Influence of the polarizing power, distribution, size and number of exchangeable cations

https://doi.org/10.1088/1468-6996/9/1/013007

“The adsorption of gas by the zeolites is also determined by the polarizing power of the exchangeable cations and their distribution, size and number that influence the local electric field and the polarization of adsorbed molecules on the zeolites [12]. In general, the polarizing power of the cations is inversely proportional to its ionic radius [30]. For instance, the diameters of cations from Group 1A vary as following: Cs⁺ (3.3 Å) >Rb⁺ (2.9 Å) > K⁺ (2.7 Å) > Na⁺ (1.9 Å) > Li⁺ (1.4 Å) and their polarity: Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺ [31, 32]. Hence, the zeolites such as ZSM-5 and M-ZSM-5 (M=Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) which have small cations might penetrate more easily within channels, are able to interact more strongly with the CO₂ [19, 33]. This is in agreement with the results from a study performed by Yamazaki et al. [21]. They showed that energy of the interaction between the CO₂ and the cations sites of zeolites M-ZSM-5 (M=Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) decreases with the increase of the size of these cations sites. In parallel, the distribution of exchangeable cations in the different sites of the zeolites structure causes the heterogeneous character of the CO₂ adsorption [17]. For example, it has been established for faugasite-type zeolites that molecules of CO₂ might be adsorbed only in the supercage, and more specifically in sites localized within sodalite cage and/or in the hexagonal prism [22, 34, 35]. The position of extraframework cations sites in faujasite-type structure are shown in figure 3. In the case of zeolite NaX, the sodium ions that are accessible to CO₂ molecules are localized in two sites: the sites II that are strongly bound to zeolite lattice and the sites III that are less bound and responsible of heterogeneous character of CO₂ adsorption [17]. In regard with this, Khelifa et al [34, 37] have notably observed a decrease of the CO₂ adsorption affinity when Na⁺ cations of an x-type zeolite are exchanged by the M²⁺ cations (Mg²⁺, Sr²⁺, Zn²⁺, Cu²⁺). However, there is an increase of the CO₂ adsorption affinity when the degree of Na⁺ exchange increases. Khelifa et al [17] have also shown that the CO₂ adsorption on the zeolites X exchanged with Ni²⁺ and Cr³⁺ decreases as compared to that of zeolite NaX due to a decrease of the adsorbate-adsorbent interaction. This phenomenon has been associated with a depopulation of sites III and a decrease of electric field in the cavities of zeolite. Nevertheless, it has been showed that the substitution of Na⁺ cations by the M²⁺ and Cr³⁺ cations changes only the CO₂ adsorption when the rate of exchange is superior to 40–50%[17, 37, 38]. A similar effect has also been observed by Khvoshchev and Zverev [39] on the isosteric heat of CO₂ adsorption on the dehydrated faujasites (Mg, Ca)-X and (Mg, Ca)-Y following the substitution of a part of the Ca²⁺ ions by the Mg²⁺ ions. The decrease of heat of adsorption achieved during the substitution has been attributed to a greater screening of Mg²⁺ ions by the oxygen atoms electric field of zeolites as compared to that of Ca²⁺ ions. This might be caused by the penetration of Mg²⁺ ions deep within six-membered rings of the framework or to their localization principally outside the cavities of zeolites. Coughlan and Kilmartin [40] and Coughlan and McCann [41] have also showed a decrease of CO₂ affinity after an introduction of trivalent cations (Fe³⁺, Y³⁺, Cr³⁺, Co³⁺ and Tl⁺) in the X, Y, A and L type zeolites. According to these authors, this effect could be generalized to all the polyvalent cations of transition metals [41]. The number of cations which are able to interact with adsorbates has also an importance in the adsorption process. Indeed, a measurement of the heat of CO₂ adsorption in faujasites CaY and CaX has indicated that the isosteric heat of CO₂ adsorption on the zeolite CaX is higher than that of CO₂ on the zeolite CaY. This could be a consequence of a higher CO₂ adsorption on zeolite CaX than on the zeolite CaY [39]. Moreover, this difference could also be due to a greater number of Ca²⁺ cations in the zeolite CaX which are able to interact directly with the adsorbed CO₂ molecules, than in zeolite CaY.”