Investigating the location and distribution of transition metal cations in porous solids by ESE spectroscopy

Abstract

Electron spin echo methods were applied to characterize the immediate environment and the spatial distribution of paramagnetic transition metal cations in microporous solids. Electron spin echo envelope modulation (ESEEM) induced by framework 27A1 nuclei was used to investigate the different sites of Cu2+ in zeolites X, Y and A. All Cu2+ species obtained after complete dehydration showed relatively large isotropic hyperfine interactions with framework Al. This is indicative of a firm interaction of the cation with the framework oxygens. Orientation selective experiments provided additional information concerning the arrangement of the 27A1 nuclei around the Cu2+ and led to site identification. Furthermore, such measurements separated the contributions of distant and near Al nuclei to the modulation. The immediate environment of Mn2+ in MnAlPO-5 where the Mn2+ was incorporated during synthesis as compared to post synthesis introduction via impregnation was investigated. ESEEM induced by framework 31P and 27A1 nuclei was used to investigate the location of the Mn2+ cations with respect to these framework elements and the spatial distribution of the Mn2+ centers throughout the structure was studied by the ”2+1” ESE pulse sequence. All the experimental results obtained by these methods led to the conclusion that in MnAlPO-5 the Mna+ is not situated in the framework but is rather bounded to the external surface via terminal framework oxygens. © 1992 IUPAC