Kinetics and mechanism of oxidative dehydrogenation of propane on vanadium, molybdenum, and tungsten oxides

Article abstract of DOI:10.1021/jp9933875

The effect of cation identity on oxidative dehydrogenation (ODH) pathways was examined using two-dimensional VO_x, MoO_x, and WO_x structures supported on ZrO₂. The oxidative dehydrogenation of propane occurred via similar elementary steps on VO_x and MoO_x, which involve rate-determining C-H bond activation steps using lattice oxygen atoms. The activation energies for propane dehydrogenation and for propylene combustion increased in the sequence VO_x/ZrO₂ < MoO_x/ZrO₂ < WO_x/ZrO₂. In this sequence, the corresponding reactions rates decreased, suggesting that turnover rates reflect C-H bond cleavage activation energies, which are in turn affected by the reducibility of these metal oxides. Propane ODH activation energies were higher than for propylene combustion. This led to an increase in maximum alkene yields and in the ratio of rate constants for propane ODH and propylene combustion as temperature increases. This difference in activation energy (48-61 kJ/mole) between propane ODH propylene combustion was larger than between bond dissociation enthalpies for the weakest C-H bond in propane and propylene (40 kJ/mole) and it increased in the sequence VO_x/ZrO₂ < MoO_x/ZrO₂ < WO_x/ZrO₂.

Full text of DOI:10.1021/jp9933875

1292
J. Phys. Chem. B 2000, 104, 1292-1299
Kinetics and Mechanism of Oxidative Dehydrogenation of Propane on Vanadium,
Molybdenum, and Tungsten Oxides
Kaidong Chen, Alexis T. Bell,* and Enrique Iglesia*
Chemical and Materials Sciences DiVisions, Lawrence Berkeley National Laboratory, and Department of
Chemical Engineering, UniVersity of California, Berkeley, California 94720-1462
ReceiVed: September 21, 1999; In Final Form: NoVember 24, 1999
The effect of cation identity on oxidative dehydrogenation (ODH) pathways was examined using
two-dimensional VOx, MoOx, and WOx structures supported on ZrO2. The similar kinetic rate expressions
obtained on MoOx and VOx catalysts confirmed that oxidative dehydrogenation of propane occurs via similar
pathways, which involve rate-determining C-H bond activation steps using lattice oxygen atoms. The activation
energies for propane dehydrogenation and for propene combustion increase in the sequence VOx/ZrO2 <
MoOx/ZrO2 < WOx/ZrO2; the corresponding reaction rates decrease in this sequence, suggesting that turnover
rates reflect C-H bond cleavage activation energies, which are in turn influenced by the reducibility of these
metal oxides. Propane ODH activation energies are higher than for propene combustion. This leads to an
increase in maximum alkene yields and in the ratio of rate constants for propane ODH and propene combustion
as temperature increases. This difference in activation energy (48-61 kJ/mol) between propane ODH and
propene combustion is larger than between bond dissociation enthalpies for the weakest C-H bond in propane
and propene (40 kJ/mol) and it increases in the sequence VOx/ZrO2 < MoOx/ZrO2 < WOx/ZrO2. These results
suggest that relative propane ODH and propene combustion rates depend not only on C-H bond energy
differences but also on the adsorption enthalpies for propene and propane, which reflect the Lewis acidity of
cations involved in π bonding of alkenes on oxide surfaces. The observed difference in activation energies
between propane ODH and propene combustion increases as the Lewis acidity of the cations increases (V5+
< Mo6+ < W6+).
Introduction
SCHEME 1: Reaction Network in Oxidative
Dehydrogenation of Alkane Reactions
In spite of its significant economic potential as an alternate
route to alkenes1-5 and in spite of extensive scientific studies,1-22
the oxidative dehydrogenation (ODH) of alkanes to alkenes is
not currently practiced because the secondary combustion of
primary alkene products limits alkene yields to about 30% for
propane and higher alkanes.3 Alkene selectivities decrease
markedly as conversion increases.2,3 One important reason for
these yield limitations is the typically higher energies of the
C-H bonds in alkane reactants compared with those in the
desired alkene products,23 which lead to rapid alkene combustion
at the temperatures required for C-H bond activation in alkanes.
A recent literature survey of product yields in oxidation
reactions23 suggested that low yields are obtained when the
energy of the weakest bond in the products is 30-40 kJ/mol
lower than that of the weakest bond in the reactants.
