Efficient synthesis of dimethyl carbonate via transesterification of ethylene carbonate over a new mesoporous ceria catalyst

Article abstract of DOI:10.1016/j.apcata.2014.07.009

Mesoporous ceria materials (CeO₂-meso) have been prepared through a soft-templating method using cetyltrimethylammonium bromide as a template and cerium nitrate as a precursor. The synthesized CeO₂-meso materials possess narrow pore size distributions of 5.1-5.4 nm and tunable surface areas (109-182 m² g^-1). As heterogeneous catalysts in the transesterification of ethylene carbonate (EC) with methanol to dimethyl carbonate (DMC), CeO₂-meso materials demonstrate superior catalytic performance to the commercial ceria. N₂ adsorption-desorption and CO₂-TPD characterization results indicate that the catalytic activity obtained over various CeO₂-meso samples depends on their surface areas and basicity. The highest activity is achieved over CeO ₂-meso-400, affording a DMC yield as much as 73.3%, together with excellent recycling ability. Besides the transesterification of EC with methanol, CeO₂-meso is found to be able to catalyze the reactions of other cyclic carbonates and alcohols. In view of the high catalytic performance along with the convenience in catalyst preparation, CeO₂-meso-400 compares favorably with the ionic liquids as well as other ceria-based catalytic systems.

Full text of DOI:10.1016/j.apcata.2014.07.009

Applied Catalysis A: General 484 (2014) 1–7
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Efficient synthesis of dimethyl carbonate via transesterification of
ethylene carbonate over a new mesoporous ceria catalyst
Jie Xu , Kai-Zhou Long , Fei Wu , 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:
Received 9 May 2014
Received in revised form 21 June 2014
Accepted 6 July 2014
Available online 11 July 2014
Mesoporous ceria materials (CeO -meso) have been prepared through a soft-templating method using
cetyltrimethylammonium bromide as a template and cerium nitrate as a precursor. The synthesized CeO2-
2
meso materials possess narrow pore size distributions of 5.1–5.4 nm and tunable surface areas (109–182
2
−1
m g ). As heterogeneous catalysts in the transesterification of ethylene carbonate (EC) with methanol
to dimethyl carbonate (DMC), CeO2-meso materials demonstrate superior catalytic performance to the
commercial ceria. N2 adsorption–desorption and CO2-TPD characterization results indicate that the cat-
alytic activity obtained over various CeO2-meso samples depends on their surface areas and basicity. The
highest activity is achieved over CeO2-meso-400, affording a DMC yield as much as 73.3%, together with
excellent recycling ability. Besides the transesterification of EC with methanol, CeO2-meso is found to
be able to catalyze the reactions of other cyclic carbonates and alcohols. In view of the high catalytic
performance along with the convenience in catalyst preparation, CeO2-meso-400 compares favorably
with the ionic liquids as well as other ceria-based catalytic systems.
Keywords:
Dimethyl carbonate
Ethylene carbonate
Transesterification
Ceria
©
2014 Elsevier B.V. All rights reserved.
1
. Introduction
production of DMC (Scheme S1) [10,12], since cyclic carbonates can
be synthesized in a quantitative yield via cycloaddition of CO2 to
epoxides [10]. Furthermore, the co-products obtained in the trans-
esterification, i.e. ethylene glycol (EG) and propylene glycol (PG),
are also important industrial reagents [13]. A wide range of hetero-
geneous catalysts, including basic metal oxides [14], alkali-metal
hydroxide [15], anion-exchange resin [16], hydrotalcite [17], daw-
sonite [18], smectite [19], mesoporous carbon nitride [20], etc, have
been developed for the transesterification reactions. Up to now,
the most efficient catalysts proposed for the process have been
confined to ionic liquids (ILs) [4,21]. However, the thorny issue
associated with such ILs is the catalyst–product separation [22].
Although immobilization the ILs onto solid support (e.g. porous
silica materials) can solve this problem, the high cost of siliceous
coupling agents and tedious preparation for the immobilized ILs
still restrict their practical applications [20]. In this context, it is
highly desired to develop a new catalyst that can allow a robust
activity along with easy separation for the transesterification
reactions.
