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.

In this study, 2-pyrone-4,6-dicarboxylic acid (PDC), a valuable monomer for bio-degradable plastics, was synthesised using terephthalic acid (TPA) derived from PET waste by a comprehensive chemo-microbial hybrid process. Depolymerisation of PET into TPA was performed via microwave-assisted hydrolysis using biomass-derived SiO₂ catalysts with thiol functionalisation. The increased PET hydrolysis efficiency was investigated by reaction kinetics studies. The conversion of TPA obtained by depolymerisation of PET waste into PDC was achieved using two recombinant Escherichia coli strains expressing tphAabc and tphB genes for TPA degradation into protocatechuic acid (PCA) through 3,4-dihydroxy-cyclohexa-1,5-diene-1,4-dicarboxylic acid (DCD) (E. coli PCA strain) and using ligABC genes to convert PCA to PDC through 4-carboxy-2-hydroxymuconate semialdehyde (CHMS) (E. coli PDC_PCA strain). Based on the high chemical depolymerisation of PET waste into TPA (over 97% efficiency), the double-catalyst system composed of the E. coli PCA strains with TPA degradation module and PDC synthesis module enables PET waste to be used in the PDC production platform with 96.08% of the overall efficiency.