An in situ infrared study of dimethyl carbonate synthesis from carbon dioxide and methanol over zirconia

Article abstract of DOI:10.1006/jcat.2001.3411

IR spectroscopy was used to study the mechanism of dimethyl carbonate (DMC) synthesis from methanol and CO₂ over monoclinic zirconia. The dissociative adsorption of methanol occurred more slowly than CO₂ adsorption, but the species formed from methanol were bound more strongly. On adsorption, the oxygen atom of methanol binded to coordinately unsaturated Zr⁴⁺ cations present at the catalyst surface. Rapid dissociation of the adsorbed methanol resulted in the formation of a methoxide group (Zr-OCH₃) and the release of a proton, which reacted with a surface hydroxyl group to produce water. CO₂ inserted into the Zr-O bond of the methoxide to form a mondentate methyl carbonate group (Zr-OC(O)OCH₃). This process was facilitated by the interaction of C and O atoms in CO₂ with Lewis acid-base pairs of sites (Zr⁴⁺O^2-) on the surface of the catalyst. Methyl carbonate species could also be produced via the reaction of methanol with CO₂ adsorbed in the form of bicarbonate species, but this process was slower than that involving the reaction of CO₂ with methoxide species. DMC was formed by reaction of the methyl carbonate species with methanol, a process that led to the transfer of a methyl group to the carbonate and restored a hydroxyl group to the zirconia surface. DMC decomposition on monoclinic zirconia occurred via the reverse of the process described for DMC synthesis.

Full text of DOI:10.1006/jcat.2001.3411

Journal of Catalysis 204, 339–347 (2001)
doi:10.1006/jcat.2001.3411, available online at http://www.idealibrary.com on
An in Situ Infrared Study of Dimethyl Carbonate Synthesis
from Carbon Dioxide and Methanol over Zirconia
Kyeong Taek Jung and Alexis T. Bell1
Chemical Sciences Division, Lawrence Berkeley Laboratory and Department of Chemical Engineering,
University of California, Berkeley, California 94720-1462
Received March 20, 2001; revised September 4, 2001; accepted September 4, 2001
Studies by Fujimoto and co-workers (4–6) have demon-
strated recently that DMC can be synthesized by the reac-
tion of methanol with carbon dioxide:
The mechanism of dimethyl carbonate (DMC) synthesis from
methanol and carbon dioxide over monoclinic zirconia has been
investigated using in situ infrared spectroscopy. The dissociative
adsorption of methanol occurs more slowly than the adsorption of
carbon dioxide, but the species formed from methanol are bound
more strongly. On adsorption, the oxygen atom of methanol binds
to coordinately unsaturated Zr4+ cations present at the catalyst
surface. Rapid dissociation of the adsorbed methanol leads to the
formation of a methoxide group (Zr–OCH3) and the release of a
proton, which reacts with a surface hydroxyl group to produce wa-
ter. Carbon dioxide inserts into the Zr–O bond of the methoxide to
form a mondentate methyl carbonate group (Zr–OC(O)OCH3). It is
proposed that this process is facilitated by the interactions of C and
O atoms in CO2 with Lewis acid-base pairs of sites (Zr4+O2 ) on the
surface of the catalyst. Methyl carbonate species can also be pro-
duced via the reaction of methanol with carbon dioxide adsorbed in
the form of bicarbonate species, but this process is slower than that
involving the reaction of carbon dioxide with methoxide species.
DMC is formed by reaction of the methyl carbonate species with
methanol, a process that results in the transfer of a methyl group to
the carbonate and restores a hydroxyl group to the zirconia surface.
