Origin of Nb2O5 Lewis Acid Catalysis for Activation of Carboxylic Acids in the Presence of a Hard Base

Abstract

The Nb2O5 surface catalyzes the amidation of carboxylic acids with amines through Nb5+ Lewis acid activation of the C=O group. In this work, DFT calculations were applied to theoretically investigate the C=O bond activation of a model carboxylic acid (acetic acid) on θ-Al2O3(110), anatase TiO2(101), and T-Nb2O5(100) surfaces. The adsorption sites, adsorption energies, reaction energy barriers, electronic properties, and vibrational frequency of acetic acid were examined in detail. It was found that the bond activation of the carbonyl group is most efficient on Nb2O5, although the adsorption energy is larger on Al2O3 and TiO2. The most efficient C=O bond activation on Nb2O5 results in the lowest energy barrier of C−N bond formation during amidation. The Nb2O5 surface also shows larger tolerance to methylamine and water molecules than Al2O3 and TiO2 surfaces. These crucial factors contribute to the highest amidation catalytic reactivity on Nb2O5. Furthermore, the position of the mean density of states of the d-conduction band of the active metal site relative to the Fermi energy level correlates well with the efficiency in the C=O bond activation and, consequently, the catalytic activity for amidation. These results suggest that, unlike a classical understanding of strong acid sites of metal oxide surfaces, interaction of a carbonyl HOMO with an unoccupied metal d-orbital, or, in other words, covalent-like interaction between a carbonyl group and metal adsorption site, is relevant to the present system. © 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.