Synthesis of dimethyl carbonate from methanol and CO2 on the SnO2/Al2O3-based catalyst

Article abstract of DOI:10.1016/j.mencom.2016.11.012

The SnO₂ /Al₂O₃ catalyst promoted with CuCl₂, ZnCl₂, KF exhibited high activity in the synthesis of dimethyl carbonate from methanol and CO₂ (130?°C, 16?bar, 4?h) providing the 17.8% yield of the product with ca. 99% selectivity.

Full text of DOI:10.1016/j.mencom.2016.11.012

Mendeleev
Communications
Mendeleev Commun., 2016, 26, 497–499
Synthesis of dimethyl carbonate from methanol
and CO2 on the SnO2/Al2O3-based catalyst
Alexander A. Greish,* Elena D. Finashina, Olga P. Tkachenko, Elena V. Shuvalova and Leonid M. Kustov
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 119991 Moscow, Russian Federation.
Fax: +7 499 137 2935; e-mail: greish@ioc.ac.ru
DOI: 10.1016/j.mencom.2016.11.012
The SnO2/Al2O3 catalyst promoted with CuCl2, ZnCl2, KF
130 °C, 16 bar
exhibited high activity in the synthesis of dimethyl carbonate
from methanol and CO2 (130°C, 16 bar, 4 h) providing the
17.8% yield of the product with ca. 99% selectivity.
2MeOH + CO2
(MeO)2C=O
SnO2(Cu,Zn,K)/Al2O3
Yield 17.8%
Selectivity >98%
Carbon dioxide utilization attracts great interest in view of
global warming problem and the environmental protection against
waste gases of chemical and power industries. Currently there
are several strategies for CO2 utilization.1,2 Among them, a
promising approach is the direct synthesis of dimethyl carbonate
(DMC) from CO2 and methanol. DMC is widely used as a
methylation agent, a solvent and a fuel additive.3–6 In principle,
the formation of DMC from methanol and CO2 can be presented
by the equation
DMC from CO2 and MeOH. The catalyst required was prepared
on the basis of the supported SnO2/Al2O3 system promoted by
CuCl2, ZnCl2, KF additives.† The catalyst was investigated by
diffuse reflectance infrared Fourier-transform (DRIFT), X-ray
photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and
thermogravimetry combined with differential thermal analysis
(TG-DTA).
When solving the problem of the catalyst composition for the
DMC synthesis, we reasoned that the catalyst should generate
simultaneously methoxy groups from methanol and carboxylate
or carbonate species from CO2. Obviously, to meet these require-
ments, one could include metal oxides of the weak basic or
amphoteric nature into the catalyst composition. From this point
of view the SnO2/Al2O3 system seemed to be suitable, moreover,
it could be easily prepared by the incipient wet impregnation
of Al2O3 with a solution of SnCl4. To improve basic properties of
2MeOH + CO2 ® (MeO)2CO + H2O.
However, the equilibrium yield of DMC in this reaction as usual
does not exceed 1–1.5%. The main cause of that is the unfavorable
thermodynamics of the reaction. The Gibbs energy DG for this
process at 130°C is 45.5 kJ mol–1 > 0 that shows that the reaction
is not spontaneous.7 To increase the yield of DMC, the equilibrium
of the reaction can be shifted to the right by removal of water
formed in the process.8
†
The reaction was carried out in the mini 8.5 cm3 autoclave. The catalyst
(250 mg, particle size 0.2–0.4 mm) was mixed with crushed quartz (particle
size 0.5–1 mm) in the ratio of 1:3 and loaded into the 1.5 cm3 metal
gauze basket fixed in the upper part of the autoclave. A layer of calcined
B2O3 (50 mg, particle size 0.5–1 mm) was placed on the catalyst bed.
The function of B2O3 was to remove water produced during the reaction.
In the middle of the autoclave there was a micro-fan which stirred the gas
phase in the reactor with a rate of 500 min–1. Before the run the autoclave
was purged with CO2, then methanol (0.3 ml) was injected by a syringe.
