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 so lution (pH = 5.2 at 23 degrees C). After dehydration at 400 degrees C and c a. 1 x 10(-5) Torr for 2 days, its structure was determined by X-ray diffra ction techniques in its new cubic space group Fd (3) over bar m at 23 degre es 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+ io ns occupy three crystallographic sites: 30 almost fill site I', 23 nearly t hree-quarters fill site II', and the remaining three occupy site II. Each s odalite unit contains a monomeric tetrahedral aluminate anion (Al-O = 1.79( 6) Angstrom) at its center; this appears to be the first accurate crystallo graphic observation of the monomeric tetrahedral aluminate ion. Many are HA lO44- anions each coordinated by seven Zn2+ ions: three of the four alumina te 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 fra mework and its subsequent reorganization to remove the resulting vacancies occurred during the dehydration step, not during ion exchange. The number o f unit cells in the crystal decreases in this process and long-range Si/Al ordering is lost, resulting in a change of space group.