Adsorption of water vapor on X and Y zeolites exchanged with barium

The adsorption of water vapor on BaY and BaX zeolites is studied by thermogravimetry and calorimetry under isothermal or isobaric conditions. The faujasite zeolite exhibits a stronger adsorption affinity for water molecules with barium than with sodium or potassium as exchangeable cations. The adsorption-desorption isotherm at 150 degreesC shows a hysteresis loop attributed to the migration of compensating cations during their interaction with water molecules. Calorimetric measurements and the Dubinin Radushkevich's model both show that the adsorption of water occurs via a three-step mechanism: adsorption on compensating cations, formation of a monolayer on the walls and multilayer adsorption in the cavities. Water probes the complete microporosity of the zeolite. After several isobaric adsorption-desorption cycles, the structure of BaX undergoes severe damages (complete loss of the microporosity and amorphization of the solid) whereas that of BaY remains intact. (C) 2001 Elsevier Science B.V. All rights reserved.