Quantifying Impact of Intrinsic Flexibility on Molecular Adsorption in Zeolites

The majority of molecular simulations of adsorption in crystalline nanoporous materials such as zeolites and metal–organic frameworks (MOFs) are performed using a rigid framework. This assumption is made for convenience, as simulations with flexible frameworks require dedicated force fields and more computational power. Several recent studies of molecular adsorption in metal–organic frameworks have hinted that computed adsorption properties may, in many cases, be quite sensitive to the inclusion of framework flexibility associated with thermal vibration of framework atoms. It is unclear whether this conclusion is also appropriate for molecular adsorption in zeolites. To investigate this issue, we have used molecular simulation methods to quantify the impact of intrinsic framework flexibility on molecular adsorption in zeolites without volume change (ΔV = 0) and ignoring adsorbate-induced deformations. We investigated the adsorption properties (loading, Henry’s constant, and selectivity) of four molecules (CO2, CH4, N2, and butane) and two binary mixtures (CO2/N2 and butane/CH4) in different pure silica zeolite frameworks. We also performed a limited set of simulations for a cationic zeolite with Si:Al = 1. We find that in zeolites, the influence of framework flexibility on adsorption is small, particularly when compared with MOFs.