(378a) Structure of Iron in a Fe-Al2O3 Catalyst for Sour Propane Dehydrogenation

H₂S is a strong poison for noble metal catalysts and requires expensive separation from hydrocarbon feeds before catalytic transformations can be performed. H₂S is also a common component of shale gas, the light alkane fraction of which is increasingly used for on purpose dehydrogenation to produce propylene used in the polymer and chemical industries. In this work, we present the structural analysis and catalytic performance of a H₂S tolerant propane dehydrogenation catalyst consisting of iron impregnated alumina. Cofeed of and pretreatment in H₂S improved the catalyst stability and dehydrogenation rate. The dehydrogenation rate as a function of iron loading showed a plateau corresponding to the predicted monolayer coverage of iron on the support. Synchrotron X-Ray Diffraction (XRD), Pair Distribution Function (PDF), and X-ray Absorption Spectroscopy (XAS) were used to study the iron structure in the as-synthesized catalyst. XRD and PDF ruled out the formation of crystalline or nanocrystalline iron oxide or mixed oxide phases. XAS showed the isolated nature of the iron sites below monolayer coverage. Extended X-ray Absorption Fine Structure (EXAFS) modeling showed that iron atoms have on average one long Fe-O bond and four short Fe-O bonds, which is distinct from the oxygen environment of Fe in Fe₂O₃. Additionally, EXAFS modeling showed that the second nearest neighbor of iron was aluminum, confirming the single site nature of iron. H₂ Temperature Programmed Reduction (TPR) showed that iron sites are reduceable. Additionally, H₂ TPR measurements after H₂S pretreatment led to the evolution of H₂S at 380°C, suggesting formation of an Fe-S bond. Normalizing the dehydrogenation rate using the reduceable iron fraction gave a propylene formation turnover frequency that is constant across iron loadings above and below monolayer coverage. Based on these results it is proposed that the active iron sites are dispersed in nature.