Catalyst Materials Based on Nb₂O₅ Supported on SiO₂−Al₂O₃:  Preparation and Structural Characterization

Abstract

Supported niobium pentoxide materials have been effective catalysts for a variety of acid and redox reactions (e.g., dehydration of alcohols, esterifications, etc.). To the best of our knowledge, there are no reports about Nb 2 O 5 supported on silica-alumina. Catalysts of Nb 2 O 5 /SiO 2 -Al 2 O 3 were prepared with 2, 5, 10, 15, 20, and 25 wt % of Nb 2 O 5 by aqueous solution impregnation using ammonium niobium oxalate on silica-alumina. The materials, after being dried at 100 °C, were calcined at 800 °C and characterized by several methods. Investigation through X-ray diffraction showed the typical patterns of crystalline Nb 2 O 5 , which were composed of mixtures of orthorhombic and monoclinic phases (T, M, and H, respectively) present in the materials with content higher than 10 wt %. DTA curves displayed an exothermic peak at 1356 °C (average) without mass loss (confirmed by TG), which may be ascribed to a phase transition (H phase formation) of Nb 2 O 5 supported on silica-alumina. Pure Nb 2 O 5 ‚nH 2 O showed a transition from amorphous to hexagonal or orthorhombic phase (TT or T, respectively) at 567 °C. FTIR and DRIFTS results confirmed the reaction of the niobium oxide with the hydroxyl functionality of silica-alumina. The formation of surface niobium pentoxide species over the support through selective and progressive consumption of hydroxyl groups from the support and the appearance of characteristic niobium hydroxylated species on the surface were demonstrated. Raman spectra attested a two-dimensional overlayer of niobium pentoxide on silica-alumina at contents below 10 wt %. At higher concentrations, the absorptions characterize the formation of phases T and H of bulk Nb 2 O 5 .

Characterization and Catalytic Performance of Niobic Acid Dispersed over Titanium Silicalite

“…Niobic acid (sample NBA) shows a major strong band centered at 586 cm −1 and a shoulder band at 933 cm −1 . This spectrum is consistent with the skeletal vibration previously reported for amorphous niobic acid [12,21,22]. The spectrum of niobium oxide crystals (sample NBO) exhibits two strong bands at 615 and 850 cm −1 .…”

“…On the other hand, crystalline niobium oxide does not show any peak in the vibration region of hydroxyl groups indicating that crystalline niobium oxide does not contain any hydroxyl groups. A different finding was reported recently by Braga et al [12] for niobium oxide calcined at 800…”

“…They also reported that the surface niobium species on TiO 2 gave rise to acidic products. Low amount of niobium oxide loading (1 wt%) on silica is predominantly composed of isolated NbO 4 units, while the Nb cations in the supported Nb 2 O 5 /SiO 2 catalysts possess polymerized NbO x species and/or bulk Nb 2 O 5 with increasing Nb loading [10][11][12]. It has been reported that for supported niobium oxide on silica, both Brønsted and Lewis acid sites remains bound to the support surface even in the presence of water and acts as an effective catalyst for acidcatalyzed reactions [13][14][15][16][17].…”

Acidity and Characterization of 12-Tungstophosphoric Acid Supported on Silica-Alumina

“…17 It has been proposed that silica-alumina surface is formed by isolated silanols and/or hydrogen-bonded bridge silanols. 17,39 The highest frequencies of OH vibrations are found for Si-OH groups (isolated silanols) and O−Si−OH-Al groups (called aluminic pseudo-bridging silanols). 17 Thus, it may be suggested that the interaction of H 3 PW Keggin structure can take place preferentially at these sites, since most of the metal oxides interact to O−H surface sites of different supports.…”

“…17 Thus, it may be suggested that the interaction of H 3 PW Keggin structure can take place preferentially at these sites, since most of the metal oxides interact to O−H surface sites of different supports. 39,40 Nitrogen physisorption isotherm obtained for SiO 2 -Al 2 O 3 was analyzed by plotting the t-curve, i.e., 2017 amount of N 2 adsorbed versus thickness (t). 41 The linear fit from the latter showed a straight line passing through the origin, which indicates a multilayer adsorption of N 2 on the solid surface ( Figure S1, in the SI section).…”