Novel Brønsted Acidic Ionic Liquids as High Efficiency Catalysts for Liquid-Phase Beckmann Rearrangement

Abstract

Exploring environmentally friendly, efficient, cheap and recyclable catalysts are essential for the development of green, sustainable and mild processes for the liquid-phase Beckmann rearrangement. Herein, a novel caprolactam-based Brønsted acidic ionic liquid ([CPL][2MSA]) was developed for the conversion of cyclohexanone oxime (CHO) to caprolactam (CPL), not only as a catalyst, but also as a mild reaction medium. Under the reaction conditions for the reaction temperature (90 °C), reaction time (2 h) and mole ratio ([CPL][2MSA]: CHO = 3:1), [CPL][2MSA] possesses plenty of high sulfonate groups, which exhibit high conversion (100%) and selectivity (95%) without any other co-catalysts or metals. Based on the thermogravimetric (TGA) and differential scanning calorimetry (DSC) analyses, the decomposition and glass transition temperatures are gradually increased with the increase in MSA mole content, revealing the existence of hydrogen-bonded clusters. Interestingly, the occurrent route of the liquid-phase Beckmann rearrangement for CHO in [CPL][2MSA] is revealed by in situ FT-Raman. In addition, the dominating H-bond combination between CHO and [CPL][2MSA] is further confirmed by COSMO-RS model. The activation energy (Ea) of the reaction is calculated by the first-order reaction kinetics. Thus, the [CPL][2MSA] with plenty of acidic catalytic active species is an environmentally friendly and efficient candidate for the liquid-phase Beckmann rearrangement.