Ionic liquid immobilized on mesocellular silica foam as an efficient heterogeneous catalyst for the synthesis of dimethyl carbonate via transesterification

Article abstract of DOI:10.1016/j.apcata.2013.06.016

Functionalized basic ionic liquid, 1-(triethoxysilyl) propyl-3- methylimidazolium hydroxide ([SmIm]OH) was immobilized on the mesocellular silica foams (MCF) material and then applied in the catalytic synthesis of dimethyl carbonate (DMC) via transesterification of ethylene carbonate (EC) with methanol (MeOH). Combined characterization results of FT-IR, ²⁹Si NMR, N₂ adsorption-desorption, and TEM suggest that [SmIm]OH has been successfully grafted on the MCF material, and the three-dimensional mesoporosity with large pore size (～34 nm) of MCF has been well retained during the immobilization procedure. The influences of reaction temperature, time, and the molar ratio of MeOH/EC have also been investigated in detail. The catalytic tests demonstrated that MCF-[SmIm]OH could catalyze the transesterification of EC and MeOH with high efficiency, allowing a stable and high DMC yield as much as 83% under mild reaction conditions, superior to those results of the heterogeneous catalysts reported previously.

Full text of DOI:10.1016/j.apcata.2013.06.016

Applied Catalysis A: General 464–465 (2013) 357–363
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Ionic liquid immobilized on mesocellular silica foam as an efficient
heterogeneous catalyst for the synthesis of dimethyl carbonate
via transesterification
Jie Xu , Hai-Tao Wu , Cheng-Ming Ma , Bing Xue , Yong-Xin Li , Yong Caob
a
a
a
a
a,∗
a
Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, College of Chemistry and Chemical Engineering, Changzhou University, Gehu
Road 1, Changzhou, Jiangsu 213164, PR China
b
Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University, Shanghai 200433, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Functionalized basic ionic liquid, 1-(triethoxysilyl) propyl-3-methylimidazolium hydroxide ([SmIm]OH)
was immobilized on the mesocellular silica foams (MCF) material and then applied in the catalytic synthe-
sis of dimethyl carbonate (DMC) via transesterification of ethylene carbonate (EC) with methanol (MeOH).
Received 7 April 2013
Received in revised form 6 June 2013
Accepted 15 June 2013
Available online xxx
29
Combined characterization results of FT-IR, Si NMR, N2 adsorption–desorption, and TEM suggest that
[SmIm]OH has been successfully grafted on the MCF material, and the three-dimensional mesoporosity
with large pore size (∼34 nm) of MCF has been well retained during the immobilization procedure. The
influences of reaction temperature, time, and the molar ratio of MeOH/EC have also been investigated in
detail. The catalytic tests demonstrated that MCF-[SmIm]OH could catalyze the transesterification of EC
and MeOH with high efficiency, allowing a stable and high DMC yield as much as 83% under mild reaction
conditions, superior to those results of the heterogeneous catalysts reported previously.
© 2013 Elsevier B.V. All rights reserved.
Keywords:
Dimethyl carbonate
Ethylene carbonate
Transesterification
Ionic liquid
Mesocellular silica foam
1
. Introduction
Owing to its high oxygen content, low toxicity, and excel-
(Scheme 1) for the production of DMC [10–12], since the manu-
facturing techniques of EC and PC have been well established [8].
Moreover, the co-products obtained in the transesterification, i.e.
ethylene glycol (EG) and propylene glycol (PG), are also impor-
tant industrial reagents. A variety of heterogeneous catalysts, such
as basic metal oxides [13,14], metal cyanide [15], alkali-metal
hydroxide [16], basic anion-exchange resin [17], hydrotalcite [18],
dawsonite [19,20], smectite [4], and polymers [10], have been
developed for the process. Unfortunately, most catalytic systems
suffer from low catalytic activity, and thus harsh reaction condi-
tions, e.g. high temperature and long time, are frequently required
to obtain a high EC yield [5,11]. In this context, it is imperative to
develop a new catalyst that can allow efficient and stable activity
for the transesterification of EC with MeOH to DMC.
