Methane Partial Oxidation over Vanadyl Pyrophosphate and the Effect of Fe and Cr Promoters on Selectivity

Article abstract of DOI:10.1006/jcat.1997.1840

Partial oxidation of methane by molecular oxygen over Cr- and Fe-promoted, as well as unpromoted, vanadyl pyrophosphate catalysts has been studied in the temperature range of 573-698 K and atmospheric pressure. Carbon monoxide, carbon dioxide, and water were found to be the principal reaction products over unpromoted vanadyl pyrophosphate. Over both the Fe- and Cr-promoted catalysts high formaldehyde selectivity was observed at very low methane conversion levels with HCHO space time yields in the range of 0.5 to 2.0 g/kg-h. As the extent of the reaction was increased selectivity to formaldehyde decreased rapidly and changes in selectivities with conversion indicate a sequential conversion of methane to formaldehyde, CO, then CO₂, Macrokinetic parameters for the methane oxidation reaction were determined for the unpromoted catalyst. A simple power law rate expression fit the rate data well over the whole temperature range. The rate of reaction of methane was 0.08 ± 0.02 order in oxygen partial pressure, 0.73 ± 0.07 order in methane partial pressure, and the activation energy was found to be 102 ± 6 kJ/mole. Activation energy was unchanged by promotion with Fe and Cr. Analysis by XPS and ICP-AA indicates that promoters were incorporated equally into the bulk and surface of these catalysts. XPS indicates an increase in the average surface oxidation state of vanadium in the promoted catalysts and XRD shows that promotion causes oxidation of a small fraction of the pyrophosphate to form α_II-VOPO₄. ³¹P NMR spin-echo mapping confirms the enhanced formation of V⁵⁺ in the promoted samples. The presence of V⁵⁺ may therefore be required for the formation of selective products. It is proposed that the role of promoters is to increase the rate of formation of V⁵⁺ during activation, or to stabilize V⁵⁺ containing domains under the highly reducing methane oxidation conditions.

Full text of DOI:10.1006/jcat.1997.1840

JOURNAL OF CATALYSIS 172, 160–169 (1997)
ARTICLE NO. CA971840
Methane Partial Oxidation over Vanadyl Pyrophosphate and the Effect
of Fe and Cr Promoters on Selectivity
,1
ꢀ
ꢀ
ꢀ
Robert L. McCormick, Gokhan O. Alptekin, Andrew M. Herring, T. R. Ohno,† and Steven F. Dec‡
ꢀ
Department of Chemical Engineering and Petroleum Refining, †Department of Physics, and ‡Department of Chemistry and Geochemistry,
Colorado School of Mines, Golden, Colorado 80401-1887
Received April 4, 1997; revised July 23, 1997; accepted July 24, 1997
oxidation reactions of methane to methanol and formalde-
hyde are thermodynamically favorable (1Gꢃ = ꢄ291,
Partial oxidation of methane by molecular oxygen over Cr- and
Fe-promoted, as well as unpromoted, vanadyl pyrophosphate cata-
lysts has been studied in the temperature range of 573–698 K and
atmospheric pressure. Carbon monoxide, carbon dioxide, and wa-
ter were found to be the principal reaction products over unpro-
moted vanadyl pyrophosphate. Over both the Fe- and Cr-promoted
catalysts high formaldehyde selectivity was observed at very low
methane conversion levels with HCHO space time yields in the
range of 0.5 to 2.0 g/kg-h. As the extent of the reaction was increased
selectivity to formaldehyde decreased rapidly and changes in selec-
tivities with conversion indicate a sequential conversion of methane
to formaldehyde, CO, then CO2, Macrokinetic parameters for the
methane oxidation reaction were determined for the unpromoted
catalyst. A simple power law rate expression fit the rate data well
over the whole temperature range. The rate of reaction of methane
was 0.08 ꢁ 0.02 order in oxygen partial pressure, 0.73 ꢁ 0.07 order
in methane partial pressure, and the activation energy was found
to be 102 ꢁ 6 kJ/mole. Activation energy was unchanged by promo-
tion with Fe and Cr. Analysis by XPS and ICP-AA indicates that
promoters were incorporated equally into the bulk and surface of
these catalysts. XPS indicates an increase in the average surface
oxidation state of vanadium in the promoted catalysts and XRD
shows that promotion causes oxidation of a small fraction of the
pyrophosphate to form ꢀII-VOPO4. 31P NMR spin-echo mapping
confirms the enhanced formation of V5+ in the promoted samples.
