Role of surface atomic arrangements of well defined phosphates in partial oxidation and oxidative dehydrogenation reactions

Abstract

Various vanadium and iron phosphates have been synthesized and characterized physically before and after catalytic reaction has taken place. Vanadium phosphate was used for butane oxidation into maleic anhydride and iron phosphate for isobutyric acid dehydrogenation to methacrylic acid. Vanadium phosphate catalyst in its working state corresponds to a mixture of the pyrophosphate phase (VO)₂P₂O₇ and a given VOPO₄ phase (α_II, β or δ). ³¹P MASNMR spectra showed that the most efficient catalysts contained VOPO₄ entities in interaction with (VO)₂P₂O₇, while an X-ray electronic radial distribution study showed that VOPO₄ entities are present but not necessarily organized in sufficiently long-range order to be detected by classical X-ray diffraction. The iron phosphates Fe₂P₂O₇, FePO₄ and Fe₇(PO₄)₆ were transformed under catalytic reaction into Fe₇(PO₄)₆ and an unknown phase. The latter phase was tentatively assigned from Mössbauer spectroscopy to Fe₆P₆O₂₃ with an Fe³⁺/Fe²⁺ ratio equal to two. The best catalysts including the industrial catalyst were those containing the Fe₆P₆O₂₃ phase. A similarity has been found in the behaviour of vanadium and iron phosphates. The best catalysts correspond to those which exhibit short- or long-range order between the reduced pyrophosphate phase and a more oxidized form as VOPO₄ or Fe₆P₆O₂₃. It is suggested that the ease of transformation of two PO₄ groups, at least locally, into one P₂O₇ group is the clue to obtaining an efficient catalyst. This is a case of topotactic transformation.