Mechanisms of Initial Propane Activation on Molybdenum Oxides: A

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Mechanisms of Initial Propane Activation on Molybdenum Oxides: A Density Functional Theory Study Gang Fu, Xin Xu,* Xin Lu, and Huilin Wan* Department of Chemistry and Institute of Physical Chemistry, State Key Laboratory for Physical Chemistry of Solid Surfaces and Center for Theoretical Chemistry, Xiamen UniVersity, Xiamen 361005, China ReceiVed: October 4, 2004; In Final Form: January 28, 2005

We report the first detailed density functional theory study on the mechanisms of initial propane activation on molybdenum oxides. We consider 6 possible mechanisms of the C-H bond activation on metal oxides, leading to 17 transition states. We predict that hydrogen abstraction by terminal ModO is the most feasible reaction pathway. The calculated activation enthalpy and entropy are 32.3 kcal/mol and -28.6 cal/(mol/K), respectively, in reasonably good agreement with the corresponding experimental values (28.0 kcal/mol and -29.1 cal/(mol/K)). We find that activating the methylene C-H bond is 4.7 kcal/mol more favorable than activating the methyl C-H bond. This regioselectivity is correlated with the difference in strength between a methylene C-H bond and a methyl C-H bond. Our calculations suggest that a combined effect from both the methylene and the methyl C-H bond cleavages leads to the experimentally observed overall kinetic isotopic effects from propane to propylene on the MoOx/ZrO2 catalysts.

Introduction Hydrocarbons, especially lower alkanes, are the main constituents of natural gas and crude oil. Selective oxidation of lower alkanes is of fundamental and technological interest because it provides a potential route to effectively transform lower alkanes to higher, value-added chemicals.1-3 Although it has been widely accepted that the oxidation of alkanes begins with the activation of the C-H bond, which is generally believed to be the rate-determining step for most catalytic processes, much remains obscure for the molecular details of the activation mechanisms.4-6 Oxidative dehydrogenation (ODH) of lower alkanes offers an attractive route to alkenes.7-18 While nonoxidative dehydrogenation is endothermic and leads to the concurrent formation of carbon, which, in turn, deactivates the catalysts, ODH is exothermic by forming water as a byproduct and eliminating the coke formation, leading to stable catalysts. Several metal oxides have been examined as catalysts for ODH, and vanadiumbased materials are among the most active.17 Molybdenum-based oxides are the other promising catalysts for ODH.13,14 Although with less activity, some Mo-based catalysts show higher propylene selectivity as well as better thermostability.18 Reaction mechanisms and detailed elementary steps have been proposed, and isotopic tracer studies have been performed to check the reversibility of the proposed elementary steps.12,14 It was concluded that ODH on V-based and Mo-based catalysts follow the same mechanism and that two lattice oxygens participate in the irreversible activation of the C-H bonds in propane via a Mars-van Krevelen redox mechanism.12,14 Quantum mechanical methods at various levels have been applied to the study of V- and Mo-based catalysts.19-23 Emphasis has been laid on the study of the electronic structure and bonding properties of geometrically inequivalent surface oxygen sites on V2O5 and MoO3 solids.19-22 Although contro* Authors to whom correspondence should be addressed. E-mail: [email protected]; [email protected].

versy exists, Hermann and Witko et al. concluded, based on a recent density functional theory (DFT) study with large embedded cluster models, that H atoms are bound most strongly to the singly coordinated vanadyl/molybdenyl (VdO/ModO) sites such that the most active oxygen is the terminal oxygen (dO).22 A mechanistic study using DFT also appeared recently. Bell’s group has explored the oxygen nucleophilicity and studied the ODH of propane on V2O5 using cluster models.23 It was shown that the energetically preferred initial step is the dissociative adsorption of propane to form i-propoxide and hydroxyl species and that the active site is two VdO groups bonded by a V-O-V bridge.23 Here, we report the first detailed DFT study on the mechanisms of initial propane activation on the molybdenum oxides. We considered 6 possible ways of C-H bond activation involving 17 kinds of transition states. We predict that hydrogen abstraction by ModO is the most feasible reaction pathway. We find that activation of the methylene C-H bond is easier than activation of the methyl C-H bond in the initial step of the C-H activation. Our calculations infer that methyl and methylene groups in propane should make a comparative contribution to the experimentally observed overall kinetic isotopic effects from propane to propylene on the MoOx/ZrO2 catalysts. Computational Details In this report, we chose Mo3O9 as a model compound to provide various kinds of terminal (dO) active sites. Such a model is consistent with the experimental observation that a metal oxide cluster itself can serve as an active component in both heterogeneous and homogeneous catalysis.24,25 It was shown that Mo3O9 was one of the primary products of MoO3 vaporization such that Mo3O9+ has been generated to study the gas-phase reaction with small alcohols by using Fourier transform ion cyclotron resonance (FTICR).26 Bell and Iglesia examined the ODH of propane over MoO3/ZrO2.14 They found that the structure of the MoOx species for 11 wt % MoO3 loading

