Effects of zeolite structure and composition on the synthesis of dimethyl carbonate by oxidative carbonylation of methanol on Cu-exchanged Y, ZSM-5, and Mordenite

Article abstract of DOI:10.1016/j.jcat.2007.07.018

The aim of this work was to establish the effects of zeolite structure/chemical composition on the activity and selectivity of Cu-exchanged Y (Si/Al = 2.5), ZSM-5 (Si/Al = 12), and Mordenite (Si/Al = 10) for the oxidative carbonylation of methanol to DMC. Catalysts were prepared by solid-state ion-exchange of the H-form of each zeolite with CuCl and were then characterized by FTIR and X-ray absorption spectroscopy (XAS). The XANES portion of the XAS data showed that all of the copper was present as Cu⁺ cations, and analysis of the EXAFS portion of the data shows the Cu⁺ cations had a Cu{single bond}O coordination number of ～2.1 on Cu-Y and ～2.7 on Cu-ZSM-5 and Cu-MOR. Dimethyl carbonate (DMC) was observed as the primary product when a mixture of CH₃OH/CO/O₂ was passed over Cu-Y, whereas dimethoxy methane was the primary product over Cu-ZSM-5 and Cu-MOR. The higher activity and selectivity of Cu-Y for the oxidative carbonylation of methanol can be attributed to the weaker adsorption of CO on the Cu⁺ cations exchanged into Y zeolite. In situ IR observations revealed that under reaction conditions, adsorbed CO was displaced by methoxide groups bound to the Cu⁺ cations. The kinetics of DMC synthesis suggests that the rate-limiting step in the formation of this product was the insertion of CO into Cu{single bond}OCH₃ bonds. The yield of DMC decreased with methanol conversion, likely due to the hydrolysis of DMC to methanol and carbon dioxide.

Full text of DOI:10.1016/j.jcat.2007.07.018

Journal of Catalysis 251 (2007) 443–452
www.elsevier.com/locate/jcat
Effects of zeolite structure and composition on the synthesis
of dimethyl carbonate by oxidative carbonylation of methanol
on Cu-exchanged Y, ZSM-5, and Mordenite
Yihua Zhang, Daniel N. Briggs, Emiel de Smit, Alexis T. Bell ∗
Department of Chemical Engineering, University of California, Berkeley, CA 94720-1462, USA
Received 22 May 2007; revised 15 July 2007; accepted 17 July 2007
Available online 4 September 2007
Abstract
The aim of this work was to establish the effects of zeolite structure/chemical composition on the activity and selectivity of Cu-exchanged Y
(Si/Al = 2.5), ZSM-5 (Si/Al = 12), and Mordenite (Si/Al = 10) for the oxidative carbonylation of methanol to DMC. Catalysts were prepared by
solid-state ion-exchange of the H-form of each zeolite with CuCl and were then characterized by FTIR and X-ray absorption spectroscopy (XAS).
+
The XANES portion of the XAS data showed that all of the copper was present as Cu cations, and analysis of the EXAFS portion of the data
+
shows the Cu cations had a Cu–O coordination number of ∼2.1 on Cu-Y and ∼2.7 on Cu-ZSM-5 and Cu-MOR. Dimethyl carbonate (DMC)
was observed as the primary product when a mixture of CH OH/CO/O was passed over Cu-Y, whereas dimethoxy methane was the primary
3
2
product over Cu-ZSM-5 and Cu-MOR. The higher activity and selectivity of Cu-Y for the oxidative carbonylation of methanol can be attributed
+
to the weaker adsorption of CO on the Cu cations exchanged into Y zeolite. In situ IR observations revealed that under reaction conditions,
+
adsorbed CO was displaced by methoxide groups bound to the Cu cations. The kinetics of DMC synthesis suggests that the rate-limiting step in
the formation of this product was the insertion of CO into Cu–OCH bonds. The yield of DMC decreased with methanol conversion, likely due
3
to the hydrolysis of DMC to methanol and carbon dioxide.
© 2007 Elsevier Inc. All rights reserved.
Keywords: Dimethyl carbonate; Methanol; Oxidative carbonylation; Cu-Y; Cu-ZSM-5; Cu-MOR
1. Introduction
Cu-X has greater activity and selectivity for DMC formation
than Cu-ZSM-5. The authors proposed that CO adsorption has
a negative effect on the formation of DMC. However, the mech-
anism by which zeolite structure and chemical composition af-
fect the adsorptive and catalytic properties of the catalyst have
not been investigated.
The present study was undertaken with aim of assessing the
effects of zeolite structure on the activity and selectivity of Cu-
exchanged zeolites for the synthesis of DMC. Catalysts were
prepared by solid-state ion exchange of H-Y, H-ZSM-5, and
H-MOR with CuCl under conditions chosen to minimize the
retention of occluded CuCl [8–10]. All catalysts were character-
ized after preparation by infrared (IR) spectroscopy, to establish
the extent to which Brönsted acid OH groups had been ex-
changed for copper cations, and by XANES, to establish the
valence of the exchanged copper cations. The coordination of
Cu to framework oxygen atoms was probed by EXAFS. The ac-
Dimethyl carbonate (DMC) can be used as a fuel additive
to replace methyl tert-butyl ether (MTBE), a precursor for syn-
thesis of carbonic acid derivatives, as a methylating agent to
replace methyl halides and dimethyl sulfate, and as an interme-
diate in the synthesis of polycarbonates and isocyanates [1–3].
