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, Q_st, 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, Q_st, ≈ ΔH_vap, the molar enthalpy of vaporization. In such cases, the molar loading q_i at any temperature T and pressure p_i, is a unique function of (p_i/P_i^sat) where P_i^satis the saturated vapor pressure at T. For most guest/host combinations, however, the Q_st, values are significantly higher than ΔH_vap because of strong interactions of guest molecules with the material framework. For several guest/host combinations, a reasonable estimation of Q_st, values is obtained by addition of the Toth potential to ΔH_vap.