Intrinsic mechanism of active metal dependent primary amine selectivity in the reductive amination of carbonyl compounds

Abstract For the catalytic reductive amination of carbonyl compounds, the kind of active metal used is the most important factor determining the catalytic selectivity in heterogeneous catalysis systems. However, a detailed understanding of the intrinsic mechanism is still lacking. In this work, by evaluating the reductive amination of butyraldehyde and the hydrogenation reaction of secondary imine on various metal catalysts, the competitive hydrogenation reactions of primary imine and secondary imine are proven to be the key factors determining primary amine selectivity. DFT calculations verify that Co, Ni, and Ru, rather than Fe, Rh, Pd, and Pt, tend to show high primary amine selectivity in terms of adsorption energy and activation energy. It should be noted that the different stable adsorption modes of secondary imine on metal surfaces (C&N adsorption mode on Fe, Co, Ni, Ru, and Rh, and N adsorption mode on Pd and Pt) are key factors affecting these reaction characteristics. Moreover, microkinetic simulation proves that the coadsorption of NH3 improves primary amine selectivity on Co, Ni, and Ru surfaces. Combining the theoretical and experimental results, it is clearly verified that the active metal determines the catalytic selectivity of the primary amine by affecting the competitive hydrogenation reactions of the primary imine and secondary imine.