Improvement of Stability of CeO2–Based Catalysts by Mn Doping for the Synthesis of 2-Imidazolidinone from Ethylenediamine Carbamate

Cerium oxide (CeO2) was recently reported to function as a highly active heterogeneous catalyst in the flow synthesis of 2-imidazolidinone (EU) without gas-phase CO2 from ethylenediamine carbamate (EDA-CA), the latter of which can be synthesized easily from ethylenediamine (EDA) and ambient CO2, in an EDA solvent. However, catalyst deactivation due to the surface deposition of polyurea-like compounds has remained a grand challenge. In this study, in an attempt to develop new catalysts with better stability than conventional CeO2 by tuning surface properties, the addition of a second metal to CeO2 was examined in the kinetic region. Mn-doped CeO2 (Mn content = 1 wt %) prepared via a coprecipitation method was found to be a more stable catalyst in the EU production with a lower deactivation rate constant of 0.018 h–1 than the pure CeO2 (0.058 h–1). Mn species incorporated in the CeO2 lattice exhibited good resistance against their leaching during the reactions operated in the EDA solvent, leading to a higher stability of Mn-doped CeO2 catalysts than the pure CeO2 and also Mn-loaded CeO2 that readily underwent the leaching of Mn species during the reaction. The origin of the better stability of Mn-doped CeO2 than pure CeO2 was suggested from various characterization data to be the decreased density of the acid sites. The high density of acid sites of the pure CeO2 possibly leads to the multipoint adsorption of polyurea-like compounds, resulting in the catalyst deactivation. In contrast, the low density of acid sites has been suggested to retard such undesirable interactions with catalyst poisons and thus improved the catalyst stability in the EU production from EDA-CA in the EDA solvent.