13328
J. Phys. Chem. 1994, 98, 13328-13333
Crystal Structures of Encapsulates within Zeolites. 1. Krypton in Zeolite A Nam Ho Heo,* Kee Heon Cho, and Jong Taik Kim Department of Industrial Chemistry, Kyungpook National University, Taegu, 702-701 Korea
Karl Seff Department of Chemistry, University of Hawaii, Honolulu, Hawaii 96822-2275 Received: April 20, 1994; In Final Form: August 26, 1994@
Atoms of Kr were encapsulated in the cavities of fully dehydrated Cs3NagH-A by treatment with 635 atm of Kr at 400 "C for 5 days, followed by cooling at pressure. The crystal structures of the zeolite before (under vacuum) and after treatment (at 1 atm) were determined by single-crystal X-ray crystallography in the cubic space group Pm3m at 21 "C. In the crystal structures of dehydrated Cs3NagH-A ( a = 12.256(1) A, R1 = 0.054, R2 = 0.051) and Cs3NagH-A(SKr) ( a = 12.260(3) A, R1 = 0.050, R2 = 0.047), nearly three Cs+ ions per unit cell were found at the centers of 8-oxygen rings, each 3.39 A from four framework oxygens and 3.57 A from four others. Eight Naf ions per unit cell are located near the centers of 6-oxygen rings, each 2.29 8, from three framework oxygens. Encapsulated Kr atoms are found at three crystallographically distinct positions in Cs3NagH-A(5Kr). Each unit cell contains one Kr atom at a threefold-axis site in the sodalite unit (Kr(l)), two opposite 4-oxygen rings in the large cavity (Kr(2)), and two on threefold axes in the large cavity (Kr(3)). They interact weakly with the zeolite (and with each other) by polarization in its electrostatic field. The atom at Kr( 1) is near the center of a sodalite unit, consistent with a theoretical calculation of London dispersion energy in sodalite. Kr(1) is 3.39(4) 8, from a Na+ ion and 3.87(2) 8, from the three framework oxygens; Kr(3) has a similar environment with distances of 3.23(2) and 4.16(2) 8,. Each atom at Kr(2) is 3.81(3) 8, from four 4-ring oxygens. The four Kr atoms in the large cavity form a rhombus [-Kr(2)-Kr(3)-Kr(2)Kr(3)-] with Kr(2)-Kr(3) = 4.67(3) A and Kr(2)-Kr(3)-Kr(2) = 95.6(5)'. The charge dipoles induced on the Kr(2) and Kr(3) atoms by their interactions with the zeolite alternate around the rhombus.
Introduction Gas molecules with kinetic diameters somewhat larger than those of zeolite windows can nonetheless be sorbed by heating the zeolite.1,2 At high pressures, large quantities can be sorbed at equilibrium. This gas can be entrapped (encapsulated) by quenching to ambient temperature while the pressure is m a i ~ ~ t a i n e d . Unlike ~ - ~ chemi- or physisorbed gas molecules: the encapsulated gas molecules in the zeolitic cavities can maintain high-pressure concentrations without leakage at room temperature. These gases can be controllably released by the relaxation of window blockage by reheating the zeolite and/or by exposing the zeolite to small polar molecules (decaps~lation).~-'~ Encapsulation of argon (kinetic diameter = 3.40 A) and krypton (3.60 A)'' into the 14-hedral sodalite-type cages in zeolites A and X was first examined by G . A. Cook;12 these atoms must enter through 6-ring windows of free diameter ca. 2.2 A. A number of qualitative experimental and theoretical studies on the encapsulation and diffusion of such gases in zeolites and other aluminosilicates were performed by R. M. Barrer and D. E. W. V a ~ g h a n . ~ . ' ~Since - ' ~ Fraissard's pioneering work17J8xenon (kinetic diameter = 3.96 A)'' sorbed into various zeolites has also been studied by NMR spectroscopy to probe the internal structure of cavities in zeolite A, 19-22 zeolite Rh0,18323,24 and other zeolite~.2',~~-~8 In a series of encapsulation studies for developing a storage medium for small nonpolar gas molecules, including hydrogen and methane, it was found that both cavities (the a-and @-cages) of zeolite A can be utilized as microcontainers. For this purpose, it is necessary that the 8-ring windows of zeolite A be blocked @Abstractpublished in Advance ACS Abstracts, November 15, 1994.
by large monopositive cations such as Cs+, Rb+, or K+.29-35 In these cases, the concept of effective window diameter involves the mobility of the blocking cations, which is sensitive to temperature. Experimental and theoretical studies of the electrostatic fields in the cavities of zeolites, and their effects on sorbed molecules, have been conducted for several decades by many zeolite s ~ i e n t i s t s . ~ ~ Structural , ~ ~ ~ ~ ~information, -~* i.e the positions of sorbed atoms and molecules within zeolites, is needed for the development of theoretical methods. An understanding of the electrostatic fields within zeolites is crucial for understanding the equilibrium positions and dynamic behavior of sorbed atoms and molecules. In this work, we sought to entrap krypton atoms in the cavities of fully dehydrated Cs3NagH-A and to identify their sorption sites within the zeolite crystallographically. Although only weak interactions are expected, precise Kr coordinates, sensitive to the electrostatic field in a relatively unperturbed zeolite, would be seen. Perhaps interesting clusters of Kr atoms would be found.
