Dimethyl carbonate synthesis via transesterification of ethylene carbonate and methanol using ionic liquid catalysts immobilized on mesoporous cellular foams

Article abstract of DOI:10.1007/s11164-015-2396-4

Dimethyl carbonate (DMC) was synthesized via transesterification of ethylene carbonate and methanol with ionic liquid catalysts. For this reaction, 1,4-diazobicyclo[2.2.2]octane (DABCO), [Choline]OH, and [BMIM]Cl were used as a homogeneous catalyst, and hydrotalcite, [DABCO]OH@MCF, [DABCO]Cl@MCF, and DABCO/MCF were used as a heterogeneous catalyst. To support the ionic liquids, mesoporous cellular foam (MCF) was prepared and characterized by SEM, TEM and BET surface area analyzer. The average cell and window sizes of the prepared MCF were 34.4 and 21.3 nm, respectively. The prepared MCF had a well structured three-dimensional structure. Among the homogeneous catalysts used, DABCO showed the highest DMC yield about 84 %, and among the heterogeneous catalysts, [DABCO]OH@MCF showed the highest DMC yield about 77 %. In the reusability test of the used catalysts, there was only 8 % point decrease in DMC yield with [DABCO]OH@MCF, whereas 58 percent point decrease in DMC yield with DABCO/MCF after four times recycling tests. The effects of an anion on the catalytic activity were investigated. The optimum reaction condition for DMC synthesis was also investigated with [DABCO]OH@MCF catalyst.

Full text of DOI:10.1007/s11164-015-2396-4

Res Chem Intermed (2016) 42:109–121
DOI 10.1007/s11164-015-2396-4
Dimethyl carbonate synthesis via transesterification
of ethylene carbonate and methanol using ionic liquid
catalysts immobilized on mesoporous cellular foams
1
1
•
Kwang-Hyeok Lee
1
•
Seungjea Lee
1
Dongil Shin
•
Hyun-Sik Hahm
Received: 9 July 2015 / Accepted: 12 December 2015 / Published online: 12 January 2016
Ó Springer Science+Business Media Dordrecht 2016
Abstract Dimethyl carbonate (DMC) was synthesized via transesterification of
ethylene carbonate and methanol with ionic liquid catalysts. For this reaction, 1,4-
diazobicyclo[2.2.2]octane (DABCO), [Choline]OH, and [BMIM]Cl were used as a
homogeneous catalyst, and hydrotalcite, [DABCO]OH@MCF, [DABCO]Cl@MCF,
and DABCO/MCF were used as a heterogeneous catalyst. To support the ionic
liquids, mesoporous cellular foam (MCF) was prepared and characterized by SEM,
TEM and BET surface area analyzer. The average cell and window sizes of the
prepared MCF were 34.4 and 21.3 nm, respectively. The prepared MCF had a well
structured three-dimensional structure. Among the homogeneous catalysts used,
DABCO showed the highest DMC yield about 84 %, and among the heterogeneous
catalysts, [DABCO]OH@MCF showed the highest DMC yield about 77 %. In the
reusability test of the used catalysts, there was only 8 % point decrease in DMC
yield with [DABCO]OH@MCF, whereas 58 percent point decrease in DMC yield
with DABCO/MCF after four times recycling tests. The effects of an anion on the
catalytic activity were investigated. The optimum reaction condition for DMC
synthesis was also investigated with [DABCO]OH@MCF catalyst.
Keywords Dimethyl carbonate Á Ionic liquids Á Mesoporous cellular foam Á
Ethylene carbonate Á Reusability
&
Hyun-Sik Hahm
hahm@mju.ac.kr
1
Department of Chemical Engineering, Myongji University, Yongin, Gyeonggi-do 449-728,
Korea
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K.-H. Lee et al.
