X-ray Absorption Fine Structure Characterization of the Local Structure of Fe in Fe−ZSM-5

The local structure of Fe in Fe−ZSM-5 prepared by solid-state exchange was investigated with XAFS. Fe K-edge spectra were taken at liquid nitrogen temperature for samples with Fe/Al ratios of 0.33, 0.66, and 0.80. The radial structure function (RSF) of He- and CO-pretreated Fe−ZSM-5 shows two main peaks, one at 1.6 Å and the other at 2.5 Å. To interpret the origin of these peaks, RSFs were simulated for a number of mono- and di-iron structures obtained from quantum chemical calculations. By this means, the peak in the RSF at 1.6 Å is clearly identified with backscattering from O atoms coordinated to an Fe atom. The peak at 2.5 Å has been previously ascribed to Fe−Fe scattering and has been used to argue for the presence of di-iron−oxo species; however, the origin of this peak and its interpretation remains an open question. The imaginary part of the Fourier transformed data for the peak at 2.5 Å has the same characteristics as that generated theoretically for Fe−Al backscattering and is distinctly different from that generated theoretically for Fe−Fe backscattering. This evidence strongly suggests that the iron in Fe−ZSM-5 is present as isolated cations associated with framework aluminum. Further evidence for such a structure is the absence of any change in the magnitude of the peak near 2.5 Å with sample treatment. The RSFs and the information obtained from curve-fitting demonstrate that the structure of Fe in Fe−ZSM-5 does not change significantly with Fe/Al ratio. For both He- and CO-pretreated sample, the Fe−O coordination number is about 4 and correspondingly the Fe−Al coordination number is about 1, regardless of Fe/Al ratio. Therefore, the structure of Fe in Fe−ZSM-5 is best described as either Z-[Fe(O)2]+ or Z-[Fe(OH)2]+, where Z- represents the charge-exchange site in the zeolite. Upon O2 pretreatment, a new feature appears at about 1.1 Å in the RSF, which may be due to a migration of some of the Fe into the zeolite framework. This interpretation is qualitatively consistent with the observed RSF for Fe−silicalite.