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

Cerium oxide (CeO2) was recently reportedto functionas a highly active heterogeneous catalyst in the flow synthesis of2-imidazolidinone (EU) without gas-phase CO2 from ethylenediaminecarbamate (EDA-CA), the latter of which can be synthesized easilyfrom ethylenediamine (EDA) and ambient CO2, in an EDA solvent.However, catalyst deactivation due to the surface deposition of polyurea-likecompounds has remained a grand challenge. In this study, in an attemptto develop new catalysts with better stability than conventional CeO2 by tuning surface properties, the addition of a second metalto CeO2 was examined in the kinetic region. Mn-doped CeO2 (Mn content = 1 wt %) prepared via a coprecipitation methodwas found to be a more stable catalyst in the EU production with alower deactivation rate constant of 0.018 h(-1) thanthe pure CeO2 (0.058 h(-1)). Mn speciesincorporated in the CeO2 lattice exhibited good resistanceagainst their leaching during the reactions operated in the EDA solvent,leading to a higher stability of Mn-doped CeO2 catalyststhan the pure CeO2 and also Mn-loaded CeO2 thatreadily underwent the leaching of Mn species during the reaction.The origin of the better stability of Mn-doped CeO2 thanpure CeO2 was suggested from various characterization datato be the decreased density of the acid sites. The high density ofacid sites of the pure CeO2 possibly leads to the multipointadsorption of polyurea-like compounds, resulting in the catalyst deactivation.In contrast, the low density of acid sites has been suggested to retardsuch undesirable interactions with catalyst poisons and thus improvedthe catalyst stability in the EU production from EDA-CA in the EDAsolvent.