Synthesis of V-containing Keggin polyoxometalates: Versatile catalysts for the synthesis of fine chemicals?

Vanadium-containing polyoxometalates (POM): H4PVMo11O40, H5PV2Mo10O40 and H6PV3Mo9O40 were synthesized by two distinct procedures and characterised by XRD, 31P MAS-NMR, FT-IR, BET, UV–vis and SEM. These vanadium-substituted POM were shown to be able to oxidize benzene into phenol with nearly 100% selectivity in the presence of H2O2. Furthermore, other aromatics were also oxidised to the corresponding phenols or ketones at appreciable degrees of conversion. However, the question of their stability toward leaching arises and has been investigated. While, there appears to be a correlation for liquid-phase reactions between the yields toward oxygenates and the vanadium content in the Keggin structure, we were also able to observe a decrease in the stability of these POM after successive catalytic runs. Furthermore, POM crystals having a regular size and shape, with peculiar orientation have been prepared and hence exhibited the highest catalytic activity.