Lewis Acid Catalysis of Nb2O5 for Reactions of Carboxylic Acid Derivatives in the Presence of Basic Inhibitors.

This review focuses on catalytic applications of niobium pentoxide (Nb2O5) and niobic acid (Nb2O5 ⋅ nH2O). These materials serve as heterogeneous acid catalysts that show excellent activity and selectivity for various types of reactions including hydrations, dehydrations, and oxidations. Most recently, they have been used for dehydration reactions of polyols and nucleophilic substitution reactions of carboxylic acid derivatives. A special focus is placed on the latter reactions, where superficial Nb5+ cations act as Lewis‐acidic sites that activate oxygen‐containing functional groups of substrates even in the presence of basic inhibitors such as water or amines. This unique behavior of Nb‐oxide‐based catalysts is a consequence of their Lewis acidity, which is characterized by their ability to simultaneously activate C=O bonds but not to engage in strong competitive adsorption of base molecules. Both experimental and theoretical investigations are presented in order to assess the characteristic features of the Nb‐oxide‐based catalysts together with their structural features. Note well the niobium: Catalytic properties of niobium pentoxide (Nb2O5) and niobic acid (Nb2O5 ⋅ nH2O) are reviewed. A special focus is placed on the role of superficial Nb5+ cations that act as Lewis‐acidic sites activating oxygen‐containing functional groups of substrates even in the presence of basic inhibitors such as water or amines. [ABSTRACT FROM AUTHOR]