Predicting Large CO₂ Adsorption in Aluminosilicate Zeolites for Postcombustion Carbon Dioxide Capture

Abstract

Large-scale simulations of aluminosilicate zeolites were conducted to identify structures that possess large CO 2 uptake for postcombustion carbon dioxide capture. In this study, we discovered that the aluminosilicate zeolite structures with the highest CO 2 uptake values have an idealized silica lattice with a large free volume and a framework topology that maximizes the regions with nearest-neighbor framework atom distances from 3 to 4.5 Å. These predictors extend well to different Si:Al ratios and for both Na + and Ca 2+ cations, demonstrating their universal applicability in identifying the best-performing aluminosilicate zeolite structures.P orous materials such as zeolites and metal−organic frameworks are seen as promising candidates for carbon capture from postcombustion gas streams because of their selective CO 2 adsorption and large capacities. 1−5 Specifically in aluminosilicate zeolites, the addition of cations changes the adsorption properties of the zeolite structure in two important ways: (1) it creates stronger adsorption sites as a result of the additional interactions between the CO 2 molecules and the cations, and (2) it decreases the saturation uptake of CO 2 because of the reduction in the free volume. In the adsorption isotherm, the modification is reflected as an increase in the CO 2 uptake at low pressures and a decrease in the uptake at high pressures. For the purpose of postcombustion CO 2 /N 2 separation, these property changes can present an attractive tradeoff.There have been many experiments conducted on aluminosilicate zeolites that have shown significant uptake enhancement relative to the pure-silica zeolites. 6−9 Moreover, there has been simulation work that can reproduce the experimental data on a few selected International Zeolite Association (IZA) zeolites with different Si:Al ratios. 3,10,11 However, to the best of our knowledge, there is not a theory available that can tell us which properties of the pure-silica zeolites can lead to the best aluminosilicate zeolite structures. When referring to the "best" structures in this work, we use the pure-component CO 2 uptake at 0.15 bar as the quantity to be maximized, as this is of interest to the postcombustion CO 2 separations community.In our simulations, we utilized grand canonical Monte Carlo (GCMC) simulations to obtain the CO 2 uptake values at different pressures. 12−14 The aluminosilicate zeolite structures were generated by randomly replacing Si atoms with Al while adhering to Loẅenstein's rule. 15 For the Na + cations, all of the Lennard-Jones interaction parameters and atomic charges were taken from the force field of Garcı́a-Sańchez et al., 10 which has been shown to be transferrable to multiple aluminosilicate zeolites with varying Si:Al ratios. For Ca 2+ , the same LennardJones force field of Garcı́a-Sańchez et al. was used, but the charge was doubled. The positions of the cations were generated in two different ways: (1) fixed cations, where the cations were inserted one by one into the unit cell at the gl...

New materials for methane capture from dilute and medium-concentration sources

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“…This method has been successfully used for these purposes for decades and it is proven to provide trustworthy results [15]. However, as it is the case in all types of simulations, the quality of the results can only be as good as the model used.…”

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