Changes in the sieving action and thermal stability of zeolite a produced by ion-exchange

Na-A and K-A zeolites have been ion-exchanged with Ca2+ and Zn2+. The molecular sieving properties of the ion-exchanged zeolites were measured as functions of their composition. Abrupt changes in sieving action occur at 75 % exchange in the (K, Ca)-A series, and at 75 and 83 % exchange in the (K, Zn)-A series, as measured with chemically unreactive molecules. The sieving action of (K, Zn)-A zeolite for some polar molecules and unsaturated hydrocarbons changes gradually with its composition.The thermal stability of molecular sieve 3A is greatly improved by introducing a large number of divalent cations into this K-A zeolite.These properties are discussed in terms of site-selectivities of the exchangeable cations. From these, are derived diagrams of cation distribution on three kinds of site in the crystal.