Zirconia-supported MoOx catalysts for the selective oxidation of dimethyl ether to formaldehyde: Structure, redox properties, and reaction pathways

Article abstract of DOI:10.1021/jp0221744

Dimethyl ether (DME) reacts to form formaldehyde with high selectivity at 500-600 K on MoO_x-ZrO₂ catalysts with a wide range of MoO_x surface density (0.5-50.1 Mo/nm²) and local structure (monomers, oligomers, MoO₃ crystallites, and ZrMo₂O₈). Reaction rates (per Mo-atom) increased markedly as MoO_x surface density increased from 2.2 to 6.4 Mo/nm₂ and two-dimensional polymolybdates and MoO₃ clusters became the prevalent active species. The rate of incipient stoichiometric reduction of MoO_x-ZrO₂ samples in H₂ also increased with increasing MoO_x surface density, suggesting that catalytic turnovers involve redox cycles that become faster as the size and dimensionality of MoO_x domains increase. DME reaction rates (per Mo-atom) decreased as MoO_x surface densities increased above 6.4 Mo/nm², as MoO₃ and ZrMo₂O₈ clusters with increasingly inaccessible MoO_x species form. On MoO_x and ZrMo₂O₈, areal reaction rates reach a constant value at MoO_x surface densities above 10 Mo/nm², as the exposed surfaces become covered with the respective active species. ZrMo₂O₈ surfaces were more reducible in H₂ than MoO_x surfaces and showed higher areal reaction rates. Reaction rates were nearly independent of O₂ pressure, but the reaction order in DME decreased from one at low pressures (<40 kPa) to zero at higher pressures (>60 kPa). DME reacts via primary pathways leading to HCHO, methyl formate, and CO_x, with rate constants k₁, k₂, and k₃, respectively, and via secondary HCHO conversion to methylformate (k₄) and CO_x (k₅). Primary HCHO selectivities (and k₁/(k₂ + k₃) ratios) increased with increasing MoO_x surface density on MoO_x-containing samples and reached values of 80-90% above 10 Mo/nm². Kinetic ratios relevant to secondary HCHO reactions (k₁/[(k₄ + k₅)C_AO]; C_AO inlet DME concentration) also increased with increasing MoO_x surface density to values of a??0.1 and 0.8 on MoO_x and ZrMo₂O₈ structures (at the constant inlet DME concentration C_AO), respectively. Thus, increasing the coverage of ZrO₂ surfaces with MoO_x or ZrMo₂O₈ leads to more selective structures for HCHO synthesis from DME.

Full text of DOI:10.1021/jp0221744

4118
J. Phys. Chem. B 2003, 107, 4118-4127
Zirconia-Supported MoOx Catalysts for the Selective Oxidation of Dimethyl Ether to
Formaldehyde: Structure, Redox Properties, and Reaction Pathways
Haichao Liu, Patricia Cheung, and Enrique Iglesia*
Department of Chemical Engineering, UniVersity of California at Berkeley, Berkeley, California 94720
ReceiVed: October 1, 2002; In Final Form: January 31, 2003
Dimethyl ether (DME) reacts to form formaldehyde with high selectivity at 500-600 K on MoOx-ZrO2
catalysts with a wide range of MoOx surface density (0.5-50.1 Mo/nm2) and local structure (monomers,
oligomers, MoO3 crystallites, and ZrMo2O8). Reaction rates (per Mo-atom) increased markedly as MoOx
surface density increased from 2.2 to 6.4 Mo/nm2 and two-dimensional polymolybdates and MoO3 clusters
became the prevalent active species. The rate of incipient stoichiometric reduction of MoOx-ZrO2 samples
in H2 also increased with increasing MoOx surface density, suggesting that catalytic turnovers involve redox
cycles that become faster as the size and dimensionality of MoOx domains increase. DME reaction rates (per
Mo-atom) decreased as MoOx surface densities increased above 6.4 Mo/nm2, as MoO3 and ZrMo2O8 clusters
with increasingly inaccessible MoOx species form. On MoOx and ZrMo2O8, areal reaction rates reach a constant
value at MoOx surface densities above 10 Mo/nm2, as the exposed surfaces become covered with the respective
active species. ZrMo2O8 surfaces were more reducible in H2 than MoOx surfaces and showed higher areal
reaction rates. Reaction rates were nearly independent of O2 pressure, but the reaction order in DME decreased
from one at low pressures (<40 kPa) to zero at higher pressures (>60 kPa). DME reacts via primary pathways
leading to HCHO, methyl formate, and COx, with rate constants k1, k2, and k3, respectively, and via secondary
HCHO conversion to methylformate (k4) and COx (k5). Primary HCHO selectivities (and k1/(k2 + k3) ratios)
increased with increasing MoOx surface density on MoOx-containing samples and reached values of 80-
90% above 10 Mo/nm2. Kinetic ratios relevant to secondary HCHO reactions (k1/[(k4 + k5)CAo]; CAo inlet
DME concentration) also increased with increasing MoOx surface density to values of ∼0.1 and 0.8 on MoOx
and ZrMo2O8 structures (at the constant inlet DME concentration CAo), respectively. Thus, increasing the
coverage of ZrO2 surfaces with MoOx or ZrMo2O8 leads to more selective structures for HCHO synthesis
from DME.
1. Introduction
weak C-O bonds in DME (CH3OCH3),16,17 but previous reports
consist of only a few patents.18-22 The limited scope of these
previous reports reflects the historically higher cost of DME
relative to methanol; direct synthesis gas routes to DME,
however, have been recently developed and they provide more
favorable thermodynamics and economics than methanol
synthesis.23-25 A potential DME distribution infrastructure in
some developing countries and the benign properties of DME
as a fuel24,25 also render it attractive as an alternate intermediate
for the synthesis of chemicals currently produced from methanol.
These considerations led us to examine the catalytic chemistry
of DME and specifically its oxidative conversion to HCHO on
supported metal oxides. Well-dispersed domains of metal oxides,
predominately present as two-dimensional structures, provide
an effective balance between reactivity and accessibility of oxide
surfaces. Our preliminary studies identified MoOx and VOx as
the preferred active oxides.26 MoOx domains supported on
Al2O3, ZrO2, or SnO2 showed high primary HCHO selectivities
(80-98%).26,27 HCHO formation rates on these catalysts were
significantly higher than previously reported,18-22 even at
temperatures much lower than in these earlier studies.
