Reusable citric acid modified V/AC catalyst prepared by dielectric barrier discharge for hydroxylation of benzene to phenol

Abstract

Direct hydroxylation of benzene with a high selectivity and durability for phenol production is still a challenge. Herein, a reusable V_xO_y/AC(cit)-DBD catalyst was synthesized using a facile and environmentally benign method in which a mixture of citric acid, NH₄VO₃ and activated carbon (AC) was treated using dielectric barrier discharge (DBD) at room temperature and under an N₂ atmosphere. Compared with V_xO_y/AC-DBD, which is prepared without citric acid, more vanadium species were exposed on the surface of V_xO_y/AC(cit)-DBD. The residues of citric acid formed by DBD treatment can stabilize the vanadium species on the surface of AC by restraining the leaching of vanadium from the catalyst during the reaction. Moreover, the addition of citric acid increased the ratio of V⁴⁺/V⁵⁺ and benzene adsorption on the catalyst surface, which are favorable for the hydroxylation of benzene. Additionally, the resulting vanadium particles are smaller (less than 10 nm) and amorphous owing to the inhibiting effect of citric acid residues on particle growth and the low temperature characteristics of DBD treatment. Under the optimized reaction conditions, V_xO_y/AC(cit)-DBD showed a good stability and recyclability in the hydroxylation of benzene and achieved a benzene conversion of 21.5% and phenol selectivity of 99.1%.