Over the past decade, because of their negligible vapor pres-
sure and excellent thermal stability, the use of ionic liquids (ILs)
as environmentally benign media for catalytic process has been
widely recognized and accepted [21,22]. In the transesterification
of EC/PC with MeOH, it is reported that ILs can catalyze the reac-
tion with extremely high activity [23]. Nevertheless, the critical
problem associated with the use of IL lies in the difficulty in sepa-
rating as well as recovering the IL catalysts [24]. To circumvent the
problem, one of practical and economical approaches is to immo-
bilize IL onto solid support. Recently, Park et al. have successfully
immobilized ILs on amorphous silica [1] and MCM-41 [11] and sub-
sequently employed them as recyclable catalysts for the synthesis
lent biodegradability, dimethyl carbonate (DMC) has been used
extensively as an environmentally friendly building block in the
synthesis of pharmaceutical and agricultural chemicals [1–3]. DMC
is also a promising substitute for highly toxic phosgene and con-
ventional dimethyl sulfate in the carbonylation and methylation
reactions [4,5]. The industrial manufacture of DMC is mainly based
on the phosgenation of methanol and oxidative carbonylation of
methanol, which involves high-risk compounds including COCl2
and CO [6,7]. Recently, the utilization of CO2 as an economical
C1 building block to synthesize valuable chemicals has attracted
considerable attention [8,9]. Therein, DMC can be produced via one-
step reaction from CO2 and methanol, or one-pot synthesis from
CO , methanol, and epoxides. However, due to the high inertness of
CO , the DMC yield of the first process is relatively low. Likewise, the
2
2
one-pot process undergoes several undesired side reactions which
inevitably result in a poor selectivity to DMC [5,8,10].
The transesterification of cyclic carbonate (e.g. ethylene car-
bonate (EC) or propylene carbonate (PC)) with methanol (MeOH)
has been regarded as a suitable and sustainable synthetic pathway
∗ Corresponding author. Tel.: +86 519 86330135; fax: +86 519 86330135.
E-mail addresses: liyxluck@126.com, liyxluck@163.com (Y.-X. Li).
0
926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.06.016

3
58
J. Xu et al. / Applied Catalysis A: General 464–465 (2013) 357–363
◦
O
O
toluene, and the mixture was refluxed at 90 C for 24 h under argon.
Afterwards, the white solid was filtered and washed with toluene
(30 mL) for three times to remove excessive [SmIm]Cl. The resultant
O
O
HO
HO
(a)
(b)
O
O
O
O
+ 2 H
3
C OH
H
3
C
CH
3
+
+
OH
OH
O
O
O
O
◦
solid was then dried at 70 C under vacuum and designated as MCF-
SmIm]Cl. MCF-[SmIm]OH was synthesized from MCF-[SmIm]Cl
via ion-exchange process. In brief, 1 g of MCF-[SmIm]Cl was added
into 100 mL of tetramethylammonium hydroxide/MeOH solution
[
+
2
H
C OH
H C
CH3
3
3
CH3
−1
(0.2 mol L
) and the dispersion was stirred vigorously for 15 min.
H C
3
Then, the white solid was filtered and washed with MeOH (100 mL)
for several times until the pH value of the methanolic filtrate
reached ca. 7.0.
Scheme 1. Transesterification of EC/PC with MeOH to DMC and EG/PG.
of DMC via the transesterification of EC with MeOH. However, con-
cerning the large molecular sizes of immobilized ILs, the natures
in terms of rigid crossing-linking of amorphous silica and narrow
mesopores (3–5 nm) of MCM-41 material may restrain the immobi-
lization amount of ILs as well as the diffusion of reactants/products
in the pore of silica materials [25].
2
.2. Samples characterization
◦
Nitrogen adsorption–desorption at −196 C was measured using
a Micromeritics ASAP 2010C after the samples were degassed
−
2
◦
(
1.33 × 10 Pa) at 200 C overnight. The specific surface area (SBET)
was calculated using the Brunauer–Emmett–Teller (BET) method.
