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

Abstract

The major objective of this communication is to evaluate procedures for estimation of the isosteric heat of adsorption, Qst, in microporous materials such as zeolites, metal organic frameworks (MOFs), and zeolitic imidazolate frameworks (ZIFs). For this purpose we have carefully analyzed published experimental data on adsorption isotherms at different temperatures, T, for a variety of guest molecules (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 analysis shows that when the binding energy with the structural framework is dominated by molecule–molecule interaction forces, Qst≈ΔHvap, the molar enthalpy of vaporization. In such cases, the molar loading qi at any temperature T and pressure pi, is a unique function of (pi/Pisat) where Pisat is the saturated vapor pressure at T.For most guest/host combinations, however, the Qst values are significantly higher than ΔHvap because of strong interactions of guest molecules with the material framework. For several guest/host combinations, a reasonable estimation of Qst values is obtained by addition of the Tóth potential to ΔHvap.