Evaluation of Ideal Adsorbed Solution Theory as a Tool for the Design of Metal−Organic Framework Materials

As a class of porous materials, metal−organic frameworks (MOFs) show promise for the adsorption-based
separation of mixtures of gases. The design of any process involving selective adsorption requires knowledge of mixture
adsorption isotherms. Ideal adsorbed solution theory (IAST) predicts mixture adsorption equilibria using only single-component
data, thereby minimizing the need for experimental adsorption data. In this work we perform a systematic study of the
applicability of IAST to MOFs by using grand canonical Monte Carlo (GCMC) simulations to investigate the suitability of IAST
for the prediction of the adsorption of mixtures of molecules of differing sizes, asphericities, and polarities in a range of
structurally different MOFs. We show that IAST is generally accurate for MOFs. Where we find IAST is less accurate, deviations
result from both mixture effects, in the form of nonidealities in the adsorbed phase, and characteristics of the adsorbent
structures. In terms of the MOF structure, departures from IAST are a consequence of heterogeneities both on the scale of the
unit cell and on shorter length scales, whereby competition for adsorption sites has a strong influence.