Crystal Structures of Encapsulates within Zeolites. 2. Argon in Zeolite A

Abstract

The positions of Ar atoms encapsulated in the cavities of fully dehydrated Cs 3 Na 8 H-A (Cs 3 -A) have been determined. Cs 3 -A was exposed to 660 atm of argon gas at 400°C, followed by cooling at pressure to encapsulate Ar atoms; a second crystal was treated similarly at 1000 atm. The resulting crystal structures, of Cs 3 -A(5Ar) (a ) 12.253(2) Å, R 1 ) 0.055, R 2 ) 0.056) and Cs 3 -A(6Ar) (a ) 12.253(2) Å, R 1 ) 0.064, R 2 ) 0.065), have been determined by single-crystal X-ray diffraction techniques in the cubic space group Pm3 hm at 21(1)°C and 1 atm. The five and six Ar atoms per unit cell, respectively, are distributed over three crystallographically distinct positions: one Ar atom at Ar(1) lies opposite a six-ring in the sodalite unit, two Ar atoms at Ar(3) occupy a similar position in the large cavity, and two or three Ar atoms in Cs 3 -A(5Ar) and Cs 3 -A(6Ar), respectively, are at Ar(2) opposite four-rings in the large cavity. Relatively strong interactions of Ar(1) and Ar(3) atoms with six-ring Na + ions are observed: Na-Ar(1) ) 3.26 and Na-Ar(3) ) 3.00 Å. The four Ar atoms in the large cavity of Cs 3 -A(5Ar) form a rhombus with Ar(2)-Ar(3) ) 4.75(8) Å and Ar(2)-Ar(3)-Ar(2) ) 88(1)°. The five Ar atoms in the large cavity of Cs 3 -A(6Ar) have a trigonal bipyramidal arrangement with three Ar(2) atoms at equatorial positions and two Ar(3) atoms at axial positions; Ar(2)-Ar(3) ) 4.63(9) Å. Both of the above arrangements are stabilized by alternating dipoles induced on Ar(2) and Ar(3) by four-ring oxygens and six-ring Na + ions, respectively.

Investigation of the possibility of low pressure encapsulation of carbon dioxide in potassium chabazite (KCHA) and sodium chabazite (NaCHA) zeolites

“…However, more encapsulation studies have been carried out quantitatively in the following decades for different gases on a wide range of zeolites, including Ar in 3A [27,28], Ar in 4A [25,29,30], Kr in 3A [28,31,32], Kr in 4A [25,29], Kr in 5A [32], Xe in 4A [33], He and Ne in 3A and 5A [34], Ne in heulandite and stilbite [35], H 2 in CsA [36,37], H 2 in 4A [38], H 2 in 5A [39], CH 4 in 4A [25,[40][41][42], N 2 in 3A [27], N 2 in CsA [36]. A similar successful encapsulation was made on some other low-porosity zeolites by quantitative measurements of Ar in sodalite and cancrinite [43], Kr in sodalite and cancrinite [43], and He and Ne in tridymite and cristobalite [44].…”

Crystal Structures of Encapsulates within Zeolites. 3. Xenon in Zeolite A

“…In recent years, the Kr 4, Ar 4 , and Ar 5 clusters, formed by confinement in the cavities of Cs 3 Na 8 H-A (Cs 3 -A), 26,27 were characterized crystallographically. 28,29 In the structures of Cs 3 -A(5Kr) and Cs 3 -A(5Ar), four Kr or Ar atoms in each large cavity form a rhombus with interatomic distances of 4.67 (3) ) 78(2)°. In these clusters, charge dipoles induced on the rare gas atoms by their interactions with the zeolite alternate around the ring, showing the effect of the electrostatic fields in the zeolite cavity.…”

“…2.2 Å. 1,12,21,31 However, in the crystal structures of Cs 3 -A(xAr), x ) 5 or 6, 29 and Cs 3 -A(5Kr), 28 one Ar (kinetic diameter ) 3.40 Å) 30 or Kr atom (3.60 Å) 30 was always and unambiguously found near the center of each sodalite unit, indicating that one or more dynamic processes exist for the passage of those atoms through six-rings, at least at elevated temperatures, even though their aperture is formally considered to be too small.…”

“…In this work, xenon atoms were encapsulated at various pressures in the cavities of fully dehydrated Cs 3 -A 26-29 in order to observe their positions and to further characterize the confinement effect 28,29 crystallographically. Both the Cs + and Na + ions of Cs 3 -A were expected to contribute to the effective encapsulation of Xe atoms by blocking the eight-and six-rings, respectively.…”

“…Both the Cs + and Na + ions of Cs 3 -A were expected to contribute to the effective encapsulation of Xe atoms by blocking the eight-and six-rings, respectively. 19,[27][28][29][32][33][34][35][36] Although weak interactions were expected, the precise coordinates of the encapsulated Xe atoms, sensitive to the electrostatic field in a relatively unperturbed zeolite, would be seen. Perhaps interesting clusters of Xe atoms in the large cavity, and a Xe atom despite its size in the sodalite unit, would be seen.…”