Unraveling the role of water in catalytic glycolysis of PET

Abstract

The chemical recycling of polyethylene terephthalate (PET) via glycolysis is a promising route for recovering the monomer bis(2-hydroxyethyl) terephthalate (BHET), which can be used for virgin-grade PET production. However, the influence of water—an inevitable impurity and potential byproduct—on this process is complex and not fully elucidated. This study systematically investigates the effect of water content (0–22.2 vol%) on PET glycolysis using selected heterogeneous catalysts (ZnO and Mn₂O₃) and homogeneous catalysts (zinc acetate (ZnAc₂), 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)). Product distribution and reaction kinetics were quantified by HPLC and in situ IR spectroscopy, respectively. The heterogeneous catalysts (ZnO and Mn₂O₃) and homogeneous ZnAc₂ retained high PET conversion (>95%) even at elevated water concentrations. Nevertheless, the BHET yield and selectivity for these systems decreased significantly due to a competing hydrolytic side reaction, promoted by water, which yields terephthalic acid (TPA). Notably, ZnAc₂ exhibited a more rapid decline in BHET selectivity compared to ZnO. Conversely, the organic base catalysts TBD and DBU experienced complete deactivation in the presence of water, resulting in a drastic reduction in both PET conversion and BHET yield, with DBU showing greater susceptibility. In situ IR experiments corroborated that the deactivation mechanism for TBD involves protonation by water. These results emphasize that water's influence is a function of the catalyst's chemical nature, modulating product selectivity for metal-based systems while causing the deactivation of organic bases. Understanding these divergent effects is critical for the optimization of industrial PET glycolysis and the rational design of water-tolerant catalytic systems.