Efficient heterogeneous functionalized polymer ionic liquid catalyst for the synthesis of ethylene carbonate via the coupling of carbon dioxide with ethylene oxide

Article abstract of DOI:10.1039/c4ra02689f

A series of functionalized polymer ionic liquids (PILs) were synthesized and immobilized onto a 4 A molecular sieve. The catalytic activity of the resulting heterogeneous catalyst toward the synthesis of ethylene carbonate (EC) via the cycloaddition reaction of ethylene oxide (EO) and CO₂ was studied. The effects of the reaction conditions such as reaction temperature, pressure, time, and the amount of catalyst used, were systematically investigated. A high yield of EC and excellent selectivity could be obtained under optimized conditions. The catalyst is thermally stable and shows good reusability. Based on the experimental results, a plausible reaction mechanism has been proposed for the catalytic reaction. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra02689f

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PAPER
Efficient heterogeneous functionalized polymer
ionic liquid catalyst for the synthesis of ethylene
carbonate via the coupling of carbon dioxide with
ethylene oxide†
Cite this: RSC Adv., 2014, 4, 20506
Bo Zhang,a Lei Zhang,a Qinghai Wu,b Quanxi Wang,b Baoan Song,a Wenjun Wu,c
b
b
*
Bin Lu and Tianrui Ren
A series of functionalized polymer ionic liquids (PILs) were synthesized and immobilized onto a 4 A˚
molecular sieve. The catalytic activity of the resulting heterogeneous catalyst toward the synthesis of
ethylene carbonate (EC) via the cycloaddition reaction of ethylene oxide (EO) and CO2 was studied. The
effects of the reaction conditions such as reaction temperature, pressure, time, and the amount of
catalyst used, were systematically investigated. A high yield of EC and excellent selectivity could be
obtained under optimized conditions. The catalyst is thermally stable and shows good reusability. Based
on the experimental results, a plausible reaction mechanism has been proposed for the catalytic reaction.
Received 27th March 2014
Accepted 23rd April 2014
DOI: 10.1039/c4ra02689f
www.rsc.org/advances
reusability. Hence, it is crucial to identify more efficient
heterogeneous catalysts.
Introduction
Supported ionic liquids (ILs) are considered promising as
efficient heterogeneous catalysts for the synthesis of cyclic
carbonates, because they retain important physical and chem-
ical features of ILs, such as nonvolatility, nonammability, and
good thermal stability, as well as high catalytic activity and
selectivity. Compared to traditional ILs, the functionalized ionic
liquids (e.g., those with hydroxyl or carboxyl groups) were found
to show much better catalytic efficiency than single ILs toward
the synthesis of cyclic carbonates.17–19 For example, Sun et al.
prepared a series of carboxyl IL catalysts containing Brønsted
acidic sites and Lewis basic sites, and this type of catalysts
showed high catalytic activity with no use of a cocatalyst and
organic solvent.20 The immobilization of hydroxyl- or carboxyl-
functionalized ionic liquid on a solid support as effective
heterogeneous catalysts for the synthesis of cyclic carbonate
from the coupling of CO2 with epoxides have exhibited excellent
catalytic activity and selectivity.21–24 The excellent catalytic
performance can be ascribed to the synergetic effects among
OH group (or COOH group) and the halide anions. For example,
Han and co-workers23 found that compared with silica-graed
imidazolium-based ILs with hydroxyl groups and with no
functional group, carboxylic acid-functionalized imidazolium-
based ILs immobilized on silica show the highest activity and
selectivity for the synthesis of cyclic carbonates via the cyclo-
addition of epoxides with CO2, since the –COOH group in the IL
cation can greatly accelerate the reactions and showed a
synergistic effect with the halide anions. Therefore, the incor-
poration of carboxylic acid and hydroxyl moieties into ILs could
provide an effective alternative for improving the catalytic
Cyclic carbonates, e.g., ethylene carbonate (EC), are valuable
chemical products that are used as polar aprotic solvents and
electrolytic elements of lithium secondary batteries. Cyclic
carbonates can serve as precursors for the formation of poly-
carbonates as well as intermediates in the production of phar-
maceuticals, ne chemicals and agricultural chemicals.1–3 One
of the most effective routes for the synthesis of cyclic carbonates
is the cycloaddition of CO2 with epoxides.
