Enhancement of D₂/H₂ 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 α-cages in this material are accessible only to D 2 molecules.