Synthesis and multifunctional properties of superparamagnetic iron oxide nanoparticles coated with mesoporous silica involving single-site Ti-oxide moiety

A new multifunctional nanocomposite (FexOy@Ti-hexagonal mesoporous silica (HMS)) involving superparamagnetic iron oxide nanoparticles, ordered mesoporous channels, and isolated and a tetrahedral Ti-oxide moiety has been developed via the coating of as-synthesized iron oxide nanoparticles with an amorphous silica layer followed by the sol-gel polymerization using tetraethyl orthosilicate, tetrapropyl orthotitanate, and a structure-directing reagent. The formation of a hexagonally packed mesoporous structure was confirmed by low-angle X-ray diffraction (XRD) and N-2 adsorption-desorption. Characterization by wide-angle XRD, Raman, transmission electron microscopy (TEM), high-resolution TEM, and Fe K-edge X-ray absorption fine structure (XAFS) measurements demonstrated the encapsulation of the iron oxide nanoparticles having a mean diameter of ca. 9.0 nm within a silica matrix. Superconducting quantum interference device magnetization studies indicated that the FexOy@Ti-HMS exhibited superparamagnetic behavior at 300 K and a transition to ferrimagnetic at 5 K. Ti K-edge XAFS spectra confirmed that the Ti-oxide moiety was proven to exist in the isolated and tetrahedral form. The FexOy@Ti-HMS acted as an efficient heterogeneous catalyst for the liquid-phase selective oxidation reactions of organic compounds using hydrogen peroxide (H2O2) as an oxidant. Recovery of the FexOy@Ti-HMS from the reaction mixture could be facilely attained by application of an external permanent magnet, and the spent catalyst could be recycled without any appreciable loss of its catalytic activity.