Crystal structures of zeolite x exchanged by two different cations. Structures of Cd32Cs28-X and Cd28Rb36-X (X = Si100Al92O384)

Two anhydrous crystal structures of fully dehydrated Cd 2 + - and Cs + -exchanged zeolite X, Cd 3 2 Cs 2 8 Si 1 0 0 Al 9 2 O 3 8 4 (Cd 3 2 Cs 2 8 -X: a = 24.828(11) A) and fully dehydrated Cd 2 + - and Rb + -exchanged zeolite X, Cd 2 8 Rb 3 6 Si 1 0 0 Al 9 2 O 3 8 4 (Cd 2 8 Rb 3 6 -X: a = 24.794(2) A), have been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd3 at 21(1)°C. The structures were refined to the final error indices, R 1 = 0.058 and R 2 = 0.065 with 637 reflections for Cd 3 2 Cs 2 8 -X and R 1 = 0.086 and R 2 = 0.113 with 521 reflections for Cd 2 8 Rb 3 6 -X for which I > 3σ(I). In the structure of Cd 3 2 Cs 2 8 -X, 16 Cd 2 + ions fill the octahedral sites I at the centers of the double six rings (Cd-O = 2.358(8) A and O-Cd-O = 90.8(3)°). The remaining 16 Cd 2 + ions occupy site II (Cd-O = 2.194(8) A and O-Cd-O = 119.7(4)°) and six Cs + ions occupy site II opposite to the single six-rings in the supercage; each is 2.322 A from the plane of three oxygens (Cs-O = 3.193(13) and O-Cs-O = 73.0(2)°). About ten Cs + ions are found at site II', 1.974 A into the sodalite cavity from their three oxygen plane (Cs-O = 2.947(8) A and O-Cs-O = 80.2(3)°). The remaining 12 Cs + ions are distributed over site III' (Cs-O = 3.143(9) and O-Cs-O = 59.1(2)°). In the structure of Cd 2 8 Rb 3 6 -X, 16 Cd 2 + ions fill the octahedral sites I at the center of the double-six rings (Cd-O = 2.349(15) and O-Cd-O = 91.3(5)°). Another 12 Cd 2 + ions occupy two different II sites (Cd-O = 2.171(18)/2.269(17) A and O-Cd-O = 119.7(7)/113.2(7)°). Fifteen Rb + ions occupy site II (Rb-O = 2.707(17) A and O-Rb-O = 87.8(5)°). The remaining 21 Rb + ions are distributed over site III' (Rb-O = 3.001(16) A and O-Rb-O = 60.7(4)°). It appears that the smaller and more highly charged Cd 2 + ions prefer sites I and II in that order, and the larger Rb + and Cs + ions, which are less able to balance the anionic charge of the zeolite framework, occupy sites II and II' with the remainder going to the least suitable site in the structure, site III'. The maximum Cs + and Rb + ion exchanges were 30% and 39%, respectively. Because these cations are too large to enter the small cavities and their charge distributions may be unfavorable, cation-sieve effects might appear.