Dimethyl carbonate (DMC) has been used for a variety of appli-
cations in the chemical industries owing to its versatile chemical
reactivity and unique physical properties [1,2]. It is a potential
additive to fuel oil because of its high octane number and rapid
biodegradability [3,4]. More importantly, DMC is an environmen-
tally friendly and promising alternative to highly toxic phosgene
and conventional dimethyl sulfate in carbonylation and methyla-
tion reactions, respectively [5,6]. Traditionally, DMC is synthesized
via the phosgenation or oxidative carbonylation of CH OH, which
involves high-risk compounds including COCl and CO [7,8]. To cir-
cumvent this issue, much effort has been devoted to the direct
manufacture of DMC from CO2 and CH OH in the presence of
3
2
3
organometallic complexes, inorganic bases, etc. Unfortunately, due
to the high inertness of CO2 as well as the thermodynamic limita-
tions, the DMC yield of the process is relatively low [9–11].
The transesterification of cyclic carbonate (e.g. ethylene car-
bonate (EC) or propylene carbonate (PC)) with CH OH has been
3
regarded as a clean and sustainable synthetic route for the
Ceria is one of the most important rare earth oxides. As a key
redox component, ceria has been extensively used in for many
catalytic processes [23], such as three-way-catalyst for exhaust
treatment, low-temperature CO oxidation [24], and water-gas shift
[25]. Moreover, ceria is also a typical Lewis-base catalyst, which is
∗ Corresponding authors. Tel.: +86 519 86330135.
E-mail addresses: shine6832@163.com (J. Xu), liyxluck@163.com (Y.-X. Li).
http://dx.doi.org/10.1016/j.apcata.2014.07.009
0
926-860X/© 2014 Elsevier B.V. All rights reserved.

2
J. Xu et al. / Applied Catalysis A: General 484 (2014) 1–7
responsible for its application in several base-catalyzing processes.
Tomishige et al. [26] have reported the utilization of high-surface-
area ceria (40–131 m g ) for the direct synthesis of organic
Transmission electron microscopy (TEM) experiments were
conducted on a JEOL 2010 electron microscope operating at 200 kV.
Before being transferred into the TEM chamber, the samples
dispersed in ethanol were deposited onto holey carbon films sup-
ported on Cu grids.
2
−1
carbonate from the reaction of CO2 with CH OH. Wherein, the cat-
3
alytically active sites were attributed to the (1 1 1) faces of ceria.
Adopting SBA-15 as a hard template through a nanocasting path-
way, Liu et al. [27] have prepared a series of mesoporous ceria
Diffused reflection infrared Fourier transform (DRIFT) spectra
were collected using a Bruker Tensor 27 spectrometer. The samples
2
−1
◦
(
SBET = 135 m g ) catalysts loaded with NaOH/KOH. As a super-
were placed in in-situ cell and pretreated with heating at 200 C for
base, the synthesized mesoporous ceria material showed excellent
1 h under N2 flow.
catalytic performance (DMC yield = 65%) in transesterification of EC
The basicity of the samples was measured by CO2 temperature-
◦
with CH OH at 65 C. Corma et al. [28] also revealed that cerias
programmed desorption (CO -TPD) on a Quantachrome ChemBET-
3
2
◦
loaded with nanosized gold could promote the transesterification
3000 analyzer. A 200 mg of the sample was pretreated at 300 C for
◦
−1
◦
of PC with CH OH to DMC at 140 C, affording a maximum DMC
1 h in dry He flow (30 mL min ), cooled to 50 C, and then exposed
to CO2 for 0.5 h. After purging the sample with He for 0.5 h, the
3
yield of 35%. Also, ceria-based mixed oxides (e.g. MgO-CeO2 [29])
have been reported to be able to catalyze the transesterification of
cyclic carbonates.
◦
TPD data was recorded from 50 to 550 C with a ramping rate of
◦
−1
.
