarbonates5,6
and polyurethanes7,8 as shown in Scheme 2.
Because carbon dioxide contains carbon in its most oxidized
state, it has inherent thermodynamic stability and kinetic
aState Key Laboratory of Urban Water Resource and Environment, Harbin Institute of
Technology, Harbin 150080, China. E-mail: sunjm@hit.edu.cn; Tel: +86 451
86403715
bThe Academy of Fundamental and Interdisciplinary Science, Harbin Institute of
Technology, Harbin 150080, China
cSchool of Life Science and Technology, Harbin Institute of Technology, Harbin
150080, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra14111c
Scheme 1 Cycloaddition reaction of CO2 with epoxides.
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and 3.5 MPa.34 Sun35 and Zhou et al.36 have reported that
hydroxyl-functionalized (HFILs)/carboxyl-functionalized ionic
liquids (CFILs) could coordinate with the oxygen atom of
epoxides and thus lead to the activation of the epoxide.
Furthermore, a theoretical study also indicated that the
formation of hydrogen bonds between catalyst and epoxide
could accelerate the ring-opening reaction of the epoxide,
enhancing the activity signicantly.37 However, the catalytic
activity was unsatisfactory and needed further improvement.
Previous literature also reported that zinc halides could coor-
dinate with the oxygen atom to activate the epoxide by forming
the zinc-epoxide adduct.15,38,39 Herein, we designed novel Zn-
based task-specic ionic liquids composed of ZnBr2 and
hydroxyl/or carboxyl group-functionalized ILs, which are
expected to be excellent catalysts for the CO2 cycloaddition
reaction. The Zn-based task-specic ionic liquids (Zn-TSILs)
were facilely obtained from the reaction of ZnBr2 with HFILs/
or CFILs. The catalytic activities were examined for CO2
coupling with PO as well as other epoxides, especially the less
active epoxides, such as styrene oxide and cyclohexene oxide.
Systematic investigations on the effects of CO2 pressure and
catalyst concentration on the target reaction were also con-
ducted. Additionally, the contrast calculations for the activation
energies were provided over HFIL and Zn-HFIL catalysts. A
plausible reaction mechanism was proposed based on the
experimental results.
Scheme 2 Synthesis of polyurethanes via cyclic carbonates.7
Scheme 3 Synthesis process for Zn-based task-specific ionic liquids.
(PO) to PC catalyzed by 1-butyl-3-methylimidazolium
(BmimBF4) ionic liquid. Thereaer, series of imidazolium
salts,23–25 quaternary ammonium salts,26 quaternary phospho-
nium salts,27 pyridinium salts,28 guanidinium salts,29 poly-
meric30,31 and other immobilized ILs32,33 were developed as
catalysts for the cycloaddition of CO2 into epoxides. Among
these, Kim's group found that imidazolium zinc tetrahalides
showed surprisingly high activities for the coupling reaction of
CO2 and ethylene oxide or propylene oxide to produce corre-
sponding cyclic carbonates and the highest TOF were up to 3545
2. Experimental
2.1. General information
Carbon dioxide with purity of 99.99% was purchased from
Harbin Qinghua Industrial gases Co. Ltd. PO, sodium
hydride (60%), imidazole (99%), N-methylimidazole (99%), n-
butyl bromide ($99%), 2-bromoethanol (96%), bromoacetic
acid (98%), tetrahydrofuran (extra dry, >99%) were purchased
hꢀ1 for 1-butyl-3-methylimidazolium bromide-ZnBr2 at 100 C
ꢁ
a
Table 1 Effect of various catalysts on the cycloaddition reaction of PO and CO2
Entry
Catalyst
Conversionb (%)
Yieldb (%)
TOFc (hꢀ1
)
1
2
3
4
5
6
7
8
None
ZnBr2
[Emim]Br
[Bmim]Br
BrCH2CH2OH
BrCH2COOH
[(CH2CH2OH)Mim]Br
[(CH2COOH)Mim]Br
[(CH2CH2OH)Bim]Br
[(CH2COOH)Bim]Br
[(CH2CH2OH)Mim]ZnBr3
[(CH2COOH)Mim]ZnBr3
[(CH2CH2OH)Bim]ZnBr3
[(CH2COOH)Bim]ZnBr3
[(CH2CH2OH)Bim]ZnBr3
[(CH2CH2OH)Bim]ZnBr3
[(CH2CH2OH)Bim]ZnBr3
Trace
Trace
33
Trace
Trace
32
—
—
276
302
—
35
35
Trace
Trace
54
49
55
53
90
83
93
85
86
91
92
Trace
Trace
53
48
54
51
89
81
92
83
86
90
91
—
457
414
466
440
768
699
794
716
742
776
523
9
10
11
12
13
14
15d
16e
17f
a
Reaction conditions: 34.5 mmol PO, 0.04 mmol Cat., T ¼ 120 ꢁC, P (CO2) ¼ 2.0 MPa, t ¼ 1.0 h. b Based on GC analysis. c Moles of PC produced per
mole of catalyst per hour. d T ¼ 100 ꢁC. e The fourth run. The h run, t ¼ 1.5 h.
