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.
Heat of adsorption is an important factor in determining the utility of a porous material for gas separation and storage applications. Although theoretically the heat of adsorption can depend on temperature, it is common practice to assume that this dependence is so weak that it can be ignored. In this paper, we challenge this common wisdom. We simulated the adsorption isotherms and heats of adsorption of small molecules (CO 2 , CH 4 , and N 2 ) in reference siliceous (LTA, CHA, MFI) and cation-exchanged (LTA-4A, Na-LTA Si/Al = 2,5) zeolites and found very significant temperature dependence of the isosteric heat of adsorption for CO 2 at low loadings for some systems. In cation-exchanged LTA zeolites, we found more than a 15 kJ/mol decrease over a 300 K range (∼30% variation). We also found remarkable temperature dependence for CO 2 in some siliceous zeolites with eight-membered-ring windows (e.g., ITQ-29). A weak temperature dependence was observed for CO 2 on silica MFI and for CH 4 and N 2 adsorption in all materials. Concurrent adsorption microcalorimetry measurements on cationic 4A and siliceous ITQ-29 (LTA) zeolites fully support the theoretical predictions. Our results demonstrate how the temperature dependence of the isosteric heat is related on the microscopic level to the redistribution of adsorption sites with changes in temperature. A wider implication of our findings is that many porous materials exhibit distinct populations of adsorption sites that can lead to significant temperature dependence of the isosteric heats of adsorption. Therefore, care should be exercised when reporting isosteric heats of adsorption on such materials. For some systems, the significant temperature dependence of the isosteric heats of adsorption may need to be accounted for in process design.
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.
Manipulation of materials exhibiting stepped-shaped isotherms using simple and scalable methods is key to realizing their utility in advanced separations schemes. Through the judicious synthetic tuning of the zeolitic imidazolate...
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