Saturation loadings on 5A zeolite above and below the critical conditions: monatomic species data evaluation and modeling

Isotherms for monatomic species (He, Ne, Ar, Kr and Xe) adsorption on 5A zeolite are extracted from the literature and analyzed for consistency and saturation. The saturation data below the critical adsorbate reduced temperature, TCAR, is modeled using the Rackett equation and the zeolite structure. Consideration is made regarding whether adsorbates can enter β cage in addition to the α cage. The data for argon and krypton suggest that both species may also enter the β cage, although there is not enough information to be conclusive. Xenon does not enter the β cage. The limited data establishes the need for further study on this point. The subTCAR data for both 5A and 13X are relatively consistent with the proposed subTCAR model. Although there is not sufficient data to determine this for helium or neon, their size and the literature suggest that both molecules enter the β cage. The data are divided into subTCAR and superTCAR regions, where TCAR is the critical reduced temperature for the adsorbed species, which often differs from 1. The subTCAR model predictions for argon and krypton are excellent, while that for xenon requires a steric factor of 0.76. For superTCAR data, helium has only one data point. The superTCAR data for argon, krypton and xenon are linear with excellent regression coefficients. Intersection of the linear superTCAR plots with the subTCAR model prediction curves gave TCAR’s of 0.996 for argon, 0.80 for krypton and 0.66 to 0.86 for xenon. Literature models Ruthven et al. (Chem Eng Sci 28:701–709, 1973), Ozawa et al. (J Colloid Interface Sci 56(1):83–91, 1976), and Dubinin (in: Caldenhead (ed) Prog. surface and membrane science, Academic Press, New York, 1975) expressed in terms of qmax are also fitted to the data with some success. A modified Ozawa et al. model fits the data reasonably well. Data for 4A, 5A and 13X are observed to be very similar.