Effects of Cofeeding Hydrogen on Propane Dehydrogenation Catalyzed by Isolated Iron Sites Incorporated into Dealuminated BEA

Iron sites dispersed on nonacidic siliceous supports have been reported to be catalytically active for propane dehydrogenation (PDH), yet the precise relationship between site structure and catalytic activity remains elusive. This study provides a comprehensive understanding of the catalytic performance of iron supported on dealuminated BEA (DeAlBEA) zeolites for PDH. Using XAS, UV-vis, and IR spectroscopy of adsorbed pyridine and deuterated acetonitrile, it was found that, at an Fe/Al₀ of 0.04, isolated Fe sites form. These isolated sites exhibit a forward rate of PDH of 213 mol propene/mol Fe·h at 823 K and a feed containing 15 kPa propane. When 15 kPa of H₂ is added to the feed, the forward rate of PDH rises to 391 mol of propene/mol of Fe·h. In both cases, the propene selectivity is over 99%. IR spectroscopy of <i>d</i>₃-acetonitrile suggests that the open Lewis acid site ((-Si-O-)₂Fe³⁺-OH) serves as the active site responsible for PDH, while Brønsted acid sites (≡Fe³⁺-O(H)-Si≡) contribute to propane cracking with increasing Fe/Al₀ ratios. Kinetic analysis of the effects of H₂ addition to the propane feed on PDH kinetics shows that H₂ enhances the activity of 0.04FeDeAlBEA primarily by enhancing the strength of the propane adsorption.