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 Å, 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.

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Monte Carlo simulations on the adsorption of the two model chlorocarbons, chloroform and trichloroethylene, in three faujasite type zeolites, NaX, NaY, and siliceous faujasite (Si/Al ) 1.2, 3.0, and ∞, respectively), are compared with the results of calorimetric measurements on the same systems. At low sorbate loadings, the heats of adsorption increase with increasing polarity of the zeolite host (siliceous faujasite < NaY < NaX), underlining the importance of the dipolar nature of the sorbates. For all six systems, quantitative agreement is found between the calculated and observed heats as a function of sorbate loading. Decomposition of the calculated heats into the short-range and long-range contributions shows that sorbate-sorbate interactions are important in all three hosts and lead to an increase in the adsorption heat with coverage in NaY and siliceous faujasite. The SIII′ cations play a specific role in the case of zeolite NaX, where a cancellation between the short-range and electrostatic interactions leads to a heat of adsorption that is invariant with coverage.

Microcalorimetric measurements were performed at 195 K for CO and N2 on zeolite H−MFI and on a siliceous MFI in order to characterize the interaction of these molecules with the Brønsted acid sites. On the siliceous MFI, the differential enthalpies of adsorption (isosteric heats) for CO and N2 were found to be essentially constant with coverage at a value of 16 ± 1 kJ/mol. While the isosteric heats for N2 were slightly higher on H−MFI at coverages below one per site, the additional interaction with the Brønsted sites is less than 3 kJ/mol. For CO on H−MFI, microcalorimetry provides evidence for a 1:1 adsorption complex with the acid sites in that the isosteric heats are higher on H−MFI at low coverages and approach the values for the siliceous material at higher coverages. Because the calorimetric data can be fit to a two-site, Langmuir model in which the hydrogen-bonded CO interacts with an additional 10.5 kJ/mol, the results suggest that the Brønsted acid sites in H−MFI are essentially identical in regard to how they interact with CO.