Pickering Emulsion Interfaces Enhance Photocatalytic Selective Oxidation of Aromatic C─H Bonds

Solar-driven photocatalysis provides a promising route for the green synthesis of high value-added oxygenated chemicals through oxidization of C─H bonds. However, improving the photocatalytic selectivity without compromising its activity remains a grand challenge due to the difficulty in controlling the generation of desired reactive oxygen species during the reaction. Here, based on Pickering emulsions, we constructed an interfacial photocatalytic system to enhance the photocatalytic selectivity, exemplified by the selective oxidation of ethylbenzene. Positioning a photocatalyst, bismuth bromide oxide flakes, at the outer side of Pickering emulsion droplet interfaces can regulate the concentrations of generated superoxide anion radicals and hydroxyl radicals, resulting in >99.0% ethylbenzene conversion and >99.0% selectivity towards acetophenone under visible light irradiation. Theoretical calculations and experimental investigations reveal that (i) the loose hydrogen bonding at the outer interfaces of Pickering emulsion droplets increases the dissociation barrier of water molecules, inhibiting the formation of hydroxyl radicals; (ii)  this system can create an oxygen-rich lipophilic solvation environment which facilitates the chemisorption of oxygen and the generation of superoxide anion radicals. Furthermore, this system can work well in a continuous-flow fashion with high activity and long-term operational stability (200 h), illustrating appealing potentials for practical photocatalytic synthesis.