Crystal Structure of a Zinc Sorption Complex of Cd2+-Exchanged Zeolite X Containing Tetrahedral Cd2+4(μ3-Zn0Cd2+Zn0)4 Clusters

Abstract

The sorption of Zn vapor at 480 °C onto fully dehydrated fully Cd2+-exchanged zeolite X (Cd46Si100Al92O384 per unit cell) has been studied at 23 °C by single-crystal X-ray diffraction in the cubic space group Fd3̄ (a0 = 24.895(3) Å). In this structure, Cd46Si100Al92O384·20Zn, Cd2+ ions are found at five nonequivalent sites, redistributed significantly from 16, 0, 30, 0, 0 in dehydrated Cd46−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 Cd2+4(μ3-Zn0Cd2+Zn0)4 clusters per unit cell. The remaining four Cd2+ ions at site II have captured the remaining (about four) Zn atoms. Each Cd2+4(μ3-Zn0Cd2+Zn0)4 cluster has at its center interpenetrating tetrahedra of (Cd2+)4 and (Zn0)4 in the sodalite unit, with four terminal Cd2+−Zn0 groups extending radially into the supercage from each of the above Zn0 atoms; Cd2+ ions and Zn0 atoms alternate throughout. The central four Zn0 atoms form a tetrahedral Zn40 cluster with Zn−Zn = 2.88(4) Å, somewhat longer than the 2.66 Å distance in Zn metal, due perhaps to delocalization of bonding density to adjacent Cd2+ ions. In each Cd2+4(μ3-Zn0Cd2+Zn0)4 cluster, each sodalite unit Cd2+ ion has a distorted octahedral environment, 2.566(11) Å to three framework oxygens and 2.410(11) Å to three Zn atoms; each Cd2+ ion at site II coordinates equatorially to three framework oxygens at 2.225(8) Å and axially to two Zn atoms at 2.408(23) Å and 1.97(5) Å, 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) Å. About 6 + 8 = 14 Cd2+ ions have relocated to two III‘ sites, 2.46(5) Å and 2.38(3) Å, respectively, from framework oxygens.