Fitting of Unary Isotherms in MOFs and Zeolites: Are Saturation Capacities and Freundlich Exponents Temperature-Dependent?

Microporous crystalline adsorbents such as zeolites, and metal-organic frameworks (MOFs) have potential use in a wide variety of separations applications. The evaluation of the separation performance requires the measurement of data on unary isotherms of the guest molecules in the microporous host materials; such measurements are usually conducted for a range of temperatures. The experimental data on the unary isotherms may be fitted with a variety of models such as 1-site Langmuir, 1-site Langmuir-Freundlich, and Dual-Site Langmuir-Freundlich (DSLF). The fitted isotherms are then used for calculation of the adsorption selectivity, using the Ideal Adsorbed Solution Theory (IAST). The isotherm fits at different temperatures allow the determination of the isosteric heats of adsorption. The main focus of this article is on the best strategies to adopt for fitting of the unary isotherms for each guest/host combination. The DSLF fits, for example, is described by six different parameters q A,sat , b A , v A , q B,sat , b B , v B at any chosen temperature. By careful and detailed examination of the experimental data of 78 guest/host combinations at different temperatures, we conclude that the saturation capacities q A , s a t , q B , s a t and Freundlich exponents ν A , ν B are essentially temperature-independent. For any guest/host combination, the optimal fits of the sets of unary isotherm data at different temperatures are achieved by constraining the saturation capacities and Freundlich exponents to be temperature-independent. Plotting the unary isotherm data at different temperatures as function of the surface potential, Φ , results in a unique curve.