Characterization of HNbWO₆ and HTaWO₆ metal oxide Nanosheet aggregates as solid acid catalysts

Nanosheet aggregates prepared from protonated layered tungstates HMWO ₆ (M) Nb, Ta) are examined as potential solid acid catalysts. The nanosheet aggregates are formed by soft chemical processing of the layered compound with tetra(n-butylammonium) hydroxide, and the catalytic activity and acid strength of the aggregates are compared with those for HTiNbO₅, HNb₃O₈, and a range of conventional solid acids. The catalytic activity for the Friedel-Crafts alkylation of anisole in the presence of benzyl alcohol increases in the order HTiNbO₅ < HNb ₃O₈ < HMWO₆ (M) Nb, Ta), consistent with the acid strengths determined by desorption measurements and nuclear magnetic resonance spectroscopy. Nuclear magnetic resonance spectroscopy indicates that the acid catalytic activity of the nanosheet aggregates is attributable to strong Brønsted acid sites, presumably M(OH)M' (M) Ti, Nb, Ta; M') Nb, W).