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.

We report the structural evolution of Pd-Zn alloys in a 3.6% Pd-12% Zn/Al2O3 catalyst which is selective for propane dehydrogenation. High signal-to-noise, in situ synchrotron X-ray diffraction (XRD) was used quantitatively, in addition to in situ diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) and extended X-ray absorption fine structure (EXAFS) to follow the structural changes in the catalyst as a function of reduction temperature. XRD in conjunction with DRIFTS of adsorbed CO indicated that the β1-PdZn intermetallic alloy structure formed at reduction temperatures as low as 230 °C, likely first at the surface, but did not form extensively throughout the bulk until 500 °C which was supported by in situ EXAFS. DRIFTS results suggested there was little change in the surfaces of the nanoparticles above 325 °C. The intermetallic alloy which formed was Pd-rich at all temperatures but became less Pd-rich with increasing reduction temperature as more Zn incorporated into the structure. In addition to the β1-PdZn alloy, a solid solution phase with face-center cubic structure (α-PdZn) was present in the catalyst, also becoming more Zn-rich with increasing reduction temperature.

Silica supported Pd and PdZn nanoparticles of a similar size were evaluated for neopentane hydrogenolysis/isomerization and propane hydrogenolysis/dehydrogenation. Monometallic Pd showed high neopentane hydrogenolysis selectivity. Addition of small amounts of Zn to Pd lead Pd-Zn scatters in the EXAFS spectrum and an increase in the linear bonded CO by IR. In addition, the neopentane turnover rate decreased by nearly 10 times with little change in the selectivity. Increasing amounts of Zn lead to greater Pd-Zn interactions, higher linear to bridging CO ratios by IR and complete loss of neopentane conversion. Pd NP's also had high selectivity for propane hydrogenolysis, thus were poorly selective for propylene. The PdZn bimetallic catalysts, however, were able to preferentially catalyze dehydrogenation and were not active for propane hydrogenolysis, thus, were highly selective for propylene formation. The decrease in hydrogenolysis selectivity was attributed to the isolation of active Pd atoms by inactive metallic Zn, demonstrating that hydrogenolysis requires a particular reactive ensemble whereas propane dehydrogenation does not.

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