Many recent studies have addressed the conversion of
methane to chemicals and transportation fuels.1-6 An attractive
route to chemicals involves the selective conversion of methane
to formaldehyde2,7-12 and the subsequent conversion of form-
aldehyde to larger molecules containing C-C bonds. Form-
aldehyde (HCHO) synthesis via direct oxidation of methane with
O2 leads to low yields, because of fast sequential HCHO
decomposition and combustion reactions. Most previous studies
have reported HCHO yields below 5%,7-10 but HCHO yields
as high as ∼18% have been recently reported.5,11,12 As a result,
indirect routes for methane conversion to chemicals and liquid
fuels are industrially practiced; for example, formaldehyde is
produced via the synthesis of methanol from synthesis gas (H2/
CO) and its subsequent oxidative dehydrogenation to HCHO.
Here, we report an alternate route to HCHO via the selective
oxidation of dimethyl ether (DME) produced from H2/CO
mixtures.
Fe-Mo oxides and Ag-based catalysts13,14 are currently used
for methanol oxidation to HCHO. The pathways and catalyst
requirements for this reaction have been widely studied.14,15
Lattice oxygen atoms are used in rate-limiting steps involving
hydrogen abstraction from adsorbed methoxide species (CH3O-δ).
These methoxide species can also be formed via cleavage of
Here, we describe a detailed study of the reducibility,
structure, and catalytic properties of ZrO2-supported MoOx
samples with a wide range of MoOx densities; surface densities
were varied by changing the Mo content and the thermal
treatment conditions. These samples consist of MoOx domains
of varying size and dimensionality or of surface layers of
* Author to whom correspondence should be addressed. Tel: (510) 642-
9673. Fax: (510) 642-4778. E-mail: iglesia@cchem.berkeley.edu.
10.1021/jp0221744 CCC: $25.00 © 2003 American Chemical Society
Published on Web 04/04/2003

Dimethyl Ether Oxidation to Formaldehyde on MoOx
J. Phys. Chem. B, Vol. 107, No. 17, 2003 4119
TABLE 1: Surface Areas and MoOx Surface Density for MoOx/ZrO2 Catalysts Treated at 723, 773, and 873 K
723 K
773 K
873 K
MoO3 content
(wt %)
surface area
(m2/g)
MoOx surface
surface area
(m2/g)
MoOx surface
surface area
(m2/g)
MoOx surface
sample
density (Mo/nm2)
density (Mo/nm2)
density (Mo/nm2)
1MoOx/ZrO2
6MoOx/ZrO2
11MoOx/ZrO2
21MoOx/ZrO2
29MoOx/ZrO2
37MoOx/ZrO2
44MoOx/ZrO2
1.0%
5.7%
11.0%
20.7%
29.3%
37.0%
44.0%
118.8
130.0
145.9
153.7
114.0
99.6
0.3
1.8
3.2
105.6
110.3
132.6
136.3
96.9
73.9
60.2
0.4
2.2
3.5
85.6
97.1
103.4
102.7
64.6
49.3
36.7
0.5
2.5
4.5
5.6
6.4
8.4
10.7
15.5
22.0
12.6
20.9
30.6
20.0
31.4
50.1
83.5
MoOx-ZrO2 mixed oxides, depending on the thermal treatment
and the MoOx surface density. This study addresses the structural
and site requirements for primary and secondary pathways
relevant to DME oxidation reactions. It also explores the redox
nature of the catalytic sequence involved in HCHO synthesis
and the fundamental relationships among the size, structure, and
reducibility of MoOx domains and their catalytic reactivity.
rates and selectivities were measured at 453-553 K, typically
using a mixture of DME (80 kPa; 99.5%, Praxair), O2 (18 kPa),
and N2 (2 kPa) (Praxair, certified O2/N2 mixture). For kinetic
measurements, DME, O2, and H2O partial pressures in the
reactant stream were varied in the ranges 5-80 kPa, 5-40 kPa,
and 0-15 kPa, respectively. Homogeneous DME reactions were
detected only above 593 K in empty reactors. Water was added
to DME/O2 reactant mixtures by passing a controlled flow of
20% H2/He over a bed of CuO/Al2O3 (13 wt %, Aldrich) held
at 623 K in order to combust all the H2 to H2O using the lattice
oxygen atoms in CuO. All transfer lines after the introduction
of H2O were kept above 393 K in order to prevent condensation.
The reactor effluent was analyzed by on-line gas chroma-
tography (Hewlett-Packard 6890 GC) using a methyl silicone
capillary column (HP-1, 30 m × 0.25 mm × 0.25 µm film
thickness) connected to a flame ionization detector and a
Porapak Q packed column (80-100 mesh, 1.82 m × 3.18 mm)
connected to a thermal conductivity detector. Methanol, form-
aldehyde (HCHO), methylformate (MF), CO, CO2, H2O, and
traces of dimethoxymethane (DMM) were the only products
detected. Detailed HCHO calibrations were carried out in order
to ensure that neither decomposition nor oligomerization of
HCHO occurred in the transfer lines, the on-line sampling valve,
or the chromatograph injector or column.
Reaction rates and product selectivities were measured as a
function of DME conversion, which was varied through changes
in reactant flow rate between 1.5 and 10 cm3 (STP)/g s-1. DME
conversion levels were kept below 10% and DME reaction rates
and selectivities were extrapolated to zero residence time in
order to obtain primary reaction rates and selectivities. DME
reaction rates and selectivities are reported in two ways. One
method considers CH3OH as a reaction product, while the other
lumps any CH3OH formed with unreacted DME and considers
selectivities on a CH3OH-free basis. The latter approach seems
appropriate in view of the available pathways for DME-CH3-
OH interconversion and for CH3OH conversion to HCHO. All
selectivities are reported on a carbon basis as the percentage of
the carbon atoms in the reacted DME appearing as each product.
