Insights into the effect of oxygen vacancies on the epoxidation of 1-hexene with hydrogen peroxide over WO3−x/SBA-15

Abstract

Linear terminal olefin epoxidation is a significant oxidation reaction for the synthesis of valuable industrial intermediates. As typical olefin epoxidation catalysts, supported tungsten oxides are rich in oxygen vacancies, which have been investigated for improving catalytic performance in other reactions, but the influence on olefin epoxidation is rarely studied. Herein, a series of WO_3−x/SBA-15 catalysts with different oxygen vacancy concentrations were obtained by impregnation and sequent hydrogen reduction at 300–600 °C. Their catalytic performances for 1-hexene epoxidation with hydrogen peroxide at 70 °C were studied. It was found that an appropriate concentration of oxygen vacancies could improve epoxidation (the epoxide productivity increased from 135 to 196 μmol_epoxide g_W⁻¹ min⁻¹) and reduce the apparent activation energy (from 51.6 to 46.9 kJ mol⁻¹). The oxygen vacancies are also beneficial for cycloolefin and other linear olefin epoxidation. In particular, the productivity of cyclooctene oxide was promoted from 4161 to 5744 μmol_epoxide g_W⁻¹ min⁻¹. This promotion effect was attributed to the increase in the number of Lewis acid sites caused by introducing moderate oxygen vacancies, which could improve the activation of hydrogen peroxide and facilitate epoxidation. In situ FT-IR spectroscopy and density functional theory calculations further indicated that increasing the oxygen vacancies contributes to reducing the energy barrier of the transfer of electrophilic oxygen atoms in active intermediate W–OOH to 1-hexene, thereby promoting epoxidation.