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 CO 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-NH metal-organic frameworks (UZN) to perform cycloaddition of CO 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 CO is adsorbed on the exposed Zr sites of [ZrO] units and reduced to CO 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·h, 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 CH-BrC-O intermediates, which react with CO, finally producing propylene carbonate (PC) by dehalogenation. The findings provide general guidance to design efficient photocatalysts for CO fixation and green organic photosynthesis.