Studies of Fe(III) incorporated into AlPO4-20 by X- and W-band EPR spectroscopies

Abstract

The incorporation of Fe(III), during the synthesis, into aluminosilicate sodalite (FeSOD) and aluminophosphate sodalite, AlPO4-20 (FAPO), was investigated by continuous wave (CW) and pulsed electron paramagnetic resonance (EPR) techniques at X- and W-band. Specifically, the effect of the framework composition and the presence of occluded template molecules (tetramethyl ammonium hydroxide, TMAOH) in the β cages on the distribution of the Fe(III) species was explored. The X-band CW EPR spectrum of FAPO shows the existence of two types of species, one with a large (g ≈ 6.3,4) and the other with a small (g ≈ 2) zero field splitting (ZFS) interaction. These species were also found in FeSOD synthesized with TMAOH. The X-band field-sweep echo-detected (FS-ED) EPR spectrum shows contributions only from the Fe(III) species in the more symmetric environment (g ≈ 2). The other was not detected due to fast relaxation. This spectrum is very broad and suffers from distortions due to the nuclear modulation effect. In contrast, the W-band FS-ED EPR spectrum of the same species was significantly narrower and free from distortions. Analysis of the temperature dependence of the width and relative intensity of the peak corresponding to the |−½ 〉 | +½ 〉 EPR transition shows that the g ≈ 2 signal arises from a number of Fe(III) species with a distribution of ZFS parameters. Calcination significantly reduces the ZFS parameter, D, suggesting that the distortions of the T sites are due to specific interactions with the template. Electron spin echo envelope modulation (ESEEM) experiments shows the presence of weak dipolar interaction between Fe(III) and template 14N and 1H template nuclei, as well as framework 27Al and 31P nuclei. This indicates that the species characterized by small ZFS are well dispersed and are located within the inner structure of the zeolite. These g≈ 2 species are most probably Fe(III) in framework sites. A small fraction that occupies highly asymmetric sites (g ≈ 6.3,4), situated at ‘defect’ framework or extraframework sites, and some Fe(II) produced due to the reduction of Fe(III) by the organic template (detected by Mossbauer spectroscopy), were found as well. The possible presence of some extraframework Fe(III) with a g ≈ 2 signal cannot be excluded. Copyright © 1999 John Wiley & Sons, Ltd.