Sorption of nitrogen, oxygen, and argon in silver-exchanged zeolites

Sorption of nitrogen, oxygen, and argon on silver-exchanged zeolites, A, X, Y, mordenite, BEA, L, and ZSM-5 at 288.2 and 303.0 K were studied. The nitrogen adsorption capacity, selectivity, and heat of adsorption in the low-pressure region are very high for silver-exchanged zeolites compared to other cation-exchanged samples, showing strong interactions between nitrogen molecules with the silver cations. Heat of nitrogen adsorption decreases with the increase in the adsorption equilibrium pressure in all zeolites except zeolite A. However, zeolite AgA shows N-2 adsorption capacity of 20.8 molecules per unit cell for nitrogen at 101.3 kPa and N-2/O-2 selectivity in the range of 5-14.6 at 303 K, the highest known so far for any zeolite A type of adsorbent. Other zeolites also show increased adsorption capacities for nitrogen on silver exchange, but these are smaller compared to those observed for zeolite A. Furthermore, unlike other cation-exchanged zeolites which show small oxygen selectivity over argon, silver-exchanged zeolites display argon selectivity over oxygen. The stronger interaction of nitrogen molecules with silver cations present inside zeolite cavities is attributed to pi-complexation of N-2 molecules with silver cations. Selective adsorption of argon is explained in terms of its interaction with silver cations through Ar(p sigma)-Ag(d sigma) bonding molecular orbital.