Rules for Selective Oxidation Exemplified by Methanol Selective Oxidation on Iron Molybdate Catalysts

Abstract

A number of simple rules are proposed which dictate for high selectivity to formaldehyde during methanol oxidation on iron molybdate catalysts. The reaction is of the Mars-van Krevelen type, that is, it is surface lattice oxygen that is the active species. However, we also show that the material is quite a flexible one, in that the bulk oxygen anions become mobile enough at moderate temperatures to exchange rapidly with the surface vacancies that are created during the reaction. Further, and essential to high selectivity, is the fact that Mo is preferentially segregated to the surface layer of the catalyst, with no Fe present there. Finally, it can also be shown that the important oxidation state for the cations at the surface is the highest one (6 ? for Mo), with lower oxidation states being detrimental to selectivity. It is likely that these basic rules are also important for a range of other catalytic processes/catalysts.

Preparation of iron molybdate catalysts for methanol to formaldehyde oxidation based on ammonium molybdoferrate(II) precursor

“…However, the catalyst thus prepared is contaminated with iron(III) oxide as the individual phase which reduces the selectivity of methanol oxidation. It is well established that high selectivity performance of iron(III) molybdate is in contrast to the selectivity performance of Fe 2 O 3 , which shows essentially zero selectivity across the temperature range 200-400°C and the dominant product is always CO 2 [9,11,13,14]. In order to remove the iron oxide a long (up to 48 h) heat treatment of catalyst precursor at 500°C is required [9,10].…”

Partial Oxidation of Methanol on MoO3 (010): A DFT and Microkinetic Study

“…However, even in the absence of gas-phase oxygen, at a sufficiently high temperature, migration of oxygen atoms from the bulk to the surface ensures that the surface remains oxidized, thereby acting as a temporary buffer against surface reduction by methanol . Bowker and coworkers demonstrated the importance of the highest Mo oxidation state in determining selectivity to formaldehyde by contrasting the rates and selectivity on MoO 3 (Mo in +VI state) and MoO 2 (Mo in +IV state) using TPD. , MoO 3 shows complete selectivity to the partial oxidation product, CH 2 O, but when it is placed under anaerobic conditions, it shows a reduced selectivity to CH 2 O. The Mo +VI atoms in MoO 3 under anaerobic conditions are reduced to the +IV oxidation state, possibly also leading to surface reconstruction, which is responsible for the shift in selectivity.…”

Effect of synthesis pH on the structure and catalytic properties of FeMo catalysts

“… 43,44 The strong acid sites improved the ability to break the C–H bond with the absorbed methanol than weak acid sites, which has been generally accepted to be rate determining step. 45 However, the desorption of reaction products will be harder in a strong than a weak acid site. 46 If the acid sites are too strong, the reaction species have enough time to form DME species.…”