Introduction
It is very attractive to synthesize dimethyl carbonate (DMC) from ethylene
carbonate (EC) and methanol (MeOH) in terms of the reuse of carbon dioxide,
because ethylene carbonate (EC) is synthesized from carbon dioxide and easily
converted to DMC via the transesterification reaction of EC and MeOH. Owing to
the high oxygen content, low toxicity, and high chemical stability, DMC is an
emerging substitute for phosgene, solvent, and gasoline additives [1–3]. DMC has
been produced industrially by the methyl chloroformate reaction of phosgene and
methanol. However, this process has a problem of phosgene’s toxicity; therefore
DMC is produced industrially by a transesterification of ethylene carbonate or
propylene carbonate with methanol [4–7].
For the transesterification reaction, homogeneous or heterogeneous, acid or base
catalysts have been used. As a heterogeneous catalyst, metal oxides or inorganic
materials such as perovskite, hydrotalcite, and ZnO were used [8, 9]. As a
homogeneous catalyst, several ionic liquids and 1,4-diazobicyclo[2.2.2] octane
(
DABCO) were used [10, 11]. The heterogeneous catalysts have some advantages of
separation and reusability, while they have a problem of low catalytic activity. On
the other hand, the homogeneous catalysts have an advantage of high catalytic
activity, while they have a problem of separation after the reaction. To solve these
problems, there has been an attempt in which the homogeneous liquid catalysts were
supported on some supports such as SBA-15, MCM-41, and mesoporous cellular
foams (MCF) [12–14]. The supports with micro-pores can hinder the mass transfer
of reactants and products in this reaction, thus mesoporous supports are necessary.
There are some kinds of mesoporous materials that have been synthesized such as
MCM-41, SBA-15, HMM-33, and FSM-16 [15–18]. MCF has a three dimensional
structures in which spherical cells are interconnected. The cell and window sizes
can be controlled in the preparation steps.
In the present work, MCF is prepared and characterized, and then several ionic
liquids are supported on the MCF to prepare heterogeneous catalysts. We choose
MCF as the support because it has three dimensional meso-sized pores and
stable structures in this reaction system. DMC is synthesized at various reaction
conditions using the prepared catalysts, and the reusability of the used catalysts is
tested. The effects of anions of ionic liquids are also investigated.
Experimental
Materials
For the preparation of MCF, trimethyl benzene (TMB, 98 %), soduium silicate
solution (reagent grade), and Pluronic P123 (poly(ethylene glycol)-block-
poly(propylene glycol)-block-poly(ethylene glycol), M * 5800) were purchased
n
from Sigma-Aldrich, and acetic acid (99.5 %) was from Daejung Chemical and
Metals Co. For the synthesis of DMC, methanol (99.9 %) and ethylene carbonate
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Dimethyl carbonate synthesis via transesterification of…
111
(
98 %) were also purchased from Sigma-Aldrich. The purchased reagents were used
without further purification. 1,4-diazobicyclo[2.2.2]octane (DABCO), 1-butyl-3-
methylimidazolium chloride ([BMIM]Cl, 98 %), and choline hydroxide ([Cho-
line]OH, 45 wt% in methanol) were purchased from Sigma-Aldrich to use as a
catalyst. 3-chloropropyl trimethoxy-silane (CPTMS, 97 %), potassium hydroxide
(90 %), ethyl ether (98 %), anhydrous toluene (99.8 %), and acetonitirile (99.8 %)
were also purchased from Sigma-Aldrich.
Preparation of the MCF support
For the preparation of MCF, tetraethyl orthosilicate (TEOS) has been used as a
silica source; however, in the present work inexpensive sodium silicate was used as
the silica source. Pluronic P123 was used as a surfactant. The pore size of MCF was
controlled by the weight ratio of TMB to Pluronic P123.