f
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2.2. Synthesis of Zn-based task-specic ionic liquids
Four ILs composed of ZnBr3ꢀ anions and different cations were
prepared according to Scheme 3. The detailed synthesis proce-
dures are described in ESI† and characterization results are as
follows.
[(CH2CH2OH)Mim]ZnBr3: yield: 97%; 1H NMR (400 MHz,
D2O): d ¼ 8.78 (s, 1H), 7.54 (m, 1H), 7.48 (m, 1H), 4.51–4.21 (m,
2H), 3.99–3.94 (m, 2H), 3.93 (s, 3H). FT-IR: 3497, 3152, 2955,
1619, 1567, 1384, 1165, 1061, 826, 748, 643 cmꢀ1
.
[(CH2COOH)Mim]ZnBr3: yield: 93%; 1H NMR (400 MHz,
D2O): d ¼ 8.66 (s, 1H), 7.43–7.22 (m, 2H), 4.66 (s, 2H), 3.87 (s,
3H). FT-IR: 3400, 3134, 2617, 1747, 1593, 1171, 1082, 974, 834,
749, 623 cmꢀ1
.
[(CH2CH2OH)Bim]ZnBr3: yield: 95%; 1H NMR (400 MHz,
D2O): d ¼ 8.78 (s, 1H), 7.70–7.11 (m, 2H), 4.35–4.25 (m, 2H), 4.18
(t, J ¼ 7.2 Hz, 2H), 3.95–3.84 (m, 2H), 1.92–1.71 (m, 2H), 1.37–
1.19 (m, 2H), 0.87 (t, J ¼ 7.4 Hz, 3H). FT-IR: 3449, 3142, 2959,
Fig. 1 Effect of CO2 pressure on the synthesis of PC. Conditions:
34.5 mmol PO, 0.04 mmol [(CH2CH2OH)Bim]ZnBr3, T ¼ 120 ꢁC, t ¼
1.0 h.
2871, 1937, 1614, 1562, 1458, 1161, 1057, 947, 832, 749 cmꢀ1
.
[(CH2COOH)Bim]ZnBr3: yield: 90%; 1H NMR (400 MHz,
D2O): d ¼ 8.75 (s, 1H), 7.54–7.33 (m, 2H), 4.71 (s, 2H), 4.16 (t, J ¼
7.3 Hz, 2H), 1.93–1.71 (m, 2H), 1.35–1.13 (m, 2H), 0.84 (t, J ¼ 7.4
Hz, 3H). FT-IR: 3432, 2959, 2285, 1746, 1616, 1572, 1168, 1081,
838, 751, 621 cmꢀ1
.