A wide range of homogeneous catalysts such as alkali metal
salts,4 organophosphorous compounds,5 organic bases,6 and
organometallic compounds7 have been developed, and have
exhibited excellent catalytic performance toward the coupling
reaction. Unfortunately, those reactions were conducted in
homogeneous catalytic systems, which suffer from poor catalyst
recovery and product separation. Although heterogeneous
catalysts (e.g., metal oxides,8–10 molecular sieves,11–13 and sup-
ported catalysts14–16) have been explored, their application is
limited because of low activity, and/or poor stability and
aState Key Laboratory Breeding Base of Green Pesticide and Agricultural
Bioengineering/Key Laboratory of Green Pesticide and Agricultural Bioengineering,
Ministry of Education, Guizhou University, Guiyang, 550025, P. R. China
bThe Key Laboratory of Resource Chemistry of Ministry of Education, The Development
Centre of Plant Germplasm Resources, College of Life and Environmental Science,
Shanghai Normal University, 100 Guilin Road, Shanghai, 200234, P. R. China.
E-mail: trren@shnu.edu.cn; Fax: +86 21 64328850; Tel: +86 21 64328850
cSchool of Chemistry and Molecular Engineering, East China University of Science and
Technology, 130 Meilong Road, Shanghai, 200237, P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra02689f
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performance of ILs toward the cycloaddition of CO2 and bromide was also synthesized using 1-bromoethane instead of
epoxides.
We synthesized a novel hydroxyl and carboxyl functionalized
2-bromoethanol.
The NMR data are as follow: 1-hydroxyethyl-3-vinyl imida-
polymer ionic liquid (PIL) via copolymerization with 1-hydroxy- zolium bromide: 1H NMR (600 MHz, D2O) d: 3.84 (m, 2H), 4.26
ethyl-3-vinyl imidazolium bromide, methyl acrylic acid, (m, 2H), 5.32 (dd, 1H), 5.71 (dd, 1H), 7.05 (dd, 1H), 7.49 (s, 1H),
hydroxyethyl acrylate and styrene as monomers, and immobi- 7.68 (s, 1H), 8.97 (s, 1H). 1-Ethyl-3-vinyl imidazolium bromide:
lized them onto a 4 A molecular sieve surface, forming hetero- 1H NMR (600 MHz, DMSO-d6): 1.48 (t, 3H); 4.25 (q, 2H); 5.42
˚
geneous catalysts for the cycloaddition reaction of CO2 and EO. (dd, 1H); 5.96 (dd, 1H); 7.31 (dd, 1H); 7.98 (s, 1H); 8.24 (s, 1H);
The catalyst is active and selective for the reaction without any 9.61 (s, 1H).
use of cocatalyst and organic solvent.
Secondly, the functionalized PILs were synthesized accord-
ing to the procedures described in Scheme 1. In a typical reac-
tion, methacrylic acid (5.0 g) was neutralized to pH 6 using 0.1
mol Lꢁ1 NaOH, and then 1-hydroxyethyl-3-vinyl imidazolium
bromide (10 g), hydroxyethyl acrylate (10 g), styrene (1.0 g) were
dissolved in the above solution (called solution A). Ammonium
persulfate (0.55 g) was dissolved in double-distilled water
(10 mL) (called solution B). ZNX chain extender (0.054 g) and
double-distilled water (10 mL) were added to a three-necked
ask equipped with an electric stirrer, and heated at 80 ꢀC.
Solution A and solution B were added into the ask stepwise.
Then, the reaction mixture was maintained at 80 ꢀC for 2 h.
Subsequently, the reaction mixture was cooled down, the
product 2a as a light yellow foam was obtained and dialysed to
remove low molecular weight components and other impuri-
ties. The obtained foam became solid aer freeze-drying.
Following a similar procedure, the other functionalized PILs
were synthesized.
Experimental
Materials
˚
4 A molecular sieves, styrene, absolute ethanol, dodecyl sodium
sulfate (SDS), ammonium persulfate, 1-vinylimidazole, methyl
acrylic acid, hydroxyethyl acrylate, sodium hydroxide, 2-bro-
moethanol, 1-bromoethane, ethylene oxide and ZNX chain
extender were purchased from the Sinopharm Chemical
Reagent Co., Ltd. CO2 (99.999%) was provided by Successful Gas
Industry Co., Ltd. All chemicals were analytic grade and were
used without further purication.
