The Selective Oxidation of Methanol to Formaldehyde on Iron Molybdate Catalysts and on Component Oxides

Abstract

The reaction of methanol with an industrial iron molybdate catalyst, and with Fe₂O₃ and with MoO₃, has been investigated with a pulsed flow reactor and temperature-programmed desorption (TPD). The molybdena-based samples show only formaldehyde in TPD as the carbon-containing product, arising from the decomposition of a surface methoxy species. In contrast, haematite yields no formaldehyde, only CO₂ and H₂, which evolve coincidently at 290 °C, and indicates the presence of a formate intermediate on the surface. In turn, the reactor work shows high selectivity to formaldehyde for the molybdate materials and zero for haematite. The iron molybdate sample is more active than the molybdena, conversion beginning at 150 °C for the former and 270 °C for the latter. These data are discussed in terms of a global mechanism for the reaction and a tentative reaction enthalpy profile is proposed. The main differences between the iron and molybdenum samples arise from the stronger binding of oxygen in the former and the higher concentration of cation sites.