Construction of highly dispersed mononuclear iron-oxo species in the supercages of Y zeolite by use of surface organometallic chemistry

Y zeolite containing well-dispersed iron (Fe/Y-G) was prepared by grafting reaction of the ferrocene molecule (Cp₂Fe) on the acidic sites of HY zeolite followed by thermal treatment in oxygen. In situ FTIR and chemical analysis revealed that Cp₂Fe molecule adsorbed on the zeolite surface lost one cyclopentadienyl group leading under vacuum at 423 K to the formation of a surface iron-cyclopentadienyl complex. Fe/Y-G was then obtained by calcination at 773 K in flowing oxygen. This solid was characterized by XPS, XAFS, CO adsorption FTIR, UV-vis DRS, and catalytic oxidation reaction of CO with oxygen. The characteristic oxygen-to-metal charge-transfer transition bands at ca. 215 nm and 250 nm showed existence of Fe³⁺ of Fe/Y-G in isolated tetrahedral sites. Both the 2P_3/2 and 2P_1/2 electron binding energies of iron and the Fe K edge energy were higher than those of Fe₂O₃, suggesting that the iron ions contained in Fe/Y-G were highly dispersed and had higher spin-orbital splitting, due to the hybridization of iron 4p atomic orbitals with partly 3d molecular orbitals. The Fe/Y-G showed only one ν(CO) infrared adsorption band at 2194 cm^-1, very different from a Fe/Y-I sample prepared by impregnation with FeCl_3. The Fe/Y-G zeolite showed a higher catalytic activity in the oxidation of carbon monoxide with molecular oxygen than the Fe/Y-I sample. This behavior was ascribed to the special coordination geometry and electronic structure of the iron atom in the zeolite.