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Heterogeneous Ni Catalysts for N‑Alkylation of Amines with Alcohols

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journal contribution
posted on 03.05.2013, 00:00 by Ken-ichi Shimizu, Naomichi Imaiida, Kenichi Kon, S. M. A. Hakim Siddiki, Atsushi Satsuma
Nickel nanoparticles loaded onto various supports (Ni/MOx) have been prepared and studied for the N-alkylation of amines with alcohols. Among the catalysts, Ni/θ-Al2O3 prepared by in situ H2-reduction of NiO/θ-Al2O3 shows the highest activity, and it acts as reusable heterogeneous catalyst for the alkylation of anilines and aliphatic amines with various alcohols (benzyl and aliphatic alcohols) under additive free conditions. Primary amines are converted into secondary amines and secondary amines into tertiary amines. For the reaction of aniline with an aliphatic alcohol the catalyst shows higher turnover number (TON) than precious metal-based state-of-the-art catalysts. Mechanistic studies suggest that the reaction proceeds through a hydrogen-borrowing mechanism. The activity of Ni catalysts depends on the nature of support materials; acid–base bifunctional supports give higher activity than basic or acidic supports, indicating that acid–base sites on supports are necessary. The presence of basic (pyridine) or acidic (acetic acid) additive in the solution decreased the activity of Ni/θ-Al2O3, which suggests the cooperation of the acid–base site of θ-Al2O3. For a series of Ni/θ-Al2O3 catalysts with different particle size, the turnover frequency (TOF) per surface Ni increases with decreasing Ni mean particle size, indicating that low-coordinated Ni species and/or metal–support interface are active sites. From these results, we propose that the active site for this reaction is metal–support interface, where low-coordinated Ni0 atoms are adjacent to the acid–base sites of alumina.