One-pot realisation of reaction-extraction coupling: Innovative application and mechanistic insights of InCl3 deep eutectic solvents in Beckmann rearrangement reactions

Based on the principles of green chemistry, deep eutectic solvents (DESs) have been widely employed as novel green solvents in the Beckmann rearrangement of cyclohexanone oxime for the synthesis of caprolactam. However, existing catalytic systems face challenges like difficult separation, limited recyclability, high activation energy, low reaction rates, and suboptimal performance. To address these, this study synthesized eight novel indium-based acidic DESs using InCl3, a metal with strong coordination ability, as the hydrogen bond acceptor. The [InCl3][Urea] system (molar ratio 1:1) achieved high caprolactam conversion (98.2 %) and selectivity (98.8 %) at 80 °C for 2.5 h. The DES showed excellent reusability, maintaining stable performance over six cycles. The product was efficiently extracted into the organic phase, enabling a one-step reaction-extraction process and efficient DES recycling. DFT calculations revealed the DES’s molecular structure and bonding characteristics, while kinetic and thermodynamic studies indicated a final step energy barrier of 40.2 kcal mol−1 and activation energy (Ea) of 28.25 kJ mol−1. The coupled reaction-extraction technology, combined with the In-based DES catalyst, reduced activation energy and significantly shortened reaction time, providing a green, direct, and efficient pathway for caprolactam production under mild conditions.