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 mol_BHET·mol_cat^–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.