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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 15, Number 3
BKCSDE 15(3)
March 20, 1994 

 
Title
Crystal Structures of Fully Dehydrated Zeolite Cd6-A and of Rb13.5-A, the Product of its Reaction with Rubidium, Containing Cationic Clusters
Author
Se Bok Jang, Yang Kim, Karl Seff*
Keywords
Abstract
The crystal structures of Cd6-A evacuated at 2 x 10-6 Torr and 750 ℃ (a=12.216(l) Å), and of the product of its reaction with Rb vapor (a= 12.187(l) Å), have been determined by single-crystal x-ray diffraction techniques in the cubic space group Pm3m at 21(l) ℃. Their structures were refined to the final error indices, R1=0.055 and R2=0.067 with 191 reflections, and R1=0.066 and R2=0.049 with 90 reflections, respectively, for which I>3σ(I). In dehydrated Cd6-A, six Cd2+ ions are found at two different threefold-axis sites near six-oxygen ring centers. Four Cd2+ ions are recessed 0.50 Å into the sodalite cavity from the (111) plane at O(3), and the other two extend 0.28 Å into the large cavity from this plane. Treatment at 250 ℃ with 0.1 Torr of Rb vapor reduces all Cd2+ ions to give Rb13.5-A. Rb species are found at three crystallographic sites: three Rb+ ions lie at eight-oxygen-ring centers, filling that position, and ca. 10.5 Rb+ ions lie on threefold axes, 8.0 in the large cavity and 2.5 in the sodalite cavity. In this structure, ca. 1.5 Rb species more than the 12 Rb+ ions needed to balance the anionic charge of zeolite framework are found, indicating that sorption of Rb0 has occurred. The occupancies observed can be most simply explained by two "unit cell" compositions, Rb12-A·Rb and Rb12-A·2Rb, of approximately equal population. In sodalite cavities, Rb12-A·Rb would have a (Rb2)+ cluster and Rb12-A·2Rb would have a triangular (Rb3)+ cluster. Each of the atoms of these clusters must bind further through a six-oxygen ring to a large cavity Rb+ to give (Rb4)3+ (linear) and (Rb6)4+ (trigonal). Other unit-cell compositions and other cationic cluster compositions such as Rb8n+ may exist.
Page
236 - 241
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