Efficient synthesis of imine from alcohols and amines over different crystal structure MnO_X catalysts

The redox properties of different crystal structure MnO₂ in organic reactions at low temperature of 60 °C were disclosed. Among the three crystal structures (α-, β- and γ-types), γ-MnO₂ showed unusual high heterogeneous catalytic performance, reusability and stability for imine formation from benzyl alcohol and aniline under mild conditions. The yield of imine over γ-MnO₂ reached up to 98.1% in 12 h under air atmosphere and its formation rate per surface area was 27.9 μmol h⁻¹ m⁻², and various imines could be obtained in excellent yield over γ-MnO₂. Based on series of catalyst characterizations, it was found that the high activity of γ-MnO₂ was mainly attributed to its high concentration of low-valence manganese ions. Furthermore, the catalytic mechanism was also disclosed. This research may have certain reference value for heterogeneous catalytic reactions by manganese.