External surface adsorption on silicalite-1 zeolite studied by molecular simulation

We have studied the adsorption of ethane, propane, and their mixtures on the external surface of silicalite-1 zeolite by molecular simulation using a classical force field. The ideal adsorbed solution theory (IAST) was successfully used to describe mixture adsorption, both on the external surface and inside the zeolite. Propane is preferentially adsorbed inside the zeolite for fugacities from 103 to 108 Pa, while ethane is favored at higher fugacities. On the zeolite surface, propane is always preferentially adsorbed. We also determined the surface excess concentration for pure ethane and propane, as well as the surface excess of both components in an equimolar mixture. The surface excess concentration is negative or close to zero until the fugacity is approximately 105 Pa, when condensation of guest molecules on the external surface becomes important. The surface excess adsorption for ethane in a mixture is different than that for pure ethane, while for propane they are identical.