For the preparation of MCF, 9.8 g of Pluronic P123 and 200 mL of deionized
water were put into a Duran bottle and the mixture was stirred at room temperature
to dissolve. And then, 4.47 mL of acetic acid was added into the prepared Pluronic
P123 solution, and the mixture was stirred at room temperature for 30 min. After
that, 5.88 g of TMB was added to the prepared solution, and the mixture was
vigorously agitated at 60 °C for 2 h. A 200 mL sodium silicate solution in which
1
6 mL of sodium silicate was dissolved in deionized water was added to the
solution, and the mixture was vigorously agitated at 60 °C for 20 h. And then, the
prepared emulsion, stored in a capped Duran bottle, was further aged in a
convection oven at 100 °C for 24 h. The aged mixture was then filtered and the
solid part was washed with 200 mL of deionized water. Finally, the washed solid
part was dried in a drying oven at 80 °C for 12 h, and calcined in an air environment
at 550 °C for 8 h.
Preparation of catalysts
Ionic liquids were supported on MCF support by an impregnation and chemical
immobilization methods. To distinguish the prepared catalysts, the catalysts
prepared by the impregnation method were denoted by ‘/’ like DABCO/MCF,
and the catalysts prepared by the chemical immobilization method were denoted by
‘
@’ like [DABCO]OH@MCF.
DABCO and [BMIM]Cl that have been used as a homogeneous catalyst were
impregnated on MCF. DABCO/MCF stands for ‘‘DABCO impregnated on MCF’’.
For the preparation of the DABCO/MCF catalyst, 2.5 g of MCF, dried in an oven at
1
00 °C for 12 h, was mixed with 15 mL of anhydrous toluene and DABCO
corresponding to 1 mol% of ethylene carbonate), and the mixture was stirred at
(
room temperature for 20 h. Then, the mixture was filtered, and the solid part was
dried in an oven at 60 °C for 12 h.
[DABCO]OH@MCF stands for ‘‘[DABCO]OH immobilized on MCF by a
chemical bonding’’. [DABCO]OH was immobilized on MCF by the anchoring
action of CPTMS. For the preparation of [DABCO]OH@MCF, we referred to the
work of Moghaddam [19]. For the preparation of [DABCO]OH@MCF, 2.5 g of
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K.-H. Lee et al.
MCF, dried in an oven at 100 °C for 12 h, was mixed with 15 mL of anhydrous
toluene and CPTMS (corresponding to 1.5 mol% of ethylene carbonate), and the
mixture was refluxed at 60 °C for 48 h to prepare chemically bonded CPTMS on
MCF. Then, the mixture was filtered, and the solid part was washed with 50 mL of
anhydrous toluene and 50 mL of dichloromethane, respectively, and dried in an
oven at 60 °C for 12 h. After that, 2 g of the dried solid part was mixed with the
solution that was prepared by the dissolving of a predetermined amount of DABCO
in 30 mL of anhydrous toluene, and the mixture was stirred for 48 h. In order to
exchange chlorine ions for hydroxyl ions, the stirred mixture was mixed with a
KOH solution (in which the amount of KOH was corresponding to 2 mol% of
ethylene carbonate), and the mixture was stirred for 48 h. Finally, the mixture was
filtered, and the solid part was washed with 100 mL of deionized water and dried in
an oven at 60 for 8 h. Through this process, the [DABCO]OH@MCF catalyst was
prepared. [DABCO]Cl@MCF referred to the catalyst that did not exchange the
chlorine ions for hydroxyl ions.
DMC synthesis reaction
DMC was synthesized by the transesterification of ethylene carbonate and
methanol in an autoclave (Parr 4843). The prepared catalyst was mixed with the
pre-calculated amounts of ethylene carbonate and methanol, in which the amount
of catalyst (ionic liquid) was controlled to be 1 mol% of the limiting reactant,
ethylene carbonate. The reaction mixture was stirred in a rate of 450 rpm, and the
pressure was maintained at 37–41 psi by their vapor. The reaction was carried out
at 70–150 °C for 7 h. The reactants molar ratio (MeOH/EC) was varied from 10 to
2
5.
To examine the reusability of the used catalysts, the used catalyst was collected
by filtration after the reaction, and washed with 100 mL of deionized water, and
then dried in an oven at 60 °C for 8 h. The dried used catalyst was reused for the
reaction.