Preparation of functionalized PIL
The routes to synthesize the functionalized PILs are presented
in Scheme 1. Firstly, 1-hydroxyethyl-3-vinyl imidazolium
bromide was prepared according to literature.25 1-Vinyl-
imidazole (9.41 g) and absolute ethanol (30 mL) were added to a
Synthesis of heterogeneous functionalized PILs
˚
three-necked ask equipped with a magnetic stirrer. The 4 A molecular sieve (10 g), functionalized PIL 2a (5 g) and
mixture was heated at 70 ꢀC under a nitrogen atmosphere. A double-distilled water (20 mL) were added to a three-necked
ꢀ
mixture of 2-bromoethanol (12.52 g) and absolute ethanol ask under continuous shaking for 24 h at 30 C followed by
(20 mL) was dropped very slowly into the ask. The reactants vaporization of the water under vacuum. The nal product was
were magnetically stirred for 24 h. Then, the reaction mixture dried in vacuum at 80 ꢀC for 12 h, and the heterogeneous
was cooled down, and the unreacted starting materials were functionalized PIL 2a (HFPIL2a) was obtained. Following a
removed by distillation in vacuum. Aer drying under vacuum, similar procedure, a series of heterogeneous functionalized
1-hydroxyethyl-3-vinyl imidazolium bromide was obtained. PILs (HFPIL1a, HFPIL1b and HFPIL2b) were immobilized onto
˚
Following a similar procedure, 1-ethyl-3-vinyl imidazolium 4 A molecular sieve.
Scheme 1 Synthesis of the functionalized PILs.
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demonstrated in Fig. 1. The functionalized PIL displays a typical
strong peak corresponding to –OH stretching frequency
centered at about 3380 cmꢁ1. The peaks at 3140, 2950 and
1080 cmꢁ1 are assigned to stretching vibrations of the C–H
bonds in the imidazole ring, C–H bonds in alkane, and C–O
bonds in monohydric alcohol, respectively. The peak at
1730 cmꢁ1 is related to the carboxyl groups. The characteristic
peaks of the imidazole ring and the benzene ring are present at
1570, 1450 and 1400 cmꢁ1. The peaks at 1270 and 1170 cmꢁ1
belong to C–N stretching bands of imidazole ring. The out-of-
plane bending vibration absorption peak of monosubstituted
benzene derivatives appeared at 750 cmꢁ1. The above result is a
clear indication that the functionalized PIL 2a has been
synthesized successfully.
The chemical compositions of the functionalized PIL 2a have
been determined using XPS. The XPS spectrum of the func-
tionalized PIL 2a (Fig. 2) shows the existence of carbon,
nitrogen, oxygen, bromine and sodium, illustrating that the
functionalized PIL 2a has been successfully synthesized by the
four monomers. The above results correlate well with the
normal FTIR spectra of the functionalized PIL 2a.
Cycloaddition reaction
EC was synthesized by a coupling reaction between EO and CO2
in the presence of heterogeneous functionalized PILs. Reaction
was performed in a 30 mL stainless-steel reactor with a
magnetic stirrer. For a typical catalytic reaction, heterogeneous
functionalized PIL (0.5 g) was added to the reactor without any
co-solvent. EO (4.4 g) was introduced into the reactor by high-
pressure liquid pump. The reactor was placed under a constant
pressure of CO2 and then heated to the required temperature.
Aer the appropriate time, the reactor was cooled to room
temperature, depressurized and the resulting reaction mixture
was separated by ltration. For recycling studies the solid
catalyst was washed with absolute ethanol (3 ꢂ 10 mL), and
dried under vacuum. The products were analyzed on a gas
chromatograph (Beifen Co., Ltd. Model, SP-3420) equipped with
a capillary column (HP-5, 30 m ꢂ 0.32 mm ꢂ 0.5 mm) using a
ame ionized detector (FID).
