Thermochemical methanolysis-based recyclability of polyethylene terephthalate (PET) via pseudo-catalytic depolymerisation

Abstract

The global production and consumption of polyethylene terephthalate (PET) have continued to rise, accounting for approximately one-quarter of total plastic output. Currently, PET is derived from fossil-based resources, and the majority of used PET follows a linear, single-use trajectory, ultimately contributing to environmental burdens. While PET has contributed to modern convenience, the alarming statistic that nearly one million PET bottles are discarded every minute emphasises the urgent need to reassess the sustainability of its current production and disposal practices. In this study, a catalyst-free depolymerisation technology, termed pseudocatalytic transesterification, was developed as a sustainable valorisation approach for the recovery of resources from PET. This approach enabled rapid depolymerisation, achieving high yields of dimethyl terephthalate (DMT, 75.0 %) and ethylene glycol (EG, 17.0 %) within 1 min at 400 degrees C. Dimethyl carbonate (DMC) was introduced as a (co) solvent to enhance monomer recovery. A 1:1 M ratio of methanol:DMC yielded the highest DMT and EG recoveries outperforming methanol alone. However, excessive DMC promoted ethylene carbonate (EC) formation, thereby reducing the EG yield. Under optimised conditions, the process achieved a recycled PET (rPET) conversion of 68 % with an estimated economic value of $4392 per tonne of rPET. Although the production cost remains higher than that of fossil-based PET, this approach offers substantial environmental benefits by reducing 2 tonnes of CO2 emissions per tonne of recycled PET waste.