Dimethyl carbonate synthesis from carbon dioxide and methanol over CeO 2versus over ZrO2: Comparison of mechanisms

Article abstract of DOI:10.1039/c4ra03081h

A comparison of the mechanisms of dimethyl carbonate (DMC) formation directly from carbon dioxide and methanol over CeO₂versus over ZrO₂ is made through in situ Fourier transform infrared spectroscopy (FTIR). During the reaction involving methanol and CO₂ adsorption over CeO₂, a new band appears at around 1295 cm^-1. Combining this result with in situ FTIR results of methyl formate adsorption, this band is assigned to carbomethoxide, which is taken as the intermediate in DMC formation over the ceria surface. Carbomethoxide originates from the reaction of methanol and adsorbed carbon dioxide; its formation is followed by reaction with a methoxy group to form DMC. This mechanism differs from that occurring on zirconium oxide, in which DMC is formed by the reaction between monodentate methyl carbonate and methanol. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra03081h

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Dimethyl carbonate synthesis from carbon dioxide
and methanol over CeO2 versus over ZrO2:
comparison of mechanisms
Cite this: RSC Adv., 2014, 4, 30968
*
Lei Chen, Shengping Wang, Jingjie Zhou, Yongli Shen, Yujun Zhao and Xinbin Ma
A comparison of the mechanisms of dimethyl carbonate (DMC) formation directly from carbon dioxide and
methanol over CeO2 versus over ZrO2 is made through in situ Fourier transform infrared spectroscopy
(FTIR). During the reaction involving methanol and CO2 adsorption over CeO2, a new band appears at
around 1295 cmꢀ1. Combining this result with in situ FTIR results of methyl formate adsorption, this band
is assigned to carbomethoxide, which is taken as the intermediate in DMC formation over the ceria
surface. Carbomethoxide originates from the reaction of methanol and adsorbed carbon dioxide; its
formation is followed by reaction with a methoxy group to form DMC. This mechanism differs from that
occurring on zirconium oxide, in which DMC is formed by the reaction between monodentate methyl
carbonate and methanol.
Received 7th April 2014
Accepted 18th June 2014
DOI: 10.1039/c4ra03081h
www.rsc.org/advances
ceria surface. It would be helpful if CO2 could be activated more
easily in DMC formation. When CO2 activation is involved in the
Introduction
mechanism, the elementary steps should be different from that
on zirconium; however, most researchers do not take this into
account when they present the mechanism occurring on the
ceria catalyst.8,14
In this paper, CeO2 and ZrO2 were prepared by the sol–gel
method. In situ Fourier transform infrared spectroscopy (FTIR)
was applied to track each reaction step to identify any differ-
ences between the mechanisms involving ceria catalyst and
zirconium oxide.
Carbon dioxide conversion has been attracting great interest
because of concerns about global warming and sustainable
development in society. There are several strategies for CO2
utilization;1ꢀ4 among these, a promising approach is direct
synthesis, from CO2 and methanol, of dimethyl carbonate
(DMC), which is widely used in industrial processes, for
example as a fuel additive, methylation agent and solvent.
Converting CO2 into environmentally friendly compounds can
not only relieve ‘greenhouse’ damage, but also provide CO2 as a
carbon source alternative to coal, natural gas and petroleum.
Both homogeneous and heterogeneous catalysts have been
applied in this reaction system.5ꢀ9 Zirconium oxide has been
proven to be a useful heterogeneous catalyst, on which the
mechanism of DMC formation is well studied based on acidic
and basic properties.10 In recent years, cerium oxide has been
applied in many elds,11,12 and also has been demonstrated to
be an effective catalyst in DMC formation as a result of the
acidity and basicity on the catalyst surface.8,13 However, the
presently understood mechanism of DMC formation on ceria
refers to that occurring on zirconium oxide, in the absence of
comprehensive investigation,8,14 which is not favorable for
further modifying the catalyst surface and enhancing catalytic
activity. Recently, CeO2 has been attracting interest for its high
oxygen storage capacity in CO2 activation, and some
researchers15,16 have conrmed that CO2 can be activated on a
Experimental design
Catalyst preparation
The CeO2 and ZrO2 were prepared by the sol–gel method with
Ce(NO3)3$6H2O and Zr(NO3)4$4H2O as precursors. Solutions
were made from 50 ml deionized water in which 5.211 g
Ce(NO3)3$6H2O or 1.288 g Zr(NO3)4$4H2O was dissolved; then
the solutions were transferred into 250 ml beakers and 5.04 or
1.26 g citric acid was added to the cerium solution or zirconium
solution, respectively. The mixed solutions were stirred for 3 h
under 353 K, then vaporized at the same temperature to remove
the water until sols were obtained. The sols were dried at 373 K
overnight and then calcinated at 873 K for 5 h.
