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