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

Abstract

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 MnOx and red mud is presented. With the catalysis of active red mud, the synthesis of imines over MnOx 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 MnOx. 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 MnOx 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.