The presence of V5+ may therefore be required for the formation
of selective products. It is proposed that the role of promoters is to
increase the rate of formation of V5+ during activation, or to sta-
bilize V5+ containing domains under the highly reducing methane
c
ꢄ111 kJ/mole, respectively), but combustion products are
more so (1Gꢃ = ꢄ544, ꢄ801 kJ/mole for CO and CO2, re-
spectively). Thermodynamic instability of the desired prod-
ucts with respect to those of combustion requires a catalyst
and reaction conditions that facilitate the formation of oxy-
genates without oxidizing them further. A large number of
catalysts have been studied in the literature, with most of
the attention focused on molybdenum and vanadium-based
systems. Several reviews (1–4) have been published in re-
cent years and provide an overall view of the progress made
in this field. High selectivities toward the desired products
are usually achieved only at very low methane conversion
and yields are too low to be of economic interest.
Vanadyl pyrophosphate, with the formula (VO)2P2O7
(referred to as VPO in this paper), combined with related
– –
V P O phases is used commercially to catalyze the selec-
tive oxidation of n-butane to maleic anhydride (5). Pure
vanadyl pyrophosphate contains only VIV but the presence
of VV is required for selective hydrocarbon oxidation to
proceed (6) and to complete the VIV/VV redox couple. The
exact state of VV in these materials (i.e., isolated surface
sites or macroscopic domains of a specific phosphate phase)
is an area of active research (7–11). It is clear, however,
that the most active and selective catalysts are largely com-
posed of vanadyl pyrophosphate (11, 12). VPO exhibits a
unique ability to activate and selectively oxidize alkanes.
There are published reports on VPO indicating moderate
to high selectivity in oxidation of ethane (13), propane (14)
and pentane (15), as well as butane (5). It has been shown
that for saturated hydrocarbons, the first step in activat-
oxidation conditions.
ꢂ 1997 Academic Press
INTRODUCTION
–
ing the molecule is the dissociation of the C H bond in a
Methane partial oxidation to methanol and formalde-
hyde is one of the most difficult challenges in catalysis.
Severe reaction conditions are required to activate this oth-
erwise verystable species, and under such conditionsthe de-
sired oxygenates can be oxidized to COx and water. Partial
manner similar to the producion of hydrocarbon free rad-
icals (16). Strong Lewis acid sites present on the surface
of VPO may initiate alkane activation (17). Vanadyl py-
rophosphate undergoes a phase change to VOPO4 at tem-
peratures above 773 K in the presence of oxygen (18). Five
polymorphsofVOPO4 are known. Methane oxidation, over
what was probably ꢀI-VOPO4, has been investigated by
1 To whom correspondence should be addressed. E-mail: rlmccorm@
mines.edu.
160
0021-9517/97 $25.00
c
Copyright ꢂ 1997 by Academic Press
All rights of reproduction in any form reserved.

METHANE PARTIAL OXIDATION
161
Lopez Granados and Wolf (19) and formation of formalde- by diffuse reflectance on a BioRad FTS-40 instrument us-
hyde at low yield was reported in the temperature range ing a Harrick diffuse reflectance attachment and samples
873–993 K.
mixed with KBr. Chemical analysis was performed by ICP-
In the work reported here we have examined vanadyl py- AA on samples digested in concentrated nitric acid and
rophosphate for activity and selectivity in the methane par- then diluted before analysis. A Kratos Electronics spec-
tial oxidation reaction at atmospheric pressure and in the trometer with monochromatic Al Kꢀ radiation was used to
temperature range 573–698 K. A variety of cations have obtain the X-ray photoelectron spectra (XPS). The binding
been added to vanadium phosphate catalysts to improve energy of C 1s (284.6 eV) was used as a reference in these
catalyst activity or selectivity (20). For n-butane oxidation measurements. Deconvolution of the V 2p3/2 binding en-
beneficial effects have been claimed for promotion by many ergy envelope was performed by fitting two Gaussian peaks
of the first row transition metals (20–24), although negative separated by1to 1.1eV with a nonlinear regression package
effects have also been observed (24). Vanadyl pyrophos- varying intensity and peak width. Wide-line (nonspinning)
phate promoted with Fe and Cr has also been examined for 31P NMR experiments were conducted on a Chemagnet-
the methane partial oxidation reaction in this work.
ics CMX Infinity 400 instrument (7.5 mm probe, 31P spec-
tral frequency of 162.0 MHz) using a spin-echo mapping
approach similar to that described by Li and co-workers
(7). In our implementation of this method the carrier fre-
quency was varied in increments of 62.5 kHz above and be-
low the resonance frequency of 31P in 85% H3PO4 to cover
the complete range where spectral intensity was observed.
