Oxidative functionalization of propane over FeMFI zeolites effect of reaction variables and catalyst constitution on the mechanism and performance

The effect of the iron distribution in steam-activated FeZSM-5 and Fe-silicalite on their catalytic performance for direct NO decomposition and for the oxidative dehydrogenation of propane (ODHP) with N2O and 02 was investigated by means of steady-state and transient pulse experiments. Steam-activated FeZSM-5, containing a large fraction of iron as oligonuclear FexOy clusters, is more active and selective in ODHP and direct N2O decomposition than steam-activated Fe-silicalite, where isolated iron species predominate. This suggests that the same type of iron centers is involved in both conversions. The higher activity of steam-activated FeZSM-5 is associated to the higher mobility of atomic oxygen species attached to oligonuclear iron species, which lead to a faster oxygen desorption in direct N2O decomposition as well as to a faster transfer to propane in the ODHP reaction. Steam-activated FeMFI zeolites were found to be highly effective catalysts for ODHP with N2O, with initial propene yields up to 24%. Relatively high yields of propionaldehyde are also obtained, which mainly originates from the selective oxidation of propene. The unique ODHP performance of iron zeolites requires the use of N2O as monooxygen donor. Such species cannot be created by O-2. Only a fraction of oxygen species deposited over iron sites of FeMFI upon N2O activation was found to be active towards propane oxidation under transient vacuum conditions. This is explained by a fast transformation of short-living highly reactive atontic oxygen species into less reactive ones. (C) 2004 Elsevier B.V. All rights reserved.