Ru/NiPO_(x)高效电催化醛还原胺化反应的研究

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

电催化还原胺化是一种环境友好且可持续的合成胺途径.本工作建立了一个以2Ru/NiPO_(x)为阴极催化苯甲醛和环己胺电化学还原胺化反应的体系,该体系可以高效获得95.1%的N-苄基环己胺产率,以及64.7%的法拉第效率,其中活化二甲基亚砜(DMSO)还原胺化与DMSO自身反应之间具有相匹配的活性是实现定向氢转移还原胺化的关键.最后,通过漫反射红外傅里叶变换光谱(DRIFTS)表征、自由基捕获实验和DMSO-D6同位素标记实验探索了反应的机理.

Electro-catalytic reductive amination is an environmentally friendly and sustainable route to synthesize amines.In this paper,we prepared the 2Ru/NiPO_(x) 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/NiPO_(x) 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 NiPO_(x) 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/NiPO_(x) 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/NiPO_(x) was weaker than those of the other Ru-based catalysts,resulting in its high Faraday efficiency.The C=N adsorbed on 2Ru/NiPO_(x) 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/NiPO_(x) 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.