GRAND ENSEMBLE MONTE-CARLO SIMULATION OF SIMPLE MOLECULES ADSORBED IN SILICALITE-1 ZEOLITE

Grand ensemble simulations have been performed for Ar, Kr, and Xe adsorbed in silicalite-1. Two potential models were employed to model the adsorbate zeolite interactions: one originating from the work of Kiselev which has been widely used in previous work, and one developed in this work, which includes high order two-body dispersion, induced and three-body interactions, denoted PN1. Simulation data have been compared with experimental isotherms and isosteric heat curves obtained from microcalorimetry and also with neutron scattering data. Neither of the potential models used was able to predict the transition observed in experimental isotherms, although the heat curve transitions are reproduced. These transitions are associated with adsorption at the substep preceding the transition. Decomposition of simulated heats into wall and molecule components facilitates an interpretation of these curves. From an analysis of neutron data and the thermodynamic data, we conclude that the step observed in the argon and krypton isotherms must be attributed to an adsorbent transition which possibly induces a rearrangement of the adsorbed phase. Therefore, this transition is not due to a disordered fluid to crystalline solid adsorbate transformation nor to commensurate-incommensurate phase changes. We demonstrate that the distorted lattice cannot be identified with the para-form which occurs during p-xylene adsorption. We conclude with a brief description of current ideas bearing on adsorbent lattice transformation and propose a tentative mechanism. A full atomic level interpretation awaits further investigation.