Phosphonium ionic liquids-ZnO heterogeneous catalyst for PET glycolysis: Ultrasonication-assisted synthesis and performance evaluation

With global plastic production soaring, developing efficient chemical recycling methods for polyethylene terephthalate (PET) is critical. While synergistic systems of ionic liquids and metal salts have shown promise for PET glycolysis, they often face significant challenges in catalyst separation and recyclability, hindering practical application. To address these limitations, this study introduces a heterogeneous catalyst [P 66614 ]Cl-ZnO, synthesized through an ultrasonication-assisted method. Comprehensive characterization reveals that the ionic liquid forms a stable composite through electrostatic interactions, creating a bifunctional acid catalyst with both Lewis and Brønsted acid sites, as confirmed by pyridine-FTIR. Critically, the mesoporous architecture of [P 66614 ]Cl-ZnO (surface area = 57.5 m 2 /g, mean pore diameter = 35.48 nm) is more than double that of pure ZnO and specifically designed to facilitate rapid diffusion of bulky PET polymer chains through catalyst pores, overcoming typical mass-transport limitations in polymer depolymerization. Under optimized conditions (195°C, 80 min), the catalyst achieves complete PET conversion and an 89.89% yield of bis(2-hydroxyethyl) terephthalate (BHET), while reducing the activation energy to 102.3 kJ·mol −1 . The heterogeneous nature of the catalyst allows for easy recovery, demonstrating consistent performance in ten consecutive cycles. The high purity of the BHET product was verified by NMR, MS, and elemental analysis. This work provides a robust, efficient, and reusable catalytic system for PET glycolysis, highlighting the importance of tailored textural and acidic properties in catalyst design.