New insights on the selective oxidation of methanol to formaldehyde on FeMo based catalysts

The selective oxidation of methanol has been studied in detail, with particular focus on gaining insights into the surface active sights responsible for directing the selectivity to formaldehyde. Various Fe and Mo containing oxides have been investigated for their reactivity with methanol, to gain an understanding of the different roles of these components in the industrial catalyst employed, which is a mixed phase comprised of MoO3 and Fe2(MoO4)3. Catalysts have primarily been tested through using TPD (temperature programmed desorption) and TPPFR (temperature programmed pulsed flow reaction). The reactivity of Fe2O3 is dominated by combustion products, with CO2 and H2 produced via a formate intermediate adsorbing at the catalyst surface. For MoO3 however, the surface is populated by methoxy intermediates, so that the selectivity is almost 100 % directed to formaldehyde. When a mixture of isolated Fe and Mo sites co-exist, the surface methoxy becomes stabilised, resulting in a dehydrogenation reaction to CO and H2. CO and CO2 can also be observed on Mo rich surfaces, however here a consequence of the further oxidation of formaldehyde, through a linear pathway. TPD and DRIFTS identify these intermediates and products forming. Since the structure of the industrial catalyst is relatively complex, in that it contains both MoO3 and Fe2(MoO4)3, it is difficult to identify the active site for the reaction with methanol. A novel approach to understanding this further, has involved the synthesis of a series of MoOx modified Fe2O3 catalysts in an attempt to make core-shell oxidic materials of the type MoOx/Fe2O3. Various monolayer loadings are investigated. It is conclusively shown that for all coverages the Mo stays in the surface region, even after annealing to high temperatures, only reacting with the iron oxide surface when the material is annealed above 400 C. From drying at 120 C to calcining at 500 C, the Mo converts from a MoO3-like octahedral layer to Fe2(MoO4)3, with Mo in a tetrahedral structure. Although changes in the Mo phase are clearly evident, it is shown that for all catalysts a one monolayer equivalent of amorphous octahedral MoOx also remains at the surface, regardless of the calcination temperature employed. It is this layer which is deemed as the surface active layer, since all catalysts at varying monolayer overages and anneal temperatures show a similar reaction with methanol. This overlayer is unique, and is suggested to be comparable to the surface terminating layer in bulk catalysts such as Fe2(MoO4)3. Successive work involved studying the reactivity of this upper layer, with suggestions of a two site Mo-Mo surface species forming on adsorption of methanol. Concluding work involves an investigation into the redox properties of Fe2(MoO4)3, to address the significance of this mixed oxide in commercial materials. Fe2(MoO4)3 forms the majority of the industrial catalyst, and although it shows a superior performance in terms of its activity, it cannot compete with the near 100 % selectivity of MoO3 to formaldehyde. Other supports have been trialled for their performance under reaction with methanol. It is shown that Fe2(MoO4)3 has increased bulk lattice oxygen mobility. Under normal reaction conditions, the reaction is carried out aerobically. However if oxygen supplies are restricted, Fe2(MoO4)3 is able to demonstrate a satisfactory performance above 300 C, as lattice oxygen is able to replace lost surface oxygen. This can continue for some time, until reduced phases containing Mo(IV) form. At this point formaldehyde selectivity drops, matched by a rise in CO production. High oxidation states are crucial to catalyst performance, with the reaction continuously cycling between Mo(VI) and Mo(VI), with a very short lifetime for the Mo(IV) species.