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

Abstract

Steady-state isotopic transient kinetic analysis (SSITKA) was used in this study to compare the adsorption behavior of pure N2 and O2 with the adsorption of binary N2−O2 mixtures at different PN2/PO2 ratios on a series of NaCaX zeolites with different degrees of Ca2+ exchange. A sharp increase in relative N2 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 N2 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 N2 and Ca2+. A similar increase was observed for O2 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 N2, 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 N2 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 N2 uptake in the presence of O2 at a PN2/PO2 ratio of 1/4 was significantly smaller than the one measured for pure N2. Similarly, for the same zeolite, the O2 uptake in the presence of N2 at a PN2/PO2 ratio of 4/1 was significantly smaller than the one measured for pure O2. These phenomena are attributed to competition between N2 and O2 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.