Nature of active sites in the Fe-TON Zeolites: Mossbauer and IR studies

Results on the stability of framework Fe atoms obtained for the ZSM-22 zeolites of different iron contents - members of the TON family - are presented. They show how the formation of extraframework iron species can modify the parent acidity of the Fe-TON zeolites. Mossbauer spectroscopy revealed that iron atoms were partially extracted from the framework during both high-temperature template elimination and activation of the NH(4) form prior to the reaction. The extent of the Fe removal depended on the iron content and was higher for samples of low Si/Fe ratios. Unexpectedly, strength of the Bronsted sites was found to be higher also for iron rich samples. It was postulated that an interaction between the extraframework iron species of a cationic character and the neighbouring bridging Si-OH-Fe groups enhanced the acidity of the existing Bronsted sites.