The decomposition of DMC on monoclinic zirconia has also been
investigated and has been observed to occur via the reverse of the
c
2 CH3OH + CO2 → (CH3O)2CO + H2O.
Of the various catalysts tested only ZrO2 was found to be
active. The effectiveness of this catalyst was attributed to
the presence of both acidic and basic sites. It was proposed
that basic sites are required to activate methanol and CO2,
and that acidic sites are required to supply methyl groups
from methanol in the last step of the reaction mechanism
(6). Based on evidence from infrared spectroscopy (6), it
wasproposedthatmethoxidespeciesareformedonadsorp-
tion of methanol on basic sites and that these species are
rapidly converted to methyl carbonate species. Methanol
adsorption at acidic sites is thought to produce methyl car-
bocations, which then react with the anionic methyl carbon-
ate to produce DMC. The general features of this mecha-
nism are supported by in situ Raman studies of the reaction
mechanism for both DMC formation and decomposition
(7). Here we report a detailed investigation of the DMC
synthesis over monoclinic zirconia (m-ZrO2) undertaken
for the purpose of identifying the nature of the reaction sites
required for each step and the extent to which strongly
and weakly adsorbed CO2 are involved in the formation
of methyl carbonate. In situ infrared spectroscopy was used
to follow the dynamics of each reaction step.
processes described for the synthesis of DMC.
2001 Elsevier Science
INTRODUCTION
Dimethyl carbonate (DMC) is a nontoxic molecule that
can be used for a variety of applications. For example, DMC
has been proposed as a methylating agent for aromatic com-
pounds, to replace methyl halides and dimethyl sulfate,
which are both toxic and corrosive, and as an intermediate
in the synthesize polycarbonates and isocyanates, thereby
avoiding the need to use phosgene (1, 2). DMC is also a
candidate for replacing tertiary butyl ether (MTBE) as an
oxygen-containing additive for gasoline (3). For these rea-
sons, there has been a growing interest in examining novel
approaches to the synthesis of DMC (1, 3).
EXPERIMENTAL
Monoclinic zirconia was prepared by boiling a 0.5 M so-
lution of zirconyl chloride (ZrOCl2 8H2O, Aldrich) under
reflux at 373 K for 240 h, while maintaining the pH at 1.5.
The precipitated material was recovered by vacuum filtra-
tion and washed repeatedly by redispersion in deionized
water in order to remove chloride. After each wash, a few
drops of AgNO3 solution were added to the filtrate liquid.
The washing process was repeated until no evidence for
a precipitate of AgCl could be observed. The freshly pre-
pared material was dried in air at 383 K and then calcined at
1 To whom correspondence should be addressed. E-mail: bell@cchem.
berkeley.edu.
339
0021-9517/01 $35.00
2001 Elsevier Science
All rights reserved
c

340
JUNG AND BELL
FIG. 1. Infrared spectra taken during exposure of m-ZrO2 to CH3OH at 298 K.
573 K for 3 h. The BET surface area of the calcined zirconia minal, bibridged, and tribridged OH groups, respectively,
was 110 m2/g. X-ray powder diffraction by powder diffrac- present on the surface of m-ZrO2 (9–12). The growth in
tometry showed that only the monoclinic phase of zirconia intensity of the bands at 2923 and 2817 cm 1, associated
was present in the calcined material.
with C–H stretching vibrations of monodentate and biden-
In situ transmission infrared spectroscopy was performed tate of methoxide groups, is seen in Fig. 1B and the growth
using a 2-cm-diameter catalyst disk weighing 50 mg. The in intensity of the bands at 1157 and 1032 cm 1, associated
catalyst disk was contained in a low dead-volume infrared with bending vibrations for both types methoxide groups, is
cell (8). Infrared spectra were collected using a Nicolet seen in Fig. 1C (13–20). Bands can also be seen in Fig. 1B at
Magna 750 series II Fourier transform infrared spectrom- 2943 and 2831 cm 1 for molecularly adsorbed CH3OH [see
1
eter. Signals were detected using a narrow-band MCT de- (21, 22) and references therein] and at 2965 and 2881 cm
tector. Satisfactory signal-to-noise was obtained by collect- for bidentate formate species (15, 19, 23–32). The dynam-
1
ing 21–64 scans at 4 cm resolution. Electrical resistance ics of OH group consumption and CH3O group formation
heaters were used to heat the cell and an Omega Series are presented in Fig. 2. The rates of hydroxyl group con-
CN-2010 programmable temperature controller was used sumption and methoxide group formation are essentially
to control the cell temperature.
