Synthesis and characterization of mesoporous Ta-W oxides as strong solid acid catalysts

Several mesoporous Ta_xW_10-x mixed oxides prepared from TaCl₅ and WCl₆ in the presence of poly block copolymer surfactant Pluronic P-123 were examined as potential solid acid catalysts. Amorphous wormhole-type mesopores were observed in samples with x values from 3 to 10, whereas W-rich samples (x = 0-2) formed nonmesoporous structures with crystallized tungsten oxide (WO₃). The acid strength increased with addition of W for mesoporous Ta_xW_10-x oxides, and mesoporous Ta₃W₇ oxide exhibited the highest acid catalytic activity for the Friedel-Crafts alkylation of anisole and the hydrolysis of disaccharides. The results are compared with those of nonporous Ta₂O₅-WO₃, HTaWO₆ nanosheets, which were a solid acid obtained by exfoliation and aggregation of layered HTaWO ₆ with strong Brønsted acid sites in the interlayer, and a range of conventional solid acid catalysts. Mesoporous Ta-W oxides exhibited a turnover rate higher than that of nonporous Ta₂O₅-WO ₃ and HTaWO₆ nanosheets, indicating that the mesoporous structure is an advantageous environment for the strong acid sites, because of the high surface area and easy reactant accessibility.