High-efficiency construction of ZnO/ZIF-8 nanoheterojunctions enriched with oxygen vacancies for chemical recycling of plastic

Rotating packed bed (RPB) is firstly used for chemical synthesis of heterojunction. • ZnO/ZIF-8 heterojunctions achieve a record catalytic activity in PLA methanolysis. • RPB product has more oxygen vacancies and stronger Lewis acid than counterpart. • Lewis acid of ZnO and high surface area of ZIF-8 endow catalyst high activity. • Excellent activity covers various real-life polyester wastes and their mixtures. The chemical recycling of plastic is crucial for achieving a circular economy, and developing heterogeneous catalysts that combine high activity with structural stability represents a pivotal challenge. Herein, a high-gravity-assisted approach was proposed to efficiently construct ZnO/ZIF-8 (RPB-ZnO/ZIF-8) nanoheterojunctions enriched with oxygen vacancies (Ov) for polylactic acid (PLA) methanolysis to methyl lactate (MeLA). Compared to the stirred tank (ST), high-gravity synthesis drastically reduces coordination reaction time from 1 h to an extremely short time (< 1 s). The as-prepared RPB-ZnO/ZIF-8 catalyst achieved 100% PLA conversion at 100 °C in 0.5 h with a MeLA yield of 95.6%, reaching the highest activity compared to previous works so far. More importantly, it demonstrated excellent cycle stability, maintaining >92% of its initial activity after five cycles. Catalytic mechanistic investigations reveal that the Ov increases Lewis acidity of the heterojunction surface, thereby facilitating the reactants adsorption and activation, which leads to a reduction in the energy barrier for PLA methanolysis. Furthermore, RPB-ZnO/ZIF-8 catalyzes the depolymerization of other plastics effectively (e.g., polyethylene terephthalate and polycarbonate) and demonstrates remarkable versatility in converting various real-life polyester wastes and their mixtures. This study presents an efficient strategy for advancements in sustainable plastic recycling.