All gases were purified prior to use and delivered to the
infrared cell via Tylan Model FC-280 mass flow controllers
at a flow rate of 60 cm3/min. He gas was passed through
an oxysorb (CrO2) trap to remove O2 and then through a
a
molecular sieve trap (3 ADavison grade 564) to remove
water. A He stream containing either 1% CO2 or CH3OH
was used for studies of adsorption and reaction. DMC
(0.01%) diluted in He was used to investigate the adsorp-
tion and decomposition of DMC.
RESULTS
Infrared spectra taken during the adsorption of methanol
on zirconia at 298 K are illustrated in Fig. 1. In all cases the
spectra are referenced to the spectrum of zirconia observed
prior to the start of methanol adsorption. On adsorption,
methanol reacts with the surface hydroxyl groups of zirco-
nia to form methoxide groups with the concurrent release of
FIG. 2. Intensities of OHs and methoxy features for m-ZrO2 taken
during the experiments shown in Fig. 1. Band intensities are normalized
water. Figure 1A shows the decrease in the absorbance of
to those observed at the beginning of the transient for hydroxyl groups,
bands at 3768, 3745, and 3672 cm associated with ter- and to the values observed at the end of transient for methoxy groups.
1

AN IN SITU INFRARED STUDY OF DIMETHYL CARBONATE SYNTHESIS
341
FIG. 3. Infrared spectra taken during exposure of m-ZrO2 containing preadsorbed CH3OH to CO2 at 298 K.
equivalent. It is also evident that terminal and tribridged ated with different metals (33, 34). It is also noted that the
OH groups react at equivalent rates and that the rate of positions of the bands observed in Fig. 3 are nearly identical
formation of m-CH3O groups is slightly faster than the rate to those previously observed on ZrO2 under similar condi-
of formation of b-CH3O groups. The apparent first-order tions (see Table 1) (6, 10). Evidence for the formation of
time constant for the consumption of m-CH3O–Zr species methyl carbonate species can also be obtained by examin-
3
1
is 2.0 10
s
and that for b-CH3O–Zr species is 2.9
ing the C–H stretching portion of the spectrum. As seen in
Fig. 3A, with increasing exposure of the sample to CO2, the
3
1
10
s .
The exposure of m-ZrO2 containing methoxide groups to bands for CH3O–Zr species (2923 and 2817 cm 1) decrease
a stream of CO2 in He results in a significant change in the and new features appear at 2965 and 2883 cm 1. This pat-
appearance of the infrared spectrum of adsorbed species. tern is similar to that reported previously (6, 10), and as sug-
As seen in Fig. 3B, the bands for m- and b-CH3O (1157 gested by Table 1, the new features appearing in the spec-
and 1032 cm 1) decrease and new features appear at 1600, trum are characteristic of m-CH3OCOO–Zr. Figure 3A
1497, 1474, 1389, 1370, 1200, and 1113 cm 1. As indicated in indicates that m-CH3O species react more rapidly and ex-
Table 1, these features can be assigned to adsorbed mon- tensively than do b-CH3O–Zr species, and Fig. 4 shows that
odentate methyl carbonate species (m-CH3OCOO–Zr) thedynamicsofm-CH3Oconsumptionandm-CH3OCOO–
based on the closeness of the band positions seen in Fig. 3 Zr formation are nearly identical. It is also evident that not
with those reported for methyl carbonate species associ- all m-CH3O–Zr species react at an equivalent rate, since
TABLE 1
Comparison of Bands Observed on Reaction of CO2 with Adsorbed Zr–OCH3 with Those for Monodentate Methyl Carbonate Species
a
b
b
c
d
e
Na–O(CO)OCH3
Sn–O(CO)OCH3
Pb–O(CO)OCH3
Zr–O(CO)OCH3
Zr–O(CO)OCH3
Zr–O(CO)OCH3
1631
1618
1474
1378
1193
1100
1090
935
1631
1610
1453
1372
1202/1190
1182
1623
1602
1449
1322
1187
1165
1092
925
—
1583
1478
1372
1223/1200
1120
—
—
—
1600
1474
1389/1370
1200
1113
—
1600
1474
1370
1223
1112
—
1071
937
943
—
900
1
Note. All frequencies shown in cm
a From Ref. (34).
