All Green Microwave Assisted 99% Depolymerisation of Polyethylene Terephthalate into Value Added Products via Glycerol Pre-treatment and Hydrolysis Reaction

Energy-efficient and fast depolymerisation technologies present as new sustainable and green recycling routes for achieving a circular economy for plastics. Herein, we present a highly efficient 2-step microwave-based (MW) degradation of polyethylene terephthalate (PET). Initially, a MW-assisted pre-treatment was evaluated using glycerol as a non-toxic reagent for the conversion of PET into a modified form that makes it easily depolymerised. Box Behnken Design was employed to determine the optimised pre-treatment conditions attaining maximum PET weight loss and favourable crystallinity and carbonyl indices for the pre-treated PET. Glycerol of 12 mL volume and 3 min of 182W MW irradiation resulted in 11% PET weight loss at onset temperature of degradation and gave rise to carbonyl index up to 4.22 and 33% crystallinity of pre-treated PET. MW assisted hydrolysis of the pre-treated PET was then performed in the presence of sodium bicarbonate and ethylene glycol as depolymerizing agents. Within 3 min, the proposed depolymerisation methodology provided 99.9% conversion of PET into 79.1% terephthalic acid (TPA), 17.6% monohydroxyethyl terephthalate (MHET), and 1.8% bis (2-hydroxyethyl) terephthalate (BHET). The obtained TPA was separated from the monomers mixtures and its purification was evaluated via different characterization techniques against a standard TPA. A purity of 95%, 82.4 APHA colour value, 645.3 mgKOH/g acid number and acceptable heavy metal content indicated that the purified TPA can be repolymerised as virgin PET. Graphical Abstract