Structures of cobalt(II)-exchanged zeolite X

Exhaustive Co2+-exchange of single crystals of zeolite X (Na92Si100Al92O384 ) was attempted from aqueous solution at 23 degrees C (crystal 1), 50 degre es C (crystal 2), and 80 degrees C (crystal 3). After partial dehydration a t 23 degrees C and ca. 10(-3) Torr for 3 days, their structures were determ ined by X-ray diffraction techniques in the cubic space group Fd (3) over b ar at 23 degrees C [a=24.920(7), 24.950(9), and 24.905(5) Angstrom, respect ively]. 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 sig ma(F-o). The number of Co2+, Na+, H3O+, and OH- ions in the three crystal s tructures, respectively, are about 38, 24, 0, 8; 38, ii, 18, 13 and 46, 8, 0, 8 per unit cell. Thus, in no case were all Na+ ions replaced by Co2+, in all cases, overexchange due to uptake of OH- occurred, and in one case, H3 O+ and OH- ions coexisted within the crystal structure: in no case did simp le ion-exchange occur. In crystal 1, eight Co2+ ions per unit cell are at s ite I', 20 at site II, and the remaining 10 at site III'; three Na+ ions ar e at site I, 16 at site I', and the remaining five at site II. Each of eigh t OH- ions bridges between two site-I' Na+ ions and a site-I' Co2+. I, crys tal 2, 10 Co2+ 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 H3O+ ions are at site I' and eight are at site II'. In five of the eight sodalite ca ges, two site-I' H3O+ ions and two site-I' Co2+ ions bind to a water molecu le on a twofold axis to give a (H3O+)(2)H2O(Co2+)(2) cluster; the remaining three sodalite cages each contain only H3O+ ions. In crystal 3, Co2+ ions occupy sites II, II and III' (16, 20 and 10 per unit cell, respectively), a nd eight Na+ ions are found at site I. The I' sites are occupied by only 16 Co2+ ions to avoid electrostatic repulsion with these eight Na+ ions. Half of these Co2+ ions each coordinate additionally to a OH- ion. In all three crystals, Co2+ ions prefer site II. (C) 1999 Elsevier Science B.V. Ail nig hts reserved.