Efficient aerobic oxidation of amines to imines catalyzed by two-dimensional ultrathin δ-MnO2

The selective oxidation of amines to imines is a key reaction in heterogeneous catalysis. However, the reported catalysts suffer from high cost, complicated preparation processes, low catalytic activity, and poor selectivity. Herein, two-dimensional (2D) δ-MnO₂ is reported for the first time as a robust catalyst for the aerobic oxidation of amines to imines while avoiding over-oxidation of amines at full conversion. Compared with δ-MnO₂ after first exfoliation and bulk δ-MnO₂, δ-MnO₂ after second exfoliation (S-MnO₂) displayed a 2D ultrathin nanosheet birnessite-type structure, larger specific surface, higher content of active lattice oxygen and surface hydroxyl groups, and higher Mn³⁺/(Mn⁴⁺+Mn²⁺) ratio. Among the three catalysts, S-MnO₂ not only gave 95.0 % imine selectivity at 99.1 % conversion, but also exhibited the highest catalytic activity with a turnover frequency (TOF) as high as 2.29 h⁻¹, which was far superior to other existing protocols. The Mn³⁺/(Mn⁴⁺+Mn²⁺) ratio and active lattice oxygen were found to play pivotal roles in enhancing the catalytic activity. A plausible mechanism for benzylamine oxidation catalyzed by 2D ultrathin δ-MnO₂ was proposed. This study offers novel insights into the Mn-catalyzed aerobic oxidation of amines, serving as a significant complement to the applications of 2D manganese oxides.