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-N2/P-O2, ratios on a series of NaCaX zeolites with different degrees of Ca2+ exchange. A sharp increase in relative N-2 selectivity was observed when more than 49% of Na+ was exchanged with Ca2+. For the single adsorbate case, there was also a sharp increase in N-2 uptake at this point. This was probably due to Ca2+ cations starting to be in accessible locations in the zeolite for adsorption and the stronger field-quadrupole interactions between N-2 and Ca2+. 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 Ca2+ 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 Ca2+ and 6.3 kcal/mol on the one having 94% of the Na+ replaced by Ca2+. For the zeolite with 94% of Na+ exchanged, the N-2 uptake in the presence of O-2 at a P-N2/P-O2 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-N2/P-O2 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.