Abstract
The structure of the orthorhombic phase in the MoVNbTeO propane ammoxidation catalyst system has been characterized and refined using a combination of TEM, synchrotron X-ray powder diffraction (S-XPD), and neutron powder diffraction (NPD). This phase, designated as M1 by Ushikubo et al. [1], crystallizes in the orthorhombic space group Pba2 (No. 32) with a = 21.134(2) A, b = 26.658(2) A, and c = 4.0146(3) A. The formula unit is Mo7.5V1.5NbTeO29. Bond valence sum calculations indicate the presence of d 1 metal sites neighbored by d 0 metal sites. The d 1 sites are occupied by a distribution of Mo5+ and V4+, whereas the d 0 sites are occupied by a distribution of Mo6+ and V5+. Out-of-center distortions in d 0 octahedra are consistent with the second-order Jahn–Teller effect and lattice effects. We argue that the V5+–O–V4+/Mo5+ moieties adjacent to Te4+ and Mo6+ sites in the [001] terminal plane provide a spatially isolated active site at which the selective ammoxidation of propane occurs.
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