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strongly influence the formation of the silver clusters.9,10,13.14 The effect of the dehydration conditions on extraframework silver can be assessed by looking at the effects of dehydration treatment on sample color and nitrogen adsorption. Temperature effects. Color changes upon vacuum dehydration of silver- exchanged A-type zeolite were found to be related to the formation of metallic clusters within the sodalite cage or the 6-prism of the zeolite.9,10,11 These color changes are also evident in X-type zeolite. A summary of some dehydration conditions and the resulting color changes in the zeolites of interest in this study are given in Table 2. The Ag-X and Ag-LSX behaved very similarly, both becoming deep golden yellow with thermal vacuum dehydration. Gellens et al. observed that the yellow color occurred in Ag-A when, on average, only one cluster per sodalite cage was formed.13,14 The Ag-X and Ag- LSX zeolites maintain the yellow color because there is no interaction between silver clusters. These authors also noted that more Ag present in the faujasite-type zeolite resulted in brighter color upon thermal vacuum dehydration. Effects of dehydration temperature. In order to determine the effect of temperature on silver cluster formation and the subsequent adsorptive characteristics, samples of the air-dried Ag-LSX zeolite were heated to various temperatures in a vacuum. The N2 adsorption isotherm was then measured for each of the heat-treated materials. The results are shown in Figure 2. In this plot the Ag-LSX sample shows a considerable increase in N2 capacity after dehydration at 450 C over that of the same sample dehydrated at 350 C (at 1 atm, 22 molecules of N2 and 19 molecules of N2 per unit cell respectively). This increase cannot be attributed to a loss of water since all but the most tenaciously held water is removed by 350 C;2 and there is no increase in the N2 17PDF Image | PSA USING SUPERIOR ADSORBENTS
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