Efficient imine synthesis via oxidative coupling of alcohols with amines in an air atmosphere using a mesoporous manganese–zirconium solid solution catalyst

Abstract

Direct oxidative coupling of alcohols with amines using a non-precious metal oxide catalyst under mild conditions is highly desirable for imine synthesis. In this work, a mesoporous Mn₁Zr_xO_y solid solution catalyst prepared by a co-precipitation method showed excellent catalytic performance in imine synthesis from primary alcohols and amines without base additives in an air atmosphere. XRD, N₂ physisorption, H₂-TPR, O₂-TPD, EPR and XPS were comprehensively used to unravel its structural, redox and amphoteric properties that closely depended on the interaction between MnO_y and ZrO₂ with a variable Zr ratio. The Mn₁Zr_0.5O_y catalyst presented the highest fractions of Mn³⁺ ions and reactive oxygen species on the surface, and the highest concentrations of acidic–basic sites, which were disclosed to play important roles in activating alcohols and molecular O₂ in the rate-determining step. In the model reaction of oxidative coupling of benzyl alcohol with aniline, such enhanced features of the Mn₁Zr_0.5O_y catalyst can promote the intrinsic catalytic activity (iTOF of 1.87 h⁻¹) and boost benzylideneaniline formation (5.56 mmol g_cat.⁻¹ h⁻¹) based on a >99% yield at 80 °C respectively at a fast response. It can also work effectively at a room temperature of 30 °C, as well as for the gram-grade synthesis. This is one of the best results among all the MnO_y-based catalysts in the literature. Moreover, this catalyst showed good stability and a wide substrate scope with good to excellent yields of imines.