Structural Characterization of the Orthorhombic Phase M1 in MoVNbTeO Propane Ammoxidation Catalyst

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) Å, b = 26.658(2) Å, and c = 4.0146(3) Å. The formula unit is Mo_7.5V_1.5NbTeO₂₉. 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.