Continuous Flow Synthesis of 2-Imidazolidinone from Ethylenediamine Carbamate in Ethylenediamine Solvent over the CeO₂ Catalyst: Insights into Catalysis and Deactivation

Synthesis of 2-imidazolidinone (EU) from ethylenediamine carbamate (EDA-CA), which was produced via chemical absorption of CO₂ in ethylenediamine (EDA), with a fixed-bed flow reactor was performed using CeO₂ and EDA as a heterogeneous catalyst and reaction solvent, respectively. In the previously reported batch system for the same reaction, the yield of EU was low (62%) due to its sequential reaction with EDA into N,N′-bis(2-aminoethyl)urea (LU) with 38% yield in the liquid phase under the optimized reaction conditions at 413 K. In stark contrast, the flow reaction enabled the highly selective and high-yielding production of EU (94% yield) under the optimized reaction conditions at 363 K. This difference in the selectivity of EU originated from the lower ratio of the amount of EDA to that of the CeO₂ catalyst in the current flow reaction system compared to the previous batch system. The highly crystallized CeO₂ surface bearing both acid and base sites was found to be responsible for its high catalytic performance. The combination of the kinetics, density functional theory calculations, and adsorption experiments demonstrated that EDA-CA is preferentially adsorbed onto the surface of CeO₂ even in the presence of an excess amount of EDA, which is used as a solvent, to be activated and transformed into EU. The solid-state ¹³C magic angle spinning/cross polarization nuclear magnetic resonance spectroscopy revealed that the CeO₂ catalyst was deactivated gradually in a long-term reaction operation due to the formation and deposition of polyurea-like organic compounds on the catalyst surface.