(636h) Advancing Closed-Loop Recycling of Nylon 6 Waste through a Nucleophile-Assisted Depolymerization

Nylon 6, a major component of polyamides, has found wide applications in fishing, textile, packaging, medical, and automotive industries. Nylon 6 production process in the United States releases about 4.1 kg of carbon dioxide equivalent and requires 117 MJ of energy for every kg of nylon 6 [1]. However, less than 2% of nylon 6 is produced from chemically recycled monomer, ε-caprolactam (CPL), owing to the high operation temperatures and sensitivity to contaminants or additives of existing monomer recovery processes [2]. In this study, we presented a nucleophile-assisted approach to efficiently depolymerize nylon 6 waste to CPL (>75% yield) with a Brønsted acid catalyst at 210 ℃ in less than 3 hours. With primary alcohols being the solvent, we posited that the Brønsted acid catalyst can coordinate with the amide carbonyl, facilitating nucleophilic attack by alcohols to cleave the amide bond. The detection of alkyl esters of 6-aminocaproic acid supported our hypothesis and revealed that the nucleophile-assisted depolymerization started with alcoholysis which was followed by a cycloelimination step. We proposed a pseudo-homogeneous first-order kinetic model for this depolymerization process, which matched well with the experimental data. From the model, we found that the alcoholysis step is the rate-determining step. Additionally, an equilibrium exists for the cycloelimination step, where the transformation of alkyl esters of 6-aminocaproic acid towards CPL is more kinetically favorable. This nucleophile-assisted approach after further optimization is applicable to nylon 6 commodities and post-consumer nylon 6 waste such as fishing nets, T-shirts, and carpets without compromising the CPL yield, demonstrating its robustness. Lastly, techno-economic analysis and life cycle assessment revealed that the recycled nylon 6 obtained through this approach has a minimum selling price lower than the market price of virgin nylon 6 and can reduce greenhouse gas emissions compared to the conventional way. These results demonstrate that the whole process has the promise to be economically feasible and environmentally friendly to close the loop of nylon 6 recycling.

References

1. Nicholson, S. R., Rorrer, N. A., Carpenter, A. C., et al. Joule 5, 673-686 (2021).

2. Coates, G. W., and Getzler, Y. D. Y. L. Nat. Rev. Mater. 5, 501-516 (2020).