Steering the Au−FexCo1Oy interface for efficient imine synthesis at low temperature via oxidative coupling reaction

Abstract

Low-temperature imine formation through oxidative coupling of alcohols with amines under base-free conditions is very important for fine-chemical synthesis. Supported-Au catalysts show superior performances toward selective oxidation reactions but are scarcely reported for the above application. In this work, the finely-dispersed Au nanoparticles (ca. 2.4 nm) loaded on magnetic FexCo1Oy oxides were developed as novel, efficient and reusable heterogeneous catalyst. Varying the Fe/Co molar ratio allowed tuning the Au−FexCo1Oy interface and further boosting the selective oxidation performance of Au. The optimum Au/Fe0.25Co1Oy catalyst showed >99% yield of benzylideneaniline and 20.1 molimine molAu−1 h−1 productivity using the model reaction at 30 °C under air atmosphere without alkaline additives. This was the best result compared to benchmark Au-based catalysts in the literature. In addition, this catalyst exhibited superior performances towards gram-scaled synthesis and diverse substrates. XRD, N2-physisorption, TEM, H2-TPR, O2-TPD, EPR, pyridine-adsorbed FT-IR, and kinetic investigations were used to study this Au catalytic system. It was found that the synergy between Fe2O3 and Co3O4 can induce formation of abundantly mobile and reactive oxygen species (i.e., O2− and O− related to oxygen vacancies) and electron-rich metallic Au species on the surface of mesoporous FexCo1Oy oxides. These interfacial features were disclosed to be vital to activating molecular O2 and O H bond in an alcohol. Besides, it was demonstrated that the surface Lewis acidic sites associated with coordinatively unsaturated Co3+ ions can help facilitate the tandem oxidation-condensation reactions.