Acid properties of protonated titanate nanotubes

Abstract

Acid properties of protonated titanate nanotubes were investigated by comparing the catalytic properties of various titanate materials with different morphology. Titanate nanotubes with a scroll-like layered structure exhibited excellent catalytic performance for Friedel-Crafts alkylation and Beckmann rearrangement of cyclohexanone, exceeding the activities of titanate nanorods and nanosheets with similar crystal structures to that of the titanate nanotubes. The ion-exchange capacity of titanate nanorods is much larger than that of titanate nanotubes, but the ion-exchange sites (i.e. Brønsted acid sites) of titanate nanorods are located at the interspace of layered structures and strongly interact with the anionic titanate layers, i.e., the Brønsted acid sites are not exposed on the surface. FT-IR analyses and ion-exchange experiments revealed that titanate nanotubes possess both Brønsted and Lewis acid sites on the catalyst surface. In addition, the Brønsted acidity of titanate nanotubes is much higher than that of titanate nanosheets due to lattice distortion caused by scrolling of the lamellar titanate nanosheets. The synergic effect between Brønsted and Lewis acid sites is important in the high catalytic performance of titanate nanotubes for various acid-catalyzed reactions.