(490d) Electrocatalytic Reductive Amination for Synthesis of Primary Amines

Ammonia is one of the largest volume commodity chemicals due to its use in the production of fertilizers, polymers, and specialty chemicals. Electrochemical routes to ammonia utilization in industrial chemical synthesis are appealing due to increasingly available distributed sources of renewable electricity. In this work, we investigate electrochemical utilization of ammonia to form carbon-nitrogen bonds. In particular, we explore an electrochemical analogue to reductive amination where a carbonyl group is converted into an amine. To study electrochemical reductive amination, we use benzaldehyde as a model carbonyl substrate and produce benzyl amine at ambient conditions by applying a reductive potential; the industrial equivalent conversion of benzaldehyde to benzyl amine proceeds at 100-150°C and 40-65 bar hydrogen gas. Previous research on electrochemical reductive amination focuses on outer-sphere processes for proof-of-concept synthesis of amines and the production of secondary amines, whereas we investigate an inner-sphere route.

In this work, we found that ammonia and benzaldehyde can be reacted under a reductive electrochemical potential to form benzyl amine. This reaction proceeds through an inner-sphere pathway that is extremely solvent and catalyst dependent. We achieved partial currents across metals ranging from 0.5 mA/cm² to over 3 mA/cm² and Faradaic efficiencies ranging from below 10% to above 70%, and we have developed a theoretical descriptor that describes the influence of catalyst on electrochemical activity. We also interrogated the mechanism for this reaction and found that the first electron transfer is the rate-determining step and that the reaction exhibits non-linear dependencies on some reactants such as the benzaldehyde substrate, indicating complexities arising from surface adsorbates and secondary reaction products. Through mechanistic understanding of this inner-sphere reaction, we can suggest routes toward improving its selectivity and expanding substrate scope to other nitrogen-containing chemicals.