INTERMOLECULAR POTENTIAL FUNCTION FOR THE PHYSICAL ADSORPTION OF RARE-GASES IN SILICALITE

We present a simple method for the derivation of two-body and three-body dispersion coefficients for incrystal atoms from the knowledge of their dipole polarizability and effective number of electrons. This method is checked by comparison with results from quantum mechanical calculations and used to derive two- and three-body coefficients for the dispersion interaction of argon, krypton, and xenon adsorbed in the siliceous zeolite silicalite-l. Repulsive parameters for the argon/silicalite system are obtained by fitting experimental data over a wide range of temperature and the full scale potential constructed in this way (referred to as PN1) is shown to perform well in predicting other argon data. The parameters for the repulsive energy for the krypton/silicalite and xenon/silicalite systems obtained using combination rules and the PN1 potential for these adsorbates (without any parameter adjustment) are also found to be successful in predicting low coverage properties. We compare the performance of the PN1 function with the Kiselev adsorption potential, widely used in the field of modeling adsorption in zeolite cavities, and show that the latter tends to overestimate thermodynamic properties and also predicts a wider pore than the new potential. The energetics of adsorption are discussed in terms of site location and shape and the effective size of zeolite oxygen atoms.