Reusable Task‐Specific Ionic Liquids for a Clean ε‐Caprolactam Synthesis under Mild Conditions

Abstract Brønsted‐acidic ionic liquids that bear a sulfonic acid group, known as Forbes acids, show a good catalytic activity for the Beckmann rearrangement, used to prepare ε‐caprolactam, which is a precursor of Nylon 6. The activity essentially stems from the acidity of the sulfonic acid group. Although these task specific ionic liquids suffer from a high viscosity, this drawback can be circumvented at higher temperatures. A combination of the hydrogen sulfate anion and the sulfonic acid group of the cation is needed to obtain the rearrangement product rapidly under mild conditions. When using an excess of ionic liquid, we postulate that the internal pressure of the ionic medium, generated by the high viscosity and the high number of hydrogen‐bonds, is strong enough to contribute to a decrease of the thermodynamic barrier. In accordance with the “Principles of Green Chemistry,” we have developed a synthesis of ε‐caprolactam that requires no additional chemicals except cyclohexanone oxime and the reusable TSIL.