Insight into Synergistic Brønsted and Lewis Acidic Deep Eutectic Solvent for Beckmann Rearrangement of Cyclohexanone Oxime

Abstract

ε-Caprolactam (CPL) is industrially produced by Beckmann rearrangement of cyclohexanone oxime (CHO) under fuming sulfuric acid, resulting in corrosive and environmental issues. Herein, we prepared triethylamine hydrochloride (TEAHC) and ZnCl₂ formed deep eutectic solvent (DES) [TEAHC:2ZnCl₂] with Brønsted and Lewis acid sites for efficient liquid rearrangement, achieving 100% conversion of CHO and 95.5% yield of CPL at 80 °C for only 1 h. The results show that ZnCl₂ in [TEAHC:2ZnCl₂] can promote the detachment of proton, which acts as Brønsted acid site combined with another ZnCl₂ molecule to synergistically catalyze the reaction. In the Brønsted acid catalyzed process, the nitrogen atom in CHO as reactive site can be readily attacked by the proton to form protonated CHO, which subsequently undergoes rearrangement. By adding ZnCl₂ into TEAHC to obtain [TEAHC:2ZnCl₂], the formation of ZnCl₂-CHO complex results in a significant reduction in reaction energy barrier through synergistic effect of Brønsted and Lewis acids. Particularly, the fitted reaction kinetics and low activation energy also confirm the rearrangement can occur under low reaction temperature. Thus, the DESs with efficient catalytic performances for ketoxime rearrangements provide a potential method to design active sites for Beckmann rearrangements of oximes under mild reaction conditions.