Photoelectrochemical Generation of Value-Added Organics via C–H Bond Activation at WO3 Photoelectrodes

The production of value-added molecular scaffolds using sustainable routes and mild conditions currently represents an important challenge in chemical sciences. In the present work, we report on the oxidation of cyclohexane (CH) to cyclohexanol (CH-ol) and cyclohexanone (CH-one) and of tetrahydrofuran (THF) to 2-hydroxotetrahydrofuran (2HTHF) and gamma-butyrolactone (GBL) by exploiting WO3 electrodes and visible light. Photoelectrochemical experiments as well as transient absorption spectroscopy measurements were performed to investigate the relevant mechanistic aspects associated with these reactions. For this purpose, we also compare the activity of two different electrode types: one obtained by deposition of a colloidal suspension (type I) and one prepared via solvothermal synthesis (type II). We show that, while production of CH-ol and CH-one is comparable with both electrodes (total Faradaic efficiencies in the order of 30% after 6 h of irradiation at an applied bias of +1.15 V vs Fc+/Fc), more efficient formation of 2HTHF and GBL can be achieved using type I electrodes (total Faradaic efficiencies up to 90% for type I electrodes after 6 h of irradiation at an applied bias of +1.45 V vs Fc+/Fc in comparison to values of ca. 30% for type II electrodes). The different behaviors can be rationalized on the basis of the different morphologies and structural characteristics of the two WO3 materials, highlighting the strong relationship between material design and target reaction. All in all, this study presents a synthetic strategy that expands the application of WO3 beyond its conventional aqueous-phase use, enabling the profitable oxidation of organic substrates into value-added products.