The reaction products were analyzed with a gas chromatograph (DS6200)
equipped with a FID detector and a capillary column (HP-INNOWAW 30 m,
0.25 mm, 0.25 lm). For the quantitative analysis, we used an internal standard
method, in which acetone was used as the internal standard.
Characterization of catalysts
The specific surface area and pore size distribution of the prepared MCF and
supported catalysts were characterized by a surface area analyzer (BELSORP-
max). The specific surface area was calculated by the BET method and the pore
size distribution was calculated by the Barrett–Joyner–Halenda (BJH) method.
The surface and inner structures of MCF were observed by SEM (scanning
electron microscope, JSM 6700F) and TEM (transmission electron microscope,
JEM 2100F).
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Dimethyl carbonate synthesis via transesterification of…
113
Results and discussion
BET surface areas and pore size distribution
The N adsorption/desorption isotherms of the prepared MCF are depicted in Fig. 1.
2
o
There is an adsorption–desorption hysteresis observed in the P/P range of 0.9–1,
which means that MCF has meso-sized pores [15]. The BET surface area of the
2
prepared MCF was 733 m /g. The pore size distribution of the prepared MCF was
analyzed by the BJH method and presented in Fig. 2. As shown, the average pore
sizes of the cell and window of the MCF are 34.4 and 21.3 nm, respectively. The
pore sizes of the cell and window of the MCF are very important, because small
pores can hinder the mass transfer of the reactants and products. The prepared MCF
shows a proper pore sizes for this reaction. The average surface area, cell size,
widow size, and specific volume of the prepared MCF and supported catalysts are
summarized in Table 1. The surface area, cell and window pore sizes decrease with
the amount of ionic liquids loaded. Especially, DABCO/MCF, prepared by the
impregnation method, shows the lowest these values. This can be explained by the
reason that DABCO/MCF is a physically supported one, while the others are
chemically supported ones, therefore, in the case of DABCO/MCF, the multi-
layered ionic liquids may cause the decrease of these values.
SEM and TEM analysis
The surface structure of the prepared MCF was observed with SEM, and the result
was presented in Fig. 3. The low resolution SEM image (Fig. 3a) shows a porous
granule-like structure; however, the high resolution image (Fig. 3b) shows relatively
1
1
800
200
ads
des
600
0
0
.0
0.5
1.0
Relative pressure, P/Po
Fig. 1 N
2
adsorption/desorption isotherms for the prepared MCF
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K.-H. Lee et al.
6
5
4
3
2
1
0
1
10
100
1000
Pore diameter (nm)
Fig. 2 Pore size distribution of the prepared MCF; cell size: 37.4 nm, window size: 21.3 nm
Table 1 Characteristics of the prepared catalysts
2
2
Catalyst
S
BET (m /g)
Cell size (nm)
Window size (nm)
Vtotal (cm /g)
MCF
732.8
477.9
528.2
493.9
34.4
10.4
28.1
27.5
31.3
7.0
2.2
1.4
1.9
1.8
DABCO/MCF
[
[
DABCO]Cl@MCF
DABCO]OH@MCF
18.5
16.9
uniform cells with an average size of about 34 nm, that is coincident with the result
of BJH analysis.
The structure of the prepared MCF was also observed with TEM, and the result
was presented in Fig. 4. From Fig. 4a, it is found that the prepared MCF has a
relatively well-developed three dimensional cell structures, and the cells are
relatively uniform. From Fig. 4b, a well-developed window structures can be
observed. From the TEM images, it is proved that the average sizes of the cells and
windows are about 34 and 21 nm, respectively.
DMC synthesis reaction
In an autoclave, DMC was synthesized by the transesterification reaction of
methanol and ethylene carbonate. For this reaction, DABCO, [Choline]OH, and
[
BMIM]Cl were used as a homogeneous catalyst, and hydrotalcite, [DAB-
CO]OH@MCF, [DABCO]Cl@MCF, and DABCO/MCF were used as a heteroge-
neous catalyst.
