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A CNDO study of the electronic structure of faujasite type six-rings as influenced by the placement of magnesium and by the isomorphous substitution of aluminum for silicon

par Mortier, Wilfried;Geerlings, Pau̧l;Van Alsenoy, Christian;Figeys, Hubert
Référence Journal of Physical Chemistry (1952), 83, 7, page (855-861)
Publication Publié, 1979

Article révisé par les pairs

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Résumé :

A CNDO study has been made of the electronic structure of the main cation-exchange site in faujasite-type zeolitic structures, the framework six-ring. The following isolated molecules and ions were considered as models: Si6O18H12, (Si6O18H12Mg)2+, (Si3Al3O18H12)3-, and (Si3Al3O18H12Mg)-. Some general statements can be made concerning the properties of the aluminosilicate framework and the six-ring site. First, isomorphous substitution of Al for Si, or the presence of an exchangeable cation, results in only small variations in framework oxygen charges and a considerable delocalization of these charges is observed. The charge on Si is more highly variable than that on Al This explains the difference in sensitivities of the SiKβ and AlKβ emission energies upon isomorphous substitution. Second, protons attached to aluminate tetrahedra are more acidic than those attached to silicate tetrahedra. Finally, the molecular electrostatic potential (MEP) seems to be very sensitive to residual charges. This influence, as indicated by the features of the MEP pattern, accounts for the fundamentally different adsorption properties of X and Y zeolites toward CO and CO2 Further, the increase in the CO stretching frequency observed when passing from X to Y zeolites, both containing divalent cations, is also explained by the MEP. © 1979 American Chemical Society.