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

doi:10.6023/A24020054

Abstract

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.

Key words: electro-catalytic reductive amination, N-benzylcyclohexylamine, Ru-based catalyst, nickel phosphate, free radical reaction

Cite this article

Hao Liu, Xuli Xu, Yong Guo, Xiaohui Liu, Yanqin Wang. Efficient Electro-catalytic Reductive Amination of Aldehyde over Ru Deposited on Nickel Phosphate[J]. Acta Chimica Sinica, 2024, 82(5): 477-485.

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Fig. & Tab. 19

Figure 1. Nyquist plots of NiPOx before and after loading Ru

Figure 2. H2-TPR characterization of 2Ru/NiPOx and NiPOx

Figure 3. XRD pattern of 2Ru/NiPOx prepared at 150 ℃

Figure 4. SEM images of (a) NiPOx and (b, c) 2Ru/NiPOx

Figure 5. (a) HAADF-STEM image of 2Ru/NiPOx; EDS elemental mapping images for 2Ru/NiPOx: (b) Ru, (c) Ni, (d) P and (e) O

Figure 6. CV curves of 2Ru/NiPOx and CP in DMSO

催化剂	2b产率/%	FE/%
2Ru/NiPO_x	95.1	64.7
2Ru/Nb₂O₅	82.9	54.4
2Ru/Co₃O₄	87.2	51.5
2Ru/Al₂O₃	86.0	60.3
2Ru/C	76.4	50.6
CP	65.6	59.0

Table 1. Reductive amination using Ru catalysts with different supports

催化剂	2b产率/%	FE/%
NiPO_x	84.7	56.5
0.5Ru/NiPO_x	87.5	60.8
1Ru/NiPO_x	88.5	63.1
2Ru/NiPO_x	95.1	64.7
5Ru/NiPO_x	97.0	57.8

Table 2. Reductive amination using Ru/NiPOx with different Ru loading

Figure 7. Activity of 2Ru/NiPOx prepared at different temperatures

Figure 8. CV curves in the presence of the reductive amination reactants

Figure 9. Time course of the reductive amination over the 2Ru/NiPOx electrode

Figure 10. Reusing the 2Ru/NiPOx electrode in the reductive amination

Figure 11. DRIFT spectra of DMSO on Ru-based catalysts

Figure 12. DRIFT spectra of the reductive amination reactants on Ru-based catalysts

Figure 13. Free radical trapping experiments

溶剂	2b产率/%	FE/%
DMF	74.7	52.3
MeOH	44.3	5.6
Acetone	27.5	10.3
Toluene	—	—
p-Xylene	—	—
THF	—	—

Table 3. Reductive amination activity of 2Ru/NiPOx in different solvents

Figure 14. The proposed mechanism of reductive amination over 2Ru/NiPOx electrode

Figure 15. Mass spectra of the production of N-benzylcyclohexylamine when using (a) DMSO-D6 and (b) DMSO as the solvent

Figure 16. Electrochemical reductive amination at larger scale

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Ru/NiPO_x高效电催化醛还原胺化反应的研究