The pore diameters (Dpore) were determined from the adsorption
branches. The pore size distributions (PSDs) of MCF and SBA-15
derived materials were determined by a modified Broekhoff de Boer
method and Barret–Joyner–Halenda method, respectively.
Unlike MCM-41 and SBA-15 which have 2D mesoporous struc-
tures and moderate pore size (3–10 nm), mesocellular silica foam
(
MCF) materials possess hydrothermally stable 3D mesostructures
with ultra-large mesopores (20–50 nm) and pore volumes (up
3
−1
to 2 cm g ) [26,27]. In this sense, MCF materials have obvious
advantages in terms of better diffusion of reactant and product,
thereby allowing them to overcome internal mass transfer limi-
tations [28,29]. Despite the important potential uses in catalytic
applications, to the best of our knowledge, there are few reports
dealing with the immobilization of ILs with MCF. In the present
work, we have made the first attempt in the development of a new
efficient MCF supported IL (MCF-IL) catalyst, featuring an ultra-
large mesoporous structure. The MCF-IL catalyst exhibited very
high DMC productivity for the catalytic transesterification of EC
with MeOH.
Transmission electron microscopy (TEM) analysis was carried
out with a JEOL 2010 electron microscope operating at 200 kV.
Before being transferred into the TEM chamber, the samples dis-
persed in ethanol were deposited onto a holey carbon-coated
copper grid and then quickly moved into the vacuum evaporator.
Fourier transform infrared (FT-IR) spectra of the samples were
measured on a Bruker Tensor spectrometer via the usual KBr pel-
let technique. These IR spectra were normalized using the band at
−
1
1
079 cm as an internal reference band, which is usually assigned
to the asymmetric vibration of the Si Si bond.
O
Thermal gravimetric (TG) measurements of the catalysts
were conducted on a Perkin-Elmer TGA 7 analyzer. The sam-
ple was placed in an ␣-Al O cumber and heated in flowing
air (50 mL min ) from room temperature to 850 C at a rate of
2
2
2
2
. Experimental
2
3
−
1
◦
.1. Catalyst preparation
◦
2
−1
0 C min .
9
Si solid-state NMR experiments were performed on a Bruker
.1.1. Preparation of ILs
DSX300 spectrometer with a frequency of 59.63 MHz, a recycling
delay of 600 s, a radiation frequency intensity of 62.5 kHz, and a
reference sample of [(CH ) SiO] Si O12.
3
g
of 3-choropropyltriethoxysilane and 1 g of N-
◦
methylimidazole were mixed and refluxed at 80 C under N2
atmosphere for 48 h. The resultant mixture was purified by
washing three times with ether to remove unreacted impu-
rities. Then, the solution was dried under vacuum to remove
excess ether. The finally functionalized IL, 1-(triethoxysilyl)
propyl-3-methylimidazolium chloride was denoted as [SmIm]Cl.
3
3
8
8
2.3. Catalytic test
Catalytic tests for the transesterification of EC with MeOH were
conducted in a two-neck round bottomed flask (50 mL). MeOH and
EC (a total volume of 15 mL) were mixed well. 0.5 g of immobi-
lized IL catalyst was added into the mixture and the tests were
2.1.2. Preparation of the MCF materials
The MCF material was prepared as described previously using
◦
performed at 65 C for 5 h. After the reaction, the mixture was cen-
Pluronic P123 triblock copolymer (EO20PO70EO20, Mav = 5800,
Aldrich) as a template and 1,3,5-trimethylbenzene (TMB) as a
swelling agent [26]. Briefly, 4.0 g of Pluronic P123 was dissolved
trifuged and analyzed by GC–MS. The filtered catalyst was washed
with methanol (50 mL) for two times, dried overnight, and then
investigated for its next running. The turnover frequency (TOF) for
each catalyst was calculated as follows:
−1
in 150 mL of 1.6 mol L HCl solution. Then 4.0 g of TMB was added,
◦
followed by elevation of the reaction temperature to 40 C. After
mEC,converted
Wcatal. × t
nEC × Cov.%(EC) × M
Wcatal. × t
EC
that, 8.8 g of TEOS was added, and the solution was further stirred at
r =
=
◦
4
0 C for 24 h. The milky solution was transferred into an autoclave
◦
◦
and aged at 130 C for 24 h. The white precipitates were filtered off
where nEC, MEC, t, and Wcatal. are the molar amount (mol), formula
−1
and dried at 80 C under vacuum overnight. Finally, the P123 was
weight (g mol ) of EC, reaction time (h), and the mass of overall
◦
removed by Soxhlet extraction with ethanol at 120 C for 48 h. For
catalyst (g), respectively.
the sake of comparison, SBA-15 material was also fabricated and
the detailed preparation was supplied in the supporting materials.