Cu-exchanged zeolites have been shown to be active catalysts
for the oxidative carbonylation of methanol to dimethyl car-
bonate, with the principal byproducts being dimethoxymethane
(DMM) and methyl formate (MF) [4–9]. Previous studies have
shown that the structure and chemical composition of the zeo-
lite influence the activity and selectivity of Cu-exchanged ze-
olites. For example, Anderson and Root [7] have reported that
*
Corresponding author. Fax: +1 510 642 4778.
E-mail address: bell@cchem.berkeley.edu (A.T. Bell).
0021-9517/$ – see front matter © 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2007.07.018

444
Y. Zhang et al. / Journal of Catalysis 251 (2007) 443–452
tivity and selectivity of Cu-Y, Cu-ZSM-5, and Cu-MOR were
then explored for various conditions. In situ IR and XANES
measurements also were carried out to identify the nature of
the adsorbed species associated with the copper cations and the
oxidation state of the copper cations during the oxidative car-
bonylation of methanol.
allows minimization of the residuals between both the mag-
nitude and the imaginary part of the Fourier transforms of
the data and the fit. R-space fitting has significant advantages
over the usually applied fitting in k-space, as has been ex-
plored in depth by Koningsberger et al. [15]. Theoretical phase
shift and backscattering amplitude for the Cu–O absorber-
scatterer pair, generated using the FEFF8 code, were used in
EXAFS data analysis. The theoretical references were cali-
brated using experimental data for Cu2O as described previ-
ously [10,16].
2. Experimental
2.1. Catalyst preparation and characterization
2.2. Measurement of catalyst activity and selectivity
Cu-exchanged zeolites were prepared by solid-state ion ex-
change (SSIE) of the protonated form of each zeolite (Y:Si/Al =
2.5, Strem; ZSM-5:Si/Al = 12, ALSI-PENTA; MOR:Si/Al =
10, Zeolyst) with CuCl at elevated temperature in a flow of He.
Details of this procedure have been described elsewhere [8–11].
Freshly prepared catalysts, designated as Cu-Y, Cu-ZSM-5, and
Cu-MOR, were stored in a drybox before use. The level of pro-
ton exchange was evaluated by observing the intensity of the IR
band for residual Brönsted acid sites.
Measurements of catalyst activity and selectivity were car-
ried out using 150 mg of catalyst loaded into a 10-mm-i.d.
quartz tubular flow reactor. Before being exposed to reactants,
the catalyst was pretreated at 873 K for 1 h in a stream of high-
purity He (99.999%). For the catalytic experiment, a CO/O2
mixture (25.0% CO, 2.5% O2, balance He) and He (99.999%)
were used. Methanol (CH3OH) was introduced by passing the
CO/O2 mixture through a saturator maintained at a constant
temperature of 293 K. The reaction products were analyzed
by gas chromatography equipped with a capillary column (All-
tech, AT aquawax; polyethylene glycol stationary phase) con-
nected to a flame ionization detector (FID) and by a packed
column (Alltech, Haysep DB packing) connected to a ther-
mal conductivity detector (TCD). The effects of feed space
velocity were investigated using a CH3OH/O2/CO/He mixture
(4.0/1.0/9.0/19.3). The flow rate of this mixture was varied
from 3 to 80 cm3/min while the catalyst temperature was main-
tained at 403 K. A second series of experiments designed to
study the effects of CO, CH3OH, and O2 partial pressures on re-
action rate and product selectivity was carried out using a fresh
sample of Cu-exchanged zeolite. In this case, the total flow rate
of feed was maintained at 20 cm3/min, and the catalyst temper-
ature was 403 K.
The oxidation state of Cu in the Cu-exchanged zeolites was
determined by Cu K-edge X-ray absorption near-edge spec-
troscopy (XANES). These data were acquired in transmis-
sion mode at the Stanford Synchrotron Radiation Laboratory
(SSRL) on beamline 2–3. This beamline is equipped with a
double-crystal monochromator, Si(111), detuned to 70% inten-
sity to minimize the presence of higher harmonics. The sam-
ples were pressed into self-supporting wafers (calculated to
have an absorbance of 2.5) and then mounted in a controlled-
atmosphere cell operated at 101 kPa. Following characteriza-
tion by XAS, each sample was cooled to 298 K before expo-
sure to a particular gas or gas mixture. In separate experiments,
pretreated Cu-exchanged zeolites were exposed to mixtures of
He/CH3OH, He/CO, and He/CO/O2/CH3OH. Methanol vapor
was introduced using a gas saturator containing liquid methanol
at 298 K. To examine the effects of gas adsorption, the sample
was exposed to a flow of gas at room temperature, and the tem-
perature was then raised at 10 K min−1 to 403 K and held at
this level for 1 h. All XAS measurements were made in situ.
Cu XANES data were analyzed using the IFEFFIT package
[12,13]. Pre-edge absorptions due to the background and de-
tector were subtracted using a linear fit to the data in the range
of −200 to 50 eV relative to the sample edge energy (E0). Each
spectrum was normalized by a constant determined by the aver-
age absorption in the range of 100–300 eV relative to E0. The
edge energy of each sample and reference was taken at the first
inflection point beyond any pre-edge peaks.
Product selectivity was determined using the following
equations:
ꢀ
ꢁ
SDMC/CO = [DMC]/ [DMC] + [CO2]
and
ꢀ
ꢁ
Si/CH OH = n[i]/ 2[DMC] + 2[MF] + 3[DMM] + 2[DME] ,
where i is DMC, MF, DMM, and DME and n is the number of
3
carbon atoms derived from methanol.
