Red mud-mediated cross-coupling of alcohols and amines to imines over MnOx catalysts

The development of new application scenarios for the red mud, a huge by-product from the aluminum manufacture, has become urgent because of the increasingly stringent demands of environmental protection. Herein, a very simple and sustainable approach to the design of a synergistic catalytic system for the direct oxidative cross-coupling of alcohols and amines to (un)symmetric imines by physically mixing MnO x and red mud is presented. With the catalysis of active red mud, the synthesis of imines over MnO x can achieve an excellent (>98%) yield at 60 °C, air as oxidant for 8 h Without this catalysis, the imine yield reaches only 25%. The red mud clearly assists the reaction, resulting in a 4 ∼ 10-folds higher increase over reactions using only MnO x . From detailed experiments and characterizations, the synergistic effects of the extraordinary performance of this catalytic system were confirmed to be a combination of the reactivity of MnO x in the oxidation of alcohols and the enhancement by the active red mud in the condensation of aldehydes and amines. This work demonstrates a facile strategy for revival of the hazardous red mud. It could turn “waste” into wealth and greatly promote the catalytic applications of red mud.