Catalytic test
Direct formation of DMC from CO2 and methanol was carried
out in an autoclave. Catalyst (0.1 g) and 15 ml CH3OH were put
into the autoclave. The reactor was pressurized with 5 MPa CO2.
Then the reactor was heated to 413 K and stirred for 2 h. As an
Key Laboratory for Green Chemical Technology, School of Chemical Engineering and
Technology, Tianjin University, Collaborative Innovation Center of Chemical Science
and Engineering, Tianjin 300072, China. E-mail: spwang@tju.edu.cn; Fax: +86-22-
87401818; Tel: +86-22-87401818
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internal standard substance, 1-propanol was added to quantify deoxidized in deoxidizing tube. DMC, methanol, formic acid
the mixture. Products were analyzed with a gas chromatograph and methyl formate were introduced into the reaction unit by
(GC, Agilent 4890 D).
the He stream through a bubbling sampler, which was main-
tained at 298 K.
Catalyst characterization
X-ray diffraction was measured with a Rigaku D/max 2500
diffractometer equipped with Cu Ka radiation at a diffractiꢀo1n
Results
angle ranging from 10ꢁ to 90ꢁ with a scanning rate of 8ꢁ min
.
Table 1 shows the catalytic activities of CeO2 and ZrO2 for the
synthesis of DMC from CO2 and methanol. DMC is the only
product under the experimental conditions. It can be clearly
seen that CeO2 exhibits more favorable activity in DMC
synthesis than ZrO2, which is consistent with the results of
other researchers.17,18
Fig. 1 shows the XRD patterns of CeO2 and ZrO2 catalysts. As
shown in Fig. 1(a), diffraction peaks at 2q ¼ 28.6ꢁ, 33.1ꢁ, 47.6ꢁ
and 56.4ꢁ are associated with the cubic uorite phase of CeO2,
corresponding to the planes of (111), (200), (220) and (311),
respectively. In Fig. 1(b), diffractions shown at 2q ¼ 28.2ꢁ and
31.5ꢁ are assigned to the monoclinic phase of ZrO2, and the
peak at 2q ¼ 30.2ꢁ is attributed to the tetragonal phase of ZrO2.
This implies that ZrO2 is a mix of monoclinic and tetragonal
phases, which accords with the result of Parghi.19 There are no
other diffraction peaks in ZrO2 or CeO2, indicating that ZrO2
and CeO2 are both at high purity.
In situ FTIR was carried out using a Nicolet spectrometer
with an MCT detector. Four scans were averaged with a reso-
lution of 4 cmꢀ1. Before the experiment, the samples were
pressed into a disc and transferred to a homemade reaction cell
that used ZnSe windows. The temperature of the cell was
controlled with a programmable temperature controller.
The disc was pretreated under a He stream at 673 K for 1 h;
then the sample was cooled to reaction temperature in the He
stream before exposure to reactant stream. The entire process
was conducted at 0.1 MPa. The ow rates of He and CO2 were
15 ml minꢀ1 and 10 ml minꢀ1, respectively. The He stream was
passed through a drying agent to remove water, and was
Table 1 Catalytic activity of different catalysts in DMC formation
ZrO2
0.21
CeO2
5.34
Fig. 2 shows the IR spectra of methanol adsorption on ceria
catalysts. Methanol interacts with the surface of ceria catalyst
via the oxygen atoms. Features for n(CO) of methoxy grow
simultaneously at 1097, 1054 and 1012 cmꢀ1, attributed to on-
top methoxy, bridging methoxy and three coordinate methoxy
groups, respectively.20 The intensity of the on-top methoxy band
rises more slowly than those of the others. The band around
1031 cmꢀ1 may be ascribed to bridging methoxy or physisorbed
methoxy. In the n(CH3) region there are two types of vibration:
bands at 2806 and 2915 cmꢀ1 are associated with the modes of
n(CH3) and nas(CH3) of chemisorbed methoxy; 2834 and 2934
cmꢀ1 are attributed to symmetric and asymmetric CH3
stretching modes of physisorbed methanol. Bands observed at
1575, 1453 and 1369 cmꢀ1 represent monodentate methyl
mmol DMC per g cat
carbonate (MMC). A band also emerges around 1295 cmꢀ1
,
Fig. 1 XRD patterns of different catalysts (a) CeO2 (b) ZrO2.
