Efficient chemical recycling of poly(3-hydroxybutyrate) to crotonic acid via a sodium crotonate/stearyl alcohol catalytic system

Abstract

Poly(3-hydroxybutyrate) (PHB) is a biodegradable, bio-based polymer with significant application potential. The development of efficient chemical recycling strategies for PHB is essential for advancing a circular bioeconomy. However, existing methods for depolymerizing PHB into crotonic acid (CA), a valuable platform chemical, often rely on harsh reaction conditions, suffer from low selectivity, or require high energy input. Herein, we report a novel catalytic system based on sodium crotonate (NaCA) and stearyl alcohol (StOH), which enables high-yield conversion of PHB to CA under mild conditions (150 °C, 3 Torr). Owing to the synergistic action of NaCA and StOH, rapid depolymerization of PHB is achieved, affording a CA yield of 80.8% with 99.6% purity. Further enhancement using the NaCA/StO-CA synergistic system increases the CA yield to 96.1% with 99.4% purity. Mechanistic investigations, supported by NMR, FTIR, and density functional theory (DFT) calculations, reveal that StOH serves dual roles: it acts as a transesterification agent to reduce the molecular weight of PHB and as a high-boiling solvent to promote continuous removal of CA. Meanwhile, NaCA catalyzes C–O bond cleavage and double bond formation, enabling sustained CA production. The catalytic system was successfully applied to post-consumer PHB waste and PHB in plastic blends, achieving CA yields exceeding 94%. Moreover, a sequential batch recycling strategy was developed for PLA/PHB mixed plastics, in which PLA is first depolymerized to lactide, followed by the selective conversion of PHB to CA, thereby avoiding cross-contamination. The catalytic system exhibits excellent recyclability, maintaining high efficiency over multiple cycles, and thus offers a promising and sustainable approach for closed-loop recycling of bio-based polyesters.