Bridging the structural gap of supported vanadium oxides for oxidative dehydrogenation of propane with carbon dioxide†

Abstract

As an extensively used industrial catalyst for oxidation reactions, supported vanadium oxide (VO_x) is a promising candidate for oxidative dehydrogenation of propane with carbon dioxide (CO₂-ODP). Although the structure of VO_x is found to be a key factor in determining the catalytic activity and stability of supported VO_x for CO₂-ODP, the essential reason still remains elusive at the molecular level. To shed some light on this fundamental issue, VO_x/(−)SiO₂ catalysts with narrow distributions of V loading while well-defined structures of VO_x species, i.e., monomeric VO_x, less polymeric VO_x, highly polymeric VO_x and V₂O₅ crystallites, were purposely synthesized by appropriate methods, including one-pot hydrothermal synthesis, incipient wetness impregnation and physical grinding. We found that the catalytic activity and stability of VO_x species decrease in the order of monomeric VO_x > less polymeric VO_x > highly polymeric VO_x > crystalline V₂O₅, which coincides with the ability for the re-oxidation of the correspondingly reduced VO_x species by CO₂. As a result of the most facile re-oxidation of the reduced monomeric VO_x species by CO₂, a well matched redox cycle of V⁵⁺/V⁴⁺ oxides during CO₂-ODP can be maintained with increasing the time on stream, leading to an improved stability of the catalyst with more monomeric VO_x. These mechanistic findings on the redox properties of VO_x with different structures can be guidelines for developing a high-performance VO_x-based catalyst for CO₂-ODP.