Calorimetric study of adsorption of alkanes in high-silica zeolites

Isosteric heats and adsorption isotherms were measured for combinations of three alkanes (methane, ethane, propane) on a series of six high-silica zeolites (TON, MTW, UTD-1, MFI, FER, FAU). Three of these zeolites (TON, MTW, UTD-1) have one-dimensional channels, two (MFI, FER) have two-dimensional intersecting channels, and FAU has spherical cavities. Since the adsorbate molecules are nonpolar and the zeolites possess a high Si/Al ratio, electrostatic energies are small in comparison to van der Waals energies. Heats of adsorption of methane at the limit of zero coverage are 27.2, 20.9, and 14.2 kJ/mol in TON, MTW, and UTD-1, respectively. This homologous series of zeolites has one-dimensional channels composed of 10-, 12-, and 14-membered rings of average effective diameter 5.0, 5.9, and 8.8 Angstrom, respectively. Short-range gas-solid interactions between spherical methane molecules and the oxygen atoms composing the pore walls are explained by a smoothed integration of the Lennard-Jones 12-6 potential for cylindrical pores.