Effect of Framework Flexibility on C8 Aromatic Adsorption at High Loadings in Metal-Organic Frameworks

Many simulations of adsorption in nanoporous materials such as metal-organic frameworks (MOFs) treat the adsorbing materials as rigid. We show that the use of this approximation to describe the multicomponent adsorption of C-8 aromatics in MOFs under industrial conditions gives results that differ dramatically from descriptions that include local framework flexibility. To address this issue, we develop an efficient method for capturing the effect of framework flexibility on adsorption in nanoporous materials. Our flexible snapshot method uses GCMC simulations to model adsorption in snapshots collected using fully flexible MD simulations and can be applied to any framework-adsorbate system for which reliable force fields are available. Our method gives considerably better agreement with experiments for multicomponent C-8 aromatic selectivities in multiple MOFs than more traditional calculations using a single rigid framework. The rotation of organic linkers in the MOFs has a strong influence on selectivities in these systems. Because many MOFs contain this structural feature, we expect that using simulations that incorporate this kind of internal flexibility will be important in obtaining accurate adsorption predictions in a range of circumstances. This is especially true for many industrially relevant separations in MOFs, in particular, those that exploit high pore loadings of adsorbed species.