CRYSTAL-STRUCTURE OF DEHYDRATED METHYLAMMONIUM-EXCHANGED ZEOLITE-A, (CH3NH3)(10)NA-2-A

The crystal structure of methylammonium-exchanged zeolite A dehydrated at 100 degrees C and 2 x 10(-6) Torr has been determined by single-crystal X-ray diffraction techniques in the cubic space group Pm $($) over bar$$ 3m at 22(1)degrees C (a = 12.280(2) Angstrom. (CH3NH3)(10)Na-2-A was prepared from Na-12-A by flow methods using aqueous 0.05 M methylammonium chloride. The structure was refined to the final error indices R(1) = 0.066 and R(2)(weighted) = 0.053, using 224 independent reflections for which I > 3 sigma(I). Methylammonium ions are found at three different crystallographic sites: one per unit cell is found on a threefold axis in the sodalite unit, six are on threefold axes opposite 6-oxygen rings in the large cavity, and three are associated with g-rings. The CH3NH3+ position with the highest occupancy shows the most acceptable geometry: C-N = 1.58(3) Angstrom and N...O = 2.72(1) Angstrom. The corresponding distances for the sodalite and 8-ring methylammonium ions are 1.24(7) and 3.15(8) Angstrom and 1.28(6) and 3.30(2) Angstrom, respectively. The two unexchanged Na+ ions remain nearly on 6-ring planes, where each coordinates to three oxygens. Two methylammonium ions must share a single 6-ring with C-N bonds collinear and N...N of ca.3.34 Angstrom. Further exchange of methylammonium has not occurred because the zeolite appears structurally to be fully packed.