Effective and recoverable homogeneous catalysts for the transesterification of dimethyl carbonate with ethanol: Lanthanide triflates

Article abstract of DOI:10.1134/S0023158409050073

The catalytic activities of metal triflates were tested for the transesterification of dimethyl carbonate (DMC) with ethanol. It was found that yttrium triflate was the most efficient homogeneous catalyst. When the transesterification reaction was catalyz

Full text of DOI:10.1134/S0023158409050073

ISSN 0023ꢀ1584, Kinetics and Catalysis, 2009, Vol. 50, No. 5, pp. 666–670. © Pleiades Publishing, Ltd., 2009.
Published in Russian in Kinetika i Kataliz, 2009, Vol. 50, No. 5, pp. 693–697.
Effective and Recoverable Homogeneous Catalysts
for the Transesterification of Dimethyl Carbonate with Ethanol:
Lanthanide Triflates1
F. M. Mei, E. X. Chen, and G. X. Li
School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology,
Wuhan, Hubei, 430074 P.R. China
eꢀmail: ligxabc@163.com
Received July 27, 2008
Abstract—The catalytic activities of metal triflates were tested for the transesterification of dimethyl carbonꢀ
ate (DMC) with ethanol. It was found that yttrium triflate was the most efficient homogeneous catalyst. When
the transesterification reaction was catalyzed by yttrium triflate at 76–80°C, 7 h, ethanol to DMC in 6 : 1
molar ratio, 0.35 mol % of catalyst based on DMC, the conversion of DMC was 89.2%, the selectivities of
diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) were 85.1 and 13.6%, respectively. Yttrium triꢀ
flate was reused 5 times for the transesterification without loss of its catalytic activity.
DOI: 10.1134/S0023158409050073
1
In recent years, organic carbonates have been and found that two ionꢀexchange resins, Lewatit
attracted a great attention due to their ubiquitous K1221 and Nafion SACꢀ13, had better catalytic activꢀ
applications as intermediates for fine chemicals, bioꢀ ities than other heterogeneous catalysts, but the reacꢀ
logical and medicinal reagents, plasticizers, synthetic tion rate over these catalysts was slow. We found that a
lubricants, monomers for organic glass [1]. In addiꢀ tertꢀbutyl oxide interlayered MgꢀAl hydrotalcite was
tion, dimethyl carbonate (DMC), ethyl methyl carꢀ an effective heterogeneous catalyst for the transesꢀ
bonate (EMC) and diethyl carbonate (DEC) are terification of DMC with ethanol, but it was expenꢀ
excellent organic solvents [2–4], and are widely used sive [15].
as additives in lithium batteries [5–7]. Most imporꢀ
tantly, DMC and DEC are both considered to be the
best alternatives for methyl tertꢀbutyl ether (MTBE) as
oxygenꢀcontaining fuel additive. In comparison with
DMC, DEC will reduce emissions from gasoline and
diesel engines, and gasoline/water distribution coeffiꢀ
cients are more favorable for DEC [8].
In terms of activity and life time of the catalysts,
both homogeneous and heterogeneous catalysts for
transesterification of DMC with ethanol had some
kinds of drawbacks. Thus, our aim is to find a catalyst
which has both the advantage of homogeneous catalyst
with high activity and the advantage of heterogeneous
catalyst with easy separation from reaction mixture.
Triflate salts such as lanthanide triflate, copper triꢀ
flate and zinc triflate are new kinds of Lewis acids
which are widely used as catalysts for a variety of
organic transformations such as Diels–Alder addition
[16–18], Friedel–Crafts reaction [19–21], Michael
reaction [22], nitroꢀaldol (Henry) reaction [23], Manꢀ
nich and aldol reactions [24], carbon–carbon bondꢀ
forming reaction [25], cationic copolymerization
[26], cycloaddition reactions [27, 28], epoxide ringꢀ
opening and esterification reactions [29]. In previous
study, we found that samarium triflate was active for
the transesterification of DMC with phenol to methyl
phenyl carbonate and diphenyl carbonate [30]. Triꢀ
flate salts can be used both in organic and in aqueous
medium, and easily recovered from reaction mixture.
