Depolymerization of polyethylene terephthalate (PET) under mild conditions by Lewis/Brønsted acidic deep eutectic solvents

Modern society urgently needs new recycling methods to handle the impressive amount of plastic items that are annually discarded. Deep Eutectic Solvents (DESs) have shown interesting results in the depolymerization of polyethylene terephthalate (PET), but most of the procedures still need harsh conditions of temperature and pressure. In this contribution, we propose a bifunctional Lewis/Brønsted acidic DES composed of FeCl3·6H2O, cheap and scarcely toxic, in combination with a variety of acids, both mineral and organic, including some of natural origin (citric and acetic acid). We show that the LBDES formed with methanesulfonic acid and para-toluenesulfonic acid are capable of quantitatively depolymerizing PET under mild conditions, with a temperature of 100 °C and a reaction time of 1 h, affording high purity terephthalic acid in high yield. For acetic acid, a reaction time of 3 h are necessary to obtain a quantitative depolymerization. Different strategies to optimize the PET/LBDES ratio has been successfully tested, as the consecutive addition of multiple aliquots of PET or the filtration and reuse of the solvent. The best solvent has been characterized through the comparison of theoretical and experimental eutectic phase diagram, confirming its nature of DES.