Redox properties of niobium oxide catalysts

Dehydration of Fructose to 5-Hydroxymethylfurfural: Effects of Acidity and Porosity of Different Catalysts in the Conversion, Selectivity, and Yield

There is a demand for renewable resources, such as biomass, to produce compounds considered as platform molecules. This study deals with dehydration of fructose for the formation of 5-hydroxymethylfurfural (HMF), a feedstock molecule. Different catalysts (aluminosilicates, niobic acid, 12-tungstophosphoric acid—HPW, and supported HPW/Niobia) were studied for this reaction in an aqueous medium. The catalysts were characterized by XRD, FT-IR, N2 sorption at −196 °C and pyridine adsorption. It was evident that the nature of the sites (Brønsted and Lewis), strength, quantity and accessibility to the acidic sites are critical to the conversion and yield results. A synergic effect of acidity and mesoporous area are key factors affecting the activity and selectivity of the solid acids. Niobic acid (Nb2O5·nH2O) revealed the best efficiency (highest TON, yield, selectivity and conversion). It was determined that the optimum acidity strength of catalysts should be between 80 to 100 kJ mol−1, with about 0.20 to 0.30 mmol g−1 of acid sites, density about 1 site nm−2 and mesoporous area about 100 m2 g−1. These values fit well within the general order of the observed selectivity (i.e., Nb2O5 > HZSM-5 > 20%HPW/Nb2O5 > SiO2-Al2O3 > HY > HBEA).

Hydroxyapatite supported molybdenum oxide catalyst for selective oxidation of methanol to formaldehyde: studies of industrial sized catalyst pellets

Promising alternative catalysts for the Formox process as industrial sized pellets and the influence of pellet density on catalyst performance.

Niobium-Titanium-Based Photocatalysts: Its Potentials for Free Cyanide Oxidation in Residual Aqueous Effluent

Organic compounds are employed as additives to increase the dissolution speed of gold, in concentrations around 1 g/L when using cyanidation, thereby forming a residual aqueous effluent with high amounts of free cyanides and organic compounds, which generate metallic complexes difficult to degrade. To increase the photodegradation efficiency, promising niobium and titanium porous materials are proposed as photocatalysts, due to their role in simultaneous oxidation and reduction reactions. In the process of cyanide oxidation, NbO₅ 0.3H₂O was doped with titanium oxalate (IV) of 0.5, 1, and 1.5%; and HTiNbO₅ were synthesized, from the mixture of NbO₅ with TiO₂ Degussa-P25, by coprecipitation, impregnation, and solid state. The determination of its elemental composition, morphological and textural properties were carried out by using various XRD techniques, Raman spectroscopy, SEM/EDS and acidity by pyridine. The experiments of photocatalytic oxidation of cyanide used one semibatch reactor with ultraviolet irradiation 125 W in a pH range of 9.5–12. The catalyst with the highest percentage of degradation was HTiNbO₅ 93.7%, which is attributed to the microstructure of the double layer and Lewis acidity sites, followed by NbTi-1% 92.9% and the Nb₂O₅.3H₂O 82.4%, being the majority product cyanate, proposing its mechanism of reaction. Characterization experiments indicated Nb-O-Ti bridges that have been associated with the control of redox properties of the niobium species and Ti-O-Nb = O, which could be generating a greater number of e–H +pairs, increasing the photocatalytic activity. It is considered that the method of synthesis has a strong influence in changing the morphology of the particles such as porosity, specific surface and factors such as the acidity of niobium–based catalysts, which are important to achieving efficiency in degradation. Niobium-Titanium photocatalysts proved to be an excellent new breakthrough in Advanced Oxidation Technologies (AOT), to eliminate cyanide in wastewater from mining activities.

A Quantitative Fourier Transform Infrared Study of the Grafting of Aminosilane Layers on Lithium Niobate Surface

Due to its impressive optical properties, lithium niobate (LiNbO₃) is considered to be one of the most important ferroelectric materials. Its uses in sensing platforms require functionalization at the surface to enable the capture and quantifying of molecules. The current paper aims to demonstrate the covalent bonding of aminosilane layers to the LiNbO₃ surface. Fourier transform infrared (FT-IR) analysis reveals the presence of an NbO-Si bond observable as a shoulder at the same wavenumber (975 cm^-1) on the surfaces of LiNBO₃ as well as on those of Nb₂O₅, using 3-(aminopropyl)trimethoxysilane (APTMS) or 3-(aminopropyl)methyldimethoxysilane (APDMS) precursors. This covalent bonding is confirmed by the insolubility of the silane coating in dimethyl sulfoxide (DMSO). A kinetic study of the aminosilane layer growth obtained by quantitative FT-IR analysis is also carried out.

Generating active and non-selective oxidizing species by previous treatment of niobium-doped mesoporous silica with hydrogen peroxide

Active catalysts for oxidation of non-selective organic compounds were produced by doping silica with niobium followed by treatment with hydrogen peroxide.