In the exploration of homogeneous catalysts for the
transesterification of DMC with ethanol, we found
that lanthanide triflates were efficient homogeneous
catalysts for the transesterification of DMC with ethaꢀ
The transesterification of DMC with ethanol is
considered to be a clean process in which nonꢀtoxic
DMC is used for the substitution of highly toxic phosꢀ
gene. In general, transesterification reaction is cataꢀ
lyzed by homogeneous base catalysts such as alkali
metal methoxide [9], sodium methoxide/triethanolaꢀ
mine [10]. These catalysts have good activity, but they
are toxic, flammable, corrosive and sensitive to air and
water. Due to the easy separation from reaction mixꢀ
ture and recovery, many heterogeneous catalysts were
developed for the transesterification of DMC with
ethanol. Group IIIB metal oxides such as Sm2O3 were
used as solid catalysts for the transesterification of
DMC with ethanol, but their activities were low [11].
K2CO3 and modified K2CO3 were found to be effective
catalysts for this reaction [12, 13]. ZielinskaꢀNadolska
et al. [14] tested a variety of heterogeneous catalysts,
1
The article is published in the original.
666

EFFECTIVE AND RECOVERABLE HOMOGENEOUS CATALYSTS
667
nol. They are stable to air, water and reaction mixture,
General Procedure for Transesterification Reaction
and reuseable and easy to handle. Lanthanide triflates
could catalyze the transesterification of DMC with
ethanol for a long time without loss of their activities.
The experiments were done in a thermostatted
threeꢀneck flask equipped with a magnetic stirrer, a
thermometer, a tube for sample withdrawal and a recꢀ
tification column which was filled with small cylinder
cerams. Thus, a reactive distillation under atmoꢀ
spheric pressure was used to remove the byꢀproduct
methanol to shift the equilibrium to the direction of
products.
EXPERIMENTAL
Catalyst Preparation
DMC was analytic grade. It was fractionally disꢀ
tilled and stored over molecular sieves (4 Å) prior to
In a typical experiment, DMC 18.0 g (0.2 mol),
ethanol 55.2 g (1.2 mol), 0.35 mol % of catalyst based
on DMC were introduced into the flask, and heated to
desired temperature. At certain intervals, samples of
reacting mixture were collected and analyzed by gas
chromatography (GC). The catalytic activity of cataꢀ
lyst was assessed based on the yield and selectivity of
DEC and EMC.
use. ZnO, CuO, Y2O3, La2O3, Sm2O3, Yb2O3 were
purchased from Shanghai Reagent Company. Lanꢀ
thanide triflates [Ln(CF3SO3)3] and Y(CF3SO3)3 were
prepared by following method: in aqueous solution of
trifluoromethane sulfonic acid, a little excess amount
of Ln2O3 or Y2O3 was added in several times with stirꢀ
ring. The mixture was reacted at 60°C for about half an
hour, then filtrated to remove unreacted Ln2O3
(or Y2O3). The mother liquor was evaporated by a
rotation evaporator to remove water.
RESULTS AND DISCUSSION
Cu(CF3SO3)3
,
Zn(CF3SO3)2 were prepared
according to literature [31]. The other reagents were
used as received. The hydrated H2O in metal triflates
was removed by refluxing with toluene (dehydrating
reagent) using a Dean–Stark apparatus.
Thermodynamical Analysis
The transesterification of DMC with ethanol to
DEC is a twoꢀstage process:
O
O
H3CO–C–OCH3 + C2H5–OH
H3CO–C–OC2H5 + CH3–OH,
(I)
DMC
O
EMC
O
H3CO–C–OCH5 + C2H5–OH
H5C2O–C–OC2H5 + CH3–OH.
(II)
EMC
DEC
The stoichiometric equation for the transesterification of DMC with ethanol to DEC is presented as reaction (III):
O
O
(III)
H2CO–C–OCH3 + 2C2H5–OH
H5C2O–C–OC2H5 + 2CH3–OH.