Extraction of the EXAFS data from the measured absorp-
tion spectra was performed with the XDAP code [14]. The
pre-edge was subtracted using a modified Victoreen curve. The
background was subtracted using cubic spline routines with a
continuously adjustable smooth parameter. Normalization was
performed by dividing the data by the height of the absorption
edge at 50 eV.
2.3. IR characterization of adsorbed species
IR spectra were recorded on a Nicolet Nexus 670 FTIR
spectrometer equipped with an MCT-A detector. Measurements
were obtained at a resolution of 4 cm−1. The catalysts were
pressed into 15-mg self-supporting pellets and placed into an
IR cell equipped with CaF2 windows. Before absorbate expo-
sure, the catalyst was heated in He at 673 K.
Data analysis was performed by multiple-shell fitting in R-
space using the EXAFS data analysis program XDAP, which

Y. Zhang et al. / Journal of Catalysis 251 (2007) 443–452
445
3. Results
Cu K-edge XANES was used to determine the oxidation
state of copper cations exchanged into each zeolite. Fig. 2 dis-
plays the XANES spectra of Cu-Y, Cu-ZSM-5, and Cu-MOR
along with their corresponding first derivative curves. A sin-
gle, well-defined pre-edge peak at 8983.8 eV can be seen on
all three catalysts. This feature has been assigned to the de-
generate 1s–4px, y electron transition of Cu+ cations, which are
doubly or triply coordinated in a linear or planar configuration,
respectively [10,21,22]. The absence of Cu2+ and Cu0 features
rules out the presence of CuO or Cu metal species in copper-
exchanged samples. The absence of a peak at 8987.3 eV in the
XANES region also suggests the absence of any bulk CuCl. The
greater intensity of the pre-edge feature for Cu-Y compared
with Cu-ZSM-5 or Cu-MOR implies that Cu+ sites in Cu-Y
have a more unsaturated coordination to the zeolite framework.
The local environment of Cu+ cations in Cu-exchanged
zeolites was further analyzed using Cu K-edge EXAFS. The
Fourier transforms of the k-weighted scattering function, χ(k),
for Cu-exchanged zeolites are shown in Fig. 3. The main con-
tributions can be seen to lie in the range of 1.0–2.0 Å due
3.1. Catalyst characterization
Fig. 1 illustrates the IR spectrum for the O–H stretching re-
gion. Spectra are shown for zeolites before and after SSIE. Two
bands were located at 3640 and 3540 cm−1 for H-Y, attribut-
able to the Brönsted acid O–H stretching vibrations located in
the supercage and sodalite cage, respectively [8,17,18]. The
small band at 3750 cm−1 was due to the silanol groups in
zeolite Y. The IR spectrum of H-ZSM-5 exhibited only a sin-
gle band at 3600 cm−1 characteristic of the Brönsted acid OH
groups, along with a small silanol band at 3740 cm−1 [19]. The
IR spectrum of H-MOR was very similar to that of H-ZSM-5,
exhibiting bands at 3600 and 3740 cm−1 for Brönsted acid OH
and silanol groups, respectively [20]. As shown in Fig. 1, bridg-
ing hydroxyl stretches disappeared after H-Y, H-ZSM-5, and
H-MOR were exchanged with CuCl, suggesting that the Brön-
sted acid sites were replaced quantitatively by Cu+ cations.
Fig. 1. IR spectra of catalysts before and after exchange of the zeolite with CuCl: (a) H-Y, Cu-Y; (b) H-ZSM-5, Cu-ZSM-5; (c) H-MOR, Cu-MOR.
Fig. 2. Cu K-edge XANES of (a) Cu-Y; (b) Cu-ZSM-5; (c) Cu-MOR.

446
Y. Zhang et al. / Journal of Catalysis 251 (2007) 443–452
Fig. 3. Fourier transforms of experimental data for Cu-exchanged zeolites
−1
1
(k weighted, ꢀk = 3–13 Å , phase-uncorrected).
Fig. 4. CO temperature-programmed desorption spectra for Cu-Y (a), Cu-
ZSM-5 (b), and Cu-MOR (c).
Table 1
−1
Fit parameters of EXAFS spectra (ꢀk: 3–13 Å , ꢀR: 1.0–2.0 Å) and vari-
ances for model spectra of Cu-Y, Cu-ZSM-5 and Cu-MOR
2
Catalyst
Cu-Y
Cu-ZSM-5 O
Cu-MOR
Scatterer N
R
ꢀσ
ꢀE
0
2
−3
(
10%) (Å, 0.02 Å) (10 Å , 5%) (eV, 10%)
O
2.1
1.99
1.96
1.96
3.7
3.3
3.3
1.1
−1.5
−1.9
2.7
2.7
O
to backscattering from framework oxygen atoms. The EXAFS
patterns for Cu-ZSM-5 and Cu-MOR were nearly identical and
differed from that for Cu-Y. Analysis of the EXAFS data for
copper-exchanged zeolite catalysts was performed by multiple
shell fitting in R space over the range of 1 < R < 2 Å. Cl
backscattering was not included for the fitting, for which the
contribution has been proven negligible [10]. The EXAFS co-
ordination parameters are given in Table 1. For Cu-Y, the Cu–O
first shell coordination number was 2.1, and the Cu–O bond
distance was 1.99 0.02 Å, compared with respective values
of 2.7 and 1.96 0.02 Å for Cu-ZSM-5 and Cu-MOR. These
observations agree with the XANES findings showing a more
intense pre-edge feature on Cu-Y than on Cu-ZSM-5 or Cu-
MOR, implying more unsaturated local coordination for Cu+
sites in Cu-Y.
