Structure of Flame-Made Vanadia/Silica and Catalytic Behavior in the Oxidative Dehydrogenation of Propane

Catalyst based on supported vanadia showed promising results for the ODH reaction. Here vanadia/silica particles with a specific surface area up to 330 m²g^-1 and a V₂O₅ content up to 50 wt.% or V surface density up to 27.6 V nm^-2 were prepared by flame-spray-pyrolysis. Selected samples were tested for the ODH of propane. The as-prepared flame-made catalysts showed dominantly isolated monomeric VO_x surface species for V-loadings exceeding even the typical “monolayer coverage” (2 V nm^-2) of classic wet-impregnated materials as confirmed by Raman and NMR analysis. The turnoverfrequency of flame-made catalysts decreased with increasing vanadia loading while the presence of small V₂O₅ crystallites seemed to be less critical. The conversion (global activity), however, showed a maximum for the 20 wt.% V₂O₅/SiO₂ (4.6 V nm^-2) catalyst. Catalysts consisting dominantly of isolated monomeric VO_x-species exhibited higher selectivity to propylene than those where crystalline V₂O₅ prevailed. Nevertheless, the highest propylene yield was measured for catalysts with high V surface density containing both monomeric VO_x and crystalline V₂O₅. Flame-spray-pyrolysis proved to be a very versatile method for synthesis of V₂O₅/SiO₂ catalysts with high dispersion and well defined structure of the active sites.