10 C min
The effort above has verified the catalytic capability of ceria for
the transesterification reactions of cyclic carbonates to DMC. In the
continued search for a more effective ceria for the transesterifica-
tion of cyclic carbonates to DMC, there is a definite demand for an
economic and facile method for the preparation of ceria that can
allow efficient activity for the production of DMC. In our previous
work [30], we reported the synthesis of mesoporous ceria materials
using a soft-templating approach, which showed potential catalytic
application for the oxidative dehydrogenation of ethylene benzene
to styrene. Herein, we employ the mesoporous ceria catalyst (CeO2-
meso) as a new catalyst for the transesterification reaction of cyclic
2.3. Catalytic test
The transesterification reactions of EC with CH OH were carried
out in 80 mL stainless steel autoclave equipped with a magnetic
stirrer. 25 mmol of EC and 250 mmol of CH OH were mixed well,
followed by the introduction of 0.1 g of the catalyst. The reactor
3
3
◦
was pressurized with CO2 to 0.6 MPa and heated to 140 C under
stirring for 2 h. After the reaction, the autoclave was cooled down
in ice water and the mixture was centrifuged and analyzed by a
GC equipped with a PEG-2000 capillary column coupled with a FID
detector. The quantity of reagents and products are calculated by
an area-normalization method. The carbon balance was 100 ± 5%. In
carbonates and alcohol. The CeO -meso materials exhibit high and
2
steady activity in the transesterification of EC with CH OH. Based on
3
the characterization results of N2 adsorption–desorption and CO2-
TPD, the high surface area and the abundant basicity are responsible
the transesterification of EC with CH OH, DMC and 2-hydroxyethyl
3
methyl carbonate (HEMC) is the target molecule and by-product,
respectively. The glycol is co-product. The conversion (Conv.) of EC
and selectivity (Sel.) to DMC are calculated as follows:
for its high catalytic activity. Besides EC, CeO -meso could also
2
promote a series of transesterification of other cyclic carbonates.
nEC, fed − n
nDMC
EC, uncoverted
Conv.% =
, and Sel.% =
nEC, fed − nEC, uncoverted
n
EC, fed
2
. Experimental
The filtered catalyst was washed with methanol (50 mL) for two
times, dried overnight, and then investigated for its next running.
The catalytic activity for each catalyst is based on its specific reac-
2.1. Catalyst preparation
−
1
−1
h
Mesoporous ceria materials were synthesized via
a
tion rate (rs, gEC gcatal.
), which is calculated as follows:
template-assisted precipitation method [30,31]. Typically, 4.34 g
mEC,converted
nEC × Conv.%(EC) × M
EC
−
1
Ce(NO ) ·6H O was added into 200 mL of 0.03 mol L
solution
H N(CH ) Br, CTAB),
33
rs =
=
3
3
2
Wcatal. × t
W
× t
catal.
of cetyltrimethylammonium bromide (C
16
3 3
and stirred vigorously for 3 h. Next, a NaOH solution (2 g in 300 mL
of water) was added into the above solution and then stirred
overnight. After that, the mixture was transferred into an auto-
where nEC, MEC, t, and Wcatal. are the molar amount (mol), formula
weight (88 g mol ) of EC, reaction time (h), and the mass of the
catalyst (g), respectively.
−
1
◦
clave and aged at 90 C for 12 h. The as-received yellow precipitate
◦
was filtered and washed with hot water (ca. 80 C) for several
3. Results and discussion
times to remove the residual CTAB. The resultant yellow powder
◦
was dried at 110 C for 6 h and then calcined for 4 h. The resultant
3.1. Catalyst characterization
ceria samples are denoted as CeO -meso-T, where T stands for the
2
calcination temperature.
XRD patterns of the commercial and mesoporous ceria samples
are shown in Fig. 1. All samples present intensive diffraction peaks
at 2ꢁ = 28.5 , 33.1 , 47.46 , and 56.3 , corresponding to the charac-
teristic diffractogram of cubic fluorite structure (JCPDS: 34-0394).
The commercial ceria shows very strong and sharp diffraction
lines. Calculated by Scherrer equation based on the (1 1 1) plane,
◦
◦
◦
◦
2.2. Sample characterization
X-ray diffraction patterns were recorded with
a Rigaku
D/max 2500 PC X-ray diffractometer equipped with a graphite
the crystallite size of CeO -com is ca. 54.7 nm (Table 1). By con-
2
monochromator (40 kV, 40 mA) using Ni-filtered Cu-K␣ radiation
(
trast, the diffraction peaks acquired over CeO -meso materials are
2
ꢀ = 1.5418 A˚ ).
broader and weaker than CeO -com, suggesting that the average
2
Nitrogen adsorption–desorption isotherms were measured at
particle sizes of ceria are rather small. As the calcination temper-
ature increases from 400 to 600 C, the diffraction peaks become
◦
◦
−
196 C using a Micromeritics ASAP 2020 analyzer. Prior to the
−
2
◦
analysis, the samples were degassed (1.33 × 10 Pa) at 150 C for
at least 4 h. The specific surface area was calculated according to the
Brunauer–Emmet–Teller (BET) method, and pore size distribution
was determined by the Barret–Joyner–Halenda method.
sharper, indicating that the crystallinity is improved. Meanwhile,
the corresponding crystallite sizes of cerias, calculated by Scher-
rer’s equation (based on the (1 1 1) plane), increase from 8.8 to
◦
9.5 nm as the annealing temperature is raised from 400 to 600 C.

