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
M
S
= −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.
