Predictions of Stepped Isotherms in Breathing Adsorbents by the Rigid Adsorbent Lattice Fluid

Adsorbents that undergo structural changes in the presence of adsorbate molecules are an interesting new class of materials, which could offer enhanced selectivity, purity and recovery in separation technology. To date however, their application in such technology is hampered by the lack of a simple, consistent thermodynamic framework, which can effectively describe and predict their
adsorption behaviour under a range of conditions. This becomes especially true for their behaviour in multicomponent adsorbate mixtures, for which experimental data is limited and cumbersome to obtain. Here we present how the relatively simple Rigid Adsorbent Lattice Fluid model successfully and accurately predicts stepped isotherms in the breathing MOF, MIL-53 (Al) in the presence of CO2 and CH4. Breathing transitions are predicted solely on the basis of the different densities of the material’s two structural configurations and their associated Gibbs energies. Hysteresis effects can easily be included by considering the structures’ osmotic stress, which can be calculated readily from the lattice fluid expressions. The model can be parameterised with a minimum of experimental or simulated data, subsequently becoming predictive, and since the model has its origins in statistical mechanics, no prior assumptions, such as Langmuir type behaviour are required, presenting a major advantage over existing (semi)-empirical models. The approach shown in this study should
furthermore be generic and should equally well apply to other flexible adsorbents.