The mechanism of dimethyl carbonate synthesis on Cu-exchanged zeolite Y

Article abstract of DOI:10.1016/j.jcat.2008.01.033

The mechanism of dimethyl carbonate (DMC) synthesis from oxidative carbonylation of methanol over Cu-exchanged Y zeolite has been investigated using in situ infrared spectroscopy and mass spectrometry under transient-response conditions. The formation of DMC is initiated by reaction of molecularly adsorbed methanol with oxygen to form either mono- or di-methoxide species bound to Cu⁺ cations. Reaction of the mono-methoxide species with CO produces monomethyl carbonate (MMC) species. DMC is formed via two distinct reaction pathways-CO addition to di-methoxide species or by reaction of methanol with MMC. The rate-limiting step in DMC synthesis is found to be the reaction of CO with mono-methoxide or di-methoxide species. The first of these reactions produces MMC, which then reacts rapidly with methanol to produce DMC, whereas the second of these reactions produces DMC directly. Formaldehyde was identified as an intermediate in the formation of dimethoxy methane (DMM) and methyl formate (MF). Both byproducts are thought to form via a hemiacetal intermediate produced by the reaction of methanol with adsorbed formaldehyde at a Cu⁺ site.

Full text of DOI:10.1016/j.jcat.2008.01.033

Journal of Catalysis 255 (2008) 153–161
www.elsevier.com/locate/jcat
The mechanism of dimethyl carbonate synthesis on Cu-exchanged zeolite Y
Yihua Zhang, Alexis T. Bell ∗
Department of Chemical Engineering, University of California, Berkeley, CA 94720-1462, USA
Received 3 December 2007; revised 21 January 2008; accepted 22 January 2008
Abstract
The mechanism of dimethyl carbonate (DMC) synthesis from oxidative carbonylation of methanol over Cu-exchanged Y zeolite has been
investigated using in situ infrared spectroscopy and mass spectrometry under transient-response conditions. The formation of DMC is initiated
+
by reaction of molecularly adsorbed methanol with oxygen to form either mono- or di-methoxide species bound to Cu cations. Reaction of
the mono-methoxide species with CO produces monomethyl carbonate (MMC) species. DMC is formed via two distinct reaction pathways—CO
addition to di-methoxide species or by reaction of methanol with MMC. The rate-limiting step in DMC synthesis is found to be the reaction
of CO with mono-methoxide or di-methoxide species. The first of these reactions produces MMC, which then reacts rapidly with methanol to
produce DMC, whereas the second of these reactions produces DMC directly. Formaldehyde was identified as an intermediate in the formation
of dimethoxy methane (DMM) and methyl formate (MF). Both byproducts are thought to form via a hemiacetal intermediate produced by the
+
reaction of methanol with adsorbed formaldehyde at a Cu site.
© 2008 Elsevier Inc. All rights reserved.
Keywords: Dimethyl carbonate; Methanol oxidative carbonylation; Cu-Y zeolite; Infrared spectroscopy
1. Introduction
When CO is present in the gas phase, new bands appeared
which were attributed to carbomethoxide species (CH3OCO–);
however, a definitive assignment of these features could not be
made. DMC was envisioned to form upon reaction of methanol
with the carbomethoxide species. The rate-limiting step was
proposed to be CO addition to adsorbed methoxide species to
form carbomethoxide species. These findings are supported by
the studies of Anderson and Root [6,7], which show that the
rate of DMC over Cu-X is first order in CO partial pressure
and nearly zero order in both methanol and oxygen partial pres-
sures. More recently, we have carried out studies of the kinetics
of DMC synthesis on a fully characterized sample of Cu-Y pre-
pared by dry exchange of H-Y with CuCl [8–10]. Analysis of
the catalyst after preparation showed that all of the protons in
H-Y had been exchanged one-for-one by Cu, and XANES data
confirmed that all of the exchanged Cu was present as Cu+. In
agreement with the work, of Anderson and Root, it was also
shown that the kinetics of DMC formation are nearly first order
in CO partial pressure, whereas the dependencies on methanol
and oxygen partial pressures are nearly zero order. In situ in-
frared spectra revealed that while CO is strongly adsorbed by
extra-framework Cu+ cations, this adsorbate is almost com-
Dimethyl carbonate (DMC) has many potential applications.
