A novel polyoxometalate-functionalized mesoporous hybrid silica: synthesis and characterization

Abstract

Novel polyoxometalate (POM)-functionalized mesoporous hybrid silicas, SiW₁₁/MHS, have been synthesized by a co-condensation route and employment of Keggin-type monovacant SiW₁₁ as POM precursor in the presence of block copolymer EO₂₀PO₇₀EO₂₀ (P123) under acidic conditions. This synthesis allows a bulky inorganic metal–oxygen cluster to be grafted directly on the surface of ordered mesoporous silica with SBA-15 architecture. The as-obtained SiW₁₁/MHS samples were characterized by FT-IR, UV-vis/DRS, ²⁹Si CP MAS NMR, XRD, TEM, N₂ adsorption–desorption measurement and elemental analysis. These materials possess not only hexagonal mesoscopic order and homogeneous pore sizes, but also intact Keggin units immobilized in channels by covalent linkages with the mesopore walls. This covalent bond results from the reaction of TEOS with SiW₁₁, in which Si species are inserted into the vacancy of lacunary SiW₁₁ to form Si–O–W bonds and an intermediate SiW₁₁Si₂, by which the POM is bound onto the framework of mesoporous silica. Investigation of the synthesis conditions indicates that the quality of the product depends on the initial SiW₁₁ concentration, the aging temperature of the sample and the prehydrolysis time of TEOS. A higher SiW₁₁ concentration can enhance the loading of SiW₁₁ to a certain extent, but leads to a disordered structure owing to the salting-out effect of POM if the prehydrolysis time is not long enough. Increased aging temperature is favorable for the formation of ordered mesostructures. However, hydrolysis of Si–O–W bonds and consequent removal of SiW₁₁ from the silica framework also occur at such temperatures. Finally, the chemical linkage between the POM and the surface of mesoporous silica is further confirmed by the higher stability of the hybrid materials in water-leaching experiments compared to the impregnated samples, suggesting that the hybrid materials and the strategy for synthesizing these materials are significant for the acquisition of supported POM catalysts of high efficiency and practicability in applications using polar solvents as media.