Nylon 6 depolymerization in supercritical alcohols studied by the QM/MC/FEP method

Nylon 6 depolymerization in supercritical (SC) alcohols was theoretically studied using the QM/MC/FEP method. All geometry optimizations and the Gibbs free energy calculation in the gas phase were calculated at the B3LYP/6-31++G(d,p) level of theory. The effect of different types of alcohols, methanol (MeOH) and isopropanol (iPrOH), is the focus on this investigation. There are two candidates for the depolymerization mechanism. One is an intermolecular mechanism (Rxn A), which produces aminocaproic ester as the major product. The other is an intramolecular mechanism (Rxn B), directly giving ε-caprolactam. The activation free energies (ΔG‡rxn) of Rxn A in the two alcohols are lower by 8–9 kcal mol−1 than those of Rxn B. Thus, the calculated rate constants (k) of Rxn A are much larger than those of Rxn B. The cyclization route to the aminocaproic ester is preferable by more than 15.0 kcal mol−1 to the alkylation at the amino group, and aminocaproic ester exclusively produces ε-caprolactam. The further reaction of ε-caprolactam, such as N-alkylation, is able to proceed only in MeOH. The activation free energies of the cyclization of aminocaproic acid are almost the same as those of the aminocaproic ester in the SC alcohols.