Fig. 5. IR spectra of Cu-Y in the presence of different gases (from top to bottom,
CO; CO/CH OH; CO/CH OH/O ).
3
3
2
3.3. Adsorbed species on Cu-exchanged zeolites
The nature of adsorbed species present on the Cu-exchanged
zeolites was investigated by IR spectroscopy. IR spectra for
Cu-Y, Cu-ZSM-5, and Cu-MOR are presented in Figs. 5, 6,
and 7 for three scenarios: in the presence of CO, CO plus
methanol, and CO plus methanol and oxygen. As shown in
Fig. 5 for Cu-Y, when CO (20.2 kPa) was the only adsorbate,
carbonyls were formed on Cu+ cations, which exhibited a broad
stretching band at 2139 cm−1 [8]. After addition of CH3OH
(12.12 kPa), features for molecularly adsorbed methanol ap-
peared at 2951 and 2844 cm−1, attributable to antisymmetric
and symmetric C–H stretching vibrations, respectively [4,8]. At
the same time, a red shift from 2139 to 2120 cm−1 appeared in
the C–O stretching vibration. This pattern can be attributed to
the co-adsorption of methanol on the Cu+ cation to which CO is
3.2. CO-TPD
Fig. 4 compares CO-TPD spectra for Cu-Y, Cu-ZSM-5, and
Cu-MOR. The TPD spectra for Cu-ZSM-5 and Cu-MOR were
very similar, exhibiting a well-resolved peak at ∼370 K fol-
lowed by a very broad tail to 720 K on Cu-ZSM-5 and to 680 K
on Cu-MOR. The total amount of CO adsorbed per Cu+ cation
was 0.82 for Cu-ZSM-5 and 0.70 for Cu-MOR. The TPD spec-
trum for Cu-Y was quite different from that for Cu-ZSM-5 and
Cu-MOR, exhibiting two peaks at 388 and 425 K and complete
CO desorption at 560 K. In this case, the amount of CO ad-
sorbed per Cu+ cation was 0.51.

Y. Zhang et al. / Journal of Catalysis 251 (2007) 443–452
447
Fig. 6. IR spectra of Cu-ZSM-5 in the presence of different gases (from top to
bottom, CO; CO/CH OH; CO/CH OH/O ).
Fig. 7. IR spectra of Cu-MOR in the presence of different gases (from top to
bottom, CO; CO/CH OH; CO/CH OH/O ).
3
3
2
3
3
2
adsorbed. As a consequence of electron donation from adsorbed
methanol, the electron density on the Cu+ cations increased and
resulted in a greater degree of back-donation of electron den-
sity from the d-orbitals of Cu to the antibonding orbital of CO.
The net effect of the electron flow was to decrease the force
constant of the C–O bond, which in turn caused a red shift in
the C–O stretching frequency. When O2 was introduced into
shown in Figs 8a, 8d, and 8g, when the catalysts were exposed
to CH3OH at 403 K, the pre-edge peak of Cu+ remained at the
position observed under He but slightly decreased in intensity.
This suggests that the molecular adsorption of CH3OH on Cu+
sites did not affect the oxidation state of Cu and influenced its
coordination only slightly. When He-treated Cu-exchanged ze-
olites were exposed to 20.2 kPa of CO at 403 K, the resonant
peak at ∼8983 eV associated with coordinatively unsaturated
Cu+ observed in He and/or methanol treated samples was re-
placed by a new feature at ∼8981 eV (Figs. 8b, 8e, and 8h).
These changes in the near-edge region are due to the changes
in the coordination of Cu+ cations resulting by the forma-
tion of carbonyl complexes [22,24]. Thus, the formation of the
new feature can be considered a characteristic of CO adsorp-
tion. When Cu-Y was exposed to a CH3OH/CO/O2 mixture
(12.12/20.2/2.02 kPa) at 403 K, the first derivative spectrum
(Fig. 8c) shows that the peak at ∼8983 eV was restored to a
level very similar to that observed when the catalyst was un-
der He or CH3OH. The peak at ∼8981 eV due to CO adsorp-
tion became negligible. The XANES data together with the IR
spectra shown in Fig. 5 suggest that on exposure of Cu-Y con-
taining preadsorbed CO to a CH3OH/CO/O2 mixture (DMC
synthesis conditions), CO was displaced significantly by ad-
sorbed methanol and methoxide species. A different pattern
can be seen on Cu-ZSM-5 and Cu-MOR for similar condi-
tions. The first derivative spectrum (Figs. 8f and 8i) shows two
well-defined peaks at ∼8981 and ∼8983 eV. The positions of
these two features were identical to those observed under CO or
CH3OH, respectively. In agreement with the IR spectra shown
in Figs. 6 and 7, the XANES data recorded on exposure of Cu-
ZSM-5 or Cu-MOR to a CH3OH/CO/O2 mixture showed no
significant evidence for the displacement of CO by methanol
and methoxide species, as was seen for Cu-Y. Thus, for Cu-
ZSM-5 and Cu-MOR, the principal species observed under re-
action conditions were CO and CH3OH co-adsorbed on Cu+
cations.
the system, two new bands appeared at 2925 and 2823 cm−1
,
assigned to symmetric and antisymmetric vibrations of C–H
bonds in methoxide species [4,8]. When both methanol and
oxygen were present, the CO band remained at the frequency
seen in the presence of CO plus methanol, but the band inten-
sity was ∼10% of that observed when CO alone was present.
This observation indicates that once formed, methoxide species
displaced the adsorption of CO. Recent quantum mechanical
calculations support this conclusion [23].
