Highly Dispersed Vanadia Anchored on Protonated g-C3N4 as an Efficient and Selective Catalyst for the Hydroxylation of Benzene into Phenol

The direct hydroxylation of benzene is a green and economical-efficient alternative to the existing cumene process for phenol production. However, the undesired phenol selectivity at high benzene conversion hinders its wide application. Here, we develop a one-pot synthesis of protonated g-C3N4 supporting vanadia catalysts (V-pg-C3N4) for the efficient and selective hydroxylation of benzene. Characterizations suggest that protonating g-C3N4 in diluted HCl can boost the generation of amino groups (NH/NH2) without changing the bulk structure. The content of surface amino groups, which determines the dispersion of vanadia, can be easily regulated by the amount of HCl added in the preparation. Increasing the content of surface amino groups benefits the dispersion of vanadia, which eventually leads to improved H2O2 activation and benzene hydroxylation. The optimal catalyst, V-pg-C3N4-0.46, achieves 60% benzene conversion and 99.7% phenol selectivity at 60 oC with H2O2 as the oxidant.