Mean field lattice model for adsorption isotherms in zeolite NaA

Using a lattice model for adsorption in microporous materials, pure compone nt adsorption isotherms are obtained within a mean field approximation for methane at 300 K and xenon at 300 and 360 K in zeolite NaA. It is argued th at the increased repulsive adsorbate-adsorbate interactions at high coverag es must play an important role in determining the adsorption behavior. Ther efore, this feature is incorporated through a "coverage-dependent interacti on'' model, which introduces a free, adjustable parameter. Another importan t feature, the site volume reduction, has been treated in two ways: a van d er Waal model and a 1D hard-rod theory [van Tassel et al., AIChE J. 40, 925 (1994)]; we have also generalized the latter to include all possible adsor bate overlap scenarios. In particular, the 1D hard-rod model, with our cove rage-dependent interaction model, is shown to be in best quantitative agree ment with the previous grand canonical Monte Carlo isotherms. The expressio ns for the isosteric heats of adsorption indicate that attractive and repul sive adsorbate-adsorbate interactions increase and decrease the heats of ad sorption, respectively. It is concluded that within the mean field approxim ation, our simple model for repulsive interactions and the 1D hard-rod mode l for site volume reduction are able to capture most of the important featu res of adsorption in confined regions. (C) 1999 American Institute of Physi cs. [S0021-9606(99)70515-5].