The distribution of Al in cationic zeolites is important in controlling the performance of these materials, but it is challenging to measure this distribution experimentally. We examine the effect of Al ordering in a range of industrially relevant zeolites on CO 2 adsorption using molecular simulations based on high quality force fields. A Reverse Monte Carlo method is used to generate Al distributions with specified degrees of order or randomness. Results for a range of Na + -exchanged zeolites show that CO 2 adsorption isotherms can be used to distinguish between different degrees of Al ordering. Good agreement with experiments is observed in the limited number of cases where information is available independently about Al ordering. The differences that exist among adsorption isotherms as a function of Al ordering can be understood in terms of the existence of single-and dual-cation sites as quantified by Na−Na radial distribution functions. Our results show that measurements of CO 2 adsorption combined with molecular simulations can be used to determine Al ordering in zeolites without relying on the more challenging experimental techniques previously available for this task.

Institute of Standards (NIST) reported CO 2 adsorption isotherms measured independently by 11 groups on reference material RM 8852, an ammonium ZSM-5 zeolite. Good reproducibility and high reliability of this experimental data provide a strong test for the ability of atomically detailed models to predict adsorption of CO 2 in zeolites. We developed force fields for CO 2 in ammonium zeolites based on first-principles calculations and also independently performed experiments with RM 8852 by microcalorimetry. At low pressures good agreement was obtained between predictions and experiments. At high pressures, however, deviations were observed. We show that the charge-balancing cations in the experimental material are the predominant source of the discrepancy between simulation and experiment at high pressures; the experimental sample treatment causes deammoniation. In addition, accounting for a small amount of noncrystalline mesoporosity in the zeolite brings predictions into much better agreement with experiments.

We present a transferable force field for hydrocarbons (linear and branched olefins and paraffins) and small adsorbates (CO 2 , O 2 , N 2 , and H 2 O) in pure silica zeolites. The fitting procedure is based on adsorbate−adsorbent interaction energies obtained from periodic density functional theory calculations and corrected using coupled-cluster 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 topologies, showing good agreement between theoretical predictions and a range of experimental measurements of adsorption and diffusion.

With rising CO 2 levels, it is important to develop new methods to capture CO 2 directly from air. Currently, most direct air capture (DAC) adsorbents, such as amines, rely on chemisorption. However, finding materials with weaker, reversible adsorption could improve the regenerability of these adsorbents. The low concentration of CO 2 relative to other components of air and the presence of humidity limit the effectiveness of physisorbent materials for DAC. In this work, we screened the 2014 computation-ready, experimental metal−organic framework (CoRE MOF) database along with silica zeolites, aluminophosphate (AlPO) zeolites, and gallophosphate (GaPO) zeolites for DAC under humid conditions based on heat of adsorption criteria. After the initial assessment, AlPO and GaPO zeolites appeared to be promising adsorbents for DAC. To assess the accuracy of these initial predictions, we compared adsorbate−adsorbent interaction energies predicted by the force field used in screening (UFF) and dispersion-corrected density functional theory. To improve the accuracy of these predictions, first-principles force fields were fit for AlPO and GaPO zeolites. More accurate CO 2 and H 2 O heats of adsorption and adsorption isotherms were computed and showed that AlPO and GaPO zeolites are not suitable for DAC applications.

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.