The oxidation of light alkanes to alkenes occurs via parallel
and sequential oxidation steps (Scheme 1).1 Alkenes are primary
ODH products while CO and CO2 (COx) can form via either
secondary combustion of alkenes or direct combustion of
alkanes. Selective poisoning of sites responsible for direct
combustion of alkanes using SiOx has been reported.24 The
activation of C-H bonds in alkane ODH reactions, however,
appears to require the same sites as the C-H bond activation
steps involved in the combustion of alkenes,25,26 making
selective poisoning strategies ineffective in improving alkene
yields. On VOx-based catalysts, the evolution of oxide structures
from monovanadate to polyvanadate species as VOx surface
density increases leads to a similar increase in ODH and propene
combustion rates, apparently because similar sites are required
for the two reactions.26 Propane combustion rates, however, are
less affected by VOx surface density, which leads to a decrease
in k2/k1 ratios as surface density increases.25,26 Initial selectivities
greater than 90% can be achieved for many ODH reactions.2,3
The k3/k1 (propene secondary combustion/propane primary
dehydrogenation) ratio, which causes the yield losses observed
as conversion increases, is large (5-40) on VOx-based cata-
lysts.25,26 The greater reactivity of propene arises in part because
the weakest C-H bond in propane (at the methylene group) is
significantly stronger than the allylic C-H bond in propene.23
If the relative C-H bond dissociation enthalpies were the only
factor responsible for the k3/k1 ratio, this ratio would not depend
on the chemical identity or the local structure of the active oxide.
The differences observed among VOx, MoOx, WOx and other
oxides suggest that other factors, such as differences in
* Authors to whom correspondence should be addressed. E-mail:
iglesia@cchem.berkeley.edu; bell@cchem.berkeley.edu.
10.1021/jp9933875 CCC: $19.00 © 2000 American Chemical Society
Published on Web 01/22/2000

Oxidative Dehydrogenation of Propane
J. Phys. Chem. B, Vol. 104, No. 6, 2000 1293
TABLE 1: Structure of MOx/ZrO2 (M ) V, Mo, W) Catalysts
catalyst
10 wt % VOx/ZrO2
773
170
3.9
11 wt % MoOx/ZrO2
773
133
3.5
20 wt %WOx/ZrO2
873
124
4.2
treatment temp (K)
BET surface area (m2/g)
MOx surface densitya (M atom/nm2)
predominant MOx structurea,b
polyvanadate
polymolybdate
polytungstate
b
a M ) V, Mo, W. Inferred from Raman and UV-visible spectroscopy data.
adsorption enthalpy or entropy between alkanes and alkenes,
and therefore the acid-base properties of active oxides and of
supports, may also influence k3/k1 ratios and alkene yields.
VOx-based materials are among the most active and selective
oxidative dehydrogenation catalysts.1-5 Polyvanadate structures
on ZrO2 lead to reducible and accessible VOx domains and to
more active catalysts than VOx dispersed on other supports.26
ZrO2-supported MoOx species have also been examined as
alkane ODH catalysts;18,27,28 ODH rates tend to be lower on
MoOx than on VOx domains, but MoOx-based catalysts retain
their dispersion and structure at significantly higher temperatures
than VOx-based catalysts. Catalysts based on WOx domains are
seldom used because of their low ODH activity. This study
addresses the chemical and structural basis for the observed
differences in ODH rates and in rate constants for individual
reaction steps on VOx, MoOx, and WOx catalysts. Detailed ODH
kinetics, including activation energies and entropies for primary
dehydrogenation and secondary combustion, are reported. They
are used to identify relevant factors influencing k3/k1 and k2/k1
ratios for reactions of propane and propene on these catalysts.
Figure 1. Effect of C3H8 pressure on C3H6 formation rate on 11 wt %
MoOx/ZrO2 (703 K, 1.3 kPa O2, balance He).
Experimental Methods
V2O5/ZrO2 (10 wt % V2O5), MoO3/ZrO2 (11 wt % MoO3),
and WO3/ZrO2 (20 wt % WO3) catalysts were prepared by
incipient wetness impregnation of ZrOx(OH)4-2x with solutions
of NH4VO3,25 (NH4)2Mo2O7,27 and (NH4)6H2W12O40,29,30 re-
spectively. Surface areas were measured by N2 physisorption
at its boiling point using a Quantasorb surface area analyzer
(Quantachrome Corp.) and BET analysis methods.
used in this study are shown in Table 1. All three samples were
also characterized by X-ray diffraction and by X-ray absorption,
Raman, and UV-visible spectroscopies.25-27,29,30 These meth-
ods were used to establish that the dispersed oxide species
present in these samples were two-dimensional oligomers. Such
oligomeric structures minimize the number of M-O-Zr link-
ages accessible to gas phase reactants and M-O-Zr linkages
appear to be active for undesired propane combustion reac-
tions.25,27
Catalytic measurements were carried out in a packed-bed
quartz microreactor with plug-flow hydrodynamics25 using
0.03-0.3 g catalyst samples. Quartz powder was used to dilute
the bed and to minimize temperature gradients. Propane (Airgas,
99.9%) and O2 (Airgas, 99.999%) were used as reactants and
He (Airgas, 99.999%) was used as an inert diluent. Water was
introduced into C3H8-O2 reactant streams by flowing a 20%
H2/Ar mixture through a CuO bed (0.5 m long, 150 g of CuO)
held at 623 K. Water was formed by CuO reduction to Cu metal.
