, Volume 23, Issue 1-4, pp 137-143

Mechanisms of Iron Activation on Fe-Containing Zeolites and the Charge of α-Oxygen

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Abstract

According to previous Mössbauer data [1] α-sites formation at the activation of Fe-containing zeolites is accompanied by irreversible “self-reduction” of the iron, proceeding without participation of an external reducing agent. Reduced Fe2+ ions are inert to O2 but are reversibly oxidized to Fe3+ by N2O, generating the α-oxygen species, Oα, which provide selective oxidation of hydrocarbons.

In this work, the mechanism of α-sites formation was studied via quantitative measurement of the dioxygen amount desorbed into the gas phase at the step of “self-reduction”. A prominent role of the zeolite matrix chemical composition has been revealed. For example, with zeolites of Al–Si composition (FeZSM-5 and Fe-β), heating to 900 °C in a closed vacuum space leads to irreversible evolution of O2, which is accompanied by the immediate formation of α-sites. Similar heating of B–Si and Ti–Si zeolites also leads to dioxygen evolution; however, this evolution is reversible and is not accompanied by formation of α-sites. Activation of these zeolites occurs only in the presence of water vapor. Stoichiometric measurements showed that in terms of charge one regular O2- ion, removed at the activation, is equivalent to two α-oxygen atoms. So, α-oxygen is identified as an ion-radical species Oα -., whose unique oxidation properties still distinguish it from the generally observed O-. radicals.

The mechanism of α-sites formation is proposed, in which the process of strong chemical stabilization of reduced Fe2+ atoms in the zeolite structure is a key step, making impossible the reoxidation of the iron with O2.