Efficient Halogen Radical-Mediated Photosynthesis of Cyclic Carbonates over Perylene Diimide-Grafted Zirconium Metal–Organic Frameworks with Visible Light Irradiation

This work demonstrates an efficient halogen radical-mediated photocatalytic route for overcoming the kinetic bottleneck of CO2 cycloaddition with epoxides to produce cyclic carbonates. A novel organic–inorganic nanohybrid photocatalyst is smartly designed and prepared by grafting of perylene-3,4,9,10-tetracarboxylic diimide (PDI) molecules with UiO-66-NH2 metal–organic frameworks (UZN) to perform cycloaddition of CO2 to propylene oxide (PO) under visible light (λ ≥ 420 nm) irradiation. The PDI and UZN moieties are linked by an amide bridge to form a type II heterojunction interface, and the former serves as the hole collector, where halogen anions including Br–, Cl–, F–, and I– are directly oxidized to radicals, and the latter acts as the electron acceptor, where CO2 is adsorbed on the exposed Zr sites of [ZrO6] units and reduced to CO2–• radicals. The optimal PDI–UZN photocatalyst achieves an impressive propylene carbonate (PC) yield of 99.4% at a production rate of 34.1 mmol·g–1·h–1, with a benchmark apparent quantum efficiency of 35.9% at 400 nm. The combination of characterization results and density functional theory calculation clearly reveals that the formed Br• radicals are preferential to attacking the C–O bonds of PO adsorbed over the PDI moiety to generate C2H6–BrC–O• intermediates, which react with CO2–•, finally producing propylene carbonate (PC) by dehalogenation. The findings provide general guidance to design efficient photocatalysts for CO2 fixation and green organic photosynthesis.