Characterization of iron counter-ion environment in bulk and supported phosphomolybdic acid based catalysts

Extended X-ray absorption fine structure (EXAFS) was used to characterize the environment of iron counter-cations in Keggin type phosphomolybdic compounds used as catalysts for oxidation reactions. Iron doped compounds corresponding to bulk acid and to acid supported on the cesium salt were prepared and studied. Iron formed hexa-hydrated complex of both Fe3+ and Fe2+ in the bulk acid, whereas it was present as Fe(OH)2+ hydroxy-cations in the acid supported on the cesium salt. Upon heating the hexa-hydrated complex lost one molecule of water to bind to the Keggin anion through a terminal oxygen. (Fe-O-Mo bond). The environment of the iron hydroxy-cation changed upon heating while its closer coordination append to remained unchanged.