O₂-Bridged Multicopper(II) Complex in Zeolite for Catalytic Direct Photo-oxidation of Benzene to Diphenols

Abstract

The photo-oxidation of benzene with hydrogen peroxide is catalytically promoted by copper(II)-exchanged BEA zeolite (Cu-BEA), resulting in the selective formation of diphenols. Analysis by UV−vis, Cu K-edge X-ray absorption fine structure (XAFS), and Raman spectroscopies are used to identify the active species in Cu-BEA treated with the reaction mixture or a H2O2 solution. The reaction of monomeric Cu(II) cations in BEA zeolite with H2O2 yields O2-bridged multicopper(II) complexes, possibly the (μ-η2:η2-peroxo)dicopper(II) species, which is characterized by the Cu−Cu distance of 3.24 Å, the UV−vis band around 370 nm, and the Raman band at 830 cm-1. Extended XAFS, in situ UV−vis, and O2 temperature programmed desorption results show that thermal desorption of the bridged O2 on the multicopper(II) site results in the formation of monomeric Cu(II) cations. In situ UV−vis under photo-irradiation shows that the complex is photolabile in the presence of benzene. The effective wavelength range for the benzene hydroxylation (300−420 nm) is in the wavelength range of ligand-to-metal charge transfer bands of the complex, suggesting that the photo-excitation of the complex is involved in the important step of the selective benzene hydroxylation.