Application of Reaction-Extraction Coupling Technology: Highly Efficient Preparation of ε‑Caprolactam over a Deep Eutectic Solvent under Mild Reaction Conditions

A coupling reaction-extraction technology was developed to produce ε-caprolactam over a Cr-based deep eutectic solvent (DES). The used DES, prepared by simple mixing and reacting under mild reaction conditions using CrCl₃ and urea, had adjustable Lewis acid property and excellent coordination ability enchanced by the hydrogen bonding microenvironment. High cyclohexanone oxime (CHO) conversion (100%) and ε-caprolactam selectivity (98.7%) were obtained under mild conditions, such as DESs 2.5 mmol, CHO 1 mmol, and 80 °C for 4 h. DES [CrCl₃]₂[Urea] was easily prepared, recovered, and reused simply for six runs without noticeable loss of catalytic performance. The activation energy (E_a) of the reaction was 45.70 kJ mol^–1, attributed to the excellent coordination ability of DES [CrCl₃]₂[Urea]. Meanwhile, the application of coupling reaction-extraction technology enabled the real-time separation of the product ε-caprolactam from the DES catalyst phase during the reaction process, thereby avoiding its further side reactions and improving its selectivity. Additionally, DES [CrCl₃]₂[Urea] had good reusability and general applicability for the rearrangement reaction. This study provided a facile, intriguing, and scalable approach to the highly efficient preparation of ε-caprolactam.