Iron-nased Lewis/Brønsted deep eutectic solvents enable controlled hydrolysis of nylon 6,6 for monomer recovery

A new study published in Engineering demonstrates the controlled hydrolysis of polyamide 6,6 (PA 66) using iron‑based Lewis/Brønsted acidic deep eutectic solvents (LBDESs), converting the widely used polymer into its monomers under relatively mild conditions. PA 66 accounts for approximately one‑third of global nylon production, with an annual output of 2.5 million tonnes, yet its limited biodegradability and underdeveloped end‑of‑life recycling strategies leave most post‑consumer waste directed toward energy recovery or landfilling.

Researchers from the University of Pisa and the University of the Basque Country UPV/EHU report that two iron‑based LBDES systems, FeCl₃·6H₂O/methanesulfonic acid (1:1 molar ratio, A1_1) and FeCl₃·6H₂O/acetic acid (1:1 molar ratio, B1_1), promote quantitative depolymerization of PA 66 at 180 °C within 5 h. The process breaks down the polyamide chains to yield adipic acid and hexamethylenediamine, with the latter recovered as diammonium dichloride. Following optimization of reaction parameters and work‑up procedures, overall monomer recovery exceeds 85%, with individual monomer yields reaching high levels after purification.

The method shows consistent performance across different PA 66 types, including virgin pellets, virgin fibers, and real post‑consumer 100% nylon hosiery in 15‑denier and 20‑denier forms, supporting practical applicability beyond laboratory‑scale materials. Key reaction improvements include an alkaline treatment step to remove iron impurities, followed by selective extractions using ethyl acetate and ethanol to separate and isolate the two monomers with improved purity. The dual acidic character of LBDESs is believed to facilitate amide bond activation, with iron(III) coordinating to carbonyl oxygen and protons protonating amide nitrogen to promote hydrolytic chain scission.

Comparative assessment of environmental metrics, including the environmental factor E, energy economy coefficient ε, and energy impact factor ξ, indicates the LBDES‑based approach is competitive with previously reported depolymerization methods, such as supercritical water hydrolysis, microwave‑assisted acid hydrolysis, ammonolysis, and hydrogenolysis processes. While the acetic acid‑containing system B1_1 offers advantages in cost, corrosivity, and sustainability potential, both LBDES formulations achieve full PA 66 conversion under the optimized conditions.

The study supports chemical recycling as a viable route to preserve material value without the property degradation associated with mechanical recycling, providing a monomer‑level recycling pathway that can produce PA 66 matching the quality of virgin polymer. Further refinements may focus on enhancing monomer separation purity and scaling solvent recycling to strengthen the circularity of the overall process.

The paper “Iron-Based Lewis/Brønsted Deep Eutectic Solvents for the Hydrolysis of Nylon-6,6,” is authored by Marco Rollo, Francesca Rastelli, Marta Ximenis, Elisa Martinelli, Gianluca Ciancaleoni, Haritz Sardon. Full text of the open access paper: https://doi.org/10.1016/j.eng.2026.02.001. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.