Effect of the nature and the distribution of vanadium Species on the catalytic behavior of vanadium-based silica catalysts

摘要

V-containing hexagonal mesoporous silica catalysts were prepared by direct synthesis and impregnation method in a wide range of V loading (0.68 - 7.8 wt. %). Investigation of VOx complex distribution by means of H2-TPR reveals that direct synthesis method led predominantly to formation of material with very narrow distribution of VOx complexes, whereas impregnation methods resulted in materials with broad distribution of VOx species including VOx units with a lower degree of dispersion. The catalytic results in propane ODH (Oxidative dehydrogenation) were much better for synthesized catalysts compared to impregnated catalysts. Synthesized samples exhibited higher activity as well as selectivity to propene. This activity and selectivity of synthesized catalysts was ascribed to well-dispersed VOx units characterized by low-temperature reduction peak in H2-TPR. Presence of more polymeric or bulk-like vanadium oxide species, reflected in high-temperature reduction peak, led to decrease in selectivity and/or activity of the catalysts in C3-ODH reaction.