Moessbauer spectroscopic study of iron phosphate catalysts used in selective oxidation

Three iron phosphate compounds FePO4, Fe2P2O7 and Fe7(PO4)6 have been synthesized and studied as catalysts for the oxidative dehydrogenation of isobutyric acid to methacrylic acid. Moessbauer spectroscopic characterization of the solids before and after catalytic reaction has allowed us to show that the starting phases were transformed in the conditions of reaction and that a new phase was formed. This new phase is a mixed Fe3+ and Fe2+ phosphate with a Fe3+/Fe2+ ratio of 1.8 ± 0.2 with Moessbauer parameters: δ1 = 0.47 ± 0.05, Δ1 = 0.68 ± 0.02 mm.s-1 and δ2 = 1.20 ± 0.05, Δ2 = 2.73 ± 0.02 mm.s-1. This new phase appears to be selective for methacrylic acid formation. (orig.)