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Small amounts of silver were also located in sodalite six-ring sites (SI', SII, and SII*). There was no silver associated with the single six-ring (SI) or inside the sodalite cage (SII'). The silver site II is approximately in the plane of the six-ring. A new site, which is labeled SII* to distinguish it from the conventional SII (see Figure 3) was found in this sample. Site II* is still observed on the supercage side of the six-ring, but is shifted further away from the plane of the six-ring. Only about 70% of the expected silver were located using Rietveld refinement. One explanation for this result is that there may be reduced silver crystallites on the exterior of the zeolite. This was discussed in a previous work for near-fully Ag-exchanged faujasites (Hutson et al., 2000). This explanation also helps to explain the higher than expected lithium. A schematic of the sodalite cage and double six-ring (hexagonal prism) showing the sites for lithium and silver occupancy is shown in Figure 3. Fourier difference analysis indicated the presence of scattering density at the center of the sodalite cage and approximately 1.7 Å away from this site. One likely explanation for this extra scattering density is the presence of extraframework alumina in the center of the sodalite cage. This was also noted in a neutron diffraction structural analysis of near-fully exchanged Ag-X and Ag-LSX zeolites. Even when carefully dehydrated, X and LSX tend to lose framework alumina through self-steaming to void spaces in the zeolite (the sodalite cage and supercage). An extraframework tetrahedral [AlO4]- unit was added to the model with the Al3+ located at the center of the sodalite cage and oxygen atoms along the (111) axis. The fractional occupancies for these two atoms were constrained to be the same. Isotropic atomic displacement parameters were constrained to be the same as that of the extraframework silver. 95PDF Image | PSA USING SUPERIOR ADSORBENTS
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