Titania as an Early Transition Metal Oxide with a High Density of Lewis Acid Sites Workable in Water

Abstract

Anatase TiO 2 prepared by a simple sol–gel reaction of Ti(i-pro) 4 was studied as an early transition metal oxide with Lewis acid sites workable in water. Fourier transform infrared (FT-IR) measurements revealed that TiO 2 has a significantly higher density of effective Lewis acid sites in water than niobic acid (Nb 2 O 5 · n H 2 O), a heterogeneous water-tolerant Lewis acid, because most Lewis acid sites on TiO 2 can maintain Lewis acidity even in the presence of water, which is in contrast to those on Nb 2 O 5 · n H 2 O. TiO 2 as an easily separable and reusable heterogeneous catalyst that exhibits higher catalytic performance for the transformation of pyruvaldehyde into lactic acid and the allylation of benzaldehyde with tetraallyltin in water than Nb 2 O 5 · n H 2 O, and the catalytic activity of TiO 2 is comparable to that of scandium trifluoromethanesulfonate (Sc(OTf) 3 ), a highly active homogeneous water-tolerant Lewis acid, despite the lower amount of effective Lewis acid sites. The high catalytic performance of TiO 2 for these reactions can be attributed to a high density of Lewis acid sites that can function in water.