Scalable green sustainable photosynthesis of H2O2: catalyst-free H radical generation and ORR under sunlight

Abstract

Sunlight-driven green hydrogen peroxide (H₂O₂) synthesis is in demand, but current photo-catalysis methods fail to meet industry demand due to unsatisfactory rates and yields, and the natural drawbacks of photocatalysts. This study proposes a novel green H₂O₂ photochemistry approach to improve energy efficiency and selectivity, and achieve renewable by-products. Mild selective homolytic cleavage of photoexcited O–H/C–H bonds and subsequent spontaneous intramolecular hydrogen atom transfer (HAT) are demonstrated. Through four typical molecule electronic structures and property comparison, phenethyl alcohol (PA) is found to be the optimized molecule oxidized by O₂ with the unique by-product acetophenone (ACP), releasing hydrogen radicals, which react with O₂ via an oxygen reduction reaction (ORR) to generate H₂O₂. ACP is then easily reduced back to PA by hydrogen, thereby achieving an oxidation–hydrogenation green closed loop. At 20 °C, PA achieved an H₂O₂ yield of 53.1 mM h⁻¹ with a 45% apparent quantum yield (AQY) at a wavelength of 313 nm, indicating that it is promising for application in industry. After 60 cycles, the H₂O₂ yield hardly reduced, and the accumulation of ACP in PA was negligible. The straightforward ORR photoreaction was conducted under mild conditions without a photocatalyst, providing a reference strategy for green H₂O₂ production. This study reports a green and scalable synthesis of H₂O₂ with H atoms from hydroxy and benzyl group selective homolytic cleavage and a stable intramolecular HAT process. The reaction path is unique, and the recyclable products can take part in mild and sustainable closed-loop production for all-weather application.