Reduction and reoxidation of iron phosphate and its catalytic activity for oxidative dehydrogenation of isobutyric acid

The freshly calcined iron phosphate catalysts with a P/Fe atomic ratio of 1.0 similar to 1.3 consist of amorphous phase, quartz-type FePO4 and/or tridymite-type FePO4 depending on the calcination temperature, though they contain only Fe3+ ions. In the absence of oxygen, the P/Fe > 1.2 catalyst samples are reduced with hydrogen or isobutyric acid to form Fe2P2O7 via Fe-3(P2O7)(2). While the P/Fe = 1.0 samples are reduced to Fe2P2O7 not passing through Fe-3(P2O7)(2). Fe2P2O7 is oxidized with air to an unknown compound (Y phase), but not to Fe-3(P2O7)(2) nor FePO4. The Y phase is reduced to Fe2P2O7. During the use in the oxidative dehydrogenation of isobutyric acid, the freshly calcined catalysts are reduced to Fe-3(P2O7)(2), the Fe2P2O7 catalysts are in part oxidized to the Y phase, and the Y phase is in part reduced to Fe2P2O7, though the rates are very slow. The catalytic activity and selectivity are scarcely affected with the variation in the structure of the iron phosphate catalyst and in the oxidation states of iron ions. (C) 1996 Academic Press, Inc.