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 VO x , MoO x , and WO x structures supported on ZrO 2 . The similar kinetic rate expressions obtained on MoO x and VO x 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 VO x /ZrO 2 < MoO x /ZrO 2 < WO x /ZrO 2 ; 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 VO x /ZrO 2 < MoO x /ZrO 2 < WO x /ZrO 2 . 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 (V 5+ < Mo 6+ < W 6+ ). Introduction In spite of its significant economic potential as an alternate route to alkenes 1-5 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 reactions 23 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 CO 2 (CO x ) can form via either secondary combustion of alkenes or direct combustion of alkanes. Selective poisoning of sites responsible for direct combustion of alkanes using SiO x 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 VO x -based catalysts, the evolution of oxide structures from monovanadate to polyvanadate species as VO x 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 VO x surface density, which leads to a decrease in k 2 /k 1 ratios as surface density increases. 25,26 Initial selectivities greater than 90% can be achieved for many ODH reactions. 2,3 The k 3 /k 1 (propene secondary combustion/propane primary dehydrogenation) ratio, which causes the yield losses observed as conversion increases, is large (5-40) on VO x -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 k 3 /k 1 ratio, this ratio would not depend on the chemical identity or the local structure of the active oxide. The differences observed among VO x , MoO x , WO x 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. SCHEME 1: Reaction Network in Oxidative Dehydrogenation of Alkane Reactions 1292 J. Phys. Chem. B 2000, 104, 1292-1299 10.1021/jp9933875 CCC: $19.00 © 2000 American Chemical Society Published on Web 01/22/2000