Enhancement of D 2 /H 2 Selectivity in Zeolite A through Partial Na–K Exchange: Single-Gas and Coadsorption Studies at 45–77 K
Résumé
We report D2/H2 adsorption selectivities under cryogenic temperatures (45-77 K) in Na and K-Na exchanged zeolites A (LTA structural code) measured by co-adsorption technique. These values are systematically compared with Ideal Adsorbed Solution Theory (IAST) predictions based on the single gas adsorption isotherms. For NaA zeolite the evolution of the selectivity as a function of total pressure and gas mixture composition at 77 K shows ideal behavior. In contrast, as temperature decreases, D2/H2 selectivity rises exponentially and its values can no longer be predicted by IAST. We found that in these conditions the evolution of the selectivity can be described by a simple quantum sieving model based on a spherical particle in a one-dimensional cylindrical pore. Partial substitution of Na by K influences significantly the properties of LTA zeolite in D2/H2 adsorption at 77 K. The adsorbed amounts decrease with K content, but the D2/H2 selectivity rises attaining the maximum value in K1.6Na10.4A, which surpasses that measured in NaA. A detailed study of D2/H2 mixture adsorption in zeolite revealed its non-ideal behavior even at 77 K. Upon the decrease of temperature the uptakes of D2 and H2 in K1.6Na10.4A decrease and the selectivity rises more steeply than in the case of NaA. Due to this effect at 48 K for 25%D2 + 75%H2 mixture the D2/H2 selectivity of 23.0 is attained in K1.6Na10.4A (P = 648 hPa, loading-1.9 mmol.g-1) whereas the value of 8.5 is obtained in NaA (P = 574 hPa, loading-10.3 mmol.g-1). The improved D2/H2 selectivity in K1.6Na10.4A zeolite is explained by the fact that some -cages in this material are accessible only to D2 molecules.
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