CLUSTER-MODELS FOR BRONSTED ACID CENTERS IN FAUJASITES

Cluster models for Bronsted acid centres in zeolites are proposed which would be suitable for intermediate quantum chemical calculations. Our extended models for different environments of framework hydroxyls in faujasites include explicitly the second cationic coordination sphere of the bridging unit and are based upon real atoms as saturators of dangling bonds at cluster boundaries. For such models MNDO-type investigations with geometry optimization of the entire cluster could be undertaken. The adequacy of the proposed cluster models is checked on the basis of deprotonisation energies and their direct dependence on the number of aluminium atoms in the neighbourhood of the bridging hydroxyl.