Developing a Highly Efficient and Magnetically Recoverable Nanocatalyst for Glycolytic Depolymerization of Various Polyesters

The synthesis of a new recyclable magnetic catalyst consisting of silica-coated magnetite nanoparticles (Fe₃O₄@SiO₂) with a zinc-containing ionic liquid anchored to the surface is described. An in-depth characterization was performed using different techniques, which demonstrated that Fe₃O₄@SiO₂@(mim)-[ZnCl-(OH)₂] (mim: methylimidazolium) depicts the actual structure of the nanocatalyst. This system exhibits an outstanding performance as a magnetically recoverable catalyst for the glycolysis of different polyesters in ethylene glycol, such as polyethylene terephthalate (PET), poly-(1,4-butylene terephthalate) (PBT), and bisphenol A polycarbonate (BPA-PC). The depolymerization of PET and PBT into bis-(2-hydroxyethyl)-terephthalate (BHET) was carried out with nearly 100% selectivity and yield over 12 reaction cycles at 170 °C without tedious workup or purification processes. Similar behavior was observed in the glycolysis of BPA-PC into bisphenol A (BPA), which was obtained with more than 80% yield during 12 consecutive runs. Indeed, the nanocatalyst remained active with only a small loss of activity in the 20th cycle of recovery and reuse, demonstrating the high potential of this catalytic system for the chemical recycling of plastics. Besides, the unique catalytic and magnetic properties of this hybrid material have allowed us to develop gram-scale experiments. Finally, an in-depth characterization of the recovered catalyst showed that its overall structure was preserved after the glycolysis process. Only a loss of Cl^- ions of the Zn-based ionic liquid, caused by a ligand exchange process with ethylene glycol species and OH^- ions, was observed.