Atomistic Simulation of Water Intrusion¿Extrusion in ITQ-4 (IFR) and ZSM-22 (TON). The role of Silanol Defects

Abstract

[EN] Water in pure silica zeolites (zeosils) may behave as a "bumper" by absorbing mechanical energy of the intruded water, as a "spring" by restoring after extrusion of the energy spent in intrusion, or as "shock absorber" by dissipating the energy. The understanding of how the structure and topology of the zeosils are responsible of such behavior has not yet been fully clarified. Molecular dynamics and molecular mechanics simulations of IFR- and TON-type zeosils have been performed in an attempt to elucidate the energetics of these materials after water intrusion-extrusion. We aim our simulations to capture the experimentally observed "bumper" and "spring" water-zeosil behavior of IFR and TON, respectively The excess energy with respect to dry zeosil was calculated, and this relates to the energetic response of the zeosil after water intrusion. We found that the excess energy of water-TON is larger than the energy of bulk water at any loading. The Small opening of the TON channel prevents the formation of energetically stable bulky water clusters. The Water content-was shown to be stabilized On a-certain loading range in Water-IFR was shown that any silanol defects' in IFR framework channels stabilize systems. Defect positions(silanol groups), which make the water-IFR system energetically stable, are found Silanol groups increase the hydrophilicity of IFR-type zeosil, initially hydrophobic. There are two factors explaining the bumper behavior (under high pressure, Water penetrates into the zeosil Channels and remains there even. after the pressure is released) of water-IFR systems: channel size and hydrolisis leading to framework breaking under large hydrostatic pressure. Silanol groups in channels are centers of water clusterization. The chemical stability of TON framework and its small channel size explain its spring behavior.