One-pot synthesis of Pd-promoted Ce–Ni mixed oxides as efficient catalysts for imine production from the direct N-alkylation of amine with alcohol

Abstract

Ce–Ni mixed oxides with different Ni/Ce molar ratios, promoted by a rather low amount of Pd (0.2 wt%), were prepared by a simple one-pot precipitation method. The thus-synthesized CeNi_XO_Y and Pd–CeNi_XO_Y catalysts were applied respectively to the direct N-alkylation of amine and alcohol via oxidative coupling, under O₂ in the absence of basic additives. The CeNi_XO_Y catalyst could provide a >99% yield of imine in the model alkylation reaction of aniline with benzyl alcohol at 100 °C. The Pd-promoted Pd–CeNi_XO_Y catalyst surprisingly showed an enhanced imine yield (>99%) at a mild temperature of 60 °C. This catalyst exhibited good reusability, and moreover, demonstrated high performance towards imine synthesis from various amines with alcohols. The presence of Pd species and the Ni/Ce molar ratio showed a synergistic impact on the conversion of aniline, as well as on the product selectivity, which was believed to be related to the improvement on the intrinsic activity of the oxidation of benzyl alcohol to benzaldehyde. Various physicochemical techniques, including ICP-MS, XRD, N₂ adsorption–desorption, UV-Raman, H₂-TPR, TEM, EPMA and XPS, were employed to study mainly the composition, structure and redox properties of the catalysts. The proportion of Ce³⁺ species and oxygen vacancies, which can be manipulated by the Ce–Ni redox system and the interaction between Ce and Ni, was crucial to the selective activation of alcohols in the presence of reactive amines. The activation of alcohol on the highly reactive Pd⁰ species was the other crucial factor.