10.1021/jp0454974 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/11/2005

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is two-dimensinal MoOx oligomers. Theoretically, Mo3O9 fulfills the requirements of the stoicheometry principle, neutrality principle, and coordination principle such that it is a good model of choice.27 In fact, Goddard’s group has used this model to investigate the (amm)oxidation mechanism of propylene.28,29 The quantum calculations were performed using hybrid density functional theory at the level of B3LYP.30-34 Full geometry optimizations with no fixed atoms and analytical frequency calculations are performed with basis sets of double-ζ quality (6-31G)35 for the main group elements. The final energies are calculated with polarization functions being included (631G**).36 Hay’s effective core potential37 (Lanl2dz denoted in Gaussian 98)38 was used for Mo, which includes the relativistic effects. All energies reported here are enthalpies at the experimental temperature 688 K. Geometry optimizations were also performed for some testing cases with a 6-31G** basis set. No sensible changes were found for the optimized geometries as compared to those from 6-31G basis set. Full geometry optimizations of the Mo3O9 cluster along the reaction pathways may lead to an overestimation of the relaxation effect as compared to the more restricted situation of MoO3 solids. Testing calculations were performed for methane activations with a partially fixed Mo3O9 geometry. Our calculations show that there are differences within 1-2 kcal/ mol between partially fixed and fully relaxed Mo3O9 clusters for the estimation of the activation barriers. We considered 6 possible mechanisms of C-H bond activation (Figure 1). We have the 2 + 2 pathway, which leads to carbide or hydride formation, depending on how the C-H bond is approaching to the ModO bond (Figure 1). We have the 3 + 2 and 5 + 2 pathways. Both of them lead to the formation of hydroxyl and alkoxy directly. These pathways are correlated with homolytic cleavage of the C-H bond, where Mo is formally reduced from VI to IV in 3 + 2, whereas each Mo is formally reduced from VI to V in 5 + 2. Figure 1 also illustrates the oxenoid insertion pathway, which directly leads to the formation of alcohol. While the above four pathways involve activation of a single C-H bond, both methyl and methylene C-H bonds are simultaneously involved in the 2 + 4 pathway. We also examined the hydrogen-abstraction pathway, which distinguishes itself as a one-electron process, leading to the radical formation. We considered hydrogen abstraction both via anti mode and syn mode (Figure 1). Since either the methyl C-H bond or the methylene C-H bond or both of them can be involved in the propane activation, we together have located 17 transition states. As compared to the extended MoO3 surface, our Mo3O9 cluster model may be still too restricted. Thus, some of reaction paths on the surfaces may be neglected, e.g., our model does not distinguish the symmetric bridging surface oxygens from the nonsymmetric ones and our model does not include the three-fold coordinated oxygens.19-22 To make a direct comparison with the experimental data of the isotopic tracer studies at 688 K,14 we have also estimated the kinetic isotopic effect of each reaction pathway based on the transition state theory (TST).39 We replaced hydrogen with deuterium for either methyl hydrogens or methylene hydrogens or both. We calculated the Gibbs free energy of activation at 688 K (∆Gq) and computed the TST rate of reaction using Eyring equation39

kTST(T) )

kBT -∆Gq/RT e h

(1)

where h is Planck’s constant, kB is the Boltzmann constant, and

Figure 1. Possible mechanisms of C-H bond activation. For n + m pathways, cyclic structures are involved in the transition states, where n indicates how many atoms in the active sites of the substrate are involved and m indicates how many atoms in the adsorbate are involved. Here, no change of the valence state of the metal center occurs in 2 + 2 and 2 + 4 such that these two pathways may be considered as acidbase reactions, while change of the valence state of the metal center happens in the other pathways such that these pathways may be considered as redox reactions. The hydrogen-abstraction reaction is a one-electron-transfer process, leading to radical formation; the other pathways are two-electron-transfer processes.