Characterization
Field emission scanning microscopy (FESEM) was taken with
Hitachi S-4800 operating at 1.5 kV. Fourier transform infrared
(FT-IR) spectra were collected on a FTIR EQUINO 55 apparatus
using KBr pellets. X-ray photoelectron spectroscopy (XPS)
spectra were obtained with a Perkin Elmer PHI-5000C ESCA
system. The elemental analysis was carried out on a Vario EL III
instrument. NMR measurements were conducted on Varian
INOVA-400 spectrometer at room temperature. The thermog-
ravimetricꢁa1nalysis was made in a Shimadzu TG-50, at a rate of
Depicted in Fig. 3 is the TGA curve of the functionalized PIL
2a. It is clear that it is stable up to 250 ꢀC. Upon heating to
700 ꢀC, there is the pyrolysis and complete decomposition of
PIL. The results sꢀhow that the functionalized PIL 2a is thermally
stable up to 250 C.
The morphology and structure of the samples were investi-
gated by FESEM micrograph (Fig. 4). The FESEM images in
˚
Fig. 4a and b reveal the morphology of 4 A molecular sieves.
ꢀ
ꢀ
˚
10 C min from ambient temperature to 900 C.
Obviously, 4 A molecular sieves have a rough surface with
˚
porous structure at the surface, suggesting that 4 A molecular
sieves have a relatively large adsorption capacity. The adsorp-
Results and discussion
Characterization of catalyst structure
˚
tion kinetics studies of functionalized PIL 2a onto 4 A molecular
sieves showed that the absorption equilibrium achieved aer 24
To conrm the synthesis of the functionalized PIL 2a via h (ESI Fig. S1†), and the adsorption isotherms results obtained
copolymerization with 1-hydroxyethyl-3-vinyl imidazolium at various concentrations showed the adsorption reaches
bromide, methyl acrylic acid, hydroxyethyl acrylate and styrene maximum at about 400 mg gꢁ1 when the equilibrium concen-
tration is about 0.6 g mLꢁ1 (ESI Fig. S2†). The functionalized PIL
as monomers, FTIR a spectroscopic study was rstly carried out.
The FTIR spectrum of the functionalized PIL (II) is
˚
2a was adsorbed to the surfaces of 4 A molecular sieves to form
Fig. 1 FT-IR spectra of the functionalized PIL 2a.
20508 | RSC Adv., 2014, 4, 20506–20515
Fig. 2 The wide survey XPS spectrum of the functionalized PIL 2a.
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Table 1 Catalysts screening results for the conversion of CO2 and EO
into ECa
Entry
Catalyst
EO conversion (%)
EC selectivity (%)
1
2
3
4
5
6
7
1-Bromoethane
1-Vinylimidazole
2-Bromoethanol
HFPIL1a
HFPIL1b
HFPIL2a
Trace
Trace
Trace
40
87
93
—
—
—
97
98
97
97
HFPIL2b
54
a
Reaction conditions: EO 0.1 mol, catalyst 0.4 g, initial CO2 pressure 2.5
MPa, time 4 h, temperature 130 ꢀC.
bromoethanol are ineffective catalyst for this reaction (Table 1,
entries 1–3).
Fig. 3 TGA curve of the functionalized PIL 2a.
The activities of HFPIL2a with carboxyl and hydroxyl groups,
HFPIL1b with hydroxyl but without carboxyl groups and
HFPIL1a in the absence of functional groups were investigated
(Table 1, entry 6 vs. 5 and 4). HFPIL2a showed higher activity
than HFPIL1b and HFPIL1a. The better catalytic activity of
HFPIL2a catalyst is most likely related to the abundant carboxyl
and hydroxyl groups, which could provide efficient synergistic
sites in accelerating the ring opening of epoxide.17–24 Moreover,
the HFPIL2b with carboxyl group but without hydroxyl group
(Table 1, entry 7) exhibits higher catalytic activities than
HFPIL1a in the absence of functional groups (Table 1, entry 4).
It can be seen that the introduction of carboxylic acid moieties
into the catalytic system can greatly enhance the catalytic
performance of the heterogeneous functionalized PIL catalyst
toward the cycloaddition reaction.
Catalytic performance
Fig. 4 FESEM images of 4 A˚ molecular sieves (a and b) and hetero-
geneous functionalized PIL (c and d).
The catalytic performance of the heterogeneous functionalized PIL
catalyst was evaluated for the synthesis of EC from EO and CO2.
Fig. 5 depicted the inuence of different catalyst–ethylene oxide
(EO) mass ratios on cycloaddition reaction of EO and CO2. As
so-called heterogeneous functionalized PIL catalyst. In
comparison to the original molecular sieves, irregular akes
occurred on the surface of them (Fig. 4c and d), suggesting that
the ionic liquid has been successfully immobilized on the
support.
