Supramolecular purification of aromatic polyester monomers from chemical depolymerization†

Abstract

The separation of poly(ethylene terephthalate) (PET) comonomers produced from the chemical recycling and depolymerization of PET is a critical step in realizing polyester material circularity. Various grades of PET (e.g., fiber, bottle, tire-cord) each require precise degrees of crystallinity which is controlled by the introduction of isomeric phthalates, comonomers which then contaminate secondary monomer streams. The use of supramolecular interactions as a method of extracting and separating these comonomers is herein described. The binding constants (K_b) between various-sized cyclodextrins with dimethyl terephthalate/dimethyl isophthalate and bis-2-hydroxyethyl terephthalate/bis-2-hydroxyethyl isophthalate are measured by fluorescence quenching and titration measurements. It is shown that beta-cyclodextrins most favorably bind the monomers (K_b up to 830 M⁻¹) and that methyl ester comonomers bind more strongly than the glycolysis products. Competitive binding measurements between isomeric aromatic esters indicate alpha cyclodextrin binds dimethyl terephthalate (K_b = 328 M⁻¹) 6-times more strongly than the dimethyl isophthalate isomer (K_b = 55 M⁻¹). Crosslinking of the cyclodextrins with diisocyanate or perfluorinated bis-cyanobenzene moieties resulted in particles that could effectively remove the monomers from depolymerization solutions (up to 0.6 mmol of monomer per gram of particle). The results suggest supramolecular extraction and separation may be a scalable means of improving the purity and atom recovery of secondary recycled polyester feedstocks.