2. Experimental Section
2.1. Synthesis of Catalytic Materials. MoOx/ZrO2 was
prepared by incipient wetness impregnation of precipitated ZrO-
(OH)2 with aqueous (NH4)2Mo2O8 solutions (99%, Aldrich).28,29
ZrO(OH)2 was prepared via precipitation of zirconyl chloride
solutions (98%, Aldrich) at a pH of 10, maintained constant by
continuous addition of NH4OH (29.8%, Fisher Scientific). The
precipitated solids were washed with a NH4OH solution of pH
∼8 until Cl ions were no longer detected by AgNO3 in the
filtrate and then dried overnight in ambient air at 393 K. The
impregnated solids were also dried overnight in ambient air at
393 K and then treated in flowing dry air at 723, 773, or 873
K for 3 h. The Mo content was set by the Mo concentration in
the impregnating solution.
These MoOx/ZrO2 samples were characterized using X-ray
diffraction (XRD) and UV-visible, Raman, X-ray absorption
near-edge (XANES), and fine structure (XAFS) spectros-
copies.28,29 BET surface areas were measured using N2 at its
normal boiling point. MoOx surface densities are reported as
Mo/nm2 using the measured Mo contents and BET surface areas.
Table 1 shows a summary of the Mo contents, BET surface
areas, and surface densities for the samples used in this study.
2.2. Stoichiometric Reduction of MoOx/ZrO2 in H2.
Reduction rates of MoOx/ZrO2 were measured using H2 as the
reductant and a Quantasorb surface area analyzer (Quantachrome
Corporation) modified with electronic mass flow controllers.
MoOx/ZrO2 samples (10 mg Mo) were placed in a quartz cell
(4 mm-i.d.) containing a quartz thermowell in contact with the
samples. The temperature was increased linearly from 298 to
1253 K at 0.167 K s-1 in flowing 20% H2/Ar (1.33 cm3s-1
)
(Matheson UHP). The H2 content in the effluent was measured
by thermal conductivity after the H2O formed during reduction
was removed from the effluent stream using a 13X molecular
sieve trap held at ambient temperature. The conductivity detector
response was calibrated from the complete reduction of CuO
powders (99.995%, Aldrich). Reduction rate constants were
obtained from the initial rates of H2 consumption using
previously reported kinetic analysis protocols.30,31
2.3. Dimethyl Ether Catalytic Oxidation Rates and Se-
lectivities. Dimethyl ether reaction rates and selectivities were
measured in a fixed-bed quartz microreactor using MoOx/ZrO2
samples (0.3 g) diluted with quartz powder (1.0 g) in order to
avoid temperature gradients within the catalyst bed. Samples
were treated in flowing 20% O2/He (0.67 cm3 s-1) for 1.5 h at
723, 773, or 873 K before catalytic measurements. Catalytic
3. Results
3.1. Effects of Reactant Residence Time. DME conversion
rates are shown in Figure 1 as a function of reactant residence
time at 513 K on 20.7% MoOx/ZrO2 (21MoOx/ZrO2 in Table
1) (treated in air at 773 K; 6.4 Mo/nm2 surface density). The
ZrO2 support in this sample is predominately covered by two-
dimensional polymolybdate species.28,29 DME conversion rates
decreased slightly with increasing residence time as a result of
the weak H2O inhibition effects discussed below. The primary
DME reaction rate was 12.2 mol/g-atom Mo-h. CH3OH, HCHO,
methylformate (MF), dimethoxymethane (DMM), and COx (CO
+ CO2) were detected in the reactor effluent. The selectivity
for the predominant HCHO product decreased with increasing
residence time and DME conversion (Figure 1), while methyl-

4120 J. Phys. Chem. B, Vol. 107, No. 17, 2003
Liu et al.
Figure 1. DME reaction rates and selectivities as a function of reactant
residence time at 513 K on 21MoOx/ZrO2 (6.4 Mo/nm2) treated at 773
K (80 kPa CH3OCH3, 18 kPa O2, 2 kPa N2).
Figure 3. Effect of DME partial pressure on primary DME reaction
rates and selectivities to HCHO, MF, and COx at 493 K on 44MoOx/
ZrO2 (50.1 Mo/nm2) treated at 873 K (9 kPa O2, 1 kPa N2, He balance).
parallel path, possibly involving hydrogen transfer among the
methoxide species also involved in HCHO formation. In view
of these parallel pathways and of the catalytic routes available
for the concurrent dehydration and oxidative dehydrogenation
of CH3OH, we consider CH3OH products as part of the DME
reactant pool and we report hereinafter rates and selectivities
on a CH3OH-free basis.
3.2. Effects of CH3OCH3, O2, and H2O Concentrations
on Reaction Rate and Selectivity. The effects of varying CH3-
OCH3, O2, and H2O partial pressures on DME conversion rates
and selectivities were examined on several MoOx/ZrO2 (3.5,
6.4, 8.4, and 50.1 Mo/nm2) samples with MoOx or ZrMo2O8
structures. The observed kinetic responses were very similar
on all samples. Here, we show detailed kinetic results only on
44MoOx/ZrO2 (50.1 Mo/nm2; treated at 873 K); this sample
contains predominately ZrMo2O8.
Figure 3 shows primary DME conversion rates and primary
selectivities to HCHO, MF, and COx at 493 K and 9 kPa O2 as
a function of DME partial pressure (10-80 kPa) without water
in the reactant mixture. Primary DME conversion rates increased
almost linearly with DME partial pressure from 10 to 40 kPa,
and then more weakly at higher DME pressures, until reaction
rates became nearly independent of DME pressure above 60
kPa. This behavior suggests surface saturation by one or more
DME-derived reactive intermediates as DME pressure increases.
Primary MF (14.8%) and DMM (1.0%) selectivities were almost
independent of DME pressure, while primary COx selectivities
decreased from 6.0% to a constant value of 1.5% as the DME
pressure increased from 10 to 40 kPa. Concurrently, HCHO
selectivities increased from 78.3 to 82.7% (Figure 3).
