Crystal structure of zeolite X nickel(II) exchanged at pH 4.3 and partially dehydrated, Ni-2(NiOH)(35)(Ni4AlO4)(2)(H3O)(46)Si101Al91O384

Complete Ni2+ exchange of a single crystal of zeolite X of composition Na92 Si100Al92O384 per unit cell was attempted at 73 degreesC with flowing aqueo us 0.05 M NiCl2 (pH = 4.3 at 23 degreesC). After partial dehydration at 23 degreesC and approximate to 10(-3) Torr for two days, its structure; now of composition Ni-2(NiOH)(35)(Ni4AlO4)(2)(H3O)(46)Si101Al91O384 per unit cell , was determined by X-ray diffraction techniques at 23 degreesC (space grou p Fd3, a(0) = 24.788(5) Angstrom). It was refined using all intensities; R- 1 = 0.080 for the 236 reflections for which F-o > 4 sigma (F-o), and wR(2) = 0.187 using all 1138 unique reflections measured. At four crystallographi c sites, 45 Ni2+ ions were found per unit cell. Thirty of these are at two different site III' positions. Twenty of those are close to the sides of 12 -rings near O-Si-O sequences, where each coordinates octahedrally to two fr amework oxygens, to three water molecules which hydrogen bond to the zeolit e framework, and to an OH- ion. The remaining 10 are near O-Al-O sequences; only three members of a likely octahedral coordination sphere could be fou nd. In addition, two Ni2+ ions are at site I, eight are at site I', and fiv e are at site II. Forty six H3O+ ions per unit cell, 24 at site II' and 22 at site II, each hydrogen bond triply to six rings of the zeolite framework . Each of the 22 H3O+ ions also hydrogen bonds to a H2O molecule that coord inates to a site III' Ni2+ ion. Six of the eight sodalite cages each contai n four H3O+ ions at site II'; the remaining two each contains a tetrahedral orthoaluminate anion at its center. Each tetrahedral face of each orthoalu minate ion is centered by a site I' Ni2+ ion to give two Ni4AlO4 clusters. The five site II Ni2+ ions each coordinate to a OH- ion. With 46 H3O+ ions per unit cell, the great tendency of hydrated Ni2+ to hydrolyze within zeol ite:X is demonstrated. With a relatively weak single-crystal diffraction pa ttern, with dealumination of the zeolite framework, and with an apparent de crease in long-range Si/Al ordering likely due to the formation of antidoma ins, this crystal like others treated with hydrolyzing cations appears to h ave been damaged by Ni2+ exchange and partial dehydration. (C) 2000 Elsevie r Science B.V. All rights reserved.