Tailoring Zn-based diacidic functionalization of Deep eutectic solvent Catalyst: Green and efficient synthesis of ε-Caprolactam under mild conditions

A novel Zn-based acidic functionalized deep eutectic solvent (DES) catalyst were prepared by a “tailoring” mixing procedure of Brønsted acidic hydrogen bond donors (HBD) and Lewis acidic hydrogen bond acceptor (HBA). It is found that the hydrogen bonded cluster with thermal stability and adjustable acidity has an excellent flexible hydrogen bond “assembly mode”, providing a suitable coordination microenvironment for Zn active center. The functionalized DESs were investigated to produce caprolactam from cyclohexanone oxime. The results showed that DES [ZnCl2][ChCl][TCA]2 had an excellent catalytic performance for the reaction with 100% cyclohexanone oxime conversion and ε-caprolactam 99.4% selectivity at 80 °C for 1.5 h, attributing to the strong complexing ability of Zn active center and the synergistic effect of BrØnsted and Lewis acid sites of DES, which also decreased the energy barriers of the intermediates and transition states, thus reducing the reaction energy (Ea = 29.22 kJ mol−1 and ΔG = –32.8 kJ mol−1). Otherwise, the recovered DES [ZnCl2][ChCl][TCA]2 was reused seven times with no significant decrease in its catalytic performance. The advantages of this method, such as high yield, short reaction time, and easy catalyst recovery, make the DES catalytic system a potential method for Beckmann rearrangement.