One-step selective hydroxylation of benzene to phenol with hydrogen peroxide catalysed by copper complexes incorporated into mesoporous silica–alumina

Abstract

Benzene was hydroxylated with hydrogen peroxide (H₂O₂) in the presence of catalytic amounts of copper complexes in acetone to yield phenol at 298 K. At higher temperatures, phenol was further hydroxylated with H₂O₂ by catalysis of copper complexes to yield p-benzoquinone. The kinetic study revealed that the rate was proportional to concentrations of benzene and H₂O₂, but to the square root of the concentration of a copper(II) complex ([Cu(tmpa)]²⁺: tmpa = tris(2-pyridylmethyl)amine). The addition of a spin trapping reagent resulted in formation of a spin adduct of hydroperoxyl radical (HO₂˙), as observed by EPR spectroscopy, inhibiting phenol formation. HO₂˙ produced by the reaction of [Cu(tmpa)]²⁺ with H₂O₂ acts as a chain carrier for the radical chain reactions for formation of phenol. When [Cu(tmpa)]²⁺ was incorporated into mesoporous silica–alumina (Al-MCM-41) by a cation exchange reaction, the selectivity for production of phenol was much enhanced by prevention of hydroxylation of phenol, which was not adsorbed to Al-MCM-41. The high durability with a turnover number of 4320 for the hydroxylation of benzene to phenol with H₂O₂ was achieved using [Cu(tmpa)]²⁺ incorporated into Al-MCM-41 as an efficient and selective catalyst.