Crystallographic Studies in the V-P-As-O System

Abstract

<p> The crystal structures of VOP2SiO8, VO(PO3)2, V(PO3)3, (VO)2P2O7 and VAsO5 have been determined with the help of x-rays, and are compared with the known structures in the V-P-As-O system. All the vanadium atoms in the mixed oxides are octahedrally coordinated and the V^+4 O6 and V^+5 O6 octahedra are all characterized by one short vanadyl bond. V^+3 O6 groups are nearly regular. The tetrahedral phosphorus is found in structural elements ranging from infinite metaphosphate chains (V(PO3)3, VO(PO3)2), to pyrophosphate groups ((VO)2P2O7) to isolated tetrahedra (VAsO5 and VOP2SiO8). Both structural and substitutional disordering is evident in the V-P-As-O system, and is discussed together with a detailed model for stacking faults in VAsO5.</p> <p> Some of the phases in the V-P-As-O system are known to catalyze the oxidation of butene to maleic anhydride, and certain structural features of the compounds are related to this catalytic activity. α-VPO5 can be related to (VO)2P2O7 through the formation of shears in a manner similar to shear formation in V2O5, and such a mechanism is proposed as a means whereby an α-VPO5 catalyst can change into (VO)2P2O7, the known composition of the spent catalyst.</p>