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.

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Chemo-biological upcycling of poly(ethylene terephthalate) (PET) developed in this study includes the following key steps: chemo-enzymatic PET depolymerization, biotransformation of terephthalic acid (TPA) into catechol, and its application as a coating agent. Monomeric units were first produced through PET glycolysis into bis(2-hydroxyethyl) terephthalate (BHET), mono(2-hydroxyethyl) terephthalate (MHET), and PET oligomers, and enzymatic hydrolysis of these glycolyzed products using Bacillus subtilis esterase (Bs2Est). Bs2Est efficiently hydro-lyzed glycolyzed products into TPA as a key enzyme for chemoenzymatic depolymerization. Furthermore, catechol solution produced from TPA via a whole-cell biotransformation (Escherichia coli) could be directly used for functional coating on various substrates after simple cell removal from the culture medium without further purification and water-evaporation. This work demonstrates a proof-of-concept of a PET upcycling strategy via a combination of chemo-biological conversion of PET waste into multifunctional coating materials.

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