Effect of Ca2+ Exchange on Adsorption of N2−O2 Mixtures by NaCaX Zeolite

Steady-state isotopic transient kinetic analysis (SSITKA) was used in this study to compare the adsorption behavior of pure N 2 and O 2 with the adsorption of binary N 2 −O 2 mixtures at different P N 2 / P O 2 ratios on a series of NaCaX zeolites with different degrees of Ca 2+ exchange. A sharp increase in relative N 2 selectivity was observed when more than 49% of Na + was exchanged with Ca 2+ . For the single adsorbate case, there was also a sharp increase in N 2 uptake at this point. This was probably due to Ca 2+ cations starting to be in accessible locations in the zeolite for adsorption and the stronger field-quadrupole interactions between N 2 and Ca 2+ . A similar increase was observed for O 2 uptake for zeolite samples with more than 71% of the Na + exchanged. It is suggested that, during the exchange process, the first Na + cations to be replaced are the ones located at sites I and I‘ which are inaccessible to either of the adsorbate molecules. At exchange degrees higher than 49%, the drastic increase in adsorption uptake, particularly for N 2, suggests that the Ca 2+ also replaces Na + cations located at sites II and III in the supercages of the X zeolite. The differences in molecular interactions between adsorbate molecules and cationic sites are reflected by differences in the isosteric heats of adsorption of N 2 which were found to be around 3.8 kcal/mol on the Na zeolite with no Ca 2+ and 6.3 kcal/mol on the one having 94% of the Na + replaced by Ca 2+ . For the zeolite with 94% of Na + exchanged, the N 2 uptake in the presence of O 2 at a P N 2 / P O 2 ratio of 1/4 was significantly smaller than the one measured for pure N 2 . Similarly, for the same zeolite, the O 2 uptake in the presence of N 2 at a P N 2 / P O 2 ratio of 4/1 was significantly smaller than the one measured for pure O 2 . These phenomena are attributed to competition between N 2 and O 2 adsorption. They were not observed at lower levels of Na + replacement (higher Na + content), probably due to lower adsorbate coverages under the conditions studied which minimized the competitive adsorption effects.