Synthesis of niobium-doped titanate nanotubes as solid acid catalysts

Niobium-doped titanate nanotubes (Nb–TiNTs) with various Nb contents were prepared by hydrothermal treatment of Nb-doped TiO2 with an aqueous NaOH solution and subsequent proton exchange. Nb–TiNTs effectively catalyzed the Friedel–Crafts alkylation of toluene with benzyl chloride or benzyl alcohol and the activity increased with the Nb content, reaching a maximum at 5%. Furthermore, Nb–TiNTs exhibited much higher catalytic activity than other Nb-containing solid acid catalysts such as Nb2O5·nH2O, Nb–Ti mixed oxides and Nb–Ti nanosheet materials. Fourier transform-infrared (FT-IR) spectroscopy measurements revealed that Nb–TiNTs possess both Lewis and Bronsted acid sites, and the amounts of these acid sites are comparable to those of non-doped TiNTs. The high catalytic performance of Nb–TiNTs is attributable to their higher Bronsted acid strength and to the enhanced accessibility of reactant molecules to acid sites.