Crystal Structure of Zeolite X Exchanged with Pb(II) at pH 6.0 and Dehydrated: (Pb4+)14(Pb2+)18(Pb4O4)8Si100Al92O384

Abstract

A single crystal of zeolite X was 39% overexchanged with aqueous Pb2+ at pH 6.0. After dehydration at 420 °C and 2 × 10-6 Torr, its crystal structure was determined by X-ray diffraction techniques in the cubic space group Fd3̄ at 21 °C (a = 25.238(5) Å). The structure, Pb64O32−Si100Al92O384 per unit cell, was refined to the final error indices R1 = 0.080 and R2 = 0.070 with 486 reflections for which I > 3σ(I). Pb4+ formed upon evacuation at elevated temperature by a reaction which may be written, per unit cell, as 64 Pb2+ + 36 OH- → 18 Pb2+ + 8 Pb4O4 + 14 Pb4+ + 14 H2 + 4 H2O. Pb ions occupy three nonequivalent sites. Each of 14 lead ions at site II is nearly in a six-ring plane where it coordinates to three framework oxygens at 2.32(2) Å and to a nonframework oxygen at 2.24(2) Å; these short distances indicate that these are Pb4+. Eighteen Pb2+ ions, also at site II, are 2.54(2) Å from three framework oxygens and 2.95(2) Å from a nonframework oxygen. Each sodalite cage is filled by a nearly cubic Pb4O4 cluster with Pb2+ and O2- ions alternating; the four Pb2+ ions (32 per unit cell) are at site I‘; each is 2.67(2) Å from three framework oxygens and 2.59(3) Å from three nonframework oxygens. Each Pb4O4 cube bonds radially via its oxygens through six-rings to four additional lead ions (Pb2+ and Pb4+) to give larger cationic clusters, perhaps predominantly of formula [Pb8O4]12+. The 18 Pb2+ ions which are external to the Pb4O4 cubes appear to have axial nonbonding lone pairs which extend into the supercage.