A physico-chemical study of iron(III)-containing MFI zeolitic materials. The iron (III) isomorph of zeolite ZSM-5

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Abstract
A range of iron(III)-containing MFl structured zeolites with increasing iron(III) content has been prepared using stirred and unstirred reaction mixtures. The products have been characterized by elemental analysis and cation exchange capacity, X-ray powder diffraction measurements, specific surface area determinations and scanning electron microscopy. Staining of the iron-containing MFl structured zeolitic material [FeIIIMFIZ] with methylene blue shows the presence of silicalite up to the point where the bulk iron content reaches 0.7 FeIII ions per unit cell. Measurements of (a) the ratio of the cation exchange capacity to bulk FeIII content, (b) the area under the peaks between values of 2θ from 22.5 to 25° in the X-ray powder diffraction patterns, (c) the Langmuir monolayer capacities for the adsorption of methylene blue, and (d) the intensities of the e.s.r. signals in the g= 2.0, 2.3 and 4.3 regions, when plotted against increasing FeIII contents per unit zeolitic cell, show a discontinuity when the FeIII content of the unit cell reaches 1.8. E.s.r. spectroscopy has been used to distinguish FeIII in the isomorphic substitution positions in the framework of the zeolite and the minor amount in non-framework sites. Magnetic susceptibility measurements have shown the absence of exchange interactions in zeolite material from stirred reaction mixtures and their presence due to adventitiously introduced FeIII from unstirred reaction mixtures. The sensitivity of the e.s.r. silence of the dehydrated FeIII ZSM-5 towards a variety of chemical reagents which bring about a selective restoration of the e.s.r. signals at high and low fields has been used to characterize further the chemical nature of the FeIII centres. The chemical reactivity of the FeIII MFIZ towards the representative reagents NO2, pyridine and acetyl-acetone has been studied by thermogravimetric analysis, X-ray powder diffraction measurements, and i.r. and e.s.r. spectroscopy. The results have been interpreted in terms of the interaction of the reagent with acidic sites on the zeolite and FeIII centres and compared with similar interactions occurring in AlIII–ZSM-5 and silicalite. The absence of an oxidic FeIII-containing phase in FeIIIMFIZ obtained from stirred reaction mixtures has been shown by an e.s.r. spectroscopic study of material prepared form heated compacts of FeIII oxide and silicalite.

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