Reactivity of hydroxylamine ionic liquid salts in the direct synthesis of caprolactam from cyclohexanone under mild conditions

Abstract

The reactivity of several sulfobutyl hydrosulfate hydroxylamine ionic liquid salts in the direct synthesis of caprolactam from cyclohexanone under mild conditions was investigated. The results showed that the cyclohexanone conversion was mainly affected by cation species in the molecules of the hydroxylamine ionic liquid salts, and hydroxylamine N,N,N-trimethyl-N-sulfobutyl hydrosulfate salt was a better choice for the direct synthesis of caprolactam. The optimum reaction condition was at 80 °C for 4 h, and the suitable molar ratio of cyclohexanone : hydroxylamine ionic liquid salt : ZnCl₂ was 2 : 1 : 3. Under the optimal reaction conditions, cyclohexanone was almost completely converted into caprolactam, corresponding to 99.1% cyclohexanone conversion and 92.0% caprolactam selectivity. Furthermore, the reaction medium acetonitrile, and the ionic liquid which was combined in the hydroxylamine salt, can be recovered after the reaction, achieving an eco-friendly route for the direct synthesis of caprolactam.