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.

Lignocellulosic biorefineries have received considerable attention for the purpose of producing high-value chemicals and materials. Levulinic acid (LA) is an important biomass-derived platform chemical that is produced from sugar-based biomass. Unfortunately, the catalysts reported thus far have shortcomings, such as expensive starting materials, complicated synthesis or purification operations, and a low LA yield under harsh reaction conditions. Herein, we develop a novel dual-functional catalyst, HScCl 4 , by combining Brønsted acid (HCl) and Lewis acid (ScCl 3 ) sites. The as-prepared HScCl 4 catalyst shows high efficiency and high selectivity for converting 5-hydroxymethylfurfural (HMF) to LA in a biphasic system consisting of methyl isobutyl ketone (MIBK) and water. The density functional theory (DFT) results show that the synergistic catalytic effect, originating from the Brønsted and Lewis acidic sites of HScCl 4 , significantly decreases the energy barriers of reactants and intermediates, thus facilitating the conversion of HMF to LA. Moreover, the efficient separation of LA in the water-MIBK biphasic system by extracting LA to the MIBK phase minimizes the side reactions of LA and thus the formation of humins while significantly improving the LA yield. The conversion of HMF and the selectivity for LA are 100 and 95.6% at 120 °C for 35 min, respectively. The free energy (ΔG) and activation energy (E a ) of the reaction are −30 kcal mol −1 and 13.7 kJ mol −1 , respectively. The developed process provides a green, sustainable, and efficient pathway to produce LA from biomass-derived HMF under mild conditions.

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