It can serve as an oxygen-containing fuel additive, a precur-
sor for synthesis of carbonic acid derivatives, a methylating
agent, and as an intermediate in the synthesis of polycarbon-
ates and isocyanates [1–3]. Cu-exchanged zeolites are partic-
ularly attractive catalysts for the synthesis of DMC, because
they promote the oxidative carbonylation of methanol to DMC
in the gas phase [4–9]. Previous investigations have shown
that Cu-X and Cu-Y are the most active catalysts for produc-
ing DMC, whereas Cu-ZMS-5 and Cu-MORD are less active
and exhibit a high selectivity to producing dimethoxy methane
(DMM).
Several studies of the mechanism and kinetics of DMC syn-
thesis over Cu-exchanged zeolites have been reported. Based
on evidence from in situ IR spectroscopy, King [4,5] has pro-
posed that methanol adsorbs on extra-framework Cu cations
in Cu-Y and reacts with oxygen to form methoxide groups.
*
Corresponding author.
E-mail address: bell@cchem.berkeley.edu (A.T. Bell).
0021-9517/$ – see front matter © 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2008.01.033

154
Y. Zhang, A.T. Bell / Journal of Catalysis 255 (2008) 153–161
Fig. 1. IR spectra of Cu-Y exposed to DMC (5.57 kPa): (a) 12 s and (b) 20 min; and flushing with He: (c) 1.7 min and (d) 20 min.
pletely displaced by methoxide groups when methanol and oxy-
gen are present together with CO.
673 K. The products formed during transient-response experi-
ments were monitored by mass spectrometry (MKS Mini-Lab
quadrupole). In a typical experiment, the temperature was re-
duced to 403 K prior to the introduction of CH3OH (12.12 kPa).
All flow rates were approximately 40 cm3 min−1. After 30 min
of exposure to CH3OH, 2.02 kPa O2 was added into the stream.
Once steady-state was reached, the stream was switched to
He in order to remove gas-phase CH3OH and O2 from the
atmosphere, as well as any weakly adsorbed species. A flow
of CO (20.2 kPa, balance He) was then introduced for a
fixed period of time after which the cell was again flushed
with He. Next, a flow of CH3OH/O2 (12.12 kPa CH3OH,
2.02 kPa O2) was reintroduced into the cell. Finally, a flow of
CH3OH/O2/CO/He (4.0/1.0/9.0/19.3) was passed through the
cell at the rate of 40 cm3 min−1. Since the concentration of reac-
tion products leaving the infrared cell was too small to measure
accurately, a parallel experiment was carried out using a larger
amount of catalyst (150 mg) [9]. The same protocol used for
the transient-response IR experiments was used for the observa-
tion of products formed during the transient-response reaction
experiments. Separate experiments were carried out to identify
the formation for DMM/MF. In this case, a He stream contain-
ing formaldehyde was introduced into the system, after which
was switched to one containing methanol and formaldehyde.
The flow of formaldehyde was turned off and turned on again
to identify the role of formaldehyde in the formation for DMM
and MF.
The present study was undertaken to elucidate the mecha-
nism for DMC formation on Cu-Y. The same catalyst sample
was used as in our previous work [8,10]. Transient-response IR
spectroscopy was used in combination with measurements of
the temporal evolution of products in the gas phase, to iden-
tify the sequence of elementary steps involved in the formation
of DMC. Additional transient-response experiments were con-
ducted to gain information on the pathways for forming DMM
and methyl formate (MF), the byproducts of DMC synthesis.
2. Experimental
2.1. Catalyst preparation
Cu-Y zeolite was prepared by solid-state ion exchange of
H-Y zeolite (Si/Al = 2.5) with CuCl at 923 K [8]. A detailed
discussion of the extent of cation exchange and the oxidation
state of Cu in the sample of Cu-Y used for the present study
has been presented previously [8,10]. This work demonstrated
that all of the Brönsted acid protons are replaced quantita-
tively by Cu+ cations during solid-state exchange of H-Y by
CuCl. The presence of Cu+ cations as the exclusive form of Cu
was confirmed by the observation of a well-defined, CuK-pre-
edge peak at 8983.8 eV by XANES. Analysis of EXAFS data
demonstrated that the Cu+ cations are coordinated to frame-
work oxygens with a coordination number of ∼2.1 at a distance
of 1.99 0.02 Å.
