Catalytic dehydrogenation of propane over iron-silicate zeolites

Isomorphously substituted iron-silicate zeolites were prepared and their catalytic properties for the monomolecular propane reaction were investigated. [Fe]ZSM-5 (MFI) samples with Si/Fe ratios of 26 and 48, and [Fe]beta zeolite (BEA*) with Si/Fe ratio of 15 were synthesized and characterized using X-ray powder diffraction (XRD), N2 adsorption and ultraviolet–visible (UV/Visible) spectroscopy. Unit cell volumes, determined using XRD, were used to monitor the presence of Fe in the framework and the migration of Fe species from framework to extra-framework positions. Most of the Fe atoms remain in framework positions after mild calcination (753 K in air), but most of the Fe species migrate to extra-framework positions after steaming at 973 K. Migration of Fe species from framework to extra-framework positions can occur during the monomolecular propane reaction as well. Isolated framework Fe sites are the most important sites for hydrocarbon conversion even though Fe species in extra-framework positions also showed catalytic activity for dehydrogenation. The iron-silicate zeolites exhibited nearly 20 times higher dehydrogenation rates than cracking rates while H-[Al]ZSM-5 with similar Si/Al ratio (∼26) showed low dehydrogenation selectivity (dehydrogenation–to-cracking ratio ∼0.36). The activation energy for H-[Fe]ZSM-5 is also lower than the activation energy for H-[Al]ZSM-5. A redox catalytic cycle is suggested for the hydrocarbon reaction over isolated Fe framework sites in [Fe] zeolites via the formation of propane radical cations since a protolytic mechanism seems to be unable to explain the observed differences in activation energy and selectivity.