Transesterification of cyclic carbonates to dimethyl carbonate using solid oxide catalyst at ambient conditions: Environmentally benign synthesis

Article abstract of DOI:10.1002/cssc.201000038

Continuous synthesis at ambient conditions: Dimethyl carbonate (DMC) is an important methylating and carbonylating agent. Transesterification of cyclic carbonates using methanol for the synthesis of DMC is environmentally benign. CaO-ZnO catalysts, prepared by a wet impregnation method, are effective catalysts for the transesterification of ethylene carbonate using methanol in batch and in continuous reactors. Yields of ca. 84?% DMC can be achieved at ambient conditions.

Full text of DOI:10.1002/cssc.201000038

DOI: 10.1002/cssc.201000038
Transesterification of Cyclic Carbonates to Dimethyl Carbonate Using Solid
Oxide Catalyst at Ambient Conditions: Environmentally Benign Synthesis
[
a, d]
[b, d]
[c, d]
Meenakshisundaram Sankar,*
Srikanth Satav,
and Palanichamy Manikandan*
Dimethyl carbonate (DMC) is used for a variety of applications
in the chemical industries owing to its low toxicity, high
Many basic metal oxide-based heterogeneous catalysts have
been reported for the synthesis of DMC from cyclic carbo-
[
1,2]
[5]
oxygen content, and versatile chemical nature.
DMC is a
nates. However, these reactions need severe reaction condi-
[5,9]
safe and environmentally acceptable alternative for phosgene
and dimethyl sulfate as carbonylating and methylating agents,
because of the higher reactivity of DMC towards nucleophilic
tion like high temperature and/or high CO pressure.
It is
2
rare to see reports on catalyst systems that are active at ambi-
[10]
ent conditions especially in a continuous operation. A con-
tinuous process for the synthesis of DMC from EC at room
temperature is attractive from an industrial point of view. Thus,
the objective of the present work was to develop an efficient
heterogeneous catalytic system for the above transformation
at ambient conditions with a flexibility of operating in both
batch and continuous reactors.
[
3]
molecules, such as amines and phenols. DMC is traditionally
produced from phosgene and CO routes that are environmen-
tally hazardous and rigorous safety measures are involved with
[
4,11]
these processes.
Thus, synthesis of DMC through non-toxic
and environmentally benign routes has received considerable
[
4]
attention in recent years. Two alternate green routes are
available for the synthesis of DMC. One is through the direct
Herein, we report CaO–ZnO-based catalysts with varying Ca/
Ca+Zn ratios for the synthesis of DMC at ambient conditions
both in batch and continuous processes. These catalysts were
reaction of methanol with CO where the DMC yield is restrict-
2
[
4]
ed by thermodynamics. The other route is transesterification
[
5]
[11]
of cyclic carbonates with methanol (Scheme 1). Cyclic carbo-
prepared by a wet impregnation method. These catalysts
were tested, initially in a batch reactor, for the transesterifica-
tion of EC by methanol under an atmospheric pressure and
the reaction conditions were optimized systematically. In order
to optimize the catalyst loading, the reaction was carried at
ambient conditions with varying catalyst weight in the range
0.05–0.5 g for 0.05 mol of EC. The Ca/Ca+Zn ratio of the cata-
Scheme 1. Transesterification of ethylene carbonate using methanol.
lyst was 0.5 and the EC:MeOH ratio was 10. The products at
the end of 1 h reaction time were analyzed by GC (see the
Supporting Information). The DMC yield increased steeply up
to 0.25 g catalyst loading and then reached near plateau there-
after. Thus, the catalyst weight of 0.25 g was found to give
maximum DMC yield of ca. 84 mol%. Even at the lowest cata-
lyst loading of 0.05 g, the DMC yield was 55 mol%, indicating
the efficiency of the catalyst. In the absence of catalyst, the
DMC yield was negligible even after 10 h. In order to under-
stand the effect of the reaction temperature on the DMC yield,
the reactions were carried out at different temperatures in the
range of 283–313 K and the resultant kinetic data are shown in
Figure 1.
nates can be produced quantitatively by CO insertion into ep-
2
[
6]
oxides. Recently, Asahi Kasei commercialized a process for
the transesterification of ethylene carbonate (EC) to produce
[
7]
DMC though at an elevated temperature range.
Many homogeneous and heterogeneous catalytic systems
have been reported for the transesterification of cyclic carbo-
nates with methanol, which resulted in high selectivity to
[
5,6,8]
DMC.
Although homogeneous catalyst systems are more
active, downstream catalyst separation is usually an issue.
[a] Dr. M. Sankar
School of Chemistry, Cardiff University
Cardiff, CF10 3AT (United Kingdom)
Fax: (+44)2920874030
The experiments were carried with the catalyst having a low
Ca/(Ca+Zn) ratio of 0.2 in order to collect sufficient data points
over a wider range of temperatures. As seen in Figure 1, the
catalyst was active and efficient for the entire temperature
range employed. In general, the DMC yield increased steeply
at the beginning and then reached the chemical equilibrium.
At 300 K, the DMC yield reached around 80% within 4 h. Al-
though the DMC yield increased slightly at higher tempera-
tures up to 313 K, the temperatures higher than that did not
improve the DMC yield further. In fact, slight decomposition of
cyclic carbonates occurred at higher temperatures.
E-mail: sankarncl@gmail.com
[b] S. Satav
Department of Chemical and Bioengineering
ETH Honggerberg, HCI E115, 8093 Zurich (Switzerland)
[c] Dr. P. Manikandan
Dow Chemical International Pvt Ltd, Pune (India)
Fax: (+91)2066269555
E-mail: palanimani@yahoo.com
[
d] Dr. M. Sankar, S. Satav, Dr. P. Manikandan
Catalysis and Inorganic Chemistry Division
National Chemical Laboratory
Transesterification of EC with methanol was carried out with
catalysts with different Ca/(Ca+Zn) ratios at room temperature
for a fixed amount of time. The catalytic activity increased in a
nearly linear fashion with the Ca content, indicating that CaO
Dr. Homi Bhabha Road, Pune-411 008 (India)
Fax: (+91)20-2590 2633
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201000028.
ChemSusChem 2010, 3, 575 – 578
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
575

