Selective Electrochemical Reductive Amination of Benzaldehyde at Heterogeneous Metal Surfaces

Ammonia is one of the largest volume commodity chemicals, and electrochemical routes to ammonia utilization are appealing due to increasingly available renewable electricity. In this work, we demonstrate an electrochemical analogue to reductive amination for the synthesis of benzylamine from benzaldehyde and ammonia. Previous works on electrochemical reductive amination generally focus on proof-of-concept outer-sphere routes. We demonstrate an inner-sphere route, opening a large phase space of heterogeneous electrocatalysts that can direct selectivity and drive the reaction. In our system, imine hydrogenation proceeds on a silver electrocatalyst at ambient conditions in methanol with an initial Faradaic efficiency toward the primary amine product of ~80% and partial current greater than 4 mA/cm2 at -1.96 V vs. Fc/Fc+ (-1.36 V vs. NHE). Silver was selected after evaluating diverse transition metal electrocatalysts, and with density functional theory, we found that the reaction rate on various metals is best described by the charge density distribution above the metal surface, independent of molecular adsorption energies. On silver, the catalyst that promotes amination with the highest Faradaic efficiency and one of the highest partial currents, the rate-determining step was found to be the initial electron transfer to the imine. Overall, this work on the kinetics of electrochemical reductive amination represents a step toward inner-sphere electrochemical reductive amination systems for the synthesis of amines that currently rely on thermochemical reductive amination.