D2/H2 adsorption selectivity on FAU zeolites at 77.4 K: Influence of Si/Al ratio and cationic composition

Equilibrium D-2/H-2 adsorption selectivity was determined at 77.4 K below 1000 hPa for a series of FAU type zeolites X exchanged with different cations (Li+, Na+, K+, Mg2+, Ca2+, Ba2+ and Mn2+). In addition NaY, DAY (dealuminated Y) and pure silica CHA and MFI zeolites were studied. Two experimental approaches were used to determine the D2/H2 adsorption selectivity: direct determination at the thermodynamic equilibrium from manometric coadsorption experiments and calculations by Ideal Adsorbed Solution Theory (LAST) from single gas adsorption isotherms. While these two approaches are not in quantitative agreement, they reveal similar trends. At low loading (< 20 molec/uc) exchanged zeolites X in the selectivity diminishes with increasing cation size from 5.7 for MgX to 2.4 for KX: MgX > MnX > LiX > CaX approximate to NaX > KX approximate to BaX. In contrast, at high loading the selectivity is not influenced by the material composition being around 1.5 +/- 0.2 for all studied materials. For cation-free zeolites (CHA and MFI) the latter value is observed for all loadings. The key role of cations in D2/H2 selectivity is explained by the fact that at low loading the adsorption proceeds through strong guest-cation interactions while at high loading weaker interactions are involved. Surprisingly, the selectivity in the low loading range is found to be correlated with the cation size and not with the interaction strength (estimated from the value of the Langmuir constant). This behavior of FAU type zeolites is thus different from that of MOFs or that predicted by simple theoretical models based on a one dimensional gas in a harmonic potential.