Tuning effect of amorphous Fe2O3 on Mn3O4 for efficiently atomic economic synthesis of imines at low temperature: Improving [O] transfer cycle of Mn3+/Mn2+ in Mn3O4

A facile and scaled-up synthesis route to efficient and environmental-friendly metal oxide catalysts with desirable properties is of great practical importance, owing to their excellent performance as heterogeneous catalysts in organic synthesis. Herein, a novel amorphous Fe2O3 modified Mn3O4 catalyst (Fe5Mn5-100) has been prepared by adopting simple co-precipitation method following low temperature baking. Fe5Mn5-100 showed exceptionally high catalytic activity for atomic economic production of imine from benzyl alcohol with aniline, giving a 98% imine yield at 60 oC for only 3 h, which is higher than all the reported non-noble and noble metal catalysts. Importantly, Fe5Mn5-100 could still exhibit extraordinary catalytic performance on the large-scale reaction without any solvent, including frequently-used toxic mesitylene, xylene or toluene. Interestingly, amorphous Fe2O3-100 provided none catalytic activity, and pure Mn3O4-100 showed very inferior catalytic activity towards this reaction. Further detailed characterizations and experimental results revealed that amorphous Fe2O3 plays an important part of expediting the [O] transfer cycle of Mn3+/Mn2+ in Mn3O4, and enhances the oxidation ability and acidity of Fe5Mn5-100. This discovery would provide a new avenue for atomic economic and environmental-friendly industrial synthesis of imine.