Crystal structure of a zinc sorption complex of Cd2+-exchanged zeolite X containing tetrahedral Cd2+ 4(μ3-Zn0Cd2+Zn0)4 clusters

The sorption of Zn vapor at 480 degrees C onto fully dehydrated fully Cd2+-exchanged zeolite X (Cd46Si100Al92O384 per unit cell) has been studied at 23 degrees C by single-crystal X-ray diffraction in the cubic space group Fd (3) over bar (a(0) = 24.895(3) Angstrom). In this structure, Cd46Si100Al92O384. 20Zn, Cd2+ ions are found at five nonequivalent sites, redistributed significantly from 16, 0, 30, 0, 0 in dehydrated Cd-46-X to 12, 8, 12, 6, 8 (best integers) at sites I, I', II, III', and another III', respectively. The 8 Cd2+ ions at site I' and 8 of the 12 Cd2+ ions at site II have captured 16 Zn atoms to form two Cd-4(2+)(mu(3)-(ZnCd2+Zn0)-Cd-0)(4) clusters per unit cell. The remaining four Cd2+ ions at site II have captured the remaining (about four) Zn atoms. Each Cd-4(2+)(mu(3)-(ZnCd2+Zn0)-Cd-0)(4) cluster has at its center interpenetrating tetrahedra of (Cd2+)(4) and (Zn-0)(4) in the sodalite unit, with four terminal Cd2+-Zn-0 groups extending radially into the supercage from each of the above Zn-0 atoms; Cd2+ ions and Zn-0 atoms alternate throughout. The central four Zn-0 atoms form a tetrahedral Zn-4(0) cluster with Zn-Zn = 2.88(4) Angstrom, somewhat longer than the 2.66 Angstrom distance in Zn metal, due perhaps to delocalization of bonding density to adjacent Cd2+ ions. In each Cd-4(2+)(mu(3)-(ZnCd2+Zn0)-Cd-0)(4) cluster, each sodalite unit Cd2+ ion has a distorted octahedral environment, 2.566(11) Angstrom to three framework oxygens and 2.410(11) Angstrom to three Zn atoms; each Cd2+ ion at site II coordinates equatorially to three framework oxygens at 2.225(8) Angstrom and axially to two Zn atoms at 2.408(23) Angstrom and 1.97(5) Angstrom, respectively. Each sodalite unit Zn atom coordinates tetrahedrally to four Cd2+ ions; each terminal Zn atom interacts only with one Cd2+ ion. Surprisingly, no Cd2+ ions have been reduced by Zn atoms; all Cd2+ ions are at reasonable bonding distances to framework oxygens, and all Zn atoms are far from framework oxygens. About 12 Cd2+ ions remained at site I upon Zn sorption; each coordinates octahedrally to six framework oxygens at 2.430(10) Angstrom. About 6 + 8 = 14 Cd2+ ions have relocated to two III' sites, 2.46(5) Angstrom and 2.38(3) Angstrom, respectively, from framework oxygens.