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In silico screening of CO2 adsorbents is a very powerful method for preselecting the most promising porous solids for experimental studies. Because of increased computational power, it is now possible to investigate a large number of adsorbents in a fairly short time. However, it remains difficult to rationalize structure-performance correlations because the complexity of such porous materials cannot be reduced to a few simple descriptors. In this paper, we present a different approach that was applied to the design of an optimal adsorbent for CO2 separation from gas mixtures. A CO2-CH4 mixture was used as the feed. We constructed a simplified model of the porous material by adopting a spherical pore geometry. From the dispersion-repulsion point of view, the spherical pore was modeled by homogeneously distributed Lennard-Jones sites. Besides, a charge distribution was introduced to mimic the electrostatic behavior. The selected charge distribution is constituted of negative charges homogeneously distributed over the surface of the pore. The total negative charge is counterbalanced by eight positive discrete charges placed on the corners of the cube inscribed in the sphere. This model can be characterized by two main descriptors: the pore size and the cation charge. For this model, the adsorbate-adsorbent interaction potential due to electrostatic and dispersion-repulsion interactions was determined. Then, the Henry constants of CO2 and CH4 are calculated from statistical thermodynamics and substituted in the Ruthven statistical model, which allows calculation of the binary adsorption isotherms. Finally, the adsorbed quantities are used to estimate the performance indexes of the pressure swing adsorption (PSA) process, such as the working capacity and separation factor. We show how these two parameters depend on the pore size and cation charge. For a medium-pressure PSA process, the optimal adsorbent has a large pore volume and is highly charged, which provides high working capacity and selectivity. The high cationic charge allows one to efficiently fill the large pore volume and assures high CO2 selectivity. [ABSTRACT FROM AUTHOR]

Thechoice of an appropriate adsorbent for CO2separationby pressure-swing adsorption remains a field of intense research.In this work, several FAU and LTA zeolites with different Na contents(Si/Al ratios) are studied for the separation of CO2frommixtures of CO2, CO, and CH4by means of breakthroughexperiments. The breakthrough experiments were carried out between1 and 5 bar at 303 K using two feed mixtures: 50/50 (v/v) CO2/CH4and 75/15/15 (v/v/v) CO2/CH4/CO. The most polar zeolites, i.e., those with high Na content, exhibitthe highest adsorption capacity and selectivity for CO2, but their regeneration is difficult; hence, their working capacityis low. The opposite is true for the least polar zeolites, i.e., thosewith low Na content. In order to quantify the trade-off between theselectivity and working capacity, the Ruthven statistical model (RSM)was used. It satisfactorily reproduced the experimental trends. We,therefore, used the RSM to identify the properties of the adsorbentthat lead to an optimal compromise between the working capacity andseparation factor. The critical parameter is the concentration ofextraframework cations, which, in turn, depends on the framework chargeof the zeolites FAU and LTA. The optimal trade-off zone is definedin terms of the Henry constant of CO2(KCO2). It is found that this zone is placedbetween KCO2= 5 × 10–3and 50 × 10–3molecules·bar–1·Å–3. This interval correspondsto a heat of adsorption of CO2at zero coverage between27 and 32 kJ·mol–1. In our study, this optimalrange of Henry constants was achieved for the zeolites Na-USY, SAPO-37,LTA (Si/Al = 5), and EMC-1. [ABSTRACT FROM AUTHOR]