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

Abstract

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.

Selectivity determinants for dual function catalysts: applied to methanol selective oxidation on iron molybdate

“…At very high conversion and temperature CO 2 is seen, and can become dominant, but is likely to be because of secondary CO oxidation. This behaviour is similar to what is observed for 1.5: 1 Mo:Fe catalysts, and for MoO 3 itself, which are highly selective to formaldehyde over a wide temperature range, 4,8,13 though the conversion for MoO 3 itself is much lower than for iron molybdate and only reaches 100% at much higher temperatures. 4,13 Data for iron oxide are in contrast to those in Fig.…”

“…7 is a function of bulk loading and we do not at this point know if the latter is the same as the surface concentration of Mo. Indeed, we 4,8,10 and others 5 -7,11,12 have shown that there is a strong tendency of Mo to segregate to the surface of these materials, that is, the surface concentration is higher than the bulk concentration. This may especially be important for the very low mole fraction of Mo in Fig.…”

“…4,8 -10 This is the main reason that they are so selective, since we 4,13,15 and others 16 Figure 3 shows a result to indicate the kinds of reaction profile which we see as a function of temperature for methanol oxidation on ferric molybdate (in this case Mo:Fe 2.2: 1). Here, is seen that conversion begins at , 1608C, and there is some evolution of dimethyl ether at very low conversion.…”

“…3 The latter reaction is carried out on either Ag or iron molybdate catalysts, and for the latter case it has been found that there is a very strong dependence of the product selectivity on the cation ratio in the bulk. 3,4 In turn, these changes are then because of changes in that ratio in the top layer, which is the active layer for reaction with the gas phase. This has lead to a few studies where researchers have made surface doped materials in the hope of learning what is the active configuration and what it is that makes that special for high selectivity to formaldehyde.…”

A Combined Multi-Technique In Situ Approach Used to Probe the Stability of Iron Molybdate Catalysts During Redox Cycling

“…Interestingly however, on switching to air we observed a large increase in the formaldehyde production. As there was no methanol being introduced, the source of formaldehyde is likely to be surface-bound methoxy species [25,70,71]. This sudden increase in formaldehyde production in the absence of Fe 2 (MoO 4 ) 3 suggests that in these circumstances, the oxygen necessary to complete the reaction cycle leading to formaldehyde formation must have come directly from the gas phase.…”