CHARACTERIZATION OF IRON IN ZEOLITES BY X-BAND AND Q-BAND ESR, PULSEDESR, AND UV-VISIBLE SPECTROSCOPIES

The local structure of Fe sites in a series of ferrisilicate and ferri aluminosilicate zeolites (Fe-sodalite (FeSOD), Fe-L (FeL), Fe-faujasit e (FeFAU), FeZSM5, and Fe-mazzite (FeMAZ)) in which iron was incorpora ted during zeolite synthesis was studied by X- and Q-band ESR, electro n spin echo detected ESR (ED-ESR), electron spin echo envelope modulat ion (ESEEM), and diffuse reflectance UV-vis spectroscopies. Samples we re investigated as a function of Fe content and after variable-tempera ture dehydration and rehydration. Transitions characteristic of tetrah edrally coordinated Fe3+ were observed in the diffuse reflectance spec tra of all Fe zeolites except for FeFAU. A strong g = 2 signal was obs erved in the ESR spectra of all the Fe zeolites whereas a g = 4.3 sign al was observed only for the FeFAU, FeZSM5, and FeMAZ samples. Compari son between X- and Q-band spectra indicates that only one type of Fe3 site is present in Fe-sodalite. Thus, neither the traditional assignm ent of the ESR signals of iron in the zeolite framework to a g = 4.3 s ignal nor some more recently proposed assignments could fully account for all the present results. We conclude that Fe3+ can be incorporated into framework sites in significant amounts in FeSOD, FeL, FeZSM5, an d FeMAZ and probably also in FeFAU and that in each case framework Fe3 + exhibits a g = 2 ESR signal. Since extraframework Fe3+ can also show a g = 2 signal, its appearance does not provide evidence for framewor k substitution unless combined with other physical or chemical methods . Furthermore, the absence of the g = 4.3 signal in the ESR spectrum d oes not exclude the possibility of framework substitution in zeolites. Fe-faujasite was found to have significantly more extraframework Fe3 than the other Fe zeolites studied. Combined ESR, ED-ESR, and ESEEM o n Fe-faujasite that was subjected to dehydration/rehydration cycling s how that phases with ''aggregates'' of Fe ions are generated either by dislodged framework Fe3+ and/or by migration of extraframework Fe3+.