METHODS
Catalyst Preparation
Unpromoted VPO catalyst. Vanadyl pyrophosphate
was prepared following the procedure reported by Busca
and co-workers (25). Fifteen grams of V2O5 was suspended
in 90 ml of isobutyl alcohol and 60 ml of benzyl alcohol. The
suspension was stirred under reflux for 3 h, then cooled to
room temperature and left stirring overnight. Then 16.2 g
99% anhydrous phosphoric acid was added (P : V atomic
ratio 1.00), and the mixture was refluxed for an additional
2 h. After completion of the reaction, the solid phase was
recovered by filtration, washed with isobutyl alcohol, and
dried in air at 393 K overnight. The dried catalyst precur-
sor, VOHPO4-0.5H2O, was then calcined in 1.5% butane
in air at 673 K for 18 h (GHSV 800 hꢄ1) to facilitate the
transformation to vanadyl pyrophosphate.
Catalyst Testing
Steady-state reaction studies were performed in a fixed-
bed micro-reactor. The composition (CH4, O2, He) and flow
rate of the feed gas was controlled by Brooks 5850E mass
flow controllers. The reactor was a quartz tube, 30-cm long
and 1.0-cm ID at the catalyst bed portion, mounted verti-
cally in a tubular furnace. A quartz frit was used to hold
the catalyst bed in place. Typically about 0.3 g of catalyst
(screened to 0.5–0.7 mm particle size, 0.677 g/cm3 bulk den-
sity) was loaded into the reactor and covered with 15-mm
layer of quartz beads to obtain a preheating zone and a uni-
form gas distribution. The exit diameter was decreased to
5-mm ID right after the quartz frit to allow reaction prod-
ucts to leave the heated zone more rapidly. Temperature
was monitored by two K-type thermocouples, one placed
in contact with the catalyst bed and the other right under
the frit. Prior to reaction the catalyst was calcined in situ
under helium flow (20 ml/min) at the reaction tempera-
ture for 1 h. Methane was introduced into the reactor first
so that at all times the methane–oxygen mixture was kept
well above the upper explosive limits. Methane conversion
was well below 10% for all experiments reported. Over-
all carbon balance closures obtained were within ꢁ5% and
mostly within ꢁ3% . Fractional conversion and selectivity
were defined as
Promoted VPO catalysts. Nitrate saltsofCr and Fe were
used as the source of promoter elements. Five grams of
CrIII nitrate nonahydrate or 5.05 g of FeIII nitrate nonahy-
drate was dissolved in a benzyl/isobutyl alcohol mixture.
This was then added to 45 g of previously prepared pre-
cursor that had been resuspended under reflux in a ben-
zyl/isobutyl alcohol mixture and cooled. This mixture was
then stirred at 323 K overnight. The precipitate was then
filtered and washed with more solvent and dried in a rotary
vacuum drier for 8 h at slightly greater than ambient tem-
perature. The promoted catalyst precursors were activated
in the same way described for unpromoted VPO.
Analytical Methods
Specific surface areas (BET) of the catalyst samples were
measured using a Micromeretics 2100E Accusorb Instru-
ment. Nitrogen was used as adsorbate at liquid nitrogen
temperatures, taking a value of 0.162 nm2 for the cross sec-
tion area of the adsorbed nitrogen molecule. X-ray powder
diffraction (XRD) patterns were obtained using a Rigaku
moles (HCHO + CO + CO2) formed
Conversion =
moles CH4 fed
moles product formed
Selectivity =
.
moles (HCHO + CO + CO2) formed
˚
diffractometer. Cu Kꢀ radiation (ꢁ = 1.5432 A) was used as
An on-line Hewlett-Packard 5890 Gas Chromatograph
the incident X-ray source. Infrared spectra were obtained (GC) equipped with a thermal conductivity detector was

162
McCORMICK ET AL.
used to analyze reactant and product streams. Separation of analyzed to obtain a formaldehyde peak area. The mixture
CH4, O2, CO, and CO2 was accomplished with a 6 ft ꢅ 1/8 in. wasthen passed over a 4wt% Pd/Al2O3 combustion catalyst
S.S. Carbosphere 80/100 column. For methanol, formalde- at 673 K. Complete formaldehyde conversion was obtained
hyde, and water an 8 ft ꢅ 1/8 in. S.S. Poropak-T column was and CO2 was the only combustion product. The known CO2
used. For the Carbosphere column a GC temperature pro- calibration was then used to determine the concentration
gram starting at 323 K and ending at 403 K was applied, with of formaldehyde in the feed gas.
an initial soak time of 3 min and a heating rate of 30 K/min.
RESULTS
For the Poropak-T column, the GC temperature was held
constant at 403 K. Calibration for CH4, CO, and CO2 was
accomplished using standards from Scott Specialty Gases.
