Preparation of Fe-substituted Mesoporous Silicas with Highly Isolated Iron Species in Buffer Solution

The iron-substituted mesoporous silicas were synthesized under mild acidic conditions (pH = 4.4, HOAc-NaOAc buffer solution) using tetramethoxysilane (TMOS), tetraethoxysilane (TEOS) or sodium silicate solution as silica source in the presence of block copolymer Pluronic P123 as the mesoporous template. TMOS and sodium silicate led to iron-containing silicas with ordered two-dimensional hexagonal mesoporous structure, and vesicle mesostructure was obtained using TEOS as silica precursor. UV-Vis and UV Resonance Raman spectra show that highly isolated iron species can be predominantly obtained on mesoporous iron-containing silicas synthesized using inexpensive sodium silicate and Fe(NO3)3 as precursors. The buffer solution provides a facile strategy for controlling the mesostructure of iron-containing silicas by simply varying the silica source. Mesoporous iron-substituted silicas are highly efficient in hydroxylation of phenol because of a high dispersion of active iron centers within silica matrices and a high accessibility of active iron species to reactants.