All transfer lines beyond the point of water introduction were
kept above 393 K in order to prevent condensation. Reactants
and products were analyzed with a Hewlett-Packard 6890 gas
chromatograph using analysis procedures described else-
where.25,26 C3H8 and O2 conversions were varied by changing
reactant flow rates (50-200 cm3/min); they were kept below
2% and 20%, respectively. Reaction rates and selectivities were
extrapolated to zero residence time in order to determine initial
reaction rates and selectivities. First-order rate constants for
alkane ODH (k1), primary alkane combustion (k2), and secondary
combustion of alkenes (k3) were obtained from bed residence
time effects on product distributions using previously described
procedures.25
The kinetics and reaction pathways for propane ODH on VOx-
based catalysts have been described in detail previously.31 The
ODH rate is proportional to C3H8 partial pressures and
independent of O2 partial pressures when H2O partial pressures
are low; the rates acquire a more complex dependence on C3H8
and O2 pressures when H2O is present at higher concentrations.
Similar kinetic dependencies were observed on MoOx/ZrO2.
Figure 1 shows initial propene formation rates for MoOx/ZrO2
as a function of propane partial pressure at 1.3 kPa of O2 without
added H2O. The propene formation rate increases linearly with
increasing propane partial pressure, in agreement with previous
reports.31-34 Propane ODH rates do not depend on O2 partial
pressures when H2O is not added (Figure 2), in agreement with
previous reports.31-33 The effect of H2O partial pressure on
propane ODH rates is shown in Figure 3 at 8.1 kPa of C3H8
and 1.3 kPa of O2. Water inhibits propane ODH on MoOx-
ZrO2 catalysts in a manner similar to that reported on VOx-
31
34
ZrO2 and VOx-Al2O3 catalysts. This resemblance in the
ODH kinetics on MoOx-ZrO2 and VOx-ZrO2 and comple-
mentary isotopic tracer studies of the reaction steps suggest that
the elementary steps involved in alkane ODH reactions are also
similar on MoOx and VOx catalysts. Detailed kinetics and
isotopic tracer studies31 have shown that propane ODH occurs
on VOx domains via a Mars-van Krevelen redox mechanism
Results and Discussion
Mechanism of Propane Oxidative Dehydrogenation. The
BET surface area and the MOx surface density for the catalysts

1294 J. Phys. Chem. B, Vol. 104, No. 6, 2000
Chen et al.
Figure 2. Effect of O2 pressure on C3H6 formation rate on 11 wt %
MoOx/ZrO2 (703 K, 5.1 kPa C3H8, balance He).
Figure 4. Comparison of predicted (from eq 1) and experimental C3H6
formation rates of ODH of propane on 10 wt %VOx/ZrO2 (O) and 11
wt % MoOx/ZrO2 (4) catalysts.
4. Recombination of OH groups to form water and a reduced
M center (/)
OH* + OH* a H2O + O* + /
(IV)
5. Reoxidation of reduced M-centers via dissociative chemi-
sorption of O2
O2 + / + / f O* + O*
(V)
In this scheme, O* is a lattice oxygen atom in the MOx
overlayer (e.g., M ) O, M-O-M, or M-O-Zr), OH* is a
hydroxyl group in M-O-H, C3H7O* represents an adsorbed
propoxide bonded to M (M-O-C3H7), and / represents a
surface vacancy associated with either one Mo(n-2)+ cation (n
is the highest valence of M) or two Mo(n-1)+ cations in the MOx
lattice. These elementary steps, together with pseudo-steady-
state assumptions for all adsorbed intermediates and quasi-
equilibrium assumptions for steps I and IV, lead to a rate
expression of the form:
Figure 3. Effect of H2O pressure on C3H6 formation rate on 11 wt %
MoOx/ZrO2 (703 K, 8.1 kPa C3H8, 1.3 kPa O2, balance He).
