Intrazeolitic Chemistry And Crystallography In The Faujasite System

Date
2004-12
Authors
Wang, Yijia
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Abstract
Ion-exchange of Ca2+ into zeolite X (FAU) was attempted at various pHs using aqueous Ca(N03)2, aqueous Ca(OH)2, or a mixture of the two. Two crystals prepared at (initial) pHs of 5.5 and 8.3 were damaged by this treatment. Ca(OH)2 was imbibed into the remaining two crystals prepared at pH 11.5 (crystal 1) and pH 12.0 (crystal 2). After vacuum dehydration at 350°C, Ca4O4 clusters (interpenetrating tetrahedra, symmetry 23 (Td)) were seen in the sodalite cavities in both crystals; the four Ca2+ ions are at site I'. In crystal 1, 2.4O(4) of the 8 sodalite cavities per unit cell contained a Ca4O4 cluster; in crystal 2, 4.95(4) sodalite cavities were so occupied. The Ca-O bond lengths in these Ca4O4 clusters are relatively long, 2.545(8) and 2.581(5)Å, respectively. Each Ca4O4 unit coordinates further via its oxygen atoms to four more Ca2+ ions (Ca-O = 2.376(21) and 2.328(13)Å, respectively) in the 6-rings (site II) to give the larger cationic Ca8O48+ clusters (symmetry 23 (Td)). As oxide ions were taken from H20 and OH- to form Ca8O48+, H+ ions must have formed in both crystals. The reactions of dehydrated Ca2+-exchanged zeolite X (ion exchanged at pH = 11.5) with buckminsterfullerene (C60) at 500 and 400°C were studied. After 10 and 20 days at 500°C, a carbonate ion was found in each sodalite cavity in both crystal structures. This nearly planar carbonate ion is surrounded by five Ca2+ ions (three at site l' and two at site II) similar to the arrangement in calcite. The C-O bond lengths in these carbonate ions are relatively long, 1.378(23) and 1.356(12)Å, respectively. The reaction of carbon with both the Ca4O4 clusters and the framework Si4+is perhaps responsible for the formation of these carbonates. After 20 days at 400°C, however, only four of the eight sodalite cavities per unit cell contained a carbonate ion; the other four contained the Ca4O4 clusters. Complete Na+ exchange into a large zeolite Y single crystal was attempted using aqueous 0.1 M NaCl at pH 11 by the addition of aqueous NaOH. After vacuum dehydration at 400°C, only about 61 Na+ ions were found per unit cell, fewer than the 71 needed to balance the anionic charge of its framework, Si121Al71O38471- per unit cell. NH4+-exchange into a large Na+-exchanged zeolite Y single crystal was attempted using aqueous 0.1 M NH4Cl. Because the resulting zeolite crystal was severely damaged, its crystal structure could not be determined.
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