Effect of Varying the Cation Ratio within Iron Molybdate Catalysts for the Selective Oxidation of Methanol

The adsorption and reaction of methanol with iron molybdate catalysts of widely varying Mo/Fe ratio have been investigated using X-ray diffraction (XRD), Raman, temperature-programmed desorption (TPD), and pulsed flow reactor measurements. From these data, we can conclude that Mo is segregated to the surface of all catalysts, even that with only a 1:50 Mo/Fe ratio in the bulk of the sample. There is a very marked difference between the behavior of pure ferric oxide and catalysts with even a small amount of Mo present. The former produces only CO2 as the product of methanol oxidation with no formaldehyde production, yet with only small amounts of Mo in the preparation the sample is already highly selective for formaldehyde. TPD shows the reason for this. For iron oxide, the methanol adsorbs to form a formate intermediate, resulting in CO2 evolution, whereas after addition of Mo the main intermediate seen is the selective one, that is, the methoxy intermediate, which decomposes to produce formaldehyde in the gas phase upon heating. However, the best catalysts for the reaction are still those with a ratio of Mo/Fe of greater than 3:2, even though this is the ratio for single phase ferric molybdate, Fe2(MoO4)3. This is because, although the low Mo catalysts have high selectivity at low temperature, they decline at high temperature, producing mainly CO as the product. This effect is strongly Mo-loading dependent. The surface of all the Mo-loaded catalysts manifest the behavior of MoO3 itself, implying that the surface is greatly enriched in Mo, even for those with very low loadings.