Unusual Hysteresis Loop in the Adsorption−Desorption of Water in NaY Zeolite at Very Low Pressure

We report the observation of an unusual hysteresis loop in the water adsorption−desorption isotherm in NaY in the very low pressure range. Vacuum equilibrium thermodesorption and n -pentane adsorption experiments combined with grand canonical Monte Carlo simulations show that this hysteresis phenomenon is consecutive to the trapping of water in sodalite cages. We propose that the adsorption−desorption of water in NaY occurs as follows. (i) Initial adsorption of a few water molecules on the most accessible cations located on sites II in the supercages. The solvation of these cations makes the sodalite cages accessible to water. (ii) Complete filling of supercages and sodalite cages associated with a migration of nonframework cations from D6R prisms to sodalite cages. (iii) Desorption of water from supercages with water still trapped in the sodalite cages. (iiii) Thermally activated desorption of water from sodalite cages. Thus, according to the hydration procedure used, the residual water can be located either in supercages or in sodalite cages. This provides a way to prepare zeolite samples with the same amount of residual water but with different nonframework cation and water molecule distributions. These two samples would presumably behave differently with regard to selective adsorption of hydrocarbon mixtures, a feature that could be of interest for separation technologies.