J. Xu et al. / Applied Catalysis A: General 484 (2014) 1–7
3
Table 1
Textural parameters of ceria samples.
5
00
SBET (m2
−1
Vpore (cm3
−1
)
Dparticle (nm)a
Sample
g
)
Dpore(nm)
g
CeO2-meso-400
CeO2-meso-500
CeO2-meso-600
182
149
109
4
5.41
5.29
5.18
–
0.249
0.226
0.179
<0.01
0.246
8.8
9.3
9.5
54.7
8.9
×
1/10
CeO -com
CeO2-meso-400
2
b
171
5.30
d
a
Calculated based on the (1 1 1) planes.
Spent catalyst after five consecutive runs.
b
c
b
a
porosity, CeO -meso materials possess considerably higher BET
2
surface areas as well as larger pore volumes. With the increase
◦
of calcination temperature from 400 to 600 C, the surface areas
and pore volumes decrease gradually. This is mainly attributed to
the agglomeration of ceria nanoparticles and/or partial shrinkage of
mesostructures occurring at higher calcination temperature, which
has been verified by the corresponding structural values listed in
Table 1.
20
30
40
50
60
70
80
2θ (°)
Fig. 1. XRD patterns of CeO2-meso-T (400–600: a–c) and CeO2-com (d) materials.
The mesoporous structures of ceria materials were further
investigated by TEM and a representative image of CeO -meso-400
2
The XRD information obtained over the mesoporous ceria materi-
als are similar to the mesoporous ceria materials synthesized via
hard templating routes [27,32], and cerias with other shapes [33]
reported previously.
is shown in Fig. 3. The image reveals that the ceria is composed of
nanoparticles, which are 7–9 nm sized, consistent with the crys-
tallite size calculated by XRD characterization results. Moreover,
foam-like mesopores can be also observed in the image, espe-
cially at the edge of the sample. The TEM image indicates that the
Fig. 2A depicts the N2 adsorption–desorption isotherms of
CeO -meso materials with different calcination temperatures. The
mesopores of CeO -meso mainly arise from the nanosized channels
2
2
three samples demonstrate apparent type IV curves with H1
among the nanoparticles.
hysteresis loops at p/p = 0.55–0.85. Meanwhile, in the range of
Furthermore, we have also prepared a ceria sample annealed
at lower temperature (300 C). The N2 adsorption–desorption
0
◦
high pressure, i.e. p/p > 0.90, no evident N adsorption has been
0
2
observed, suggesting that the CeO -meso materials possess exclu-
isotherm and its corresponding textual parameters are provided in
Fig. S1, and Table S1, respectively. Although the surface area of such
2
sive mesostructures. In the case of CeO -com, the quantity of N
2
2
3
−1
2
−1
CeO -meso-300 (192 m g ) is very close to that of CeO -meso-
2 2
adsorbed is very low (<3 cm g
(STP), not shown here), which
means that its pore structures are very poor. The corresponding
pore size distribution (PSD) curves are shown in Fig. 2B. The PSDs
400, the pore size is only 4.3 nm, ca. 20% less than the value acquired
over other CeO -meso samples. As mentioned above, the present
2
for the CeO -meso samples do not differ significantly; all exhibit
mesoporous ceria materials are fabricated via the soft templating
method using CTAB as the template. Unlike polyether surfactants,
such as nonionic P123 employed in the preparation of SBA-15
[34], the CTAB molecule has substantial C instead of O atoms.