r ) kIIKI[C3H8]/{1 + (KIV[H2O])0.5(kIIKI[C3H8]/
2kV[O2])0.25}2 (1)
using lattice oxygen atoms to abstract hydrogen atoms from
C3H8 in an irreversible C-H bond activation step. Propane ODH
is envisioned to occur via the following sequence:
1. Weak associative adsorption of propane on lattice oxygen
(O*)
where ki is the rate coefficient and Ki is the equilibrium constant
for step i. Figure 4 compares experimental and predicted rates
(by eq 1) at various C3H8, O2, and H2O concentrations for the
MoOx/ZrO2 and VOx/ZrO2 catalysts used in this study. It is
evident that eq 1 describes accurately propane ODH on both
catalysts at all conditions used in the study, confirming the
mechanistic resemblance of ODH reactions on MoOx-based and
VOx-based catalysts. The limited available kinetic data do not
permit a conclusive assessment of the ODH mechanism on WOx/
ZrO2, but the chemical resemblance between Mo and W oxides
leads us to expect that the reaction also occurs by the steps
leading to eq 1.
C3H8 + O* a C3H8O*
(I)
2. C-H cleavage via H-abstraction from propane using a
neighboring lattice oxygen
C3H8O* + O* f C3H7O* + OH*
(II)
3. Desorption of propene by hydride elimination from
adsorbed alkoxide species
Effects of MOx Composition on the Rate Constants for
Primary and Secondary Reactions. The effects of metal oxide
C3H7O* f C3H6 + OH*
(III)

Oxidative Dehydrogenation of Propane
J. Phys. Chem. B, Vol. 104, No. 6, 2000 1295
Figure 7. Temperature dependence of the reaction rate constant ratio
for propane primary combustion and dehydrogenation (k2/k1).
Figure 5. Temperature dependence of the propane primary dehydro-
genation reaction rate constant (k1).
of the transition-state species in step II35
r
r
kII ) (kBT/h) exp[-(∆HII - T∆SII )/RT]
(4)
where kB is the Boltzmann constant and h is Planck’s constant.
∆HIads and ∆SIads are the enthalpy and entropy of propane
r
r
adsorption (step I), and ∆HII and ∆SII are the enthalpy and
the entropy for the formation of the transition state in step II.
Combining eqs 2 and 4 leads to
r
k1 ) (kBT/h) exp[(∆SIads + ∆SII )/R] exp[-(∆HIads
+
r
∆HII )/RT] (5)
If eq 5 is rewritten as
k1 ) A1 exp(-∆E1/RT)
(6)
then ∆E1, the apparent activation energy, is given by
r
∆E1 ) ∆HIads + ∆HII
(7)
(8)
Figure 6. Temperature dependence of the reaction rate constant ratio
for propene secondary combustion and propane primary dehydrogen-
ation (k3/k1).
and A1, the preexponential factor, is given by
r
A1 ) (kBT/h) exp[(∆SIads + ∆SII )/R]
composition on the apparent rate constants for reactions 1-3
in Scheme 1 were measured at low C3H8 conversions in order
to minimize the kinetic complexity introduced by higher H2O
partial pressures. Figure 5 shows an Arrhenius plot for k1 and
Figures 6 and 7 show similar plots for k3/k1 and k2/k1,
respectively. These data can be used to examine how the identity
of the metal cations in MOx influences rates and selectivities
for propane ODH and combustion and for secondary combustion
of propene.
Thus, the apparent activation energy for propane ODH corre-
sponds to the sum of the adsorption enthalpy for step I and the
enthalpy of formation of the transition state involved in step II.
The preexponential factor reflects the sum of adsorption entropy
for step I and the entropy of formation of the transition-state
species in step II, because
∆SIads ) S(C3H8O*) - S(C3H8) - S(O*)
∆SIIr ) Sts,II - S(C3H8O*) - S(O*)
(9)
At low H2O concentrations, the rate of propane ODH given
by eq 1 becomes
(10)
r ) kIIKI[C3H8] ) k1[C3H8]
(2)
and therefore
∆S1 ) ∆SIads + ∆SIIr ) Sts,II - S(C3H8) - S(O*) - S(O*)
The apparent rate coefficient k1 is, therefore, equal to kIIKI. The
equilibrium constant (KI) can be related to the thermodynamics
of propane adsorption using
(11)
∆S1 reflects entropy differences between the reactant (C3H8) in
step I and the transition state for step II. Apparent activation
energies (∆E1) and entropies (∆S1) for the primary ODH
reaction were obtained from the data in Figure 5 and they are
KI ) exp[-(∆HIads - T∆SIads)/RT]
(3)
and the rate constant (kII) to the thermodynamics of formation

1296 J. Phys. Chem. B, Vol. 104, No. 6, 2000
Chen et al.
