Efficient imine synthesis from oxidative coupling of alcohols and amines under air atmosphere catalysed by Zn-doped Al2O3 supported Au nanoparticles

Direct oxidative coupling of alcohols and amines is regarded as an effective and green approach for imine synthesis under mild conditions. In this work, Zn-doped gamma-Al2O3 supported Au nanoparticles was demonstrated as highly active and selective heterogeneous catalyst for a series of imine productions with good to excellent yields, from alcohols and amines via direct oxidative coupling under air atmosphere without extra base additives. Various physicochemical techniques, including ICP-MS, XRD, N-2 physisorption, TEM, XPS and CO2-/NH3-TPD, were used to study the properties of the catalysts. Well-dispersed Au-0 nanoparticles with a mean size of ca. 2.9 nm were found highly effective in activating alcohols in the presence of reactive amines. The amount of Zn2+ dopant and the calcination temperature of support during catalyst preparation showed crucial impact on tuning the intrinsic activity for oxidation of benzyl alcohol to benzaldehyde (i.e., the rate-determining step for the model reaction), which was disclosed to be related with the active surface oxygen species and the acidic-basic property of support. The 0.4% Au/Zn0.02Al2O3 catalyst calcined at 400 degrees C exhibited the highest TOF (39.1 h(-1)) at 60 degrees C based on a >99% yield to benzylideneaniline among all the ever-reported Au-based catalysts. Moreover, this catalyst could afford 98% yield to benzylideneaniline at only 30 degrees C and work effectively for the gram-scale synthesis. It also showed considerable stability after five consecutive recycling. (C) 2019 Elsevier Inc. All rights reserved.