Journal of the American Chemical Society, Vol.116, No.14, 6344-6353, 1994
Characterization of Iron in Zeolites by X-Band and Q-Band ESR, Pulsed ESR, and UV-Visible Spectroscopies
The local structure of Fe sites in a series of ferrisilicate and ferrialuminosilicate zeolites (Fe-sodalite (FeSOD), Fe-L (FeL), Fe-faujasite (FeFAU), FeZSM5, and Fe-mazzite (FeMAZ)) in which iron was incorporated during zeolite synthesis was studied by X- and Q-band ESR, electron spin echo detected ESR (ED-ESR), electron spin echo envelope modulation (ESEEM), and diffuse reflectance UV-vis spectroscopies. Samples were investigated as a function of Fe content and after variable-temperature dehydration and rehydration. Transitions characteristic of tetrahedrally coordinated Fe3+ were observed in the diffuse reflectance spectra of all Fe zeolites except for FeFAU. A strong g = 2 signal was observed in the ESR spectra of all the Fe zeolites whereas a g = 4.3 signal was observed only for the FeFAU, FeZSM5, and FeMAZ samples. Comparison between X- and Q-band spectra indicates that only one type of Fe3+ site is present in Fe-sodalite. Thus, neither the traditional assignment of the ESR signals of iron in the zeolite framework to a g = 4.3 signal 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, and 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 framework substitution unless combined with other physical or chemical methods. Furthermore, the absence of the g = 4.3 signal in the ESR spectrum does 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 on Fe-faujasite that was subjected to dehydration/rehydration cycling show that phases with "aggregates" of Fe ions are generated either by dislodged framework Fe3+ and/or by migration of extraframework Fe3+.
Keywords:ELECTRON-PARAMAGNETIC-RES;SPIN-ECHO MODULATION;ALPHA-QUARTZ;ISOMORPHOUS SUBSTITUTION;FOURIER-TRANSFORM;ENVELOPE MODULATION;COMPENSATED CENTER;EPR;FE-3+;RESONANCE