.
b From Ref. (33).
c From Ref. (10).
d From Ref. (6).
e From this work.

342
JUNG AND BELL
He stream containing CH3OH. The bands associated with
b-HCO3–Zr (1625, 1437, 1330, and 1225 cm 1) disappear
within the first 40 s of CH3OH exposure and new bands ap-
pear that are characteristic of m- and b-CH3O–Zr (1157 and
1032 cm 1, respectively) and of m-CH3OCOO–Zr (1608,
1464, and1340cm 1). Abandisalsoseenat1530cm 1, even
after exposure of the sample to CH3OH for 2 days. This
feature is best assigned to bidentate carboxylate species
(b-CO2–Zr). Comparison of Figs. 2 and 6 indicates that the
intensities of the bands for m-CH3OCOO–Zr are consid-
erably lower when this species is formed via the reaction of
CH3OH with adsorbed CO2.
The spectrum of adsorbed species evolves in a complex
fashion when a mixture of CH3OH and CO2 are passed
over the catalyst surface. Figure 7A shows that in the first
few seconds of exposure at 298 K, the only features seen are
those for b-HCO3–Zr (1625, 1450, and 1225 cm 1) and b-
FIG. 4. Intensities of m-CH3O–Zr and m-CH3OCOO–Zr features for
m-ZrO2 taken during the experiments shown in Fig. 3. Band intensities CO3–Zr(1325cm 1). Therapidappearanceofthese species
are normalized to those observed at the beginning of the transient for
m-CH3O–Zr, and to the value observed at the end of transient for m-
CH3OCOO–Zr.
is attributable to the rapid rate of CO2 adsorption, which
is tenfold faster than the rate of CH3OH adsorption. With
increasing time, the bands for b-HCO3–Zr and b-CO3–Zr
are replaced by those characteristic of m- and b-CH3O–Zr
the rate of m-CH3O–Zr consumption decreases rapidly and (1157 and 1032 cm 1) and m-CH3OCOO–Zr (1600, 1497,
then much more slowly after about 60% of the original in- 1474, 1389, 1370, 1200, and 1113 cm 1). Figure 7B shows
ventory of m-CH3O has been consumed.
that the same pattern is observed at 423 K, the principal
Infrared spectra taken during the exposure of the sam- differences being that at the higher temperature the rate
ple to CO2 are shown in Fig. 5A. Bands rapidly appear at of appearance of m-CH3OCOO–Zr is faster and the ratio
1625, 1437, and 1225 cm 1, characteristic of bidentate bi- of m-CH3O–Zr to m-CH3OCOO–Zr is higher than at the
1
carbonate species (b-HCO3–Zr), and a band at 1325 cm
is characteristic of bidentate carbonate species (b-CO3–Zr)
lower temperature.
Figure 8 shows the changes occurring in the intensity of
(12, 14, 15, 19, 23, 24, 35–39). As seen in Fig. 5B, the adsorp- bands for m-CH3O–Zr and m-CH3OCOO–Zr as functions
tion of CO2 is very rapid and the apparent first-order rate of time under different circumstances. In each case, the sur-
1
coefficient is 3.2 10 2 s
.
face concentration of m-CH3OCOO–Zr was achieved by
Figure 6 illustrates the evolution of spectral features exposing m-ZrO2 to a mixture containing CH3OH and
when ZrO2 containing preadsorbed CO2 is exposed to a CO2. When the initial gas mixture was swept out with He,
FIG. 5. Infrared spectra taken during exposure of m-ZrO2 to CO2 at 298 K.

AN IN SITU INFRARED STUDY OF DIMETHYL CARBONATE SYNTHESIS
343
FIG. 6. Infrared spectra taken during exposure of m-ZrO2 containing preadsorbed CO2 to CH3OH at 298 K.
the initial concentration of m-CH3OCOO–Zr declined to-
ward zero, whereas the concentration of m-CH3O–Zr in-
creased relative to its initial value. When the mixture of
CH3OH and CO2 was swept out with CO2, the surface con-
centration of m-CH3OCOO–Zr grew, whereas the surface
concentration of m-CH3O–Zr declined. A rapid decrease
in the surface concentration of m-CH3OCOO–Zr occurred
when the initial gas mixture was swept out with CH3OH.
