Light-enhanced direct epoxidation of propylene by molecular oxygen over CuOx/TiO2 catalyst

Abstract

Direct epoxidation of propylene (DEP) by molecular oxygen is an ideal way to synthesize propylene oxide (PO), yet it remains quite challenging. We demonstrated here that the PO formation rate and selectivity could be enhanced simultaneously through photo-thermo-catalysis over the CuO_x/TiO₂ catalyst. At 180 °C, by introducing light, the PO formation rate increased more than 20-fold (from 8.2 to 180.6 μmol g⁻¹ h⁻¹) and the corresponding selectivity improved more than 3-fold (from 8 % to 27 %), breaking the traditional perception that the semiconductors exhibit very low reactivity for this reaction. Kinetic study results showed that the apparent activation energy for PO formation could sharply decrease under light irradiation (from 95 to 40 kJ mol⁻¹). In situ electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were applied to characterize the dynamics of the valence state of the copper oxide species and the activation intermediates of molecular oxygen. Evidence for the activation of oxygen, which could direct to the PO formation pathway, was captured. The light-driven electrons could promote the formation of active Cu⁺, which could form the side-on μ-peroxo Cu(II)₂ structure, weaken the O–O bond, and improve the PO formation rate and selectivity. This work paves a new way for designing semiconductor-supported photocatalysts for DEP reactions with molecular oxygen.