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Molecular‐Graphics Modelling of the Acid Strength in Zeolites: Influence of the Aluminium Distribution in Offretite
Author(s) -
Goursot Annick,
Fajula François,
Figueras François,
Daul Claude,
Weber Jacques
Publication year - 1990
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19900730112
Subject(s) - zeolite , chemistry , tetrahedron , aluminium , crystallography , molecular sieve , chemical physics , catalysis , computational chemistry , organic chemistry
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 AlO 4 and 16 SiO 4 tetrahedra, differ only by the relative positions of the Al‐atoms occupying the same crystallographic sites ( T 2 ) 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.