Oxidative Dehydrogenation of Ethane and Propane By CO2

A series of vanadium catalysts (V loading in the 0.7 ~ 16 V/nm² range) supported on silica have been prepared by incipient wetness impregnation method. The activity of ODH was studied in ethane and propane oxidative dehydrogenation in the presence of O₂, CO₂ and their mixture from 350 to 600^oC and at atmospheric pressure. The products of oxidative dehydrogenation of paraffins were corresponding olefins, CO and CO₂, in the propane reaction a small part was cracked to ethylene and methane.

The vanadia silica catalysts at sub-monolayer VO_x coverage were found to be active and selective in propane ODH by O₂ and CO₂, while VO_x multilayers and nano-sized clusters showed lower activity and selectivity. Moreover, due to its acidic character of silica, it showed undesirable carbon deposition (coking) in propane ODH by CO₂ in a short period of reaction time and as a result catalyst deactivated quickly. Around 20 % weight loss observed for the used catalyst after 30 hours propane ODH by CO₂ reaction under conditions of TGA in air. On the other hand, the same catalyst employed to propane ODH by O₂, almost no carbon deposition was observed. Our experimental observations are in agreement with recent theoretical calculations indicating that in this catalytic system, carbon deposition is thermodynamically favorable, unless the surface character is modified by basic additives, such as Na⁺. Moreover, basic additives were recently reported to enhance the monolayer coverage of VO_x species on silica which also improved their activity and selectivity in propane ODH by CO₂.

The catalytic activity of these species may be optimized by varying the surface coverage of VO_x species and the nature of the support and promoters.