MoO3Fe2(MoO4)3 catalysts for methanol oxidation

Abstract The present state of research on the FeMo oxide catalyst is summarised. The preparation of the catalyst is described and the nature of the precipitated amorphous, hydrous iron molybdate is discussed. After thermal dehydration and crystallisation a Fedefective iron molybdate is obtained, as shown by X-ray diffraction studies. The preparation of other trivalent metal molybdates is also discussed. The results of our kinetic studies are summarised and compared with those of other research groups. A reaction mechanism is proposed that satisfactorily accounts for the kinetic data. Surface acidity is emphasised as a most important property of the FeMo oxide catalyst. A correlation was found between surface acidity and catalytic activity. I.r. studies showed this acidity to be connected with Lewis sites at the usual reaction temperatures. Such sites, where methanol chemisorption should take place, can be described as anionic vacancies produced by dehydroxylation. Evidence is reported that suggests that these acid sites are connected with Mo6+ ions. Finally the role of Fe3+ and Mo6+ ions in this oxidation is discussed. It seems that the function of Mo6+ ions, in octahedral coordination, should be that of providing methanol chemisorption sites able to direct the subsequent oxidation towards formaldehyde, while the presence of Fe3+ ions should make the dehydroxylation of the catalyst surface easier, thus increasing the concentration of methanol adsorption sites in stationary conditions. E.p.r. studies showed that Fe3+ ions are reduced to Fe2+ by methanol; however the reoxidation of Fe2+ ions is slower, so that the redox process should be carried out by the Mo ions, Fe2+ ions probably being present at the surface.