R is the ideal gas constant. We estimated the tunneling effects based on the Wigner correction40

κ≈1+

( )

1 hVq 24 kBT

2

(2)

where ν* is the magnitude of the imaginary frequency of motion along the reaction coordinate at the transition state. Thus, the rate constant, k, is given by

k ) κkTST

(3)

Results and Discussions Figure 2 and Table 1 summarize the optimized geometry and the calculated energy of the transition state of each pathway. TS 1, TS 2, and TS 3 belong to the 2 + 2 addition. The active site for all of them is a single ModO bond. The differences between them are the orientation of the C-H bond and whether CH2 or CH3 is involved. In the TS 3 transition state, propane approaches the ModO bond from the O end, with H attacking the metal and the alkyl group the oxygen. We refer to it as the hydride formation pathway because a Mo-H bond is going to form. In the TS 1 and TS 2 transition states, propane approaches

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Fu et al.

Figure 2. Optimized geometries for the transition states of propane activation by ModO groups. (Bond length in Å; imaginary frequency in cm-1).

the ModO bond with its alkyl group attacking the metal. We refer to it as the carbide formation pathway because a Mo-C bond is going to form. In the energetic viewpoint, while the activation barrier for the 2 + 2 process of carbide formation is around 52 kcal/mol, the 2 + 2 process for hydride formation must be ruled out for too high of a reaction barrier (74.0 kcal/ mol). Electrostatic factors were claimed to be the main reason for this 22 kcal/mol difference between TS 3 and TS 1 or TS 2.41 In TS 3, the interaction between the dipoles associated with the C-H and M-O bonds tends to raise the energy. To contrast this effect, the C-H bond polarizes toward the product charge distribution, forming a negatively charged hydrogen. Thus, this

kind of hydride transfer process is an unfavorable pathway according to our calculations. In TS 1 or TS 2, the electrostatic interaction lowers the energy, and no further rearrangement of charges occurs.41 Furthermore, the product of the carbide pathway is 5.3 kcal/mol more stable than that of the hydride pathway because oxygen has a higher binding ability to the hydrogen atom than to the alkyl group.41 The present level of theory leads to the bond enthalpy difference of 4.1 kcal/mol between the methylene and methyl C-H bonds.42 For the carbide formation pathways, it is interesting to see that our calculations show that breaking a methyl C-H bond (TS 2) is slightly (1.1 kcal/mol) more favorable than breaking a methylene

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TABLE 1: Reaction Barriers and the Corresponding Active Sitesa TS

activation mode

type of H cleavage

active site

attribution of reaction

∆Hq (688 K) (kcal/mol)

∆Sq (688 K) (cal/(mol/K))

∆Hr (688 K) (kcal/mol)

TS1 TS2 TS3

2+2

CH2 CH3 CH3

ModO pair

acid-base/2e-

50.3 49.2 72.4

-38.0 -30.2 -31.6

20.2 20.8 25.5

TS4 TS5

3+2

CH2 CH3

dioxo

redox/2e-

59.3 63.0

-34.6 -35.4

19.3 23.2

TS6 TS7

5+2

CH2 CH3

dual ModO

redox/2e-

49.0 53.1

-35.1 -37.2

10.5 14.2

TS8 TS9

oxenoid insertion

CH2 CH3

terminal oxygen

redox/2e-

52.0 59.5

-29.2 -30.8

15.5 20.5

TS10 TS11 TS12 TS13

2+4

both CH2 and CH3

ModO pair

acid-base/2e-

46.1 46.5 48.3 49.2

-33.9 -34.3 -37.2 -36.5

37.8 37.8 37.8 37.8

TS14 TS15 TS16 TS17

H abstraction

CH2 (syn) CH3 (syn) CH2 (anti) CH3 (anti)

terminal oxygen

redox/1e-

39.2 43.1 32.3 37.0

-25.3 -24.0 -28.6 -29.4

31.9 36.9 31.9 36.9

exptl

H abstraction

CH2

lattice oxygen

redox

28.0

-29.1

a

The best theoretical predictions and the experimental data are in boldface.

