Production of dimethyl carbonate from ethylene carbonate and methanol using immobilized ionic liquids on MCM-41

Article abstract of DOI:10.1016/j.cattod.2010.11.010

Ionic liquids on ordered mesoporous silica were prepared and their catalytic performance in the synthesis of dimethyl carbonate (DMC) was investigated. The ionic liquids were immobilized on chloropropyl-functionalized MCM-41 (CP-MS41) through the quaternization of trialkylamines. The supported ionic liquids were proven to be an effective heterogeneous catalyst for the synthesis of DMC from transesterification of ethylene carbonate (EC) with methanol. The immobilized quaternary ammonium salt (QCl-MS41) catalysts with longer alkyl chains showed higher EC conversion and turnover number (TON). Higher temperatures and longer reaction times were favorable for the reactivity of QCl-MS41. However, carbon dioxide pressure showed a maximum for catalytic activity. The catalyst can be reused for reactions in up to three consecutive runs with only a slight decrease in catalytic activity.

Full text of DOI:10.1016/j.cattod.2010.11.010

Catalysis Today 164 (2011) 556–560
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Production of dimethyl carbonate from ethylene carbonate and methanol using
immobilized ionic liquids on MCM-41
a
b
c
c
a,∗
Dong-Woo Kim , Dong-Ok Lim , Deug-Hee Cho , Jae-Cheon Koh , Dae-Won Park
a
Division of Chemical Engineering, Pusan National University, Busan 609-735, Republic of Korea
Seki-Arkema Co. Ltd., Gyungsangnamdo 637-940, Republic of Korea
Korea Research Institute of Chemical Technology, Daejeon 305-600, Republic of Korea
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 July 2010
Received in revised form 1 November 2010
Accepted 2 November 2010
Available online 3 December 2010
Ionic liquids on ordered mesoporous silica were prepared and their catalytic performance in the synthesis
of dimethyl carbonate (DMC) was investigated. The ionic liquids were immobilized on chloropropyl-
functionalized MCM-41 (CP-MS41) through the quaternization of trialkylamines. The supported ionic
liquids were proven to be an effective heterogeneous catalyst for the synthesis of DMC from trans-
esterification of ethylene carbonate (EC) with methanol. The immobilized quaternary ammonium salt
(
QCl-MS41) catalysts with longer alkyl chains showed higher EC conversion and turnover number (TON).
Keywords:
Higher temperatures and longer reaction times were favorable for the reactivity of QCl-MS41. However,
carbon dioxide pressure showed a maximum for catalytic activity. The catalyst can be reused for reactions
in up to three consecutive runs with only a slight decrease in catalytic activity.
© 2010 Elsevier B.V. All rights reserved.
Dimethyl carbonate
Ethylene carbonate
Methanol
Immobilization
Ionic liquid
1
. Introduction
The synthesis of dimethyl carbonate (DMC) is a promising reac-
Several researchers have reported the direct synthesis of DMC;
however, most of those works are at a preliminary stage for prac-
tical application due to low DMC yields [7,8]. A more promising
approach to DMC synthesis is based on transesterification of cyclic
carbonates with methanol. The cyclic carbonates can be synthe-
sized in a quantitative yield via cycloaddition of CO2 to epoxides
[9,10]. Using transesterification for DMC synthesis is desirable in
terms of green chemistry and sustainable development, as CO2 is a
greenhouse gas and an abundant carbon resource [11].
