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3.1.4 Exchangeable Cations The type of exchangeable cations in a zeolite significantly impacts its selectivity for nitrogen. Papai et al.19 studied how different exchangeable cations affect zeolite selectivity. Their conclusion was that substituting Li+ for Na+ increases the binding energy for nitrogen molecules more than it does for oxygen molecules, which increases selectivity for nitrogen. Figure 3.2 compares the potential energy curve of Li+N2 with that of Na+N2. The potential energy difference is explained by the lack of core electrons for Li+. The core electrons of Na+ provide a lower charge density than Li+, which also has a smaller ionic radius. The higher charge density of Li+ enhances interaction with nitrogen molecules. A smaller ionic radius also enhances the attraction due to short range interaction forces. The smaller cation increases potential well depth and decreases separation between the ion and nitrogen molecule (i.e. collision diameter). Figure 3.2 compares the location of the minimum potential for Li+N2 to the minimum potential of Na+N2. Use of Li+ ions in zeolites was studied as early as 1964 by Mckee.20 Chao further explored the extent of Li+ cation exchange necessary for effective nitrogen adsorption.21 His invention showed a greater Li+ ion exchange, preferably around 90%, increased zeolite capacity and selectivity for nitrogen. He further noted that a Si/Al ratio near 1.0 significantly increased adsorption capacity and selectivity. Ion exchange using Li+ does have some limitations. Highly Li+ exchanged zeolites are expensive to produce since ion exchange with Li+ is less thermodynamically favorable than with Na+ or Ca2+. However, the cost of producing highly exchanged Li-X 35PDF Image | LIMITS OF SMALL SCALE PRESSURE SWING ADSORPTION
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