The effects of O2 partial pressure were examined at DME
pressures of 20 and 80 kPa (Figure 4a,b), which correspond to
regimes in which DME reaction orders are near-first and -zero,
respectively (see Figure 3). Primary DME conversion rates at
20 kPa DME were unaffected by O2 partial pressure in the range
5-40 kPa (Figure 4a). An increase in O2 pressure from 5 to 20
kPa at 80 kPa DME led to a slight increase in primary DME
conversion rates (Figure 4b); thus, reaction rates are nearly
independent of O2 partial pressure throughout this DME
concentration range. Primary selectivities were also essentially
unaffected by O2 partial pressure (Figure 4). The addition of
water (15 kPa) to DME/O2 reactant mixtures at levels higher
Figure 2. DME reaction rates and selectivities as a function of reactant
residence time at 513 K on 44MoOx/ZrO2 (50.1 Mo/nm2) treated at
873 K (80 kPa CH3OCH3, 18 kPa O2, 2 kPa N2).
formate, dimethoxymethane, and COx selectivities concurrently
increased. The selectivity to CH3OH was essentially independent
of residence time. The primary selectivities to CH3OH (23.0%),
HCHO (57.1%), MF (15.5%), DMM (0.3%), and COx (4.1%)
were all nonzero, indicating that these products can be formed
directly from DME.
Figure 2 shows the effects of residence time on DME
conversion rates and selectivities at 513 K on 44MoOx/ZrO2
treated at 873 K (50.1 Mo/nm2). This sample consists predomi-
nately of ZrMo2O8 and ZrO2.28,29 The primary products and the
effects of residence time on DME conversion rates were similar
to those observed on 21Mo/ZrO2 (6.4 Mo/nm2; Figure 1), which
contains predominately two-dimensional polymolybdate struc-
tures. The effects of residence time on HCHO selectivities were
much weaker on 44Mo/ZrO2 (50.1 Mo/nm2) than on 21Mo/
ZrO2 (6.4 Mo/nm2). Thus, it appears that ZrMo2O8 structures
catalyze secondary HCHO reactions less effectively than the
polymolybdate domains prevalent in 21Mo/ZrO2 (6.4 Mo/nm2)
(cf. Figures 1 and 2).
CH3OH selectivities remained almost constant with changes
in residence time on all catalysts. CH3OH appears to form in a

Dimethyl Ether Oxidation to Formaldehyde on MoOx
J. Phys. Chem. B, Vol. 107, No. 17, 2003 4121
Figure 5. Effect of water partial pressure on primary DME reaction
rate and selectivities to HCHO, MF, and COx at 493 K on 44MoOx/
ZrO2 (50.1 Mo/nm2) treated at 873 K (40 kPa CH3OCH3, 9 kPa O2, 1
kPa N2, He balance).
by the monotonic decrease in the UV-visible edge energy
observed with increasing MoOx surface density.28
Figure 6a shows the effects of MoOx surface density and of
the concurrent structural and compositional changes on primary
DME reaction rates (per Mo-atom). On samples containing
predominately MoOx structures, these rates increased with
increasing MoOx surface density; they reached a maximum value
at ∼6.4 Mo/nm2, and then decreased at higher Mo surface
densities. On ZrMo2O8 species, DME reaction rates decreased
monotonically as the MoOx surface density increased from 8.4
to 50.1 Mo/nm2. At each (nominal) surface density, ZrMo2O8
samples showed higher rates (per Mo-atom) than MoOx/ZrO2
samples consisting of polymolybdate domains and MoOx
clusters. Treatment of 11MoOx/ZrO2 at 873 K led to a 2-fold
increase in DME reaction rates relative to those measured after
treatment at 723 or 773 K (Figure 6a), even though Mo surface
densities were essentially unchanged by this treatment (3.2-
4.5 Mo/nm2) (Table 1).
Figure 4. Effect of O2 partial pressure on primary DME reaction rate
and selectivities to HCHO, MF, and COx at 493 K at 20 kPa DME (a)
and 80 kPa DME (b) (He balance, without added water) on 44MoOx/
ZrO2 (50.1 Mo/nm2) treated at 873 K.
DME reaction rates normalized per (BET) total surface area
are shown in Figure 6b for all samples. These rates increased
monotonically with increasing MoOx surface density and reached
a constant value of 0.08 mmol/m2-h at surface densities above
6.4 Mo/nm2 for MoOx species (two-dimensional polymolybdates
or MoO3 crystallites). These areal rates were unaffected by
surface density on ZrMo2O8-containing samples (0.45 mmol/
m2-h), but they were significantly higher than on samples
containing MoOx species.
than those prevalent during DME reactions led to a decrease in
DME conversion rates (from 1.4 to 1.0 mol/g-atom Mo-h)
without significant changes in primary selectivities (Figure 5).
These water inhibition effects are consistent with the small
decrease in DME conversion rates observed with increasing
reactant residence time and DME conversion (Figures 1 and
2).
3.3. Effects of MoOx Surface Density on DME Reaction
Rate and Selectivity. The structure of the MoOx domains
present on MoOx/ZrO2 depends on MoOx surface density and
thermal treatment.28,29 X-ray diffraction and UV-visible and
Raman spectroscopic studies have shown that MoOx/ZrO2
samples contain two types of MoOx structures. For surface
densities above 5 Mo/nm2 and thermal treatments at 873 K or
higher temperatures, samples contain predominately ZrMo2O8
structures; similar treatments led to two-dimensional polymo-
lybdates and MoO3 crystallites (denoted as MoOx structures)
for samples with lower surface densities.28,29 At all surface
densities, MoOx/ZrO2 samples treated in air at 723 or 773 K
contained predominately two-dimensional polymolybdates and
MoO3 crystallites.28,29 The structural evolution of MoOx domains
from monomolybdates, to two-dimensional polymolybdates, and
ultimately to three-dimensional MoO3 crystallites was confirmed
The effects of MoOx surface density and local MoOx structure
on primary selectivities are shown in Figure 7a-c. On MoOx-
containing samples, primary HCHO selectivities increased with
increasing MoOx surface density and they reached a value of
∼90% for surface densities above 10 Mo/nm2. HCHO selectivi-
ties on ZrMo2O8 species were slightly lower (∼80%); they were
not influenced by MoOx surface density (Figure 7a). Methyl-
formate selectivities on MoOx-containing samples decreased
with increasing surface density to a constant value of ∼7% for
surface densities above 10 Mo/nm2. On ZrMo2O8 samples,
methylformate selectivities were very similar (∼15%) at all Mo
surface densities (8.4-50.1 Mo/nm2) (Figure 7b). Primary COx
selectivities decreased to ∼4% at Mo surface densities above
10 Mo/nm2 on MoOx samples; they were also ∼4%, but they

4122 J. Phys. Chem. B, Vol. 107, No. 17, 2003
Liu et al.