3. Results and discussions
2
.1.3. Preparation of ILs immobilized on MCF
MCF was activated at 150 C under vacuum for 4 h to remove
3.1. Catalyst characterization
◦
physisorbed water. Next, 5.0 g of MCF was added into the previously
well-mixed solution containing 1.6 g of [SmIm]Cl and 100 mL of dry
The porous nature of MCF and MCF-IL samples was investi-
gated by N2 adsorption–desorption technique. The parent MCF

J. Xu et al. / Applied Catalysis A: General 464–465 (2013) 357–363
Fig. 1. N2 adsorption–desorption isotherms and the PSD (inset). (a) MCF, MCF-[SmIm]Cl, and MCF-[SmIm]OH; (b) SBA-15, and SBA-15-[SmIm]OH.
Fig. 2. TEM images of MCF (a), MCF-[SmIm]OH (b), and SBA-15-[SmIm]OH (c).
359
material displays type IV curves with a steep H1 hysteresis loop
at p/p = 0.85–0.95 (Fig. 1a), indicating that MCF possesses typical
The mesoporous structures of the two siliceous materials and
◦
their supported IL catalysts were further investigated by TEM. The
TEM image (Fig. 2a) of MCF reveals 3D mesoporosity with uniform-
sized spherical cells (30–40 nm) interconnected by windows, which
is the characteristic structure of the MCF material [30,31]. As for
MCF-[SmIm]OH, it can be found the foam-like morphology of the
parent material has been well retained (Fig. 2b), proving that the
mesostructure of MCF has not been destroyed during the immobi-
lization procedure. By comparison, SBA-15 and SBA-15-[SmIm]OH
present a 2D hexagonal mesostructure with pore sizes of 8–9 nm
(Figs. 2c and S1). Comparing MCF and SBA-15 in terms of their TEM
images along with the textural information mentioned above, it can
be learnt that MCF, featuring 3D mesostructure and an ultra-large
pore size, may promise a distinct advantage in mass transfer for
heterogeneous reactions.
mesoporous structure with a large pore size [29]. Correspondingly,
the inset of Fig. 1a shows MCF has a narrow PSD centered at 35.2 nm.
After the immobilization of ILs, the pore volume and surface area of
MCF decrease appreciably (Table 1). Despite this, steep H1 hystere-
sis loops can be still observed. This suggests that the mesoporous
structure of MCF has been well reserved. For SBA-15, the material
also presents type IV isothermal curves (Fig. 1b) whereas the hys-
◦
teresis loop is only limited in the medium p/p of 0.65–0.8, showing
the pore size of SBA-15 is much lower than that of MCF. After the
introduction of [SmIm]OH, the pore volume and surface area of
3
−1
the parent SBA-15 material drastically decrease to 0.51 cm g and
2
2
−1
53 m g (Table 1), respectively, implying that some mesopores
of SBA-15 have been blocked after the introduction of [SmIm]OH.
