The acetate anion promotes hydrolysis of poly(ethylene terephthalate) in ionic liquid–water mixtures

Abstract

A circular plastic economy reduces raw material consumption and discourages pollution. Chemical recycling upgrades the quality of recyclate and is a complementary approach to thermomechanical recycling of plastic waste. This study investigated the use of aprotic and protic ionic liquids (ILs) as solvents for chemical recycling by the hydrolysis of the most common polyester plastic, poly(ethylene terephthalate) (PET). Combinations of three types of cations (aprotic 1-alkyl-3-methylimidazolium, protic 1-methylimidazolium and protic 1,5-biazocyclo-[4.3.0]non-5-enium) combined with a range of anions (acetate, chloride, methanesulfonate, hydrogen sulfate, methyl sulfate, trifluoromethanesulfonate and chlorozincate) were used to hydrolyse PET in the presence of 15 wt% water as the co-solvent and reagent. PET conversion under the screening conditions (180 °C, 3 h, 5% PET loading) varied between 1 and 100%, with ILs containing the acetate anion enabling >97% PET conversion irrespective of the cation. Acidification with aqueous HCl recovered crude crystallised terephthalic acid (TPA). Significant crude yields (46–93%) were only observed for the acetate ILs. The purity of the crude TPA was 34–98%, with 1-ethy-3-methylimidazolium acetate, [C₂C₁im][OAc], and 1-methylimidazolium acetate, [C₁Him][OAc], yielding more and purer TPA than 1,5-biazocyclo-[4.3.0]non-5-enium acetate, [DBNH][OAc]. TPA solubility, PET conversion and TPA yield generally correlated well with increasing pK_a and higher hydrogen bond acceptor strength of the IL anion, suggesting that the depolymerisation mechanism in the acetate IL water mixtures is base catalysed. The screening identifies aqueous mixtures of the (pseudo)-protic IL [C₁Him][OAc] as promising catalytic solvent component for the chemical recycling of PET at an industrially feasible temperature, due to high isolated TPA yields and purity achieved at a low solvent cost ($1.74–2.15 per kg). However, an effective separation approach for the monomers TPA and ethylene glycol from the solvent remains to be developed.