MOLECULAR-GRAPHICS MODELING OF THE ACID STRENGTH IN ZEOLITES - INFLUENCE OF THE ALUMINUM DISTRIBUTION IN OFFRETITE

Using extended Hückel wave functions, molecular electrostatic potentials (MEP's) have been calculated for several model clusters representative of zeolites of offretite type. The clusters studied, which are all made of a central unit comprising 2 AlO4 and 16 SiO4 tetrahedra, differ only by the relative positions of the Al‐atoms occupying the same crystallographic sites (T2) within the zeolite framework. using the MEP values as a color‐coded acidity index for the various clusters, three‐dimensional representations of their molecular surfaces are generated as solid models on a performing computer graphics system. Important differences in acidity are predicted for the clusters which can he classified into two types according to the distribution of Al‐atoms: the first one is characterized by nearly independent acid sites localized around the main channel of zeolite (Figs. 3 and 6) whereas the second one exhibits interacting acid sites located longitudinally along the channel of the same gmelinite cage (Figs. 4 and 5). The possible relationship between the structure of the clusters and their catalytic activity towards organic species is discussed. Copyright © 1990 Verlag GmbH & Co. KGaA, Weinheim