2
mesopores centered at 5.0–5.5 nm with a narrow distribution. The
textual parameters of various ceria samples are summarized in
Table 1. In sharp contrast to the commercial ceria with a poor
1
1
1
1
75
50
25
00
0.12
(
A)
(B)
0.10
0.08
0.06
0.04
0.02
0.00
7
5
2
5
0
5
0
0.0
0.2
0.4
0.6
0.8
1.0
5
10
15
20
25
p/p0
Pore size (nm)
Fig. 2. N2 adsorption–desorption isotherms (A) and pore size distributions (B) of CeO2-meso-T (•: 400; ꢀ: 500; ꢁ: 600) materials.

4
J. Xu et al. / Applied Catalysis A: General 484 (2014) 1–7
d
c
b
a
100
150
200
250
300
350
400
450
500
550
Temperature (°C)
Fig. 3. TEM image of CeO2-meso-400.
Fig. 4. CO2-TPD profiles of CeO2-com (a), CeO2-meso-400 (b), CeO2-meso-500 (c),
and CeO2-meso-600 (d) materials.
Simultaneously, the interaction between CTAB and inorganic Ce
species is electrostatic force [35], generally stronger than the
hydrogen bond between P123 and silicate compounds. Therefore,
to eliminate CTAB, washing procedure with hot water was con-
ducted after the hydrothermal step. Despite this, there were still
some CTAB molecules remaining in the as-synthesized ceria mate-
3
.2. Catalyst activity
Initially, a blank experiment without catalyst was carried out
for the transesterification of EC with CH OH (entry 1, Table 2).
3
The result reveals that the reaction can proceed with a minor
conversion under such circumstance, affording an EC conver-
sion of 18.0%. Wherein, the monoester, namely HEMC, accounts
for a large proportion in the products. After the introduction of
CeO -com, the catalytic conversion is slightly enhanced (entry 2).
Upon employing the CeO -meso catalyst, the conversion increases
drastically, together with a high selectivity (96.1%) to the target
molecule, DMC. Additionally, the variation of catalytic perform-
ances obtained over different mesoporous ceria sample has been
found. As listed in entries 3–5, the catalytic activity of CeO -meso
decreases monotonously with the increase of their corresponding
calcination temperatures, and the prominent activity is achieved
rials. As shown in Fig. S2, the pure CTAB reveals a sharp weight loss
◦
(
ca. 97 wt%) in the range of 200–350 C. Obviously, this is attributed
to the decomposition of CTAB in air. In the case of the as-synthezied
CeO2 before calcination, it also demonstrates an apparent weigh
loss in the same temperature range, contributing to ca. 9 wt% of
2
2
overall weight. On the other hand, the synthesized CeO -meso
2
◦
shows no significant weigh loss (ca. 0.5 wt%) at ca. 200–350 C.
The comparison strongly verifies the existence of residual CTAB in
the as-synthezied CeO2 sample. In this sense, the introduction of
subsequent calcination plays a crucial role to obtain mesoporous
ceria with high surface area. First, the temperature value ought
2
over CeO -meso-400, which offers the highest EC yield of 73.3%.
◦
2
to be higher than the decomposition temperature (ca. 350 C) of
As stated above, it has been reported that ceria can catalyze
CTAB, thus guaranteeing the complete elimination of CTAB. A low
calcination temperature is inevitably unfavorable to remove the
template, resulting in a lower pore size and poor crystallinity (Fig.
the esterification of CO2 with CH OH to DMC [26]. To examine the
possible contribution of the catalytic esterification to the present
transesterification system of EC with CH OH, we also tested the
esterification reaction over CeO -meso without the addition of EC.
The result (entry 6) confirms that, under the present reaction cir-
cumstance, CeO -meso could hardly promote the esterification of
CO with CH OH. That is, the DMC gained in the study solely results
from the transesterification of EC with CH OH. In addition to the
3
3
S3), compared with those of CeO -meso samples prepared using
2
2
high calcination temperatures. Second, calcination is also neces-
sary for the crystallization from the amorphous ceria to cubic
fluorite structure. Third, the temperature should not be too high;
higher temperatures would induce inevitable aggregation of ceria
nanoparticles and consequent decrease in surface areas and pore
volumes of the final mesoporous ceria materials.
2
2
3
3
high-temperature evaluation, the catalytic behavior under a low
The basicity of the mesoporous ceria samples was analyzed
Table 2
using CO -TPD measurement. As displayed in Fig. 4, all meso-
2
Catalytic performances of ceria catalyst in the transesterification reaction of EC with
CH3OH.
porous samples exhibit broad desorption peaks centered at ca.
