Effects of differences in the structures of iron phosphates on the catalytic action in the oxidative dehydrogenation of lactic acid to pyruvic acid

Oxidative dehydrogenation of lactic acid to pyruvic acid was performed at 230°C over iron phosphates, with a P/Fe atomic ratio of 1.2, consisting of different structures and oxidation states: these included FePO4, Fe2P2O7, Fe3(P2O7)2, and a phase designated as Y-phase. The catalysts consisting of both Fe2+ and Fe3+ ions showed a higher activity than FePO4 and Fe2P2O7. During use in the reaction for more than 24 h, FePO4 was transformed to another unknown phase consisting of both Fe2+ and Fe3+ ions, which is designated here as M-phase. The best performances were obtained with the catalyst consisting of M-phase. The optimum oxygen feed rate to form the M-phase is in the range of 10 to 15 mmol/h, when the feed of lactic acid is 20 mmol/h. The M-phase was stable under the reaction conditions used and was not reoxidized by air even at 300°C.