In situ studies of atomic, nano- and macroscale order during VOHPO₄·0.5H₂O transformation to (VO)₂P₂O₇

Transformation of VOHPO₄·0.5H₂O precursor to well-crystallized (VO)₂P₂O₇, for n-butane oxidation to maleic anhydride was studied by in situ Raman and XRD techniques. Atomic scale changes observed in the precursor structure at 583 K provided new insights into its transformation to (VO)₂P₂O₇. In addition to (VO)₂P₂O₇, nanocrystalline oxidized δ-VOPO₄ invisible to XRD was detected during transformation in n-butane/air, possibly due to the specificity of the in situ conditions. Under catalytic reaction conditions the disordered nanocrystalline (VO)₂P₂O₇ in the fresh catalysts (ca. 10-20 nm domains) gradually transformed into well-crystallized (VO)₂P₂O₇ in the equilibrated VPO catalysts (>30 nm domains) with time on stream. Simultaneously, a disordered layer ca. 2 nm thick which was covering the surface (100) planes of (VO)₂P₂O₇ disappeared yielding a solid with high steady-state catalytic performance. Only (VO)₂P₂O₇ was observed both at room temperature and reaction temperature in the equilibrated VPO catalysts. Specific surface termination of the (100) planes of (VO)₂P₂O₇ in the equilibrated VPO catalysts is believed to be responsible for high activity and selectivity of these catalysts for maleic anhydride formation.