Selective Modification of the Product Profile of Biocatalytic Hydrolyzed PET via Product-Specific Medium Engineering

Over the past years, enzymatic depolymerization of PET, one of the most widely used plastics worldwide, has become very efficient leading to the end products terephthalic acid (TPA) and ethylene glycol (EG) used for PET re-synthesis. Potent alternatives to these monomers are the intermediates BHET and MHET, the mono- and di-esters of TPA and EG which avoid total hydrolysis and can serve as single starting materials for direct re-polymerization. This study therefore aimed to selectively prepare those intermediates through reaction medium engineering during the biocatalytic hydrolysis of PET. After a comparative pre-screening of 12 PET-hydrolyzing enzymes, two of them (LCC^ICCG, IsPETase^wt) were chosen for detailed investigations. Depending on the reaction conditions, MHET and BHET are predominantly obtainable: (i) MHET was produced in a better ratio and high concentrations at the beginning of the reaction when IsPETase^wt and 10 % EG was used; (ii) BHET was produced as predominant product when LCC^ICCG and 25 % EG was used. TPA itself was nearly the single product at pH 9.0 after 24 h due to the self-hydrolysis of MHET and BHET under basic conditions. Using medium engineering in biocatalytic PET-hydrolysis, the product profile can be adjusted so that TPA, MHET or BHET is predominantly produced.