Modulating the Energy Band to Inhibit the Over-oxidation for Highly Selective Anisaldehyde Production Coupled with Robust H2 Evolution from Water Splitting

Abstract

Aromatic aldehydes are important precursors for pharmaceutical, agricultural, and synthetic chemistry. Compared to thermal catalysis and organic solvent synthesis systems, photocatalytic selective conversion of aromatic alcohol to the corresponding aldehyde coupled with H2 evolution in pure water represents an alternative due to the mild reaction conditions and the renewable energy production. However, this coupling reaction in aqueous solution is still limited by the low selectivity and conversion due to the over-oxidation. Herein, we demonstrated that the typical type-II heterojunction photocatalysts with a moderate energy band can restrain over-oxidation. This strategy effectively separates the photoexcited charge and hampers the complicated radical oxidation process, thus favoring the generation of aldehyde with the robust evolution of H2 under visible light illumination without any cocatalyst. The system realizes an anisaldehyde (AA) selectivity of over 99% and a 4-methoxybenzyl alcohol (4-MBA) conversion of over 99% in a low concentration of 4-MBA. Furthermore, when the amount of 4-MBA is expanded to the mL level, the system is able to maintain a robust H2 evolution rate of 291.8 mmol·g–1·h–1 at over 99% selectivity for AA. Mechanistic investigation reveals that the photoexcited hole with a moderate potential directly reacts with 4-MBA by promoting the formation of a benzyl carbon radical with abstraction of a hydrogen atom due to the inhibition of ·OH formation. Moreover, the effective contact between water, photocatalysts, and 4-MBA also contributes to enhance the conversion activity and improve the practical application potential. The method can be further extended to various aromatic alcohols and offers a concept for highly selective synthesis of high-added value chemicals coupled with H2 evolution in pure water.