which so far has not been assigned. Fig. 3 shows the spectra of
Fig. 2 FTIR spectra of methanol adsorption on CeO2.
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Fig. 3 FTIR spectra of methanol adsorption on ZrO2.
Fig. 4 FTIR spectra following the introduction of CO2 after methanol adsorption on CeO2.
successive methanol adsorption on ZrO2 surface, which is 3674 cmꢀ1) is associated with different types of OH groups on
identical with the results of other researchers,21 except for the the ZrO2 surface.22
intensity. Bands at 1162 and 1032 cmꢀ1 are associated with the
Exposure of ceria catalysts containing preadsorbed meth-
bending vibration of methoxy groups, and bands at 2822 and anol to the CO2 stream changes the infrared spectra remark-
2924 cmꢀ1 with increasing intensity are ascribed to the C–H ably. As seen in Fig. 4, the band representing on-top methoxy
stretching vibration of bidentate and monodentate methoxy decreases, but the features for MMC (1574, 1457 and 1374 cmꢀ1
)
groups. The decreasing absorbance of bands (3768, 3686 and increase. The band at 1295 cmꢀ1 also increases, although it
does not have a specic assignment. Bands at 1468 and 1356
Fig. 5 FTIR spectra following the introduction of CO2 onto ZrO2
containing preadsorbed methanol.
Fig. 6 FTIR spectra of CO2 adsorption on CeO2.
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cmꢀ1 are associated with monodentate carbonate. Peaks at 1549 formation of MMC can also be conrmed through the C–H
and 1018 cmꢀ1 indicate the appearance of bidentate stretching vibration. Bands (2924 and 2818 cmꢀ1) for methoxy
carbonate.23,24 Infrared spectra taken during exposure of ZrO2 species decrease, and new bands at 2960 and 2880 cmꢀ1
containing preadsorbed methanol to CO2 are shown in Fig. 5. appear.10 The peak around 1295 cmꢀ1 does not appear during
Bands for on-top and bridging methoxy (1158 and 1032 cmꢀ1
decrease and new peaks appear at 1596, 1497, 1471, 1391, 1362,
)
this process.
Fig. 6 shows the infrared spectra recorded on adsorption of
1200 and 1110 cmꢀ1, which are ascribed to MMC.10 The CO2 on CeO2. Bands for nas(CO2) in gas phase are clearly
observed at 2358 and 2340 cmꢀ1. Bands at 1560, 1289 and 1011
cmꢀ1 stem from n(CO3) of bidentate carbonate,23,24 whereas
1467, 1352 and 1088 cmꢀ1 are features for n(CO3) of mono-
dentate carbonate.23,25 Peaks at 1598 and 1413 cmꢀ1 are
ascribed to hydrogen carbonate.23,26 Spectra of ZrO2 reacting
with CO2 at 413 K are shown in Fig. 7. The clear bands at 1221,
1425, 1626 and 1683 cmꢀ1 are assigned to bicarbonate, and
1366 cmꢀ1 may indicate the presence of bidentate carbonate.27
Fig. 8 displays the spectra of methanol successive introduc-
tion to CeO2 aer CO2 adsorption. Peaks appear in n(CH3) and
n(CO) regions representing all different kinds of methoxy. New
weak bands at 1572, 1454 and 1369 cmꢀ1 are assigned to MMC.
The unassigned band at 1295 cmꢀ1 arises, too. Features (1602
and 1410 cmꢀ1) for hydrogen carbonate decrease in the meth-
anol stream. The intensity of bands (1463 and 1354 cmꢀ1) for
monodentate carbonate reduces, too. Fig. 9 illustrates the
transformation of spectra when methanol starts to adsorb on
Fig. 7 FTIR spectra of CO2 adsorption on ZrO2.
