Abstract
A method is described for the preparation of Fe 3+-zeolites of the ZSM-5 type, where Fe 3+ is incorporated in the zeolite lattice. Samples with overall Si Fe ratios of 10 and 65 have been synthesized and characterized by 57Fe Mössbauer Spectroscopy, e.p.r. and 29Si MAS n.m.r. This paper deals mainly with the interpretation of our Mössbauer results. All samples have room temperature Mössbauer spectra, consisting of broad quadrupole split doublets. By lowering the temperature and applying external magnetic fields at least two spectral components can be recognized and assigned: 1. (1) Fe 3+ ions incorporated in tetrahedric sites in the zeolite framework, causing slow electronic relaxation phenomena in the spectra. 2. (2) Fe(III) oxides, precipitated as small particles on the zeolite surface, exhibiting superparamagnetic relaxation. Mössbauer spectra are presented of some samples after dehydration. This causes an increase of the average quadrupole interaction, indicating an increase of the asymmetry of the charge distribution around the iron nucleus. Dehydration at temperatures less than 250°C is reversible.
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