Molecular simulation of adsorption and diffusion in pillared clays

Using grand-canonical-ensemble Monte Carlo and molecular dynamics simulations, adsorption equilibria and diffusion of finite-size molecules in model pillared clays are studied. Our simulations show that, at moderate and high porosities, clustering of the pillars and their spatial distribution do not have a significant effect on the adsorption isotherms. However, the dependence of the adsorption isotherms on the porosity is different at low and high pressures. At low pressures, the equilibrium loading increases as the porosity decreases, whereas at high pressures it increases with increasing porosity. The difference is due to the competition between the adsorption surface and the accessible volume of the system, which are the two most important factors that control the adsorption behavior of the system. At low enough temperatures and at any porosity, a first-order phase transition (condensation) occurs. The self-diffusivity D is found to be a monotonically increasing function of the temperature. Unlike adsorption isotherms, however, clustering of the pillars does have a strong effect on the diffusivity of the molecules. Moreover, over the entire loading range studied, D increases monotonically as the porosity increases. Copyright (C) 1996 Elsevier Science Ltd.