CRYSTAL-STRUCTURES OF THE ETHYLENE AND ACETYLENE SORPTION COMPLEXES OF FULLY CA2-EXCHANGED ZEOLITE-X()

The crystal structures of the ethylene (Ca46Si100Al92O384.30C(2)H(4), a = 25.003(5) Angstrom) and acetylene (Ca46Si100Al92O384.30C(2)H(2), a = 25.013(5) Angstrom) sorption complexes of dehydrated fully Ca2+-exc hanged zeolite X have been determined by single-crystal X-ray diffract ion techniques in the cubic space group Fd (3) over bar at 21(1) degre es C. Their complexes were prepared by dehydration at 380 degrees C an d 2 x 10(-6) Torr for 2 days, followed by cooling and exposure to 300 Torr of ethylene or 250 Torr of acetylene gases, respectively, both at 21(1) OC. The structures were determined in these atmospheres and ref ined to the final error indices, R-1 = 0.050 and R-2 = 0.051 with 371 reflections, and R-1 = 0.049 and R-2 = 0.045 with 415 reflections, res pectively, for which I> 3 sigma(I). In each structure, as in dehydrate d Ca-46-X, 16 Ca2+ ions per unit cell fill site I at the centers of th e double six-rings, and the remaining 30 are at site II in the superca ges. Thirty ethylene or 30 acetylene molecules are sorbed, all in the supercages, one coordinating laterally to each site II Ca2+ ion. These Ca2+ ions are recessed ca 0.41 Angstrom into the supercage in the eth ylene sorption structure (0.42 Angstrom for acetylene) from the planes of their three nearest oxygens. The C-C bond lengths, 1.39(6) Angstro m for ethylene and 1.30(5) Angstrom for acetylene, cannot here be said to have been altered by sorption. The bonds between Ca2+ and ethylene /acetylene are relatively long (Ca2+-C = 2.98(4)/2.87(3) Angstrom, res pectively); they are ascribed to the polarization of the molecular pi- electron density by the electric field gradient at Ca2+.