Evaluation of procedures for estimation of the isosteric heat of adsorption in microporous materials

The major objective of this communication is to evaluate procedures for estn. of the isosteric heat of adsorption, Qst, in microporous materials such as zeolites, metal org. frameworks (MOFs)​, and zeolitic imidazolate frameworks (ZIFs)​. For this purpose we have carefully analyzed published exptl. data on adsorption isotherms at different temps., T, for a variety of guest mols. (water, methanol, ethanol, dimethylether, cyclohexane, benzene, toluene, xylene, ethylbenzene, ethyne, propene, propane, iso-​butane, 1-​butene, n-​hexane) in several zeolites, MOFs, and ZIFs. Our anal. shows that when the binding energy with the structural framework is dominated by mol.-​mol. interaction forces, Qst≈ΔHvap, the molar enthalpy of vaporization. In such cases, the molar loading qi at any temp. T and pressure pi, is a unique function of (pi/Pisat) where Pisat is the satd. vapor pressure at T. For most guest​/host combinations, however, the Qst values are significantly higher than ΔHvap because of strong interactions of guest mols. with the material framework. For several guest​/host combinations, a reasonable estn. of Qst values is obtained by addn. of the To´th potential to ΔHvap.