◦
1
45 C, attributed to the basic sites associated with weak chem-
ical and/or even physical adsorption of acidic CO2 molecules. In
Entry
Catalyst
Conv.(%)
Sel.(%)
DMC
Yield (%)
contrast, CeO -com gives a weak TCD signal of CO2 adsorbed, indi-
2
HEMC
cating that the basic species in it are very scarce. Moreover, based
1
2
3
4
5
6
7
8
9
–a
18.0
23.6
76.3
67.4
60.8
–
21.5
36.3
43.2
46.2
50.3
96.1
94.7
93.9
–
17.2
37.8
77.7
53.8
49.7
3.9
5.3
6.1
8.3
on the peak areas integrated, the density of the base sites for CeO -
a
2
CeO2-com
11.9
73.3
63.8
57.1
Trace
3.7
−
1
meso-400 is 212 ␮mol CO g
. Upon increasing the calcination
CeO -meso-400a
2
catal.
2
a
temperature, the basic quantity decreases. This variation is in good
agreement with the finding in terms of surface areas described
CeO2-meso-500
CeO2-meso-600
a
b
CeO2-meso-400
above. On the other hand, the difference between the CeO -com
2
c
–
82.8
62.2
22.3
and CeO -meso probably originates from their significant distinc-
c
2
CeO-com
13.7
33.6
CeO-meso-400c
tion in terms of the textual properties. Wherein, high surface areas
along with mesoporous architecture would offer abundant exposed
a
◦
2
50 mmol of CH3OH, 25 mmol of EC, T = 140 C, Wcatal. = 0.1 g, and t = 2 h.
basic sites as well as favorable mass transfer in the CO -TPD
b
c
250 mmol of CH OH, T = 140 ◦C, W
= 0.1 g, and t = 2 h.
2
3
catal.
◦
analysis.
250 mmol of CH3OH, 25 mmol of EC, T = 70 C, Wcatal. = 0.2 g, and t = 12 h.

J. Xu et al. / Applied Catalysis A: General 484 (2014) 1–7
5
of the transesterification. As shown in Fig. 6B, both conversion and
selectivity undergo a notable enhancement in the range of
◦
◦
1
00–140 C. In particular, the temperature of 120 C is found to
be a critical threshold in gaining a high content of DMC. Since
the reaction of EC with CH OH essentially involves two-step
3
of transesterification (EC + CH OH → HEMC → DMC), mild reaction
3
conditions such as low temperature and/or short time easily
impede the occurrence of the second stage. Consequently, a low
selectivity to the desired DMC would be received. This phenomenon
has also been reported in transesterification of EC over other cat-
alytic systems [3,4,20]. Given the results above, a temperature of
◦
1
40 C and reaction time of 2 h were chosen as optimal reaction
conditions.
Besides the reaction time and temperature, another issue of
practical importance for the evaluation of a heterogeneous catalyst
is the recycling capability. In view of this, a series of consecu-
tive experiments have been performed. As described in Fig. S5,
the selectivity in each cycle is above 96%. As for the conversion,
no apparent loss (± 5%) has been observed during the five repeti-
tious runs. N2 adsorption–desorption and FT-IR techniques were
further employed to analyze the physiochemical properties of the
Fig. 5. Correlation between the catalytic activity and the basicity in CO2-TPD profiles
of various ceria catalysts.
◦
temperature has also been surveyed. At 70 C and reaction time for
used CeO -meso-400 catalyst subjected to five runs. The surface
2
2
−1
1
2 h, CeO -com catalyzes the reaction of EC with CH OH with a
area and pore size of the spent CeO -meso-400 are 171 m g
2
3
2
moderate EC conversion and the DMC selectivity is very low (entry
). Under the same reaction condition, its mesoporous counterpart
provides a higher catalytic conversion and remarkable selectivity
77.7%). Combining the catalytic performance in high and low tem-
and 5.30 nm, respectively (Table 1 and Fig. S6). Moreover, the XRD
pattern demonstrates no clear change in the crystallinity com-
8
pared with the fresh CeO -meso-400 sample (Table 1). In the
2
(
case of chemical composition, the fresh and spent ceria samples
show identical DRIFT spectra (Fig. S7). The bands at ca. 3660
perature, it can be seen that the mesoporous ceria materials can
−
1
catalyze the transesterification of EC with CH OH with high activ-
and 3550 cm
are attributed to the O–H stretching modes of
3
ities.
bridged OH on the ceria [26], and adsorbed water, respectively.
