Application of Reaction-Extraction Coupling Technology:
Highly Efficient Preparation of ε‑Caprolactam over a
Deep Eutectic Solvent under Mild Reaction Conditions
A coupling
reaction-extraction technology was developed to produce
ε-caprolactam over a Cr-based deep eutectic solvent (DES). The
used DES, prepared by simple mixing and reacting under mild reaction
conditions using CrCl3 and urea, had adjustable Lewis acid
property and excellent coordination ability enchanced by the hydrogen
bonding microenvironment. High cyclohexanone oxime (CHO) conversion
(100%) and ε-caprolactam selectivity (98.7%) were obtained under
mild conditions, such as DESs 2.5 mmol, CHO 1 mmol, and 80 °C
for 4 h. DES [CrCl3]2[Urea] was easily prepared,
recovered, and reused simply for six runs without noticeable loss
of catalytic performance. The activation energy (Ea) of the reaction was 45.70 kJ mol–1, attributed to the excellent coordination ability of DES [CrCl3]2[Urea]. Meanwhile, the application of coupling
reaction-extraction technology enabled the real-time separation of
the product ε-caprolactam from the DES catalyst phase during
the reaction process, thereby avoiding its further side reactions
and improving its selectivity. Additionally, DES [CrCl3]2[Urea] had good reusability and general applicability
for the rearrangement reaction. This study provided a facile, intriguing,
and scalable approach to the highly efficient preparation of ε-caprolactam.