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

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 [Fe^IIIMFIZ] with methylene blue shows the presence of silicalite up to the point where the bulk iron content reaches 0.7 Fe^III ions per unit cell. Measurements of (a) the ratio of the cation exchange capacity to bulk Fe^III 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 Fe^III contents per unit zeolitic cell, show a discontinuity when the Fe^III content of the unit cell reaches 1.8. E.s.r. spectroscopy has been used to distinguish Fe^III 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 Fe^III from unstirred reaction mixtures. The sensitivity of the e.s.r. silence of the dehydrated Fe^III 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 Fe^III centres. The chemical reactivity of the Fe^III 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 Fe^III centres and compared with similar interactions occurring in Al^III–ZSM-5 and silicalite. The absence of an oxidic Fe^III-containing phase in Fe^IIIMFIZ obtained from stirred reaction mixtures has been shown by an e.s.r. spectroscopic study of material prepared form heated compacts of Fe^III oxide and silicalite.