3. Results
3.1. Infrared spectra of adsorbed DMC and DMM
2.2. Transient-response IR and mass spectrometry experiments
Transient-response IR experiments were carried out using
15 mg of catalyst pressed into a 20 mm-diameter pellet. The
pellet was placed into an in situ IR cell, and spectra were
recorded using a Thermo-Nicolet NEXUS 670 FTIR spectrom-
Infrared spectra of DMC and DMM adsorbed on Cu-Y were
recorded to aid the interpretation of the spectra observed un-
der transient-response conditions. Spectra of DMC adsorbed
on Cu-Y at 403 K are shown in Fig. 1. After a contact time
of 12 s, bands appear at 3026 and 2963 cm−1, due to the anti-
symmetric and symmetric C–H stretching vibrations of the CH3
eter. Eight scans were averaged using a resolution of 4 cm−1
.
Prior to adsorbate exposure, the catalyst was heated in He at

Y. Zhang, A.T. Bell / Journal of Catalysis 255 (2008) 153–161
155
Fig. 2. IR spectra of Cu-Y exposed to DMM (43.4 kPa): (a) 0.8, (b) 1.1, and (c) 20 min.
in DMC, respectively [11,12]. A weak band is also seen at
2842 cm−1, which is likely due to adsorbed methanol formed
via the decomposition of DMC on Cu-Y. The bands at 1656,
1499, and 1354 cm−1 are attributable to C=O stretching, C–H
bending, and O–C–O stretching modes of monomethyl carbon-
ate (MMC) species associated with the Cu+ cations in Cu-Y.
This assignment is supported by the work of Beutel [12], who
investigated the adsorption of DMC on Na-Y. Since the C–H
stretching vibrations for DMC are totally coincident with those
of MMC, it is impossible to identify the presence of DMC by
inspection of the C–H stretching portion of the spectrum. Ev-
idence for adsorbed DMC is given by the bands observed at
1459 and 1431 cm−1, which can be assigned to asymmetric and
symmetric bending vibrations of CH3 groups in adsorbed DMC
[11,13]. The C=O and O–C–O signals for this species exist as
shoulders located on the high- and low-frequency sides of the
bands for MMC, respectively. It is noted, though, that the band
at 1431 cm−1 could also be due to CH3 deformation vibrations
of MMC.
After 20 min of exposure to DMC, the intensity of the C–H
stretching bands at 3026 and 2963 cm−1 increased, together
with the intensities of the C=O and C–O stretching features, in-
dicating a higher occupancy of Cu+ cations by adsorbed DMC
and MMC. At the same time, a set of weak bands appeared
above 2854 cm−1, which are tentatively assigned to a combina-
tion of symmetric CH3 stretching bands for gaseous DMC. The
band at 1710 cm−1 due to C=O vibration of DMC adsorbed on
Cu+ became well resolved as well as the band at 1317 cm−1
for asymmetric O–C–O vibrations. Physically adsorbed DMC
exhibits a C=O band at 1754 cm−1. After 20 min of flushing
in He, physisorbed DMC was removed, as was confirmed by
the disappearance of the band at 1754 cm−1. The bands at 1710
and 1317 cm−1 diminished upon He flushing, and concurrently,
a band appeared at 2828 cm−1 due C–H stretching vibrations of
methoxide species [12].
C–H vibrations of CH3 groups, and the doublet at 2945 is due
to a mixture of anti-symmetric C–H vibrations of CH2 groups
and a combination of the fundamental for the symmetric C–H
vibrations of CH3 groups and the overtone of methyl defor-
mation of methoxide groups. The band at 2898 cm−1 is as-
signed to symmetric C–H vibrations of CH2 groups [14]. The
band at 2835 cm−1 cannot be assigned unambiguously, but may
be due possibly to an overtone of deformation vibrations. In
the C–H deformation region, features at 1452 and 1403 cm−1
are attributable to anti-symmetric vibrations of CH3 and CH2
groups, respectively. A new band appeared at 1590 cm−1 after
20 min of exposure to DMM, which can be assigned to formate
species [15].