Fig. 6 shows IR spectra for a series of experiments using Cu-
ZSM-5, which are similar to those shown in Fig. 5 for Cu-Y.
The carbonyl species on Cu-ZSM-5 was located at 2150 cm−1
[9]. When methanol and CO were present together, the posi-
tion of the CO band red shifted to 2126 cm−1. However, in
contrast to what was observed for Cu-Y, the addition of oxy-
gen to the gas stream present over the catalyst did not alter
the spectrum any further, and very little evidence for the for-
mation of methoxide species could be observed in the C–H
stretching region of the spectrum. This clearly indicates that
the concentration of surface methoxide species was much lower
on Cu-ZSM-5 than on Cu-Y. Fig. 7 shows that the behavior of
Cu-MOR was similar to that of Cu-ZSM-5 and, again, different
from that of Cu-Y.
3.4. Cu XANES for Cu-exchanged zeolites in DMC synthesis
The oxidation state and coordination of Cu were probed
by Cu K-edge XANES under reaction conditions. Because
changes in the XANES features were observed more clearly
in the derivative mode, only such data are presented here. As

448
Y. Zhang et al. / Journal of Catalysis 251 (2007) 443–452
Fig. 8. The first derivative curves of Cu K-edge XANES on Cu-Y (a–c); Cu-ZSM-5 (d–f); Cu-MOR (g–i) under different atmosphere.
3.5. Catalytic activity and selectivity
of zero, the selectivity to MF was highest on Cu-MOR (25%)
and lowest on Cu-Y (5%). Only small changes in the selectiv-
ity to MF were observed for any of the catalysts with increasing
methanol conversion. DME was formed to a very limited extent
on Cu-Y; however, modest selectivity to this product was seen
for Cu-ZSM-5 and Cu-MOR. The selectivity to DME increased
with methanol conversion for both Cu-Y and Cu-ZSM-5 but re-
mained almost constant for Cu-MOR.
The influence of reactant partial pressures is illustrated in
Figs. 10–12. All of these data were obtained at 403 K, a total
pressure of 101 kPa, and a fixed space-time. An increase in CO
partial pressure caused a strong increase in the rate of DMC and
CO2 formation on Cu-Y but had little effect on the formation
of DMM, MF, or DME. A similar pattern with respect to DMC
and CO2 formation was found for Cu-MOR, but in this case, the
rate of DMM formation was relatively insensitive to CO partial
pressure, whereas the rates of MF and DME formation declined
with increasing CO partial pressure. For Cu-ZSM-5, the rates of
DMC and CO2 formation showed a more modest increase with
increasing CO partial pressure than was seen for Cu-Y or Cu-
MOR. However, in contrast to the other catalysts, the rates of
DMM, MF, and DME formation decreased with increasing CO
partial pressure.
Fig. 9 displays the effects of methanol conversion on the
rates of product formation and the corresponding product se-
lectivities for Cu-Y, Cu-ZSM-5, and Cu-MOR. All experiments
were carried out at 403 K and 101 kPa with a feed contain-
ing CH3OH/CO/O2 in the ratio 12.12/20.2/2.02 kPa. In each
case, 150 mg of catalyst was used, and the methanol conversion
was varied by changing the flow rate of the feed mixture. The
carbon-containing products observed included DMC, DMM,
MF, DME, and CO2. The overall activity at a given methanol
conversion was greatest for Cu-Y and declined in the order
Cu-Y > Cu-ZSM-5 ꢀ Cu-MOR. The distribution of products
also was a strong function of zeolite structure. For an extrap-
olated conversion of zero, the selectivity to DMC was about
84% for Cu-Y, 50% for Cu-ZSM-5, and only 35% for Cu-MOR.
However, with increasing methanol conversion, the selectivity
to DMC for all catalysts decreased, paralleled by an increased
rate of CO2 formation.
DMM was the second most prevalent product formed from
methanol. At zero conversion, the selectivity to DMM from
methanol was 10% for Cu-Y, 40% for Cu-ZSM-5, and 35%
for Cu-MOR. The selectivity to this product rose with increas-
ing methanol conversion. Small amounts of MF and DME were
formed on all three catalysts. For an extrapolated conversion
The rate of DMC formation for Cu-Y and Cu-ZSM-5 passed
through a broad maximum with increasing methanol partial

Y. Zhang et al. / Journal of Catalysis 251 (2007) 443–452
449
Fig. 9. Selectivities and production rates as a function of methanol conversion (achieved by varying the feed residence time) on Cu-Y (a, d), Cu-ZSM-5 (b, e),
Cu-MOR (c, f) at 403 K under CH OH/CO/O (12.12/20.2/2.02 kPa).
3
2
Fig. 10. Effect of reactant partial pressure on oxidative carbonylation of methanol at 403 K on Cu-Y: (a) CO (CH OH 12.12 kPa; O 2.53 kPa); (b) CH OH (CO
3
2
3
25.25 kPa, O 2.53 kPa); (c) O (CO 25.25 kPa, CH OH 12.12 kPa).
2
2
3

450
Y. Zhang et al. / Journal of Catalysis 251 (2007) 443–452
Fig. 11. Effect of reactant partial pressure on oxidative carbonylation of methanol at 403 K on Cu-ZSM-5: (a) CO (CH OH 12.12 kPa; O 2.53 kPa); (b) CH OH
3
2
3
(CO 25.25 kPa, O 2.53 kPa); (c) O (CO 25.25 kPa, CH OH 12.12 kPa).
