Structural Effect on Molecular Simulations of Tight-pore Systems

Adsorption of benzene and p-xylene on silicalite at infinite dilution at 20 "C has been investigated via direct integration and Monte Carlo techniques. A small uncertainty in crystal structure causes a very large difference in the values of the Henry constants, while the internal energy of adsorption is less sensitive to this uncertainty. The sensitivity of the Henry constant to structural detail is magnified in these systems due to the tight fit of these molecules in the silicalite pore structure. This high sensitivity casts a shadow of doubt on the applicability of the common practice of determining potential parameters from data at infinite dilution while assuming that the structure is rigid and completely defined. The calculations reveal that the preferential adsorption sites for both benzene and p-xylene are the channel intersections. Benzene is somewhat mobile between straight channels and intersections while p-xylene is almost completely localized at the intersections at 20 "C. In tight-pore systems the most preferred adsorption site is not necessarily the location with the highest adsorption potential, contrary to adsorption on flat surfaces and in large pores.