3.2. Transient-response studies of DMC, DMM, and MF
formation
The infrared spectrum of Cu-Y exposed to methanol
(12.12 kPa) in He at 403 K is shown by the circled points in
Fig. 3a. Bands are seen at 2952 and 2844 cm−1 due to the
anti-symmetric and symmetric C–H stretching vibrations of
molecularly adsorbed methanol, at 1458 cm−1 for the deforma-
tion of methyl groups, and at 1366 cm−1 for the deformation
of OH groups [16]. Upon addition to the flow of methanol, O2
reacts rapidly with the adsorbed methanol to form methoxide
species, which exhibit asymmetric and symmetric CH3 stretch-
ing vibrations at 2925 and 2824 cm−1, respectively. The band
at 1458 cm−1, attributed to methyl deformation, increases in in-
tensity and is accompanied by a decrease in intensity of the OH
deformation band for molecular methanol at 1366 cm−1
.
The products observed during the course of an analogous
experiment carried out with a larger amount of catalyst are
shown in Fig. 5. DMM is the principal product formed, together
with smaller amounts of MF and CO2. The turnover frequency
(TOF) for DMM formation is 7.3 × 10−6
s
−1, that for MF is
Fig. 2 displays the spectra observed during exposure of Cu-Y
to DMM at 403 K. Bands are seen at 2995, 2945, 2898 and
2835 cm−1. The band at 2995 cm−1 is due to anti-symmetric
1.9 × 10−6 s−1, and that for CO2 is 1.0 × 10−6 s−1. The rate of
DMC formation is nearly two orders of magnitude lower than
that of DMM formation (1.1 × 10−7
s
−1). Fig. 4a displays the

156
Y. Zhang, A.T. Bell / Journal of Catalysis 255 (2008) 153–161
Fig. 3. IR spectra recorded during transient-response experiments: (a) exposure to CH OH (12.12 kPa) and O (2.02 kPa) for 1.68 ks; (b) He flushing for 1.8 ks;
3
2
(c) exposure to CO (20.2 kPa) for 1.72 ks; (d) He flushing for 1.8 ks; (e) exposure to CH OH (12.12 kPa) and O (2.02 kPa) for 1.7 ks; (f) exposure to CH OH
3
2
3
(12.12 kPa), CO (20.2 kPa), and O (2.02 kPa) for 1.8 ks.
2
rate of methoxide species formation rate as a function of time
(after the addition of O2 to the flow of methanol) based on the
growth in the intensity of the band at 2824 cm−1 associated with
C–H symmetric stretching vibration of Cu–CH3O. The forma-
tion of methoxide species on Cu-Y in the presence of O2 occurs
Flushing the infrared cell with He at 403 K removes CH3OH
and O2 from the gas phase as well as weakly adsorbed CH3OH.
The disappearance of the molecularly adsorbed methanol is
evidenced by the progressive decrease in the intensities of
the bands at 2952, 2844, 1458, and 1366 cm−1, as seen in
Fig. 3b. By contrast, the bands at 2925 and 2824 cm−1 as-
with an apparent first-order rate coefficient of 1.7 × 10−2 s−1
.

Y. Zhang, A.T. Bell / Journal of Catalysis 255 (2008) 153–161
157
Fig. 3. (continued)
sociated with adsorbed methoxide species remain essentially
constant in intensity. A new band appears at 1580 cm−1
Passage of CO (20.2 kPa) over the catalyst previously ex-
posed to CH3OH and O2 and then flushed with He produces
significant changes in the infrared spectrum, as seen in Fig. 3c.