TABLE 2: Kinetic Constants for MOx/ZrO2 (M ) V, Mo,
W) Catalysts in ODH of Propane
catalyst
10 wt %
VOx/ZrO2
11 wt %
MoOx/ZrO2
20 wt %
WOx/ZrO2
∆E1 (kJ/mol)
∆E3 (kJ/mol)
99
51
117
60
57
-121
-179
126
65
61
-120
-178
∆E1 - ∆E3 (kJ/mol) 48
∆S1 (J/(mol K))
∆S3 (J/(mol K))
-123
-177
reported in Table 2. The values of ∆S1 obtained are very similar
on all three catalysts. Since S(C3H8) is a molecular property
that does not depend on the nature of the catalyst, the similar
values of ∆S1 obtained on the three catalysts suggest that values
of Sts,II, and thus the structure of the activated complex, are also
similar on MoOx, VOx, and WOx domains. This confirms the
conclusion reached from measurements of the reaction kinetics
and from isotopic tracer studies that propane ODH proceeds
via the same mechanism on VOx/ZrO2 and MoOx/ZrO2,28,31 and
possibly also on WOx/ZrO2. The measured activation energies
(99-126 kJ/mol) are similar to values previously reported on
V2O5-Nb2O5 (72-110 kJ/mol),36 and they increase in the
sequence
Figure 8. Speculated transition state structures of activation of propane
(a and a′) and propene (b and b′).
difference in activation energy between the two steps (∆E1 -
∆E3) is 48-61 kJ/mol and it increases in the sequence
VOx/ZrO2 < MoOx/ZrO2 < WOx/ZrO2
Propane ODH rates show the opposite trend and decrease in
this order; this suggests that the higher ODH rates reported on
VOx-based catalysts reflect the lower activation energy for C-H
bond activation on VOx-based catalysts compared to those on
catalysts based on MoOx and WOx.
The k3/k1 ratio decreases with increasing temperature on all
three catalysts (Figure 6). Assuming Arrhenius forms for k1 and
k3, their ratio can be written as
VOx/ZrO2 (48 kJ/mol) < MoOx/ZrO2 (57 kJ/mol) <
WOx/ZrO2 (61 kJ/mol)
On selective ODH catalysts, alkene yield limitations pre-
dominantly reflect the value of the k3/k1 ratio, because k2/k1 ratios
tend to be very small (0.1-0.2).25-27 The value of ∆E1 - ∆E3
strongly influences k3/k1 ratios (eq 12). In what follows, we
examine the specific elementary steps that influence the
measured values of k1 and k3, as well as catalyst properties that
can influence these relevant rate constants. The apparent
activation energy for propane ODH reactions corresponds to
the sum of the adsorption enthalpy in step I and the enthalpy
of formation of the transition state species in step II (eq 7). A
similar treatment can be used for propene activation by
proposing that molecular adsorption of propene precedes
activation of allylic C-H bonds; adsorption and C-H bond
activation steps are followed by fast (and kinetically insignifi-
cant) steps required in order to complete a combustion turnover.
k3/k1 ) (A3/A1) exp[(∆E1 - ∆E3)/RT]
(12)
∆E3 and ∆S3 values (for secondary propene combustion) were
calculated from Figures 5 and 6 and they are shown in Table 2.
∆E3 increases in the same order as ∆E1 (VOx/ZrO2 < MoOx/
ZrO2 < WOx/ZrO2). Activation entropies for propene combus-
tion (∆S3) over VOx/ZrO2, MoOx/ZrO2, and WOx/ZrO2 are
similar (Table 2), suggesting that combustion pathways are also
similar on these catalysts.
The similar site requirements for reactions 1 and 3 (suggested
by the effects of VOx structure on selectivity)25,26 and the
markedly different values of ∆S3 and ∆S1 measured (Table 2)
indicate that propane ODH (reaction 1) and propene combustion
(reaction 3) pathways involve transition states with different
structures. The weakest C-H bond in alkanes and alkenes is
usually cleaved in the initial step of oxidation reactions;38-40
thus, C-H bonds at the secondary carbon and allylic C-H
bonds in propene are expected to be involved in rate-determining
C-H activation steps. This was also confirmed by the observed
kinetic isotopic effects when comparing reactions of CH3CH2-
CH3/O2 and CH3CD2CH3/O2.41 As a result, transition states for
activation of C-H bonds in propane and propene would differ
markedly, as suggested by the possible transition state structures
shown in Figure 8.
The apparent activation energy for propene combustion (∆E3)
is lower than for propene dehydrogenation (∆E1) on all three
catalysts (Table 2). This leads to a decrease in the k3/k1 ratio
(and an increase in propene yields) as temperature increases.
The C-H bond activated in propane is stronger than the allylic
C-H bond in propene; consequently, propane reaction rates are
more sensitive to temperature than propene reaction rates. The
C3H6 + O* a C3H6O*
C3H6O* + O* f C3H5O* + OH*
......
