Studies of Framework Iron in Zeolites by Pulsed ENDOR at 95 GHz

Abstract

High frequency (95 GHz, W-band) pulsed ENDOR measurements were carried out on the 57Fe-containing zeolites: Fe-sodalite (FeSOD), Fe-L (FeLTL), Fe-mazzite (FeMAZ), and Fe-ZSM5 (FeMFI), where 57Fe(III) was introduced during synthesis. The echo-detected EPR spectra of all zeolites investigated, recorded at 1.8 K, show mainly the |−5/2〉 to |−3/2〉 EPR transition. Accordingly, the ENDOR spectra exhibit only two 57Fe ENDOR transitions at 67.8−68.8 and 39.0−39.6 MHz, corresponding to MS = −5/2 and −3/2, respectively. From these frequencies isotropic hyperfine couplings of −29.0, −29.3, −29.5, and −29.6 MHz were derived for 57FeSOD, 57FeL, 57FeMAZ, and 57FeMFI, respectively. On the basis of an earlier assignment of the g = 2 signal in FeSOD to Fe(III) in tetrahedral framework sites it is concluded that hyperfine couplings in the range −29.0 to −29.6 MHz are characteristic of 57Fe(III) in zeolite frameworks. In contrast to the X-band 57Fe ENDOR signals, the W-band signals are free from second- and third-order contributions of the hyperfine and zero-field splitting (ZFS) interactions and are thus significantly simpler to assign and interpret. The ZFS contributions caused excessive inhomogeneous broadening of the X-band ENDOR spectra of 57FeL, 57FeMAz, and 57FeMFI and the detection of the ENDOR spectra was practically impossible: All zeolites studied exhibited ENDOR signals from 27Al and 57FeSOD showed also clear 23Na ENDOR signals. The hyperfine interaction of the 23Na was significantly larger than that of the 27Al, confirming the assignment of the Fe(III) to framework sites, substituting for Al. Moreover, the value obtained for the 23Na anisotropic hyperfine component, 0.53 MHz, corresponding to a distance of 3.4 Å, is in good a agreement with the known structure of sodalite where the distance between a framework atom and the Na+ cations in the center of the six rings is 3.35 Å. This work demonstrates the power and potential of high-field ENDOR in terms of resolution, signal assignment, and spectral analysis.