氧化铌催化苯甲醇与苯胺N-烷基化反应的研究

doi:10.6023/A25010007

摘要/Abstract

通过固体催化剂实现醇的N-烷基化是一条环境友好的胺类化合物合成路线. 本工作将水热法制备的氧化铌应用于苯甲醇和苯胺的N-烷基化反应中, 与几种金属氧化物和传统的三氯化铝催化剂相比, 氧化铌的催化活性最优, 苯甲醇转化率100%, N-苄基苯胺收率65.2%. 进一步通过反应级数、溶剂和空间位阻效应以及碳正离子的相关实验对氧化铌催化苯甲醇N-烷基化的反应路径和机理进行研究, 结果表明反应经历的是苯甲醇生成碳正离子的S_N1反应机制. 此外, 苯甲醇和苯胺的原位红外吸脱附实验证明, 氧化铌能活化苯甲醇的C—O键以及苯胺的N—H键, 促进了碳正离子的生成以及苯胺N—H键的断裂, 从而具有很好的苯甲醇N-烷基化催化活性.

关键词: 氧化铌, 苯甲醇, 苯胺, N-烷基化, S_N1

The N-alkylation of alcohols over solid catalysts represents an environmentally benign approach for the synthesis of amine compounds. In this study, niobium oxide synthesized via a hydrothermal method was employed for the N-alkylation of benzyl alcohol with aniline. Compared to various metal oxides and conventional aluminum trichloride catalysts, niobium oxide demonstrated superior catalytic activity, achieving complete conversion of benzyl alcohol (100%) and a 65.2% yield of N-benzylaniline under optimized conditions (180 ℃, 1 MPa N₂, 4 h). To elucidate the reaction pathway and mechanism of benzyl alcohol N-alkylation over niobium oxide, comprehensive investigations were conducted, including reaction kinetics, solvent polarity effects, steric hindrance studies, and carbocation-trapping experiments. Kinetic analysis revealed a reaction order of 1.04 for benzyl alcohol and 0.008 for aniline, indicating a first-order overall reaction predominantly governed by the alcohol. Solvent polarity significantly influenced the reaction outcome: in toluene (a polar solvent), the yield remained stable at 65.0%, while in nonpolar solvents (cyclohexane and dodecane), the yields drastically decreased to 3.2% and 4.4%, respectively. This highlights the critical role of polar media in stabilizing carbocation intermediates. Steric effects were evaluated using diphenylmethanol as a bulky substrate, which surprisingly yielded 77% of the target product, suggesting minimal steric hindrance under the S_N1 mechanism. Definitive evidence for carbocation intermediate was obtained through experiments with 1-phenylcyclopropylmethanol, where only products derived from rearranged carbocations were detected. These results demonstrated that the reaction proceeds via the S_N1 mechanism involving carbocations generated from benzyl alcohol. Furthermore, in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis of benzyl alcohol and aniline revealed that niobium oxide activates the C—O bond of benzyl alcohol and the N—H bond of aniline, facilitating the formation of carbocations and the cleavage of the N—H bond of aniline, thereby exhibiting good catalytic activity for benzyl alcohol N-alkylation. The niobium oxide catalyst also exhibited exceptional stability, maintaining its activity even after six cycles. This study not only provides a comprehensive mechanistic understanding of Nb₂O₅-catalyzed N-alkylation but also establishes a green and practical strategy for sustainable amine synthesis.

Key words: Nb₂O₅, benzyl alcohol, aniline, N-alkylation, S_N1

引用此文

夏新宇, 侯善凯, 郭勇, 刘晓晖, 王艳芹. 氧化铌催化苯甲醇与苯胺N-烷基化反应的研究[J]. 化学学报, 2025, 83(3): 212-220.

Xinyu Xia, Shankai Hou, Yong Guo, Xiaohui Liu, Yanqin Wang. Catalytic N-Alkylation of Benzyl Alcohol and Aniline on Nb₂O₅ Catalyst[J]. Acta Chimica Sinica, 2025, 83(3): 212-220.