The bands for methoxide species (2931 and 2824 cm−1) de-
crease in intensity and new features appear at 1690, 1664, 1498,
1485, 1463, 1433, 1348, and 1327 cm−1, as well as a shoulder
at 1354 cm−1. The bands at 1690, 1463, and 1327 cm−1 can be
,
attributable to the formation of small amount of formate
species [15]. Fig. 5 shows that during He flushing of the
larger charge of catalyst, the rates of DMM and MF forma-
tion rapidly decline, as do the rates of DMC and CO2 forma-
tion.

158
Y. Zhang, A.T. Bell / Journal of Catalysis 255 (2008) 153–161
Fig. 4. Dynamics of band appearance or disappearance during transient-response experiments. Reactions conditions are those given in the caption for Fig. 3: (a) the
formation rate of Cu methoxide species (Q); (b) the disappearance of methoxide species (") and the formation of MMC (Q); (c) the formation rate of methoxide
species (Q) and the disappearance MMC (").
Fig. 5. Rates of product formation during transient-response studies aimed at understanding the mechanism of DMC formation. The reaction conditions used for
each panel are identical to those used for the experiments shown in Figs. 3a–3f.
assigned to adsorbed DMC, whereas the bands at 1664, 1485,
and 1348 cm−1 can be assigned to MMC. It is also noted that a
carbonyl band (Cu+–CO) appears at 2129 cm−1. With increas-
ing exposure of the sample to CO, the bands for adsorbed DMC,
MMC and CO increase in intensity. The band at 1348 cm−1
slowly evolves into a shoulder on the high frequency side of
the band of 1333 cm−1. The bands at 1664 and 1348 cm−1
do not show any changes in intensity during the last several
scans, suggesting that the formation of MMC has reached a
maximum. However, slow increases in the intensity of the C=O
and O–C–O stretching vibrations for DMC are still detectable.
The frequency of the C–O stretching band for adsorbed CO is
red shifted by 9 cm−1 relative to that observed when CO is ad-
sorbed on Cu-Y in the absence of other species. As discussed
earlier [10], the red shift in the position of the CO band is
due to the coadsorption of methanol or methoxide species. The
band of constant intensity appearing at 1582 cm−1 during car-
bon monoxide exposure is assigned to asymmetric vibrations of
O–C–O in formate ions [15] formed during He flushing. Fig. 4b
shows the dynamics of methoxide consumption and MMC for-
mation. The rates of methoxide species consumption and MMC
formation are comparable and both are significantly lower than

Y. Zhang, A.T. Bell / Journal of Catalysis 255 (2008) 153–161
159
the rate of methoxide group formation. The apparent first-order
species (2927 and 2824 cm−1), and adsorbed CO (2118 cm−1).
No features for MMC or DMC are observed under steady-
state reaction conditions in agreement with our previous report
[8,10]. The observation of only a small, red-shifted band for
adsorbed CO at 2118 cm−1 indicates that the formation of
methoxide groups inhibits the adsorption of CO. Fig. 5 shows
that upon introduction of the CH3OH/CO/O2 mixture, the rate
of DMC formation rises rapidly to 2.7 × 10−5 s−1, whereas the
rates of DMM and MF formation rate rise to 5.5 × 10−6 s−1
rate coefficient for the MMC formation is 2.5 × 10−3
s
−1. As
seen in Fig. 5, the introduction of CO causes a small increase
in the rates of DMM and MF formation, which then slowly
decline, whereas the rate of DMC formation increases steadily
with time. Over the same interval of time, the rate of CO2 for-
mation increases to 6.0 × 10−5 s−1 and then steadily declines.
After the passage of CO over the catalyst, it was again
flushed with He. The infrared spectrum taken at the onset of He
flushing is illustrated by circled points in Fig. 3d. A number of
changes in the spectrum are observed: (1) the position of the CO
band shifts to 2144 cm−1 caused by the desorption and/or con-
sumption of methanol/methoxide species; (2) the bands at 1690,
1463, and 1333 cm−1, attributable to adsorbed DMC, decrease
and 2.8 × 10−6
s
−1, respectively. CO2 is formed at a rate
only slightly lower than that for the formation of DMC (1.8 ×
10−5 s−1).
