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 (Ea) 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.