Methanol and water calibrations were performed using
Catalyst Characterization
X-ray diffraction data obtained for the precursor show
standard solutions of reagent grade methanol and deion- the peaks of vanadyl phosphate hemihydrate VOHPO4-
ized water prepared in volumetric flasks. Calibration of the 0.5H2O. A poorly crystalline vanadyl pyrophosphate was
GC for formaldehyde is more complex. Paraformaldehyde the only phase observed by XRD in the activated, unpro-
was placed in a heated flask, purged by the feed gas. A feed moted sample in agreement with the results reported for
gas of oxygen–helium was passed through the system and this preparation method (25) as shown in Fig. 1. Given the
FIG. 1. X-ray diffraction patterns of unpromoted and promoted vanadyl pyrophosphate catalysts.

METHANE PARTIAL OXIDATION
163
than in the others. After use in methane/oxygen the relative
intensity is comparable to that for the unpromoted VPO.
These data indicate that Fe promotion enhances the rate
of development of crystallinity and suggest that one reason
for the use of Fe as a promoter for industrial butane oxi-
dation catalysts (20) is because equilibrium catalysts are
more rapidly obtained.
Surface areas are listed in Table 1 and are nearly identical
for all three catalysts. XPS results, also in Table 1, indicate
that roughly one tenth of the surface metal atoms are pro-
moter. Bulk analysisyieldssimilar resultsindicatingthat the
method of introducing promoters modifies both the surface
and bulk composition. Surface P : V ratios are well above
1.0 as has been reported for commercial butane oxidation
catalysts. Bulk P : V ratios are slightly below 1.0 and slightly
below the ratio employed in the catalyst synthesis.
XPS binding energies for vanadium (2p3/2), phosphorus
(2p), oxygen (1s), and the promoter atoms are listed in
Table 2. Reported binding energies in the vanadyl py-
rophosphate phase (P/V = 1.0–1.2) range from 516.6 to
517.9 eV (26, 27). For reference, VV compounds show bind-
ing energies of 518.2 eV for ꢂ-VOPO4 (21) and 517.1 eV
for V2O5 (26). The binding energy of 516.9 eV reported in
Table 2 is in good agreement with literature values when
differences in the C 1s reference binding energy are con-
sidered. Binding energies in this range were also observed
for the promoted catalysts although peaks are significantly
braodened. Deconvolution of the V 2p3/2 peak envelope
leads to the average vanadium oxidation state reported in
Table 2. Promotion has resulted in oxidation of a significant
fraction of the surface vanadium atoms.
An additional method for discerning vanadium oxidation
states employs the difference in binding energy of the O 1s
and V 2p3/2 signals (1O 1s ꢄ V 2p) which is correlated with the
surface oxidation states (26, 28, 29). This approach elimi-
nates the need for a reference binding energy such as C 1s.
The binding energy difference for pure vanadyl pyrophos-
phate, known to contain mostly VIV, varies from 14.9 to 15.2
for P/V = 1.0 to 1.2. For the VV containing ꢂ-VOPO4 the
binding energy difference is between 13.0 to 13.9 and for
V2O5 is 12.9 eV (27). This binding energy difference shifts
TABLE 1
Characterization Results for Vanadyl Pyrophosphate
and Promoted Catalysts
Surface
area
Surface
P : V
ratio
Bulk
P : V
ratio
Surface
Bulk
Catalyst
VPO
(m2/g)
promoter: V
promoter: V
32.1
33.8
32.6
—
0.13
0.09
—
0.08
0.11
1.23
1.58
1.16
0.97
0.99
0.92
–
Cr VPO
–
Fe VPO
broadness of some of the XRD peaks the presence of other
VPO phases cannot be completely ruled out, however. Af-
ter exposure to methane oxidation conditions for 20 h the
(200) reflection at 22.8ꢃ has increased in intensity, suggest-
ing that order in the layer stacking direction has increased.
