Thermally Switchable Deep Eutectic Solvents for Sustainable ε-Caprolactam Synthesis: Synergistic Proton Transfer and Phase-Controlled Catalysis

ε-Caprolactam (CPL) is an important organic feedstock for Nylon-6 and other high-performance polymers. Conventional production relies on highly corrosive catalysts and generates substantial byproducts, highlighting the urgent need for greener, more sustainable alternatives. Herein, we report a thermally switchable deep eutectic solvent (DES) strategy, including the synthesis of a novel Bro̷nsted-acidic DES [PEG600][CF3COOH]3. Crucially, this study represents the first demonstration of a temperature-controlled phase-transfer catalytic system applied to the liquid-phase Beckmann rearrangement of cyclohexanone oxime to caprolactam. Under optimized conditions (110 °C, 1.0 h), the process achieves 99.7% conversion of cyclohexanone oxime and 95.9% selectivity to caprolactam. The product spontaneously partitions into the organic phase after the reaction, enabling efficient catalyst recovery and reuse over multiple cycles. Density functional theory (DFT) calculations reveal that the flexible PEG chain conformation and the proton of CF3COOH synergistically create dynamic Bro̷nsted-acid active sites, lowering the reaction activation energy to 23.90 kJ mol–1. Kinetic and thermodynamic analyses further corroborate the reaction mechanism and validate the efficacy of the thermally switchable system across both molecular and macroscopic scales. This work provides a new route to green, sustainable CPL production and opens new directions for integrating deep eutectic solvents into combined catalysis–separation processes, thus advancing process intensification with significant industrial relevance.