This change was accompanied by an increase in the sur-
face concentration of m-CH3O–Zr. A qualitatively similar
pattern was observed at 423 K, a temperature very close
to that at which the formation of DMC over ZrO2 is re-
ported to be a maximum (6). In this case, though, the rates
of m-CH3OCOO–Zr disappearance in He and CH3OH are
comparable. It is also noted that raising the temperature
increases the former process to a much greater degree than
the latter.
FIG. 8. Intensities of m-CH3OCOO–Zr for m-ZrO2 taken after swit-
FIG. 7. Infrared spectra taken during exposure of m-ZrO2 to the mix- ching from a mixture of CO2 and CH3OH to a mixture of CH3OH, CO2,
ture of CO2 and CH3OH at (A) 298 K and (B) 423 K.
and He at (A) 298 K and (B) 423 K.

344
JUNG AND BELL
FIG. 9. Infrared spectra taken during the exposure of m-ZrO2 to DMC at 298 K. Four distinguishable features shown in panel A were obtained
1
from exposure times of 87–174 min. The inset shows that the peak at 2350 cm increased with the exposure time from 57 min (*) to 87 min (**) and
then decreased.
The series of spectra shown in Fig. 9 were obtained on
exposure of the sample to a gas stream containing DMC.
At low exposure times (<67 min) the only features ob-
served in Fig. 9B are those for b-HCO3–Zr and b-CO3–Zr
species. However, as the exposure time increases, the spec-
trum evolves and ultimately becomes characteristic of that
for a mixture of m-CH3OCOO–Zr and m- and b-CH3O–Zr.
Comparison of the last spectrum in the sequence appearing
in Fig. 9 with that appearing at long times in either Fig. 3
or 6 shows that they are essentially identical, the only dif-
ference being that the intensities of the peaks associated
with m-CH3OCOO–Zr are smaller in Fig. 9. The transient
evolution of CO2 is also observed, as evidenced by the ap-
pearance of a weak band at 2350 cm 1, as seen in Fig. 9A.
Figure 10 shows a plot of band intensities versus the time
of exposure of m-ZrO2 to DMC.
DISCUSSION
The interactions of CH3OH with the surface of ZrO2
reported in the present study are qualitatively consistent
with those described previously (6, 7, 10). It is envisioned
that the initial adsorption of CH3OH on m-ZrO2 occurs
via the interaction of the O atom of CH3OH with a coor-
dinately unsaturated Lewis acid Zr4+ center on the surface
of m-ZrO2. This interaction can then result in the transfer
of the H atom associated with the OH group of the alco-
hol to a coordinately unsaturated O2 center on the surface
of m-ZrO2 or reaction with a surface OH group. The first
of these two cases would result in the formation of bridg-
ing (tri- or bibridging) OH group and a methoxide group,
whereas the latter case would result in the release of water
andtheformationofamethoxidegroup. Figure2showsthat
the disappearance of surface OH groups and the appear-
ance of CH3O–Zr groups are preceded by a short induc-
tion period, after which the dynamics of the two processes
are virtually identical. This suggests that the formation of
CH3O–Zr groups and the release of H2O occur concur-
rently. Therefore the overall process can be described as in
Scheme 1.
As was noted above, the rates of consumption of ter-
minal and tribridging OH groups is identical and, corre-
spondingly, so are the rates of formation of monodentate
(terminal) and bidentate (bridging) CH3O–Zr groups.
FIG. 10. Plot of band intensities taken during exposure of m-ZrO2 to
DMC at 298 K.
