A calorimetric and statistical mechanics study of water adsorption in zeolite NaY

Adsorption isotherms and differential heats of adsorption of water in the zeolites NaY and NaY(Br) were measured at 303 K. The latter zeolite was obtained from NaY by introducing NaBr into the β-cages thus rendering them impenetrable to water molecules.The results obtained lead to the conclusion that initially water is accomodated in the small β-cages up to 4 H2O/cage before adsorption in the large supercages sets in and proceeds to a maximum of 26 H2O/supercage. The experimentally obtained adsorption and heat data for water in both types of cages can be quantitatively reproduced on the basis of the statistical-mechanical concept of independent subsystems which are identified with the β-cages and, on the other hand, with the supercage-Na+ ions. For each of the one-to-four H2O/β-cage units and each of the monomeric to hexameric H2O/Na+ ion clusters residing in the supercages, the partition functions and energetic quantities such as the energies of formation and of successive addition as well as entropies are evaluated. Potential energies of the oligomeric clusters are estimated. The data for the zeolite water/cation clusters are compared with experimental and theoretical (ab initio) results of free (gaseous phase) water/cation clusters available in the literature.