Fig. 3 displays FT-IR spectra of the MCF and MCF-IL samples. All
−
1
spectra exhibit sharp bands at 465, 802, and 1107 cm . Accord-
ing to the literature [25,32], the three bands are attributed to the
Table 1
Textural properties of mesoporous silica materials and their supported catalysts.
condensed silica (Si
O Si) network. Besides, a shoulder band at
−
1
SBET (m2
−1
Dpore a (nm)
Vpore (cm3
−1
)
970 cm can also be detected in the three spectra, corresponding
to the stretching vibrations of Si OH bonds [25,31]. Obviously,
the intensity of the band becomes weaker after the immobiliza-
tion of ILs. Simultaneously, in comparison with MCF, MCF-[SmIm]Cl
Sample
g
)
g
MCF
375
279
284
581
253
35.2
34.4
34.5
9.9
2.17
1.45
1.93
1.11
0.51
MCF-[SmIm]Cl
MCF-[SmIm]OH
SBA-15
and MCF-[SmIm]OH show several new bands at 3150, 3100, and
SBA-15-[SmIm]OH
8.6
−1
1
570 and 1461 cm , which are attributed to the C H stretching
a
Determined according to the adsorption branch.
[33], and C H bending vibrations, respectively. This demonstrates

3
60
J. Xu et al. / Applied Catalysis A: General 464–465 (2013) 357–363
00
1
Si-O-Si
(a)
9
0
0
W = 1.6%
(H-O-H)
8
(c)
(
(
(
c)
b)
a)
(-CH2)
70
W = 25.2%
(-CH2)
60
50
40
30
20
10
0
(b)
(H-O-H)
0
.2
(Si-OH)
W = 90.4%
1
00
200
300
400
500
600
700
800
3500
2000
1500
1
1000
500
Temperature (°C)
Wavenumber (cm )
Fig. 5. TG curves of MCF (a), [SmIm]OH (b), and MCF-[SmIm]OH (c).
Fig. 3. FT-IR spectra of MCF (a), MCF-[SmIm]Cl (b), and MCF-[SmIm]OH (c). The ꢀ,
ı, and ꢁ indicate the vibrations of stretching, bending, and rocking, respectively.
of [SmIm]OH, the TG profile of MCF-[SmIm]OH also shows grad-
ual weight losses at 250–750 C (ꢂW = 25.2 wt%). Compared with
◦
the nominally immobilization amount of MCF-[SmIm]OH (32 wt%;
note: the nominal amount is calculated by WIL/WMCF while the
weight loss of TG analysis is based on the total catalyst.), it can
be concluded that most [SmIm]OH species have been loaded on
the MCF support. This is also indirect evidence of the success of
immobilization of the IL on MCF.
that the terminal Si OH groups located on the parent MCF mate-
29
rial have been condensed with the functionalized IL. Si NMR is
a powerful technique for studying the chemical environment of
Si species for the silica-based materials. Three distinct resonances
(
Qn = Si(OSi)n(OH)4 , n = 2–4), originating from different siloxanes,
−n
are presented for the parent MCF material (Fig. 4). After the immo-
bilization of ILs, the Q3 peak diminishes drastically, implying that
a majority of Si OH species in MCF have been coupled with the ILs
3.2. Catalyst activity
[
25,33]. Combing the information of 29Si NMR and FT-IR spectra,
The catalytic performances of the siliceous supports IL-derived
it can be speculated that the [SmIm]Cl and [SmIm]OH have been
successfully grafted on the host system of MCF.
Furthermore, TG analysis was employed to probe the weight-
loss profiles of the MCF and IL-based samples (Fig. 5). The TG curve
catalysts are listed in Table 2. Both of the two functionalized ILs
exhibit high catalytic activity for the catalytic transesterification of
EC with MeOH. Notably, the EC conversion of [SmIm]OH is higher
than that of [SmIm]Cl. Since the transesterification of EC with MeOH
is an equilibrium reaction [19], the phenomenon could be explained
of the bare MCF support presents a minor weight loss in the range of
◦
8
0–150 C, which is attributed to the release of physisorbed water.
−
−
by taking into account the higher basic intensity of OH than Cl ,
which is beneficial to the thermodynamic equilibrium of the trans-
esterification toward DMC production. In sharp contrast, the bare
MCF powder gives a poor EC conversion, showing that the pure
silica material is almost inactive to the transesterification reaction.
