Protonated Titanate Nanotubes with Lewis and Brønsted Acidity: Relationship between Nanotube Structure and Catalytic Activity

Abstract

Nanostructured titanate materials with different morphologies, including layered materials, nanosheets, and nanotubes, were examined as solid acid catalysts to elucidate the relationship between the structure and the catalytic properties. The titanate nanotube consists of a scroll-like layered structure derived from lamellar titanate nanosheets that exhibits excellent catalytic performance for the Friedel–Crafts alkylation of toluene with benzyl chloride near room temperature, exceeding the activities of layered titanates (H2Ti3O7, H0.7Ti1.825□0.175O4·H2O) and nanosheets that have similar crystal structures to that of the titanate nanotubes. Fourier transform infrared (FT-IR) spectroscopy and 31P magic angle spinning nuclear magnetic resonance (31P MAS NMR) spectroscopy using basic probe molecules have revealed that these titanate materials possess both Brønsted and Lewis acid sites, and the Brønsted acid strength of the titanate nanotubes is higher than that of the titanate nanosheets. The strong Brønsted acidity of the titanate nanotubes is attributed to lattice distortion due to scrolling of the lamellar titanate nanosheet, which is evidenced by Raman spectroscopy and density functional theory calculations. Furthermore, the mesoporous structure of the titanate nanotubes is advantageous for acid catalysis, because the reactant molecules are confined within the nanotubes.