One-Pot Chemo-bioprocess of PET Depolymerization and Recycling Enabled by a Biocompatible Catalyst, Betaine

Abstract

Poly(ethylene terephthalate) (PET) has been widely used in various industries due to its unique physical properties. However, PET causes major environmental problems globally due to its low degradability and recycling rate. Since it is nearly impossible to replace PET with other materials, an efficient approach for PET recycling is necessary for a circular economy. Herein, for a paradigm shift toward the approach for resource recovery of PET components, we developed an integrated process for depolymerizing PET and converting PET monomers to high-value products in a one-pot process. The key of our approach is the use of the biocompatible catalyst betaine in a glycolysis process that enables whole PET glycolysis slurry as a substrate to be directly applied to further bioprocesses. Based on the density functional theory (DFT) analysis, betaine effectively catalyzed PET depolymerization by two strong hydrogen interactions between betaine, EG, and PET as well as by the synergetic effect between the anion and cation groups of betaine. Through the glycolysis of PET with betaine and the optimized enzymatic hydrolytic process for the PET glycolysis slurry, PET was depolymerized to terephthalate (TPA, 31.0 g/L, 62.8%, mol/mol) and ethylene glycol (EG, 11.7 g/L, 63.3%, mol/mol) at high titers and high yields. This process was further applied to the bioconversion of TPA and EG present in the PET hydrolysate to protocatechuic acid (PCA) and glycolic acid (GLA), respectively. This one-pot chemo-bioprocess integrating chemical glycolysis, enzymatic hydrolysis, and bioconversion for PET depolymerization and recycling was suggested to be highly applicable to the upcycling of waste PET.