High-yield and high-efficiency conversion of cyclohexanone oxime to ε-caprolactam in a green and facile reaction process over deep eutectic solvents

ε-Caprolactam (CPL) is mainly produced through Beckmann rearrangement of cyclohexanone oxime (CHO) catalyzed by oleum in industry, which suffers the problems such as serious corrosion problems. Herein, we developed a green and facile reaction process to produce CPL with deep eutectic solvents (DESs), which were investigated as catalysts and solvents in the reaction. The DFT results showed that the synergistic effect significantly decreased the energy barriers of reactants and intermediates, thus facilitating the CHO conversion to CPL. High conversion (100%) and selectivity to CPL (98.5%) were successfully obtained using [ZnCl2][Urea]2.0 (n(ZnCl2):n(Urea) = 1:2) at 130 °C for 2 h without other catalyst. The free energy (ΔG) and activation energy (Ea) of the reaction were −18.4 KJ mol−1 and 23.15 KJ mol−1, respectively. The reusability and general applicability of the DES were outstanding. The developed process provides an environmentally friendly and efficient candidate for rearrangements under mild conditions.