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 Ov 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.