Selective chemical recycling of polyhydroxybutyrate into high-value hydroxy acid using Taurine organocatalyst

Abstract

Polyhydroxyalkanoates (PHAs) are receiving significant attention due to their biobased origin, biodegradability, and excellent barrier properties. However, their high cost compared to traditional plastics necessitates the development of recycling technologies to retain their value post-use. Despite being thermoplastics, PHAs are difficult to recycle mechanically due to their narrow processing window, particularly for polyhydroxybutyrate (PHB), and conventional chemical recycling routes often lead to non-selective degradation and dehydration to crotonic acid, yielding complex product mixtures with limited valorization potential. In contrast, this study presents a selective chemical recycling method for PHB that suppresses dehydration pathways by using naturally occurring taurine as an organocatalyst. Taurine outperforms other catalyst families, such as Brønsted acids and bases, in terms of depolymerization yield and selectivity, achieving 98% enantiomerically pure 3-hydroxybutyric acid (HBA) in the optimized process. Density functional theory (DFT) calculations provided insights into the pH-dependent HBA elimination mechanisms demonstrating that taurine does not play a role in this process under very basic nor acidic conditions. A liquid-liquid extraction technique was developed to separate HBA from by-product crotonic acid, successfully maintaining the R-enantiomeric form of the recovered HBA. This method is applicable to both synthetic PHB and biological PHB samples, including copolymers and blends. Overall, this taurine-catalyzed PHB depolymerization approach, inhibits the formation of dehydrated by-products and offers a promising solution for selective recycling of PHB into valuable chiral building blocks.