Niobium–silica catalysts for the selective epoxidation of cyclic alkenes: the generation of the active site by grafting niobocene dichloride

Abstract

Niobium-containing silica materials obtained by deposition via liquid-phase grafting or dry impregnation of niobocene(iv) dichloride are active and selective catalysts in the epoxidation of alkenes in the presence of aqueous hydrogen peroxide. The generation of the catalytically-active Nb species was followed step-by-step, and investigated using a combined DR-UV-Vis, NIR, Raman, XRD, XANES and EXAFS analyses. At the end of the grafting procedure, the nature of the surface active species can be described as an oxo-Nb(v) site, tripodally grafted onto the silica surface in close proximity to other Nb(v) centres. The liquid-phase methodology provides a better dispersion of the metal sites onto the siliceous support than the dry-impregnation approach. The niobium-silica catalysts were then tested in the epoxidation of cyclohexene and 1-methylcyclohexene, as model substrates.

The role of niobia location on the acidic and catalytic functionalities of heteropoly tungstate

“…Besides, a strong band centered at ca. 700 cm −1 generated, associated to the stretching modes of different niobia polyhedra [15,18]. These results are in accordance with those obtained by FT-IR and XRD.…”

Rate and Selectivity Control in Thioether and Alkene Oxidation with H2O2 over Phosphonate‐Modified Niobium(V)–Silica Catalysts

“…Upon calcination, diffuse–reflectance UV/Vis (DRUV/Vis) spectroscopy of Nb‐SiO 2 , Ti‐SiO 2 , and Zr‐SiO 2 show strong absorbance only in the UV range (Figure ), evidence of highly dispersed metal oxide centers . The small shoulder at about 340 nm for Nb‐SiO 2 suggests a small extent of aggregation . Further, all three materials have optical edge energies of 3.7–3.9 eV, as calculated from a separate Tauc plot, in agreement with previous edge values for highly dispersed oxides reported by us and by others for similar catalysts.…”

Niobium(V) Saponite Clay for the Catalytic Oxidative Abatement of Chemical Warfare Agents

“…The coordination state of the Nb V species in Nb‐SAP and Na/Nb‐SAP was investigated by diffuse reflectance UV/Vis (DR‐UV/Vis) spectroscopy (Figure 2). In both cases, no absorption bands typical for Nb 2 O 5 large domains were observed 10. Both samples showed two absorption maxima at 245 nm (assigned to a charge‐transfer transition between oxygen and the Nb V centers in tetrahedral coordination) and at 275 nm.…”

“…A synthetic saponite clay was selected as support for the dispersion of Nb V ions. The conventional synthetic protocol used to obtain saponite clays8 was modified to allow the insertion of Nb V ions within the framework, aiming to obtain a novel solid (Nb‐SAP) with both oxidizing9, 10 and Brønsted acid properties (Figure 1).…”

“…In fact, niobium centers provide the clay with oxidizing properties in the presence of H 2 O 2 that are peculiar for Nb V sites evenly dispersed in inorganic oxide matrices 9. 10 Then, the presence of protonic sites in the interlayer sites of the saponite helps to increase the abatement activity through acid‐catalyzed degradation pathways 16. 17 In fact, when the protonic sites were replaced by Na + , the abatement capability was largely reduced (cf.…”