To prove the tolerance of the present catalyst system for
higher homologous of EC, transesterification reaction of pro-
pylene carbonate (PC) with methanol was also tested. With the
CaO–ZnO catalyst (Ca/(Ca+Zn) ratio=0.4), the transesterifica-
tion reaction of propylene carbonate yielded 47 mol% DMC in
4
h at room temperature (Table 1). The DMC yield from the PC
reaction was less than that of the EC reaction. As proposed in
other literature, the low activity with propylene carbonate may
[5]
be due to the steric hindrance associated with the PC.
A possible continuous operation of a reaction with a hetero-
geneous catalyst is not only highly desirable to increase the
productivity but also shows the efficiency of the solid catalyst.
The versatility of the present catalyst system was demonstrat-
ed with a continuous DMC production from EC using a simple
fixed-bed continuous-flow reactor setup (Figure 2). Reactant
Figure 1. Kinetic plots for various temperatures (&: 283 K , *: 293 K, ~:
03 K, and !: 313 K); batch reaction; EC=0.05 mol, Ca/Ca+Zn ratio=0.2,
3
MeOH=0.5 mol, catalyst wt=0.25 g.
is the active component in the present reaction. A similar reac-
tion with pure ZnO showed no trace of DMC (Table 1, entry 1),
which substantiates that CaO is the active component. Howev-
Table 1. Catalytic performance of the present catalytic systems, catalyst
[
a]
reusability, and efficiency for different substrates
.
Entry Catalyst Substrate Time [h] DMC yield [%]
1
2
3
4
5
ZnO
EC
EC
4.0
4.0
4.0
4.0
4.0
0
Figure 2. Schematic representation of the reactor used for the continuous
synthesis of DMC. A: reservoir for reactants; B: liquid feed pump; C: catalyst
CaO–ZnO (0.4) (fresh)
83
82
83
47
bed; D: product collection point; T
1
:water inlet from thermostat to maintain
andW : cooling water circulation
reactor temperature; T : water outlet; W
2
1
2
CaO–ZnO (0.4) (1st recovery) EC
CaO–ZnO (0.4) (2nd recovery) EC
for condensing the products; solid arrows indicate the flow of reactants and
products (upward flow).
CaO–ZnO (0.4)
PC
mixture containing MeOH and EC was taken in the reservoir
with a required molar ratio and was pumped by a feed pump
in an up-flow manner. The results showed that the catalyst
was active and efficient at ambient conditions even with the
continuous mode of operation. Experiments to study the effect
of MeOH:EC molar ratio (2 to 10) on the DMC yield were car-
ried out at ambient conditions at a liquid hourly space velocity
[
0
a] Substrate=4.4 g, p-xylene=1 g, MeOH=16 g, T=300 K, catalyst wt=
.25 g. The values in parentheses represent the ratio of Ca/Ca+Zn.
er, a difficulty encountered in the catalyst separation, when the
reaction was carried out with pure CaO, supports the binding
effect of ZnO to CaO in facilitating the catalyst separation in
the present system.
À1
(LHSV) of 2 h . EC conversion increased with increase in the
MeOH:EC molar ratio. The DMC yield of 70 mol% was obtained
at the MeOH:EC ratio of 2 and it increased gradually on in-
creasing the MeOH:EC ratio (reached 88% mol when the ratio
was 10). This shift towards a higher DMC yield at higher
MeOH:EC was expected for any equilibrium controlled reaction
in which higher MeOH:EC ratio shifts the equilibrium towards
The reusability of the catalyst was demonstrated as follows:
the present CaO–ZnO catalyst was filtered after the reaction,
washed with methanol to remove organic reactant and prod-
ucts, dried at 373 K for 5 h, and then reused for a fresh batch
of reactants. The cycle was repeated twice and the activity of
the catalyst was retained without loss in activity (Table-1, en-
tries 3 and 4). To the best of our knowledge, the DMC yield of
[12,13]
the synthesis of DMC.
Higher the LHSV, larger the produc-
tivity for a given catalyst amount. Hence, experiments were
carried out to monitor the DMC yield with 6 g catalyst loading
at different LHSVs and the results were plotted (Figure 3).
It is interesting to find that the DMC yield is more or less the
8
4% obtained with the present catalyst system is the highest
yield produced by any reported heterogeneous catalyst sys-
tems at ambient conditions. A table is presented in the Sup-
porting Information, which compares the activity of the pres-