After the immobilization with ILs, the two heterogeneous catalysts,
i.e. MCF-[SmIm]Cl and MCF-[SmIm]OH, demonstrate remarkably
higher EC conversions than the bare MCF powder. More interest-
ingly, the two conversions are close to the values acquired over
those corresponding [SmIm]Cl and [SmIm]OH samples. Bearing in
the mind that the nominal weight of the immobilized IL over MCF
As for [SmIm]OH, a significant weight loss, contributing 90.4 wt%
◦
of the sample, can be detected from 50 to 850 C. It is clear that the
large loss originates from the decomposition and oxidation of the
organic groups of the [SmIm]OH molecules and the residual weight
of 9.6 wt% could be attributed to the silica which results from the
oxidation of the Si element of [SmIm]OH. In comparison with that
Q4
(
0.5 g × 32%) is the same to the pure IL (0.16 g, Table 2), in this sense,
Q3
it is reasonable to conclude that the IL is the essentially catalytic
component while the MCF powder mainly plays a role as a catalytic
support.
2
0
(a)
(b)
(c)
Table 2
Catalytic performances of the pure MCF and IL catalysts.
Catalyst
Conv. (%)
Sel. (%)
DMC yield (%)
MCFa
MCF-[SmIm]Cl
MCF-[SmIm]OH
SBA-15-[SmIm]OH
<5
–
–
a
37.1
85.0
68.3
43.6
89.4
69.8
97.5
96.9
71.4
96.3
25.9
82.9
66.2
31.1
86.1
a
a
b
[
[
SmIm]Cl
SmIm]OH
b
0
-50
-100
-150
-200
a
◦
Reaction
condition:
T = 65 C,
t = 5 h, VEC+MeOH = 15 mL,
t = 5 h, VEC+MeOH = 15 mL,
Chemical shift (ppm)
nMeOH:nEC = 10:1, Wcatal. = 0.5 g.
b
◦
Reaction
condition:
T = 65 C,
Fig. 4. NMR spectra of MCF (a), MCF-[SmIm]Cl (b), and MCF-[SmIm]OH (c).
nMeOH:nEC = 10:1, Wcatal. = 0.16 g.

J. Xu et al. / Applied Catalysis A: General 464–465 (2013) 357–363
361
1
00
100
100
80
60
40
20
0
(
a)
(b)
9
8
7
6
5
4
3
2
0
0
0
0
0
0
0
0
80
60
40
20
0
Conv.
Sel.
Conv.
Sel.
Conv.
Sel.
Conv(2).
35 40 45 50 55 60 65
6:1
10:1
12:1
14:1
Temperature (°C)
nMeOH/nEC
1
2
3
4
5
6
7
t (h)
Fig. 7. Effects of the reaction temperature (a) and the nMeOH/nEC ratio (b) on the
catalytic performance of MCF-[SmIm]OH.
Fig. 6. Effect of the reaction time on the transesterification reaction.
nMeOH/nEC is therefore usually required to achieve much higher
Also, the influence of the immobilization amount of IL on the
catalytic behaviors has been investigated. Table S1 reveals that
with the increase of the immobilization amount, the catalytic activ-
ity increases gradually. However, as the immobilization amount
is elevated from 32% to 40%, the EC conversion increases slowly.
In view of the cost of catalyst as well as a potential blockage of
mesopores (Table S1) under excessive immobilization amounts, the
MCF-[SmIm]OH-32 wt% sample was chosen as the optimal catalyst
for further study.
Besides MCF, SBA-15 has also been utilized for the catalytic sup-
port for [SmIm]OH and its corresponding catalytic performance
was evaluated under identical reaction conditions. As listed in
Table 2, the EC conversion for SBA-15-[SmIm]OH is only 68.3%,
inferior to the value achieved for MCF-[SmIm]OH. Given the vari-
ations of textural parameters (Table 1) and TEM images (Fig. 2) of
MCF and SBA-15 materials before and after the introduction of IL,
it is reasonable to infer that the well retained 3D mesostructure
of MCF-[SmIm]OH ensures benign mass transfer in the heteroge-
neous catalysis, consequently enhancing its catalytic performance
for the transesterification reaction.
conversion [6,19]. However, taking into account the TOF value, the
−1
catalytic activity under the nMeOH/nEC of 10:1 (0.96 h ) is appre-
−1
ciably higher than those under higher ratios (0.80–0.87 h ). This
is mainly because an excessive amount of MeOH would impede the
adsorption of EC on the surface of catalyst, thus undermining cat-
alytic efficiency. Considering the results above, a temperature of
◦
6
5 C and nMeOH/nEC of 10:1 are the optimal reaction conditions for
achieving the highest catalytic activity.