−
1
To gain further insight into the basic property of the mesoporous
ceria catalysts in relation to their activity in the transesterifi-
cation, the basicity intensity based on the CO -TPD results is
plotted against the catalytic activity (gEC
for the CeO -meso-400/500/600 sample. As depicted in Fig. 5,
the basicity intensity exhibits a good correlation with its corre-
sponding catalytic activity. Moreover, considering the variation
of the three mesoporous ceria samples in terms of their textual
The strong peak at ca. 660 cm is due to the stretching frequency
of Ce–O–Ce [37]. In addition, the bands in the frequency region
−
1
from 1200 to 1700 cm are assigned to carbonate species formed
by coordination of CO2 molecules onto the coordinatively unsat-
urated CeO2 surface [38]. Obviously, no band associated with the
reagent/product of the transesterification reaction has been dis-
covered. Therein, the steady textual and chemical properties are
responsible for the excellent reproducibility of the mesoporous
ceria catalyst.
2
−
1
−1
gcatal.
h )
converted
2
properties, the basicity intensity for each CeO -meso is further
2
calculated regarding the surface area. The resultant intensity for
In addition to the transesterification reaction of EC with CH OH,
3
−
2
CeO -meso-400/500/600 is 1.15, 1.13, 1.13 ␮mol CO m , which
the reaction between PC and CH OH has also been tested in the
2
2
3
are fairly close. Thus, it can be confirmed again that the varia-
tion of the basicity of the mesoporous CeO2 materials originates
from their surface areas. On the other hand, it is widely reported
that ceria is a typically amphoteric metal oxide, featuring both
acid and base nature [36], and the transesterification reactions
can also be promoted by either acid or base. Therein, to examine
the possible involvement of acid property for the transesterifi-
cation in the present mesoporous ceria catalyst, we have also
presence of CeO -meso-400 (Table 3). The result (entry 2) indi-
2
cates that the catalytic reaction undergoes with a minor conversion.
Generally, the transesterification of cyclic carbonate is a typical
nucleophilic addition reaction that can be catalyzed by either acid
or base. In the previous work, the basic hydroxyl (Ce–O–H) groups
has been recognized as the key site to adsorb and activate CH OH
3
−
molecules [26], yielding highly active CH O anion. Due to the
3
steric hindrance, the nucleophilic addition of the anion to PC is
more difficult than EC and consequently lower yield is obtained in
the transesterification of PC. Nevertheless, prolonging the reaction
or elevating the temperature can facilitate the catalytic transes-
used NH -TPD measurement (see supporting information for the
3
detailed experiment) to probe the acidity of the three CeO -meso
2
samples above. As shown in Fig. S4, there is no apparent rele-
vance between the acid density and the catalytic activity in the
mesoporous CeO2 materials. Based on the CO /NH -TPD profiles
terification of PC with CH OH (entries 3 and 4) and then receive a
3
higher conversion. Furthermore, EC and PC can also be catalytically
transesterified with C H5OH over CeO -meso-400 (entries 5 and 6)
2
3
above, it can be concluded that the basic sites of CeO -meso con-
2
2
2
◦
tributes directly to the activity in the transesterification of EC with
at 160 C.
CH OH.
Table 4 compares the performances of various representa-
tive catalysts, including ILs and ceria-based materials. Note, for
a fair comparison, all the performances collected are based on
the batch reactor. The best catalytic activities were achieved
over ILs (entries 1–4). Especially, DMIC afforded a top activity
3
In order to elucidate the dependence of the catalytic perfor-
mance on the reaction conditions, the catalytic conversions and
selectivities at different reaction time and temperatures have been
investigated. At the first 0.5 h, the reaction proceeds with a mod-
erate EC conversion as much as 46.6%, and the selectivity to
DMC received is 72.8% (Fig. 6A). As the reaction is prolonged,
the conversion and selectivity increase progressively but level
off after 2 h. On the other hand, the reaction temperature also
exhibits a noticeable relationship with the catalytic performance
−
1
−1 ◦
at 110 C, and the performance
as much as 38.75 gEC gcatal.
h
was largely owing to the intrinsically homogeneous nature that
is beneficial to the contact of active sites with the substrates.
