Investigation and structural characterization of crystalline phases in mixed-metal oxide systems for the selective oxidation and ammoxidation of light alkanes

Mixed-metal oxide systems were investigated to determine structural details of crystalline phases that may lead to selective (amm)oxidation catalysis of propane and n-butane. Crystalline phases representing solid solutions were found in the SbVPO and MoVNbTeO systems. Structural details of these phases were refined from synchrotron X-ray powder diffraction (S-XPD) and/or neutron powder diffraction (NPD) data using the Rietveld method. The initial models of the SbVPO phases were taken to be those of the parent compounds. The initial models of the MoVNbTeO phases were determined using selected area electron diffraction (SAED) and high-resolution electron microscopy (HREM) imaging.; Solid solutions found in the SbVPO system are those with a SbOPO 4-type structure and a NASICON-type structure. The SbOPO4 solid solution contains minute amounts of vanadium at the nominal compositions studied here: however, the proposed phase diagram suggests that higher amounts may be present along the compositional line Sb1-xVx OPO4 with 0 ≤ x ≤ 0.25. The NASICON solid solution has the formula Sb2-xVx(PO 4)3 with 0.53 ≤ x ≤ 0.76. Vanadium substitution in both phases results in a complex interplay between metal-oxygen bond lengths, metal-metal repulsions, and angles between polyhedra. A phase diagram is proposed for the SbVPO system.; In the MoVNbTeO system, the structures of the crystalline phases M1 and M2 were refined and the metal distributions and oxidation states in the bulk were determined from site occupancies and bond valence sum calculations. The structure of M1 with the formula unit Mo7.8V1.2NbTe0.937O28.9 was found to be Pba2 (No. 32) with a = 21.134(2) A, b = 26.658(2) A, c = 4.0146(3) A and Z = 4. The structure of M2 with the formula unit Mo4.25V1.36Te 1.81Nb0.39O19.81 was found to be Pmm2 (No. 25) with a = 12.6294(6) A, b = 7.29156(30) A, c = 4.02010(7) A and Z = 4. Bulk bond valence sums for each phase were found to be consistent with electron paramagnetic resonance spectra. X-ray photoelectron spectroscopy suggests that the surfaces of both phases are oxidized relative to the bulk. Whereas the chemical topology of M1 provides possible active sites for propane ammoxidation, M2 may provide suitable sites for propene ammoxidation.