Valorization of polycaprolactone for the production of nylon-6 monomers

Abstract

Upcycling biodegradable polyester waste into value-added chemicals can not only avoid CO₂ emissions but also achieve sustainable carbon recycling. Herein, we report a one-pot catalytic process to deconstruct polycaprolactone into 6-aminocaproic acid (EACA) and caprolactam (CPL), two monomers of nylon-6, using aqueous ammonia and Ru catalysts. No additional hydrogen is required, and the total yield of EACA and CPL could reach 90.2% using a ternary catalyst, Ru/CeO₂/RGO, at 140 °C. It is verified that 6-hydroxyhexanoic acid and 6-hydroxyhexanoamide are the initial intermediates resulting from polycaprolactone decomposition, which undergo dehydrogenation, amination and re-hydrogenation to transform into EACA and 6-aminohexanoamide over the Ru catalyst. The hydrolysis of 6-aminohexanoamide produces EACA, and the reverse reaction between dehydration of EACA and hydrolysis of CPL results in their coexistence. The protocol demonstrated here provides a novel route to recycle polycaprolactone to valuable chemicals.