Comparative Study of Manganese Oxides at Different Oxidation States for Halogen-Free Synthesis of Cyclic Carbonate from CO₂ and Epoxides

Manganese oxides with different valences, such as MnO, MnO2, Mn2O3, and Mn3O4, were successfully prepared and evaluated by the halogen-free cycloaddition reaction with CO2 and epoxides. The manganese oxides showed the following order of catalytic performance: Mn3O4 > MnO2 > Mn2O3 > MnO, and the Mn3O4 catalyst could achieve >99.9% propylene carbonate yield in N,N-dimethylformamide solvent at 393 K, 3 MPa CO2, with the maximum initial TOF of 28.6 h–1 among reported metallic oxide catalysts. Furthermore, the Mn3O4 catalyst could be reused at least 4 times and showed a broad substrate scope of applicability. In addition, the characterization and experiment results showed that the catalytic performance was related to the amount of acidic–basic active sites with moderate strength, and Mn3+ and oxygen vacancy sites worked as acidic and basic sites, respectively. Moreover, the possible reaction mechanism was proposed.