Zeolites are crystalline inorganic microporous materials, with pores that are of a size comparable to that of many small molecules. Therefore, they can selectively discriminate between molecules with relatively small differences in size and shape. This feature makes zeolites ideal candidates for membrane separations, catalysis, and sequestration. One important environmental application for zeolites is the separation of CO2 from gaseous mixtures containing N2 or CH4.1-3 Experimental studies have been performed to analyze the adsorption and diffusion of these mixtures in zeolite membranes, a well-established example being ZSM-5.4, 5 This zeolite contains a two-dimensional network of intersecting channels running in the x and y-directions, and having a size of ~5.5 Å. ZSM-5 preferentially adsorbs CO2 over N2 due to the strong quadrupole moment of CO2.1 The CO2-favored selectivity of ZSM-5 depends significantly on the Si/Al ratio that defines the zeolite composition. The presence of Al substitutions into the framework requires the presence of charge-balancing cations, such as Na+ and Rb+. The cation and CO2 strongly interact, which causes increased adsorption and decreased diffusion.6 We have performed molecular simulations of CO2 and N2 in ZSM-5 to study the impact caused by the presence of Na+ on diffusion and adsorption. The roles of temperature and Si/Al ratio are considered.
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5 – Zhu, W. et al. Ind. Eng. Chem. Res. 2006, 45, 767-776.
6 – Wirawan, S. and Creaser, D. Micro. Meso. Mat. 2006, 91, 196-205.