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A method for the quantitative determination of isolated Fe 3+ ions in FeHZSM-5 by EPR was devised. The method is based on the comparison of signals, taken at −196°C, from pyridine-impregnated catalysts with signals from solutions of Fe(III)-phthalocyanine in pyridine. FeCl 3 was introduced into HZSM-5 having Si/Al ratios of 15 and 25. The amount of Fe was between 0.5 and 5.5 wt%. The samples were prepared either by sublimation of FeCl 3 into the zeolite in an N 2 flow or by impregnation with FeCl 3 water solutions, and subsequently calcined at 400–800°C. The measurement of “ESR-visible Fe 3+ ” in FeCl 3 /HZSM-5 calcined at 400°C in vacuum demonstrates that virtually all ferric ions, present as isolated FeCl + 2 -species, contribute to the ESR signal in samples with Fe/Al ratios 3 sublimation in an N 2 flow results in the stabilization of the same amount of isolated FeCl + 2 species with the excess forming aggregates. The number of isolated Fe 3+ ions decreases sharply after treatment in an air flow at T ≥500°C. Irrespective of the starting concentration of iron, after oxidative calcinations at 500–520°C, the amount of isolated low-coordinated Fe 3+ cations in FeHZSM-5 does not exceed Fe/Al∼0.15; the rest aggregates into ESR-invisible species or forms bulk α-Fe 2 O 3 . Calcination of FeHZSM-5 with the lower Si/Al ratio of 15 at 800°C causes further loss of ESR-visible isolated Fe 3+ ions, whereas the higher silica samples, with Si/Al=25, are tolerant to this treatment. Formation of aggregated iron species is minor in the most diluted, low-loaded sample, 0.5% Fe/HZSM-5(Si/Al=25), where the majority of Fe 3+ (∼2/3) remains well isolated and contributes to the ESR signal even after oxidative treatment at 800°C. Redox treatment of this sample even under mild conditions significantly redistributes the iron, with irreversible loss of of isolated Fe 3+ ions and formation of ferromagnetic aggregates of Fe 3 O 4 . |
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