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

Abstract

Vanadia/titania particles with a specific surface area (SSA) up to 195 m 2 g −1 and a V 2O 5 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 TiO 2 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 TiO 2 beyond a theoretical “monolayer” (>8–10 V nm −2) while retaining amorphous (monomeric and polymeric) VO x surface species. Controlling liquid precursor and dispersion gas feed rates, precursor concentration, and V 2O 5 content allowed the tailoring of SSA and the population of the different VO x species in these vanadia/titania mixed oxides. For comparison, vanadia/titania catalysts containing 10% (w/w) V 2O 5 with comparable SSA and V coverage were prepared by impregnation, resulting in typical amorphous (<10 V nm −2) and crystalline (>10 V nm −2) VO x species. Catalysts containing 7, 10, and 20% (w/w) V 2O 5 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 VO x species seemed to be less critical. In contrast, selectivity to phthalic anhydride was significantly affected by the nature of the VO x species; amorphous species exhibited higher selectivity for conversion <90% compared with catalysts containing crystalline V 2O 5.