Synthesis, characterization, and catalytic performance of single-site iron(III) centers on the surface of SBA-15 silica.

Abstract

A new molecular precursor strategy has been used to prepare a series of single-site catalysts that possess isolated iron centers supported on mesoporous SBA-15 silica. The iron centers were introduced via grafting reactions of the tris(tert-butoxy)siloxy iron(III) complex Fe[OSi(O(t)Bu)(3)](3)(THF) with SBA-15 in dry hexane. This complex reacts cleanly with the hydroxyl groups of SBA-15 to eliminate HOSi(O(t)Bu)(3) (as monitored by (1)H NMR spectroscopy) with formation of isolated surface species of the type identical with SiO-Fe-[OSi(O(t)Bu)(3)](2)(THF). In this way, up to 21% of the hydroxyl sites on SBA-15 were derivatized (0.23 Fe nm(-)(2)), and iron loadings in the range of 0.0-1.90% were achieved. The structure of the surface-bound iron species, as determined by spectroscopic methods (electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), UV-vis, and in situ infrared measurements) and by elemental analyses, contains a pseudotetrahedral iron(III) center. The THF ligand of this surface-bound complex was quantitatively displaced by acetonitrile (by (1)H NMR spectroscopy). Calcination of these materials at 300 degrees C for 2 h under oxygen resulted in removal of all organic matter and site-isolated iron surface species that are stable to condensation to iron oxide clusters. Spectroscopic data (UV-vis and EPR) suggest that the iron centers retain a mononuclear, pseudotetrahedral iron(III) structure after calcination. The calcinated, iron-grafted SBA-15 materials exhibit high selectivities as catalysts for oxidations of alkanes, alkenes, and arenes, with hydrogen peroxide as the oxidant.