A comparison of iron molybdate catalysts for methanol oxidation prepared by copreciptation and new sol-gel method

Abstract

Iron molybdates and silver catalysts are nowadays equally used as industrial catalysts for formaldehyde production. Many metal molybdates catalyze the reaction under consideration and the active sites are widely believed to be associated with surface Mo atoms in octahedral co-ordination. Octahedral co-ordination of Mo is only achieved in Fe-defective iron molybdates that is in accordance with the fact that maximum activity is obtained with catalysts with Mo/Fe atomic ratio greater than 1.5. Iron molybdates, with atomic ratio Mo/Fe=3, were prepared by coprecipitation and sol-gel techniques in acid medium. Characterization results show that sol-gel catalysts have much higher surface areas than coprecipitated catalysts. Several methods have been used to prepare iron molybdates, the most part of them based on coprecipitation techniques. Previous studies have evidenced that the catalytic behavior of Mo–Fe oxides depend on many variables of the coprecipitation procedure: starting compounds, concentration of parent solutions, pH and temperature of coprecipitation step, order of addition of parent solutions, ripening, etc. In principle, any metal that forms stable oxides can be forced to copolymerise with other metals in sol-gel procedures to provide mixed metal oxides. To investigate the influence of preparation methods on catalytic properties of iron molybdates catalysts with controlled excess of MoO3 were prepared by coprecipitation and solgel techniques. Their properties and performances were compared with an industrial catalyst.