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capacity for other zeolite forms (Li+, Na+, K+, etc.) with dehydration at temperatures beyond 350 C. Ag-LSX zeolites which had been dehydrated in vacuum at 550 C and 600 C had N2 capacities which were considerably lower than those dehydrated in vacuum at 450 - 500 C. Similarly, N2 adsorption isotherms were measured for the Ag-X zeolites after thermal vacuum dehydration at 350 C and 450 C. This sample did not show the same increase in N2 adsorption capacity with the higher temperature dehydration (at 1 atm both samples adsorbed approximately 15.5 molecules of N2 per unit cell). Effects of duration of dehydration. Migration of silver ions, most likely to or from the sodalite cage and hexagonal prism, and possibly from one unit cell to another, is necessary to form the charged metallic clusters. Using a theoretical model of diffusion into a sphere, Cvjeticanin and Petranovic showed that the diffusion of Ag+ is the rate determining step in silver cluster formation.19 In order to determine the effect of dehydration time on cluster formation, and the resulting adsorptive characteristics, the Ag-LSX sample was vacuum dehydrated at 450 C at various times; and the resulting N2 adsorption isotherms were measured. These are shown in Figure 3. From these results one can see that the N2 adsorption capacity increased sharply between 2 and 3 hours of dehydration and finally peaked at about 4 hours. No further enhancement of the adsorption capacity was seen after the peak at 4 hours. The initial sharp increase in the adsorptive capacity was due to loss of water in the zeolite. It also appears that the silver clusters, which presumably are responsible for the enhanced adsorptive capacity of the 450 C dehydrated sample, are formed within the first 4 hours of dehydration. Because of this, all subsequent samples were dehydrated for a minimum of 4 hours (but usually 18PDF Image | PSA USING SUPERIOR ADSORBENTS
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