Catalytic N-Alkylation of Benzyl Alcohol and Aniline on Nb2O5 Catalyst

doi:10.6023/A25010007

Abstract

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

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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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Fig. & Tab. 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

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

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

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

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

Figure 4. Reaction process of SN1 and SN2

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

Table 2. Effect of solvent on amination of benzyl alcohol

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

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

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

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

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

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

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

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氧化铌催化苯甲醇与苯胺N-烷基化反应的研究

Catalytic N-Alkylation of Benzyl Alcohol and Aniline on Nb₂O₅ Catalyst

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