Figure 1 also reports X-ray diffracion data for both tran-
sition metal-promoted VPO catalysts after activation and
after use for 20 h in methane oxidation. These data show
that additional peaks are evident for Fe- and Cr-promoted
ꢃ
˚
VPO. In the Cr-promoted sample, peaks at 24.9 (3.58 A),
ꢃ
ꢃ
˚
˚
29.15 (3.06 A), and 40.71 (2.21 A) in the activatꢃed sample
˚
correspond to ꢀII-VOPO4 (7). The peak at 12.2 (7.26 A)
could not be assigned. These are reduced in intensity after
exposure to methaꢃne oxidation conditions and a new peak
˚
is observed at 21.9 (4.21 A). The crystallinity of this mate-
rial does not appear to have developed during catalysis, as
relative peak intensities are essentially unchanged. These
data indicate that addition of Cr by the method outlined in
the experimental section causes the predominantly vanadyl
pyrophosphate starting material to be oxidized. For the Fe-
ꢃ
ꢃ
˚
˚
promoted sample, peaks at 27.1 (3.29 A) and 30.4 (2.94 A)
after activation do not correspond to obvious oxide phases
– –
of the promoter or of V P O. These peaks might corre-
spond to iron phosphates but with only two peaks a defini-
tive assignment cannot be made. After exposure to methane
oxidation conditions peaks corresponding to a VOPO4
ꢃ
ꢃ
˚
˚
phase are observed at 25.8 (3.95 A), 29.2 (3.06 A), and
ꢃ
ꢃ
˚
40.7 (2.24 A). It is notable that the (200) reflection at 22.8
is much more intense in this catalyst following activation
TABLE 2
XPS Binding Energies and Estimated Average Vanadium Oxidation Statea Observed for Promoted
and Unpromoted Catalysts
Catalyst
V 2p3/2
O 1s
P 2p
1O 1s ꢄ V 2p
Vox-Fit
Vox-Corr
Vox-NMR
(VO)2P2O7
516.5 (1.9)b
517.5 (1.8)
517.4 (2.4)
532.0 (2.2)
531.4 (1.6)
531.3 (1.8)
134.1 (1.9)
134.0 (1.8)
134.0 (1.8)
15.1
13.9
13.9
4.15
4.22
4.38
3.55
4.38
4.38
4.03/4.06
4.12/4.12
4.00/4.10
–
Cr VPO
–
Fe VPO
a Vox-Fit is by the deconvolution approach described in the experimental section. Vox-Corr is based on the correlation with 1O 1s ꢄ V 2p presented
in Ref. (29). Vox-NMR is based on integration of the 31P NMR intensities after activation/and after catalysis of methane oxidation.
b Full width at half maximum shown in parenthesis.

164
McCORMICK ET AL.
towards lower energy values with addition of promoter el-
ements consistent with an increase in the amount of VV on
the surface. Coulston and co-workers (29) present a cor-
relation vanadium average oxidation state and 1O 1s ꢄ V 2p
.
This correlation has also been used to estimate the average
vanadium oxidation state and these values are reported in
Table 2 for comparison with estimates based on deconvo-
lution. The correlated value for the unpromoted catalysts
is significantly lower than estimated by our deconvolution
approach, and the values for the promoted catalysts are
slightly higher. This approach again confirms the difference
in average surface oxidation state for the promoted and un-
promoted materials.
The (VO)2P2O7 phase reported in the literature (27)
yields an O 1s binding energy of 531.1 to 532.8 eV with a nar-
row signal (FWHM 2.4 eV). More oxidized VV phases with
and without phosphorus exhibit a much narrower O 1s sig-
nal (1.7–2.0 eV), shifted slightly downwards in some cases
such as 531.2 (ꢂ-VOPO4) and 530.0 eV (V2O5). The data
in Table 2 indicate O 1s binding energies comparable to
pure (VO)2P2O7 for the unpromoted as well the Fe- and
Cr-promoted catalysts. The signal is significantly narrower
for the promoted materials consistent with a higher average
surface oxidation state for vanadium.
Binding energies for the promoter elements were also
examined by XPS. For the Fe-promoted catalyst, the Fe
2p3/2 binding energy value of 714.2 eV indicates very lit-
tle possibility of presence of FeO or Fe2O3 on the surface.
The Fe 2p3/2 energy is 709.5 eV for FeO, 710.8 eV for Fe2O3,
and 711.5 eV for FeCl3. For Fe promoted vanadyl pyrophos-
phate the binding energy is much greater. Wang and Otsuka
(30) report a similarly high binding energy of 713.2 eV for
FePO4 suggesting the presence of FeIII in a phosphate ma-
trix in the Fe promoted sample. For Cr2O3, the 2p3/2 energy
is reported as 576.8 eV. The observed Cr 2p3/2 energy of
578.3 eV for the Cr-promoted catalyst is very close to the
value of 578.5 reported for ꢂ-CrPO4 (31), indicating CrIII
in a phosphate matrix.
FIG. 2. Wideline 31P NMR spectra of promoted and unpromoted
vanadyl pyrophosphate: (a) freshly activated in butane/air; (b) after 20 h
use in methane oxidation.
