Crystal Structures of Vacuum-Dehydrated Ni2+-Exchanged Zeolite Y (FAU, Si/Al=1.69) Containing Three-Coordinate Ni2+, Ni8O4 center dot xH(2)O(8+), x <= 4, Clusters with Near Cubic Ni4O4 Cores, and H+

Abstract

Five single crystals of vacuum-dehydrated Ni2+-exchanged zeolite Y (Ni-Y) were variously prepared by the exchange of Na-Y or K-Y (Na-71- or K-71-Si121Al71O384, Si/Al = 1.69) with Ni2+ using flowing aqueous 0.05 M Ni(NO3)(2) at 294 or 353 K, followed by vacuum dehydration at 2.0 x 10(-6) Torr and 623 or 723 K. Their crystal structures and chemical compositions were determined using synchrotron X-radiation and energy dispersive X-ray (EDX) analyses to give Ni-n-Y, where 25.3 <= n <= 34.1 per unit cell: [NiuMvHw[Ni8O4 center dot xH(2)O](y)[Al(OH)(4)](z)l [Si121+zAl71-zO384]-FAU, where 21.4 <= n <= 24.2, M = Na and/or Ca, 3.3 <= v <= 13.1, 9.5 <= w <= 21.6, 2 <= x <= 4, 0.3 <= y <= 3.2, and 0 <= z <= 2.5. In each of the five crystal structures (space group Fd(3)m; mean a = 24.47 A), Ni2+ is found at sites I, I&apos;, a second I&apos;, II&apos;, and sometimes 11; unexpectedly, Na+ and Ca2+ are found at another site It. Two extra-framework oxygen positions (O-f), one in the sodalite cavity and the other nearby in the supercage, are seen on 3-fold axes in all crystals. They bond to Al3+ and Ni2+ ions in the sodalite cavities to form Al(OeH)(4)(-) and Ni-8(O-e)center dot xH(2)O(e)(8+) clusters with Ni-4(O-e)(4) cores, and with framework oxygens (O-f) to give trigonal bipyramidal Ni(O-f)(3)(O-e)(2)(2+) and trigonal pyramidal Ni(O-f)(3)(O-e)(2)(2+.) At site I (centers of double 6-rings) in the Na-Y crystal that was Ni2+-exchanged at 294 K, similar to 14 Ni2+ ions per unit cell each coordinate octahedrally to six framework oxygen atoms. The Ni2+ ions at the first F site are 3-coordinate near planar in all crystals. The Ni2+ ions at the second Y site are members of the Ni4O4 cores, tetrahedrally distorted cubes with Ni-O = 2.199(13) angstrom and O-Ni-O = 79.6(9)degrees in a representative crystal; these Ni2+ ions are distorted octahedral with three O-e&apos;s of the cluster and three Of&apos;s. The Ni2+ ions at sites II&apos; and II are 3-, 4-, or 5-coordinate. Al(OH)(4)(-) from framework dealumination centers some sodalite cavities in four of the five crystals; their number increased with both ion-exchange and dehydration temperatures, suggesting that dealumination occurred during both processes. The number of Ni-2+ ions per unit cell increases with Ni2+-exchange temperature and is greater with K-Y than with Na-Y, perhaps because the larger K+ ions are more loosely held. The leaving cation affects the Ni2+ distribution over the available sites, perhaps via the level of Ni2+-exchange. Both a greater degree of Ni2+-exchange and a higher dehydration temperature cause more Ni8O4 center dot xH(2)O(8+) clusters to form, leaving fewer Ni2+ ions at sites I and II. As more Ni8O4 center dot xH(2),O8+ formed, more H+ ions were produced. Some H+ and some 3- and 4-coordinate Ni2+ ions are easily accessible for catlysis.