Effect of structural heterogeneity on adsorption of benzene, p-xylene and their vapor mixture on silicalite

The effect of structural heterogeneity on observed non-ideality of physical adsorption is studied via experiment and molecular simulation. The systems investigated are benzene, p-xylene and their mixture on silicalite.; Adsorption equilibrium of benzene/p-xylene vapor mixture on silicalite is measured at 70{dollar}spcirc{dollar}C with a specially designed cyclic volumetric apparatus at pressure levels of 2.53 kPa and 1.20 kPa. The isobaric isotherms are S-shaped, and selectivity curves at different pressures cross over; the p-xylene selectivity at 2.53 kPa is higher than that at 1.20 kPa over a certain composition range. The Heterogeneous Ideal Adsorbed Solution (HIAS) model implemented on two patches qualitatively predicts highly unusual behavior. The observed extraordinary phenomena are attributed to structural heterogeneity, a result of the tight-fit of sorbate molecules in silicalite pores. The success of HIAS attests to the paramount importance of adequate representation of heterogeneity in adsorption models.; Adsorption of benzene and p-xylene on silicalite at low occupancy at 20{dollar}spcirc{dollar}C is also investigated via direct integration and Monte Carlo techniques. A small uncertainty in crystal structure causes a large difference in the values of Henry's constants, while internal energy of adsorption is less sensitive to the uncertainty. The sensitivity of Henry's constant to structural detail is magnified in these systems due to the very-tight-fit in the pores. This high sensitivity raises questions about the validity of the common practice of determining potential parameters from the data at zero coverage 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{dollar}spcirc{dollar}C. It is shown that the most preferential adsorption site in such tight-pore systems is not necessarily the location with the highest adsorption potential. This finding is contrary to adsorption on flat surfaces and in large pores.