Fig. 6 shows the products formed during transient-response
experiments aimed at elucidating the pathways leading to
DMM and MF. DMM and MF are detected when formalde-
hyde alone is passed over the catalyst. The rate of DMM
formation reaches a maximum of 8.7 × 10−6 s−1 and then de-
creases monotonically with further reaction time. The highest
in intensity; and (3) the bands at 1664, 1498, and 1433 cm−1
,
due to adsorbed MMC, retain their intensity. The removal of
CO from the gas phase causes an immediate decrease in the
production of DMC, DMM, MF, and CO2, as seen in Fig. 5.
Fig. 3e shows that when the sample is exposed to a stream
containing CH3OH and O2, a number of changes occur in
the infrared spectrum. The bands for molecularly adsorbed
methanol (2951, 2842, 1457 cm−1) grow in intensity, whereas
the band for adsorbed CO first shifts to 2133 cm−1 and then
rapidly disappears. These changes are accompanied by a loss in
rate of MF formation is 1.3 × 10−6
s
−1. After reaching a max-
imum, the rate decreases to 4.8 × 10−6 s−1 and then increases
slowly again. When methanol is added in the feed stream to-
gether with formaldehyde, the rates of DMM and MF formation
increases rapidly to 2.1 × 10−4 s−1 and 2.3 × 10−5
s
−1, re-
spectively. Without the presence of formaldehyde, the rates
of DMM and MF formation decrease. The rates of formation
of DMM and MF recover when formaldehyde is re-introduced
in the system together with methanol. The data shown in Fig. 6
demonstrate that DMM and MF are formed at significant rates
only when methanol and formaldehyde are present together.
the intensity of the bands for MMC (1654, 1498, 1354 cm−1
)
and the concurrent increase and then decrease in the intensity of
the bands due to adsorbed DMC (1690, 1463, and 1333 cm−1),
as well as the loss in intensity of the bands for gas-phase DMC
(1716 and 1310 cm−1). The latter pattern suggests that DMC is
formed via the consumption of adsorbed MMC with methanol.
With increasing time, the bands for adsorbed DMC diminish
completely and the surface becomes dominated by the fea-
tures of molecularly adsorbed methanol and methoxide species.
Fig. 4c shows that the rates for the disappearance of adsorbed
MMC and the formation of methoxide groups are rapid and es-
sential equivalent. The disappearance of the adsorbed MMC has
4. Discussion
The results presented in Figs. 3–5 suggest that the mech-
anism of DMC formation proceeds via the steps shown in
Fig. 7. Methanol first adsorbs molecularly on extra-framework
Cu+ cations, species 1, to form species 2. When oxygen and
methanol are present in the gas phase, molecularly adsorbed
CH3OH reacts to form methoxide species, as evidenced by the
appearance of bands for these species in the infrared spectra
shown in Fig. 3a. The elementary processes by which methox-
ide groups are formed cannot be identified from the results of
the present study. However, it is reasonable to suppose that upon
reaction with O2, adsorbed methanol forms species 3, in which
a methoxide and a hydroxide group are both attached to a single
Cu cation. The reaction of 3 with a second molecule of CH3OH
results in the release of water and the formation of species 5,
which contains two methoxide groups. As shown in Fig. 5, the
formation of methoxide groups is relatively rapid and occurs
with a time constant of 1.7 × 10−2 s.
an apparent first-order rate coefficient of 6.3 × 10−2 s−1
.
Fig. 5 demonstrates that when the sample is exposed to a
mixture of CH3OH and O2, after previously being exposed sep-
arately to mixture of CH3OH and O2 and then to CO, a burst
of DMC, DMM and MF formation occurs, after which the rate
of formation of all three products declines monotonically, with
that of DMC occurring most rapidly. The rate for DMC forma-
tion quickly reaches a maximum of 3.8 × 10−5 s−1 and then
decreases to 3.1 × 10−6
s
−1. Maxima of 1.6 × 10−5 s−1 and
6.1 × 10−6 s−1 are detected in the rates of DMM and MF
formation, respectively. After ∼10 min, the formation rates ap-
proached a constant value of 6.4×10−6 s−1 and 2.5×10−6 s−1
for DMM and MF, respectively. Over the same period the rate
for CO2 formation rises rapidly to 1.0 × 10−5 s−1 and then
rapidly decreases at a rate paralleling the decrease in the DMC
formation.
