New Fe/SiO2 materials prepared using diiron molecular precursors: Synthesis, characterization and catalysis

Several well-defined diiron siloxide complexes have been synthesized, isolated, characterized, and used as molecular precursors for the grafting of well-defined isolated iron species on the surface of mesoporous silica SBA-15. The precursors have bridging siloxide ligands, a diamine linker, or a μ-oxo diiron core, and their binuclear structures have been confirmed by elemental analysis, solution molecular weight measurements, and nuclear magnetic resonance spectroscopy. All precursors react with SBA-15 to immobilize the iron centers and produce silanol; the various stoichiometries and structural implications of these grafting reactions are discussed. Calcination of the grafted iron complexes yields materials largely devoid of organic components, and these calcined catalysts are active in the oxidation of hydrocarbons by hydrogen peroxide. The catalytic activities and selectivities of these materials are compared with each other and with those of other Fe/SiO2 catalysts. Issues including iron-loading dependence and grafting conditions are discussed. Characterization of the catalysts by diffuse-reflectance ultraviolet-visible, X-band electron paramagnetic resonance, Mössbauer, and extended X-ray absorption fine structure spectroscopies indicates that although the diiron structures of the precursors are usually maintained during the initial grafting process, calcination results in their conversion to monoiron centers on the support. This theory is also consistent with the generally similar catalytic behaviors of materials prepared from diiron and monoiron precursors. The implications of these findings for the generality of molecular precursor techniques are discussed.