Process optimization by NMR-assisted investigation of chemical pathways during depolymerization of PET in subcritical water

Abstract

Chemical recycling of polymers to monomers and chemicals is a promising pathway to valorize plastic waste that cannot be mechanically recycled, thus potentially minimizing resource consumption and the overall CO₂ impact of the polymer industry. Among chemical recycling technologies, solvolytic depolymerization of poly(ethylene terephthalate) stands out as a selective process that maximizes monomer recovery. However, many challenges still remain regarding the optimization of these recycling technologies. Addressing these challenges could lead to these technologies becoming truly environmentally advantageous compared to alternative waste management solutions. Subcritical water has proven to be an outstanding media for a broad variety of reactions and its potential as a green solvent for PET depolymerization is reassessed based on a new nuclear magnetic resonance quantification method allowing for product characterization to a degree never reported before. In order to study the intrinsic product composition at every reaction condition (280 to 340 °C and 0 to 45 min reaction time), depolymerization experiments were performed in agitated micro-batch reactors, and NMR analysis was conducted prior to any alkaline-based purification. The highest recovery of terephthalic acid was achieved after 45 min at 340 °C, however, under these conditions ethylene glycol experiences a high degree of degradation. Collected data was then used to compare the environmental performance of different case scenarios leading to the preferable conditions to be 5 min at 310 °C, where the recovery of terephthalic acid, ethylene glycol, mono(2-hydroxyethyl) terephthalate and bis(2-hydroxyethyl) terephthalate is as high as 0.9 g g⁻¹ PET.