Structure of dehydrated Zn2+-exchanged zeolite X.: Overexchange, framework dealumination and reorganization, stoichiometric retention of monomeric tetrahedral aluminate

Complete Zn2+-exchange of a single crystal of zeolite X (Na92Si100Al92O384, space group Fd (3) over bar) was attempted at 80 degrees C from aqueous solution (pH = 5.2 at 23 degrees C). After dehydration at 400 degrees C and ca. 1 x 10(-5) Torr for 2 days, its structure was determined by X-ray diffraction techniques in its new cubic space group Fd (3) over bar m at 23 degrees C (a = 24.718(6) Angstrom). It was refined to the final error index R-1 = 0.119 with 183 reflections for which F-o > 4 sigma(F-o). About 56 Zn2+ 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) Angstrom) at its center; this appears to be the first accurate crystallographic observation of the monomeric tetrahedral aluminate ion. Many are HAlO44- anions each coordinated by seven Zn2+ ions: three of the four aluminate oxygens each bridge between a site-I' Zn2+ and a site-II' Zn2+ ion; the aluminate OH group coordinates only to a site I' Zn2+ cation. Other AlO45- ions may have fewer Zn2+ 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.