MOLECULAR SIMULATION OF ADSORPTION AND DIFFUSION IN PILLARED CLAYS

Using grand-canonical-ensemble Monte Carlo and molecular dynamics simu lations, adsorption equilibria and diffusion of finite-size molecules in model pillared clays are studied. Our simulations show that, at mod erate and high porosities, clustering of the pillars and their spatial distribution do not have a significant effect on the adsorption isoth erms. However, the dependence of the adsorption isotherms on the poros ity is different at low and high pressures. At low pressures, the equi librium loading increases as the porosity decreases, whereas at high p ressures it increases with increasing porosity. The difference is due to the competition between the adsorption surface and the accessible v olume of the system, which are the two most important factors that con trol the adsorption behavior of the system. At low enough temperatures and at any porosity, a first-order phase transition (condensation) oc curs. The self-diffusivity D is found to be a monotonically increasing function of the temperature. Unlike adsorption isotherms, however, cl ustering of the pillars does have a strong effect on the diffusivity o f the molecules. Moreover, over the entire loading range studied, D in creases monotonically as the porosity increases. Copyright (C) 1996 El sevier Science Ltd.