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

The liquid phase amination of the cyclohexanone (CyO) with aniline (PhNH2) was studied on a Pd/C catalyst using NaCO2H 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 degrees C) and H-2-equivalents availability (0, 2 and 4 H(2)equivalents with respect to CyO). The results indicate that, the use of NaCO2H 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 PhNH2, 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).

Automated summary

The use of NaCO2H has little impact on reactivity but does affect the selectivity to secondary amines in the liquid phase amination of cyclohexanone and aniline. The limited effectiveness of H2 transfer, hydrogen solubility in toluene, and mass transfer from liquid to catalyst surface are key factors influencing selectivity. The experimental observations support a multi-step mechanism involving condensation of ketone and amine to form an imine, followed by disproportionation into secondary amines with the surface reaction to form the intermediary imine being the rate limiting step.