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

Abstract

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.

Niobium pentoxide nanomaterials with distorted structures as efficient acid catalysts

“…iobium pentoxides show high potential as acid catalysts for the sustainable production of fuels and chemicals from inedible biomass sources, but their performance needs to be improved for competing with catalysts currently used in industry [1][2][3][4][5] . The physical and chemical properties of niobium oxides, including the acid properties, depend on their structure 1,2,4,6 .…”

Impact of Thermal Treatment of Nb2O5 on Its Performance in Glucose Dehydration to 5-Hydroxymethylfurfural in Water

“…Nevertheless, Nb 2 O 5 _300_air still exhibited a mostly amorphous structure, with XRD pattern strongly resembling that of Nb 2 O 5 ·nH 2 O (see Figure 1 ). Further increase in the temperature to 550 °C clearly adjusted the degree of structural order of niobium oxide and peaks typical of TT-niobium oxide phase (JCPDS, 28-317) [ 35 , 40 , 47 , 48 ] were observed for both samples (i.e., Nb 2 O 5 _550_N 2 and Nb 2 O 5 _550_air), regardless of the atmosphere used during the thermal treatments. On the other hand, as it can be seen in Figure 1 , the commercial Nb 2 O 5 (Nb 2 O 5 _SA) has a multiphase character and contains in majority H-niobium oxide phase (ICDD card No 37-1468), together with some T-niobium oxide (ICDD card No 27-1003) and a minor addition of M-niobium oxide (ICDD card No 32-0711) [ 49 , 50 ].…”

“…In distorted polyhedrons of Nb 2 O 5 , a part of the surface –OH groups functions as BA sites, while NbO 4 tetrahedra function as LA sites [ 33 ]. As a result, niobium pentoxide (Nb 2 O 5 ) and niobic acid (hydrated form of Nb 2 O 5 ) serve as highly acidic heterogeneous catalysts that display excellent activity in various reactions [ 31 , 32 , 34 , 35 , 36 ]. In this regard, a variety of niobium-based catalysts has been successfully applied in dehydration of hexoses in biphasic solvent media.…”

Metal‐Terpyridine Complexes in Catalytic Application – A Spotlight on the Last Decade

“…Niobium, as vanadium's heavier homologue, has also been combined with tpy – though in a completely different context. Nb 2 O 5 is known to be a water‐compatible, Lewis‐acidic catalyst that was used in various organic reactions, such as the formation of cyclic imides from dicarboxylic acids . The metal oxide is typically prepared by calcination of niobic acid which, in turn, can be prepared by hydrolyzing NbCl 5 or Nb(OC 2 H 5 ) 5 .…”