Redox processes of surface of vanadyl pyrophosphate in relation to selective oxidation ofn-butane

Abstract

Oxidation and reduction processes of the surface of vanadyl pyrophosphate ((VO)2P2O7) have been studied by means of Raman spectroscopy, X-ray photoelectron spectroscopy, thermogravimetry, and “micro-pulse” reaction ofn-butane in relation to the selective oxidation ofn-butane. Oxidation of (VO)2P2O7 by O2 was carried out to controlled extents by changing the reaction temperature and period. When (VO)2P2O7 was oxidized at 733 K for 2 h, about one layer of the surface of (VO)2P2O7 was oxidized to the X1 phase, which was reported previously by the authors, as deduced from the amount of O2-uptake and spectroscopic changes. Reductions of the surface X1 phase and β-VOPO4 were carried out by a “micro-pulse” reaction with n-butane at 733 K. The reduction byn-butane changed the surface X1 phase to (VO)2P2O7 and, at the same time, gave maleic anhydride with selectivities ranging from 13 to 40%. The selectivity was less than 10% in the case of β-VO)2PO4. These results suggest that the redox cycle between the X1 phase and (VO)2P2O7 in the surface layer is involved in the catalytic oxidation ofn-butane to maleic anhydride.