Many homogeneous and heterogeneous catalysts for transester-
ification have been developed, such as tertiary amines [12], alkali
metals, or alkali metal compounds [13]. Knifton and Duranleau
[14] used free organic phosphines supported on partially cross-
linked polystyrene for the reaction. Heterogeneous catalysts such
as alkali-treated zeolite [15,16], basic metal oxides [17,18], and
hydrotalcite [19] were also reported. However, most of the reported
heterogeneous catalysts require severe experimental conditions,
e.g., high temperature and/or pressure, to obtain even a moderate
DMC yield [16,20–24].
tion for the use of naturally abundant carbon dioxide. DMC has
gained considerable interest owing to its versatile chemical reac-
tivity and unique properties such as high oxygen content, relatively
low toxicity, and excellent biodegradability [1,2]. It can be effec-
tively used as an environmentally benign substitute for highly toxic
phosgene and dimethyl sulfate in carbonylation and methylation
reactions, as a monomer for several types of polymers, and as an
intermediate in the synthesis of pharmaceutical and agricultural
chemicals [3]. Hence, it is regarded as an environmentally benign
building block [4]. Furthermore, DMC has also been used as an elec-
trolyte in lithium batteries and an octane booster because of its high
dielectric constant [5] and high oxygen content, respectively [6].
Conventionally, DMC has been synthesized using phosgene and
methanol. Several nontoxic DMC synthetic processes have been
suggested: (1) oxidative carbonylation using CO, O , and methanol;
2
(
(
2) direct synthesis of DMC from carbon dioxide and methanol; and
3) transesterification of methanol with cyclic carbonates. How-
The use of ionic liquids (ILs) as environmentally benign media
for catalytic processes or chemical extraction has become widely
recognized and accepted [25]. ILs have negligible vapor pressure,
excellent thermal stability, and special characteristics compared
with conventional organic and inorganic solvents. Many high-
performance reactions catalyzed with ILs have been reported
ever, the oxidative carbonylation of methanol suffers from a low
production rate, the need for corrosion resistant reactors, and the
toxicity and potential explosiveness of CO.
[
26–30].
In our previous work [30–36], we studied the performance of
ILs in the synthesis of cyclic carbonate from epoxide and carbon
∗ Corresponding author. Tel.: +82 51 510 2399; fax: +82 51 512 8563.
E-mail address: dwpark@pusan.ac.kr (D.-W. Park).
0
920-5861/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2010.11.010

D.-W. Kim et al. / Catalysis Today 164 (2011) 556–560
557
dioxide. ILs immobilized onto MCM-41 showed excellent activity
for carbon dioxide insertion reactions [36].
system purchased from Micromeritics. The solvent-extracted sam-
◦
−5
ples were degassed at 110 C and 10 torr overnight before the
adsorption experiments. The mesoporous volume was estimated
from the amount of nitrogen adsorbed at a relative pressure of
0.5 by assuming that all mesopores were filled with condensed
nitrogen in the normal liquid state. The pore size distribution was
estimated using the Barrett, Joyner, and Halenda (BJH) algorithm
software from Micromeritics built into the ASAP2010 system. Ele-
mental analysis was performed using a Vario EL III in which 2.0 mg
In the present work, we report the synthesis of DMC from ethy-
lene carbonate (EC) and methanol using a quaternary ammonium
salt IL immobilized onto MCM-41 prepared by one-pot synthesis
under relatively mild, simple synthesis conditions. The effects of
reaction time, carbon dioxide pressure, reaction temperature, and
amount of catalyst are discussed to increase understanding of the
reaction mechanism. A recycle test of the catalyst was also con-
ducted.
◦
of each sample was subjected to 1100 C with sulfanilic acid used
as the standard.
Fourier transform infrared (FT-IR) spectra of THA-MS41 were
collected on a 960981(A) (Bruker A.M GMBH) instrument at the
Korea Basic Science Institute (KBSI) using a KBr pellet technique.
2
. Experimental
2.1. Materials
1
0 mg of the sample was ground with approximately 200 mg of
spectral-grade KBr to form a pellet under hydraulic pressure, and
Commercially available tetraethyl orthosilicate (TEOS, Aldrich),
-chloropropyltriethoxysilane (ClPTES, Aldrich), cetyltrimethy-
−
1
13
29
the IR spectrum was recorded at 4000–400 cm
state NMR experiments were conducted at frequencies of 100.6 and
9.5 MHz, respectively, on a UNITY INOVA-400WB spectrometer
equipped with a commercial 5-mm T3 hydrogen exchange yield
HXY) magic-angle spinning (MAS) probe. All spectra were mea-
.
