The catalytic properties of isomorphously substituted iron-silicate zeolites for the dehydrogenation of propane and for the hydrogenation of propene have been investigated on Fe-ZSM-5 and Fe-Beta zeolites. Characterization of the samples show that isolated framework Fe sites are kinetically the most important sites for hydrocarbon conversion even though Fe species in extra-framework positions also showed some catalytic activity for dehydrogenation. The iron-silicate zeolite samples exhibited nearly 20 times higher dehydrogenation rates than cracking rates while, in comparison, H-[Al]ZSM-5 with similar Si/Al ratioshowed low dehydrogenation selectivity. The high dehydrogenation selectivity indicates that [Fe] zeolites could be a potential catalyst for the large-scale dehydrogenation of propane reaction. Since activation energies for H-[Fe]ZSM-5 were lower than those for H-[Al]ZSM-5; it is likely that a different reaction mechanism is required to explain this large difference in selectivity and measured activation energy. A redox catalytic cycle is suggested as a possible mechanism for the hydrocarbon reaction over the Fe sites isolated in the framework of [Fe] zeolites via formation of a propyl radical cation. The reverse reaction, hydrogenation of propene over H-Fe zeolites is also investigated. As expected from microscopic reversibility, ratios of reaction rate constants are equal to the equilibrium constant of the reaction and the apparent activation energy for hydrogenation is negative.

Catalytic hydrogenation/dehydrogenation of propane and propene over Al- and Fe-zeolites

247th National Meeting (2014)

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