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 N-14(2) (or O-16(2)) and N-15(2) (or O-18(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 Ca2+ 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-N2 + P-O2) was varied. While the amount of N-2 adsorbed increased with an increase in P-N2, no significant change in nitrogen selectivity, alpha(N2), was able to be observed. This suggests, that the competition for adsorption between N-2 and O-2 depends solely on the P-N2/P-O2 ratio rather than on the absolute value of P-N2.