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Dimethyl carbonate synthesis via transesterification of…
115
Fig. 3 SEM images of the prepared MCF. a Low resolution. b High resolution
DMC yield was calculated as the percentage of the ratio of the amount of DMC
produced to the amount of DMC calculated. EC conversion was also calculated as
the percent of the ratio of the amount of EC converted to the amount of EC
presented initially.
DMC synthesis results obtained with various catalysts are depicted in Fig. 5. As
shown, hydrotalcite, [BMIM]Cl, and [DABCO]Cl@MCF catalysts show a low
DMC yield around 10 %. However, DABCO, DABCO/MCF, and [DAB-
CO]OH@MCF show a relatively high DMC yield around 80 %. Among the
homogeneous catalysts used, DABCO shows the highest DMC yield of 84 %, and
among the heterogeneous catalysts used, [DABCO]OH@MCF shows the highest
DMC yield of 77 %.
123

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K.-H. Lee et al.
Fig. 4 TEM images of the prepared MCF
Effects of temperature and reaction time on DMC yield
In order to examine the effects of temperature and reaction time on DMC yield, the
transesterification reaction of ethylene carbonate and methanol was carried out with
[
DABCO]OH@MCF catalyst, and the result was depicted in Fig. 6. As shown, the
time to reach equilibrium decreases with increasing the reaction temperature. The
times to reach equilibrium are around 6, 5, 4, and 3 h at 70, 80, 100, and 150 °C,
respectively. The highest DMC yield is obtained at 100 °C. The lower DMC yield at
1
50 °C may be due to the decomposition of ethylene carbonate at higher
temperature or the degradation of immobilized species at the higher temperature.
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23

Dimethyl carbonate synthesis via transesterification of…
00
117
1
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
DABCO
Hydrotalcite
[
[
BMIM]Cl
Choline]OH
DABCO/MCF
[
[
DABCO]Cl@MCF
DABCO]OH@MCF
0
1
2
3
4
5
6
7
8
Time (h)
Fig. 5 DMC yields with various catalysts
100
90
80
70
60
50
40
30
20
10
0
7
8
1
0 °C
0 °C
00 °C
150 °C
0
1
2
3
4
5
6
7
8
Time (h)
Fig. 6 DMC yields with the reaction time at various temperatures
Effects of the molar ratio of reactants on DMC yield and EC conversion
In order to test the effects of the molar ratio of reactants on DMC yield and EC
conversion, the transesterification reaction was carried out at a molar ratios of
123

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K.-H. Lee et al.
MeOH/EC = 10–25, and the results were depicted in Fig. 7. Because of the alkoxyl
group exchange in this reaction system, excess methanol was used. Both the highest
DMC yield and the highest EC conversion were obtained at MeOH/EC = 15. From
this experiment, it was found that the proper reactants molar ratio (MeOH/EC) was
1
5.
Reusability test of the used [DABCO]OH@MCF and DABCO/MCF
catalysts
Although homogeneous ionic liquid catalysts show a high DMC yield, there is a
problem of separation of the catalysts from the products. This problem may be a
significant drawback in the industrialization of the process. Therefore, in the present
work, ionic liquids were supported on MCF so as to separate the used catalyst
easily.
The reusability test of the used [DABCO]OH@MCF and DABCO/MCF catalysts
were performed. The changes in DMC yield and EC conversion with the used
catalysts were tested and depicted in Figs. 8 and 9. As shown in Fig. 8, there are
about 8 percent point decrease in DMC yield and about 7 percent point decrease in
EC conversion with the chemically immobilized [DABCO]OH@MCF catalyst after
4
times reuses. On the other hand, the DMC yield and EC conversion decrease by
about 58 and 64 percent points, respectively, with the physically impregnated
DABCO/MCF catalyst after 4 times reuses, as shown in Fig. 9. From these results, it
is proved that the chemically immobilized [DABCO]OH@MCF catalyst is much
more stable than the physically impregnated DABCO/MCF catalyst. [DAB-
CO]OH@MCF catalyst shows a possibility for the industrial application of the
chemically immobilized ionic liquid catalysts.
