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

The oxidative dehydrogenation of alkanes (C2H6, C3H8, i-C4H10, and n-C4H10) was investigated on VOx supported on Al2O3. 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 Bronsted-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 oil 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 COx 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 pi-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 oil the large heats of adsorption of alkenes and only slightly on the presence of weak allylic C-H bonds. The high rate of C2H4 combustion is thus a consequence of its high heat of adsorption.