Ultrasound-Assisted Alkaline Hydrolysis of Waste Poly(Ethylene Terephthalate) in Aqueous and Non-aqueous Media at Low Temperature

This work demonstrates an attempt to depolymerize the waste poly(ethylene terephthalate) (PET) at low temperature using 20 kHz frequency ultrasound. Alkaline hydrolysis of PET was carried out in an aqueous as well as in methanolic medium. Ultrasound has shown a significant enhancement in the rate of alkaline hydrolysis of PET in methanol compared to an aqueous medium. The rate of alkaline hydrolysis of PET increases with an increase in the temperature and alkali concentration. PET conversion of 69% and 46% was observed with and without ultrasound, respectively, using 10% (w/w) sodium hydroxide (NaOH)-methanol solution at 50°C temperature in 60 min. Pure terephthalic acid was recovered from the PET and characterized using Fourier-transform infrared spectroscopy and acid number. The shrinking core model was proposed for the reaction mechanism with surface reaction as the rate of the controlling step. Kinetic analysis indicates almost equal activation energy and yet shows a significant difference in the frequency factor with and without ultrasound-assisted alkaline hydrolysis. Thus, ultrasound does not alter the intrinsic mechanism of PET hydrolysis. The reason for enhancement in the rate of reaction may be the ultrasound-induced micro-mixing and thereby the frequency of interaction between PET and NaOH molecules.