C and Si solid-
3
lammonium bromide (CTMABr, Aldrich), tetramethylammonium
hydroxide (25% aqueous TMAOH, Aldrich), EC (Aldrich), and
methanol (Aldrich) were used without further purification. Com-
mercially available carbon dioxide with a purity of 99.99% (Hanyu
Chemicals) was also used.
7
(
sured at room temperature under the following conditions: MAS
at 5 kHz, ꢀ/2 pulse, 6.5 ␮s, and a repetition delay of 60 s, 3928
29
scans. The Si MAS spectra were referenced to tetramethylsilane.
2
.2. Preparation of immobilized ionic liquid
13
1
The C{ H} cross-polarization (CP) spectra were measured with a
recycle delay of 5 s, 1024 scans, and a CP contact time of 1.5 ␮s under
MAS at 5 kHz and a ꢀ/2 pulse at 7 ␮s. The spectra were referenced
to tetramethylsilane.
The immobilized quaternary ammonium salt on MCM-41 was
prepared according to our previous report [37]. In a typical syn-
thesis, TEOS (20.36 mL) was mixed with ClPTES (3.81 mL), and the
solution was added to a solution of 18.35 g of TMAOH and 18 g of
2.4. Catalytic activity test
H O and stirred for 5 min to form solution A. This was added to
2
solution B (9.1 g of CTMABr dissolved in a mixture of 30 g of H O
2
The transesterification reaction was conducted in a 50-mL stain-
and 45 mL of EtOH) and stirred vigorously at room temperature
for 45 min. The resulting mixture was then heated in a water bath
less steel autoclave equipped with a magnetic stirrer. For each
reaction, the reactor was typically charged with QCl-MS41 cata-
lyst (0.2 g), EC (25 mmol), and excess methanol (200 mmol), and
CO2 was introduced at room temperature to a preset pressure. The
reaction was started by stirring when the desired temperature and
pressure were attained. The reaction was performed in a batch
operation mode. The products and reactants were analyzed using
a gas chromatograph (HP 6890N) equipped with an FID and a cap-
illary column (HP-5, 5% phenyl methyl siloxane). The selectivities
to DMC and ethylene glycol (EG) were calculated on the basis of EC
as a limited reactant.
◦
at 80 C for 45 min to remove the added alcohol as well as that
produced by alkoxide hydrolysis. Finally, 54 g of H O was added
2
slowly to the solution, during which the clear liquid became a turbid
gel. The gel was aged for 20 h (pH ∼12.1) with continuous stirring.
The pH of the resulting gel was adjusted to 11.3 with 4 N HCl. The
resulting solution was stirred for 4 h and then heated in a Teflon-
◦
coated stainless steel autoclave at 140 C for 48 h [37]. 1.5 g of the
as-synthesized hybrid material was extracted with 210 mL of EtOH
◦
and 20 mL of 35% HCl at 65 C for 24 h [38]. The resulting solid was
filtered and washed with 500 mL of EtOH, followed by washing with
◦
2
L of distilled water and drying in air at 100 C. The same procedure
was repeated again to ensure maximum extraction.
3. Results and discussion
Grafting experiments were carried out using standard Schlenk
techniques under
a nitrogen atmosphere (99.99%) accord-
3
.1. Characterization of immobilized ionic liquid on MCM-41
ing to the following procedure. The mesoporous molecular
sieves (chloropropyl-functionalized MCM-41, CP-MS41) were pre-
The N2 adsorption–desorption isotherm and pore size distri-
◦
activated at 175 C under a vacuum for 4 h to remove the
bution of THA-MS41 are illustrated in Fig. 1. THA-MS41 shows
characteristic type IV isotherms with apparent hysteresis loops,
which are typical features of mesoporous materials according to
the IUPAC classification. Fig. 1 also shows a very narrow pore size
distribution with mesopores of relatively small diameter. The sur-
face area, pore size, and pore volume were 695.9 m /g, 2.43 nm, and
0
analyzed by elemental analysis, and the amount of immobilized IL
is shown in Table 1. The amount of amine for the immobilized IL on
MS41 was 0.36–0.40 mmol/g-cat.
