CALORIMETRIC STUDY OF ADSORPTION OF ALKANES IN HIGH-SLICA ZEOLITES

Isosteric heats and adsorption isotherms were measured for combination s of three alkanes (methane, ethane, propane) on a series of six high- silica zeolites (TON, MTW, UTD-1, MFI, FER, FAU). Three of these zeoli tes (TON, MTW, UTD-1) have one-dimensional channels, two (MFI, FER) ha ve two-dimensional intersecting channels, and FAU has spherical caviti es. Since the adsorbate molecules are nonpolar and the zeolites posses s a high Si/Al ratio, electrostatic energies are small in comparison t o 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-dimensiona l channels composed of 10-, 12-, and 14-membered rings of average effe ctive diameter 5.0, 5.9, and 8.8 Angstrom, respectively. Short-range g as-solid interactions between spherical methane molecules and the oxyg en atoms composing the pore walls are explained by a smoothed integrat ion of the Lennard-Jones 12-6 potential for cylindrical pores.