Effect of Impregnation on the Structure of Niobium Oxide/Alumina Catalysts Studied by Multinuclear Solid-State NMR, FTIR, and Quantum Chemical Calculations

Abstract

Multinuclear solid-state 1 H, 27 Al, and 93 Nb NMR experiments and DFT calculations were carried out for structural characterization of alumina-supported niobium oxide catalysts with high niobium content following an every stage in the catalyst preparation. It was found that the first stage of the impregnation procedure plays a key role in determining the catalyst structure and acidity. In order to monitor the presence in catalysts of aluminum niobate phase, AlNbO 4 , a series of 27 Al and 93 Nb NMR experiments was performed for several different individual AlNbO 4 samples. Aluminum and niobium NMR parameters were determined for AlNbO 4 , which crystal structure contains two different crystallographic sites for each element. The compound was investigated through a combination of experimental 93 Nb and 27 Al NMR spectroscopy methods at several magnetic field strengths (9.4, 11.7, 19.4, and 21.1 T) and complemented by ab initio quantum chemical calculations of NMR parameters for these nuclei. The chemical shielding and the quadrupole coupling tensor parameters were determined for both 93 Nb and 27 Al.

Highly porous niobium-containing silica glasses applied to the microwave-assisted conversion of fructose into HMF

“…Table S2, Supporting Information, lists the average isotropic chemical shifts <δ iso > and quadrupolar coupling parameters <C Q > obtained when modeling the spectra with the Czjzek distribution [60]. The data are comparable to values of six-coordinate niobium found in the literature [61][62][63][64], suggesting that the Nb-surface oxides have a similar local environment featuring six-coordinated Nb. Nevertheless, the different lineshapes observed for the 1.8%Nb-MG and the 2.8%Nb-MG indicate that the local structure of the Nb sites depends on the loading level.…”

An Environmentally Friendly Nb–P–Si Solid Catalyst for Acid-Demanding Reactions

“…Indeed, in the case of alumina impregnated with niobium oxide, it has been proposed that a broad resonance at 8 ppm but 10 ppm wide corresponds to Nb–O­(H)–Nb Brønsted acid sites. Such a broad line would encompass the 1 H chemical shifts seen for 2.5NbP-500 . Upon exposure to water, the resonance at 2 ppm remains unchanged, as expected given the inertness of silicate networks to hydrolysis.…”

“…Such a broad line would encompass the 1 H chemical shifts seen for 2.5NbP-500. 37 Upon exposure to water, the resonance at 2 ppm remains unchanged, as expected given the inertness of silicate networks to hydrolysis. However, new resonances appear around 1.8 ppm typical of terminal Si−OH.…”

Multinuclear Solid-State NMR Investigation of Hexaniobate and Hexatantalate Compounds

“…Most of the works published on 93 Nb MAS NMR were performed on niobium oxides or mixed-metal oxides that are used as catalysts, piezoelectric components, or optoelectronic materials. ,− …”