Mars-van-Krevelen Mechanism in Anaerobic and Aerobic Reaction of Bimetallic Polyoxometalates: Theory and Experiment in Concert

Molecular oxygen is a very attractive oxidant for most industrially important reactions. However, its direct interaction with organic compounds, particularly in oxidation of hydrocarbons, is characterized by low selectivity and tendency to over-oxidation. Selective liquid phase activation and oxidation of hydrocarbons could be achieved in presence of polyoxometalates. In such reactions, substrates are dehydrogenated or oxygenated by oxygen atoms from the catalyst bulk, and then the reduced catalyst is reoxidized by molecular oxygen. This spatiotemporal separation between the substrate oxidation and the catalyst regeneration is responsible for high selectivity of such reactions.

It has been shown that phosphovanadomolybdates, notably the H₅PV₂Mo₁₀O₄₀ polyoxometalates of the α-Keggin structure, conduct oxidative dehydrogenation and oxygenation of various substrates by an electron transfer−oxygen transfer type mechanism. There are three key steps to these reactions: (1) initiation of substrate activation by electron transfer (ET) to the polyoxometalate; (2) a sequence of reactions leading to dehydrogenation of the substrate or to oxygen transfer from the polyoxometalate to the substrate with formation of the oxygenated product; and (3) the reoxidation of the reduced catalyst with O₂.

In this presentation, we will discuss our recent results on the mechanisms of oxygenation and oxidative dehydrogenation of strong sp²- and sp³-C-H bonds in anaerobic conditions followed by aerobic reoxidation of the reduced polyoxometalates. While an outer-sphere mechanism of the reoxidation reaction is possible for several polyoxometalates, we will show that both reduction and oxidation of phosphovanadomolybdates follow the Mars-van-Krevelen mechanism typical for solid metal oxides.