Heterogeneous Lewis Acid Catalysts Workable in Water

The Lewis acidities of Nb2O5·nH2O, anatase TiO2, and Ti-deposited mesoporous silica (TDMS) in the presence of water have been studied to devise new catalytic systems for the environmentally benign production of chemicals. These Groups 4 and 5-based oxides exhibit high catalytic performance for Lewis acid-catalyzed reactions in aqueous media such as the conversion of glucose into 5-(hydroxymethyl)furfural (HMF), lactic acid production from pyruvaldehyde, the allylation of benzaldehyde with tetraallyltin, and the Mukaiyama–aldol condensation of benzaldehyde with 1-trimethylsilyloxycyclohexene. Lewis acid catalysis of these materials in water can be attributed to MOx unsaturated coordination spheres such as NbO4 and TiO4 terrahedra: they are not decomposed in water and water molecules interacting with such Lewis acid sites can be easily exchanged with nucleophilic reactants. Lewis acid-catalyzed reactions on TiO2 and TDMS revealed that Lewis acid sites on TDMS are quite distinct from those of TiO2 in catalysis.