Visible-light-induced photocatalytic oxidation of C–H bonds with O2 employing simple porphyrins as photocatalysts under solvent-free conditions

Abstract

To insert oxygen atoms into C–H bonds through efficient and green process with lower energy consumption and lower carbon emission, porphyrins and metalloporphyrins as model compounds of chlorophyll were utilized as photocatalysts and applied to the oxidation of C–H bonds with O₂ under irradiation of visible light at room temperature and solvent-free conditions. The generation efficiency of oxygen-containing products reached up to the millimolar level (mmol (g_cat.⁻¹ h⁻¹)) with excellent substrate tolerance. In cyclohexane oxidation as the model reaction, substrate conversion reached up to 3.18 mmol (g_cat.⁻¹ h⁻¹) with a selectivity of 99.9% towards cyclohexyl hydroperoxide, utilizing optimized tetrakis(4-carboxyphenyl)porphyrin (T(4-COOH)PP) as the photocatalyst. Based on characterization studies of UV-vis absorption spectroscopy, photoluminescence emission spectroscopy, time-resolved photoluminescence spectroscopy, transient photocurrent spectroscopy and electrochemical impedance spectroscopy, the source of excellent catalytic performance of T(4-COOH)PP was investigated, and it was ascribed to its higher performance in visible-light absorption, charge separation, production of photo-generated electrons, and lower impedance during charge migration under irradiation of visible light. The reactive species and reaction mechanism in this work were investigated in detail as well. The production efficiency of oxidation products at the millimolar level (mmol (g_cat.⁻¹ h⁻¹)) was higher than most of the reports in current literature. Thus, the oxidation method developed in this work was an efficient, sustainable and low-energy consumption strategy for oxidative functionalization of C–H bonds and would be a valuable reference for the development of an efficient, sustainable, low-energy consumption and low-carbon emission chemical process.