ISOTOPIC TRANSIENT STUDY OF MULTICOMPONENT N-2 AND O-2 ADSORPTION ON CAX ZEOLITE

Steady-state isotopic transient kinetic analysis, previously used exte nsively 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 wit h the adsorption of binary N-2/O-2 mixtures on a CaX zeolite. By varia tion 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 a ble to be measured at equilibrium conditions using isotopic switches b etween N-14(2) (or O-16(2)) and N-15(2) (or O-18(2)). The N-2 uptake w as about 1 order of magnitude higher than the O-2 uptake on CaX, mainl y 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 phas e were also measured between 303 and 338 K. The N-2 uptake in the pres ence 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 molec ules and the Ca2+ adsorption sites. On the other hand, when the O-2 pa rtial pressure was varied at constant N-2 partial pressure, a systemat ic 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 onl y to be due to the competitive adsorption of N-2 and O-2 molecules. Ad ditionally, N-2 and O-2 uptakes were measured from mixtures having a c onstant N-2 to O-2 ratio of 4/1 where only the overall adsorbate press ure (P-N2 + P-O2) was varied. While the amount of N-2 adsorbed increas ed with an increase in P-N2, no significant change in nitrogen selecti vity, alpha(N2), was able to be observed. This suggests, that the comp etition 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.