Switching Product Selectivity in Immobilized Tungstate Catalysts by Control of Hydrophobicity of Mesoporous Silicate Supports

Abstract

Novel mesoporous silicates modified with imidazolium cation were applied as the support for peroxotungstate. Stepwise introduction of multiple functional groups to SBA-15 was achieved by the combination of the direct and post-synthesis procedures. The co-condensation of tetraethoxysilane and 3-(imidazolyl-1-yl)-propyltriethoxysilane in the presence of the micelles of an amphiphilic surfactant yields the hydrophilic SBA-15 type silicate support with the imidazole modifier locating into the mesopores. The hydrophobic environment of the supports was imparted by capping the residual silanols on the silicate surface with trimethylsilyl groups. The fluoroalkyl- or alkyl-imidazolium cation-anchored supports were derived from the corresponding imidazole-functionalized precursors. The anion-exchanging reaction between peroxytungstate and the imidazolium cation-anchored supports yielded the corresponding tungstate-immobilized catalysts. The hydrophilic catalyst with the fluoroalkyl-imidazolium mediated alkene epoxidation with H₂O₂ in less polar solvents. The hydrophobic one, in contrast, produced diol through the epoxidation and following hydration in an aqueous solvent by the action of the highly Lewis acidic tungsten. The hydrophobic catalysts had the advantage of being more tolerant than hydrophilic catalysts, as evidenced by reusability tests. The activities of the alkyl-imidazolium catalysts were lower than those of the corresponding fluoroalkyl-imidazolium catalysts, indicating that the local structures surrounding the tungstate site affected the catalytic performance.