Transient and steady state investigation of selective and non-selective reaction pathways in the oxidative dehydrogenation of propane over supported vanadia catalysts

Abstract

Mechanistic aspects of the formation of C₃H₆, CO and CO₂ in the oxidative dehydrogenation of propane over VO_x/γ-Al₂O₃ materials have been investigated by means of steady state and transient isotopic tests. The materials possessed highly dispersed and polymerised VO_x species as well as bulk-like V₂O₅. Propene was primarily formed via oxidative dehydrogenation of propane by lattice oxygen of VO_x species. It was suggested that non-selective consecutive propene oxidation is initiated by the breaking of the C–C bond in the molecule by the lattice oxygen, forming formaldehyde as a side product, which is further oxidised to CO and CO₂. The following order of initial steady state propene selectivity (at a zero degree of propane conversion) as a function of the nature of VO_x species was established: a mixture of bulk-like V₂O₅ and polymerised VO_x > polymerised VO_x > highly dispersed VO_x species. The low propene selectivity over highly dispersed VO_x species was explained by the fact that these species do not fully cover the bare acidic surface of γ-Al₂O₃ where propene adsorption and further oxidation take place. Thus, two different locations of CO_x formation were considered: (i) in the vicinity of acidic sites of the support and (ii) on VO_x species. The propene selectivity over samples possessing polymerised VO_x species and bulk-like V₂O₅ strongly decreased with an increasing degree of propane conversion. Contrarily, highly dispersed VO_x species showed the lowest ability for consecutive propene oxidation.