2
2
3
Fig. 12. Effect of reactant partial pressure on oxidative carbonylation of methanol at 403 K on Cu-MOR: (a) CO (CH OH 12.12 kPa; O 2.53 kPa); (b) CH OH
3
2
3
(CO 25.25 kPa, O 2.53 kPa); (c) O (CO 25.25 kPa, CH OH 12.12 kPa).
2
2
3
pressure, whereas for Cu-MOR, the rate of DMC formation
increased monotonically with increasing methanol partial pres-
sure. For Cu-Y, the rates of DMM and MF formation showed
negligible changes with increasing methanol partial pressure,
whereas the rates of CO2 and DME formation decreased with
increasing methanol partial pressure. The dependence of the
rate of DMM formation on Cu-ZSM-5 was similar to that ob-
served for DMC, whereas the rates of all other products de-
creased with increasing methanol partial pressure. In contrast,
for Cu-MOR, the rate of MF formation passed through a shal-
low minimum, but the rates of DMM, DME, and CO2 formation
increased with increasing methanol partial pressure.
The rate of DMC formation decreased slightly with increasing
O2 partial pressure for Cu-Y and Cu-MOR but increased for
Cu-ZSM-5. However, the rates of DMM, MF, and CO2 forma-
tion increased with increasing O2 partial pressure for all three
catalysts.
4. Discussion
4.1. Catalyst characterization
Complete exchange of Brönsted acid protons in H-Y, H-
ZSM-5, and H-MOR by copper cations was achieved by SSIE,
as evidenced by disappearance of the IR band for Brönsted
acid hydroxyl groups. Cu K-edge XANES demonstrated that
independent of the zeolite framework structure and Si/Al ra-
The effects of O2 partial pressure on the rates of product
formation were more similar for all three catalysts than those
observed for changes in the CO and methanol partial pressures.

Y. Zhang et al. / Journal of Catalysis 251 (2007) 443–452
451
tio, all of the exchanged copper was present as Cu+ in the
as-prepared catalyst. Differences in the Cu–O coordination and
bond lengths were demonstrated by Cu K-edge EXAFS. The
increased. This trend is likely due to the reaction of DMC and
H2O, both of which are formed under reaction conditions, to
produce methanol and CO2 via reaction (1) [1].
Cu–O coordination number decreased in the order NCu-ZSM-5
NCu-MOR > NCu-Y, whereas the average Cu–O bond distance
increased in the order RCu-ZSM-5 = RCu-MOR < NCu-Y
=
H2O + (CH3O)2CO → 2CH3OH + CO2.
(1)
Because the ꢀG0 for this reaction is −12.2 kcal/mol, it should
.
The IR spectroscopy of adsorbed CO provided limited evi-
dence for difference in the properties of Cu+ cations exchanged
into zeolites Y, ZSM-5, and MOR. As shown in Figs. 5–7, the
IR spectrum of adsorbed CO consisted of two or more overlap-
ping bands in the region of 2139–2150 cm−1. The band for CO
adsorbed on Cu-Y was broader than that for CO adsorbed on ei-
ther Cu-ZSM-5 or Cu-MOR, particularly on the low-frequency
side, and the bands for CO adsorbed on Cu-ZSM-5 and Cu-
MOR were of very similar shape and position. DFT calculations
for CO adsorbed on Cu-ZSM-5 and Cu-Y showed that the vi-
brational frequencies for the two zeolites were very similar and
that there was no obvious correlation of band position with the
zeolite framework structure or Si/Al ratio [25].
occur spontaneously.
Insights into the processes involved in the formation of DMC
can be gained from the in situ IR and XANES spectra given in
Figs. 5 and 8, respectively. Fig. 5 shows that under DMC syn-
thesis conditions, the amount of adsorbed CO decreased con-
siderably relative to what was observed in the presence of CO
alone and that the position of the CO band was red-shifted by
22 cm−1. Features of adsorbed methanol and methoxide species
also can be clearly seen under the reaction conditions. The
derivative XANES patterns shown in Fig. 8 confirm that un-
der DMC synthesis conditions, the fraction of the Cu+ cations
interacting with CO diminished significantly relative to that ob-
served in the presence of CO alone. These observations suggest
that under reaction conditions, methoxide species were formed
at the Cu+ sites, and that the formation of these species inhib-
ited the adsorption of CO. Quantum calculations indicate that
CO could adsorb weakly on a Cu+ cation containing a sin-
gle methoxide species, but not on one containing a methoxide
species and a second ligand, such as OH, OOH, or OCH3 [23].
It also is noted that when the formation of methoxide species
is limited (as is the case on Cu-ZSM-5 and Cu-MOR), the
rate of DMC formation is reduced significantly (see Figs. 9e
and 9f). Therefore, the formation of methoxide species ap-
pears to be an essential first step on the pathway to DMC
formation. As shown below, DMC may form via one of two
pathways. The first is by the addition of gas-phase or weakly
adsorbed CO to a species such as Z−[Cu+(OH)(OCH3)] to
produce Z−[Cu+(OH)(O(CO)CH3)] [reaction (3)]. The addi-
tion of CH3OH to the latter species would produce DMC and
H2O [reaction (4)]. This pathway is similar to that proposed
by King [4,5] and by Anderson and Root [6,7]. Alternatively,
CH3OH may be first added to Z−[Cu+(OH)(OCH3)] to pro-
duce Z−[Cu+(OCH3)2] plus water via reaction (5), after which
CO reacts with the dimethoxide to form DMC [reaction (6)].