However, as discussed above, the main drawback for such IL-
catalyzed system lies in the inconvenience in the separation of

6
J. Xu et al. / Applied Catalysis A: General 484 (2014) 1–7
1
00
100
80
60
40
20
0
8
6
4
2
0
0
0
0
0
Conv.
Sel.
(
A)
(B)
1
2
3
4
100
110
120
130
140
Reaction time (h)
Reaction temperature (°C )
Fig. 6. Effect of reaction time (A) and temperature (B) on the transesterification over CeO2-meso-400 catalyst. Reaction conditions: 250 mmol of CH3OH, 25 mmol of EC, and
Wcatal.= 0.1 g.
the catalysts. Furthermore, the supported ILs (entries 5 and 6)
suffer from high-cost and complicated immobilization procedure.
As far as ceria-based catalysts concerned, although the NaMC and
Au/CeO demonstrate superior activity to CeO -meso, the synthetic
a soft-templating approach which is facile, simple and cheap. More
importantly, no further loading is demanded. At this juncture, the
comparison in Table 4 strongly suggests that the single CeO -meso
2
catalyst, featuring low-cost preparation and high catalytic activity,
can serve as a promising heterogeneous catalyst for the synthesis
2
2
routes associated with the two ceria-based samples is complex. In
sharp contrast, the present CeO -meso materials are fabricated via
of DMC via transesterification of EC with CH OH.
2
3
Table 3
Catalytic performance of CeO2-meso-400 in various transesterification of cyclic carbonates with alcohols a.
◦
Sel.(%) b
Entry
Cyclic carbonate
Alcohol
T ( C)
t (h)
Conv.(%)
Yield (%)
1
2
3
4
5
6
EC
PC
PC
PC
EC
PC
CH3OH
CH3OH
CH3OH
CH3OH
C2H5OH
C2H5OH
140
140
140
160
160
160
2
2
6
6
6
6
76.3
18.0
37.5
59.8
80.6
34.0
96.1
91.9
98.6
99.2
92.5
97.9
73.3
16.5
37.0
59.3
74.5
33.3
a
Reaction condition: 250 mmol of alcohol, 25 mmol of cyclic carbonate, Wcatal. = 0.1 g.
The sole byproduct is monoester.
b
Table 4
a
Comparison of the catalytic performances of ILs and ceria-based catalysts for the transesterification of EC with CH3OH to DMC.
◦
−1 −1
)
Entry
Catalyst
Wcatal. (g)
nEC (mmol)
nCH3OH/nEC
T ( C)
t (h)
EC conv. (%)
rs (gEC gcatal.
h
1
2
3
4
5
6
7
8
9
[BmIm]Cl [21]b
[BmIm]Cl [7]
0.348
0.348
0.0187
0.028
0.2
25
25
10
20
25
32
100
10
25
5:1
8:1
140
120
70
110
180
65
65
140
140
6
0.25
6
1.33
4
5
3
6
2
74.1
59.1
90
82
78.4
85
64.5
63
76.3
0.78
14.94
7.06
38.75
2.16
0.96
23.65
0.80
8.4
b,c
d
DABCO-derived IL [3]
15:1
10:1
8:1
10:1
5:1
DMIC [4]e
IL on MCM-41 [39]f
g
MCF-[SmIm]OH [13]
0.5
h
NaMC [27]
0.08
0.115
0.1
Au/CeO2 [28]i
10:1
10:1
CeO2-meso-400
a
◦
Unless specified, the reactions with temperatures over 70 C were carried out in an autoclave under CO2 atmosphere (0.5–1.2 MPa).
b
c
d
e
f
[
BmIm]Cl:1-n-butyl-3-methylimidazolium.
Microwave (100 W) heating.
DABCO: 1-n-butyl-1,4-diazabicyclo[2.2.2]octane.
DMIC: 1,3-dimethylimidazolium-2-carboxylate.
IL: 1-(triethoxysilyl) propyl-tri-n-hexylammonium chloride.
g
h
i
[SmIm]OH: 1-(triethoxysilyl) propyl-3-methylimidazolium hydroxide.
Mesoporous ceria loaded with NaOH.
Transesterification of PC with CH3OH.

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