SCHEME 1

AN IN SITU INFRARED STUDY OF DIMETHYL CARBONATE SYNTHESIS
345
The changes in the infrared spectrum observed when
methoxide groups interact with CO2, shown in Fig. 3, are
qualitatively similar to those reported earlier for similar in-
teractions occurring over both ZrO2 (6, 7, 10) and Al2O3
(40). Also in agreement with previous results is the observ-
ation that m-CH3O–Zr species are more reactive than
b-CH3O–Zr species (10). While the insertion of CO2 into
metal–hydrogen, metal–carbon bonds, and metal–nitrogen
bonds of various compounds is well-known, the analogous
insertion of CO2 in M–OR bonds has few documented
precedents (41). An exception, though, is a recent descrip-
tion of CO2 insertion into the M–OCH3 bonds of
methoxide-containing complexes based on Mg or Ca (42).
The site requirements and the mechanism of CO2 insertion
into metal alkoxide bonds are also poorly understood. It has
been proposed that the first step in the insertion of CO2 into
a Zr–OCH3 bond involves an electron donor–acceptor in-
teraction in which the alkoxy oxygen lone pairs act as the
donor and the CO2 carbon atom as the acceptor (10). A
more likely scenario is that several donor acceptor interac-
tions occur simultaneously, as suggested in Scheme 2.
In this instance a coordinately unsaturated surface oxy-
gen atom donates charge to the carbon atom of CO2, and
concurrently, an oxygen atom of CO2 donates charge to the
Zr4+ cation to which the methoxide group is attached. The
proximity of the methoxide oxygen atom to the CO2 carbon
atom would then enable the transfer of the alkoxide group
to the carbon atom of CO2 and the simultaneous formation
of a Zr–O bond between the Zr4+ cation and one of the
oxygen atoms of CO2. The formation of m-CH3OCOO–
Zr species, as opposed to bridging, chelating, or chelating
bridging methyl carbonate species, is suggested by the close
agreement between the observed infrared bands and the
positions of the bands in well-defined methyl carbonate
compounds, as shown in Table 1. It should be noted that
the reaction in Scheme 2 is readily reversible. As shown in
Fig. 8, in the absence of gas-phase CO2, m-CH3OCOO–Zr
decomposes to release CO2 and produce m-CH3OH–Zr.
In a previous study, we suggested on the basis of evidence
from Raman spectroscopy that methyl carbonate species
are formed on the surface of ZrO2 via the reaction of bi-
carbonate species with methoxide species (7). The experi-
ment shown in Fig. 6 was undertaken in an effort to confirm
the occurrence of this process. As was already noted, the
results of this experiment are difficult to interpret, since
several processes appear to take place concurrently. The
SCHEME 3
1
appearance of bands at 1608, 1464, 1340, and 1200 cm
does confirm the formation of m-CH3OCOO–Zr species,
but the intensity of these bands is considerably smaller than
those seen when methoxide groups are allowed to interact
with CO2. The appearance of a broad band in the region
of 1540–1550 cm 1 suggests that b-CO3–Zr species may be
formed, possibly by transfer of an H atom from HCO3–
Zr to either methanol or a methoxide species. Thus, it is
possible to envision a pathway to m-CH3OCOO–Zr from
b-HCO3–Zr, as shown in Scheme 3.
This reaction appears to occur much more slowly than
that in Scheme 2. It is also interesting to observe that the
preexistence of b-HCO3–Zr species leads to an increase
in the surface concentration of b-CH3O–Zr relative to m-
CH3O, the more reactive form of methoxide. The reason
for this is most likely that b-HCO3–Zr species are bound
in a chelating manner to a single Zr4+ cations, thereby de-
creasing the availability of sites for the stabilization of m-
CH3O–Zr species, a phenomenon similar to that reported
for formate groups (43).
The experiments in which CH3OH and CO2 are passed
concurrently over the catalyst also support the hypothesis
that methyl carbonate species are formed predominantly
via CO2 addition to methoxide species. Figure 7 shows
convincingly that while the initial adsorbed species are b-
HCO3–Zr because of the more rapid adsorption of CO2
than CH3OH, as soon as methoxide species are formed they
rapidly react with CO2 to form m-CH3OCOO–Zr species.
This suggests that under steady state reaction conditions
the primary route to methyl carbonate species will be the
reaction in Scheme 2.
