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(oCp) 2 . The grafting and evolution of the surface species was elucidated by 1 H 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(oCp) 2 to release one diene ligand (oCpH), generating a SiO 2 -bound Fe II (oCp) species, 1-FeoCp. Subsequent treatment with H 2 at 400 °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 °C, forming an isolated, ligand-free Fe II on silica, 1-Fe II , 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, 2-NP-Fe 0 , were independently prepared by grafting Fe[N(SiMe 3 ) 2 ] 2 onto silica, 2-FeN*, and reducing it at 650 °C in H 2 . 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.

A Mesoporous Cobalt Aluminate Spinel Catalyst for Nonoxidative Propane Dehydrogenation

“…Silica‐supported isolated transition metal ions (that is, Co, Fe) were found by Hock et al. to be catalytically active and selective for propane dehydrogenation . A similar catalytic dehydrogenation activity was also reported by Copéret et al.…”

Intermetallic Compounds as an Alternative to Single‐atom Alloy Catalysts: Geometric and Electronic Structures from Advanced X‐ray Spectroscopies and Computational Studies

“…The effect of conversion on the propylene selectivity of the PtFe 3 and Pt catalysts is shown in Figure . Under these conditions, the mono‐metallic Fe catalyst showed no propane conversion, consistent with the much lower reported activity compared to Pt . When tested with a feed containing 2.5 % C 3 H 8 in N 2 , the PtFe 3 catalyst was 98 % selective towards propylene at 10 % conversion, constituting a significant improvement over Pt, which was 90 % selective (Figure a).…”

“…Under these conditions, the mono-metallic Fe catalyst showed no propane conversion, consistent with the much lower reported activity compared to Pt. [29] When tested with a feed containing 2.5 % C 3 H 8 in N 2 , the PtFe 3 catalyst was 98 % selective towards propylene at 10 % conversion, constituting a significant improvement over Pt, which was 90 % selective ( Figure 3a). As the propane conversion increased, there was an even larger difference in selectivity between the two catalysts; the PtFe 3 IMC catalyst maintained high propylene selectivity (94 %) while that of Pt decreased to 55 %.…”

Tetrahedral Nickel(II) Phosphosilicate Single‐Site Selective Propane Dehydrogenation Catalyst

“…The reaction of SiO 2 ‐supported Ni II with H 2 likely occurs following the mechanism shown in Scheme –. Heterolytic cleavage of the H−H and Ni−O bonds generates O−H and Ni−H bonds.…”