Li and co-workers (7) introduced the use of 31P NMR
spin-echo mapping to determine the presence of bulk VIV
and VV species in vanadium phosphate catalysts. These
spectra are characterized by a broad peak centered at about
2500 ppm (relative to H3PO4) and assigned to phosphorus
in the vicinity of VIV, and a narrower peak near 0 ppm as-
signed to phosphorus near VV. Spectra of promoted and
unpromoted samples, both freshly activated and after use
in methane oxidation, are shown in Fig. 2. For the activated,
unpromoted catalyst there is a small peak for VV but 97% of
the spectral intensity is in the VIV peak at 2500 ppm. Promo-
tion with Cr leads to an increase in the fraction of VV, which
amounts to 12% of the signal intensity. Promotion with
Fe produces a sample with essentially no VV after activa-
tion. After use in methane oxidation, the unpromoted and
Fe-promoted samples exhibit increased VV signals, 6% for
unpromoted and 10% of signal intensity for Fe-promoted.
The spectrum of the Cr-promoted sample is essentially un-
changed at 12% VV. Average vanadium oxidation state
estimated by integration of the NMR peak intensities are
reported in Table 2 for comparison with XPS values. The
NMR (bulk) values are slightly lower than the XPS (sur-
face) values determined using the deconvolution approach.
However, the results are consistent with the XPS and XRD
data, which indicate that promoters enhance the concentra-
tion of VV or oxidized VPO phases, even under the highly
reducing conditions employed for methane oxidation.
Diffuse reflectance infrared spectra were obtained for
both promoted and unpromoted catalysts. Band positions
in the 700 to 1700 cmꢄ1 range agreed closely with those

METHANE PARTIAL OXIDATION
165
reported elsewhere (32). Consequently, the observations in
the presence of catalyst appear to be primarily the result
of heterogeneous process with very little or no intrusion
of purely gas phase reactions. A number of experiments
and calculations were performed to ensure that the mea-
sured reaction rates were not confounded by mass transfer
effects (33). Experiments varying catalyst particle size and
varying gas velocity at constant space velocity indicate no
internal or external limitations. Calculations indicate that
at 698 K, the highest temperature employed in this study,
the isothermal effectiveness factor is greater than 0.95.
Unpromoted VPO. The principal products of methane
oxidation over VPO were found to be carbon monoxide
and carbon dioxide. Formaldehyde was found only in trace
quantities. Figure 4 presents selectivities to these products
as a function of conversion, varied by varying tempera-
ture and GHSV for two different methane to oxygen ra-
tios. As shown, carbon monoxide is the primary product
and CO selectivity decreases as the conversion increases.
At zero conversion, CO selectivity approaches 100% sug-
gesting that methane is oxidized directly to carbon monox-
ide and that any methoxy or formate surface intermediate
is very rapidly converted under these conditions. Carbon
dioxide was never a significant product at very low methane
conversion levels, which is an indication of no direct oxida-
tion route from methane to carbon dioxide. CO selectivity
is higher at the higher methane to oxygen ratio, as expected
because less oxygen is available.
–
–
FIG. 3. Infrared spectra (diffuse reflectance) showingthe PꢄO1 P sym-
metric stretch at 742 cmꢄ1 and the V (O V) stretch at 795 cm
.
–
==
reported by others for vanadyl pyrophosphate (9, 25) with
==
an absorbance maximum at 975 cmꢄ1 assigned to V
O
To determine the reaction orders, methane partial pres-
sure was changed from 21 to 65 kPa, keeping the oxygen
partial pressure constant at 8 kPa. During these runs, gas
hourly space velocity (GHSV) was also kept constant at
2700 hꢄ1. In a similar set of experiments at the same GHSV
stretching. This intense band was broad and not well re-
solved in these samples. The only significant differences
observed in the spectra are for vibrations of the linkages
between the layers of the (VO)2P2O7 structure and this re-
gion of the spectrum is shown in Fig. 3. For the Cr- and
ꢄ1
==
–
Fe-promoted catalysts the (V O) V band at 795 cm hꢄas1
– –
a greater intensity relative to the P O P band at 742 cm
than is observed in the unpromoted catalyst. Examination
of the entire infrared spectrum suggests that the band at
742 cmꢄ1, corresponding to the P O P stretch, has de-
– –
creased in intensity. Thus the promoter atoms are somehow
affecting the layer linkages, perhaps by becoming interca-
lated between the layers. This intercalation, if it occurs, does
not appear to have produced disorder in the layer stacking
as shown by XRD. Ordering and crystallinity may have ac-
tually increased for the Fe-promoted sample.
Catalyst Testing
Prior to any measurement, the contribution of gas phase
reactions was evaluated by performing a series of exper-
iments with the empty reactor packed with quartz beads.
In the absence of a catalyst and at the temperatures and
flow rates employed in the present work, conversions of
FIG. 4. Product selectivity (% ) as a function of methane conver-
sion (% ) for unpromoted vanadyl pyrophosphate catalyst. GHSV = 2700–
methane, methanol, and formaldehyde were negligible as 13,500 hꢄ1, PCH = 43 kPa, PO2 = 5 kPa, T = 573–698 K.
