Ionic-liquid-functionalized polyoxometalates for heterogeneously catalyzing the aerobic oxidation of benzene to phenol: Raising efficacy through specific design

By combining nitrile-tethered pyridinium-based ionic liquid dication with the polyoxometalate anion of Keggin H₅PMo₁₀V₂O₄₀ (PMoV₂) through precipitation in aqueous solution, an organic-inorganic hybrid catalyst 1,1′-(butane-1,4-diyl)-bis(3-cyanopyridine)-PMoV₂ was prepared for heterogeneous hydroxylation of benzene to phenol with O₂ and ascorbic acid as the oxidant and reductant, respectively. The hybrid catalyst not only gave the high phenol yield of 13.0% under the optimized reaction conditions, but also exhibited high potential for reusability. The catalyst and its analogue samples were characterized by elemental analysis, Fourier transform infrared spectroscopy, electron spin resonance spectroscopy, X-ray diffraction, scanning electron microscopy, and nitrogen adsorption-desorption analysis. The structure-activity relationship is discussed according to the characterization and density functional theory calculation results.