2.3. General procedure for PC synthesis from PO and CO2
The cycloaddition reaction of PO and CO2 was conducted in a 50
mL high pressure stainless-steel reactor equipped with a
magnetic stirring bar. In a typical run, the reactor was purged
with CO2 to evacuate air, and then Zn-TSIL catalyst (0.035
mmol, 0.12 mol% of PO) and PO (34.5 mmol) were added
successively, then the reactor was heated using an oil bath. At a
xed reaction temperature such as 120 ꢁC, CO2 was introduced
into the reactor and the pressure was adjusted to 2.0 MPa. The
autoclave was heated at that temperature for a designated
period of time. Aer the reaction was completed, the reactor
was cooled to 0 ꢁC in an ice-water bath, and the remaining CO2
was released. The product was extracted with ethyl acetate and
analyzed by GC and Zn-TSILs were separated by centrifugation,
washed with ethyl acetate (3 ꢂ 5 mL), dried under vacuum and
then reused directly for the next run.
Fig. 2 Dependence of catalyst concentration on the synthesis of PC.
Conditions: 34.5 mmol PO, T ¼ 120 ꢁC, P (CO2) ¼ 2.5 MPa, t ¼ 1.0 h.
Scheme 4 Possible side reactions of PO.
3. Results and discussion
3.1. Catalyst screening for the cycloaddition of CO2 to PO
from Aladdin Chemical Co. ZnBr2 (CP) was purchased from
Adamas Reagent Co. Ltd. All other chemicals were purchased
from Beijing InnoChem Science & Technology Co. Ltd. and
were of analytical grade and used without further
purication.
FT-IR spectra were measured on a Perkin-Elmer Spectrum
100 FT-IR Spectrometer. 1HNMR spectra were detected on a
Bruker 400 spectrometer in D2O (4.79 ppm) at ambient
temperature. GC analyses were performed on an Agilent GC-
7890A equipped with a capillary column (Agilent 19091J-413),
using a ame ionization detector.
The activities of different catalysts were tested on the reaction of
PO and CO2, and the corresponding results are summarized in
Table 1. Almost no reaction occurred in the absence of any catalyst
or in the presence of only ZnBr2 catalyst (entries 1 and 2).
Although the hydroxyl-functionalized (HFILs) and carboxyl-
functionalized ionic liquids (CFILs) showed higher catalytic
activity (entries 7–10) compared with non-functionalized ionic
liquids, such as [Emim]Br (1-ethyl-3-methylimidazolium bromide)
and [Bmim]Br (1-butyl-3-methylimidazolium bromide) (entries 3
and 4), the activities were still unsatisfactory. Almost no reactivity
was observed in the presence of BrCH2CH2OH or BrCH2COOH
(entries 5 and 6). However, the Zn-based task-specic ionic liquids
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was detected that a slight increase in PC yield resulted from
increasing the alkyl chain length of the imidazolium cation with
the same functional groups, due to the higher solubility of CO2
and PO in the IL phase.23 Compared with the imidazolium zinc
tetrahalides catalyst reported by Kim et al.,34 the same reaction
ꢁ
temperature of 100 C was used and the PC yield reached 86%
with TOF of 742 hꢀ1 under a lower reaction pressure of 2.0 MPa
(entry 15) for 1.0 h, proving that [(CH2CH2OH)Bim]ZnBr3
possessed excellent catalytic performance.
For a potential application in a green and economic process,
the recyclability of the catalyst plays an important role. Fig. S1 in
ESI† displays the recyclability of [(CH2CH2OH)Bim]ZnBr3 at
120 ꢁC and 2.0 MPa CO2 pressure. No signicant decrease in PC
yields was observed aer four cycles (entry 16), and in the 5th
reuse, the catalyst still exhibited high PO conversion and PC
yield with reaction time of 1.5 h (entry 17). Further reuse of the
spent catalyst could still achieve a high PO conversion and PC
yield, but at the expense of a little longer reaction time because
of the washing loss of the IL catalyst, demonstrating a very
feasible and reusable catalyst for the CO2 cycloaddition
reaction.
Fig. 3 Arrhenius plots for the CO2 cycloaddition to PO catalyzed by
[(CH2CH2OH)Bim]Br (-) and [(CH2CH2OH)Bim]ZnBr3 (C). Condi-
tions: 34.5 mmol PO, 0.25 mol% Cat., P (CO2) ¼ 2.5 MPa. The average
and standard deviation of ln kobs were obtained from the experiments
at each temperature.
