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

Abstract

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.

Comparative Study between 2‐Furonitrile and 2‐Cyanopyridine as Dehydrants in Direct Synthesis of Dialkyl Carbonates from CO2 and Alcohols over Cerium Oxide Catalyst

“…Meanwhile, under the “stoichiometric” (alcohol/nitrile/CH 3 CN=50 : 25 : 100) and “alcohol‐excess” (100 : 25 : 100) conditions, the possible maximum amount of dialkyl carbonates is 25 mmol. Under these three conditions, we conducted the reactions using two different amounts of CeO 2 catalyst (0.17 g and 0.68 g), which and whose related materials have been reported by many research groups to exhibit good catalytic activity for non‐reductive conversion of CO 2 [4,21–27,34–57] . The results obtained with the smaller amount of CeO 2 catalyst (0.17 g) are reflected by the reactivity of alcohols and nitrile dehydrants over the catalyst surfaces, and those with the larger amount of CeO 2 (0.68 g) provide the information about the saturation level of the product formation.…”

Direct Esterification of Alkylcarbamic Acids with Alcohols over CeO2 Catalyst

“…This result thus suggested that MoO 3 promoted the oxidative coupling of BZA to some extent; indeed, a CeO 2 -MoO 3 /SiO 2 catalyst was reported for the conversion of BZA into N-benzylidenebenzylamine in the presence of O 2 . [41] When any one of Dy 2 O 3 , Tb 4 O 7 , and Gd 2 O 3 was employed, the conversion(CO 2 ) became relatively high (entries [20][21][22]. These high values possibly originated from the broadening of 1 H NMR peak assignable to BZA-CA, which was used for the quantification of this compound, due to the leaching of paramagnetic species from each metal oxide into the reaction mixture.…”

“…From the Ce 3d XPS data in Figure S3, the proportion of the area of u''' peak, which appears away from other peaks at ~916 eV and is derived from Ce 4 + , in the total peak area was estimated (Table S6) since this value was previously used as indices of Ce 3 + content. [21,43] Due to the presence of two oxidation states of Ce species (i. e., Ce 4 + and Ce 3 + ) as well as the charge compensation, the proportion of Ce 3 + should reflect the number of oxygen vacancy. [43,44] However, for the CeO 2 catalysts calcined at different temperatures, the proportion of u''' peak was almost constant regardless of the calcination temperatures, while the yield(CO 2 ) based on S BET was apt to become higher at higher calcination temperature as described above.…”

“…In this respect, a more efficient catalytic system that enables to efficiently use CO 2 as a carbonyl source needs to be devised. In recent years, different reaction systems that do not require pressurized CO 2 have been developed for synthesizing organic urea derivatives from alkylcarbamic acids (Table S2), [17][18][19][20][21] which can be produced easily via simple chemical absorption of CO 2 into amines even at ambient pressure. [17] Likewise, Choi et al also demonstrated the esterification of alkylcarbamic acids with metal tetramethoxides such as titanium tetrabutoxide [22] and tetramethoxysilane [23] as alkoxy sources (entries 1-3 in Table S3).…”

Thermodynamic and Catalytic Insights into Non‐Reductive Transformation of CO2 with Amines into Organic Urea Derivatives

“…We recently found the Mn-doping as an effective means of reducing the acid-site density of CeO 2 and improving the catalyst stability. [129] We prepared the Mn-doped CeO 2 represented by Mn-1.0-CeO 2 -873(Co-precip) via the co-precipitation method followed by the calcination at 873 K for 3 h, and the Mn content in this catalyst was determined by X-ray fluorescence (XRF) analysis to be 0.88 wt%. As seen in Figure 19 (and also entry 22 in Table 16), the initial activity of this Mn-1.0-CeO 2 -873(Co-precip) catalyst was slightly lower than CeO 2 (HS), which was the identical catalyst to the one used for the other reactions (e. g., entry 8 in Table 10, entry 13 in Table 12, as well as Figures 14, 15, and 18), while exhibited the better stability, which was elucidated in the kinetic region away from the full conversion of EDA-CA to avoid the overestimation of catalyst stability [106] .…”