Iron molybdate and other metal oxide catalysts

2 Iron molybdate and other metal oxide catalysts 

The use of iron molybdate in industrial plants started about I960. Yields of about 90% are reported for this process, applying either excess air or excess methanol and recirculation of the latter. Carbon dioxide is the chief by-product. 

Kinetic investigations have appeared in the literature since 1965. A redox mechanism is generally accepted [254], and has been confirmed by pulse experiments which demonstrated the equal activity of the catalyst in the presence and absence of oxygen. The results of Pernicone [254] and Liberti et al. [187] seem to indicate that the rate-determining step is either hydrogen abstraction from methanol or desorption of formaldehyde. 

Another property of the iron-defective molybdate is the presence of Mo=0 double bonds on the surface. The hydrogen-abstracting capacity of the catalyst is closely related to Mo6 contained in the Mo=0 as is shown in Sect. 3. There the role of iron is also discussed. It is, however, interesting to note here that pure iron oxides accelerate combustion and that a W03—Fe2(W04)3 catalyst is practically inactive [254], Replacement of iron by chromium is possible but leads to a lower activity [253]. Baussart et al. [46] prepared stoichiometric NiMo04 which showed selective behaviour towards formaldehyde in a pulsed column below 375°C. 

There is considerable evidence that surface acidity influences the catalytic activity of iron molybdate [254]. It was found by studying the adsorption of ammonia using infrared spectroscopy that, under reaction conditions, the acidity is due to Lewis sites. The conclusion is that surface acidity is a necessary, but not a sufficient, property. 

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