We present a transferable force field for small adsorbates (CH 4 , CO 2 , O 2 , and N 2 ) in alkali metal (Li, Na, K, Rb, and Cs) exchanged zeolites. The fitting procedure is based on adsorbate−adsorbent interaction energies obtained from periodic density functional theory calculations and corrected using coupledcluster methods applied to small clusters. The fitting approach aims at accurate prediction of both adsorption and diffusion properties by using sets of configurations that sample adsorption sites and intracrystalline hopping transition states. The quality of the force field is assessed for a wide range of adsorbates in zeolites with different chemical composition and topologies, showing good agreement between theoretical predictions and experimental measurements of adsorption and diffusion.

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.