(I′)
(II′)
This treatment leads to an apparent activation energy for propene
combustion given by
r
∆E3 ) ∆HI′ads + ∆HII′
(13)
ads
r
where ∆HI′ is the propene adsorption enthalpy and ∆HII′ is
the enthalpy of formation of the transition state for allylic C-H
bond cleavage in step II′. ∆E1 - ∆E3 is the critical term
influencing k3/k1 ratios and it is given by
∆E1 - ∆E3 ) (∆HIads - ∆HI′ads) + (∆HIIr - ∆HII′ )
r
) ∆(∆Hads) + ∆(∆Hr)
∆(∆Hads) represents the difference between the adsorption
(14)

Oxidative Dehydrogenation of Propane
J. Phys. Chem. B, Vol. 104, No. 6, 2000 1297
WOx/ZrO2 < MoOx/ZrO2 < VOx/ZrO2
enthalpies for associative adsorption of propane and of propene
on a given catalyst. ∆(∆Hr) reflects the difference in the
formation enthalpies for the transition states required for C-H
bond activation in propane and propene on a given catalyst.
Since the C-H bond activation steps for propane and propene
appear to require similar sites, ∆(∆Hr) values predominantly
reflect the difference in C-H bond dissociation enthalpies
between propane and propene, while acid-base properties
would influence predominantly the adsorption enthalpies of
propane and propene (i.e. ∆(∆Hads)).
The electron-rich π bond causes propene to interact more
strongly with electron-deficient cations (or with less basic
oxygens) than propane. Therefore, ∆(∆Hads) is always positive
and it is expected to increase in the sequence
VOx/ZrO2 < MoOx/ZrO2 < WOx/ZrO2
The cleavage of allylic C-H bonds in propene occurs away
from the π bond involved in adsorption; therefore, changes of
∆(∆Hads) and in Lewis acid strength influence ∆(∆Hr) values
only weakly. For similar values of ∆(∆Hr), eq 14 predicts that
∆E1 - ∆E3 would increase as ∆(∆Hads) and Lewis acid strength
increase. The measured ∆E1 - ∆E3 values (Table 2) indeed
increase in the same sequence as the predicted Lewis acid
strength of V5+, Mo6+, and W6+ cations.
The dissociation energy for methylene C-H bonds in propane
(401 kJ/mol)42 is higher than for allylic C-H bonds in propene
(361 kJ/mol).42 The difference in C-H bond dissociation
enthalpy between these two bonds (40 kJ/mol) is smaller than
the measured ∆E1 - ∆E3 values (48-61 kJ/mol; Table 2).
Activation energies for propane ODH (∆E1 ) 99-126 kJ/mol)
and propene combustion (∆E3 ) 51-65 kJ/mol) are much
smaller than the dissociation enthalpies of the bonds broken,
suggesting that these C-H bonds are cleaved via concerted
interactions with lattice oxygen atoms. These concerted path-
ways avoid the requirement for the full dissociation energy
through the concurrent exothermic formation of C-O and H-O
bonds. Therefore, ∆(∆Hr) values are not likely to reach the full
40 kJ/mol C-H bond enthalpy difference between propane and
propene. Yet, experimental ∆E1 - ∆E3 values are larger than
40 kJ/mol on all three catalysts (Table 2). We conclude that
∆E1 - ∆E3 values must also reflect differences in adsorption
enthalpy between propane and propene ∆(∆Hads), which depend
on the nature of the catalytic surface and not just on the relative
bond strength in alkanes and alkenes. Adsorption processes are
exothermic and stronger molecule-site interactions translate into
more negative adsorption enthalpies. Propene molecules contain
a π-bond, which makes them more basic than propane and which
leads to stronger interactions with electron-deficient Lewis
centers on oxide surfaces. As a result, propene interacts more
strongly than propane with oxides containing Lewis acid
centers.2 VOx, MoOx, and WOx all contain electron-deficient
cations that act as Lewis acids; therefore, ∆(∆Hads) is always
positive and this term makes (∆E1 - ∆E3) values larger than
the ∆(∆Hr) term.
