A New Class of Non-Zeolitic Sorbents for Air Separation: Lithium Ion Exchanged Pillared Clays

Abstract

Zeolites are the only known sorbents that adsorb N{sub 2} selectively over O{sub 2}, and are used for industrial air separation. Pillared clays (PILCs) have a high Broensted acidity (k.e., high proton density). It is found in this study that when the protons are exchanged by alkali metal ions, in particular Li{sup +}, the ion exchanged pillared clays can exhibit a high N{sub 2}/O{sub 2} adsorption selectivity that rivals that of the zeolites. The first result shows a pure-component adsorption ratio of N{sub 2}/O{sub 2} = 3.2 (at 25 C and 1 atm) for Li{sup +}-exchanged PILC. The N{sub 2} capacity, however, is only 20% that of the zeolite, and remains to be improved. A systematic investigation is conducted on the effects of three factors on the N{sub 2}/O{sub 2} selectivity: (1) starting clays (tetrahedral vs octahedral isomorphous substitution and clays with different charge densities), (2) different metal oxides as pillars, and (3) different ion exchange alkali metal cations (Li{sup +}, Na{sup +}, K{sup +}, Rb{sup +}, and Cs{sup +}). The highest N{sub 2}/O{sub 2} selectivities are achieved by using clays with the highest charge densities, metal oxides forming pillars with the narrowest gallery spaces, and ion exchange cations withmore » the smallest ionic radii. Effects by all three factors are qualitatively understood. The high N{sub 2}/O{sub 2} selectivity on the Li{sup +} exchanged PILC is the result of the small ionic radius (and hence high polarizing power) of Li{sup +} and the strong quadrupole moment of the N{sub 2} molecule. Moreover, a technique is developed with which the amount of the exchanged cations can exceed that allowed by the original cation exchange capacity of the clay by using a high pH value in the ion exchange solution.« less