To further prove the presence of the functionalized PIL 2a on
˚
4 A molecular sieves surface, elemental analysis of the sup-
ported PIL catalyst were conducted by XPS (ESI Fig. S3†). The
XPS spectrum shows the presence of carbon, nitrogen, oxygen,
bromine, silicon and sodium, and the contents of them are
13.17%, 5.66%, 58.85%, 1.69%, 15.75% and 4.88%, respectively.
Catalytic performance
The reaction of EO with CO2 to produce EC was choose as the
model reaction to test the activity of the heterogeneous func-
tionalized PILs, and the results are summarized in Table 1. It
Fig. 5 Effect of catalyst amount on the cycloaddition reaction of EO.
can be seen that 1-bromoethane, 1-vivylimidazole and 2- Reaction conditions: EO 0.1 mol, initial CO2 pressure 2.5 MPa, time 4 h.
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shown in Fig. 5, when the amount of catalyst varied from 2% to
9%, conversion rate of EO signicantly raised from 73% to 92%
and selectivity of EC increased from 94% to 97%. However, the
further increasing amount of the catalyst declined EO conversion
rate and EC selectivity, which is because that the excess catalyst
could not be well dispersed in the reaction mixture and limited the
mass transfer between the active sites and reactants.26,27 Conse-
quently, the optimal catalyst amount is about 9%.
Subsequently, we investigated the effect of some parameters
on the catalytic activity of the heterogeneous functionalized PIL
catalyst. Almost all the reports on cyclic carbonate synthesis
suggest that reaction temperature and CO2 pressure are impor-
tant parameters affecting the coupling of CO2 and epoxides.23,27,28
Fig. 6 illustrates the dependence of EO conversion rate and EC
selectivity on reaction temperature. It was found that the reaction
temperature dramatically affected the conversion rate of EO. The
EO conversion increases sharply with increasing temperature
Fig. 7 Influence of CO2 pressure on the catalytic activity Reaction
conditions: EO 0.1 mol, catalyst 0.4 g, initial CO2 pressure 2.5 MPa,
time 4 h, temperature 130 ꢀC.
ꢀ
ꢀ
from 100 C to 130 C. The continued increase of temperature
beyond 130 ꢀC up to 140 ꢀC increases a little EO conversion.
However, further increase in temperature causes a decrease of EO
conversion and a slight decrease in EC selectivity, a minute
amount of ethylene glycol generated from the hydrolysis of EO is
detected as by-product owing to the higher temperature. Such an
effect of the higher reaction temperature on catalytic activity has
been observed in other catalytic systems.21,27,29 Another reason is
that the solubility of the CO2 gas phase in the reaction system
decreases with increasing temperature. Therefore, in reactions
moderate increase in EO conversion in the low-pressure region
(1.0–2.5 MPa). However, when CO2 pressure is above 2.5 MPa,
there is signicant decline in EO conversion. Such an effect of
CO2 pressure on catalytic activity has been observed in other
catalytic systems.5,21,30 For example, when immobilized ionic
liquid/ZnCl2 was used for the coupling of PO and CO2, the best
catalyst activity appeared at a CO2 pressure of 1.5 MPa at
110 C.30 A possible explanation is that acidic CO2 dissolves in
ꢀ
ꢀ
performed at temperatures greater than 140 C, CO2 solubility
basic epoxide and liquees as a result of CO2–epoxide com-
plexing.5,21,30 The high CO2 pressure promotes this kind of CO2–
EO interaction rather than enhancing the interaction between
EO and catalyst, thus leading to the low catalytic activity. It was
also reported that too high CO2 pressure may retard the inter-
action between the epoxides and the catalyst.20,28,31 Therefore,
high CO2 pressure did not benet the cycloaddition reaction
catalyzed by the heterogeneous functionalized PIL catalyst.
Additionally, the dependence of EO conversion and EC
selectivity on reaction time was also evaluated. It was found that
the reaction time had a remarkable inuence on the reaction.
As shown in Fig. 8, the EO conversion rate was faster in the
initial stage and remained almost invariant aer 4 h. The
reaction time exhibited little inuence on the selectivity of EC.
