A NEW CLASS OF NON-ZEOLITIC SORBENTS FOR AIR SEPARATION - LITHIUM ION-EXCHANGED PILLARED CLAYS

Zeolites are the only known sorbents that adsorb N-2 selectively over O-2, and are used for industrial air separation. Pillared clays (PILCs ) have a high Bronsted acidity (i.e., high proton density). It is foun d in this study that when the protons are exchanged by alkali metal io ns, in particular Li+, the ion exchanged pillared clays can exhibit a high N-2/O-2 adsorption selectivity that rivals that of the zeolites. Our first result shows a pure-component adsorption ratio of N-2/O-2 = 3.2 (at 25 degrees C and 1 atm) for Li+-exchanged PILC. The N-2 capaci ty, however, is only 20% that of the zeolite, and remains to be improv ed. A systematic investigation is conducted on the effects of three fa ctors on the N-2/O-2 selectivity: (1) starting clays (tetrahedral vs o ctahedral isomorphous substitution and clays with different charge den sities), (2) different metal oxides as pillars, and (3) different ion exchange alkali metal cations (Li+, Na+, K+, Rb+, and Cs+). The highes t N-2/O-2 selectivities are achieved by using clays with the highest c harge densities, metal oxides forming pillars with the narrowest galle ry spaces, and ion exchange cations with the smallest ionic radii. Eff ects by all three factors are qualitatively understood. The high N-2/O -2 selectivity on the Li+ exchanged PILC is the result of the small io nic radius (and hence high polarizing power) of Li+ and the strong qua drupole moment of the N-2 molecule. Moreover, a technique is developed with which the amount of the exchanged cations can exceed that allowe d by the original cation exchange capacity of the clay by using a high pH value in the ion exchange solution.