Effects of Cation Sites on Adsorption

The effects of cation sites can be best illustrated by the important system of N2/O2 on type X zeolites. NaX (or 13X) has been used commercially for air separation since the 1970s. Li-LSX is the best sorbent that is commercially available today (Chao, 1989). Mixed-cation AgLi-LSX (with 1 -3% Ag cations) has been shown to be even better than Li-LSX for air separation (Yang and Hutson, 1998 Hutson et ah, 1999 Hutson et al., 2000). 

Although Ag has a larger ionic radius (1.26 A) than Li, a weak rr-complexa-tion bond can be formed between Ag+ (in AgZ) and N2 (Chen and Yang, 1996). 

This w-complexation bond, although weak, can significantly enhance the adsorption for N2 (Yang and Hutson, 1998 Hutson et al., 1999 Hutson et al 2000). 

As discussed in 7.4.1, the sites associated with the 6-oxygen rings are not the same. Some are extended into the cavity, while others are recessed into the sodalite cage or are nearly in the same plane of the 6-ring (e.g., Firor and Seff, 1979 McCusker and Self, 1981). For the exposed sites, the distance from the plane of the 6-ring also differs for different cations. Consequently, their interactions with the adsorbate molecule are different. 

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