Optimisation of PET glycolysis by applying recyclable heterogeneous organocatalysts

Chemical depolymerisation, or solvolysis, can be a sustainable plastic recycling method, as a circular economy can be achieved by recovering the pure monomers. Polyethylene terephthalate (PET) is a ubiquitous plastic material with short-life application and slow biodegradation, so its waste management needs to be continuously improved. In this study, we tested three commercially available organocatalyst-modified silica gels in the glycolysis of PET and developed another, functionalized with triazabicyclodecene (TBD), which was also tested. Organocatalysts are efficient in PET glycolysis, but their recyclability needs to be improved for industrial application. The applied heterogeneous modified silica gels can be recovered easily by filtration. Si-TEA catalyst was chosen for reaction optimisation because it has the highest thermal stability and good catalytic activity. The PET glycolysis process was optimised by fractional factorial experimental design and response surface methodology. Under optimal reaction conditions (PET (384 mg, 2 mmol), ethylene glycol (1.41 mL, 25.2 mmol), Si-TEA (15.5 mol%), 190 °C, 1.7 h), 88.5% non-isolated bis(2-hydroxyethyl) terephthalate (BHET) monomer yield was obtained. Si-TEA and Si-TBD catalysts were recycled in five reaction cycles, and with both catalysts, high cumulative BHET yields (89 and 88%, respectively) were achieved. Additionally, environmental energy impacts were calculated for the two catalysts and were compared favourably with other organocatalysts in the literature. A process scale-up was also implemented. Finally, it has been verified that modified silica gels have much higher catalytic activities than native silica gel, as solvolytic reactions using the former catalysts took a significantly shorter time. Functionalized silica gels were applied as heterogeneous organocatalysts in PET glycolysis, and were recycled by filtration while preserving high monomer yields over five cycles.