Structure of flame-made vanadia/titania and catalytic behavior in the partial oxidation of o-xylene

Vanadia/titania particles with a specific surface area (SSA) up to 195 m2 g−1 and a V2O5 content up to 40% (w/w) or V coverage up to 59 V nm−2 were prepared by flame spray pyrolysis (FSP) under various conditions. The catalysts were characterized by nitrogen adsorption, X-ray diffraction, temperature-programmed reduction, and in situ Raman spectroscopy and tested for partial oxidation of o-xylene. Depending on vanadia content, monomeric, polymeric, and crystalline vanadia species were formed on TiO2 support particles by FSP. Increasing the high-temperature particle residence time and concentration (production rate) during FSP reduced the SSA and increased the vanadia coverage of TiO2 beyond a theoretical “monolayer” (>8–10 V nm−2) while retaining amorphous (monomeric and polymeric) VOx surface species. Controlling liquid precursor and dispersion gas feed rates, precursor concentration, and V2O5 content allowed the tailoring of SSA and the population of the different VOx species in these vanadia/titania mixed oxides. For comparison, vanadia/titania catalysts containing 10% (w/w) V2O5 with comparable SSA and V coverage were prepared by impregnation, resulting in typical amorphous (<10 V nm−2) and crystalline (>10 V nm−2) VOx species. Catalysts containing 7, 10, and 20% (w/w) V2O5 were deposited directly from the flame on ceramic foams that were tested for the partial oxidation of o-xylene to phthalic anhydride. The global activity of flame-made and conventionally impregnated catalysts depended mainly on SSA and vanadia loading (number of V surface sites), whereas the amorphous or crystalline nature of the VOx species seemed to be less critical. In contrast, selectivity to phthalic anhydride was significantly affected by the nature of the VOx species; amorphous species exhibited higher selectivity for conversion <90% compared with catalysts containing crystalline V2O5.