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

Adsorption isotherms and differential heats of adsorption of ammonia in the zeolites NaY and NaY(NaBr) were measured at 303 K. The latter zeolite which has most beta-cages blocked by NaBr, was used in order to investigate the penetration of NH3 into these small cages of NaY. The results obtained verify that such penetration does not occur. The measured adsorption isotherms and differential heats of adsorption of ammonia in the supercages of both zeolites can almost quantitatively be reproduced on the basis of the statistical mechanical model of independent subsystems which are the Na+ ions at six-ring positions accommodating up to four ammonia molecules each. However, in contrast to our previous study with water in these zeolites (Phys. Chem. Chem. Phys., 2002, 4, 4172) high-energy spurious sites have to be assumed to exist. The analysis of the body of experimental data yields the partition functions and energetic quantities such as energies of formation and entropies of the oligomeric clusters. These data are compared with results of recent density functional ab initio calculations of free ammonia/sodium cation clusters.