Iron molybdate catalysts for methanol to formaldehyde oxidation: effects of Mo excess on catalytic behaviour

Two iron molybdate catalysts have been prepared by coprecipitation, one stoichiometric (Mo/Fe atomic ratio=1.5) and the other with a typical industrial composition (Mo/Fe atomic ratio=3). Physicochemical characterisation shows that Mo excess brings about an increase in the surface area and deconvolution of NH3 TPD curves evidences some changes in acidity. On the other hand, surprisingly, Mo excess does not cause significant changes in the activity for methanol oxidation per unit surface area. This provides evidence that Fe-2(MoO4)(3) is in fact the active phase of the catalyst. The catalyst with Mo excess leads to higher selectivity for HCHO to the detriment of dimethyl ether selectivity. This result is attributable to lager oxygen availability at the catalyst surface and changes in acidity. The catalytic behaviour of the prepared catalysts was compared with that of an industrial catalyst. It was found that the prepared catalyst with an Mo/Fe composition similar to the industrial one is more active due to larger surface area. (C) 2001 Elsevier Science B.V. All rights reserved.