Structures of cobalt(II)-exchanged zeolite X

Abstract

Exhaustive Co 2+-exchange of single crystals of zeolite X (Na 92Si 100Al 92O 384) was attempted from aqueous solution at 23°C (crystal 1), 50°C (crystal 2), and 80°C (crystal 3). After partial dehydration at 23°C and ca. 10 −3 Torr for 3 days, their structures were determined by X-ray diffraction techniques in the cubic space group Fd3̄ at 23°C [ a=24.920(7), 24.950(9), and 24.905(5) Å, respectively]. They were refined to the final error indexes R 1=0.083, 0.125, and 0.131 with 549, 444, and 367 reflections, respectively, for which F o>4 σ( F o). The number of Co 2+, Na +, H 3O +, and OH − ions in the three crystal structures, respectively, are about 38, 24, 0, 8; 38, 11, 18, 13 and 46, 8, 0, 8 per unit cell. Thus, in no case were all Na + ions replaced by Co 2+, in all cases, overexchange due to uptake of OH − occurred, and in one case, H 3O + and OH − ions coexisted within the crystal structure: in no case did simple ion-exchange occur. In crystal 1, eight Co 2+ ions per unit cell are at site I′, 20 at site II, and the remaining 10 at site III′; three Na + ions are at site I, 16 at site I′, and the remaining five at site II. Each of eight OH − ions bridges between two site-I′ Na + ions and a site-I′ Co 2+. In crystal 2, 10 Co 2+ ions are at site I′, 15 at site II, and the remaining 13 at site III′; two Na + ions are at site I and nine are at site II; 10 H 3O + ions are at site I′ and eight are at site II′. In five of the eight sodalite cages, two site-I′ H 3O + ions and two site-I′ Co 2+ ions bind to a water molecule on a twofold axis to give a (H 3O +) 2H 2O(Co 2+) 2 cluster; the remaining three sodalite cages each contain only H 3O + ions. In crystal 3, Co 2+ ions occupy sites I′, II and III′ (16, 20 and 10 per unit cell, respectively), and eight Na + ions are found at site I. The I′ sites are occupied by only 16 Co 2+ ios to avoid electrostatic repulsion with these eight Na + ions. Half of these Co 2+ ions each coordinate additionally to a OH − ion. In all three crystals, Co 2+ ions prefer site II.