Methanolysis of mixed plastics catalyzed by ionic liquid: PET depolymerization and recycling of PP/PE

In the context of composite materials that elude classification via conventional pretreatment methodologies such as crushing, granulating, and flotation—specifically within the domains of food packaging, construction materials, and engineering plastics—this research employed a one-step methanolysis-separation technique for the graded recycling of polypropylene (PP), polyethylene (PE), and polyethylene terephthalate (PET) in mixed plastics. Employing methanol as the reaction solvent, the benign and eco-friendly ionic liquid BMIMOH as the catalyst for the degradation of PET. PET was methanolized to produce dimethyl terephthalate (DMT), while PP and PE remained highly stable, were separated after the reaction, and subjected to subsequent recycling. The stability of PP and PE under methanolysis conditions was revealed through structural and thermodynamic analyses. Single-factor analysis and response surface methodology were employed to determine the optimal conditions for the methanolysis reaction. Additionally, the impact of different contents of PP and PE on the recovery of PET from the mixed plastics during the swelling-condensation process at high temperatures was explored. Under the optimal conditions, the yield of DMT was 94 % for the degradation of single PET, and when the ratio of PET in the mixed plastics was 50 %, the yield of DMT could still reach 90 %.