Enhancement of D-2/H-2 Selectivity in Zeolite A through Partial Na-K Exchange: Single-Gas and Coadsorption Studies at 45-77 K

We report D-2/H-2 adsorption selectivities under cryogenic temperatures (45-77 K) in Na and K-Na exchanged zeolites A (LTA structural code) measured by a coadsorption 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, D-2/H-2 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 D-2/H-2 adsorption at 77 K. The adsorbed amounts decrease with the K content, but the D-2/H-2 selectivity rises, attaining the maximum value in K(1.6)Na(10.4)A, which surpasses that measured in NaA. A detailed study of D-2/H-2 mixture adsorption in this zeolite revealed its nonideal behavior even at 77 K. Upon the decrease of temperature, the uptakes of D-2 and H-2 in K(1.6)Na(10.4)A decrease, and the selectivity rises more steeply than in the case of NaA. Due to this effect at 48 K for 25%D-2 + 75%H-2 mixture, the D-2/H-2 selectivity of 23.0 is attained in K(1.6)Na(10.4)A (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 D-2/H-2 selectivity in K(1.6)Na(10.4)A zeolite is explained by the fact that some alpha-cages in this material are accessible only to D-2 molecules.