EFFECT OF CA2-2-O-2 MIXTURES BY NACAX ZEOLITE( EXCHANGE ON ADSORPTIONOF N)

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, rat ios on a series of NaCaX zeolites with different degrees of Ca2+ excha nge. A sharp increase in relative N-2 selectivity was observed when mo re than 49% of Na+ was exchanged with Ca2+. For the single adsorbate c ase, 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 locatio ns in the zeolite for adsorption and the stronger field-quadrupole int eractions 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+ catio ns to be replaced are the ones located at sites I and I' which are ina ccessible to either of the adsorbate molecules. At exchange degrees hi gher 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 i n molecular interactions between adsorbate molecules and cationic site s 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 th e presence of O-2 at a P-N2/P-O2 ratio of 1/4 was significantly smalle r 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 pheno mena are attributed to competition between N-2 and O-2 adsorption. The y were not observed at lower levels of Na+ replacement (higher Na+ con tent), probably due to lower adsorbate coverages under the conditions studied which minimized the competitive adsorption effects.