The autoclave was closed and filled with CO2 up to 16 bar (room tem-
perature); the MeOH:CO2 molar ratio was 1:1.3. The autoclave was
heated in the oil bath. The runs were carried out at 130°C, and the reaction
time was varied from 1 to 4 h. The products were analyzed by GLC
[Crystal 5000 gas chromatograph (Russia) equipped with FID and the
capillary column with SE-30].
Catalyst preparation. The commercial g-Al2O3 (Sasol, Germany, 0.20 mm,
150 m2 g–1) was impregnated to incipient wetness with a solution of
SnCl4 and left for 12 h at room temperature. Then the catalyst sample was
dried at 100°C for 2 h and calcined in the dry air flow at 600°C for 2 h,
the heating rate in the range of 20–600°C was 5 K min–1. The obtained
SnO2/Al2O3 sample was sequentially impregnated to incipient wetness
with the appropriate solutions of CuCl2, ZnCl2 and KF. Between last
impregnations the catalyst was dried at 100°C for 1 h. After supporting
of all constituents on the catalyst, the latter was calcined at 400°C for 2 h.
The content of SnO2, CuCl2, ZnCl2, KF in the final catalyst was 14.5, 1.2,
1.2, 6 mol% (or 20, 1.5, 1.5, 3.3 wt%), respectively. Directly before the
loading in the reactor the catalyst sample was calcined once more in dry
air at 400°C for 1 h.
Both homogeneous and heterogeneous catalysts have been
applied to this reaction.9–14 Early reports offered Bu2Sn(OAlk)2
(Alk = Me, Et, Bu) as the catalysts which provided ~100%
selectivity to DMC.15 The key step in the reaction on these
catalysts is supposed to be the insertion of CO2 into the Sn–OR
bonds of the corresponding methoxy precursors. Among hetero-
geneous systems used in carboxylation of methanol it is worth to
note the catalysts on the basis of CeO2,10,13,16,17 ZrO2,10,18,19
TiO220 or their mixed compositions.21,22 Some promotion effect
to the DMC formation was achieved by adding 0.2–0.5% reduced
copper to the ceria catalyst.23 To enhance efficiency of the process,
the original membrane reactor which combined the catalytic
reaction and separation was developed24 when KF-modified Cu
catalyst on MgO–SiO2 support was employed.
Previously25 the synthesis was carried out over the Cu–Ni
bimetal catalysts supported on the molecular sieve, which increased
the DMC yield. On using the catalyst with bimetal loading of
20 wt% (123°C, 1.1 MPa, 5 h) the DMC yield was 5%. Note-
worthy, the activity for the DMC synthesis was also raised by
employing a supported Cu–Ni/graphite bimetallic catalyst,26 the
highest yield of DMC exceeding 9% at 105°C and 1.2 MPa, and
the selectivity was ~88%.
The aim of the present work was to develop the heterogeneous
catalyst which could exhibit the higher efficiency in the synthesis of
© 2016 Mendeleev Communications. Published by ELSEVIER B.V.
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.
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Mendeleev Commun., 2016, 26, 497–499
Table 1 Results of the DMC synthesis over the SnO2(Cu,Z,K)/Al2O3
bonds in two forms of the methanol adsorption complexes. The
bands at 2954, 2830 cm–1 are related to the CH vibrations in
the Me groups of adsorbed methanol, while the bands at 2904,
2778 cm–1 can be attributed to the CH vibrations of methoxy
groups, which may appear when methanol adsorbs on the
cationic sites of the catalyst.27
Some changes are observed in the region of the C–O vibra-
tions, in particular, a decrease in the intensity of the band at
1772 cm–1 which corresponds to the stretching vibration nas C=O
in the covalently bound carbonate (O–CO2). Probably when
methanol is adsorbed on the catalyst the quantity of sites con-
taining OH groups of basic nature, which interact with adsorbed
CO2, can increase.
catalyst.