Fig. 8 FTIR spectra following the introduction of methanol to CeO2 after CO2 adsorption.
Fig. 9 FTIR spectra following the introduction of methanol to ZrO2 after CO2 adsorption.
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Fig. 10 Infrared spectra of DMC adsorption on ceria catalysts.
Fig. 11 Infrared spectra of DMC adsorption on zirconium catalysts.
ZrO2 containing preadsorbed CO2. New bands appearing at
1165 and 1033 cmꢀ1 are for top- and bridging-methoxy,
respectively. Features for 1605, 1463 and 1354 cmꢀ1 are asso-
ciated with MMC.28 There is no sign of a band around 1295
cmꢀ1 during the CO2 adsorption process.
Fig. 10 shows the infrared spectra of DMC adsorbed on ceria
catalysts. In the n(CH3) region, bands at 2910 and 2804 cmꢀ1
,
which are assigned to nas(CH3) and ns(CH3) of methoxy,
respectively, increase in intensity with time. Bands at 2965 and
2869 cmꢀ1 are associated with the nas(CH3) and ns(CH3) of DMC.
Bands at 1780 and 1767 cmꢀ1 may relate to the physisorbed
DMC because these bands can be totally removed when the
sample is exposed to the pure He stream. Features at 1109 and
1061 cmꢀ1 are associated with the n(CO) of on-top methoxy and
bridging methoxy. The unassigned peak around 1295 cmꢀ1
appears, too, with a great increase. Fig. 11 records the infrared
spectra of DMC passing over ZrO2. Bands at 1603, 1357 and
1200 cmꢀ1 indicate the appearance of MMC on zirconium.28 The
Fig. 12 Infrared spectra during exposure of CeO2 to HCOOH.
nas(OCO), 1590, 1557, d(C–H), 1371 cmꢀ1, ns(OCO), 1353, 1252
cmꢀ1. The peak at 2925 cmꢀ1 results from the combination
band of nas(OCO) and d(C–H).29 It has been indicated that the
frequency differences between nas(OCO) with ns(OCO) retain the
following sequence: monodentate formate > free formate ion >
bidentate formate. According to this empirical approach, bands
at 1590 and 1252 cmꢀ1 may result from monodentate formate
because the frequency separation is 338 cmꢀ1, which is much
on-top methoxy is evident with the rising band at 1166 cmꢀ1
.
Other types of methoxy also appear during the DMC adsorption
process.
Fig. 12 illustrates the transformation of infrared spectra
when CeO2 is exposed to HCOOH in the He stream. The bands
are ascribed to formate species,23 i.e. n(C–H), 2842 cmꢀ1
,
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Fig. 13 Infrared spectra during exposure of CeO2 to HCOOCH3.
larger than that of free formate ion (250 cmꢀ1). The remaining also considered to be the reactive methoxy by other
features (1557, 1353 cmꢀ1) are associated with bidentate researchers,8,32 decreases because of the reaction with CO2. The
formate as the frequency separation is 204 cmꢀ1, less than 250 spectra of CO2 introduction onto zirconium oxide surface aer
cmꢀ1. During the adsorption process, there is no sign of the methanol adsorption are similar to those of earlier reports,33
peak at around 1295 cmꢀ1
.
and agree with the observation that on-top methoxy is more
The spectra of methyl formate adsorption on ceria catalysts reactive than other types.10 Noticeably, bands representing
evolve in a complicated way. Bands observed at 3040, 3008, MMC are observed with both catalysts during this CO2
2967, 2947, 1766 and 1753 cmꢀ1 in Fig. 13 are assigned to adsorption process, whereas the band at around 1295 cmꢀ1 is
methyl formate physisorbed on cerium oxide.30 The peak at detected only with CeO2. It can be deduced that this band (1295
2926 cmꢀ1 is associated with the combination band of nas(OCO) cmꢀ1) on the ceria surface does not originate from single CO2
and d(C–H). Bands at 2844, 1599, 1559, 1377, 1353 and 1248 adsorption, as shown in Fig. 6. By comparison, it is noted that
cmꢀ1 are from the formate adsorbed on the surface. Shoulders the band at 1295 cmꢀ1 arises again when the ceria catalyst is
at 2915 and 2801 cmꢀ1 are similar to those peaks that result exposed to successive introduction of methanol aer CO2
from the exposure of ceria catalysts to methanol. In the region adsorption, further conrming that this band (1295 cmꢀ1
of n(CO) bands at 1032 and 1020 cmꢀ1 are associated with results from the reaction of methanol with adsorbed CO2.
