Chemical recycling of post-consumer poly(ethylene terephthalate) (PET) driven by the protic ionic liquid 2-HEAA: Performance, kinetics and mechanism

• Published in: Journal of environmental chemical engineering 2024-08, Vol.12 (4), p.113134, Article 113134
• Main Authors: Badia, J.D., Ballesteros-Garrido, R., Gamir-Cobacho, A., Gil-Castell, O., Cháfer, A.
• Format: Article
• Language: English
• Subjects: 2-hydroxyethyl ammonium acetate (2-HEAA); chemical valorisation; poly(ethylene terephthalate) (PET); protic ionic liquid
• ISSN: 2213-3437
• DOI: 10.1016/j.jece.2024.113134

The circular economy is a paradigm for the upcoming industrial era, in which plastic wastes must be focused on as new resources. Chemical valorisation permits deepening into waste-to-gate schemes by obtaining new chemical platforms to be reintroduced in the production market. In this context, the potential of the protic ionic liquid 2-hydroxyethyl ammonium acetate (2-HEAA) to be used as a homogeneous catalytic co-solvent for the glycolytic conversion of post-consumer poly(ethylene terephthalate) (PET) was validated from different perspectives. Studies were performed for the reaction triplet framed under the experimental conditions (i) temperature T [160,170,180 ºC], (ii) plastic-to-solvent mass ratio P/S [1:3, 1:4,1:5] and (iii) plastic-to-ionic liquid mass ratio P/IL [2:1, 2:2, 2:3]. The reaction was confirmed in terms of the transformation of PET into BHET (bis(2-hydroxyethyl terephthalate) and a proposal of mechanism induced by the amine group of the 2-HEAA is given. The thermal kinetics were modelled and a first-order equation and an apparent activation energy of 60.5 kJ·mol-1 were obtained. A statistical Box-Behnken design of experiments explained the relationship between the factors of the glycolysis triplet by a response surface, with a maximum around {T = 170 ºC, P/S = 1:3.75 and P/IL = 2:1.75}, being T and P/S statistically relevant, whereas only the presence of 2-HEAA and not its amount P/IL was significant. Finally, the reusability of 2-HEAA after 90-min glycolytic chemical valorisations was confirmed up to 4 cycles. The results position 2-HEAA as a promising catalytic co-solvent for the scale-up of chemical valorisation of PET. [Display omitted] •Chemical valorisation of PET into BHET was effectively catalysed by 2-HEAA.•The degradation mechanism induced by the amine group of 2-HEAA is proposed.•The PET/EG mass ratio and the temperature were significant for the conversion.•The reusability of 2-HEAA was confirmed with up to 4 valorisation cycles.