The elementary process via which DMC is formed from
methyl carbonate species cannot be identified by direct ev-
idence. As seen in Fig. 8, exposure of m-CH3OCOO–Zr
species to methanol results in a rapid loss of these species
without the formation of any new intermediates. The pres-
ence of DMC in the gas phase in such experiments could
not be detected because of the very low concentration of
the products. Thus, one can only infer that DMC is formed
via reaction of CH3OH with m-CH3OCOO–Zr. This con-
clusion is supported, though, via the concept of microre-
versibility, which requires that the decomposition of DMC
occur via the same elementary steps by which it forms.
Based on this logic, the first step of DMC decomposition
SCHEME 2

346
JUNG AND BELL
SCHEME 6
sorbed on the surface of ZrO2 would rapidly react with the
CH3O–ZrspeciestoproduceCH3OHandhydroxylspecies.
Such a scenario is quite plausible, since the adsorption en-
ergy of water is relatively high [15–30 kcal/mol (44, 45)] and
SCHEME 4
CH3O–Zr species have been observed to react very rapidly
with water at 298 K (19, 20). Once the supply of adsorbed
water is exhausted CH3O–Zr species are retained on the
catalyst surface and further decomposition of CH3COO–Zr
is halted (Schemes 5 and 6).
and the last step of DMC formation should represent the
forward and reverse directions of the same elementary step.
In addition, reference to Fig. 8A indicates that the loss of
m-CH3OCOO–Zr from the catalyst surface when it is ex-
posed to a stream containing CH3OH in He is an order
of magnitude more rapid than when the stream only con-
tains He. From this is inferred that the addition of a methyl
group from adsorbed CH3OH (see Fig. 9) is faster than the
decomposition of m-CH3OCOO–Zr to release CO2.
In Fig. 9 it was shown that the adsorption of DMC
on m-ZrO2 results in the appearance initially of b-HCO3
and weakly bound CO2. With increasing time, both of
these species disappear and new features appear for m-
CH3OCOO–Zr and m- and b-CH3O–Zr. This pattern can
be explained as follows. On adsorption, DMC dissociates
rapidly to form methoxide species and gas-phase CO2. A
part of the latter species is readsorbed as b-HCO3–Zr. The
initially formed methoxide groups are not observed be-
cause they react very rapidly with small amounts of wa-
ter vapor held by the catalyst. Previous studies have shown
that methoxide species on the surface of ZrO2 will react
very rapidly with water vapor at 298 K to form methanol.
Once the water vapor supply has been exhausted, methox-
ide species build up on the catalyst surface and so does the
inventory of m-CH3OCOO–Zr. This scenario suggests that
on adsorption DMC forms a bidentate methyl carbonate
group and transfers a methyl group to a bridging O atom
on the surface of ZrO2. The methyl group reacts, in turn,
with a hydroxyl group to form methanol and desorbs from
the surface (Scheme 4).
CONCLUSIONS
The mechanism of DMC synthesis can be envisioned to
occur via the sequence of steps shown in Fig. 11. Methanol
binds to Zr4+ Lewis acid sites and releases an H atom, which
then rapidly reacts with a surface OH group to form H2O.
CO2 is then inserted into the Zr–O bond of the CH3O–
Zr species to produce m-CH3OCOO–Zr. This process is
facilitated by interactions of C and O atoms in CO2 with
Lewis acid–base pairs of sites. Methyl carbonate species
can also be produced via the reaction of CH3OH with CO2
adsorbed in the form of bicarbonate species, but this pro-
cess is slower than that involving the reaction of CO2 with
methoxide species. DMC is formed by transfer of a methyl
group to the terminal O atom of methyl carbonate species.
The residual methyl carbonate species isomerizes to the
monodentate structure and then decomposes to a CH3O–
Zr species, with the concurrent release of CO2. Water ad-
FIG. 11. Proposed mechanism for the formation of DMC from CO2
SCHEME 5
and CH3OH.

AN IN SITU INFRARED STUDY OF DIMETHYL CARBONATE SYNTHESIS
347
The effectiveness of ZrO2 as a catalyst for the synthesis of 19. Fisher, I. A., and Bell, A. T., J. Catal. 172, 222 (1997).
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