Breaking Symmetry of Active Sites in Metal-Organic Frameworks for Efficient Photocatalytic Valorization of Polyester Plastics.

Abstract

Chemical upcycling of waste plastics offers a promising way toward achieving a circular economy and alleviating environmental pollution but remains a huge challenge. Inspired by hydrolase enzymes and aiming to overcome their intrinsic limitations, we put forward a design principle for an innovative nanozyme featuring asymmetric metal sites. This nanozyme functions as photocatalyst enabling sustainable valorization of polyester plastics. As a proof of concept, an asymmetric ligand substitution strategy is developed to construct metal-organic frameworks (MOFs) that defective MIL-101(Fe) (D-MIL-101) with asymmetric Fe3-δ/Fe3+ (0< δ <1) sites. The differential electronic configurations inherent to adjacent Fe3-δ/Fe3+ sites endow a high photocatalytic activity for the valorization of polyester plastic. Accordingly, the ester bonds of polyesters can be preferentially cleaved, contributing to the low energy barrier of upcycling plastics. As a result, the D-MIL-101 achieves a high monomer yield with terephthalic acid of ~93.9% and ethylene glycol of ~87.1% for photocatalytic valorization of poly (ethylene terephthalate) (PET), beyond the efficiency of natural enzyme and state-of-the-art photocatalysts. In addition, such a D-MIL-101 is demonstrated to be feasible for the valorization of various real-world polyester plastic wastes in a flow photocatalysis system.