Besides the reaction temperature and time, the recycling capa-
bility is also of importance for the evaluation of a heterogeneous
catalyst. Concerning this, a series of repetitive experiments have
been conducted to test the reusability of the catalyst and repro-
ducibility of catalytic behavior. As shown in Fig. 8, the selectivity
in each cycle is above 96%. For the conversion, after four runs, a
minor decline contributing ca. 10% is observed, probably because
the loss of partial catalyst along the continuous separation process.
Despite the minor drop, the fifth test provides a relatively high EC
conversion as much as 76%. Apparently, the stable performance in
the consecutive tests is owing to the solid immobilization on MCF,
as confirmed by FT-IR and NMR characterization.
Fig. 6 shows the catalytic behavior of the transesterification
reaction as a function of the reaction time. At the first 1 h, the cat-
alytic reaction proceeds with a moderate EC conversion of 62%.
Upon prolonging the reaction to 5 h, the conversion reaches the
maximum. Meanwhile, no distinct fluctuation of the selectivity has
been observed; all values are over 90% within 5 h. To probe a poten-
tial leaching problem for this heterogeneously catalytic system,
MCF-[SmIm]OH was removed at 2 h. As shown in Fig. 6, the resul-
tant conversions (Conv(2)) collected after 2 h show no apparent
increment after the removal of the catalyst. The tentative evalu-
ation excludes the possibility of the considerable leaching of the
grafted IL on MCF.
In general, the transesterification is a typical nucleophilic addi-
tion reaction, which can be catalyzed by both acid and base. To
elucidate the catalytically active sites in the transesterification
1
00
80
60
Conv.
Sel.
To gain insight into the effect of reaction conditions on the
catalytic performance of MCF-[SmIm]OH, the variations of the cat-
alytic conversions and selectivities at different temperatures were
studied and the results are depicted in Fig. 7a. As the reaction
temperature increases, the catalytic conversions increase drasti-
cally while the selectivities are all above 88%. The byproduct is
mainly ethyl methyl carbonate (EMC), probably originating from
the incomplete transesterification. Moreover, the dependence of
DMC synthesis on the molar ratios of MeOH/EC (nMeOH/nEC) was
also studied (Fig. 7b). Upon elevating the nMeOH/nEC ratio from 6 to
4
0
0
0
2
1
2
3
4
5
Running cycles
1
4, the EC conversion increases progressively. As discussed above,
the transesterification of EC with MeOH is an equilibrium reac-
tion. Accordingly, a molar ratio above the stoichiometric ratio of
Fig. 8. Evolution of the catalytic performance during five consecutive tests over
MCF-[SmIm]OH (reaction conditions: t = 5 h, Wcatal. = 0.5 g, nMeOH/nEC = 10:1).

3
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J. Xu et al. / Applied Catalysis A: General 464–465 (2013) 357–363
O
O
O
Si
O
N
N
OH
Si
O
N
N
+
H O
2
O
CH OH
CH O
3
3
HO
O
O
OH
O
CH O
3
O
O
O
CH OH
3
HO
H C
O
3
O
O
H C
CH3
3
O
O
CH OH
3
O
H C
OH +
3
CH O
O
O
3
Scheme 2. Proposed mechanism for the synthesis of DMC via the transesterification of EC and MeOH catalyzed by MCF-[SmIm]OH.
