Molecular description of active sites in oxidation reactions: Acid-base and redox properties, and role of water

Oxidation reactions in heterogeneous catalysis usually involve a Mars and van Krevelen mechanism which includes activation of the substrate on a metallic cation, insertion of oxygen from lattice oxygen ions, a redox mechanism on the catalyst surface, and the transfer of several electrons. It follows that such a reaction necessitates both acid-base and redox properties of a catalyst the acid site being of Lewis type (cations) and the basic sites being the surface O^2- or OH^- species which could exhibit electrophilic or nucleophilic properties.

The active site should be able to fulfil the following requirements: H abstraction from the substrate, oxygen insertion, and electron transfer. It has been shown to correspond to an ensemble of atoms of limited size in an inorganic molecular complex. It could correspond to local structural defects including steps, kinks, coordinatively unsaturated cations or to clusters of atoms on the surface. Some examples are described namely:

1. (i) n-butane oxidation to maleic anhydride on (VO)₂P₂O₇ catalyst where four dimers of vanadyl cations on the (100) face were suggested to form the active site;

2. (ii) isobutyric acid oxidative dehydrogenation to methacrylic acid on iron hydroxy phosphates where trimers of iron oxide octahedra were shown to constitute the most efficient and selective catalytic site while water was observed to be absolutely necessary to facilitate the reaction which corresponds to hydroxylated surface sites ensuring the redox mechanism;

3. (iii) propane oxidative dehydrogenation to propene on VMgO samples which was shown to depend both on VO_x arrangements with respect to MgO and on the basicity of the material induced by MgO while vanadium cations induced acidic features.