Structural Studies of Bulk to Nanosize Niobium Oxides with Correlation to Their Acidity

Abstract

Hydrated niobium oxides are used as strong solid acids with a wide variety of catalytic applications, yet the correlations between structure and acidity remain unclear. New insights into the structural features giving rise to Lewis and Brønsted acid sites are presently achieved. It appears that Lewis acid sites can arise from lower coordinate NbO and in some cases NbO sites, which are due to the formation of oxygen vacancies in thin and flexible NbO systems. Such structural flexibility of Nb-O systems is particularly pronounced in high surface area nanostructured materials, including few-layer to monolayer or mesoporous NbO·nHO synthesized in the presence of stabilizers. Bulk materials on the other hand only possess a few acid sites due to lower surface areas and structural rigidity: small numbers of Brønsted acid sites on HNbO arise from a protonic structure due to the water content, whereas no acid sites are detected for anhydrous crystalline H-NbO.

Niobium pentoxide nanomaterials with distorted structures as efficient acid catalysts

“…4b). In the lower wavenumber region of the spectra (<500 cm −1 ), the modes arising from the vibrations of octahedra as a whole (at 122 cm -1 ) and from the bending of Nb−O−Nb bonds (at 228 and 308 cm -1 ) are observed, as well as the stretching modes of long Nb−O bonds 17,[32][33][34][35] . The vibration of short Nb−O bonds occurs at higher wavenumbers.…”

“…Thermal treatment at high temperatures is normally employed to achieve structural modifications (crystallization and phase transformations) due to the lack of a suitable number of softchemistry routes 6 . However, thermal treatment is accompanied by dehydration/dehydroxylation of the material, drastic decrease of the surface area, disappearance of the acid sites, and consequent decrease of the catalytic activity 2 , making it difficult to correlate the type and strength of active sites with particular structures 2,12,[14][15][16][17] , which in turn hinders the development of high-performance catalysts for specific practical applications. Searching for new synthesis routes is therefore essential for producing nanosized niobium pentoxides exhibiting strong acidity, high surface area and different structural motifs, which will also allow getting insights into the structure-acidity-activity relationships.…”

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

“…On account of the Nb oxidation state (+ 5), where all 4d electrons are shared with the 2p band of oxygen, Nb 2 O 5 is generally obtained in the form of white powders and exhibits a large band gap. [68][69][70] Furthermore, Nb 2 O 5 is structurally based on 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 interconnected NbO 6 octahedra that share corners or edges with different degrees of distortion. Depending on the starting material, pressure, and temperature, Nb 2 O 5 may be obtained in one of many polymorphs.…”

“…[74] Historically, the T-Nb 2 O 5 , M-Nb 2 O 5 , and H-Nb 2 O 5 phases were named according to the temperatures (low, medium, and high, respectively) at which they are obtained. [68] A diagram for the phase transitions of Nb 2 O 5 as a function of temperature is shown in Figure 1. The H phase is the thermodynamically most stable, while the TT phase is the least stable.…”

“…However, it seems feasible to consider that the structural origin of Brønsted-acidic sites should be predominantly associated with acidic superficial hydroxyl groups as proton donors, while the structural origin of Lewis-acidic sites should be attributed coordinatively unsaturated superficial Nb 5 + cations. [68] In the interest of developing rational design guidelines for Nb-oxide catalysts, it is therefore of great interest to study the correlations between structure and acidity in more detail. [81] To investigate the Lewis acid-base interactions between the Nb sites and the carbonyl group of a model carboxylic acid, we measured in situ IR spectra of acetic acid adsorbed on Nb 2 O 5 as well as on other oxides as references.…”