The benefits of Dawson heteropoly compound reorganisation in oxidation catalysis

Heteropoly compounds are multifunctional materials of growing importance in catalysis. However, they are unstable materials under oxidation reaction conditions as they tend to transform into oxides. Taking advantage of such instability is a challenge. This work aims at determining the genesis and the evolution of the structural rearrangement that Dawson (NH4)6P2Mo18O62 heteropoly compounds undergo when submitted to the conditions of oxidation reactions. Dawson heteropoly compounds, along their decomposition pathway to a Keggin structure, generate more active species which possess the ability to form quite selectively methyl ethyl ketone from 2-butanol in the presence of O2. The redox properties of the gaseous feed are shown to influence this reorganisation and thus the formation of the truly active species. The evolution of the Dawson structural rearrangement is studied by operando Raman investigation. During the oxidation of propylene, Dawson heteropoly compounds rearranged into a not-so-well-defined Mo trioxide phase which in turn operated more efficiently for the reaction. This state was obtained when the working catalyst was in the course of rearranging between a Keggin structure and that of a Mo trioxide phase. Regarding supported Dawson heteropoly compounds on TiO2, active species for the total oxidation of propylene were formed during the Dawson rearrangement on the TiO2 support. This enhancement in total oxidation is related to the favourable development of the distribution of reduced and oxidised Mo in the supported samples. Dawson heteropoly compounds are thus promising catalysts for promoting a specific oxidation reaction provided that their a priori undesirable rearrangement during the catalytic process be adequately mastered.