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.
All Science Journal Classification (ASJC) codes
- Process Chemistry and Technology
- Physical and Theoretical Chemistry