physisorbed water. Five grams of CP-MS41 were immobilized with
triethylamine (TEA), tri-n-propylamine (TPA), tri-n-butylamine
(
TBA), or tri-n-hexylamine (THA) (1.5 mmol/g) in dry tetrahydro-
furan (30 mL/g) as the solvent under a nitrogen atmosphere at
◦
7
0 C for 24–36 h. Excess trialkylamine was removed by filtra-
2
tion followed by repeated washing with 2 L of dichloromethane.
The resulting solids were dried under a vacuum at room temper-
ature. The materials prepared from TEA, TPA, TBA, and THA on
CP-MS41 are designated as TEA-MS41, TPA-MS41, TBA-MS41, and
THA-MS41, respectively.
3
.53 cm /g, respectively. The nitrogen content in the QCl-MS41 was
CP 13C MAS-NMR spectra of THA-MS41 show typical charac-
teristic peaks corresponding to carbon atoms of the trihexylamine
attached to the chloropropyl-functionalized MCM-41, as shown in
Fig. 2.
2
.3. Characterization of catalyst
The surface area, pore volume, and pore size distribution were
Solid-state 29Si MAS-NMR spectra of THA-MS41 confirm the
presence of organo-functionalized moieties as part of the silica wall
◦
measured by nitrogen adsorption at −195 C using an ASAP2010

5
58
D.-W. Kim et al. / Catalysis Today 164 (2011) 556–560
Fig. 1. N2 adsorption–desorption isotherm of THA-MS41.
Fig. 3. 29Si CP MAS-NMR spectra of THA-MS41.
Table 1
EA and immobilized amount of amine in different QCl-MS41.
the –OH region. This was attributed to the hydrophobicity of the
organosilane in the framework. The peak at 1647 cm confirms
Catalyst
N (wt%)
C (wt%)
H (wt%)
Amount of QCl (mmol/g-cat)
−1
CP-MS41
0.01
0.56
0.53
0.51
0.52
3.075
8.00
8.23
8.47
10.69
4.139
4.56
4.50
4.47
4.04
–
the immobilization of trihexylamine on MCM-41.
TEA-MS41
TPA-MS41
TBA-MS41
THA-MS41
0.40
0.38
0.36
0.36
3.2. Synthesis of DMC from EC and methanol
DMC was synthesized by transesterification of EC and methanol
using heterogeneous QCl-MS41 catalysts in a high-pressure batch
reactor.
structure of the organically modified MCM-41 (Fig. 3). Distinct res-
n
onances are clearly observed for the siloxane [Q = Si(OSi)n(OH)
,
4−n
(
CH O) CO + 2 CH OH → (CH O) CO + (CH OH)
2
2
3
3
2
2
2
3
4
n = 2–4; Q at −101 ppm; Q at −110 ppm] and organosiloxane
m
= RSi(OSi)m(OH)3 , m = 1–3; T at −65 ppm; T2 at −61 ppm]
3
[
T
DMC and EG were the main products of the transesterification
−m
units [39].
reaction. Dimethyl ether and glycol monoethyl ether were byprod-
ucts, and small peaks of ethylene oxide from the decomposition of
EC could be detected at longer reaction times and high tempera-
tures.
