Unraveling the Synergy Effect of Bimetallic Zn–Mn-Based Catalysts for Polyethylene Terephthalate Glycolysis

The overuse of polyethylene terephthalate (PET) plastics has led to severe waste accumulation with a large portion of PET ending up in landfills or being incinerated despite ongoing recycling efforts. Chemical recycling, particularly glycolysis, offers a promising method for transforming PET waste into valuable monomers, such as bis(2-hydroxyethyl) terephthalate (BHET). This study introduces a highly efficient bimetallic Zn–Mn catalyst supported on commercial SiO2 for PET glycolysis. The synergy between Zn and Mn leads to the formation of a new active site, ZnMn2O4, which significantly improves catalytic activity and BHET selectivity compared with monometallic catalysts. Density functional theory (DFT) calculations show the bimetallic system has a smaller energy gap between the adsorption energies of ethylene glycol and PET, facilitating better interaction with the catalyst. At the optimal Zn/Mn molar ratio of 1:1, the catalyst achieved a space-time yield of 114.86 molBHET·molcat–1·h–1, surpassing those of other reported heterogeneous catalysts. The strong Zn–Mn bond reduces metal leaching, contributing to excellent catalyst stability. After five cycles, PET conversion decreased slightly from 92.08% to 88.15%, and BHET yield dropped from 88.29% to 84.97% due to some loss of active sites. Nevertheless, product selectivity remained high (95–96%), aiding in efficient separation and purification.