Conclusively, a reaction time of 4 h was appropriate for the
synthesis of EC in this study.
affects reactivity, leading to a decreased in EO conversion. Such
an effect of reaction temperature on the synthesis of cyclic
carbonates has been observed in the cycloaddition of various
epoxides with CO2.28 Therefore, 130 ꢀC is considered suitable for
the target reaction.
Fig. 7 depicts the inuence of CO2 pressure on the EO
conversion rate and EC selectivity at 140 ꢀC with a reaction time
of 4 h. It is observed that PC selectivity is always above 90.0%
and can be considered as being independent of CO2 pressure.
Interestingly, an increase in CO2 pressure resulted in a
Experiments were also conducted to examine the recycla-
bility of the heterogeneous functionalized PIL catalyst. In each
cycle, the catalyst was recovered by ltration directly. Aer
drying, the catalyst was reused for the next run. As shown in
Fig. 9, the catalyst could be reused for six runs, and the EC yield
still remained above 90%. Furthermore, it is noteworthy to
mention that the EC yield was still remained above 80% aer 10
runs, indicating that the catalyst is relatively stable and has
excellent catalytic activity. Hence despite the decline aer six
runs, the catalyst can be considered as reusable. The decline of
EC yield aer six runs could be due to the functionalized PIL
leaching. Poor reusability has been reported before over cata-
lysts of similar kind.22,30
Fig. 6 Influence of temperature on the catalytic activity Reaction
conditions: EO 0.1 mol, catalyst 0.4 g, initial CO2 pressure 2.5 MPa,
time 4 h.
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applications. The thermodynamic data of various substances
such as ethylene oxide, carbon dioxide and ethylene carbonate
are tabulated in Table 3.32,33
The changes of enthalpy, entropy and Gibbs free energy at
different temperatures in the process of the synthesis of EC
have been calculated from the following formulas respectively32
and the results are given in Table 4.
DrHm ¼ SHf(products) ꢁ SHf(reactants)
DrSm ¼ SSf(products) ꢁ SSf(reactants)
,
DrGm ¼ SGf(products) ꢁ SGf(reactants)
,
DrCpm ¼ SCpm(products) ꢁ SCpm(reactants)
Fig. 8 Influence of reaction time on the catalytic activity. Reaction
conditions: EO 0.1 mol, catalyst 0.4 g, initial CO2 pressure 2.5 MPa,
temperature 130 ꢀC.
ð
ð
T
T
ꢀ
ꢁ ꢂ
DrHT ¼ DrHq þ
DrCpdT; DrST ¼ DrSq þ
DrCp T dT
298
298
DrGT ¼ DrHT ꢁ TDrST, DrGT ¼ ꢁRT ln Kp
where Kp is the equilibrium constant of the reaction, the DrHq
and DrSq are the standard enthalpy and standard entropy
changes respectively, R is the universal gas constant, and T is
the thermodynamic temperature.
It is obvious from Table 4 that the negative of free energy
change (DrGm) is an indication of spontaneous nature of the EC
synthesis from EO and CO2. The negative values of enthalpy
change (DrHm) for the intervals of temperatures showed the
exothermic nature of the reaction. The negative values of DrSm,
DrHm and DrGm for the corresponding temperature intervals
further suggested low temperature condition is benecial for
the synthesis of EC from EO and CO2, higher temperature is
unfavorable for the forward reaction and may induce some side
reactions which is consistent with Fig. 6.
Fig. 9 EC yield (Y) got with the catalyst of different recycling number.
The reaction equilibrium constant Kp (Table 4, column 5)
within the temperature range from 298.15–413.15 K is very high,
indicating that the reaction can not only occur but also be
considered as practically irreversible.34 Owing to exothermic
reaction, the equilibrium constant Kp decreases with increasing
temperature. However, it is noteworthy to mention that the
values of Kp remained high in the temperature range of 298.15–
413.15 K, and they are thermodynamically advantageous.
Therefore, by altering the reaction temperature, the selectivity
of the reaction couldn't be improved. Based on the above
analysis, it is necessary to study reaction kinetics, thus signi-
cantly shortening the reaction time and improving the reaction
selectivity and yield, suggesting that it should focus on devel-
opment of high-activity and high-selectivity catalyst.
