Reusable Deep Eutectic Solvents for Clean ε‑Caprolactam Synthesis under Mild Conditions

A deep eutectic solvent (DES) with adjustable Brønsted acidity was prepared easily by choline chloride (ChCl) and trifluoromethanesulfonic acid (TfOH). The obtained DES [ChCl]­[TfOH]₃ exhibited great stability and excellent catalytic performance in the Beckmann rearrangement reaction as solvent and catalyst. High cyclohexanone oxime (CHO) conversion (100%) and ε-caprolactam selectivity (98.8%) were obtained under mild conditions, such as DESs 0.03 mol, CHO 0.01 mol, 100 °C for 2 h. DES [ChCl]­[TfOH]₃ was easily prepared, recovered, and reused simply for seven runs without noticeable loss of catalytic performance. The free energy (ΔG) and activation energy (E_a) of the reaction were −32.8 and 39.82 kJ mol^–1, respectively. And, DFT results showed that the intermolecular hydrogen bond between ChCl and TfOH improved the thermal stability of DES (the six-membered ring structure formed containing Cl and N atoms) and provided a good sponge effect for the active hydrogen catalytic center required for the reaction (the hydrogen bond formed by Cl and hydroxyl). This process offered several advantages including good yields, short reaction times, and easy catalyst recoveries and makes the DES catalytic system a potentially practical procedure for Beckmann rearrangement.