Efficient Electro-catalytic Reductive Amination of Aldehyde over Ru Deposited on Nickel Phosphate

Electro-catalytic reductive amination is an environmentally friendly and sustainable route to synthesize amines.In this paper, we prepared the 2Ru/NiPOx catalyst by incipient wetness impregnation using nickel phosphate as a support with Ru loading of 2% (w) and applied it for the electro-catalytic reductive amination of benzaldehyde and cyclohexylamine.The reaction was carried out in an undivided cell with tetrabutylammonium bromide (TBAB) as electrolyte and dimethyl sulfoxide (DMSO) as both H donor and solvent.When 2Ru/NiPOx on carbon paper was used as the cathode and Pt sheet as the anode, N-benzylcyclohexylamine was obtained in 95.1% yield with a Faraday efficiency of 64.7%.The Faraday efficiency started to decrease when the loading of Ru exceeded 2% (w), which is due to the fact that the excessive Ru accelerated the self-reaction of DMSO, thus decreased the Faraday efficiency.Electrochemical impedance spectroscopy (EIS) results showed that loading Ru on NiPOx could significantly improve the electron transfer rate during the reductive amination reaction.The cyclic voltammetry (CV) test showed that -4 V was the optimal potential for the 2Ru/NiPOx catalyst as the cathode during the electro-catalytic reduction of amination, which the paired anodic reaction was the oxidation of DMSO.The diffuse reflectance infrared Fourier transform (DRIFT) results showed that the self-reaction of DMSO by 2Ru/NiPOx was weaker than those of the other Ru-based catalysts, resulting in its high Faraday efficiency.The C＝N adsorbed on 2Ru/NiPOx had a higher electron cloud density, which was favorable for the attack of N by the H + generated from water splitting, thus promoting the reductive amination.Free radical trapping experiments proved that free radicals were involved in the reaction process.Isotope labeling experiments confirmed that DMSO is the H atom donor of the radical intermediate.Finally, a possible reaction mechanism was presented: the imine from benzaldehyde and cyclohexylamine reacting spontaneously was adsorbed on the surface of the 2Ru/NiPOx catalyst at the cathode, and then was attacked by H + and electrons to generate a free radical, which abstracted H from DMSO to produce the N-benzylcyclohexylamine product and one DMSO free radical, and the DMSO free radical gets H + and electrons at the cathode to regenerate DMSO.Keywords electro-catalytic reductive amination; N-benzylcyclohexylamine; Ru-based catalyst; nickel