The latter route appears to be preferred, because exposure of
Cu-Y containing methoxide species prepared by exposure of
the catalyst to a mixture of CH3OH and O2 at 403 K to CO
in He resulted in the immediate appearance of DMC in the gas
phase and the observation by IR spectroscopy of bands attribut-
able to DMC adsorbed on Cu-Y. These reactions are as follows:
The most significant effect of zeolite structure and Si/Al ra-
tio on the properties of Cu+-exchanged Y, ZSM-5, and MOR
were revealed by the CO TPD spectra presented in Fig. 4. This
clearly shows that complete desorption of CO occurred at a
significantly lower temperature for Cu-Y than for Cu-ZSM-5
and Cu-MOR, which exhibited very similar TPD spectra. The
total amount of CO adsorbed at 298 K per Cu+ cation also
reflects the zeolite structure and the Si/Al ratio; CO/Cu was
0.51 for Cu-Y, compared with 0.82 for Cu-ZSM-5 and 0.70
for Cu-MOR. These results are consistent with experimentally
measured heats of CO adsorption. Calorimetric measurements
of the heat of CO adsorption on Cu-Y are reported to be in
the range of 15.5–19.1 kcal/mol [26], whereas values of 23.8–
31 kcal/mol have been reported for Cu-ZSM-5 [27,28]. DFT
calculations have shown that the heat of CO adsorption in Cu-Y
is 15.0 kcal/mol, compared with 24–33 kcal/mol for Cu-ZSM-
5 [25]. These calculations also have shown that the strength of
CO adsorption is more sensitive to the Si/Al ratio than to the lo-
cal structure of the exchange site, which reflects the framework
structure of the zeolite. Thus, the lower heat of CO adsorption
for Cu-Y is a direct consequence of its lower Si/Al ratio (2.5)
compared with that of Cu-ZSM-5 (12) and Cu-MOR (10).
4.2. Catalyst activity and selectivity
The results presented in Figs. 9 and 10 demonstrate clearly
that Cu-Y is a more active catalyst for DMC synthesis than
either Cu-ZSM-5 or Cu-MOR. The rate of DMC synthesis in-
creased significantly with increasing CO partial pressure, was
independent of O2 partial pressure, and passed through a broad
maximum with CH3OH partial pressure. Some DMM was pro-
duced over Cu-Y, but at low methanol conversion, selectivity
to this product was only 10%. The only other product seen in
significant concentration was CO2. The formation of CO2 also
increased strongly with CO partial pressure, was insensitive to
O2 partial pressure, and decreased with increasing CH3OH par-
tial pressure. As the conversion of methanol increased, the rate
of DMC synthesis declined, whereas the rate of CO2 formation
Z[CuO] + CH3OH → Z[Cu(OH)(OCH3)],
(2)
(3)
Z[Cu(OH)(OCH3)] + CO → Z[Cu(OH)(O(CO)CH3)],
Z[Cu(OH)(O(CO)CH3)] + CH3OH → ZCu
+ (CH3O)2CO + H2O,
(4)
Z[Cu(OH)(OCH3)] + CH3OH → Z[Cu(OCH3)2] + H2O, (5)
Z[Cu(OCH3)2] + CO → ZCu + (CH3O)2CO. (6)
Compared with Cu-Y, Cu-ZSM-5 and Cu-MOR exhibited sig-
nificantly lower activity for DMC synthesis. The rates of DMC

452
Y. Zhang et al. / Journal of Catalysis 251 (2007) 443–452
formation on the latter two catalysts increased with increasing
CO partial pressure, in a manner similar to that observed for
Cu-Y. Likewise, the rate of DMC formation was not very sensi-
tive to the partial pressure of O2. Increasing the partial pressure
of O2 led to an increase in the rate of DMM formation on Cu-
ZSM-5 and Cu-MOR.
with in situ IR observations, suggest that the rate-limiting step
for the formation of DMC is likely to be the insertion of CO
into a Cu–OCH3 bond. The observed loss in DMC yield with
increasing methanol conversion is attributed to the hydrolysis
of DMC to methanol and carbon dioxide, a thermodynamically
favorable reaction.
As noted earlier, both IR spectroscopy and XANES spectra
obtained in situ demonstrate that the concentration of surface
methoxide species was much lower on Cu-ZSM-5 and Cu-
MOR than on Cu-Y and that most of the Cu+ cations were
occupied by adsorbed methanol and CO. The strongly adsorbed
CO appeared to inhibit the formation of methoxide species and
thus the formation of DMC. Therefore, reactions (4) and (6) ap-
peared to be strongly suppressed on Cu-ZSM-5 and Cu-MOR.
DMM was formed on all three catalysts. The rates of forma-
tion of this product were comparable on Cu-Y and Cu-MOR
and notably higher on Cu-ZSM-5. The participation of O2 in
the formation of this product was essential, because the overall
stoichiometry of DMM formation was 3CH3OH + (1/2)O2 →
(CH3O)2CH2 + 2H2O. Consistent with this, as shown in
Figs. 10–12, the rate of DMM formation increased with in-
creasing partial pressures of both CH3OH and O2. Because the
rate of DMM formation did not appear to correlate with the
formation of methoxide species, DMM may have formed via a
concerted reaction involving CH3OH and O2; however, the re-
sults of the present study are insufficient to provide a basis for
suggesting what this mechanism might be.
Acknowledgments
The authors thank the synchrotron laboratories of SSRL and
ALS for the allocated beam time and assistance. This work
was supported by the Methane Conversion Cooperative (MC2)
funded by BP.
References
[1] M.A. Pacheco, C.L. Marshall, Energy Fuels 11 (1997) 2–29.
[2] Y. Ono, Pure Appl. Chem. 68 (1996) 367–375.
