There and Back Again: Recovery of Terephthalic Acid from Enzymatically Hydrolyzed Polyesters for Resynthesis

Poly(ethylene terephthalate) (PET) is still a major player in the plastics industry, especially for packaging. Despite attempts to derive its basic components from biological resources, production of terephthalic acid (TPA), one of the two PET monomers, still depends on fossil resources. Alongside traditional polyesters, TPA is a building block also for biodegradable polymers, such as poly(1,4-butylene adipate-co-1,4-butylene terephthalate) (PBAT). Here, PET, PBAT, and real plastic waste were successfully depolymerized using Humicola insolens cutinase as an environmentally friendly alternative to mechanical or chemical treatments allowing recovery of TPA even from mixed plastic waste. This monomer was isolated in high purity upon acidification as confirmed by using Fourier Transform-Infrared Spectroscopy, ¹H-NMR spectroscopy, and Thermogravimetric analysis. Consequently, contaminants or residual buffer salts caused major issues during synthesis of PET precursors upon reaction with ethylene glycol (EG) and TPA. The recovered TPA was used to prepare bis(hydroxyethyl) terephthalate (BHET) and further repolymerized to PET. The resulting molecular weight of the polyesters was found to be dependent on the purity of the TPA and on the catalyst used.