Figure 6. Dependence of primary DME reaction rates normalized per
Mo atom (a) and per catalyst surface area (b) on MoOx surface density
for MoOx/ZrO2 catalysts (pretreated in air at 723 K (0), 773 K (O),
and 873 K (4, 2)) (513 K, 80 kPa CH3OCH3, 18 kPa O2, 2 kPa N2).
were not influenced by MoOx surface density on ZrMo2O8
samples (Figure 7c).
Our recent transient and isotopic studies, taken together with
the observed zero-order O2 dependence of DME rates and
selectivities (Figure 4a,b), suggest that DME reactions occur
via a redox mechanism using lattice oxygen atoms at rates
apparently limited by C-H bond cleavage in adsorbed meth-
oxide (CH3O*) intermediates.32 C-H bond activation in CH3O*
species would involve the incipient reduction of MoOx structures
and the formation of low steady-state concentrations of reduced
centers, which are continuously reoxidized by O2 in order to
complete a catalytic turnover. Therefore, the strong observed
effects of MoOx surface density and structure on DME reaction
rates may reflect the ability of MoOx domains to undergo local
reduction during each catalytic turnover. This proposal is
confirmed by measurements of the reducibility of MoOx/ZrO2
samples reported in the next section, which show a parallel
increase in the rates of stoichiometric reduction in H2 and of
catalytic DME oxidation as MoOx surface density increases.
3.4. Reduction Rates of MoOx/ZrO2 Samples in H2.
Reduction rates of MoOx/ZrO2 in H2 were measured from the
incipient rate of hydrogen consumption and oxygen removal
as the sample temperature increased from 298 to 1253 K. The
Figure 7. Dependence of primary selectivities to HCHO (a), MF (b),
and COx (c) on MoOx surface density for MoOx/ZrO2 catalysts
(pretreated in air at 723 K (0), 773 K (O), and 873 K (4, 2)) (513 K,
80 kPa CH3OCH3, 18 kPa O2, 2 kPa N2).
incipient reduction rates in H2 for a given metal oxide reflect
the removal of a few oxygen atoms, leading to surfaces with
an extent of reduction similar to that prevalent during steady-
state catalytic reactions involving redox cycles, such as the

Dimethyl Ether Oxidation to Formaldehyde on MoOx
J. Phys. Chem. B, Vol. 107, No. 17, 2003 4123
SCHEME 1: Reaction Network for Dimethyl Ether
Reactions on MoOx-Based Catalysts
reactions at 513 K on these samples.32 Thus, the very initial
stages of Mo6+ to Mo4+ reduction steps are most relevant to
the redox dynamics required in catalytic DME reactions. We
have previously reported an analysis protocol for the kinetics
of these incipient reduction processes;30,31 these methods are
used here in order to extract initial H2 reduction rates. These
rates are reported in Figure 8b as a function of MoOx surface
density for samples treated at 873 K, for 21MoOx/ZrO2 treated
at 723 and 773 K, and for 11MoOx/ZrO2 treated at 773 K (3.5
Mo/nm2). In samples with low surface densities (0.5-6.4 Mo/
nm2) and containing predominately MoOx monomers and
oligomers, initial H2 reduction rates increased markedly with
increasing surface density (Figure 8b), except for 11MoOx/ZrO2
treated at 873 K (4.5 Mo/nm2), which showed a higher reduction
rate than all other MoOx-containing samples. The ZrMo2O8-
containing sample (8.4 Mo/nm2, treated at 873 K) showed a
higher reduction rate than those containing predominately
oligomeric MoOx structures (Figure 8b).
4. Discussion
The nonzero primary selectivities for CH3OH, HCHO, MF,
DMM, and COx (Figures 1 and 2) show that these products can
be formed directly from dimethyl ether. Primary HCHO products
react in secondary reactions that lead to a decrease in HCHO
selectivity with increasing residence time. The concurrent
increase in methylformate and COx selectivities implicate these
species as products of secondary HCHO reactions. The small
effects of residence time on CH3OH selectivities indicate that
CH3OH is less reactive than HCHO or DME on these catalysts.
DME hydration reactions do not appear to be responsible for
the formation of CH3OH, because CH3OH formation rates are
unaffected by the increase in H2O concentration that ac-
companies an increase in DME conversion. Instead, it appears
that CH3OH forms via hydrogen transfer between CH3O*
adsorbed species, with the ultimate formation of one CH3OH
and one HCHO molecule.32 Similar selectivity trends were
observed on samples containing MoOx and ZrMo2O8, suggesting
that primary and secondary reaction pathways are similar on
ZrMo2O8 and MoOx domains.
The observed effects of residence time on selectivity are
consistent with the reaction pathways shown in Scheme 1. These
pathways include primary DME reactions to form CH3OH,
HCHO, MF, and COx, and reactions of HCHO (to MF and COx)
and of MF (to COx). DMM formation from DME is not included
in the kinetic analysis, because of the low DMM selectivities
observed (<1%). The weak effects of residence time on CH3-
OH selectivity led us to neglect secondary reactions of CH3OH
in our kinetic analysis.
Figure 8. (a) Temperature-programmed reduction profiles for MoOx/
ZrO2 catalysts treated at 873 K in air; (b) initial H2 reduction rate at
623 K as a function of MoOx surface density and MoOx structure for
MoOx/ZrO2 samples (pretreated in air at 723 K (2), 773 K (b), and
873 K (9)).
oxidative dehydrogenation of alkanes.31,33 The stoichiometric
reduction of oxides with H2 involves H2 dissociation and the
formation of surface OH groups similar in structure to those
formed via H-abstraction from methoxy groups during HCHO
synthesis from methanol or DME on oxide surfaces.