C-H bond (TS 1). Apparently, steric factors play an important role in determining this “abnormal” regioselectivity. TS 4 and TS 5 along with TS 6 and TS 7 correspond to 3 + 2 and 5 + 2, respectively. The former involves two terminal oxygens connected to the same metal center, using the OdMod O unit as the active site, while the latter involves two ModO units connected by a bridge O, using the OdMo-O-ModO unit as the active site. Both the 3 + 2 and 5 + 2 pathways lead to the reduction of the metal centers with the simultaneous formation of hydroxyl and alkoxy. We see that the 5 + 2 TS is 10 kcal/mol lower in energy than the corresponding 3 + 2 TS and that the 5 + 2 product is 9 kcal/mol more stable than the 3 + 2 product. In the pioneer work of Goddard on hydrocarbon oxidation by high-valent group 6 oxides, it was concluded that the second oxo group plays a central role in stabilizing critical intermediates.43,44 The same reasoning can be applied to understand why 5 + 2 is preferable over 3 + 2. There is an extra (spectator) oxo on each Mo in the 5 + 2 pathway, while no such spectator oxo remains on the metal center in the 3 + 2 pathway. It should be pointed out that the distance between two reacting terminal oxygens is 5.37 Å in Mo3O9, which is much longer than the distance (3.70 Å)45 between two nearest neighbor terminal oxygens in R-MoO3. In terms of 5 + 2, a shorter dO‚‚‚Od distance will obviously lead to a lower activation barrier. Thus, it can be anticipated that 5 + 2 may become an important C-H activation pathway for certain oxides when there are terminal oxygens within suitable distance. This result is in agreement with Bell’s deduction for ODH of propane on V2O5, where a TS similar to that in 5 + 2 was advocated.23 Our calculations predict that the barriers for 3 + 2 are 59.3 (TS 4) and 63.0 (TS 5) kcal/mol for the methylene and methyl C-H, respectively, while those for 5 + 2 are 49.0 (TS 6) and 53.1 (TS 7) kcal/mol, respectively. Thus, unlike the situation in 2 + 2, we see that TS′ involved the methylene C-H is 4 kcal/mol lower in energy that TS′ involved the methyl C-H. This trend parallels the bond strength between the C-H bonds in methylene and in methyl groups. Since terminal dO is not as electrophilic as O- or O22-, we do not expect that activation of the C-H bond via the “oxenoid insertion” mechanism is the most competitive pathway in our cases. However, we did locate two transition states here. For the oxenoid insertion into a methyl C-H bond, the calculated

barrier is 59.5 (TS 9) kcal/mol, while for the oxenoid insertion into a methylene C-H bond the calculated barrier is 52.0 (TS 8) kcal/mol. These barrier heights are higher than those of 2 + 2 and 5 + 2. Notice that, however, the methylene TS is 7.5 kcal/mol lower than the methyl TS as opposed to the 4.1 kcal/ mol calculated difference in the bond enthalpy between the methylene C-H bond and methyl C-H bond. This high regioselectivity may partly be traced back to the significant amount of charge transfer in the oxenoid insertion TS. Mulliken charge analysis showed that the amount of charge transfer reaches 0.87 or 0.75 for oxenoid insertion into the methylene or methyl C-H bond, respectively. As compared to the C-H bond activation in methane, 2 + 4 is a new mode of activation, where both methyl and methylene C-H bonds are simultaneously involved. The 2 + 4 process undergoes a concerted transfer of two hydrogens of propane to the ModO moiety through a six-membered cyclic TS, leading to the direct propylene formation.3 Four transition states (TS 10-13) have been located. The calculated activation barriers are 46.1-49.2 kcal/mol. The lowest barrier occurs when the methyl H bonds to the metal and the methylene H bonds to the terminal oxygen with the remaining methyl pointing down and away from the ModO bond (cf. TS 10). TS 14-17 are for hydrogen abstractions. These one-electron processes lead to the formation of radicals. We find that the hydrogen-abstraction pathways (barriers 39.2-43.1 kcal/mol) are generally more favorable than the two-electron processes. The hydrogen-abstraction mechanism by terminal oxygen involves only one ModO, leaving the other ModO in the same metal center as the spectator oxo.43,44 Thus, the preference for the one-electron processes can be readily attributed to the spectator oxo effects. H can approach O with its propane molecular skeleton syn or anti to the ModO bond. We find that the anti modes (TS 16/TS 17) are favored over the corresponding syn modes (TS14/TS15) by 6-7 kcal/mol, while the spectator ModO bond length is 1.718/1.717 Å in TS 16/ TS 17 as compared to 1.730/1.727 Å in TS 14/TS 15. We also find that breaking a methylene C-H bond is 3.9-4.7 kcal/mol more favorable than breaking a methyl C-H bond. Thus, our calculations show that the most favorable pathway is the anti hydrogen abstraction from the methylene group. The calculated activation energy (32.3 kcal/mol) is in reasonably