)
bridging and tri-coordinate methoxy. It is worth mentioning
that, as expected, the band at around 1295 cmꢀ1 appears.
As can be seen in Fig. 8, features for hydrogen carbonate and
monodentate carbonate all decrease during methanol adsorp-
tion on CeO2 containing preadsorbed CO2, indicating that those
kinds of adsorbed CO2 may be involved in the reaction with
methoxy. These structures lead to CO2 activation. The specic
reactive structure of CO2 is not detailed in this paper because
the activated structures depend on the type of oxygen vacancies,
crystal face and coverage of CO2. It is unreliable to propose a
specic structure merely from the results of infrared spectra.
The corresponding DFT calculation is in progress.
Further investigation was conducted to decide whether this
band (1295 cmꢀ1) represents an intermediate or not. It is a
common phenomenon that features for DMC are difficult to
observe under operating experimental conditions because of its
low concentration. However, according to the theory of micro-
reversibility, decomposition of a compound should go through
the same elementary steps as formation of that compound.
Based on this concept, decomposition of DMC would reveal the
intermediate needed to produce DMC. The decomposition of
DMC on ZrO2 surface is shown in Fig. 11. Features for MMC and
methoxy appear in the adsorption process without the band at
around 1295 cmꢀ1. Hence, MMC is the intermediate in DMC
formation over the zirconium surface. When CeO2 is exposed to
DMC, the band at about 1295 cmꢀ1 and features for carbonates
appear, as observed in Fig. 10. During this process, peaks for
MMC are invisible even aer long time exposure, which is very
different from the results seen on ZrO2. As mentioned above,
Discussion
The spectra of methanol adsorption on ceria and zirco-
nium10,20,31 are shown in Fig. 2 and 3. It is indicated that CH3OH
adsorbs on these two catalysts through O atoms. The H atom of
hydroxyl in methanol can react with OH on the metal oxide
surface, or a coordinately unsaturated O2ꢀ on the surface, to
produce hydroxyl. On the ZrO2 surface, the negative features of
n(OH) in Fig. 3 indicate that the H atom combines with the OH
to produce H2O. On the ceria surface, however, the H atom
associated with hydroxyl of methanol reacts with the coordi-
nately unsaturated O2ꢀ, resulting in formation of an OH group,
as can be observed in Fig. 2. This result is also conrmed by
those of other researchers.31 The big differences between
methanol adsorption on pure ceria and zirconium oxide are the
appearance of MMC and the band at around 1295 cmꢀ1. Peaks
of MMC and 1295 cmꢀ1 appear only when CH3OH adsorbs on
ceria catalysts, which cannot be detected on ZrO2 surface. Based
on this result, it is proposed that MMC formation on ceria
surface comes from the reaction between methanol and
adsorbed CO2, which is seldom mentioned by other
researchers.
When CO2 is introduced to ceria catalysts containing pre-
adsorbed methanol, the intensity of on-top methoxy, which is
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instead of the single adsorption, a band near 1295 cmꢀ1 origi- seen in Reaction (4). The formation of carbomethoxide is also
nates from the reaction between methoxy and CO2. Depending strongly supported by the decomposition of DMC.
on the result of DMC adsorption and the theory of micro-
reversibility, it is concluded that the peak at 1295 cmꢀ1 must be
an important feature indicative of the intermediate in DMC
formation from CO2 and methanol on the CeO2 surface,
meaning that there is a different mechanism in DMC formation
on ceria catalysts.
