Molecular structure-reactivity relationships for propane oxidation over model mixed oxide catalysts

Mixed Mo-V-Sb-Nb and Mo-V-Te-Nb oxides were recently discovered as highly active and selective catalysts for the propane oxidation to propylene and oxygenates, i.e. acrolein and acrylic acid. Fundamental information is needed for understanding the bulk and surface molecular structure-activity/selectivity relationships and providing rules of rational design of improved mixed metal oxide catalysts for the selective oxidation of propane.; The phase composition and catalytic activity of the model Mo-V-Sb-Nb-O system strongly depended on the synthesis route and composition. A number of distinct solid-state phases were observed in this system: Mo6V 9O40, MoO3, rutile SbVO4 and the defect "Mo-V-Nb-O" phases. The kinetic studies indicated that the high selectivity to acrylic acid (∼20 mol.%) was associated with the Mo 6V9O40 and the defect "Mo-V-Nb-O" phases. The synthesis of these mixed Mo-V-Sb-Nb-O system was performed using combinatorial chemistry techniques to explore the utility of liquid phase automated synthesis for the reproducible preparation of these catalysts. This study indicated good reproducibility of phase compositions and catalytic properties and provided new insights into the transformation processes that occur in these oxides with time on stream.; It was discovered in this research that the model mixed Mo-V-Te-O catalyst represents a simple model system for elucidating the molecular structure-activity/selectivity relationships in propane selective oxidation. This catalyst contains the two major crystalline phases proposed as active and selective for propane oxidation. This research elucidated the effect of the chemical composition, solution pH and thermal treatment on phase composition and catalytic performance. High resolution TEM study established the crystal structures of the hexagonal and orthorhombic phases in terms of their space groups, unit cell parameters, atomic coordinates and elemental compositions.; In order to understand the role of the structure, morphology and composition of mixed Mo-V-Te-Nb-O system, novel synthesis approaches employing ordered arrays of colloidal polystyrene spheres (0.4 mum) were successfully developed. These novel nanocrystalline catalysts with surface areas in the 80--110 m2/g range consisted of 20--50 nm particles of desirable catalytic phases and contained 6--20 nm pores. Kinetic studies demonstrated the acrylic acid yields exceeding 40 mol.% and approaching the commercial targets for the selective oxidation of propane to acrylic acid.