CRYSTAL-STRUCTURE OF A BENZENE SORPTION COMPLEX OF DEHYDRATED FULLY CD2-EXCHANGED ZEOLITE-X()

The crystal structure of a benzene sorption complex of fully dehydrate d Cd2+-exchanged zeolite X, Cd46Si100Al92-O-384. 43C(6),H-6 (a=24.880( 6) Angstrom), has been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd (3) over bar at 21 degrees C. T he crystal was prepared by ion exchange in a flowing stream of 0.05 M aqueous Cd(NO3)(2) for 3 d, followed by dehydration at 400 degrees C a nd 2x10(-6) Torr for 2 d, followed by exposure to about 92 Torr of ben zene vapor at 22 degrees C. The structure was determined in this atmos phere and refined to the final error indices R-1=0.054 and R-w=0.066 w ith 561 reflections for which I>3 sigma(I). In this structure, Cd2+ io ns are found at four crystallographic sites: eleven Cd2+ ions are at s ite I, at the centers of the double six-oxygen rings; six Cd2+ ions li e at site I', in the sodalite cavity opposite to the double six-oxygen rings; and the remaining 29 Cd2+ ions are found at two nonequivalent threefold axes of unit cell, sites II' tin the sodalite cavity) and si te II (in the supercage) with occupancies of 2 and 27 ions, respective ly. Each of these Cd2+ ions coordinates to three framework oxygens, ei ther at 2.173(13) or 2.224(10) Angstrom, respectively, and extends 0.3 7 Angstrom into the sodalite unit or 0.60 Angstrom into the supercage from the plane of the three oxygens to which it is bound. The benzene molecules are found at two distinct sites within the supercages. Twent y-seven benzenes lie on threefold axes in the large cavities where the y interact facially with the latter 27 site-II Cd2+ ions (Cd2+-benzene center=2.72 Angstrom; occupancy=27 molecules/32 sites). The remaining sixteen benzene molecules are found in 12-ring planes; occupancy=16 m olecules/16 sites. Each hydrogen of these sixteen benzenes is ca. 2.8/ 3.0 Angstrom from three 12-ring oxygens where each is stabilized by mu ltiple weak electrostatic and van der Waals interactions with framewor k oxygens.