Isolated Fe^II on Silica As a Selective Propane Dehydrogenation Catalyst

Abstract

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(<i>o</i>Cp)₂. The grafting and evolution of the surface species was elucidated by ¹H 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 Si–OH reacting with Fe­(<i>o</i>Cp)₂ to release one diene ligand (<i>o</i>CpH), generating a SiO₂-bound Fe^II(<i>o</i>Cp) species, <b>1-Fe</b><i><b>o</b></i><b>Cp</b>. Subsequent treatment with H₂ at 400 °C leads to loss of the remaining diene ligand and formation of nanosized iron oxide clusters, <b>1-C</b>. Dispersion of these Fe oxide clusters occurs at 650 °C, forming an isolated, ligand-free Fe^II on silica, <b>1-Fe</b>^<b>II</b>, which is catalytically active and highly selective (∼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, <b>2-NP-Fe</b>^<b>0</b>, were independently prepared by grafting Fe­[N­(SiMe₃)₂]₂ onto silica, <b>2-FeN*</b>, and reducing it at 650 °C in H₂. The Fe NPs were highly active for propane conversion but showed poor selectivity (∼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