Zeolites containing mixed cations for air separation by weak chemisorption-assisted adsorption

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 N-2 and O-2 were measured at 25 and 50 degrees C on these four zeolites. AgX was stable since the N-2 isotherm was not affected after prolonged exposure of the zeolite to air at 350 degrees C. Bonding of N-2 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 N-2 on AgX indicated some degree of weak pi-complexation, which was substantiated by molecular orbital calculation results using model systems. Binary N-2/O-2 selectivity (or separation factor, alpha) was calculated by using the ideal adsorbed solution theory. The high N-2/O-2 selectivities at low total pressures for AgX will result in difficult N-2 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 N-2/O-2 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 N-2 capacity for LiAgX, led to the conclusion that LiAgX can be superior to LiX for air separation.