A substrate-versatile catalyst for the selective oxidation of light alkanes II. Catalyst characterization
2003; Elsevier BV; Volume: 218; Issue: 1 Linguagem: Inglês
10.1016/s0021-9517(03)00009-5
ISSN1090-2694
AutoresChristopher J. Dillon, Joseph H. Holles, Robert J. Davis, Jay A. Labinger, Mark Davis,
Tópico(s)Catalysis and Oxidation Reactions
ResumoAbstract A highly active and selective catalyst for light alkane oxidation that is composed of a pyridine salt of niobium-exchanged molybdo(vanado)phosphoric acid (NbPMo 11 (V)pyr) is characterized using TGA–DSC, 31 P 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 31 P NMR and EXAFS. XRD shows that activated NbPMo 11 Vpyr 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 PMo 11 NbO 40 4− in the Keggin unit is verified by EXAFS for NbPMo 11 Vpyr and (VO)PMo 11 Nbpyr, 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 NbPMo 11 Vpyr and after activation that removes the pyridinium ions suggests reduction of Mo 6+ to Mo 5+ and Nb 5+ to Nb 4+ . Under hydrocarbon-rich reaction mixtures molybdenum and niobium remain in their reduced state.
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