Lewis and Brønsted Acids Synergy in Photocatalytic Aerobic Alcohol Oxidations

Photocatalytic chemical transformations for green organic synthesis has attracted much interest. However, their development is greatly hampered by the lack of sufficient reactive sites on the photocatalyst surface for the adsorption and activation of substrate molecules. Herein, we demonstrate that the introduction of well-defined Lewis and Brønsted acid sites coexisting on the surface of TiO₂ (SO₄ ^2-/N-TiO₂) creates abundant active adsorption sites for photoredox reactions. The electron-deficient Lewis acid sites supply coordinatively unsaturated surface sites to adsorb molecular oxygen, and the Brønsted acid sites are liable to donate protons to form hydrogen bonds with the OH groups of alcohols like benzyl alcohol (BA). These coexistent acid sites result in a strong synergistic effect in photocatalytic aerobic oxidation of BA. For example, the conversion of BA to benzaldehyde was found to be 88.6 %, being much higher than those of pristine TiO₂ (14.7 %), N-doped TiO₂ (N-TiO₂, 24.6 %), sulfated TiO₂ (SO₄ ^2-/ TiO₂, 35.4 %), and even their sum. The apparent quantum efficiency (AQE) was determined to be 58.1 % at 365 nm and 12.9 % at 420 nm over SO₄ ^2-/N-TiO₂. This strategy to create effective synergistic Lewis and Brønsted acids on the catalyst surfaces enables us to apply it to other semiconducting photocatalytic organic transformations.