Table 2 Kinetic parameters of CO2 cycloaddition to PO over different
catalystsa
3.2. Effects of reaction parameters
Kinetic parameters
The inuence of CO2 pressure on the PC yield was investigated
at 120 ꢁC for 1.0 h with [(CH2CH2OH)Bim]ZnBr3 catalyst. As
shown in Fig. 1, the reaction pressure had a signicant effect on
PC yield. With the increase of CO2 pressure from 0.5–1.5 MPa,
the PC yield increased rapidly. In the range of 1.5–2.5 MPa,
there was a moderate increase in yield and in the range of 2.5–
3.0 MPa, the PC yield was almost constant, but decreased
initially when the CO2 pressure was above 3.0 MPa. To our
delight, there were no obvious changes in PC selectivity. A
similar inuence of CO2 pressure on catalytic activity was
observed in other catalytic systems in the literature.15,40,41 The
reason for this could be ascribed to the phase behavior of the
CO2–PO system. There appeared two phases; a CO2-rich gas
phase and a PO-rich liquid phase in the system under experi-
mental conditions, and the reaction mainly took place in the
liquid phase, which dispersed the catalyst. The concentration of
CO2 in the liquid phase rose remarkably with increasing pres-
sure when the reaction was carried out in the low-pressure
region, thus signicantly promoting the reaction. However,
too high a CO2 pressure would decrease PC yield because of the
lower concentration of PO in the liquid phase because more PO
was extracted into the vapor phase. Therefore, 2.5 MPa was
chosen as the most suitable pressure for PC synthesis.
Additionally, the effects of the catalyst concentration on the
reaction were investigated with the molar percentage of [(CH2-
CH2OH)Bim]ZnBr3 to PO in the range 0.05–0.30 mol% (Fig. 2). It
is worth mentioning that only 0.05 mol% of catalyst showed
good catalytic performance at 85% PC yield. The PC yield
increased smoothly from 85% to around 98% with catalyst
concentration increasing from 0.05 to 0.25 mol%, and no
obvious changes in PC selectivities. Further increase in the
catalyst concentration caused a slight decrease in PC yield,
presumably due to side reactions such as isomerization of PO as
Catalyst
Ea (kJ molꢀ1
)
A (minꢀ1
)
R2
[(CH2CH2OH)Bim]Br
[(CH2CH2OH)Bim]ZnBr3
34.1
19.4
867.8
20.4
0.9733
0.9495
a
The values of kinetic parameters obtained from the linear Arrhenius
plot of ln kobs against 1/T.
realized the obvious enhancements in catalytic activity in compar-
ison with the former non Zn-based ILs, indicating the necessary
cooperative effects between Zn and IL, which stabilized interme-
diates through hydrogen bonds and Zn–O bonds for accelerating
the reaction (entries 11–14). Notably, the [(CH2CH2OH)Bim]ZnBr3
catalyst showed the best activity and PO conversion approached
93% with 92% PC yield under cocatalyst and solvent free condi-
ꢁ
tions of 120 C and 2.0 MPa CO2 for 1.0 h (entry 13), and the
high turnover frequency (TOF) value was up to 794 hꢀ1. The
catalytic activity decreased in the order of [(CH2CH2OH)Bim]-
ZnBr3 > [(CH2CH2OH)Mim]ZnBr3 > [(CH2COOH)Bim]ZnBr3
>
[(CH2COOH)Mim]ZnBr3. As hydrogen-bond donors, the –OH
and –COOH played the same roles of interacting with epoxides
to polarize the C–O ring bond and facilitate the nucleophilic
attack by the halide anion for subsequent ring opening. The
OH-containing Zn-TSILs showed better activities than the
COOH-containing Zn-TSILs (entries 11 and 13 vs. entries 12 and
14), which were accordant with the effects of HFILs and CFILs
(entries 7 and 9 vs. entries 8 and 10). The different activities
presumably were attributed to the carbonyl oxygen in COOH-
containing Zn-TSILs forming intramolecular hydrogen bonds,
thus competing with the hydrogen bonds formed between Zn-
TSILs and PO, thereby lowering the product yield.24 Besides, it
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