CO2 + CeO2 / CO2–CeO2
(1)
CH3OH + CeO2 / CH3OH–CeO2
(2)
CH3OH–CeO2 + CO2–CeO2 / CH3OCO–Ce + OH–Ce (3)
So far, the band at about 1295 cmꢀ1 has not been assigned,
but some helpful hints contribute to denition of this band. Li
et al.29 observed a band near 1300 cmꢀ1 and associated this
band with the ns(OCO). Combining the characteristic of this
reaction with the band representing ns(OCO), we propose that
this band may be a feature of carbomethoxide (CH3OCO). To
test this assumption, methyl formate adsorption was conducted
on the CeO2 surface. It is noted that a weak band appears at
around 1295 cmꢀ1 (1291 cmꢀ1), as observed in Fig. 13. It is
probable that appearance of the band at about 1295 cmꢀ1
comes from methoxy and formate because esters easily
decompose into alcohols and carboxylic acids. So, HCOOH
adsorption on CeO2 was undertaken to verify whether the band
at about 1295 cmꢀ1 stems from formic acid adsorption or not.
As exhibited in Fig. 12, bands for ns(OCO) appearing at 1353 and
1248 cmꢀ1 are far from 1295 cmꢀ1, indicating that it is hard to
link the band at about 1295 cmꢀ1 with formic acid adsorption.
Moreover, HCOOH formation from methanol on the surface of
ceria is unfavorable under the experimental conditions,34
further demonstrating that this band does not come from for-
mic acid adsorption. During the methyl formate adsorption
process, it is also noted that there are no features for on-top
methoxy, which is well accepted as the reactive methoxy in
carbomethoxide formation. Hence, the rising band at around
1295 cmꢀ1 in the methyl formate adsorption process does not
result from reaction between methoxy and CO2. Appearance of
the band at around 1295 cmꢀ1 can be explained as follows. The
C–H bond that the corresponding C atom links with two oxygen
atoms may break up to form an H atom and –OCOCH3. The H
atom can react with the OH on CeO2 surface, and the remaining
part (–OCOCH3) may attach to the ceria atom on the surface,
CH3OCO–Ce + OH–Ce + CH3OH /
(CH3O)2CO + H2O + CeO2 (4)
Conclusions
This study investigated the mechanism of DMC formation
directly from methanol and CO2 catalyzed by CeO2 compared
with that obtained over ZrO2. Based on the results of in situ
FTIR, DMC formation from CO2 and methanol over a ceria
catalyst is proposed to proceed via a new and different mecha-
nism from that on ZrO2. On a ZrO2 surface, DMC is formed by
the reaction of monodentate methyl carbonate with methanol.
In contrast, on a ceria surface, methanol reacts with the
adsorbed CO2 to produce a new intermediate, carbomethoxide.
A feature of carbomethoxide is observed at around 1295 cmꢀ1
,
which is dened through adsorption of methyl formate. Then
DMC is formed via the reaction between carbomethoxide and
methoxy produced from the dissociation of methanol. DMC
decomposition is observed to proceed via the reverse of this
route, further conrming the formation of DMC directly from
CO2 and methanol through a carbomethoxide intermediate.
Acknowledgements
The authors gratefully acknowledge nancial support by the
Natural Science Foundation of China (NSFC) (Grant no.
21176179), the Program for New Century Excellent Talents in
University (NCET-13-0411), the Scientic Research Foundation
for the Returned Overseas Chinese Scholars (MoE) and the
Program of Introducing Talents of Discipline to Universities
(B06006).
resulting in emergence of a peak at around 1295 cmꢀ1
.
Although methyl formate adsorption on ceria surface is not
direct evidence of assignment of the new band at 1295 cmꢀ1 in
DMC formation, it can explain this result to some extent.
Thus, the mechanism of DMC formation directly from CO2
and methanol over a CeO2 catalyst is described as below, which
is different from that previously suggested by other
researchers.8,14 In this mechanism, CO2 adsorbs on the CeO2
surface to produce adsorbed CO2 noted as CO2–CeO2, as shown
in Reaction (1). Reaction (2) involves adsorption of methanol on
the ceria surface. Carbomethoxide forms through interaction
between the adsorbed CO2 and methanol, as depicted in Reac-
tion (3). Methoxy will react with the –C]O in CO2 to form car-
bomethoxide. The remaining oxygen atom of CO2 combines
with the hydrogen from the hydroxyl in methanol to produce
hydroxyl on the surface of CeO2. Then the carbomethoxide
reacts with another methanol to produce DMC, which can be
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