Fig. 4 shows the FT-IR spectra of THA-MS41. The bands at 1088
−
1
and 795 cm
were assigned to the stretching vibrations of the
mesoporous framework (Si–O–Si). The comparatively weak bands
−1
around 1633, 1482, and 1440 cm
were assigned to the over-
Table 2 shows the conversion of EC and the selectivity of DMC
and EG for immobilized quaternary ammonium salt of different
tone bands of C–H stretching vibrations originating from the –CH2
groups present in the organic silane group. The –OH stretching
◦
alkyl chain lengths at 180 C and 1.17 MPa of CO2 pressure. The
−1
region (3500–3000 cm ) of THA-MS41 shows a strong stretch in
conversion of EC varied according to the trialkylamine used, which
can be explained by an inductive resonance effect. The DMC yield
and turnover number (TON) increased with increasing alkyl chain
-C-H
-C-N
-
Si-OH -Si-O-Si
-O-H
-Si-O-Si
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm )
Fig. 2. 13C CP MAS-NMR spectra of THA-MS41.
Fig. 4. FT-IR spectra of THA-MS41.

D.-W. Kim et al. / Catalysis Today 164 (2011) 556–560
559
Table 2
80
Effect of ionic liquid structure on the synthesis of DMC from EC and methanol.
TONa
70
Catalysts
EC conversion (%)
DMC yield (%)
EG yield (%)
TEA-MS41
TPA-MS41
TBA-MS41
THA-MS41
THACb
74.0
75.3
76.6
78.4
77.8
72.0
72.3
73.2
76.3
75.3
72.0
71.5
72.5
75.5
74.9
225
238
254
272
262
6
0
50
4
0
Reaction condition: EC = 25 mmol, MeOH = 200 mmol, catalyst = 0.2 g QCl-MS41,
◦
T = 180 C, reaction time = 4 h, PCO2 = 1.17 MPa.
a
30
TON = (mole of DMC)/(mole of quaternary ammonium in QCl-MS41).
Homogeneous THAC with the same amount of ionic liquid as that in 0.2 g of
b
THA-MS41.
EC conv.
DMC yield
EG yield
2
1
0
0
0
length from TEA-MS41 to THA-MS41. The electron-donating ten-
dency toward nitrogen increases with increasing chain length; this
ı+
ı−
in turn increases the electron density on nitrogen (–N → X sepa-
ration of the halogen becomes more viable), thereby increasing the
resulting ionic character of the system [40]. In contrast, a longer-
chain system shows a lower pore size and increased blocking of
the active sites due to the growth of the alkyl chain from the active
sites towards the center of the pores, which hinders the approach
of the reactant toward the active sites. The role of the immobilized
quaternary ammonium salt might be suggested by the formation
of an intermediate complex with methanol and then its reaction
with EC to form DMC [41]. Since THA-MS41 showed slightly higher
activity than TEA-MS41, the steric hindrance for reactant diffusion
became less important than the increase in anion activation for
THA-MS41. A comparison of THA-MS41 with conventional tetra-
hexylammonium chloride (THAC) revealed that THA-MS41 had a
higher conversion than THAC. Although homogenous THAC has a
high level of interaction with the reactant, it cannot provide high
conversion owing to low accessibility. However, THA-MS41 shows
higher conversion owing to the good distribution of active sites by
high surface area of the support, which provides better accessibil-
ity than a homogenous system. The synergistic effect of organic
and inorganic moieties could also be one of the advantages of the
THA-MS41 [42].
0
1
2
3
4
5
6
7
Reaction time (h)
Fig. 5. Effect of reaction time on the synthesis of DMC from EC and methanol
(reaction condition: EC = 25 mmol, MeOH = 200 mmol, catalyst = 0.2 g TEA-MS41,
◦
T = 180 C, PCO2 = 1.17 MPa).
by the reaction of methanol and the by-product intermediate pro-
duced in DMC synthesis. Therefore, the lower selectivity of EG can
be explained by a lower reaction rate of this consecutive reaction.
The transesterification of EC and methanol was performed at
◦
different temperatures in the range of 120–180 C. The observed
temperature dependence of the reaction is shown in Fig. 6. Lower
◦
◦
EC conversion and DMC yields were obtained at 120 C and 140 C.
◦
With a further increase in temperatures to 180 C, higher EC con-
version (74.0%) and higher yields of DMC (72.0%) and EG (72.0%)
were observed.
