Efficient and recyclable multi-cationic polyoxometalate-based hybrid catalyst for heterogeneous cyclohexane oxidation with H₂O₂

A polyoxometalate-based organic-inorganic hybrid was prepared by ionic self-assembly of the ionic liquid precursor N,N′-bis-2-aminoethyl-4,4′-bipyridinium dibromide dihydrobromide ([DPyAM]Br₂·2HBr) with the Keggin-structured V-substituted polyoxometalate H₅PMo₁₀V₂O₄₀ (H₅PMoV₂). The composition, electronic and porous structure of the resultant hybrid [DPyAM(H₂)]_1.25PMoV₂ were demonstrated by CHN elemental analysis, ¹H NMR, TG, SEM, XRD, FT-IR, UV-vis, ESR, and nitrogen sorption techniques. Catalytic tests showed that the hybrid was a highly active heterogeneous catalyst for cyclohexane oxidation with hydrogen peroxide, giving a yield of KA oil (a mixture of cyclohexanol and cyclohexanone) of ca. 29.4% with high turnover number (TON) of 2940 using very low catalyst dose (0.01 mol% vs. substrate). The catalyst can be conveniently separated by filtration and reused without observing a significant decrease in activity. The influences of reaction conditions were systematically investigated and a possible catalysis mechanism was proposed for understanding the highly efficient heterogeneous catalytic behavior.