Studies on novel solid acid catalysts of niobium-based oxides with mesoporous, layered and nanosheet structures

The acidities of hydrated niobium oxide (Nb₂O₅·nH₂O) and niobium mixed oxides have been widely studied as a promising substitute for liquid acid. Nb₂O₅·nH₂O which is usually called niobic acid, exhibits a remarkably high acid strength corresponding to the acid strength of 70 % H₂SO₄. In the view of catalytic activity and stability, niobic acid exhibited excellent stabilities for acid-catalyzed reactions activities in water solution reactions including hydrolysis, hydration and esterification. In this chapter, niobium-containing mixed metal oxides with different nanostructures (nanosheet aggregates, layered and mesoporous) are presented to study the effects caused by the structure and metal combination for the acid sites and acid-catalyzed reactions. Nanosheet aggregates and protonated layered niobates (HTiNbO₅, HNb₃O₈ and HNbWO₆) are examined as potential solid acid catalysts. However, as the high charge density of the oxide sheets prevents reactants from penetrating into the interlayer region, unmodified layered transition-metal oxides are generally ineffective as solid acid catalysts. Exfoliation and aggregation of layered HTiNbO₅, HNb₃O₈ and HNbWO₆ using soft chemical processing form aggregates of nanosheets with high surface areas, making possible the access of reactants to acid sites formed by the bridged hydroxyl groups, M(OH)M' (M=Ti, Nb; M'=Nb, W). The catalytic activity for the Friedel-Crafts alkylation of anisole in the presence of benzyl alcohol increased in the order HTiNbO₅ < HNb₃O₈ < HNbWO₆, consistent with the acid strengths determined by desorption measurements and nuclear magnetic resonance spectroscopy. Layered HNbMoO₆ is a very unique layered metal oxide able to intercalate different organic reactants (alcohols, saccharides, ketones, alkenes, hydroxyl acids) during the catalytic reactions. Owing to the intercalation ability and strong acidity in the interlayer, layered HNbMoO₆ functioned as a highly-active solid acid catalyst. The catalytic activity for the Friedel-Crafts alkylation, acetalization and hydrolysis of saccharides exceeded the activity of zeolites and ion-exchange resins. Mesoporous Nb_xW_(10-x) mixed oxides with different Nb and W concentrations are examined as potential solid acid catalysts. Amorphous wormhole-type mesopores are observed for samples from x = 3 to 10 whereas W-rich samples (x = 0 to 2) formed a non-mesoporous structure with presence of crystallized tungsten oxide (WO₃). The acid-catalytic activity, acid strength and mesopore structure of mesoporous Nb-W oxides changed in order of W concentrations, exhibiting a very high activity for both Friedel-Crafts alkylation and hydrolysis. The results were compared with those for non-porous Nb₂O₅-WO₃ and a range of conventional solid acids. Mesoporous Nb-W oxides obtained higher turnover rate than that of non-porous Nb₂O₅-WO₃ led to the strong acid sites and a mesoporous structure with a high surface area and easy reactant accessibility.