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 center dot), as observed by EPR spectroscopy, inhibiting phenol formation. HO2 center dot 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.