6420 J. Phys. Chem. B, Vol. 109, No. 13, 2005 good agreement with experimental value (28.0 kcal/mol) reported by Chen, Iglesia, and Bell for the ODH of propane on the MoO3/ZrO2 catalysts.14 Our calculations lead to an entropy change of -28.6 cal/(mol/K) for the anti hydrogen abstraction, which is in good agreement with the experimentally deduced value of -29.1 cal/(mol/K). This remarkable agreement may suggest the resemblance of the structures of the transition states between theory and experiment. The hydrogen-abstraction process is a one-electron-transfer redox process, leading to the formation of a radical pair, the Mo5+ center, and an alkyl radical. The formation of radical pairs appears omnipresent in various areas of chemistry involving gas-phase and matrix-isolated species, organic synthesis, homogeneous/heterogeneous catalysis, and metalloenzymes.46 For instance, Mayer and co-workers provided strong evidence that oxidation of hydrocarbons by high-valent metal oxo compounds involves hydrogen abstraction, which may be considered as a proton-coupled electron transfer (PCET).46-48 In terms of PCET, the metal center is the electron acceptor, whereas oxygen is the proton acceptor. Therefore, both the reducibility of the metal center and the basicity of the lattice oxygen play a significant role in the C-H bond activation, which is compatible with Bell and Iglesia’s proposal.12,14 Measuring kinetic isotopic effects (KIE) provides a powerful tool to determine which C-H bond is involved in the ratedetermining step.12,14 If only the methylene C-H bond is involved, CH3CD2CH3 may react more slowly than CH3CH2CH3 such that kC-H/kC-D is larger than 1.0, while replacing a methyl hydrogen with deuterium may not affect the reaction rate. However, if only the methyl C-H bond is involved, CD3CD2CD3 may react more slowly than CH3CD2CH3; hence kC-H/ kC-D is larger than 1.0. In this case, we will have similar rates for CH3CD2CH3 and CH3CH2CH3. Experimentally, Chen et al. measured KIEs at 688 K over the 11 wt % MoOx/ZrO2 catalysts.14 They reported that CH3CD2CH3 exhibits a normal KIE (1.7) with respect to CH3CH2CH3.14 This number is smaller than that obtained when using CD3CD2CD3 and CH3CH2CH3 (2.3) isotopes but greater than unity, suggesting that C-H bonds in both methyl and methylene groups can be involved in the rate-determining steps required for propane activation.14 Our calculated numbers are summarized in Table 2. Only KIEs from 2 + 4 involving both methylene and methyl groups can be compared directly with the experimentally measured numbers.14 In fact, KIEs from TS 12 are closer to the experimental data than those from TS 6 and TS7. Looking at the C‚‚‚H‚‚‚Mo moiety, structures in TS 10-TS 13 are similar, with the C-H bond being partially broken and the H-Mo bond being partially formed. Looking at the C‚‚‚H‚‚‚O moiety, however, TS 10 and 11 are much later as compared to TS 12, such that H transfer from C to O is completed in TS 10 and 11 but in the middle in TS 12. These results may be interpreted as emphasizing the importance of having the C-H bonds in both methyl and methylene groups involved in the overall kinetics. However, the calculated barrier for TS 12 (48.3 kcal/mol) is too high as compared to the experimental data (28.0 kcal/mol) to be a feasible pathway.14 As compared to the ODH of propane on the VOx/ZrO2 catalysts, experimentally measured KIEs showed that CH3CD2CH3 exhibits a normal KIE of 2.7 with respect to CH3CH2CH3, which is very close to that obtained when using CD3CD2CD3 and CH3CH2CH3 (2.8) isotopes.12 Thus, experiments demonstrated unambiguously that only the methylene C-H is involved in the rate-determining step for the VOx/ZrO2 systems.12 DFT calculations showed that the energetically preferred initial step is the dissociative adsorption of propane to form i-propoxide and hydroxyl species and that the active site is two VdO groups

