Adsorbate with Cations Effects of Cation Site, Charge, and Ionic Radius

INTERACTIONS OF ADSORBATE WITH OATIONS EFFECTS OF CATION SITE, CHARGE, AND IONIC RADIUS 

In earlier discussions, the strong or dominating contributions of cation-dipole and cation-quadrupole interactions to the total bonding energy for adsorption on zeolites are already seen (see Table 2.1). Unfortunately, except ionic radii, information on the cation sites and the charges has not been well determined. Hence they are often treated as fitting parameters in molecular simulations of adsorption. However, the strong effects of these parameters on adsorption are well established. 

Due to their importance, the most studied cation sites are for zeolites A, X, and Y. Results on cation siting in these zeolites will be summarized first. Cation sites in another main type of structure with cage-topology (i.e., chabazite) 

The cation sites in zeolites X and Y are given in Table 7.4. Sites I, T, and IF are not exposed and are not available for interaction with adsorbate molecules, with the possible exception of water, which could fit in the 6-oxygen ring (with an opening of 2.8 A). Thus, nearly one-half of the cations are not available for adsorption. Sites II and III are exposed to the cavity and are associated with, respectively, the 6- and 4-oxygen rings. Alkali metal cations occupy both of these sites but the alkaline earth metal cations tend to occupy site II only. 

Channel-type zeolites and molecular sieves also hold interest for gas separation. Cation sites in some of the SAPO4 molecular sieves have been discussed by Hartmann and Kevan (1999). Cation sites in the channel-type zeolites have been determined for a number of topologies. The cations either fully block the channels or partially block them. The cation sites in cUnoptilolite have been determined because of its practical importance (Alberti, 1975 Koyama and Takeuchi, 1977 Ackley and Yang, 1991). 

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