Enabling a Circular Materials Economy for PHAs Through Amine Catalyzed Methanolysis

Polyhydroxyalkanoates (PHAs) are a class of natural polyesters that are typically produced by microbial metabolic pathways, degrade in non-mediated environments, and are a growing contributor to a bio-based plastic supply chain. In contrast, the bulk of today's plastics are petroleum-derived, which promotes both fossil fuel depletion and the ever-increasing plastic waste problem. While the environmental degradation of PHAs is certainly an improvement over landfilling commodity plastics, chemical recycling would bring PHAs into a circular materials economy and enable recirculation of useful biogenic carbon. In this work, we apply the amine-catalyzed methanolysis (ACM) process developed for mixed polyester recycling to PHAs. We explore the conditions for model poly(3-hydroxybutyrate) (P3HB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate (PHBV) ACM and achieve near quantitative solids conversion. Additionally, monomer yields of >90% are found to be accessible with improved accounting of the many monomeric methyl ester products resulting from deconstruction. The optimized ACM conditions are applied to several formulated PHAs and PHA/poly(lactic acid) copolymers resulting in >80% monomer yields, highlighting the versatility of ACM in deconstructing PHAs and other biopolymers. We also explore the hydrolysis of the methyl ester products to their acid forms, which are found to be compatible with biological funneling back to P3HB. TEA and LCA of the deconstruction process with biological funneling were conducted to evaluate the impacts of incorporating PHA ACM as part of a circular plastics economy, with findings highlighting the importance of inclusion of more biopolymers to approach net-zero emissions.