Ab initio molecular dynamics simulation of hydration and ion-exchange processes in low Al-zeolites

First-principles molecular dynamics room-temperature simulations combined with full relaxation of atomic positions of selected structures are performed on gmelinite, a zeolite composed of hexagonal prisms like technologically important faujasites and mordenite. In low-aluminum gmelinite tone Al per cell) the location and the dynamics of the counter-ion are investigated as a function of the degree of hydration. The time evolution of the ion-exchange is simulated for the reaction Na-zeo + NH4Cl reversible arrow NH4-zeo + NaCl. The simulated IR bands of both the Na-zeolite and the ion-exchanged zeolite are consistent with the literature data. The calculated energetics shows that protonated structures are disfavored by similar to 200 kJ mol(-1) compared with Na zeolites and the NH4+ form is by -8.5 kJ mol(-1) more stable than the Na form. The adsorption energies calculated for the products of the ion-exchange reaction are higher than the energy of hydration of the Na zeolite in good agreement with experimental data. (C) 2001 Elsevier Science B.V. All rights reserved.