Isotopic Transient Study of Multicomponent N2 and O2 Adsorption on CaX Zeolite

Steady-state isotopic transient kinetic analysis, previously used extensively for the study of surface-catalyzed reactions, was used for the first time to compare the adsorption behavior of pure N2 and O2 with the adsorption of binary N2/O2 mixtures on a CaX zeolite. By variation of the partial pressure of N2 or O2 in the feed stream, N2 and O2 adsorption isotherms at temperatures between 303 and 338 K were able to be measured at equilibrium conditions using isotopic switches between 14N2 (or 16O2) and 15N2 (or 18O2). The N2 uptake was about 1 order of magnitude higher than the O2 uptake on CaX, mainly due to the higher quadrupole moment of N2. N2 adsorption isotherms in the presence of a constant partial pressure of O2 in the gas phase were also measured between 303 and 338 K. The N2 uptake in the presence of O2 was significantly smaller than the one measured for pure N2. However, the isosteric heats of adsorption of N2 calculated from the N2/O2 data were very similar to those obtained from the pure N2 data (around 6.3 kcal/mol). On the basis of these observations, it is suggested that O2 does not affect the interactions between N2 molecules and the Ca2+ adsorption sites. On the other hand, when the O2 partial pressure was varied at constant N2 partial pressure, a systematic decrease in N2 uptake could be observed with increasing O2 uptake. Thus, the smaller N2 uptake in the presence of O2 would appear only to be due to the competitive adsorption of N2 and O2 molecules. Additionally, N2 and O2 uptakes were measured from mixtures having a constant N2 to O2 ratio of 4/1 where only the overall adsorbate pressure (P N 2 + P O 2 ) was varied. While the amount of N2 adsorbed increased with an increase in P N 2 , no significant change in nitrogen selectivity, αN 2 , was able to be observed. This suggests, that the competition for adsorption between N2 and O2 depends solely on the P N 2 /P O 2 ratio rather than on the absolute value of P N 2 .