Highly efficient one-pot aerobic oxidation of cyclohexane to adipic acid using task-specific metal-based ionic liquid

Abstract

Adipic acid is a significant building block of polymers and is manufactured on a massive scale, primarily to produce nylon. However, the current route depends heavily on using corrosive HNO3 while releasing N2O as a potent greenhouse gas. Herein, we developed an efficient strategy for the highly selective aerobic oxidation of cyclohexane into adipic acid using metal-based ionic liquid as a catalyst. Particularly, the optimised catalyst 1-carboxymethyl-3-methylimidazolium chloride cobalt exhibited a high selectivity towards adipic acid (73.9%). It was discovered that the electron-donating ability of anions and the electrostatic between cations and anions could influence the charge density of Co(II) by anion coordination and then influence the oxidation performance. Meanwhile, the hydrogen bond interaction between the hydrogen-bond donor groups of the IL and cyclohexanone, as well as between the hydrogen-bond donor groups of the IL and the carboxylic radicals could promote the formation of adipic acid. This simple, efficient and green protocol has great potential for industrial applications in adipic acid production.