Measured k2/k1 ratios depend only weakly on temperature on
all three catalysts (Figure 7), suggesting that propane ODH and
propane combustion activation energies are very similar. Both
reactions use propane as the reactant and they appear to be
limited by the initial activation of a methylene C-H bond in
propane. As a result, these two rates will depend on kII and KI
values that are likely to be similar for the two reaction paths;
hence, k2/k1 ratios are almost independent of temperature. The
kinetic resemblance between ODH and combustion steps
suggests that the steps that determine the fate of adsorbed
alkoxide speciessdesorption as alkene or sequential oxidation
and ultimate desorption as COxsdepend on the nature of the
binding site, but they are not kinetically significant because they
occur after irreversible C-H bond activation steps. Previous
studies have shown that the propane ODH and combustion
reactions occur on different active sites, and that the relative
abundance of these two types of binding sites is influenced by
the structure of the MOx species, as shown by the observed
changes in k1/k2 ratio with changes in the surface density of
25,26
27
VOx
and MoOx species. The observed increase in k1/k2
values with increasing VOx and MoOx surface density appears
to be associated with the decreasing density of M-O-Zr
species, which favor strong binding of intermediate alkoxide
species and the direct combustion of propane during one surface
sojourn.25-27
Effects of Catalyst Structure and Chemical Properties on
Catalytic ODH Reactions. In ODH reactions, the redox
properties of oxide catalysts play an important role in determin-
ing reaction rates,37 and the relative C-H bond energies in
alkanes and alkenes strongly influence alkene yields.23 The
acid-base properties of oxide catalysts, however, can also
influence both reaction rates and alkene yields.37
Clearly, small k3/k1 ratios require small differences in
adsorption enthalpies between propane and propene. VOx,
MoOx, and WOx contain electron-deficient cations that act as
Lewis acid centers and WOx/ZrO2 has been widely studied as
an acid catalyst.43,44 Lewis acid strength can be related to the
electronegativity of each element in a compound and to the ratio
of the square of the charge (q) to the ionic radius (r) for a given
cation. In general higher electronegativity and larger q2/r values
are associated with stronger Lewis acids.45 The Pauling elec-
tronegativities increase in the sequence46
ODH reaction rates reflect an apparent activation energy
(∆E1) given by the sum of the enthalpy of propane adsorption
r
(∆HIads) and of the C-H bond activation energy (∆HII ) (eq 7).
V (1.63) < Mo (2.20) < W (2.36)
Generally, molecular adsorption of propane is very weak and
∆HIads values are small. As a result, ∆E1 values reflect
The ionic radii of V5+ is 0.54 Å and Mo6+ is 0.59 Å and W6+
is 0.60 Å at the same coordination number of 6.47 As a result,
q2/r values increase in the same sequence
r
predominantly ∆HII values. C-H bond activation involves a
concerted reaction with lattice oxygen atoms, which leads to
∆HIIr values much lower than C-H bond dissociation energies.
This step forms alkoxide and hydroxyl groups and it leads to a
two-electron reduction leading to either one Mn-2 or two Mn-1
cations (depending on whether bridging or terminal oxygens
V5+ (46.3) < Mo6+ (61.0) = W6+ (60.0)
r
By both criteria, the Lewis acid strength of the cations increases
in the sequence V5+ < Mo6+ < W6+. The strength of the
conjugate base (lattice oxygen atoms) decreases with increasing
cation acid strength and the basicity of the lattice oxygen atoms
therefore increases in the sequence
are involved). Therefore, the value of ∆HII must also depend
on the redox potential of the cations in their highest valence
and on the strength of the M-O-R and M-O-H bonds.
Irreversible C-H bond activation and rapid vacancy reoxidation
lead to O* most abundant surface intermediates during ODH

1298 J. Phys. Chem. B, Vol. 104, No. 6, 2000
Chen et al.
TABLE 3: Melting Point and Tamman Temperature of
Various Metal Oxides
suggesting that VOx-based catalyst would remain more active
than the other oxides at the high conversions required in order
to obtain high alkene yields.
metal oxides
melting point (K)
Tamman temp (K)
Intermediate reducibility, weak Lewis acid centers, and high
oxygen mobility represent the essential requirements for selec-
tive ODH; they are consistent with the trends in ODH rates
and in k2/k1 and k3/k1 ratios observed on VOx-, MoOx-, and WOx-
based catalysts. They are also consistent with the widespread
acceptance of VOx and MoOx as preferred ODH catalysts.1-5
A further decrease in the Lewis acidity of the cations in VOx
and MoOx by modifications with oxides or supports of moderate
or strong basicity (e.g., MgO, Cs2O), which increase the electron
density at cation sites, should lead to less favorable adsorption
of alkenes (smaller ∆(∆Hads), and to lower ∆E1 - ∆E3 and
k3/k1 value. This is consistent with the generally higher alkene
yields obtained on VOx species supported on MgO compared
with similar species supported on more acidic oxides (e.g., Al2O3
and SiO2) supported catalysts.8,9 Strongly basic promoters (e.g.,
K2O, Cs2O), however, also influence the redox properties of
the cations and can lead to an undesired decrease the rate of
V2O5
MoO3
Bi2O3
CuO
WO3
Fe2O3
TiO2
ZnO
963
1068
1098
1599
1745
1838
2128
2248
2257
2538
2983
482
534
549
780
873
919
1064
1124
1129
1269
1492
NiO
Cr2O3
ZrO2
reactions.31 Then, more reducible cations will lead to smaller
values of ∆HIIr and ∆E1, to more facile reduction, and to higher
ODH rates. The incipient cleavage of C-H bonds in alkanes,
however, also depends on the electron density (basicity) of the
lattice oxygen anions that abstract the H atoms.37 As mentioned
earlier, Lewis acidity increases in the sequence V5+ < Mo6+
<
W6+ and the electron density in the oxygen bonded to these
cations increases in the sequence of VOx/ZrO2 > MoOx/ZrO2
> WOx/ZrO2. Indeed, measured ODH turnover rates per MOx
on catalysts with fully exposed two-dimensional oxide domains
increased in the same sequence as the corresponding basicity
of the oxygen anion. The reducibility of metal centers and the
basicity of the active lattice oxygen species seem to account
for the sequence of catalytic activities obtained in this study
(VOx/ZrO2 > MoOx/ZrO2 > WOx/ZrO2; Figure 5).