On the basis of literatures [30, 32, 33], the reaction trope. In this experiment, the temperature was conꢀ
enthalpy and Gibbs free energy at 298 K, 101325 Pa trolled at 76–80°C when ethanol was used in excess.
0
for reaction (III) were calculated as
∆
=
rHm
0
The Catalytic Activity of Metal Oxides
and Metal Triflates
⎯
3.4 kJ/mol,
∆
rGm
= +12.6 kJ/mol. It is obvious that
the transesterification of DMC with ethanol to DEC is
a thermodynamically unfavorable reaction. In order to
shift the reaction to the direction of EMC and DEC,
methanol must be removed from reaction system. But
part of DMC was distilled out of flask simultaneously
with methanol during reaction because DMC and
methanol could form azeotrope system. Since the
reaction enthalpy for reaction (III) is very small, the
effect of temperature on this reaction should be small.
This conclusion was verified by Luo and Xiao [12]
when modified K2CO3 was used as catalyst for this
The catalytic activities of metal oxides and metal
triflates for the transesterification of DMC with ethaꢀ
nol are presented in Table 1.
The transesterification is almost not carried out
without catalysts. Zinc triflate and copper triflate show
moderate catalytic activity for transesterification.
When four metal oxides are used as heterogeneous catꢀ
alysts for the transesterification reaction, the converꢀ
sion of DMC is low, and EMC is the main product.
For four metal trifaltes, the conversion of DMC is
reaction. Theoretically, the reaction temperature is high, and DEC is the main product. The transesterifiꢀ
manipulated to continuously remove boiling azeoꢀ cation selectivities (DEC selectivity plus EMC selecꢀ
KINETICS AND CATALYSIS Vol. 50
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2009

668
MEI et al.
Table 1. The catalytic activity of metal triflates
gest Lewis acid among four metal triflates, and it cataꢀ
lyzes the decomposition of DMC and DEC to dimeꢀ
thyl ether and diethyl ether. Fu [34] demonstrated that
strong solid acids and bases could catalyze the thermal
decomposition of DMC to mainly give dimethyl ether.
In consideration of both the conversion and transeterꢀ
ification selectivity, Y(CF3SO3)3 is the best homogeꢀ
neous catalyst among all catalysts tested for the transꢀ
esterification reaction.
DMC conꢀ DEC selectivꢀ EMC selecꢀ
Catalyst
version, %
ity,* %
tivity,* %
No
2.4
30.0
50.0
91.6
83.2
86.5
88.2
25.0
20.8
44.0
28.0
66.7
59.7
25.8
80.7
76.6
80.9
70.2
8.8
33.3
36.3
70.6
16.8
22.6
11.9
6.0
Zn(CF3SO3)2
Cu(CF3SO3)2
Y(CF3SO3)3
La(CF3SO3)3
Sm(CF3SO3)3
Yb(CF3SO3)3
Y2O3
The Effect of the Amount of Yttrium Triflate
The effect of the amount of yttrium triflate on the
transesterification of DMC with ethanol is shown in
Table 2.
85.2
78.8
52.3
51.1
La2O3
21.2
42.9
44.6
The results in Table 2 show that the increase of
DEC selectivity and the decrease of EMC selectivity
are obvious when yttrium triflate is used in a dose from
0.175 to 0.35 mol %. When the amount of yttrium triꢀ
flate is greater than 0.35 mol %, DMC conversion,
DEC and EMC selectivities change very little. Thereꢀ
fore, 0.35 mol % of yttrium triflate is chosen as the
optimal catalyst loading for the transesterification
reaction.
Sm2O3
Yb2O3
Note: Reaction conditions: DMC (18.0 g, 0.2 mol), ethanol (55.2 g,
1.2 mol), 1.4 mol % of catalyst based on DMC, temperature
(76–80°C), time (7 h).
* Based on DMC conversion.
