Superbases as Organocatalysts in Low Temperature Glycolytic Depolymerization of Polyethylene Terephthalate (PET)

Chemical recycling offers a prospective solution for valorizing the vast amount of diverse polyethylene terephthalate (PET) waste generated from various streams. Rigid and flexible packaging such as bottles, and films, or textiles, optionally containing also comonomers like polypropylene terephthalate (PPT) and polybutylene terephthalate (PBT), are essential materials to be considered. In this study, low temperature depolymerization of PET was investigated by implementing glycolysis as the chemical recycling method. In glycolysis of PET, the polymer is depolymerized in excess ethylene glycol and presence of catalyst into bis(2-hydroxyethyl) terephthalate (BHET) monomer. The monomer can be re-utilized as feedstock for the manufacturing of recycled virgin-like PET. Herein we report utilization of novel mTBN guanidine superbase (mixture of two isomers, 7-methyl-1,5,7-triazabicyclo-[4.3.0]non-5-ene & 5-methyl-1,5,7-triazabicyclo-[4.3.0]non-6-ene) as efficient organocatalyst for glycolysis of virgin and post-consumer PET materials. Three guanidine compounds were screened as catalyst under various conditions and their efficiency was compared to zinc acetate, which is a conventionally applied transesterification catalyst. All superbases achieved high PET conversions of over 90 % at low reaction temperature of 140 °C within 2 h, while Zn(OAc) ₂ showed only 18 % conversion. Moreover, mTBN superbase was the most efficient by depolymerizing PET powder in just 20 min. Industrially viable conversions of over 90 % resulted in BHET monomer recovery of over 40 % determined by mass. Size-exclusion chromatography (SEC) results indicate that reaction temperature has significant influence on the depolymerization reaction depth. Furthermore, experiments with different PET feedstocks demonstrate the significance of materials’ surface area towards depolymerization efficiency.