Methane is activated at moderate temperature on polyoxometalates, leading to the evolution of hydrogen and the formation of a methoxy species which has been characterized by solid-state CP-MAS NMR. In the case of a molybdic polyoxometalate, the methyl group is linked to an edge-shared oxygen atom of the polyoxometalate. Upon heating, it reacts with oxygens of the polyoxometalate resulting in the formation of formyl species and then carbon dioxide and a reduction of molybdenum. Upon treatment with water, only traces of methanol can be detected. 1. Introduction Interest of the scientists in the direct methane conversion does not seem to falter as the 21st century world faces the perspective of dwindling petrol supplies [1, 2]. However, an economically feasible methane valorization system by its partial oxidation to methanol or formaldehyde, though tempting, remains still rather an elusive perspective. Indeed, methane is notorious for its chemical inertness, justified by its symmetry and high C–H primary bond stability (440？kJ·mol？1). Unfortunately, once activated, it is rare to obtain the partial oxidation products as they are more reactive than the substrate itself and the reaction proceeds rapidly leading to a CO2 release. Generally, redox properties are claimed to be responsible for the catalysts performance in oxidation reactions [3] although the acid-base strength was suggested to play a crucial role in the C–H activation initial step [4]. As the methane “neutral” chemical character makes it easily adaptable to activator’s properties, various C–H dissociation paths were evidenced [5–7]. If the task was not complicated enough, another trend should also be taken into account when developing catalytic oxidation systems—while chemistry becomes more and more environmentally aware, combinations of noble metal compounds and highly toxic reaction media [8–10] are bound to give way to “greener” and cheaper alternatives. Polyoxometalates seem a reasonable choice in this respect. The polyoxometalates constitute a well-known class of compounds, based on transition metals (Mo, W, V, and to a lesser degree Ta and Nb) and oxygen, and could be perceived as discreet analogues of metal oxide surfaces [11]. They are widely used in homogenous and heterogenous catalyses, due to their pronounced and tunable acidic and redox properties. As they are easily soluble in polar solvents, they can be used in a lot of homogeneous reactions involving polar molecules. On the other hand, there are only very limited examples of applications of pure solid heteropolyoxometalates in
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