Application of percolation theory to ion-exchanged molecular sieves A

The unit cell of dehydrated zeolite A is cubic and its {100} faces constitute “windows” of a large cavity located at the center. In sodium-form zeolite A, Na⁺ partially blocks the window, through which oxygen but not nitrogen molecules can pass. By ion-exchanging two Na⁺s with one Ca²⁺, an unblocked or “open window” can be introduced. By preparing well defined (Na, Ca)-A zeolites having various compositions, the nitrogen percolation process was quantitatively studied. A curve for the number of percolatable cells vs the open-window concentration was deduced, and compared with Hammersley's theoretical curve. The theoretical curve has a sharp cut-oft at a concentration of one open-window per unit cell. The observed curve was steeper in this transition region than the theoretical one, but has a tail in the region corresponding to lower concentrations of open windows. The first tendency is ascribed to an ordered alignment of open-windows in their spatial distribution. The second is attributed to incompleteness of the blocking action of Na⁺. Substituted Ca²⁺ and vacant cation sites change the Madelung potential at the Na⁺ site, so that the curvature of the potential valley for Na⁺ may become gentle. As a result, Na⁺ can easily be displaced and make way for a visiting nitrogen molecule.