Selective oxidation of styrene to benzaldehyde catalyzed by Schiff base-modified ordered mesoporous silica materials impregnated with the transition metal-monosubstituted Keggin-type polyoxometalates Original Research Article

A series of Schiff base-modified ordered mesoporous silica materials impregnated with the transition metal-monosubstituted Keggin-type polyoxometalates (mono-TMSPs), K10−nXn+MW11O39-Schiff-SBA-15, (X = P/Si, M = Co/Ni/Cu/Mn), was prepared using successive grafting procedures of SBA-15 silica with 3-aminopropyl-trimethoxysilane, benzaldehyde, and mono-TMSP. The composite materials were well-characterized by spectroscopy methods, X-ray diffraction analysis, transmission electron microscopy, and nitrogen sorption analysis to confirm the structure integrities of the Keggin unit and Schiff base ligand after the incorporation, to evidence formation of the coordination binding between the starting Keggin units and the Schiff base-modified silica matrix, and to investigate the morphology and surface textural properties of the composite catalysts. These materials were subsequently utilized as efficient and selective catalysts in the reaction of oxidation of styrene to benzaldehyde using H2O2 as an oxidant. For comparison, an amine-modified SBA-15 silica material impregnated with mono-TMSP (K10−nXn+MW11O39-NH2-SBA-15) and the neat Keggin unit (K10−n[Xn+M(H2O)W11O39]) was also tested. The influences of key reaction parameters, including the molar ratios of H2O2 to styrene, compositions and loadings of the neat mono-TMSPs, solvents, the catalyst amount, and reaction temperatures on the reactivity and selectivity, were also studied. Finally, the reusability of the catalysts was evaluated.