DMC
Selectivity (%)
Time/h
Yield (%)
1
2
4
3.2
12.7
17.8
99.1
98.6
98.3
the catalyst responsible for generating surface carbonate com-
pounds, we modified it with small amounts of CuCl2, ZnCl2 and
KF which could serve as the additional promoters for the CO2
activation. Therefore, the prepared catalyst contained SnO2, CuCl2,
ZnCl2, KF (14.5, 1.2, 1.2, and 6 mol%, respectively) in the final
SnO2(Zn,Cu,K)/Al2O3 sample. Table 1 shows the main results
obtained in the runs on the DMC synthesis from methanol and
CO2 with such a catalyst. Obviously, this catalyst exhibits rather
high activity in the DMC formation, the yield of DMC approaches
18%, with selectivity of 98–99%.
In the Me region there are two types of vibration: the bands
at 2778 and 2904 cm–1 are associated with the symmetric and
asymmetric modes of Me groups of chemisorbed methoxy groups;
the bands at 2954 and 2830 cm–1 are attributed to symmetric and
asymmetric Me stretching modes of physisorbed methanol.
A sharp decrease and then almost disappearance of the band
at 2348 cm–1 indicate that next to nothing of chemisorbed CO2
remains. It can be supposed that CO2 previously adsorbed on
the catalyst either takes part in any chemical transformations, or
converts to another adsorption form. Moreover, one can note a
small band at 1533 cm–1 which can attributed to the carbonate
anions CO32–.27 After 17 h exposure of the catalyst under the
methanol vapor, a band at 1373 cm–1 appears, which can be ascribed
to the stretching vibrations ns of carboxylate anion COO–.27
It is known that chemisorption of alcohols occurring on the
catalyst comprising transition metal oxides may cause formation
of the corresponding alkoxides from alcohols under the action of
coordinatively unsaturated cations or the oxygen anions.27 Two
possible mechanisms for the formation of methoxy groups during
adsorption of methanol on SnO2 are proposed: (1) through the
formation of ester; (2) due to the dissociation of methanol on the
ion pairs.32 It has been shown that methanol being adsorbed at
ambient temperature on Al2O3 did not dissociate on the oxide
surface but formed a strong hydrogen bond with the cation–anion
pair, which had strong basic character.33
In another experiment, methanol was first adsorbed on the
catalyst, and then CO2 was introduced into the ampoule. Figure 2
shows that introduction of CO2 leads to remarkable changes in
the DRIFT spectra, especially in the region of 1800–1300 cm–1.
First of all, the high intensity band at 1670 cm–1 appears, which
can be attributed to the stretching vibrations of the C=O bonds in
the acid carbonate HOCO2, while a small shoulder at 1630 cm–1
can belong to stretching vibrations of C=O (COO–) in the bidentate
carbonate.27 The bands of small intensity at 1469 and 1371 cm–1
are likely related with the stretching vibrations of the C=O
bond in monodentate carboxylate COO–, although they also can
be attributed to monodentate methyl carbonate.34 Note that after
evacuation of the catalyst, the DRIFT spectrum contained the
band at 1606 cm–1, which can characterize the stretching vibra-
tions in the bidentate carbonate bonded with the on-top metal
cations.
The great assistance in disclosure of the DMC synthesis
mechanism on the developed catalyst has been provided by the
DRIFT method, mainly due to the identification of the surface
compounds formed on the catalyst in the course of adsorption of
methanol and CO2. In our first experiment, carbon dioxide at 1 bar
and ~20°C was adsorbed on the catalyst sample placed in the IR
ampoule, after some time the ampoule with the catalyst was
evacuated and then filled with the methanol vapor, that is adsorp-
tion of methanol was performed on the catalyst with preadsorbed
CO2. The DRIFT spectra obtained in this experiment are shown
in Figure 1. One can see that adsorption of CO2 on the catalyst
leads to appearance of a series of intense bands at 1490, 1644,
1772 cm–1 as well few bands of lower intensity, which can be
attributed to the stretching vibrations of the chemosorbed carbon
dioxide and different types of carbonate structures.27–30
According to the classification,27,31 the band at 1772 cm–1
can be attributed to the stretching vibration nas C=O which is
typical of the organic carbonate (O–CO2). The bands nas 1644 and
ns 1490 cm–1 evidently correspond to the stretching vibrations
n
C=O in the bidentate and monodentate carbonates, respectively.