Passage of CO over Cu-Y previously exposed to CH3OH
and O2 results in the slow consumption of methoxide species
and the concurrent formation of MMC (see Fig. 3c). DMC also
appears in the gas phase at about the same rate as that for the
formation of adsorbed MMC (see Figs. 4b and 5). Fig. 7 il-
lustrates how adsorbed MMC (species 4) could form by the
reaction of CO with 3, and how DMC could form by reaction
of CO with 5 to form species 6, adsorbed DMC. Desorption of
In the last segment of the transient-response experiment,
all of the reactants required for DMC synthesis are introduced
into the reactor simultaneously. Fig. 3 shows that in this case
the only bands observed are those for molecularly adsorbed
methanol (2951, 2844, 1459, and 1357 cm−1), methoxide

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Y. Zhang, A.T. Bell / Journal of Catalysis 255 (2008) 153–161
Fig. 6. Rates of product formation during transient-response experiments aimed at understanding the mechanism of DMM and MF formation. The partial pressures
of CH O and CH OH are 6.06 and 0.96 kPa, respectively.
2
3
through a maximum, and then decreases slowly. Fig. 3e sug-
gests that the formation of DMC occurs as a consequence of
the reaction of adsorbed MMC reacting with CH3OH, i.e., the
reaction of 4 to 6 shown in Fig. 7.
The proposed reaction mechanism for the oxidative carbony-
lation of methanol to DMC is similar to that proposed by King
[4,5] but differs in two respects. The first and most important
is the identity of the intermediate formed upon the interaction
of CO with methoxide groups. In contrast with the interpre-
tation proposed by King, we propose that the intermediate is
MMC (see Fig. 7), rather than a carbomethoxide species. The
formation of MMC is proposed on the basis of the close sim-
ilarity in the bands observed when CO is added to methoxide
species bonded to Cu+ cations (see Fig. 3c) to those observed
upon adsorption of DMC at reaction temperature (see Fig. 1).
The scheme proposed in Fig. 7 then naturally allows for an ex-
planation of both DMC synthesis and its decomposition upon
adsorption. Another difference between the mechanism shown
here and that proposed by King is the hypothesis that DMC
may also form by CO addition to dimethoxide species. The in-
clusion of this step is supported by the observation of DMC
formation upon passage of CO over Cu-Y previously exposed
to methanol and oxygen (see Figs. 3c and 4b). The pathway via
MMC cannot explain this observation, since reaction of MMC
with gas-phase methanol is required to form DMC from MMC.
The data presented in Fig. 6 strongly suggest that the for-
mation of DMM and MF involves the reaction of CH3OH with
CH2O. While CH2O was not observed as a product during the
reaction of CH3OH with O2, it is an expected intermediate in
the oxidation of methanol to CO2. A possible mechanism for
the formation of DMM and MF is shown in Fig. 8. The coad-
Fig. 7. Mechanism for the formation DMC.
DMC from 6 results in the restoration of 1. The reaction of 3 to
form 4 is viewed as the source of adsorbed MMC seen by IR
(see Figs. 3c, 3d, and 4b), whereas the reaction of 5 to form 6 is
viewed as the source of gas-phase DMC. The similarity in the
rates of forming adsorbed MMC and gas-phase DMC suggests
that the dynamics of the elementary processes involved in these
two products are comparable. The large amount of CO2 ob-
served during this phase of the transient response sequence (see
Fig. 5) is likely due to the reaction of CO with adsorbed atoms
of O. Fig. 5 shows that the formation of DMC ceases as soon
as gas-phase CO is flushed from the reactor by He. However,
when the catalyst is next exposed to a mixture of CH3OH and
O2, the formation of DMC begins almost immediately, passes

Y. Zhang, A.T. Bell / Journal of Catalysis 255 (2008) 153–161
161
those leading to DMC. The results of Fig. 6 show that in the
absence of CO and O2, and with methanol and formaldehyde
partial pressures of 6.06 and 0.96 kPa, respectively, the rate of
DMM is more than an order of magnitude higher than during
DMC synthesis. This suggests that in the later case, the con-
centration of formaldehyde produced by the partial oxidation
of methanol may be only 0.1 kPa.