[3] Y. Ono, Appl. Catal. A Gen. 155 (1997) 133–166.
[4] S.T. King, J. Catal. 161 (1996) 530–538.
[5] S.T. King, Catal. Today 33 (1997) 173–182.
[6] S.A. Anderson, T.W. Root, J. Catal. 217 (2003) 396–405.
[7] S.A. Anderson, T.W. Root, J. Mol. Catal. A Chem. 220 (2004) 247–255.
[8] I.J. Drake, Y.H. Zhang, D. Briggs, B. Lim, T. Chau, A.T. Bell, J. Phys.
Chem. B 110 (2006) 11654–11664.
[9] Y.H. Zhang, I.J. Drake, D.N. Briggs, A.T. Bell, J. Catal. 244 (2006) 219–
229.
[10] Y.H. Zhang, I.J. Drake, A.T. Bell, Chem. Mater. 18 (2006) 2347–2356.
[11] I.J. Drake, Y.H. Zhang, M.K. Gilles, C.N.T. Liu, P. Nachimuthu, R.C.C.
Perera, H. Wakita, A.T. Bell, J. Phys. Chem. B 110 (2006) 11665–11676.
[12] M. Newville, J. Synch. Rad. 8 (2001) 322–324.
[13] B. Ravel, M. Newville, J. Synch. Rad. 12 (2005) 537–541.
[14] M. Vaarkamp, J.C. Linders, D.C. Koningsberger, Physica B 209 (1995)
159–160.
5. Conclusion
Our findings indicate that Cu in Cu-Y, Cu-ZSM-5, and Cu-
MOR prepared by solid-state exchange of H-Y, H-ZSM-5, and
H-MOR with CuCl is present exclusively as Cu+. The average
coordination number of the Cu+ is close to 2 for Cu-Y and be-
tween 2 and 3 for Cu-ZSM-5 and Cu-MOR. Under identical
reaction conditions, the activity and selectivity of these cata-
lysts for the oxidative carbonylation of methanol is a function of
the zeolite structure and Si/Al ratio. Cu-Y exhibits the greatest
activity and selectivity. Cu-ZSM-5 and Cu-MOR exhibit signif-
icantly lower DMC synthesis activity and higher selectivity for
DMM synthesis than Cu-Y. The superior activity and selectiv-
ity of Cu-Y for DMC synthesis are associated with its weaker
adsorption of CO. Under reaction conditions, CO adsorbed on
Cu-Y is gradually displaced by methoxide species attached to
the Cu+ cations. This process also occurs on Cu-ZSM-5 and
Cu-MOR but to a much lesser extent. The ability of methoxide
groups to displace adsorbed CO from the Cu+ cations of Cu-Y
is closely related to the weaker binding of CO. Theoretical cal-
culations suggest that the lower binding energy of CO to Cu+
cations in Cu-Y compared with that in Cu-ZSM-5 or Cu-MOR
is due to the lower Si/Al ratio of Cu-Y. The rate of DMC synthe-
sis was found to increase strongly with CO partial pressure and
weakly with O2 partial pressure, and to be virtually indepen-
dent of CH3OH partial pressure. These trends, in combination
[15] D.C. Koningsberger, B.L. Mojet, G.E. van Dorssen, D.E. Ramaker, Top.
Catal. 10 (2000) 143–155.
[16] Y.H. Zhang, M.L. Toebes, A. van der Eerden, W.E. O’Grady, K.P. de Jong,
D.C. Koningsberger, J. Phys. Chem. B 108 (2004) 18509–18519.
[17] U. Eichler, M. Brandle, J. Sauer, J. Phys. Chem. B 101 (1997) 10035–
10050.
[18] M.W. Anderson, J. Klinowski, Zeolites 6 (1986) 455–466.
[19] A. Zecchina, S. Bordiga, G. Spoto, D. Scarano, G. Petrini, G. Leofanti, M.
Padovan, C.O. Arean, J. Chem. Soc. Faraday Trans. 88 (1992) 2959–2969.
[20] C. Lamberti, S. Bordiga, A. Zecchina, M. Salvalaggio, F. Geobaldo, C.O.
Arean, J. Chem. Soc. Faraday Trans. 94 (1998) 1519–1525.
[21] C. Lamberti, S. Bordiga, M. Salvalaggio, G. Spoto, A. Zecchina,
F. Geobaldo, G. Vlaic, M. Bellatreccia, J. Phys. Chem. B 101 (1997) 344–
360.
[22] C. Lamberti, S. Bordiga, F. Bonino, C. Prestipino, G. Berlier, L. Capello,
F. D’Acapito, F.X.L.I. Xamena, A. Zecchina, Phys. Chem. Chem. Phys. 5
(2003) 4502–4509.
[23] X. Zheng, Y. Zhang, A.T. Bell, unpublished work.
[24] P. Kumashiro, Y. Kuroda, M. Nagao, J. Phys. Chem. B 103 (1999) 89–96.
[25] X. Zheng, Y. Zhang, A.T. Bell, J. Phys. Chem. C, submitted for publica-
tion.
[26] G.D. Borgard, S. Molvik, P. Balaraman, T.W. Root, J.A. Dumesic, Lang-
muir 11 (1995) 2065–2070.
[27] Y. Kuroda, Y. Yoshikawa, S. Emura, R. Kumashiro, M. Nagao, J. Phys.
Chem. B 103 (1999) 2155–2164.
[28] A. Gervasini, C. Picciau, A. Auroux, Microporous Mesoporous Mater.
35 (6) (2000) 457–469.