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图/表 14

催化剂	苯甲醇转化率/%	产物收率/%
催化剂	苯甲醇转化率/%	3	4	5	6	7
SiO₂	15.8	0	3.7	0	0	4.0
CeO₂	9.7	0	0	0	0	6.4
TiO₂	43.3	29.6	3.8	0	0	3.8
Al₂O₃	20.1	6.2	3.2	0	0	5.4
Nb₂O₅^a	100	65.2	2.4	10.9	5.1	0
AlCl₃	98.7	63.1	<1	1.6	<1	0

表1. 不同催化剂在苯甲醇N-烷基化反应中的性能

Table 1. The catalytic performance of different catalysts in the N-alkylation of benzyl alcohol with aniline

图1. 苯甲醇在金属氧化物催化剂上的可能反应路径图

Figure 1. The possible reaction pathways of benzyl alcohol on various metal oxides

图2. Nb2O5上苯甲醇和苯胺可能的胺化反应路径图

Figure 2. The possible amination reaction pathway of benzyl alcohol and aniline on Nb2O5

图3. Nb2O5上不同反应时间的苯甲醇N-烷基化反应产物分布

Figure 3. Products distribution versus time of benzyl alcohol N-alkylation on Nb2O5 Reaction condition: benzyl alcohol 0.2 g, aniline 0.25 g, catalyst 0.05 g, p-xylene 5 mL, 180 ℃, 1 MPa N2

图4. SN1和SN2反应历程图

Figure 4. Reaction process of SN1 and SN2

图5. Nb2O5上的苯甲醇和苯胺的反应级数

Figure 5. The reaction order of benzyl alcohol and aniline on Nb2O5 Reaction condition: catalyst 0.02 g, p-xylene 5 mL, 180 ℃, 1 MPa N2, 0.5 h

溶剂	转化率/%	收率/%
甲苯	100	65.0
十二烷	14.7	3.2
环己烷	16.2	4.4

表2. 溶剂效应对苯甲醇胺化的影响

Table 2. Effect of solvent on amination of benzyl alcohol

图6. 空间位阻对Nb2O5上醇N-烷基化反应的影响

Figure 6. The effect of steric hindrance on the N-alkylation of alcohols on Nb2O5 Reaction condition: aniline 0.25 g, catalyst 0.05 g, p-xylene 5 mL, 180 ℃, 1 MPa N2, 4 h

图7. 胺化机理的研究: 碳正离子

Figure 7. Study on the mechanism of amination: carbocation Reaction condition: aniline 0.25 g, catalyst 0.05 g, p-xylene 5 mL, 180 ℃, 1 MPa N2, 5 h

图8. (a), (b) Al2O3和(c), (d) Nb2O5上的苯甲醇原位吸脱附红外漫反射谱图

Figure 8. in-situ DRIFTS of benzyl alcohol on (a), (b) Al2O3 and (c), (d) Nb2O5

图9. (a), (b) Al2O3和(c), (d) Nb2O5上的苯胺原位吸脱附红外漫反射谱图

Figure 9. in-situ DRIFTS of aniline on (a), (b) Al2O3 and (c), (d) Nb2O5

图10. Nb2O5上苯甲醇和苯胺N-烷基化反应机理示意图

Figure 10. Mechanism of the N-alkylation of benzyl alcohol and aniline on Nb2O5

图11. Nb2O5催化剂在苯甲醇N-烷基化反应中的稳定性

Figure 11. Cyclic tests of Nb2O5 for the N-alkylation of benzyl alcohol Reaction condition: benzyl alcohol 0.2 g, aniline 0.25 g, catalyst 0.05 g, p-xylene 5 mL, 180 ℃, 1 MPa N2, 4 h

图12. (a)新鲜和(b)反应六次后Nb2O5催化剂的XRD图

Figure 12. XRD of (a) fresh and (b) after 6 reactions Nb2O5 catalysts

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