Deep Eutectic solvents and green reagent based process for polyethylene terephthalate depolymerisation

Abstract

Polyethylene terephthalate (PET) is a ubiquitous thermoplastic polymer which has numerous industrial applications including textile, construction, and packaging applications due to its versatile nature, transparency, strength, and barrier properties. According to recent studies, the production of PET is increasing at a growth rate of 8% per annum with of plastic waste rising to 8 million tons every year. It is therefore imperative to develop sustainable and scalable plastics recycling methodologies to reduce plastic waste accumulations stockpiles. Recently enzymatic treatment methodologies have emerged as promising and environment-friendly techniques for efficient PET depolymerisation. Higher selectivity of desired monomers is achievable due to specific targeting enzymes. Considerable further research and development is required to overcome the enzyme costs, sensitivity limitations and long reaction times to become amenable to industrial-scale production. Chemical recycling of PET has advantages over other recycling techniques due to increased efficiency while utilising a wide range of PET waste products and converting them into primary building blocks that could be used in repolymerisation to make virgin PET. However, the usage of harsh solvents and longer reaction conditions restrict their large scale applications. Therefore, it is important to develop a most efficient and industrial applicable PET depolymerisation techniques. Deep eutectic solvents (DESs) are mixtures of different compounds with many applications in organic, analytical, and polymer chemistry fields. Recently, DESs were proposed as green catalysts in plastics chemical recycling and have shown remarkable potential as catalysts in PET depolymerization reactions. Microwave (MW) technology has been investigated as an efficient method for fast PET depolymerization reactions. The rapid heating achievable via MW while utilising DESs as model catalysts has allowed significant shortening of reaction times while delivering acceptable monomers conversion yields. As there is insufficient knowledge in this area, this thesis aims to address PET waste pre treatments and depolymerisation reactions under new and instant MW technology coupled with DESs. Three studies were carried out to investigate DESs based MW depolymerisation technology and are detailed in the appended papers. As this research work explores the ultra-green and sustainable depolymerisation methodologies for PET, Paper I demonstrates combined ultra-green chemical and biocatalytic depolymerization of PET using DES-based MW treatment followed by enzymatic hydrolysis. A green DES with a triplet composition of choline chloride, glycerol, and urea was selected for PET depolymerization under MW irradiation without the use of additional depolymerization agents. Under the optimized conditions of 20 mL DES volume, 260 W MW power, and 3 min MW irradiation time, a significant increase in the carbonyl index and PET percentage weight loss was observed. The combined MW-assisted DES depolymerization and enzymatic hydrolysis of the treated PET residue using LCC variant ICCG resulted in a total monomer conversion of ≈16% (w/w) in the form of terephthalic acid, mono-(2- hydroxyethyl) terephthalate, and bis-(2-hydroxyethyl) terephthalate. Paper II illustrates combined green and fast glycolysis-hydrolysis depolymerization of PET under MW with excellent efficiencies. In MW-assisted glycolysis of PET, the catalytic activity of two DES based on (choline chloride-urea (DES 1)) and (choline chloride-thiourea (DES 2)) was evaluated and compared. Optimised glycolysis conditions were determined using Box Behnken Design (BBD) to attain maximum weight loss of PET, low crystallinity and increased carbonyl index of residual PET. DES volume of 4 mL, 5.5–6 mL of ethylene glycol, and 0.5 min MW irradiation time resulted in a prominent rise in PET weight loss and carbonyl index of residual PET. DES 2 showed an improved catalytic activity than that of DES 1 which is associated to its stronger interaction with EG and PET polymer chains during the course of the reaction. Residual PET obtained post glycolysis reaction was further depolymerized using MW assisted hydrolysis in the presence of weakly basic Na2CO3 and EG. Within 3-minute, the proposed sequential depolymerization technologies facilitated ≈99% conversion of PET to terephthalic acid (TPA), monohydroxyethyl terephthalate (MHET), and bis (2- hydroxyethyl) terephthalate (BHET) monomers produced at a yield of 62.79–80.66%, 17.22–34.79% and 0.54–0.59% respectively. Paper III explains a highly efficient 2-step microwave-based (MW) degradation of 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 depolymerized. 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 residual 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 residual 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 repolymerized as virgin PET. Hence, MW assisted DES technology is validated as an efficient process for boosting the depolymerisation of PET in an ultrafast and eco-friendly manner