Fu et al. TABLE 2: Calculated Kinetic Isotopic Effects (KIE) as Compared to the Experimental Dataa KIE (688 K) TS

activation mode

type of H CH3CH2CH3 CH3CH2CH3 CH3CD2CH3 cleavage CD3CD2CD3 CH3CD2CH3 CD3CD2CD3

TS 1 TS 2 TS 3

2+2

CH2 CH3 CH3

2.79 2.03 2.10

2.87 1.10 1.13

0.97 1.85 1.86

TS 4 TS 5

3+2

CH2 CH3

1.52 1.44

1.47 0.99

1.04 1.45

TS 6 TS 7

5+2

CH2 CH3

1.42 1.48

1.20 1.08

1.19 1.37

TS 8 TS 9

oxenoid insertion

CH2 CH3

1.63 1.67

1.46 1.11

1.12 1.51

TS 10 2 + 4 TS 11 TS 12 TS 13

both CH2 and CH3

2.07 2.05 2.37 2.69

1.10 1.14 1.77 1.90

1.89 1.79 1.34 1.41

TS 14 H abstraction TS 15 TS 16 TS 17

CH2 (syn) CH3 (syn) CH2 (anti) CH3 (anti)

2.98 2.79 2.61 2.32

2.99 1.02 2.50 1.06

1.00 2.73 1.04 2.19

2.3

1.7

1.4

exptl H abstraction CH2 a

The best theoretical predictions and the experimental data are in boldface.

bonded by a V-O-V bridge.23 This pathway may be envisioned as 5 + 2.14 For our Mo3O9 systems studied here, we find that breaking the methylene C-H bond (barrier 49.0 kcal/mol) is easier than breaking the methyl C-H bond (53.1 kcal/mol) in 5 + 2. However, the former leads to a KIE of 1.42 for CD3CD2CD3 versus CH3CH2CH3 and a KIE of 1.20 for CH3CD2CH3 versus CH3CH2CH3. These numbers differ greatly from the experimental KIE data either from the VOx/ZrO2 systems12 or from the MoOx/ZrO2 systems.14 A comparative theoretical study is underway in our group to explore the possible mechanistic difference for the VOx and MoOx systems. Energetically, our calculations show that the most favorable pathway is the anti hydrogen abstraction from the methylene group (barrier heights 32.3 (calcd) versus 28.0 (exptl)14) for the ODH of propane on Mo3O9. The calculated KIEs are 2.88 using CD3CD2CD3 and CH3CH2CH3 as isotopes and 2.77 using CH3CD2CH3 and CH3CH2CH3 as isotopes. These numbers can be compared favorably with the corresponding experimental numbers (2.8, 2.7) for the VOx/ZrO2 systems,12 supporting the experimental deductions that the rate-determining step involves a redox mechanism using a lattice oxygen atom to abstract a hydrogen atom from the methylene group in propane.12 These numbers (2.61, 2.50) are not in agreement with the corresponding experimental numbers (2.3, 1.7) for the MoOx/ZrO2 systems.14 For the methyl C-H bond activation, our calculations lead to a KIE of 2.32 for CD3CD2CD3 versus CH3CH2CH3 and a KIE of 1.06 for CH3CD2CH3 versus CH3CH2CH3. The comparison among the calculated KIEs from the anti hydrogen abstraction from the methylene and the methyl groups as well as that from the 2 + 4 pathway infers that both methyl and methylene groups in propane should make a comparative contribution to the experimentally observed KIEs for the MoOx/ ZrO2 systems. Experimental KIE measurements are conducted at 688 K for MoOx catalysts and 593 K for VOx catalysts, because the former is less active than the latter.12,14 Our calculations show that activating the methyl C-H bond is 4.7 kcal/mol less favorable than activating the methylene C-H bond. This regioselectivity is correlated with the difference in strength between the

