Amino alcohols as recycling-catalysts for degradation of PET to oligoethylene terephthalates

Abstract

Polyethylene terephthalate (PET) is an essential component of daily life. However, due to PET's resistance to natural degradation, a plethora of life in the ecosystem is swimming in discarded plastic causing a serious environmental disaster. Using glycolysis, a chemical recycling method, PET is largely recycled to bis(2-hydroxyethyl) terephthalate (BHET) monomers. BHET is polymerized back to a pre-polymer (∼25-35 DPn) followed by polycondensation for higher molecular weight PET, hence an energy-intensive and cost-inefficient method. This study demonstrates glycolysis of PET using Amino alcohols (AmOH's) for generating oligoethylene terephthalate (OET) of DPn value of ∼30 in high yields (98%) under mild reaction conditions; 180°C temperature, atmospheric pressure, with recyclability. PET-to-PET cycle was closed by preparing PET from generated OETs. A comparative model study on methyl benzoate (in EG) revealed the efficiency of AmOH's over Hünig's base; N, N-Ethyldiisopropylamine (DIPEA) despite being more basic. Similarly, N-Butyl-N-methylbutane-1-amine (MDBA) demonstrated slower PET conversion than Methyl diethanolamine (MDEA) AmOH. Although AmOH's such as MDEA also act as nucleophile and add to the polymer chain, to our surprise, our NMR studies revealed that there is no sign of insertion of AmOH's indicating that the AmOH's may function as catalysts. Morphological studies conducted under different reaction conditions unveiled the method of OET generation. The computational studies and Kinetic Isotopic studies explained facilitation of hydroxy group of AmOH in the degradation. Following the shrinking core model, the calculated activation energy for degradation was estimated to be 45.4 kJ mol−1.