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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 N2 adsorption capacity of 20.8 molecules per unit cell for nitrogen at 101.3 kPa and N2/O2 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 π-complexation of N2 molecules with silver cations. Selective adsorption of argon is explained in terms of its interaction with silver cations through Ar(pσ)−Ag(dσ) bonding molecular orbital.

The size of pore apertures of zeolite A was systematically controlled by silica deposition on the external surface of the zeolite A from a dilute solution of tetraethyl orthosilicate (TEOS) in dry toluene. The silica deposition on the external surface is confirmed by EDX analysis and external surface area measurements. The adsorption properties of these pore-size-engineered zeolites show that these adsorbents are useful for the size-selective separation of molecules of size in the range of 3−4 Å, such as N2/O2/Ar and H2O/CH3OH/C2H5OH. Argon and nitrogen molecules are restricted from entering the zeolite pores after depositing TEOS on the external surface of zeolite A, and their adsorption capacities decrease. Comparatively smaller oxygen molecules can still enter the pores and become adsorbed there. O2/N2 and O2/Ar selectivity increase with increase in the TEOS deposition. The adsorption properties of the pore-size-engineered zeolite A become similar to that of commercially available molecular sieve 3A after depositing around 0.20% TEOS on the external surface of NaA.

Adsorption of nitrogen, oxygen, and argon is studied in zeolite mordenite (Si/Al = 5.5) with mono-, di-, and trivalent Na+, Li+, K+, Cs+, Ca2+, Sr2+, Ba2+, La3+, and Ce3+ as extraframework cations. Adsorption capacity, selectivity, and heat of adsorption show a strong dependence on the nature as well as the extent of the extraframework cations. The cation-exchanged mordenite samples show adsorption selectivity toward nitrogen from its mixture with oxygen or argon. The electrostatic interaction between adsorbate molecules and the cations are largely responsible for the observed nitrogen selectivity of the mordenite samples because nitrogen has higher quadrupole moment than oxygen or argon. The relationships between the adsorption properties and charge densities of extraframework cations show that the interaction of adsorbate molecules depends not only on the cation type but also on the positions of these cations in the mordenite structure.

Adsorption of nitrogen, oxygen, and argon on cobalt(II)-exchanged zeolite X at 288.2 and 303.0 K was studied. The nitrogen and oxygen adsorption capacities increase upon cobalt ion exchange up to 71%, beyond which it shows a decreasing trend because of the partial degradation of the zeolite structure during the cation exchange and high-temperature vacuum dehydration processes. The magnitude of the increase in the adsorption capacities for nitrogen is much higher than that of oxygen. The nitrogen/oxygen as well as nitrogen/argon selectivities in the low-pressure region increase with an increase in cobalt exchange. Marginal oxygen selectivity over argon is observed for zeolite samples with higher cobalt exchange. The heats of adsorption values for nitrogen and oxygen increase and that for argon remain unaffected by cobalt exchange in zeolite X. The very high nitrogen adsorption capacity, selectivity, and heat of adsorption in the low-pressure region for cobalt-exchanged zeolite X compared to the parent sodium form of the zeolite show stronger interaction between nitrogen molecules with the extraframework cobalt cations of the zeolite. This stronger interaction has been explained in terms of the pi-complexation between nitrogen molecules and cobalt cations of the zeolites, as confirmed by diffuse reflectance infrared Fourier transform spectroscopy, wherein the N[triple bond]N stretching frequency at 2099 cm(-1) is observed for N2 molecules adsorbed in NaCoX.