Isomorphous substitution of iron in the framework of zeolite ZSM-23

Abstract

A crystalline ferrisilicate having the framework structure of ZSM-23 zeolite has been synthesized and characterized. The presence of iron in the zeolite lattice framework has been confirmed by spectroscopic (XRD, framework IR, ESR, and XPS), DTA/TG, magnetic susceptibility, ion exchange, and catalytic activity results. Chemical analysis and XPS show the absence of any significant quantity of Al(SiO₂/Al₂O₃ > 1400) in the ferrisilicate. XRD patterns of the as-synthesized as well as the calcined Fe/ZSM-23 matched well with that of ZSM-23 (MTT topology). Framework IR bands of ZSM-23 are shifted toward lower frequencies on incorporation of iron in the framework. The white color of the as-synthesized as well as that of calcined samples of Fe/ZSM-23, ESR (g=4.4), and magnetic susceptibility (μ=5.4-5.8, BM) data indicate that Fe-O-Fe interactions (due to oxides of iron like Fe₂O₃ or Fe₃O₄) are absent, and that well dispersed Fe 31 ions are present in a magnetically dilute environment. Fe/ZSM-23 exhibits significant ion exchange capacity and catalytic activity/shape selectivity in the metaxylene isomerization reaction. Fe²⁺ ions in zeolite lattice positions account for the ion exchange capacity and shape selective catalytic activity of this crystalline ferrisilicate.