Role of C−H Bond Strength in the Rate and Selectivity of Oxidative Dehydrogenation of Alkanes

The oxidative dehydrogenation of alkanes (C 2 H 6, C 3 H 8, i -C 4 H 10, and n -C 4 H 10 ) was investigated on VO x supported on Al 2 O 3 . Rate constants for alkane dehydrogenation ( k 1 ), alkane combustion ( k 2 ), and alkene combustion ( k 3 ) were measured, and a model was developed to describe the effects of alkane composition on these rate constants. The proposed model accounts for the effects of the number of C−H bonds available for activation and the relative strengths of these bonds in both the reactant and the product molecules. The Brønsted−Evans−Polanyi (BEP) relationship is used to relate activation energies of secondary and tertiary C−H bonds to that of primary C−H bonds. The model gives a reasonable approximation of the relative order of alkane reactivity, expressed by k 1 + k 2, and the relative ranking of alkanes with respct to combustion versus oxidative dehydrogenation, expressed by k 2 / k 1 . The ratio of k 2 / k 1 is described by the product of two components: one that depends on the nature, number, and relative strength of C−H bonds of surface alkoxides, and a second one that is independent of the alkoxide composition and structure but depends on the difference in the entropy of activation for CO x precursor versus alkene formation. The model also explains the observed variation of k 3 with alkene composition by considering two precursor states for alkenes. One is strongly bound through π-orbital interactions with Lewis acid centers, and the second weakly binds via H bonding and van der Waals interactions, similar to the binding of alkanes. As a result, the rate of alkene combustion depends strongly on the large heats of adsorption of alkenes and only slightly on the presence of weak allylic C−H bonds. The high rate of C 2 H 4 combustion is thus a consequence of its high heat of adsorption.