Crystal structure of a benzene sorption complex of dehydrated fully Ca2+-exchanged zeolite X

The crystal structure of a benzene sorption complex of fully dehydrated Ca2+-exchanged zeolite X, Ca46Si100Al92O384. 28C(6)H(6) (CI = 24.953(6) Angstrom), has been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd3 at 21 degrees C. The crystal was prepared by ion exchange in a flowing stream of 0.05 M aqueous Ca(NO3)(2) for 3 days, followed by dehydration at 400 degrees C and 2 x 10(-6) Torr for 2 days, followed by exposure to about 48 Torr of benzene vapor at 16 degrees C. The structure was determined in this atmosphere and refined to the final error indices R-1 = 0.051 and R-w = 0.049 with 358 reflections for which I > 3 sigma(I). Filling site I (at the centers of the double six-rings), 16 octahedrally coordinated Ca2+ ions (Ca-O = 2.399(7) Angstrom) are found per unit cell. The remaining 30 Ca2+ ions are found at two nonequivalent sites II tin the supercages) with occupancies of 8 and 22 ions. Each of these Ca2+ ions coordinates to three framework oxygens, either at 2.270(10) or 2.318(7) Angstrom, respectively, and extends either 0.24 or 0.55 Angstrom, respectively, 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. Twenty-two benzenes lie on 3-fold axes in the large cavities, where they interact facially with the latter 22 site-II Ca2+ ions (Ca2+-benzene center = 2.75 Angstrom; occupancy = 22 molecules/32 sites). The remaining six benzene molecules are found in 12-ring planes; occupancy = 6 molecules/16 sites. Each hydrogen of these six benzenes is ca. 3.0 Angstrom from three 12-ring oxygens.