A substrate-versatile catalyst for the selective oxidation of light alkanes: II. Catalyst characterization

A highly active and selective catalyst for light alkane oxidation that is composed of a pyridine salt of niobium-exchanged molybdo(vanado)phosphoric acid (NbPMo11(V)pyr) is characterized using TGA–DSC, 31P MAS NMR, and in situ powder XRD, XAS, and XPS. The presence of both niobium and pyridinium species strongly influences structural and redox properties of the polyoxometalate. Activation of the catalyst by heating to 420 °C in an inert atmosphere removes all of the organic species present in the solid, and structural rearrangement of the starting heteropolyanion occurs at 420 °C as evidenced by 31P NMR and EXAFS. XRD shows that activated NbPMo11Vpyr consisted of a mostly amorphous molybdenum oxide phase, the formation of which is strongly related to the composition of the catalyst. The presence of niobium as an exchange cation (NbO)3+ or a framework atom PMo11NbO404− in the Keggin unit is verified by EXAFS for NbPMo11Vpyr and (VO)PMo11Nbpyr, respectively. During activation of either catalyst, niobyl species migrate and most likely coordinate to molybdenum oxide octahedra. Comparison of near-edge electronic spectra (XANES) for as-made NbPMo11Vpyr and after activation that removes the pyridinium ions suggests reduction of Mo6+ to Mo5+ and Nb5+ to Nb4+. Under hydrocarbon-rich reaction mixtures molybdenum and niobium remain in their reduced state.