Hydrogenolysis Recycling of Polycaprolactone Plastics Promoted by Trifluoromethanesulfonates

The widespread use of plastics results in the annual generation of large amounts of waste, posing significant environmental challenges due to inefficient treatment and the resulting pollution. Therefore, the development of green and efficient recycling methods is crucial. Hydrolysis offers a promising strategy for the degradation and recycling of polyester plastics, producing either monomers or high-value-added chemicals. Polycaprolactone (PCL), a biodegradable alkyl polyester synthesized via ring-opening polymerization of the ε-caprolactone monomer, possesses desirable mechanical properties and biocompatibility, yet its recovery rate remains low. This work demonstrates the efficient hydrolysis of PCL under solvent-free conditions using a cost-effective cobalt phosphine complex and a metal trifluoromethanesulfonate as co-catalysts. Gas chromatography (GC) and proton nuclear magnetic resonance (1H NMR) spectroscopy were used to analyze the reaction products. We investigated the effects of hydrogen pressure and temperature, optimizing reaction conditions: 160 ℃, 6 MPa hydrogen pressure, 10% Co(OAc)2-triphos (1,1,1-tris(diphenylphosphinomethyl)ethane), and 10% Sc(OTf)3 (scandium trifluoromethanesulfonate) for 12 h. Under these conditions, 1,6-hexanediol was produced with 99% conversion and 99% yield. Even at reduced pressure (2 MPa) and catalyst loadings (1% each), 99% conversion and 97% yield were achieved. 1,6-hexanediol is a valuable fine chemical and an important intermediate. Catalyst recovery studies revealed limited recyclability. Mechanistically, the strong Lewis acidity of metal trifluoromethanesulfonates facilitates rearrangement, whereas weaker Lewis acids merely reduce the influence of the carbonyl group. The Co(OAc)2-triphos system catalyzes carbonyl hydrogenation, unlike the Ni(OAc)2-triphos system, which only hydrogenates C=C double bonds. Therefore, the Co(OAc)2-triphos/Sc(OTf)3 catalyst system first promotes the hydrogenation of PCL to form a hemiacetal, followed by intramolecular 1,3-hydrogen transfer, depolymerizing PCL into 6-hydroxyhexanal, which is subsequently hydrogenated to produce 1,6-hexanediol. This work provides a feasible strategy for enhancing the recycling of discarded PCL plastics, contributing to efforts to address the environmental challenges posed by plastic waste.