Efficient recycling of polyamide 6 to ε-caprolactam under mild conditions

Abstract

Polyamide 6 (PA6) is a kind of engineering plastic containing N atoms, the low activity of amide linkage makes it hard to recycle under mild conditions. In this work, a solid acid catalyst, L-Nb₂O₅ is applied to convert PA6 into ε-caprolactam and as high as 82.2 % yield was achieved at 260°C, a relatively low temperature with THF as solvent. The catalyst can be used for 5 times with little decline of catalytic activity, and can be regenerated through calcination. Catalytic mechanism investigation reveals that Lewis acidity rather than acid concentration plays a decisive role in depolymerization, whereas Brönsted acid sites exhibit an inhibitory effect on the process. Enhanced specific surface area facilitates the exposure of more acidic sites, thereby promoting depolymerization efficiency. Systematic cyclic stability assessments and scaled-up experiments confirm the system's robustness and practical viability for industrial implementation. This study ultimately establishes a facile and efficient methodology for achieving closed-loop recycling of polyamide 6 (PA6) to ε-caprolactam (CPL) through rational catalyst design.