Formic Acid: A Key Promoter for a Mild Polyamide-6 Depolymerization Approach

Polyamides (PA), mainly polyamide 6 (PA6), as fibers or thermoplastics, are widely used in several industry sectors because of their outstanding properties. However, the significant worldwide waste generation demands substantial research for the comprehension of depolymerization strategies. In this sense, alcoholysis represents a promising methodology which could allow the recovery of the monomer ε-caprolactam (CPL) or high-value derivatives, in sharp contrast with the mechanical treatment, which produces products with inferior quality. In this study, we demonstrate that the use of Brønsted acids enables the obtention of N -substituted amino esters as high-value derivatives under milder reaction conditions. The use of n -butanol as an eco-friendly alcoholysis promoter and the mildest temperature (150–180 °C) contributes to a sustainable chemical recycling of PA6. Density functional theory (DFT) calculations show that formic acid plays a key catalytic role by lowering the activation barrier of the nucleophilic addition step through a cooperative hydrogen-bonding network. Energy decomposition analysis (EDA) results further reveal that this stabilization arises from markedly stronger electrostatic and orbital interactions that outweigh the more unfavorable Pauli repulsion in the formic-acid-assisted pathway. Combined computational and experimental analyses also indicate that ε-caprolactam undergoes acid-catalyzed ring opening followed by nucleophilic addition onto formic acid, ultimately yielding the formylated product as the main species under formic-acid-assisted depolymerization conditions. Together, these insights provide a mechanistic framework for the rational design of more efficient depolymerization strategies for polyamide recycling.