Smaller in the intensity bands at 1668 and 1695 cm–1 can be
attributed to bicarbonate (HOCO2). The spectrum also contains an
intense band at 2348 cm–1 which can be ascribed to the stretching
vibrations of C–O bonds of chemisorbed CO2.
Introduction of methanol into the ampoule containing the
catalyst with preadsorbed CO2 leads to remarkable changes in
the DRIFT spectra. First of all, appearance of two pairs of the
bands in the region of CH vibrations occurs, which correspond
to the asymmetric and symmetric stretching vibrations of C–H
7
6
5
4
3
We also studied the joint adsorption of CO2 and MeOH on
the catalyst at room temperature. Adsorption of the MeOH/CO2
mixture results in appearance of bands at 4837, 5140 cm–1, which
can be ascribed to the vibrations of the methyl groups of adsorbed
methanol and the composite vibrations of water, respectively. The
latter can be regarded as an evidence of methanol carboxylation
which really occurs on the catalyst, with water being formed as
a secondary product of this reaction.
1
2
2
3
4
1
0
3200
2800
2400
2000
1600
1200
Wavenumber/cm–1
Figure 1 DRIFT spectra of adsorbed CO2 and MeOH on the sample
SnO2(Cu, Zn, K)/Al2O3: (1) after introduction of CO2 at 1 bar and 20°C;
(2) 10 min after short evacuation of the sample and introduction of MeOH at
saturated vapor pressure at 20°C; (3) after 17 h; (4) after evacuation of the
sample at 400°C for 1 h.
To get the information on elemental composition, chemical
and electronic state of the elements of the SnO2(Cu,Zn,K)/Al2O3
catalyst, we investigated it by XPS method (see Online Supple-
mentary Materials). According to the obtained XPS data, the active
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Mendeleev Commun., 2016, 26, 497–499
the hemicarbonate bonded with the coordinatively unsaturated
3
2
1
0
metal cation. The final stage of the reaction, obviously, is the inter-
action of the hemicarbonate with the second methanol molecule
that should afford the target product, DMC.
In conclusion, we prepared a new catalyst on the basis of the
supported SnO2/Al2O3 system promoted with CuCl2, ZnCl2, KF,
which showed the high efficiency in the direct synthesis of DMC
from methanol and CO2.
4
3
2
1
3200
2800
2400
2000
1600
1200
This work was supported by the Russian Science Foundation
(grant no. 14-33-00001).
Wavenumber/cm–1
Figure 2 DRIFT spectra of adsorbed MeOH and CO2 on the sample of
SnO2(Cu,Zn,K)/Al2O3: (1) 17 h after introduction of MeOH at saturated
vapor pressure at 20°C; (2) 10 min after introduction of CO2 at 1 bar and
20°C; (3) after 75 h; (4) after evacuation of the sample at 400°C for 2 h.
Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.mencom.2016.11.012.
surface of the catalyst sample was little enriched with Sn com-
pared with the calculated concentration of Sn in the catalyst,
the atomic Sn/Al ratio in the surface and subsurface layers of a
thickness of ~30 Å is 0.134 vs. 0.100. The atomic Cl/Al ratio in
the catalyst is close to the content of Cl in ZnCl2 and CuCl2. The
XPS data point out that copper exists in two electronic states,
Cu2+ and Cu+, on the catalyst surface.
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Figure 3 Postulated mechanism for the DMC formation from methanol
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Received: 29th July 2016; Com. 16/5016
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