Figure 8a shows H2 reduction rates for MoOx/ZrO2 samples
treated in air at 873 K. Two reduction peaks are apparent and
their relative intensities are consistent with the sequential
reduction of Mo6+ to Mo4+ and of Mo4+ to Mo0.31,34,35 The
low-temperature peak shifts from ∼730 K to ∼685 K as the
MoOx surface density increased from 0.5 to 8.4 Mo/nm2. An
additional shoulder appears at ∼645 K for the sample with 8.4
Mo/nm2 (Figure 8a), but the same 21MoOx/ZrO2 sample treated
at 723 K (5.6 Mo/nm2) or 773 K (6.4 Mo/nm2) lacks this feature.
This feature corresponds to the reduction of Mo6+ to Mo4+ in
Mo-O-Zr structures present in ZrMo2O8, which forms only
after treatment at 873 K. These data suggest that the rates of
Mo6+ to Mo4+ reduction steps are higher on larger MoOx
structures than on smaller ones; they are also higher on ZrMo2O8
domains than on polymolybdate structures.
At low DME and O2 conversions, DME and O2 concentra-
tions are essentially independent of residence time and at the
low H2O concentrations prevalent at these DME conversions,
In-situ X-ray absorption studies during reaction have shown
that few reduced Mo4+ centers (<5%) are present during DME

4124 J. Phys. Chem. B, Vol. 107, No. 17, 2003
Liu et al.
the inhibiting effects of water on reaction rates (Figure 5) can
be neglected. For the DME (80 kPa) and O2 (18 kPa) pressures
of this study, reaction rates for each primary DME reaction in
Scheme 1 can then be assumed to be pseudo zero-order in DME
and O2 (Figures 3 and 4b) and given by
r0 ) k0
r1 ) k1
r2 ) k2
r3 ) k3
(1)
(2)
(3)
(4)
where ri is the rate of reaction i per Mo atom and ki is the pseudo
zero-order rate constant for reaction i.
The kinetics for secondary HCHO reactions to form MF and
COx (Scheme 1) are likely to show a complex dependence on
DME partial pressure. The nearly constant DME partial pressure
along the catalyst bed allows the rates of these secondary
reactions to be expressed only as a function of the concentration
of the primary product involved in a given secondary reaction,
which increases as DME conversion increases along the catalyst
bed:
r4 ) k4CB
r5 ) k5CB
(5)
(6)
In these equations, rj is the rate of reaction j per Mo atom and
kj is the pseudo first-order rate constant for reaction j, and CB
is the HCHO concentration. The dependence of HCHO selectiv-
ity (CH3OH-free basis) (SB) at relatively low DME conversions
is then given by
1
2
SB ) SB° 1 - (k4 + k5)CAo‚υ
[
]
SB° ) k1/(k1 + k2 + k3)
(7)
Figure 9. Dependence of k1/(k2 + k3) ratios (a) and k1/((k4 + k5)CAo
)
ratios (b) (CAo: inlet CH3OCH3 concentration) on MoOx surface density
for MoOx/ZrO2 catalysts (pretreated in air at 723 K (0), 773 K (O),
and 873 K (4, 2)) (513 K, 80 kPa CH3OCH3, 18 kPa O2, 2 kPa N2).
in which SB° is the primary HCHO selectivity, CAo is the inlet
DME concentration, and υ is the g-atom Mo/inlet molar DME
rate (g-atom Mo-s/mol DME) (see details in Appendix). The
values of k1, k2, and k3 can be estimated from the primary rates
for HCHO, MF, and COx formation (CH3OH-free). The value
of [(k4 + k5)CAo] can be obtained from the dependence of SB
on υ given by eq 7.
HCHO selectivities and yields depend on the relative rates
of primary reactions that form HCHO, MF, and COx and of
secondary HCHO reactions that also form MF and COx (Scheme
1). The k1/(k2 + k3) ratio provides a measure of the primary
selectivity to HCHO, while the k1/(k4 + k5) ratio reflects the
relative rates of primary HCHO formation and of secondary
HCHO reactions to form MF and COx. The k1/(k4 + k5) ratio
can be reflected by the k1/((k4 + k5)CAo) ratio for a given inlet
DME concentration, CAo. Higher values of k1/(k2 + k3) and of
k1/((k4 + k5)CAo) lead to higher HCHO selectivities at all DME
conversion levels. Taken together, these two kinetic parameters
determine the maximum attainable HCHO yields in DME
oxidation reactions.
unaffected by MoOx surface density (8.4-50.1 Mo/nm2). These
trends in rate constant ratios parallel those discussed earlier for
primary HCHO selectivities on MoOx/ZrO2 as a function of
MoOx surface density for samples with the two predominant
types of MoOx species (Figure 7a).
Values of k1/((k4 + k5)CAo) ratios increased to constant values
of ∼0.09 and ∼0.80 as the MoOx surface density increased on
MoOx and ZrMo2O8 samples at the constant CAo, respectively
(Figure 9b). Thus, it appears that exposed ZrO2 surfaces favor
secondary HCHO reactions to form MF and COx; these
secondary reactions do not appear to reflect the presence of
Mo-O-Zr structures, which become less abundant as MoOx
domains grow with increasing MoOx surface density, because
ZrMo2O8 surfaces actually show much lower secondary reaction
rates. The k1/((k4 + k5)CAo) ratios on ZrMo2O8 domains are
much larger than on MoOx domains and they lead to the weaker
effects of residence time on HCHO selectivity observed on
ZrMo2O8 surfaces compared with MoOx surfaces (Figures 1 and
2). These effects of MoOx surface density and MoOx domain
structure differ somewhat from those reported for propane
oxidative dehydrogenation (ODH) on MoOx/ZrO2.28 In that case,
ZrMo2O8 domains led to higher ODH areal rates and to higher
COx selectivities in both primary and secondary oxidation
Figures 9a and 9b show the effects of MoOx surface density
on k1/(k2 + k3) and k1/((k4 + k5)CAo) values, respectively. As
MoOx surface density increases in MoOx-containing samples,
k1/(k2 + k3) values initially increased and then reached a constant
value (∼8) at surface densities above 10 Mo/nm2. On ZrMo2O8-
containing samples, k1/(k2 + k3) values were ∼4 and they were

Dimethyl Ether Oxidation to Formaldehyde on MoOx
J. Phys. Chem. B, Vol. 107, No. 17, 2003 4125
reactions. Also, first-order rate constants for propene combustion
(k3′) were much higher than for propane dehydrogenation (k1′)
(k1′/k3′ ∼ 0.03)28 on ZrMo2O8, while primary HCHO synthesis
rate constants (k1) are similar to those for secondary HCHO
reactions (k4 + k5) on ZrMo2O8. This reflects, at least in part,
the relative energies of the bonds involved in DME and alkane
primary and secondary reactions. While the allylic C-H bonds
in propene (361 kJ/mol) are much weaker than the secondary
C-H bonds in propane (401 kJ/mol), the difference in the
dissociation energy for C-H bonds in DME (389 kJ/mol) and
HCHO (365 kJ/mol) are smaller than between propane and
propene (24 vs 40 kJ/mol). Also, while propene is expected to
bind more strongly than propane on Lewis acid sites (Mo6+
)
because its π-bond makes it more basic than propane, the lone-
pair electrons of oxygen in DME lead to its stronger adsorption
on acid sites relative to HCHO.14,36,37 Taken together, the
molecular properties of reactants and products in these two
reactions appear to account for the higher attainable yields for
oxidative reactions of DME compared with similar reactions
of propane.
