Light-Colored rPET Obtained by Nonmetallic TPA-Based Ionic Liquids Efficiently Recycle Waste PET
Ruiqi Zhang, Xu Zheng, Xiaoqian Yao, Kuntong Song, Qing Zhou, Chunyan Shi, Junli Xu, Yi Li, Jiayu Xin, Ibrahim El Tantawy El Sayed, Xingmei Lü
Abstract
Poly(ethylene terephthalate) (PET) is well known as a needful part of comfortable and convenient life. However, the massive use of PET is a key issue in the contemporary environmental system. Glycolysis is a transesterification reaction that employs a catalyst and ethylene glycol (EG) to disintegrate PET into bis(hydroxyethyl) terephthalate (BHET). However, impurities are easily generated during the glycolysis process, and the purification of the product BHET is complicated, resulting in the yellowing of the recycled PET (rPET) obtained by repolymerization. In this research, TPA-based ionic liquids were synthesized using terephthalic acid (TPA) as an anion for the glycolysis of PET. TPA has carboxyl groups that can form hydrogen bonds with the hydroxyl group of EG. Meanwhile, the structure of TPA is similar to that of PET, which can reduce the introduction of heteroatoms in the reaction system and improve the purity of the monomer product. 1-Ethyl-3-methylimidazole terephthalate ([EMIm] 2 TPA) performed excellent catalytic activity and remarkable recycling performance after modifying the structure of ILs. Under optimization conditions, PET (5.0 g), EG (18.0 g), and [EMIm] 2 TPA (0.24 g) at 197 °C for 106 min, the conversion of PET and the yield of BHET reached 100 and 83.6%, respectively. Furthermore, [EMIm] 2 TPA could maintain excellent catalytic activity even after 15 times of utilization, showing promising recycling performance in the reported IL catalysts. The BHET acquired through catalytic depolymerization of [EMIm] 2 TPA offered a higher degree of advantages in purity. And the rPET produced by the repolymerization of the obtained BHET had a relatively lighter color.