EFFECTS OF CONSECUTIVE OXIDATION ON THE PRODUCTION OF MALEIC-ANHYDRIDE IN BUTANE OXIDATION OVER 4 KINDS OF WELL-CHARACTERIZED VANADYL PYROPHOSPHATES

Factors determining the selectivity of butane oxidation at high conver sion levels have been examined by using four kinds of well-characteriz ed vanadyl pyrophosphate catalysts(C-1-C-4) in kinetic experiments. Th e catalysts were carefully characterized by scanning electron microsco py, X-ray powder diffraction, X-ray photoelectron spectroscopy, and in frared spectroscopy and were made of a single crystalline phase of van adyl pyrophosphate, (VO)2P27. C-1 was prepared by reduction with NH2OH .HCl and consisted of large particles (5 mum) and small particles (0.2 mum). The particles of C-2 obtained from V2O4 had a size of 2 mum. C- 3, which was obtained by an organic solvent method, showed a rose-like structure. C-4 from VOPO4.2H2O had a large plate-like structure (5 mu m). While all of the catalysts exhibited similar selectivities for the formation of maleic anhydride (63-72%) at low conversion levels, the extent of selectivity decreases with an increase in the conversion and strongly depends on the catalysts. It also correlates oppositely with the catalytic activity for the oxidation of maleic anhydride, measure d separately. This indicates that the consecutive oxidation of product maleic anhydride is a crucial factor for the selectivity at high conv ersions. A simulation using a model that includes the consecutive oxid ation of maleic anhydride, in which the experimental rate constants fo r the oxidations of butane and maleic anhydride have been used, reprod uced the selectivity-conversion curves experimentally observed.