Modelling adsorption based on an isoreticular MOF‐series of IFPs–Part I: Collection of physical properties and single component equilibria

Abstract

An isoreticular series of metal organic frameworks (MOFs) of IFPs (IFP, imidazolate frameworks Potsdam) is investigated for their morphological properties and adsorption behaviour. The materials are characterized phenomenologically with respect to their particle size and tendency of agglomerate formation, and with respect to their internal structure. For this purpose, material densities, pore size distributions, specific inner surfaces, and porosities are determined. The main part of the investigation is based on the analysis of gravimetrically determined adsorption equilibria for carbon dioxide (CO2) and methane (CH4) and their modelling. In this context, two different approaches for the consideration of the buoyancy of the sample are compared. The adjusted measurement data are globally approximated as sets of isotherms at different temperatures with two different modifications of the Langmuir model. Results show that both models are well suited for the interpolation of the experimental data in the temperature range under consideration. Comparison of the heats of adsorption derived from the isosteric method with values extracted from the model equations confirms them as physically consistent. This provides the opportunity to numerically simulate the dynamic separation of CO2/CH4‐mixtures under consideration of the heat tone based on the single component data. The IFPs can be divided phenomenologically into two categories. One is exclusively microporous (IFP‐4, ‐6, ‐7, and ‐8), while the other exhibits hierarchical structures of micropores coupled with mesopores (IFP‐1, ‐2, ‐3, and ‐5). Equilibrium data indicate that the latter are better suited for the separation of CO2/CH4‐mixtures due to their higher selectivities and capacities.

Modelling adsorption based on an isoreticular MOF‐series of IFPs—Part II: Dynamic adsorption in fixed beds

“…The morphological properties as well as the experimental determination and modelling of pure component adsorption equilibria for CO 2 and CH 4 are reported for selected imidazolate framework Potsdam (IFP) species (IFP-1 to -8) in Part I of this study. [9] These results offer the possibility to simulate the dynamic and non-isothermal separation of CO 2 /CH 4 mixtures under consideration of the heat of adsorption, which is also derived from the pure component data. [9] F I G U R E 1 Flow chart of the experimental breakthrough plant consisting of the gas dosing system (I), the adsorber unit (II), and the gas analysis (III)…”

“…[9] These results offer the possibility to simulate the dynamic and non-isothermal separation of CO 2 /CH 4 mixtures under consideration of the heat of adsorption, which is also derived from the pure component data. [9] F I G U R E 1 Flow chart of the experimental breakthrough plant consisting of the gas dosing system (I), the adsorber unit (II), and the gas analysis (III)…”

“…The most common method is the theory of the ideal adsorbed solution (IAST) developed by Myers and Prausnitz in 1965, [13] which, like the isosteric method, [9] relies on the thermodynamic equilibrium condition of the change in Gibbs free energy:…”

“…Another approach modified from IAST assumes a heterogeneous distribution of adsorption sites on the solid surface, which are energetically non-equivalent and occupied by the adsorbate independently of each other. [9] An equilibrium relationship according to Langmuir-also referred to as multi-site Langmuir (MSL) model-is postulated for each site independently and also solved individually applying the IAST for each site [17,20] :…”

“…The terms for micro-and mesopore diffusion are weighted according to their volume fractions, determined from the pore size distributions, [9,38] with the micropore fraction, φ μ , and the resulting mesopore fraction, 1 À φ μ .…”