Reference potentials for adsorption of helium, argon, methane, and krypton in high-silica zeolites

Quantitative agreement with experimental data for adsorption of argon, krypton, and methane in high-silica zeolites is achieved with a molecular model, which (1) assumes pairwise additivity of intermolecular forces; (2) ignores partially concealed silicon atoms and lumps their dispersion energy with the oxygen atoms; (3) neglects energy induced by the weak electric field inside silicalite; (4) uses the Lennard-Jones 12-6 potential for gas-gas and gas-solid interactions. The parameters of the Ar-O gas-solid potential are epsilon /k = 93.0 K and sigma = 3.335 Angstrom. The gas-solid potential parameters for krypton and methane were calculated from the Ar-O reference potential using Lorentz-Berthelot mixing rules and the principle of corresponding states. The pore volume of silicalite determined from the He-O potential is 0.175 cm(3) g(-1). The local density of gases adsorbed in the pores varies with the strength of the gas-solid potential; at 306 K and 10 bar, the maximum absolute density of methane is about one half the density of liquid methane at its normal boiling point. (C) 2001 Elsevier Science B.V. All rights reserved.