Improvement of Stability of CeO<sub>2</sub>–Based Catalysts by Mn Doping for the Synthesis of 2-Imidazolidinone from Ethylenediamine Carbamate
Ryotaro Fujii, Mizuho Yabushita, Yingai Li, Yoshinao Nakagawa, Keiichi Tomishige
Abstract
Cerium oxide (CeO 2 ) was recently reported to function as a highly active heterogeneous catalyst in the flow synthesis of 2-imidazolidinone (EU) without gas-phase CO 2 from ethylenediamine carbamate (EDA-CA), the latter of which can be synthesized easily from ethylenediamine (EDA) and ambient CO 2, 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 CeO 2 by tuning surface properties, the addition of a second metal to CeO 2 was examined in the kinetic region. Mn-doped CeO 2 (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 CeO 2 (0.058 h –1 ). Mn species incorporated in the CeO 2 lattice exhibited good resistance against their leaching during the reactions operated in the EDA solvent, leading to a higher stability of Mn-doped CeO 2 catalysts than the pure CeO 2 and also Mn-loaded CeO 2 that readily underwent the leaching of Mn species during the reaction. The origin of the better stability of Mn-doped CeO 2 than pure CeO 2 was suggested from various characterization data to be the decreased density of the acid sites. The high density of acid sites of the pure CeO 2 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.