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

Abstract

One-step hydroxylation of benzene with hydrogen peroxide to produce phenol catalyzed by a copper( ii) complex.

Abstract

Benzene was hydroxylated with hydrogen peroxide (H ₂O ₂) 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 H ₂O ₂ by catalysis of copper complexes to yield p-benzoquinone. The kinetic study revealed that the rate was proportional to concentrations of benzene and H ₂O ₂, 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 (HO ₂˙), as observed by EPR spectroscopy, inhibiting phenol formation. HO ₂˙ produced by the reaction of [Cu(tmpa)] ²⁺ with H ₂O ₂ 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 H ₂O ₂ was achieved using [Cu(tmpa)] ²⁺ incorporated into Al-MCM-41 as an efficient and selective catalyst.

Notes

†Electronic supplementary information (ESI) available: Cyclic voltammogram (Fig. S1), time profiles of phenol, p-benzoquinone, H ₂O ₂, spin or benzene (Fig. S2–S5 and S9 and S10), DFT results (Fig. S6 and Tables S1 and S2), UV-Vis absorption spectra (Fig. S7) and UV-Vis DRS (Fig. S8). See DOI: 10.1039/c5sc04312c