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.