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-cr ystal X-ray diffraction techniques in the cubic space group Pm $($) ov er 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 met hylammonium chloride. The structure was refined to the final error ind ices R(1) = 0.066 and R(2)(weighted) = 0.053, using 224 independent re flections for which I > 3 sigma(I). Methylammonium ions are found at t hree different crystallographic sites: one per unit cell is found on a threefold axis in the sodalite unit, six are on threefold axes opposi te 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) Angst rom. The corresponding distances for the sodalite and 8-ring methylamm onium ions are 1.24(7) and 3.15(8) Angstrom and 1.28(6) and 3.30(2) An gstrom, respectively. The two unexchanged Na+ ions remain nearly on 6- ring planes, where each coordinates to three oxygens. Two methylammoni um 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 occurr ed because the zeolite appears structurally to be fully packed.