Bulk structure and catalytic properties of mixed Mo–V–Sb–Nb oxides for selective propane oxidation to acrylic acid

The bulk mixed Mo–V–Sb–Nb–O catalysts, which are a candidate catalytic system for the selective oxidation of propane to acrylic acid, were investigated to elucidate the bulk structure and catalytic behavior of these complex materials. These mixed oxides were prepared via a redox reaction between V5+ and Sb3+ in the presence of Mo6+ and Nb5+ and characterized by potentiometric titrations, XRD, Raman spectroscopy, electron microscopy (TEM), and bulk elemental analysis. A potentiometric titration method was used to determine concentrations of metal cations in various oxidation states. XRD and Raman spectroscopy identified Mo6V9O40, MoO3, SbVO4, and a Nb-stabilized defect phase of a V-rich molybdate as the major phases present. Electron microscopy illustrated the heterogeneity of the bulk oxide phases present in the model Mo–V–Sb–Nb–O system on the submicron scale. MoO3 comprised the bulk of this mixed metal oxide system, while the surface region of these model catalysts contained mixed Mo–V–Sb–Nb oxides. The rutile SbVO4 phase was inefficient in propane oxidation to acrylic acid, while mixed Mo–V–Nb oxides were capable of producing acrylic acid at ∼20 mol% yield.