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.