Table 4 shows the effect of CO2 pressure on the reactivity of
the THA-MS41 catalyst. The EC conversion and the DMC and EG
yields increased as the initial CO2 pressure increased from 0.10 to
0
.69 MPa. Although CO2 pressure is not required for the synthesis
of DMC, high CO pressure could inhibit the decomposition of EC to
2
This activity can be explained using the TON values in Table 3,
which show that the reaction falls on a typical heterogeneous reac-
tion trend. A comparison with different amounts of catalyst showed
that the system containing THA-MS41 catalyst (0.05–0.2 g) had a
different TON value because the suspension of a solid catalyst in
the reaction vessel is quite difficult, which reduces the activity.
Fig. 5 shows time-variant conversion of EC and the yields of
ethylene oxide and CO2 as reported previously [44,45]. However, a
further increase in CO2 pressure to 1.17 MPa slightly decreased the
catalytic activity and selectivity to DMC and EG. When the CO pres-
2
sure was 1.65 MPa, EC conversion was very low, probably owing to
the so-called dilution effect [46,47]. Earlier studies reported that
◦
DMC and EG at 180 C with TEA-MS41 catalyst under an initial CO2
80
pressure of 1.17 MPa. As the reaction time proceeded, the conver-
sion of EC increased for 2 h, and then it remained nearly constant.
The reaction time needed to arrive at equilibrium was reported to
EC conv.
DMC yield
EG yield
6
4
2
0
0
0
0
depend strongly on the type of catalyst [43]. LiOH, KOH, and K CO3
2
catalysts reached equilibrium in less than 1 h; however, KBr and
KI required more than 2 h. The selectivities to DMC and EG also
increased up to 6 h. However, DMC selectivity was higher than EG
selectivity. According to a mechanism proposed by Fang and Xiao
[
19], DMC was produced via several reaction steps involving the
−
formation of CH O and its reaction with EC. EG is believed to form
3
Table 3
EC conversion and DMC selectivity for different amount of catalysts.
Catalysts
Amount (g) EC conversion (%) DMC yield (%) EG yield (%) TONa
THA-MS41 0.05
THA-MS41 0.10
THA-MS41 0.20
52.0
69.4
78.4
50.3
67.9
76.3
49.9
68.1
75.5
699
472
272
120
140
160
180
Temperaure (ºC)
◦
Reaction condition: EC = 25 mmol, MeOH = 200 mmol, T = 180 C, time = 4 h, PCO2
=
Fig. 6. Effect of reaction temperature on the synthesis of DMC from EC and
methanol (reaction condition: EC = 25 mmol, MeOH = 200 mmol, catalyst = 0.2 g TEA-
MS41, reaction time = 4 h, PCO2 = 1.17 MPa).
1
.17 MPa.
a
TON = (mole of DMC)/(mole of quaternary ammonium in QCl-MS41).

5
60
D.-W. Kim et al. / Catalysis Today 164 (2011) 556–560
Table 4
Effect of pressure on the synthesis of DMC from EC and methanol.
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Table 5
EC conversion and yield of DMC and EG for recycle test using THA-MS41.
Run
EC conversion (%)
DMC yield (%)
EG yield (%)
TONa
1
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1
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st (0.2 g)
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◦
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Quaternary ammonium salts were immobilized on
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(
and FT-IR analyses confirmed successful immobilization of the
quaternary ammonium salts on mesoporous MCM-41. In the syn-
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ammonium salts on MCM-41 (QCl-MS41) showed good catalytic
activity without using any solvent. The quaternary ammonium
salt with longer alkyl chains showed better catalytic activity.
High temperature, long reaction time, and high CO2 pressure
also increased EC conversion and DMC and EG yield. However,
excessive pressure was unfavorable for catalytic activity. The
QCl-MS41 can be easily recovered and reused with only a slight
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Acknowledgements
1
This study was supported by the Ministry of Knowledge and
Economy of Korea through Wide Economic Region Cooperative
Program, KBSI and the Brain Korea 21 Project.
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