Figure 10. Dependence of primary DME reaction rate at 513 K on
initial H2 reduction rate at 623 K for MoOx/ZrO2 catalysts treated in
air at 723 K (2), 773 K (b), and 873 K (9).
In contrast with the strong dependence of k1/(k2 + k3) and
k1/((k4 + k5)CAo) values on MoOx surface density, these two
kinetic parameters did not change significantly with changes
in the partial pressures of DME, O2, and H2O. These changes
in k1/(k2 + k3) values led to the small parallel effects on primary
HCHO selectivity observed with changes in DME, O2, and H2O
pressures shown in Figures 3-5. The k1/((k4 + k5)CAo) values
increased from 0.26 to 0.31 and from 0.28 to 0.34 with
increasing the DME and H2O partial pressures in the range 10-
80 kPa and 0-15 kPa, respectively; the k1/((k4 + k5)CAo) values
remained almost constant (∼0.27) as the O2 partial pressure
changed from 5 to 40 kPa. This indicates that HCHO selectivi-
ties and maximum attainable yields at a given DME conversion
level depend only slightly on the partial pressures of DME, O2,
and H2O.
(8.4-50.1 Mo/nm2) (Figure 6a). On both MoOx and ZrMo2O8
structures, this reflects the nucleation of three-dimensional
clusters, which render an increasing fraction of the MoOx species
inaccessible to reactants by placing them within crystallites. The
surface density required for maximum rates (per Mo) on MoOx-
containing samples (6.4 Mo/nm2) is slightly higher than the
theoretical polymolybdate monolayer value (∼5.0 Mo/nm2).38
This reflects a compromise between the lower accessibility and
the higher reactivity of MoOx surfaces as the size and
dimensionality of MoOx domains increase.
MoOx surface density effects on areal DME reaction rates
(Figure 6b) confirmed these conclusions. Areal rates and primary
HCHO selectivities both reached constant values at Mo surface
densities of ∼10 Mo/nm2 on both MoOx (0.08 mmol/m2-h; 90%)
and ZrMo2O8 (0.45 mmol/m2-h; 80%), but the rates are
significantly higher on ZrMo2O8 surfaces. These constant values
indicate that all exposed surfaces contain predominately MoO3
and ZrMo2O8 species with active surface structures, the kinetic
behavior of which is unaffected by further increases in size or
dimensionality. This could reflect a similar surface reactivity
for two-dimensional and three-dimensional MoOx structures, or
merely the predominant contribution from polymolybdate
monolayers on ZrO2 or surface layers of ZrMo2O8 to the
exposed surfaces , without significant contributions from any
large MoO3 crystallites also present. The higher areal rates
measured on ZrMo2O8 samples are consistent with their more
reducible nature, as shown by the rate of their stoichiometric
reduction in H2 (Figure 8b). The more reducible nature of
ZrMo2O8 appears to reflect a structure consisting of two-
dimensional networks of alternating MoO4 tetrahedra and ZrO6
octahedra (corner-shared), which differs markedly from the
layers of corner-sharing MoO6 octahedra in MoO3 crystallites.
The atomic connectivity between Mo6+ and the less electrone-
gative Zr4+ cations in ZrMo2O8 may favor the electron transfer
and the activation of Mo-O bonds during the reduction in H2
and the DME reaction.
The activity of surface sites on MoOx domains for DME
oxidation to HCHO depends sensitively on the structural
evolution induced by an increase in the MoOx surface density
(Figure 6a,b). The predominant presence of MoOx monomers
and two-dimensional oligomers in MoOx/ZrO2 samples with
surface densities below 6.4 Mo/nm2 exposes most MoOx species
at surfaces, where these species are accessible to reactants. As
a result, the measured DME reaction rates (per Mo atom)
represent turnover rates (per exposed MoOx). These turnover
rates increase with increasing MoOx surface density (0.5-6.4
Mo/nm2) (Figure 6a), indicating that the reactivity of MoOx
surfaces increases as the size and dimensionality of MoOx
domains increases. These higher turnover rates parallel the
higher initial H2 reduction rates (per Mo atom) measured for
larger domains (Figure 8b). The more facile reduction of larger
domains, and the consequently faster catalytic redox cycles,
reflect more effective delocalization of the negative charge by
these larger domains. These electronic effects also lead to lower
absorption edge energies in the UV-visible spectrum of larger
domains, as a result of the effectiveness of larger domains in
delocalizing the electrons transferred from the oxygen atoms
to the metal centers in the electronic transition responsible for
the absorption edge in the UV-visible spectrum.33
Figure 10 shows the observed parallel increase in primary
DME reaction rates (per Mo atom, at 513 K) and initial
reduction rates in H2 (per Mo atom, at 623 K) with increasing
MoOx surface density. These data correspond to samples in
which most Mo species are exposed at surfaces, because only
then catalytic reaction rates and reduction rates reflect their
respective surface reaction rates. These data clearly show that
DME reaction rates (per Mo) decreased with increasing
surface density in more densely packed surfaces (>6.4 Mo/
nm2) (Figure 6a), in which polymolybdate and three-dimensional
MoO3 crystallites become the predominant structures. DME
reaction rates decreased with increasing nominal MoOx surface
density on ZrMo2O8 throughout the entire surface density range

4126 J. Phys. Chem. B, Vol. 107, No. 17, 2003
Liu et al.