4

166
McCORMICK ET AL.
FIG. 5. Effect of (a) methane partial pressure and (b) oxygen partial pressure on methane oxidation rate over unpromoted VPO.
oxygen partial pressure waschanged from 8to 32kPa, keep- positive order in oxygen of about 0.08 ꢁ 0.02, provided that
ing methane pressure constant at 42 kPa. The effect of reac- oxygen conversion was kept below 100% . Both reaction
tant partial pressures on the observed rate is shown in Fig. 5. orders were not affected by temperature over the range
The data indicate that the reaction order for methane was studied.
0.73 ꢁ 0.07 (indicated confidence intervals are standard er-
Temperature dependence of the rate was determined
ror). The rate of reaction of methane showed a very small by varying the temperature between 583 and 713 K. An

METHANE PARTIAL OXIDATION
167
Large (up to 5 cm3) GC sample loops were employed to
amplify the formaldehyde signal in these runs. Formalde-
hyde peaks were five to six times greater than the noise
level. Individual analyses as well as entire experiments were
repeated with essentially the same results to within 10% .
In both promoted catalysts, at zero conversion formalde-
hyde selectivity approaches 100% , suggesting that it is
the primary reaction product. Formaldehyde selectivity de-
creases rapidly as the extent of reaction increases. The in-
crease in selectivity to carbon monoxide indicates that this
compound is produced upon further oxidation of formalde-
hyde. Carbon dioxide became the principal product at
higher conversions. No CO2 formation at low conversions
indicates no direct oxidation route from methane to carbon
dioxide. Assuming that the rate of methane oxidation has
FIG. 6. Arrhenius plot for methane oxidation reaction over promoted
and unpromoted vanadyl pyrophosphate.
Arrhenius plot is shown in Fig. 6. The linear nature of this
plot, even at the highest conversions (in all cases well below
10% ) and temperatures confirms the absence of significant
mass transfer limitations in this system. An activation en-
ergy of 102 ꢁ 6 kJ/mole is calculated for methane conver-
sion. Arrhenius parameters are summarized in Table 3.
Transition metal-promoted catalysts. Methane oxida-
tion over the Cr- and Fe-promoted VPO catalysts was car-
ried out at a fixed GHSV of 9300 hꢄ1 and at a methane-
to-oxygen ratio of 8.3. Selectivity as a function of methane
conversion for these catalysts is shown in Fig. 7. Formalde-
hyde was observed as a significant product in the reactor
effluent for both Fe- and Cr-promoted VPO catalysts. Ad-
mittedly, conversions are very low in these experiments and
not significant in an applied sense. However, the point to be
made is that in repeated experiments, no formaldehyde was
observed over the unpromoted catalysts under nearly iden-
tical reaction conditions (lowest conversions in Fig. 4), but
formaldehyde was observed over the promoted samples.
TABLE 3
Arrhenius Parameters for Methane Oxidation over Promoted
and Unpromoted VPO Catalysts
Activation energy,
kJ/mole
Preexponential factor,
mole/g-min-atm0.81
Catalyst
VPO
102 ꢁ 6
95 ꢁ 8
96 ꢁ 6
860
1273
505
–
Cr VPO
FIG. 7. Product selectivity (% ) as a function of methane conversion
(% ) for promoted vanadyl pyrophosphate catalysts: (a) Cr; (b) Fe.
–
Fe VPO

168
McCORMICK ET AL.
the same dependency on methane and oxygen partial pres-
sures as the unpromoted catalyst, the Arrhenius relation-
ships in Fig. 6 were drawn and the Arrhenius parameters
are listed in Table 3. Activation energies are essentially the
same for the unpromoted, Fe- and Cr-promoted catalysts.
TABLE 4
Comparison of Methane Oxidation Activation Energies Reported
for Various Catalysts
Activation energy,
Reference
Catalyst
FePO4
kJ/mole
DISCUSSION
30
34
35
36
37
38
39
40
40
41
42
42
205
226
113a
188
167a
176
251
188
176a
167a
154
217
–
V2O5 SiO2
The catalyst testing data indicate a sequential reaction
path for methane oxidation over VPO and the promoted
catalysts, as has also been reported for methane oxidation
over V2O5/SiO2 (34) and other catalysts:
–
H3PMo12O40 xH2O
–
MoO3 SiO2
MoO3
–
(MoO3)3 Fe2O3
–
–
–
–
V2O5 SiO2
V2O5 Al2O3
MoO3 SiO2
V2O5 SiO2
k1
CH4 + 1O2 → HCHO + H2O
k2
HCHO + 1/2O2 → CO + H2O
Silica
k3
Vycor
CO + 1/2O2 → CO2.
a Nitrous oxide as the oxidizing agent.
