Chemo-Biological Upcycling of Poly(ethylene terephthalate) to Multifunctional Coating Materials

Abstract

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 hydrolyzed glycolyzed products into TPA as a key enzyme for chemo-enzymatic 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.

Keywords: biocatalysis; catechol; esterase; poly(ethylene terephthalate); upcycling.

Figure 1

Preparation of TPA from PET by a combination with glycolysis and enzymatic hydrolysis. (a) Three different PET‐to‐TPA hydrolysis pathways: (1) path 1 (enzymatic hydrolysis of pure BHET after crystallization), (2) path 2 (enzymatic hydrolysis of BHET mixture after filtration, (3) path 3 (enzymatic hydrolysis of BHET mixture without purification). (b) Temperature dependency of glycolysis. (c) FTIR spectra of the glycolyzed BHET in the PET‐to‐BHET conversion, reagent‐grade EG, BHET, and PET pieces. (d) Mass spectroscopy data of purified BHET after filtration. (e) ¹H and (f) ¹³C NMR data of purified BHET after recrystallization. (g–i) Enzymatic hydrolysis profiles in paths 1–3, respectively. Among three different PET‐to‐TPA hydrolysis pathways, the one‐pot enzymatic hydrolysis of glycolyzed mixtures from path 3 without filtration and recrystallization achieves a high TPA yield (116.3 %). Paths 1–3 were performed at 30 °C and 1000 rpm in 100 mm sodium phosphate (pH 7.5) buffer for 10 h. The definition of enzyme unit used in this study is described in the Supporting Information.

Figure 2

(a) Enzymatic hydrolysis profiling on 1 g l ⁻¹ of PET oligomers. (b) MALDI‐TOF pattern analysis of the hydrolysis products for 10 min, 30 min, 1 h, and 10 h. (c) Suggested mode of action toward the dimer. Molecular docking simulations of (d) dimeric substrate (EG‐TPA‐EG‐TPA‐EG), (e) BHET, and (f) MHET. 2 U mL⁻¹ of Bs2Est was used for the enzymatic hydrolysis.

Figure 3

(a) Bioconversion of TPA to catechol using an engineered E. coli strain. (b) Production of catechol from hydrolyzed TPA and reagent‐grade TPA. (c) General catechol coating process for various substrates (aluminum foil, PET, and Teflon), and the introduction of secondary functional layers on the catechol‐coating: chitosan, polylysine, and silver nanoparticle (AgNP). (d) Picture and (e) E. coli inhibition zone formation of neat PET and catechol only‐, chitosan‐, polylysine‐, and AgNP‐coated PET films. (f) (left) Escherichia coli inhibition zone diameter (negative control, 7.0 mm) and (right) relative L‐929 cell adhesion capacity of neat PET and catechol only‐, chitosan‐, polylysine‐, and AgNP‐coated PET films.