Structure of Dehydrated Zn²⁺-Exchanged Zeolite X. Overexchange, Framework Dealumination and Reorganization, Stoichiometric Retention of Monomeric Tetrahedral Aluminate

Abstract

Complete Zn 2+ -exchange of a single crystal of zeolite X (Na 92 Si 100 Al 92 O 384, space group Fd 3̄) was attempted at 80 °C from aqueous solution (pH = 5.2 at 23 °C). After dehydration at 400 °C and ca. 1 × 10 -5 Torr for 2 days, its structure was determined by X-ray diffraction techniques in its new cubic space group Fd 3̄ m at 23 °C ( a = 24.718(6) Å). It was refined to the final error index R 1 = 0.119 with 183 reflections for which F o > 4σ( F o ). About 56 Zn 2+ ions occupy three crystallographic sites: 30 almost fill site I‘, 23 nearly three-quarters fill site II‘, and the remaining three occupy site II. Each sodalite unit contains a monomeric tetrahedral aluminate anion (Al−O = 1.79(6) Å) at its center; this appears to be the first accurate crystallographic observation of the monomeric tetrahedral aluminate ion. Many are HAlO 4 4- anions each coordinated by seven Zn 2+ ions: three of the four aluminate oxygens each bridge between a site-I‘ Zn 2+ and a site-II‘ Zn 2+ ion; the aluminate OH group coordinates only to a site-I‘ Zn 2+ cation. Other AlO 4 5- ions may have fewer Zn 2+ and more H + ions. Dealumination of the zeolite framework and its subsequent reorganization to remove the resulting vacancies occurred during the dehydration step, not during ion exchange. The number of unit cells in the crystal decreases in this process and long-range Si/Al ordering is lost, resulting in a change of space group.