Synthesis and characterization of novel materials for use as adsorbents in gas separations.

Abstract

Separations using adsorption involve the preferential attraction of one or more solutes from a gas or liquid mixture to the surface of a solid adsorbent where they are held by intermolecular forces. This dissertation discusses the synthesis, characterization, and evaluation of several novel materials that may find use as adsorbents in gas separation operations. These materials include silver and mixed silver/lithium zeolites, pillared clays that had been prepared under varying conditions, ion-exchanged pillared clays, and immobilized oxygen-binding organometallic cobalt complexes. The structures of the silver-containing and mixed Li,Ag zeolites were characterized using Rietveld refinement of neutron diffraction data. In both of these cases, a novel silver site, referred to as SII*, was found. This site, because of its distance from the framework oxygen the six-ring and its low valence bond number, interacts with adsorbates such as nitrogen much more than silver in the conventional SII. One mixed Li,Ag-LSX zeolite, containing an average of only one silver per unit cell, was evaluated for its use in the adsorptive separation of air. The results of a PSA cycle simulation predicted that this material gives a >10% improvement in productivity. The oxygen-binding complexes were immobilized on various nanoporous substrates; and the adsorptive capacity and stability were evaluated. The results showed that by immobilizing these complexes on the substrates the adsorptive characteristics were affected. The adsorption isotherms were more linear and had a greater reversibility. Specific synthesis conditions and past-synthesis modification were used to control the micropore structure of alumina-pillared clays. The use of Monte Carlo (MC) techniques to the simulate the adsorption of N₂, O₂, and Ar in the Li-LSX, and the effects of residual zeolitic water on the adsorption of these atmospheric gases in lithium ion-exchanged low silica X zeolite is also discussed.