Niobic acid, Nb(2)O(5)·nH(2)O, has been studied as a heterogeneous Lewis acid catalyst. NbO(4) tetrahedra, Lewis acid sites, on Nb(2)O(5)·nH(2)O surface immediately form NbO(4)-H(2)O adducts in the presence of water. However, a part of the adducts can still function as effective Lewis acid sites, catalyzing the allylation of benzaldehyde with tetraallyl tin and the conversion of glucose into 5-(hydroxymethyl)furfural 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·nH2O), 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·nH2O. 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·nH2O, 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.

The reaction mechanism for the formation of 5-(hydroxymethyl)furfural (HMF) from glucose in water over TiO 2 and phosphate-immobilized TiO 2 (phosphate/TiO 2 ) with water-tolerant Lewis acid sites was studied using isotopically-labeled molecules and 13 C nuclear magnetic resonance (NMR) measurements for glucose adsorbed on TiO 2 . Scandium trifluoromethanesulfonate (Sc(OTf) 3 ), a highly active homogeneous Lewis acid catalyst workable in water, converts glucose into HMF through aldose-ketose isomerization between glucose and fructose involving a hydrogen transfer step and subsequent dehydration of fructose. In contrast to Sc(OTf) 3 , Lewis acid sites on bare TiO 2 and phosphate/TiO 2 do not form HMF through the isomerizationdehydration route but through the stepwise dehydration of glucose via 3-deoxyglucosone as an intermediate. Continuous extraction of the evolved HMF with 2-sec-butylphenol results in the increase in the HMF selectivity for phosphate/TiO 2 even in high concentrated glucose solution.These results suggest that limiting the reactions between HMF and the surface intermediates improves the efficiency of HMF production.

The Lewis acidity of phosphate-immobilized anatase TiO2 (H3PO4/TiO2) has been studied to develop novel environmentally benign reaction systems. Fourier transform infrared (FT-IR) measurements suggested that most Lewis acid sites on bare and H3PO4/TiO2 surface function even in water. H3PO4/TiO2 exhibits high catalytic performance for selective 5-(hydroxymethyl)furfural (HMF) production from glucose in THF/water (90/10 vol.%) solution. This is attributed to water-tolerant Lewis acid sites on H3PO4/TiO2 that promote step-wise conversion of glucose into HMF. The catalyst was easily recovered from reaction solution by simple decantation or filtration, and can be used repeatedly without significant loss of original activity for subsequent reactions.