Rt. Yang et al., ZEOLITES CONTAINING MIXED CATIONS FOR AIR SEPARATION BY WEAK CHEMISORPTION-ASSISTED ADSORPTION, Industrial & engineering chemistry research, 35(9), 1996, pp. 3093-3099
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+, t
o obtain a LiAgX zeolite. Equilibrium adsorption isotherms of pure-com
ponent 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 s
low desorption of N-2 on AgX indicated some degree of weak pi-complexa
tion, 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. T
he high N-2/O-2 selectivities at low total pressures for AgX will resu
lt 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 pressur
es and lower for LiAgX at lower pressures, due to a (relative) selecti
vity 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.