Table 2. The effect of the amount of yttrium triflate
The Effect of Reaction Time Using
Yttrium Triflate as Catalyst
DMC
DEC selecꢀ EMC selecꢀ
Catalyst,* mol %
conversion, % tivity, %
tivity, %
Because the transesterification of DMC with ethaꢀ
nol is a thermodynamically unfavorable reaction, the
conversion of DMC is very small even with increasing
temperature at a long time without catalyst. The effect
of reaction time on the transesterification of DMC
with ethanol using yttrium triflate as catalyst is given in
Table 3.
0.175
0.35
0.7
84.0
89.2
86.6
91.6
62.5
85.1
84.1
80.7
27.6
13.6
15.8
16.8
1.4
Note: Reaction conditions: DMC (18.0 g, 0.2 mol), ethanol (55.2 g,
1.2 mol), temperature (76–80°C), time (7 h).
If ethanol is used in excess, DMC can quickly react
with ethanol and upon catalysis by yttrium triflate proꢀ
duce EMC. Because DMC can form boiling azeoꢀ
trope with the produced methanol, the unreacted
DMC is distilled out of flask with methanol. This leads
to exhaustion of DMC. Thus, the conversion of DMC
changes little after 3 h, while the yield of DEC
increases and the yield of EMC decreases with
increase of time. This result indicates that reaction (I)
proceeds quickly before 3 h. After 3 h, reaction (II) is
rateꢀdetermining stage for whole reaction of transesꢀ
terification. Because the selectivities of DEC and
EMC change little after 7 h, the optimal reaction time
is set at 7 h.
* Based on DMC.
Table 3. The effect of reaction time
DMC
conversion, %
DEC selectiviꢀ EMC selectiviꢀ
Time, h
ty, %
ty, %
3
5
7
9
90.2
89.3
89.2
87.4
75.8
80.4
85.1
86.0
22.8
16.2
13.6
12.4
Note: Reaction conditions: DMC (18.0 g, 0.2 mol), ethanol (55.2 g,
1.2 mol), yttrium triflate (0.375 g, 0.7 mmol), temperature
(76–80°C).
The Effect of Molar Ratio of Ethanol to DMC
The effect of molar ratio of ethanol to DMC on the
transesterification of DMC with ethanol was tested
using yttrium triflate.
It is observed that the selectivities of DEC and
EMC depend on the molar ratio of ethanol to DMC.
When DMC and ethanol were used in equimolar ratio
tivity) of Y(CF3SO3)3 and Lа(CF3SO3)3 are higher than
those of Sm(CF3SO3)3 and Yb(CF3SO3)3. Especially
for Yb(CF3SO3)3, a considerable amount of ethers
such as diethyl ether and dimethyl ether was detected.
This can be explained that Yb(CF3SO3)3 is the stronꢀ or in excess of DMC, the transesterification of DMC
KINETICS AND CATALYSIS Vol. 50
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EFFECTIVE AND RECOVERABLE HOMOGENEOUS CATALYSTS
669
Table 4. The effect of molar ratio of ethanol to DMC
to its long duration and good activity, yttrium triflate
may be of great potential for commercial application
for the transesterification of DMC with ethanol.
In summary, yttrium triflate was proved to be active
homogeneous catalysts for the transesterification of
DMC with ethanol. It can be easily regenerated withꢀ
out loss of its activity. The transesterification of DMC
with ethanol over yttrium triflate proceeds fast. Even
when a very small amount of yttrium triflate
(0.35 mol %) is used, its catalytic activity is high. The
excellent performance of yttrium triflate makes it
potential for commercial application in the transesterꢀ
ification of DMC with ethanol.
n(EtOH) : DMC converꢀ DEC selectiviꢀ EMC selectiviꢀ
n(DMC)
sion, %
ty, %
ty, %
1 : 3*
1 : 1
3 : 1
5 : 1
6 : 1
7 : 1
8 : 1
43.5
71.3
85.8
87.4
89.2
87.9
89.2
14.9
34.5
53.1
79.7
85.1
83.0
85.1
83.4
61.3
40.4
19.2
13.6
15.6
13.9
Note: Reaction conditions: 0.35 mol % of catalyst based on DMC,
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