Isolated FeII on Silica As a Selective Propane Dehydrogenation Catalyst

We report a comparative study of isolated Fe-II, iron oxide particles, and metallic nanoparticles on silica for non-oxidative propane dehydrogenation. It was found that the most selective catalyst was an isolated Fe-II species on silica prepared by grafting the open cyclopentadienide iron complex, bis(2,4-dimethyl-1,3-pentadienide) iron(II) or Fe(oCp)(2). The grafting and evolution of the surface species was elucidated by H-1 NMR, diffuse reflectance infrared Fourier transform spectroscopy and X-ray absorption spectroscopies. The oxidation state and local structure of surface Fe were characterized by X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure. The initial grafting of iron proceeds by one surface hydroxyl SiOH reacting with Fe(oCp)(2) to release one diene ligand (oCpH), generating a SiO2-bound Fe-II(oCp) species, 1-FeoCp. Subsequent treatment with H-2 at 400 degrees C leads to loss of the remaining diene ligand and formation of nanosized iron oxide clusters, 1-C. Dispersion of these Fe oxide clusters occurs at 650 degrees C, forming an isolated, ligand-free Fe-II on silica, 1-Fe-II, which is catalytically active and highly selective (similar to 99%) for propane dehydrogenation to propene. Under reaction conditions, there is no evidence of metallic Fe by in situ XANES. For comparison, metallic Fe nanoparticles, 2-NP-Fe-0, were independently prepared by grafting Fe[N(SiMe3)(2)](2) onto silica, 2-FeN*, and reducing it at 650 degrees C in H-2. The Fe NPs were highly active for propane conversion but showed poor selectivity (similar to 14%) to propene. Independently prepared Fe oxide clusters on silica display a low activity. The sum of these results suggests that selective propane dehydrogenation occurs at isolated Fe-II sites.