Adsorption equilibria of methane and tetrafluoromethane and their binary mixtures on silicalite

Binary mixture adsorption isotherms of CH4 and CF4 on silicalite have been measured for different constant gas compositions at 298 K in the pressure range up to 1.7 MPa using a high-pressure microbalance. Additionally, the single-component adsorption isotherms were determined at 273, 298 and 323 K in the same pressure range. The gravimetric method proposed by Van Ness has been applied to obtain the compositions of the adsorbed phase as a function of gas-phase composition and pressure. The experimental mixture adsorption equilibria are compared with predictions of the ideal adsorbed solution (IAS) theory and the multisite Langmuir (MSL) model based on the single-component adsorption isotherms only. Despite the fact that the molecules of CH4 and CF4 differ greatly in size, ideal behaviour of their adsorbate mixtures on silicalite is found over a large pressure range. This result is due to the high surface homogeneity as well as the channel-like pore geometry of silicalite, where interactions between the adsorbate molecules can only occur to a small extent. Thus, both the IAS theory and the MSL model achieve a good description of the experimental data.