CRYSTAL-STRUCTURE OF AN ACETYLENE SORPTION COMPLEX OF DEHYDRATED FULLY CD2-EXCHANGED ZEOLITE-X()

The crystal structure of an acetylene sorption complex of dehydrated f ully Cd2+-exchanged zeolite X, Cd46Si100Al92O384. 28C(2)H(2) [a = 24.8 74(5) Angstrom], has been determined by single-crystal X-ray diffracti on techniques in the cubic space group Fd (3) over bar at 21(1)degrees C. Cd-46-X was dehydrated at 450 degrees C and 2 x 10(-6) Torr for 2 d, followed by exposure to ca. 250 Torr of acetylene gas at 21(1)degre es C. The structure was determined in this atmosphere and was refined to the final error indices R-1 = 0.058 and R-2 = 0.067 with 661 reflec tions for which I > 3 sigma(I). In this structure, Cd2+ ions are found at three crystallographic sites: 13 Cd2+ ions are at site I, at the c enters of the double six-oxygen rings; five Cd2+ ions lie at site I', in the sodalite cavity opposite the double six-oxygen rings; and the r emaining 28 Cd2+ ions are at site II, in the supercage near single six -oxygen rings. Each of these 28 is recessed 0.46 Angstrom into the sup ercage where it coordinates laterally to an acetylene molecule (C=C = 1.22(7) Angstrom). These Cd2+ ions have a distorted tetrahedral enviro nment, 2.192(8) A from three framework oxygens and 2.79(5) Angstrom fr om each carbon atom of an acetylene molecule (here counted as a monode ntate). As compared to dehydrated Cd-46-X, five Cd2+ ions have relocat ed from sites I and II to I', as they did in Cd-46-X . 28CO, primarily for electrostatic reasons. (C) Elsevier Science Inc. 1997.