Water–Hydrophobic Zeolite Systems

Water intrusion–extrusion in hydrophobic microporous AFI, IFR, MTW and TON pure silica zeolites (zeosils) has been investigated through molecular dynamics (MD) simulations. It was found that intruded water volumes correlate with the free volume of the zeosil unit cells. Calculated adsorption isotherms allowed us to estimate the amounts of water intruded, and deviations from experiments (lower experimental with respect to calculated intrusion pressures) have been explained in terms of connectivity defects in the synthesized materials. Water phase transitions in defectless zeosils occur in a narrow range at high pressure. On the basis of a simple model, we derived a thermodynamic equation that allows one to estimate the intrusion pressure with few parameters, which are easy to obtain, such as fractional free volume of zeosil and the intrusion pressure of a reference system. The structural properties of water clusters inside the zeosil micropores have been interpreted from the analysis of the MD simulations. Compact “bulk-like” clusters form in large channels such as those in AFI and IFR zeosils. The smaller channels of MTW and TON promote the formation of chain-like clusters, which, interestingly, are commensurate with the zeolite channel topology due to a coincidence between the distances of the crystallographic parameter, along the channel, and a maximum in the O–O radial distribution function of bulk water.