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

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 (αII, 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 Mossbauer 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.