Frontiers in mesoporous molecular sieves containing niobium: From model materials to catalysts

Nb2O5/SBA-15 catalyzed propanoic acid esterification

“…Particle sizes of crystalline niobia phases were estimated for the 32Nb/SBA-15 sample from line broadening of the 22.5° reflection in Figure 2, DRUV analyses showed that niobium incorporation resulted in the emergence of a strong and broad absorbance between 200-400 nm, consistent with that observed for pure niobia (Figure S4). This has been previously assigned to the presence of at least two different Nb species [34,48,49], with features around 220 nm arising from tetrahedral niobium species coordinated to MCM-41 and SBA-15 silica support [28,29,[50][51][52][53][54][55], and those around 330 nm due to octahedral Nb present within extra framework Nb 2 O 5 . The intensity of the higher wavelength band increased with Nb loading, in accordance with the appearance of crystalline Nb 2 O 5 observed by XRD.…”

“…Nb/SBA-15 co-functionalised with propylamine groups showed the highest activity, which was attributed to a strong interaction between Nb cations and the silica support generating Lewis acidity [26,27]. Nb/SBA-15 has also been prepared in the presence of grafted propylsulfonic acid groups for use in glycerol esterification with acetic acid, in which Nb appeared to improve the stability of sulfonic acid functions, resulting in 66 % glycerol conversion and 35 % selectivity to triacetylglycerol [28,29].…”

Epoxidation of Karanja (Millettia pinnata) Oil Methyl Esters in the Presence of Hydrogen Peroxide over a Simple Niobium-Containing Catalyst

“…For this reason, we moved from titanium(IV)-silica catalysts, that are deeply-studied, although less robust and prone to deactivation when in contact with water-containing media [25][26][27][28], towards niobium(V)-silica systems, which showed promising features in terms of activity, selectivity to epoxides and robustness, also in the presence of aqueous H 2 O 2 [29][30][31][32][33][34][35]. Niobium(V)-containing mesoporous silica catalysts in epoxidation reactions, indeed, comply with the main guidelines of Green and Sustainable Chemistry: they do not require hazardous corrosive reactants, avoiding the use of peroxoacids; they circumvent tedious separation and recovery techniques, thanks to their heterogeneous nature, and, by exploiting the oxidizing properties of hydrogen peroxide, only water and dioxygen, in the case of its decomposition, are formed [36][37][38]. Moreover, niobium-silica catalysts displaying a network of mesopores, either ordered or non-ordered, proved to be suitable solid catalysts where the epoxidation of bulky vegetable oil FAMEs can be carried out, thanks to the easy steric accessibility of their mesoporous channels to substrate molecules [39][40][41][42][43].…”

Niobium Oxide as Catalyst for the Pyrolysis of Polypropylene and Polyethylene Plastic Waste

“…Niobium oxide is an insoluble white solid that is stable in air, sometimes defined as amphoteric, but more often characterized as inert, including a mesoporous structure. Brazil has the largest reserves of niobium and produces about 92 % of the world's compounds containing niobium [24,25]. Specifically, niobium oxide has numerous applications, among them as a catalyst.…”

“…Studies of the synthesis, characterization and application of mesoporous materials containing niobium have been conducted due to their texture, structure, morphology and surface characteristics, such as ordered structure and high specific surface area [26]. The special properties (redox properties, photosensitivity, acidity and catalytic behavior) of materials containing niobium, such as niobium phosphate, niobium oxide and compounds containing niobium layers, have attracted interest in these compounds, as well as the understanding of their behavior and the use as catalysts [25].…”