Crystal structure of an ammonia sorption complex of dehydrated fully Ca2+-exchanged zeolite X

The structure of an ammonia sorption complex of dehydrated fully Ca2+-exchanged zeolite X, Ca46Si100Al92O384. . similar to 135NH(3) (a=24.904(4) Angstrom), has been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd (3) over bar at 21(1)degrees C. The complex was prepared by dehydration of a single crystal of Ca-46-X at 380 degrees C and 2 x 10(-6) Torr for 2 days, followed by exposure to 300 Torr of ammonia gas at 21(1)degrees C. The structure was determined in this atmosphere and was refined to the final error indices R-1 = 0.074 and R-2 = 0.069 with 332 reflections for which 1> 3 sigma(1). Ca2+ ions are located at two crystallographic sites. Sixteen Ca2+ ions fill the octahedral sites I at the centers of the hexagonal prisms (Ca-O=2.408(8) Angstrom). The remaining 30 Ca2+ ions are at sites II; each extends 0.87 Angstrom into the supercage tan increase of 0.57 Angstrom upon NH3 sorption) where it coordinates octahedrally to three framework oxygens at 2.382(7) Angstrom and to three ammonia molecules at 2.75(2) Angstrom. Each of the remaining 45(3) NH3 molecules hydrogen bonds via its lone pair to two coordinating NH3 molecules and is in position to associate weakly with two or more framework oxygens. To avoid an impossibly short N ... N contact, this position has a maximum occupancy of 48 per unit cell; it may be full. (C) 1999 Elsevier Science B.V. All rights reserved.