(156i) CO2 and N2 Diffusion in Zsm-5 Zeolites with Varying Si/ Al Ratios

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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