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Amidation of Carboxylic Acids with Amines by Nb2O5 as a Reusable Lewis Acid Catalyst

Among 28 types of heterogeneous and homogenous catalysts tested, Nb2O5 shows the highest yield for direct amidation of n‐dodecanoic acid with a less reactive amine (aniline). The catalytic amidation by Nb2O5 is applicable to a wide range of carboxylic acids and amines with various functional groups, and the catalyst is reusable. A comparison of the results of the catalytic study and an infrared study of the acetic acid adsorbed on the catalyst suggests that activation of the carbonyl group of the carboxylic acid by Lewis acid sites on Nb2O5 is responsible for the high activity of the Nb2O5 catalyst. Kinetic studies show that Lewis acid sites on Nb2O5 are more water‐tolerant than conventional Lewis acidic oxides (Al2O3, TiO2). In comparison with the state‐of‐the‐art homogeneous Lewis acid catalyst for amidation (ZrCl4), Nb2O5 undergoes fewer negative effects from basic additives in the solution, which indicates that Nb2O5 is a more base‐tolerant Lewis acid catalyst than the homogeneous Lewis acid catalyst.