Selective oxidation of methanol over iron molybdate catalysts

The selective oxidation of methanol to formaldehyde over iron molybdate catalysts has been investigated. It has been shown that when Fe2C 3 is present at the surface CO2 and H2 are observed from surface formates, while neighbouring pairs of molybdena sites leads to the production of formaldehyde and water from surface methoxys. When molybdenum sites are isolated then the surface methoxy is stabilised and a direct pathway to CO and H2 is created. On molybdena rich surfaces the production of CO is observed, but as a secondary oxidation product following the linear pathway: CH3OH → CH2O → CO → CO 2, established by varying bed lengths. Catalysts with addition of small amounts of molybdena added to the surface of Fe2O 3, are similar to those with a low bulk ratio of Mo:Fe showing increased activity over Fe2O3. Selectivity is dictated by the presence of isolated or pairs of molybdena sites, which guide the reaction to the primary products of CO and formaldehyde respectively. Structural analysis showed the phases of a-Fe2O3, (X-MoO3 and a-Fe2(MoO4)3, depending on the ratio of the cations present. Molybdenum has been shown to concentrate at the surface of iron molybdates by reactor results from low ratio catalysts, Raman spectroscopy, XP spectroscopy and STEM/EEL spectroscopy. The normal reaction of iron molybdates is via the Mars-van Krevelen mechanism, so tests were made without the presence of gaseous oxygen. The reduction of the surface layer can occur at temperatures as low as 200°C. At temperatures above 250°C diffusion of lattice oxygen to replace the lost surface oxygen can occur, leading to the production of further oxidised products. If the oxidation state of surface molybdenum drops below +6 then formaldehyde selectivity drops markedly, with direct production of CO and secondary production of CO2 observed