Catalytic activity of the reaction of various epoxides with CO2
Under the optimized conditions the cycloaddition of CO2 to
various epoxides using the heterogeneous functionalized PIL
was conducted at 130 ꢀC and 2.5 MPa CO2 (Table 2). The results
indicate that the catalytic system is applicable to coupling CO2
with a variety of terminal epoxides with high yield and selec-
tivity. Cyclohexene-oxide required a prolonged reaction time
compared with the terminal epoxides (Table 2, entry 5) due to
steric hindrance.21–24,27,28
Thermodynamic evaluation of the EC synthesis using EO and
CO2 as feedstocks
The kinetic study of the EC synthesis using EO and CO2 as
feedstocks
The thermodynamic phenomena describe how the systems
respond to the changes in their surroundings through the
effects of heat, work and energy on a system. Therefore, It has been veried that the synthesis of cyclic carbonate from
understanding the thermodynamics of EC formation using EO CO2 and epoxide obey rst order kinetics, the reaction are rst
and CO2 as feedstocks is highly important in seeking novel order in epoxide concentration.35,36 Therefore, dynamic equa-
synthesis ideas both in academic investigations and in practical tion of the synthesis of ethylene carbonate from CO2 and EO
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a
Table 2 Catalytic activity of the reaction of various epoxides with CO2
Results
Entry
1
Epoxide
Product
Yield (%)
Selectivity (%)
93
90
97
2
3
98
94
90
4
82
97
97
5
31.2
a
Reaction conditions: epoxide 0.1 mol, catalyst 0.4 g, initial CO2 pressure 2.5 MPa, temperature 130 ꢀC, reaction time 4 h.
Table 4 Thermodynamic properties of EC synthesis from EO and CO2
was established as described in eqn (1). Taking the logarithm of
eqn (1) leads to eqn (2).
DrHm
DrSm
DrGm
T (K)
(kJ molꢁ1
)
(J molꢁ1 Kꢁ1
)
(kJ molꢁ1
)
Kp
r ¼ ꢁd[EO]/dt ¼ k[EO]
ln[1/(1 ꢁ x)] ¼ kt
(1)
(2)
298.15
383.15
393.15
403.15
413.15
ꢁ239.631
ꢁ240.575
ꢁ240.642
ꢁ241.374
ꢁ240.750
ꢁ311.748
ꢁ314.580
ꢁ314.755
ꢁ314.902
ꢁ315.025
ꢁ146.731
ꢁ120.091
ꢁ116.943
ꢁ114.468
ꢁ110.645
5.1 ꢂ 1025
2.393 ꢂ 1016
3.497 ꢂ 1015
6.875 ꢂ 1014
9.873 ꢂ 1013
where x is conversion rate of EO, t is reaction time, k is reaction
rate constant. Under the optimal reaction conditions, the
conversion rates of EO at four temperatures between 110 ꢀC and
140 ꢀC are calculated, to get the values of ln[1/(1 ꢁ x)] and a plot
of ln[1/(1 ꢁ x)] against t (Fig. 10). Linear tting was performed
using least square method on the plot of ln[1/(1 ꢁ x)] against t,
and the reaction rate constant k and the correlation coefficient R
are obtained (Table 5). The reaction is highly exothermic for
ethylene carbonate, though the effect of raising the temperature
Table 3 Standard thermodynamic data of various substances in the reaction (298.15 K, 101.325 kPa)
Substance
DfHmq (kJ molꢁ1
)
Sfq (J molꢁ1 Kꢁ1
)
DfGmq (kJ molꢁ1
)
Cpm (J molꢁ1 Kꢁ1
)
CO2
EO
EC
ꢁ393.509
ꢁ52.63
213.74
242.53
326.69
ꢁ394.359
ꢁ13.01
26.57 + 42.258 ꢂ 10ꢁ3T ꢁ 14.25 ꢂ 10ꢁ6T2
15.21 + 0.1104T
ꢁ580.88
ꢁ96.127
ꢁ13.165 + 0.3114T ꢁ 0.9 ꢂ 10ꢁ4T2 ꢁ 0.3 ꢂ 10ꢁ7T3
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Fig. 11 Logarithm of the reaction rate constant versus the reciprocal
of the reaction temperature.
Fig. 10 Relationship of ln[1/(1 ꢁ x)] with t at different temperatures.
and hydroxyl functional groups exhibits excellent catalytic
activity (Table 1 and Fig.5), which is consistent with literature
reports.17–24 Based on our results and those of previous reports,
the proposed mechanism is shown in Scheme 2.
