Zeolites Containing Mixed Cations for Air Separation by Weak Chemisorption-Assisted Adsorption

Abstract

NaX zeolite was ion-exchanged to obtain LiX and AgX zeolites. The LiX form was further exchanged to replace 20% of the Li+ cations by Ag+, to obtain a LiAgX zeolite. Equilibrium adsorption isotherms of pure-component N2 and O2 were measured at 25 and 50 °C on these four zeolites. AgX was stable since the N2 isotherm was not affected after prolonged exposure of the zeolite to air at 350 °C. Bonding of N2 was substantially stronger on AgX than on the other zeolites. The high isosteric heat of adsorption (8.4 kcal/mol) and the relatively slow desorption of N2 on AgX indicated some degree of weak π-complexation, which was substantiated by molecular orbital calculation results using model systems. Binary N2/O2 selectivity (or separation factor, α) was calculated by using the ideal adsorbed solution theory. The high N2/O2 selectivities at low total pressures for AgX will result in difficult N2 desorption; therefore, AgX is not suitable for air separation. LiX is presently employed in industry as the sorbent for air separation by pressure-swing adsorption. Comparing LiX with LiAgX, the N2/O2 selectivities were higher for LiAgX at high total pressures and lower for LiAgX at lower pressures, due to a (relative) selectivity reversal. This result, combined with the higher N2 capacity for LiAgX, led to the conclusion that LiAgX can be superior to LiX for air separation.