Repolymerization-suppressed closed-loop recycling of PET to BHET via metal-free ammonium acetate catalysis

Although metal-based catalysts demonstrate excellent catalytic activity in polyester depolymerization, residual metal ions often induce undesirable repolymerization of depolymerization products, significantly limiting monomer recovery efficiency. To address this technical challenge, this study innovatively developed an environmentally benign glycolysis process for poly(ethylene terephthalate) (PET) using ammonium acetate as a catalyst, achieving an efficient product separation and high-value recovery of all monomer products. Under optimized conditions (200 °C, 3 h), ammonium acetate demonstrated exceptional catalytic performance, achieving a PET conversion rate of 96.3%. This remarkable performance clearly indicates a synergistic effect between the acetate anions and ammonium cations in promoting PET depolymerization. Through precisely controlled gradient vacuum distillation, ethylene glycol (EG, purity > 99%) and bis(2-hydroxyethyl) terephthalate (BHET, purity > 98%, melting point 110 °C) were sequentially recovered with 98% and high yields, respectively. Additionally, the catalyst ammonium acetate decomposed during the reaction and was subsequently removed. The analytical results (HPLC, FTIR, XRD, ¹H NMR, DSC) verified the structural consistency and high purity of the recovered BHET. This strategy not only effectively suppresses the BHET repolymerization side reactions induced by metal residues, but also significantly enhances process economics through a simplified separation workflow. The proposed approach offers an innovative solution for establishing a sustainable "depolymerization-regeneration" circular economy system for waste polyester valorization.

Graphical abstract