Ru/NiPOx高效电催化醛还原胺化反应的研究

doi:10.6023/A24020054

摘要/Abstract

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.

关键词: 电催化还原胺化, N-苄基环己胺, Ru基催化剂, 磷酸镍, 自由基反应

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

引用此文

刘浩, 徐旭莉, 郭勇, 刘晓晖, 王艳芹. Ru/NiPO_x高效电催化醛还原胺化反应的研究[J]. 化学学报, 2024, 82(5): 477-485.

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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图/表 19

图1. NiPOx负载Ru前后的Nyquist图

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

图2. 2Ru/NiPOx和NiPOx的H2-TPR表征

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

图3. 150 ℃下制备的2Ru/NiPOx的XRD图谱

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

图4. (a) NiPOx和(b, c) 2Ru/NiPOx的SEM图

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

图5. (a) 2Ru/NiPOx的HAADF-STEM图; 2Ru/NiPOx的EDS元素面扫描分布图: (b) Ru, (c) Ni, (d) P和(e) O

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

图6. 2Ru/NiPOx和CP在DMSO中的CV曲线图

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

表1. 不同载体Ru催化剂的还原胺化反应活性

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

表2. 不同Ru负载量的Ru/NiPOx催化剂的还原胺化反应活性

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

图7. 2Ru/NiPOx在不同还原温度下的反应活性

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

图8. 存在还原胺化反应物时的CV曲线图

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

图9. 在2Ru/NiPOx上进行还原胺化反应的时间曲线

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

图10. 2Ru/NiPOx循环还原胺化反应的结果

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

图11. Ru基催化剂对DMSO的DRIFT光谱

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

图12. Ru基催化剂对还原胺化反应物的DRIFT光谱

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

图13. 自由基捕获实验

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	—	—

表3. 2Ru/NiPOx在不同溶剂中的还原胺化活性

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

图14. 2Ru/NiPOx电极催化还原胺化反应的可能机理

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

图15. 分别以(a) DMSO-D6和(b) DMSO为溶剂制备N-苄基环己胺的质谱图

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

图16. 放大电化学还原胺化反应

Figure 16. Electrochemical reductive amination at larger scale

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