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

Anatase TiO2 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 TiO2 has a significantly higher density of effective Lewis acid sites in water than niobic acid (Nb2O5 center dot nH(2)O), a heterogeneous water-tolerant Lewis acid, because most Lewis acid sites on TiO2 can maintain Lewis acidity even in the presence of water, which is in contrast to those on Nb2O5 center dot nH(2)O. TiO2 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 Nb2O5 center dot nH(2)O, and the catalytic activity of TiO2 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 TiO2 for these reactions can be attributed to a high density of Lewis acid sites that can function in water.