the effects of domain size and the differences between MoOx
and ZrMo2O8 surfaces in HCHO synthesis from DME arise from
the varying ability of such species to reduce as part of the
kinetically relevant C-H bond activation steps within catalytic
cycles. Similar trends were observed for DME reactions on
MoOx species on other supports27 and on dispersed VOx
domains.39 These correlations between the reducibility of the
active oxides and their activity parallel similar effects reported
for other oxidation reactions involving MoOx and VOx lattice
oxygen atoms and Mars-van Krevelen redox cycles, such as
the oxidative dehydrogenation reactions of alkanes30,31,33,40 and
alcohols.15,41-43
sions (or at any conversion in the zero-order reaction regime)
by
dX
A ) υ‚(k1 + k2 + k3)
(A1)
dς
where
XA ) fractional DME conversion (on a DME-free basis)
υ ) g-atom Mo/inlet molar DME rate
(g-atom Mo-s/mol DME)
ki ) rate constants for primary DME reactions
5. Conclusions
(mol DME/g-atom Mo-s)
Dimethyl ether conversion to formaldehyde occurs with high
selectivity at ∼500 K on MoOx-ZrO2. Reaction rates are
significantly higher than those reported previously in the patent
literature. The catalysts used consist of MoOx oligomers and
ZrMo2O8. Dimethyl ether reaction rates (per Mo atom) increase
markedly with increasing MoOx surface density in the 2.2 to
6.4 Mo/nm2 range as two-dimensional polymolybdate domains
grow and ultimately form MoO3 clusters. The rates of reduction
of these materials in H2 increase in parallel with increasing
MoOx surface density, suggesting that redox cycles and kineti-
cally relevant reduction steps are involved in dimethyl ether
reactions and that the rate of such steps increases with increasing
reducibility of MoOx domains. Reaction rates are nearly zero-
order in O2, while the dependence on dimethyl ether decreases
from first-order at low pressures (<40 kPa) to zero-order at
higher pressures (>60 kPa). These kinetic dependences are
consistent with this proposal. Above 6.4 Mo/nm2, reaction rates
decrease with increasing surface density because MoO3 and
ZrMo2O8 (after treatment in air at 723-773 and 873 K,
respectively) with increasingly inaccessible MoOx species form.
Reaction rates (per BET surface area) approach constant values
as exposed surfaces become entirely covered with active MoOx
or ZrMo2O8. The latter are more reducible in H2 than MoOx
surfaces and show higher areal HCHO synthesis rates. DME
reacts directly to form HCHO, methyl formate, and COx, with
pseudo zero-order rate constants k1, k2, and k3. Methylformate
(k4) and COx (k5) also form in secondary reactions of HCHO.
The primary formaldehyde selectivity (and the relevant kinetic
ratio, k1/(k2 + k3)) increases with increasing MoOx surface
density on MoOx-containing samples and reaches values of 80-
90% above 10 Mo/nm2. The kinetic ratio describing the extent
of secondary reactions (k1/((k4 + k5)CAo)) also increases with
increasing MoOx surface density to values of ∼0.1 and 0.8 on
MoOx and ZrMo2O8 structures (at the constant inlet DME
concentration CAo, respectively. It appears that more selective
active sites form as ZrO2 surfaces become increasingly covered
with MoOx or ZrMo2O8 structures, which leads to more selective
structures for the synthesis of HCHO from dimethyl ether.
ς ) fractional distance along the reactor
The solution to this equation is
XA ) (k1 + k2 + k3)‚υ
(A2)
The HCHO concentration (CB) and molar rates (FB) are given
by
d(FB/FAo)
dς
1
υ
) 2k1 - (k4 + k5)CB
(A3)
where FAo is the inlet molar DME rate and the rate constants
are those for eqs 5 and 6 in the text. At relatively low DME
conversions, changes in the total number of moles can be
neglected and eq A3 becomes
dC
B ) 2k1 - (k4 + ks)CB
(A4)
1
υCAo dς
in which CAo is the inlet DME concentration and the inlet HCHO
concentration is assumed to be zero. The solution to this equation
is
2k1
CB )
[1 - exp{ - (k4 + k5)CAoυ}] (A5)
(k4 + k5)
The HCHO selectivity (SB) (CH3OH-free basis) is then given
by
CB
k1
(k1 + k2 + k3) (k4 + k5)υCAo
[1 - exp{- (k4 + k5)CAoυ}] (A6)
1
SB )
)
‚
‚
2CAoXA
When the observed selectivity changes as a result of changes
in υ are small, a series expansion of the exponential to second-
order term two gives
1
2
Acknowledgment. This study was supported by BP as part
of the Methane Conversion Cooperative Research Program at
the University of California at Berkeley. The authors acknowl-
edge helpful technical discussions with Drs. Theo Fleisch and
John Collins of BP. The authors also thank Dr. Kaidong Chen
for the synthesis of some of the supported MoOx samples.
SB ) SB° 1 - (k4 + k5)CAo‚υ
(A7)
[
]
where the primary HCHO selectivity SB° (CH3OH-free basis)
is
k1
SB° )
k1 + k2 + k3
Appendix
The conversion of DME (A) in a plug-flow reactor for the
reaction scheme in eqs 1-4 is given at relatively low conver-
The primary selectivity is obtained from the y-intercept of a
HCHO selectivity vs υ plot; the slope of the plot is - (1/2)(k4

Dimethyl Ether Oxidation to Formaldehyde on MoOx
J. Phys. Chem. B, Vol. 107, No. 17, 2003 4127
(20) Lewis, R. M.; Ryan, R. C.; Slaugh, L. H. U.S. Patent 4,439,624,
+ k5)CAo, from which the reported values of [k1/(k4 + k5)CAo
]
1984.
(21) Lewis, R. M.; Ryan, R. C.; Slaugh, L. H. U.S. Patent 4,435,602,
1984.
are readily obtained.
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