Unpromoted vanadyl pyrophosphate is a poorly selective
partial oxidation catalyst for the methane oxidation reac-
tion. Carbon oxides and water are the principal products,
indicating that the ratio k2/k1 is large. A fractional order of
0.73 in methane partial pressure was observed. In the lit-
erature, with few exceptions, methane partial oxidation is
assumed to be first order in methane and zero order in oxy-
gen. Notably, Wang and Otsuka (30) recently reported data
that we have analyzed to yield a 0.68 order for methane oxi-
dation over FePO4 in the presence of hydrogen. A reaction
order below unity indicates the existence of an inhibition
(adsorption) term in a Langmuir–Hinshelwood-type rate
Characterization of promoted catalysts indicates that
promoter elements are present on the surface and in the
bulk at roughly 10 : 1 vanadium to promoter ratio. XRD
indicates that vanadyl pyrophosphate is the main phase
present, but promotion has resulted in several new peaks
in the powder pattern, several of which can be assigned
to ꢀII-VOPO4. Examination of the (200) reflection at about
22.8ꢃ 2-theta suggests that promotion has not generated ad-
ditional disorder (additional peak broadening) in the layer
stacking and may have actually enhanced crystallinity in the
Fe-promoted sample. Promotion appearsto increase the av-
erage oxidation state (surface and bulk) of vanadium and,
based on the IR spectral data, disrupts linkages between
the layers of the structure.
model. We interpret this to mean that as PCH increases, the
4
surface becomes more reduced and sites capable of catalyz-
ing oxidation become fewer. The reaction order for oxygen
was found to be 0.08, very close to zero, as expected for
a redox or Mars and van Krevelen process with fast cata-
lyst reoxidation. Promotion of VPO with Fe or Cr leads to
the formation of measurable quantities of formaldehyde,
although space-time yields are low, typically between 0.5
and 2 g/kg-h. Low reaction temperatures and operation at
very low methane conversion are required for formation of
HCHO.
Activation energy values for methane oxidation over sev-
eral different oxide catalysts have been listed in Table 4.
Typical oxide catalysts exhibit activation energies for
methane oxidation that are 1.5 to 2 times higher than ob-
served for VPO, although much of this data is based on N2O
as the oxidant. An exception is 12-molybdophosphoric acid
studied by Ahmed and Moffat (35) which exhibits an acti-
vation energy similar to that reported here. Busca and co-
workers (17) have proposed that strong surface acid sites
are involved in the initial alkane activation step. We spec-
ulate that a surface acid site is involved in methane activa-
tion, and because of the strongly acidic nature of vanadyl
pyrophosphate and 12-molybdophosphoric acid, the acti-
vation energy for methane oxidation has a lower value.
We hypothesize that promoter atoms are incorporated
between the layers and at the edges of the layers of these
crystals rather than substituting for vanadium in the phos-
phate lattice. This is consistent with the method of promoter
addition where promoters were added to the fully formed
precursor. Perhaps the presence of these cations increases
the degree of coordinative unsaturation of the surface vana-
dium centers favoring formation of oxidized vanadium as
isolated VV, or as VV containing phase domains. Alterna-
tively, promoters might activate oxygen more readily than
VPO and catalyze oxidation of VIV to form VV. If promoter
atoms are indeed at the edges of, or in between, the vanadyl
pyrophosphate layers the promoter may enhance the ability
of the structure to incorporate additional oxygen between
the layers resulting in formation of VV. A model where ex-
cess oxygen is incorporated between the layers has been
discussed by Lopez Granados and co-workers (9).
The presence of VV is, of course, necessary for operation
of a VIV/VV redox couple. Under butane oxidation condi-
tions (1.5% butane in air) excess oxygen is present and the
formation of VV sites or phase domains is facile (11, 12).

METHANE PARTIAL OXIDATION
169
Furthermore, using in situ Raman studies of VPO activa-
tion in butane/air, Hutchings and co-workers (43) observed
the formation of VV phases simultaneously with the start
of maleic anhydride formation. Over the course of 20 h the
concentration of these phases increased, as did the selec-
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form by-products. Production of maleic anhydride ended
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selective products under butane oxidation conditions.
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90% methane and 10% oxygen) there is no excess oxy-
gen and formation of VV may be unfavorable in unpro-
moted catalysts. The observation that the reaction order
with respect to methane is less than unity may imply a
loss of VV sites capable of catalyzing oxidation as methane
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ACKNOWLEDGMENTS
This work was sponsored by the U.S. Department of Energy, Fossil En-
ergy Branch, under Contract DE-AC22-96PC92110. Acquisition of the
solid state NMR instrument was sponsored by the National Science Foun-
dation under Award CTS-9512228.
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