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methylene C-H bond and the methyl C-H bond. Because higher temperatures will tend to decrease the selectivity for activating one specific C-H bond in propane, this temperature effect has been used to explain the observed KIE difference between MoOx and VOx systems.14 We may estimate the ratio for methylene C-H bond activation over methyl C-H bond activation by using

kCH2 kCH3

1 4.7 ) exp 3 RT

( )

(4)

Thus, T ) 593 K leads to kCH2/kCH3) 18.0, while T ) 688 K leads to kCH2/kCH3 ) 10.3. While it is true that higher temperature favors the methyl C-H bond activation more than the methylene C-H bond activation, the temperature effect alone in the initial step can hardly explain how the methyl C-H bond activation can compete with the methylene C-H bond activation to make comparable contribution in the KIEs. Detailed mechanistic study is underway in our group to establish the overall kinetics for propane dehydrogenation to propylene. We anticipate that the overall kinetics involves both methylene and methyl C-H bond cleavages on the MoOx/ZrO2 catalysts. Conclusions We consider 6 possible mechanisms of C-H bond activation on metal oxides, leading to 17 transition states involving C-H activations of both the methyl and methylene groups in propane on Mo3O9. We find that the hydrogen-abstraction mechanism is the most feasible reaction pathway for initial propane activation. The calculated activation energy is 32.3 kcal/mol for the methylene C-H bond activations, in reasonably good agreement with the experimental value (28.0 kcal/mol).14 The 5 + 2 pathway using the methylene C-H, although being advocated in the related VOx/ZrO2 systems,23 leads to a barrier of 49.0 kcal/mol, which is too high, and an incorrect KIE as compared to either the VOx/ZrO2 or MoOx/ZrO2 system.12,14 The 2 + 4 pathway, where both methylene and methyl C-H bonds are involved simultaneously, gives a good KIE but leads to too high of a barrier (48.3 kcal/mol). On the basis of the comparison between the observed and the calculated KIEs, we propose that a combined effect from both methylene and methyl C-H bond cleavages leads to the experimentally observed overall KIEs for the ODH of propane to propylene on the MoOx/ ZrO2 catalysts. Acknowledgment. This work was supported by the Ministry of Science and Technology (G1999022408), NSFC (20433030, 20021002, 29973031), TRAPOYT from the Ministry of Education and the National Natural Science Foundation of Fujian (2002F010). References and Notes (1) Centi, G.; Cavani, F.; Trifiro, F. SelectiVe Oxidation by Heterogeneous Catalysis; Kluwer Academic: Dordrecht, 2001. (2) Olah, G. A.; Molna´r, A Ä . Hydrocarbon Chemistry, 2nd ed.; John Wiley & Sons: Hoboken, NJ, 2003. (3) Bettahar, M. M.; Costentin, G.; Savary, L.; Lavalley, J. C. Appl. Catal. A 1996, 145, 1. (4) Witko, M. J. Mol. Catal. 1991, 70, 277. (5) Grzybowska-Swierkosz, B. Top. Catal. 2000, 11/12, 23. (6) Haber, J.; Witko, M. J. Catal. 2003, 216, 416. (7) Oyama, S. T.; Middlebrook, A. M.; Somorjai, G. A. J. Phys. Chem. 1990, 94, 5029. (8) Kung, H. H. AdV. Catal. 1994, 40, 1. (9) Zhao, Z.; Yamada, Y.; Teng, Y.; Ueda, A.; Nakagawa, K.; Kobayashi, T. J. Catal. 2000, 190, 215. (10) Mamedov, E. A.; Cortes-Corberan, V. Appl. Catal. A 1995, 127, 1.

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