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

Zeolites are the only known sorbents that adsorb N₂ selectively over O2, and are used for industrial air separation. Pillared clays (PILCs) have a high Bronsted acidity (i.e., high proton density). It is found in this study that when the protons are exchanged by alkali metal ions, in particular Li⁺, the ion exchanged pillared clays can exhibit a high N₂/O₂ adsorption selectivity that rivals that of the zeolites. Our first result shows a pure-component adsorption ratio of N₂/O₂ = 3.2 (at 25 °C and 1 atm) for Li^+_exchanged PILC. The N₂ 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 N₂/O₂ selectivity: (1) starting clays (tetrahedral vs octahedral isomorphous substitution and clays with different charge densities), (2) diffèrent metal oxides as pillars, and (3) different ion exchange alkali metal cations (Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺). The highest N₂/O₂ selectivities are achieved by using clays with the highest charge densities, metal oxides forming pillars with the narrowest gallery spaces, and ion exchange cations with the smallest ionic radii. Effects by all three factors are qualitatively understood. The high N₂/O₂ selectivity on the Li⁺ exchanged PILC is the result of the small ionic radius (and hence high polarizing power) of Li⁺ and the strong quadrupole moment of the N₂ 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.