Insight into the effect of manganese oxidation state on the synthesis of ethylene urea from CO2 and ethylenediamine

Herein, a series of manganese oxide catalysts with different valences (Mn₃O₄, Mn₂O₃, and MnO₂) were designed and synthesized for the synthesis of ethylene urea (EU) from ethylenediamine (EDA) and carbon dioxide (CO₂). With a maximal EDA conversion of 82 % and EU selectivity of 99 % at 160 °C for 2 h, Mn₂O₃ catalysts had the best catalytic activity among them, which was superior to the reported catalysts. In the following order: Mn₂O₃ > MnO₂ > Mn₃O₄, the catalytic activity for the synthesis of EU from CO₂ and EDA decreased. Further characterization showed the Mn₂O₃ catalyst possessed a greater Mn³⁺/Mn⁴⁺ ratio and more surface oxygen vacancies than the MnO₂ and Mn₃O₄, which improved its capacity to adsorb and activate CO₂ and EDA. After four recycling runs, the EDA conversion slightly declined from 82 % to 56 % on Mn₂O₃ catalyst, while no obvious change in EU selectivity was observed. The loss of surface O_v contents and Mn³⁺ proportion were concluded as main reasons for the decrease in catalytic activity over Mn₂O₃ catalyst. This work demonstrated a metal oxide catalyst that was efficient in producing EU from CO₂ and EDA.