One-pot amination of cyclohexanone-to-secondary amines over carbon-supported Pd: Unraveling the reaction mechanism and kinetics

Abstract

• Catalytic transformation of cyclohexanone into secondary amines. • A reaction map for the catalytic amination of cyclohexanone is proposed. • A kinetic model describing the amination of cyclohexanone is elucidated. • The effect of temperature, reactant concentrations and H 2 -equivalents is assessed. The liquid phase amination of the cyclohexanone (CyO) with aniline (PhNH 2 ) was studied on a Pd/C catalyst using NaCO 2 H as a H-donor. Catalytic tests were performed in batch reactors by varying reactant concentrations (0, 0.1, 0.2 and 0.4 mol/L), the temperature (80–160 °C) and H 2 -equivalents availability (0, 2 and 4 H 2 -equivalents with respect to CyO). The results indicate that, the use of NaCO 2 H barely affect the reactivity (measured as initial reaction rate) while it does influence the selectivity to secondary amines. This effect was ascribed to the limited effectivity of the H 2 transfer, which was controlled by the water formed after the condensation of the CyO with the PhNH 2 , hydrogen solubility in toluene, and the mass transfer from the liquid to the catalyst surface. The experimental observations were consistent with a multi-step mechanism consisting in: a first step of condensation of the ketone and the amine to form an imine, followed by the disproportionation of the imine into secondary amines. The kinetic measurements at initial rate conditions were well interpreted by a Langmuir Hinshelwood (L-H) kinetic model, which suggest that the surface reaction to form the intermediary imine is the rate limiting step (R.L.S).