of EC with MeOH over MCF-[SmIm]OH, a possible mechanism
has been proposed and depicted in Scheme 2 based on the pub-
lished reports [34,35]. First, MeOH is activated by imidazolium
Table 3 summarizes the performances of different catalysts
for the transesterification of EC with MeOH; the information has
almost covered all the results to date. With respect to the yield
of DMC, there is no remarkable discrepancy between the IL-based
catalysts (entries 1–5, and 11). However, it is worth noting that
the reaction reported in this work proceeded under atmospheric
pressure, while the entries 1–3 and entry 4 were performed either
under high pressure of CO2 or assisted by microwave heating,
respectively. Additionally, the performances of other heteroge-
neous catalysts, such as the polymer and metal oxides (entries
6–10), have also been listed in Table 3. Taking into account the
corresponding DMC yields together with the reaction conditions,
it can be found that the present MCF-[SmIm]OH catalyst is more
efficient than other heterogeneous catalysts reported previously in
the catalytic transesterification reaction. This may be related to the
high surface area along with the rich mesopores of MCF-[SmIm]OH,
which render more catalytic active sites as well as benign mass
transfer. At this juncture, the comparison in Table 3 strongly sug-
gests that the present MCF supported [SmIm]OH catalyst can serve
as a promising catalyst for the synthesis of DMC via transesterifi-
cation of EC with MeOH.
−
−
via a hydrogen bond. The OH (or Cl ) anion as a nucleophilic
−
reagent attacks MeOH, forming H O (or HCl) and CH O . After
2
3
−
that, the CH O anion attacks the carbonyl C of the EC through
3
−
nucleophilic addition and thus CH O CO
O
CH2 CH2
O
(inter-
3
mediate 1) is generated. Subsequently, the MeOH and intermediate
−
1
molecules undergo a proton exchange, yielding CH O and
3
CH O CO
O CH2 CH2 OH (2-hydroxyethyl methyl carbonate,
3
intermediate 2), which is the monoester product. The as-produced
−
CH O further reacts with intermediate 2 via nucleophilic sub-
3
−
stitution and then DMC and HO CH CH2
are formed. Finally, EG and CH O are generated by a proton
O
(intermediate 3)
2
−
3
exchange between CH OH and intermediate 3. It is worthy noting
3
that the step of the formation of DMC (nucleophilic substitution)
is much more difficult than the step of the formation of interme-
diate 1 (nucleophilic addition,). As a result, the IL catalyst with
less basic anion, e.g. [SmIm]Cl and MCF-[SmIm]Cl, could give more
monoester product, namely lower selectivity to the target DMC,
and low catalytic conversion simultaneously.
Table 3
Comparison of the performances of different catalysts for the transesterification of EC with MeOH to DMC.
◦
Entry
Catalyst
Amount of EC (mmol)
nMeOH/nEC
Temperature ( C)
Time (h)
DMC yield (%)
Ref.
a
[BmIm]Cle
IL on MCM-41
1
25
25
25
25
10
25
30
N.A
N.A.
25
32
5:1
8:1
8:1
8:1
15:1
8:1
8:1
4:1
4:1
8:1
140
180
160
120
70
140
65
150
70
6
4
6
0.25
6
70
76
70
57
81
50
70
60
60
54
83
[23]
[11]
[1]
a
f
2
a
g
3
IL on amorphous silica
b
[BmIm]Cle
4
[6]
5
6
DABCO-derived IL
Poly-4-vinyl pyridine
Binary Zn–Y oxide
[BmIm][BF4]/MgO–CeO2
Na-dawsonite
[35]
[10]
[5]
[36]
[19]
[4]
c
4
7
8
2.5
N.A
6
4
5
d
9
0
1
c
1
1
Smectite containing Mg–Na
MCF-[SmIm]OH
150
65
10:1
This work
a
b
c
d
e
f
Carried out in an autoclave under CO2 atmosphere (1.0–1.2 MPa).
Microwave (100 W) heating.
Carried out in an autoclave.
Carried out in a fixed bed reactor (vertical tube).
[BmIm]Cl:1-n-butyl-3-methylimidazolium.
IL: 1-(triethoxysilyl) propyl-tri-n-hexylammonium chloride.
IL: 1-(triethoxysilyl) propyl-3-n-butylimidazolium bromide.
g

J. Xu et al. / Applied Catalysis A: General 464–465 (2013) 357–363
363
4
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4
grafted on the MCF material and the MCF-[SmIm]OH catalyst
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tion time, temperature and the ratio of nMeOH/nEC on the catalytic
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