1
00
100
90
80
70
60
50
40
30
20
10
0
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
DMC yield
EC conversion
10:1
15:1
20:1
25:1
MeOH/EC ratio
Fig. 7 DMC yield and EC conversion with various molar ratios of methanol and ethylene carbonate
1
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Dimethyl carbonate synthesis via transesterification of…
119
1
00
100
90
80
70
60
50
40
30
20
10
0
9
0
0
0
0
0
0
0
0
0
8
7
6
5
4
3
2
1
DMC yield
EC conversion
0
0
1
2
3
4
5
6
Recycle number
Fig. 8 Changes in DMC yield and EC conversion as a function of the recycle number of used
DABCO]OH@MCF
[
1
00
100
90
80
70
60
50
40
30
20
10
0
DMC yield
EC conversion
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
0
1
2
3
4
5
6
Recycle number
Fig. 9 Changes in DMC yield and EC conversion as a function of the recycle number of used DABCO/
MCF
123

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K.-H. Lee et al.
100
[
[
DABCO]Cl@MCF
DABCO]OH@MCF
90
80
70
60
50
40
30
20
10
0
0
1
2
3
4
5
6
7
8
Time (h)
Fig. 10 Changes in DMC yield with different anion-bearing catalysts
Effects of the anions of ionic liquids on DMC yield
The effects of the anions of ionic liquids on DMC yield were examined with
DABCO]OH@MCF and [DABCO]Cl@MCF catalysts, and the result was depicted
[
in Fig. 10. The [DABCO]OH@MCF catalyst shows approximately 60 percent point
higher DMC yield than [DABCO]Cl@MCF catalyst. In other words, the catalyst
-
-
containing OH anion shows higher DMC yield than the one containing Cl anion.
The reaction mechanism proposed for the transesterification of ethylene carbonate
and methanol is as follows: first, a hydrogen bond between the cation of ionic
liquids and methanol is formed, then the methanol is converted to a methoxy group
-
-
-
(
methoxy group reacts with ethylene carbonate, producing DMC and ethylene glycol
CH O ) by the anion (OH or Cl ) of the ionic liquids, finally the produced
3
-
-
[
bond with methanol, whereas the Cl anion does not form the hydrogen bond with
7, 20]. The OH anion facilitates the formation of CH O through the hydrogen
3
-
-
methanol. As a result, catalysts with OH anion show a higher activity than the one
-
with Cl anion [21]. Therefore, the [DABCO]OH@MCF catalyst showed a higher
DMC yield than [DABCO]Cl@MCF catalyst.
Conclusions
In order to prepare a supported ionic liquid catalyst, MCF was prepared as a support.
The MCF, whose average cell and window sizes are 34.4 and 21.3 nm respectively,
was prepared successfully. Furthermore, the MCF was verified to have a well-
structured three-dimensional structure by SEM and TEM analyses. Among the
1
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Dimethyl carbonate synthesis via transesterification of…
121
homogeneous catalysts used, DABCO showed the highest DMC yield of 84 %, and
among the heterogeneous catalysts, [DABCO]OH@MCF showed the highest DMC
yield of 77 %. In the reusability test of the used catalysts, there was about 8 percent
point decrease in DMC yield with the chemically immobilized [DAB-
CO]OH@MCF catalyst after four times recycling tests, whereas there was about
5
8 % point decrease in DMC yield with the physically impregnated DABCO/MCF
catalyst after four times recycling tests. The chemically immobilized [DAB-
CO]OH@MCF catalyst was much more stable than the physically impregnated
DABCO/MCF catalyst. The anions contained in the ionic liquids played an
important role in the DMC synthesis reaction. The optimum reaction conditions for
the DMC synthesis with [DABCO]OH@MCF catalyst were 100 °C, MeOH :
EC = 15 : 1 and 4 h.
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