Synergistic combination of graphitic C₃N₄ and polyoxometalate-based phase-transfer catalyst for highly efficient reductant-free aerobic hydroxylation of benzene

A facilely recyclable catalytic system towards liquid-phase reductant-free aerobic oxidation of benzene to phenol is built by simultaneously using graphitic carbon nitride (C₃N₄) and Ch₅PMoV₂. Especially, the hybrid Ch₅PMoV₂ is regarded as a temperature-controlled phase-transfer catalyst that prepared by modifying Keggin-type V-containing polyoxometalate anions (PMoV₂) with choline (Ch) cations. The combined catalyst C₃N₄-Ch₅PMoV₂ shows a high activity with 10.7% phenol yield, 8.94 h⁻¹ turnover frequency (TOF) and superior reusability under the optimized reaction conditions. Full characterizations and analyses including electron spin resonance spectroscopy (ESR), cyclic voltammetry (CV) and density functional theory (DFT) calculation are used to demonstrate the phase-transfer character and tuned redox property of Ch₅PMoV₂. Furthermore, a synergistic catalytic mechanism is proposed and discussed based on the experimental and DFT calculation results.