Direct Hydroxylation of Benzene to Phenol with Molecular Oxygen over Pyridine-modified Vanadium-substituted Heteropoly Acids.

Pyridine(Py)-modified Keggin-type mono-vanadium-substituted heteropoly acids (Py _n PMo₁₁V, n = 1–4) were prepared by a precipitation method as organic/inorganic hybrid catalysts for direct hydroxylation of benzene to phenol in a pressured batch reactor and their structures were characterized by FT-IR. Among various catalysts, Py₄PMo₁₁V exhibited the highest catalytic activity (yield of phenol 9.0%) with the high selectivity for phenol, without observing the formation of catechol, hydroquinone and benzoquinone in the reaction with 80 vol% aqueous acetic acid, molecular oxygen and ascorbic acid used as the solvent, oxidant and reducing reagent, respectively. The influences of the reaction temperature, the pressure of oxygen, the amount of ascorbic acid, the amount of catalyst, and the reaction time on the yield of phenol were investigated to obtain the optimal reaction conditions for phenol formation. Pyridine can greatly promote the catalytic activity of the Py-free catalyst (H₄PMo₁₁VO₄₀), mostly because the organic π electrons in the hydrid catalyst may extend their conjugation to the inorganic framework of heteropoly acid and thus dramatically modify the redox properties. [ABSTRACT FROM AUTHOR]