ent catalytic system with some of the known catalytic systems.
À1
same even up to a LHSV of 16 h . To the best of our knowl-
edge, this value is the highest space velocity reported for the
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Figure 3. Catalytic performance of the catalyst (Ca/Ca+Zn=0.4) at 300 K in
À1
À1
the continuous reaction for various LHSVs (2 h to 16 h ); MeOH vs EC
Figure 5. XRD pattern of Ca/Ca+Zn=0.4 catalyst: a) fresh catalyst, b) spent
deactivated) catalyst, and c) catalyst calcined at 1173 K for 5 h (active). O:
molar ratio=10; catalyst wt=6 g.
(
peaks corresponding to CaO.
cyclic carbonate transesterification reaction for the production
of DMC at ambient conditions. The commercial production of
DMC by the transesterification of EC by Asahi Kasei is being
carried out at 353–393 K, however, the present catalytic system
is effective even at 300 K. Time-on-stream study for the system
with 6 g catalyst loading was carried out at ambient conditions
showed the reflections corresponding to CaO, which are totally
absent in the spent catalyst. This deactivated catalyst was cal-
cined at 1173 K for 5 h; the XRD spectrum of this recalcined
sample showed CaO peaks but with much lower intensities
(Figure 5).
À1
at a LHSV of 2 h for several hours (Figure 4).
The reappearance of CaO peaks in the XRD pattern indicates
that part of the CaO has been transformed into another amor-
phous phase (as it is seen in the XRD pattern, Figure 5) over
the reaction time of 60 h. The Ca contents of the catalysts
were estimated using atomic absorption spectroscopy; deacti-
vated catalyst showed less amount of Ca compared to the
fresh catalyst. Thus, it is concluded that the deactivation pro-
cess may be due to the combination of leaching and formation
of dispersed CaCO or Ca(OH) .
3
2
In conclusion, we report CaO–ZnO-based catalysts for the
synthesis of DMC by the transesterification of cyclic carbonates
using methanol with DMC yields up to 84% at ambient reac-
tion conditions. The superiority of the present catalyst systems
is demonstrated by both batch and continuous reactions. With
the continuous reactor system, the catalyst was found to be
active for more than 60 h and the reaction could be carried
À1
out up to a LHSV of more than 16 h . CaO was found to be
Figure 4. Stability of catalyst in the continuous reaction for the catalyst (Ca/
Ca+Zn=0.4) at 300 K for LHSV of 2 h ; MeOH vs EC molar ratio=10; cata-
lyst wt=6 g.
the active phase for the transesterification reaction and ZnO
provided the binding effect to make the present catalyst an ef-
ficient solid catalyst. In the case of the batch reaction, the cata-
lyst was recovered at least twice without losing activity and
with the continuous mode of operation, the catalyst was
stable for more than 60 h.
À1
The catalyst was found to retain its activity up to 60 h and
thereafter, a slight decrease in the activity was detected. This
observation could be due to the disintegration of the catalyst
into finer particles of Ca(OH)2 which leaches out along the
product stream. Clear downstream products turned into a
milky solution at the end of 60 h of operation of time-on-
stream experiments, which confirmed the catalyst disintegra-
tion. The reason for the reduced activity of the catalyst after
Acknowledgements
Authors thank the Council of Scientific and Industrial Research
(
CSIR), New Delhi, for financial support (P23CMM0005). M. S.
thanks CSIR, New Delhi, for the fellowship. Authors thank Dr. C.
V. V. Sathyanarayana and Mr. T. M. Sankaranarayanan for their
help.
60 h of continuous reaction was also analyzed by powder X-
ray diffraction (XRD) of fresh and spent catalysts (recovered
after 60 h of reaction). As seen in Figure 5, the fresh catalyst
ChemSusChem 2010, 3, 575 – 578
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
577

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Keywords: calcium
·
environmental
chemistry
·
heterogeneous catalysis · transesterification · zinc
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Received: February 12, 2010
Published online on March 31, 2010
5
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