Fig. 8. Mechanism for the formation of DMM and MF.
sorption of CH2O and CH3OH produces species 7, which can
react to form a hemiacetal, species 8. The reaction of 8 with
CH3OH would produce DMM and H2O, whereas the reaction
of 8 with CH2O would produce MF and CH3OH. MF could
also be produced by the Tishcenko reaction of two molecules
of CH2O, but the data shown in Fig. 6 suggest that this process
is much less effective than the reaction of CH2O with CH3OH,
since the rate of MF formation from CH2O alone is much lower
than that when CH2O and CH3OH are both present in the gas
phase. This conclusion is further supported by the data in Fig. 5,
which show that when CH3OH and O2 are passed over the cat-
alyst the rate of DMM formation is significantly higher than the
rate of MF formation. This is also true when CO is present in
the feed gas.
It is interesting to observe that while CO readily adsorbs on
Cu+ cations in Cu-Y, the concentration of adsorbed CO de-
creases significantly when CH3OH and O2 are present in the
gas phase together with CO (see Fig. 3). This observation is
a direct consequence of the effect of methoxide groups on the
adsorption of CO. As noted in our earlier work [10], the adsorp-
tion energy for CO is decreased significantly by the presence
of methoxide species bound to Cu+ cations in Cu-Y. The red
shift in the frequency of CO coadsorbed in the presence of
CH3OH and O2 is another indicator that CO adsorbs on Cu+
sites containing either methoxide species or molecularly ad-
sorbed CH3OH, both of which serve as electron donors to Cu+.
The increased charge density on Cu+ results in greater back do-
nation of charge to the π∗ antibonding orbitals of CO, which,
in turn, causes the red shift in the C–O stretching frequency.
The infrared spectra presented in Fig. 3f show that under
steady-state conditions of DMC synthesis, the dominant species
associated with Cu+ cations are methoxide species. However,
it is not possible to determine whether these species are in
the form of species 3 (mono-methoxide) or 4 (di-methoxide).
As reported previously [10], the kinetics of DMC formation
are nearly first order in CO partial pressure and are nearly
zero-order in the partial pressures of CH3OH and CO. Taken
together, these observations strongly suggest that the rate of
DMC formation is limited by the reaction of species 3 to 4
or 5 to 6. The reaction of species 3 with CO to form species 4
results in the formation of MMC as an intermediate, and, as
discussed above, the reaction of 4 (adsorbed MMC) with gas-
phase methanol is very rapid relative to the rate at which DMC
is formed.
5. Conclusions
The elementary steps involved in the oxidative carbonylation
of methanol to form DMC on Cu-Y were investigated by com-
bining information obtained from infrared spectroscopy and
mass spectrometry under transient-response conditions. Char-
acterization of Cu-Y indicates that the Cu/Al ratio is unity and
that all of the Cu is present as Cu+ cations. Transient-response
IR experiments show that methanol adsorbs molecularly and
then reacts with O2 to form methoxide species. While CO ad-
sorbs on Cu+ more strongly than methanol, the formation of
methoxide groups inhibits the adsorption of CO significantly.
With CO present in the gas phase, the methoxide species re-
act slowly to form monomethyl carbonate species. The latter
species react rapidly with methanol to form DMC. The over-
all mechanism of DMC formation is summarized in Fig. 7.
DMM and MF are the principal byproducts of DMC synthe-
sis. The formation of both byproducts is hypothesized to pro-
ceed via a hemiacetal intermediate formed by the reaction of
formaldehyde, produced by the partial oxidation of methanol.
As shown in Fig. 8, reaction of the hemiacetal with methanol
leads to DMM, whereas reaction of this intermediate with ad-
ditional formaldehyde produces MF. These pathways are sup-
ported by transient-response experiments involving the reaction
of formaldehyde with methanol.
Acknowledgment
This work is supported by the Methane Conversion Cooper-
ative (MC2) funded by BP.
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