ROLE OF CESIUM IN IRON PHOSPHATES USED IN ISOBUTYRIC ACID OXIDATIVE DEHYDROGENATION

Iron diphosphate (Fe2P2O7) and cesium/iron diphosphate (CsFeP2O7) are known to be present in an industrial and complex iron cesium phosphate catalyst used in the oxidative dehydrogenation of isobutyric acid into methacrylic acid. It is shown that pure CsFeP2O7 presents poor catalytic selectivity for this reaction and furthermore that intimate mixtures of CsFeP2O7 and Fe2P2O7 do not exhibit any appreciable synergy effect for the same reaction, at variance with what is often encountered for multicomponent-type catalysts. However, it is observed that the presence of cesium during the preparation of the industrial-type catalyst favors the formation of a tridymite-type structure of FePO4 with respect to the stable quartz-type structure. This tridymite phase, which has never been characterized before, can be prepared in an almost pure state (95%) by using a P/Fe ratio of up to 1.32 in the preparation and in the presence or absence of cesium. These conditions, under activation or catalytic use, favor the formation of a new phase: Fe2(3+)Fe2+ (P2O7)2, which is postulated to be the most active and selective phase in its hydroxylated and partly oxidized form for methacrylic acid formation.