(665h) Grand Canonical Monte Carlo Simulations of CO2 and N2 Adsorption in Na-ZSM-5

Zeolites are microporous inorganic aluminosilicate crystals with a regular atomic-scale pore topology. One of the most studied and utilized zeolites is ZSM-5, which can serve as a molecular sieve to separate molecules based on distinct adsorption capacity or mobility, caused by differences in molecular size, shape, and molecule-zeolite interactions. ZSM-5 has distinct interactions with CO₂ and N₂, as the quadrupolar moments on CO₂ cause an increased electrostatic attraction, relative to the smaller quadrupolar moment of N₂. Hence ZSM-5 is a good candidate for CO₂ sequestration from flue gas mixtures. Nominally the zeolite framework has a SiO₂ composition, but Al atoms can replace the Si atoms. This substitution creates a charge deficit, requiring a cation, such as Na⁺, for the charge counter-balance. The Na⁺ cation creates an exposed charge site, which can have an increased interaction with the CO₂ and N₂ molecules, causing an increase in adsorption. Although many experimental and theoretical adsorption studies have been performed on all-silica ZSM-5, known as Silicalite, there are not as many systematic studies on ZSM-5 with different Si/Al ratios. This presentation discusses the adsorption of CO₂ and N₂ mixtures in the zeolite ZSM-5 by looking at the role of the Si/Al ratio (∞, 95 and 47) on three important quantities: the adsorption isotherm, the isosteric heat of adsorption, and the selectivity. Adsorption is studied using Grand Canonical Monte Carlo (GCMC) simulations, which give crucial insight into the adsorption behavior at the atomistic scale, creating reasonable quantitative predictions and useful interpretation of experimental adsorption results. The adsorption isotherms for the pure components and equimolar binary mixtures of CO₂ and N₂ are carried out at T = 280 ? 330 K and pressures up to 20 bar. The isosteric heat of adsorption of the pure components and the selectivity of the binary mixture are evaluated from the adsorption isotherms. The GCMC simulations predict a strong preference of CO₂ over N₂, even in the Silicalite form. The inclusion of Al and Na⁺ counterions increases the CO₂ and N₂ loadings at lower pressures, compared to the loadings in Silicalite, but these increases become less significant at higher pressures. This is because the CO₂ molecules preferentially surround and saturate the Na⁺ cation, minimizing its impact on additional CO₂ adsorption. The same effects occur for N₂, but they are less dramatic. Another issue is that the Na⁺ cations cause volume exclusion, taking up potential free space for guest molecule adsorption. At low loading the isosteric heat of CO₂ is Q_st ≈ 22 kJ/mol, for Silicalite, while it is Q_st ≈ 55 kJ/mol when Na⁺ is present, for both cases of Si/Al = 95 and 47. At higher loading, approaching saturation, Q_st converges to the same value of Q_st ≈ 31 kJ/mol, independent of the presence of Na⁺. For N₂ adsorption on Silicalite, Q_st ≈ 15 kJ/mol. The much larger value for CO₂ indicates its preferential adsorption over N₂. In the presence of Na⁺, due to Na⁺ ? N₂ interactions, Q_st ≈ 30 ? 45 kJ/mol at low loading, depending on the Si/Al ratio, but it also approaches the Silicalite result at higher loading. In all cases of Si/Al considered, the adsorption selectivity of CO₂ over N₂ was somewhat larger or very similar to that in Silicalite, in the range of α = 20 ? 35. The presence of Al and Na⁺ counterions increases the adsorption selectivity, but not in any dramatic manner, at least for the zeolite ZSM-5.