Degradation of Poly(ethylene terephthalate) Using Metal-Free 1,8-Diazabicyclo[5.4.0]undec-7-ene-Based Deep Eutectic Solvents as Efficient Catalysts

Abstract

Poly(ethylene terephthalate) (PET) is glycolyzed to obtain bis(hydroxyethyl) terephthalate (BHET), and repolymerization produces new PET material, making this a sustainable recycling process. However, most studies have reported common metal-based catalysts, which typically have harsh synthetic conditions and are prone to negative environmental impacts. In this study, 1,8-diazabicyclo[5.4.0]undec-7-ene-based deep eutectic solvents (DBU-based DESs) effectively degraded PET to obtain BHET due to their high activity, low price, and accessible production properties. Under the optimized reaction conditions (DES 3.2 wt %, PET/ethylene glycol (EG) 1:3, 180 °C, 70 min), PET bottle flakes were completely degraded, and the yield of BHET achieved 83%. In addition, a possible degradation mechanism was postulated based on experimental data and calculations using density functional theory (DFT). The formation of hydrogen bonds between EG and DES and between PET and DES can promote glycolysis of PET, which in turn could facilitate the attack of hydroxyl oxygen in ethylene glycol (EG) on carbonyl carbon in PET.