r
ODH reactions (by increasing ∆HII and ∆E1).28,49,50
Conclusions
The oxidative dehydrogenation of propane occurs via similar
elementary steps on VOx and MoOx. The apparent activation
energies for propane dehydrogenation and for propene combus-
tion increase in the sequence VOx/ZrO2 < MoOx/ZrO2 < WOx/
ZrO2, while the corresponding reaction rates decrease in this
same sequence. Activation energies for propane ODH are higher
than for propene combustion; this leads to a decrease in k3/k1
ratios as reaction temperature increases. This difference in
activation energy (48-61 kJ/mol) is larger than the difference
in bond dissociation enthalpy between the weakest C-H bonds
in propane and propene and it increases in the sequence VOx/
ZrO2 < MoOx/ZrO2 < WOx/ZrO2,. This suggests that the
relative adsorption enthalpies of propene and propane also
influence the relative reaction rates of these two molecules and
that these adsorption effects depend on the Lewis acidity of
the cations involved in π-bonding of alkenes on oxide surfaces.
Propane ODH and propane combustion reactions show similar
activation energies, suggesting that the steps involved in
determining the fate of adsorbed alkoxide species (desorption
vs subsequent oxidation) occur after the kinetically relevant
C-H activation steps in propane. Metal oxides having high
redox properties show high catalytic activity, and less acidic
metal oxides lead to smaller ∆E1 - ∆E3 value and smaller k3/
k1 ratio and hence higher propene yield.
Ultimately, very reducible oxides may become ineffective in
ODH reactions because surfaces become devoid of oxygen as
reduction steps become more facile than reoxidation. In such
systems, O2 chemisorption rates become rate-determining and
the rate actually decreases with increasing reducibility of the
metal cations. This leads to a maximum rates on oxides with
intermediate reducibility and to the volcano-type plots ubiquitous
in heterogeneous catalysis. Similarly, intermediate basicity of
lattice oxygens leads to maximum rates. As basicity increases,
C-H bond activation steps become faster, but the subsequent
recombination of OH groups to form water (step IV) slows down
and the surface becomes predominantly covered with OH
groups. Strong basicity can also increase the stability of alkoxide
species. This, in turn, would decrease the rate of desorption of
the desired alkenes and increase the probability that they will
instead oxidize to undesired COx products.
Oxygen mobility and the rate of reoxidation of oxygen
vacancies can also influence ODH reactions. At the low water
concentrations prevalent at low propane conversions, the reaction
rate constants depend only on KI and kII and they benefit from
weaker Lewis acids and more reducible cations. As water
concentrations increase, the term containing water concentration
in the denominator of eq 1 becomes important, the oxygen order
becomes slightly positive,31 and the apparent rate constant
contains a rate constant (kV) corresponding to the reoxidation
of vacancies by O2. The required dissociative chemisorption of
O2 requires the migration of surface oxygen species in order to
increase the probability of neighboring vacancies. In this case,
lattice oxygen mobility becomes important; the Tamman tem-
perature of metal oxides, defined as 0.5Tm (Tm, melting point
in K) provides a qualitative measurement of oxygen mobility.
Table 3 shows Tamman temperature of various metal oxides.48
Tamman temperatures increase in the sequence
Acknowledgment. This work was supported by the Director,
Office of Basic Energy Sciences, Chemical Sciences Division
of the U.S. Department of Energy, under Contract DE-AC03-
76SF00098. The authors acknowledge useful technical discus-
sions with Morris D. Argyle and Jonathan Male.
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