The coordination of the H atom of carboxyl and hydroxyl
groups of the catalyst with the O atom of EO though hydrogen
bonding results in the formation of intermediate. Due to the
polarization of the epoxide C–O bonds, the bromide anion
(Lewis base) makes a nucleophilic attack on one of the carbon
atoms of EO. Simultaneously, there is ring opening of epoxy and
generation of an oxy anion. The insertion of CO2 into the hal-
oalkoxy species would result in the formation of a linear hal-
ocarbonate that transforms into ethylene carbonate through
Table 5 Linear fitting results and reaction rate constants at different
temperatures
T/K
TK ꢂ 103 Fitting equation
ꢁ1
R
k/minꢁ1 ln k
383.15 2.61
393.15 2.54
403.15 2.48
413.15 2.42
y ¼ 0.00759x
y ¼ 0.01339x
y ¼ 0.01899x
y ¼ 0.02495x
0.9993 0.00759
0.9996 0.01339
0.9982 0.01899
0.9998 0.02495
ꢁ4.88
ꢁ4.31
ꢁ3.96
ꢁ3.69
has been ascribed to accelerating the rate of reaction (Table 5,
column 5). The correlation coefficients R were close to 1 under
the reaction conditions, indicating that the rate of the reaction
was only dependent of the concentration of the ethylene oxide
and tted rst order linear relation, which is agree with the
kinetic equation from literature.35,36
Reaction activation energy
Fig. 11 is the plots of ln k vs. 1/T. Obviously, there is a good
linear relations between the logarithm of k and 1/T.
According to the Arrhenius equation,
ln k ¼ ln k0 ꢁ Ea/RT
where k is the reaction rate constant, k0 is the exponential
factor, Ea is the activation energy, R is the gas constant and T is
the reaction temperature.
Therefore, the exponential factor k0 and activation energy Ea
were obtained as k0 ¼ 98 715.8 and Ea ¼ 51.9965 kJ molꢁ1. Thus,
the kinetic equation of this reaction is r ¼ ꢁdCEO/dt ¼
98 715.8eꢁ51.9965/RTCEO
.
Proposed mechanism of coupling reaction
Previous research suggested that Brønsted acids can accelerate
the ring-opening reaction of epoxides by forming hydrogen
Scheme 2 The proposed reaction mechanism for cycloaddition of EO
bonds.37 In our current study, functionalized PIL with carboxyl and CO2 catalyzed by heterogeneous functionalized PIL.
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intramolecular substitution of the halide and regenerates the
catalyst. Thus, in our current catalyst system, the coexistence of
hydrogen bond donors (–OH and –COOH) and halide anion
showed the synergistic effect of promoting the coupling reac-
tion. This effect might be the main contribution to their high
catalytic activity and selectivity.
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Conclusions
The functionalized PIL with carboxyl and hydroxyl functional
groups has been demonstrated as effective catalyst for the
synthesis of EC via the cycloaddition of EO with CO2. The
functionalized PIL immobilized on the molecular sieve show 10 K. Yamaguchi, K. Ebitani, T. Yoshida, H. Yoshida and
excellent catalytic activity and selectivity, since the –COOH and
–OH groups in the PIL can accelerate the reaction and showed
the synergistic effects with the halide anion. Moreover, the
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Catalysts for Cycloaddition of Carbon Dioxide to Epoxides,
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the reaction is pseudo-rst order reaction within the tempera-
ture range from 298–413 K.
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Acknowledgements
The work was nancially supported by the National Natural
Science Foundation of China (21172147), the National High
Technology Research and Development Program of China (863: 14 M. Alvaro, C. Baleizao, D. Das, E. Carbonell and H. Garcia,
2011AA100503), The National Key Technology R & D Program of
China (2011BAE06B06-4), National Program on Key Basic
Research Project (973: 2010CB126106), the Shanghai
Committee of Science and Technology (11ZR1426800).
CO2 xation using recoverable chromium salen catalysts:
use of ionic liquids as cosolvent or high-surface-area
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15 X. H. Zhang, N. Zhao, W. Wei and Y. H. Sun, Chemical
xation of carbon dioxide to propylene carbonate over
amine-functionalized silica catalysts, Catal. Today, 2006,
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