The role of lattice oxygen in the oxidative dehydrogenation of ethane on alumina-supported vanadium oxide

Abstract

The oxidative dehydrogenation (ODH) of ethane on alumina-supported vanadia was investigated with the aim of understanding the effects of lattice oxygen and vanadium oxidation state on the catalyst ODH activity and ethene selectivity. Transient-response experiments were carried out with both a fully oxidized sample of 10 wt% VO_x/Al₂O₃ (7 V nm⁻²) and a sample that had been partially reduced in H₂. The experimental results were analyzed to determine the rate coefficients for ethane ODH, k₁, and ethene combustion, k₃. The rate of ODH was found to depend solely on the concentration of reactive oxygen in the catalyst, but not on the means by which this oxygen concentration was attained (i.e., by H₂versus C₂H₆ reduction). On the other hand, the ethene selectivity observed at a given concentration of active oxygen was found to depend on the composition of the reducing agent, higher ethene selectivities being observed when H₂, rather than C₂H₆, was used as the reducing agent. It is proposed that the higher ethene selectivity achieved by H₂versus C₂H₆ reduction might be due to a lower ratio of V⁴⁺ to V³⁺ cations attained upon reduction in H₂ for a given extent of V⁵⁺ reduction. This interpretation is based on the hypothesis that ethene combustion is initiated by C₂H₄ adsorption on Vⁿ⁺ cations present at the catalyst surface and that the strength of adsorption decreases in the order V⁵⁺ > V⁴⁺ > V³⁺ consistent with the decreasing Lewis acidity of the cations.