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 (Ea) 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.