Experimental Section Colorless crystals of Na12SilzAl1~04s*27HzO (Na12-A-27H20)~~ were prepared by Charnell's method.40 Ion exchange of Cs+ was carried out dynamically (flowing stream) at 20 "C for 2 days with an aqueous solution, 0.03 M in Cs+ and 0.07 M in Na+ (pH = 5.7) made using CsN03 and NaNO3 (both Aldrich 99.99%). This solution composition was chosen so that all 8-ring sites would be occupied by Cs+ ions.33 A single crystal (crystal l), a cube 80 p m on an edge, was lodged in a fine Pyrex capillary on a vacuum line. After cautious increases in
0022-3654/94/2098-13328$04.50/0 0 1994 American Chemical Society
Krypton in Zeolite A
J. Phys. Chem., Vol. 98, No. 50, 1994 13329
TABLE 1: Positional, Thermal, and Occupancy Paramete" atom
Wyckoff position
Y
x
Z
Bllb
822
833
812
813
823
occupancyc fixed varied
0 0 0 3(5) 69(5) 0
7(2) 0 28(9) 3(5) 69(5) 0
24d 12 12 24 8 8.26(9) 2.86 2.861(14)
(a) CssNasH-A (crystal 1) (Si, Al) O(1) O(2) o(3) Na
cs (Si, Al) O(1) O(2) o(3) Na
cs Kr(1) W2) W3)
2
4
0 0 0
~
W)
12(i) 2 4 8(g) 3(c)
24w Wh) 12(i) 2 4 8(g) 3(c) 8(g) 120') 8(g)
~
1119(3) 2025(4)
0
~
1832(2) 2228(7) 2944(5) 1119(3) 2025(4) 500W
3710(2)
5000' 2944(5) 3393(4) 2025(4)
5
W
22(1) 58(7) 69(7) 38(2) 59(2) 112(2)
19(1) 51(6) 24(3) 38(2) 59(2) 83(1)
12(1) 19(5) 24(3) 37(4) 59(2) 83(1)
0 0 0 9(7) 69(5)
0
(b) CssN%H-A(SKr)(crystal 2) 0 1834(2) 3710(2) 21(1) 18(1) 9(1) 0 0 2237(7) 5W 59(7) 42(6) 25(6) 0 0 2934(5) 2934(5) 63(7) 31(4) 31(4) 0 1125(3) 1125(3) 3396(5) 36(3) 47(5) -11(7) 36(3) 2002(4) 2002(4) 2002(4) 63(2) 63(2) 63(2) 68(6) 5W 0 5000' 122(3) 81(1) 81(1) 0 407(16) 407(16) 407(16) 433(37) 433(37) 433(37) -3(64) 3004(16) 3004(16) 5000' 252(18) 252(18) 439(41) -79(55) 3522( 15) 3522(15) 3522( 15) 823(27) 823(27) 823(27) -624(39)
0
4(3)
24d 12 57(11) 12 -2(6) 24 68(6) 8 0 2.86 -3(64) 1 0 2 -624(39) 2
0
0
0 -2(6) 68(6)
0 -3(64)
0
-624(39)
8.13(9) 2.863(14) 1.02(4) 2.03(7) 2.03(9)
a Positional and anisotropic thermal parameters are given x 10". Numbers in parentheses are the estimated standard deviations in the units of Bl3hl+ the least significant figure for the corresponding parameter. The anisotropic temperature factor is exp[-@llh2 &kZ + 83312 ,&hk &kZ)]. Root mean square displacements can be calculated from pii values using the formula pi = 0.255u@i8)lnwhere u = 12.260 A. Occupancy factors are given as the number of atoms or ions per unit cell. Occupancy for (Si) = 12, occupancy for (Al) = 12. e Exactly 0.5 by symmetry.
+
+
+
temperature of 25 " C h and following complete dehydration at of each reflection (Fo)was then obtained as the square root of 350 "C and 1 x Torr for 4 days, the crystal-containing Zraw after correction for Lorentz-polarization (Lp); the contribucapillary still under vacuum was sealed off from the vacuum tion of the monochromator crystal was calculated assuming it line at room temperature. A second crystal (crystal 2) of similar to be half-perfect and half-mosaic in character. Standard dimensions was lodged in a fine capillary with both ends open. deviations (a(Fo)) of observed structure factors were assigned to individual reflections by the formula (~(1)~@Fo2))"*/2FO, This capillary was transferred to a high-pressure line connected to the vacuum line. After complete dehydration at 400 "C and where a(Z) is the standard deviation, based on counting statistics, 1x Torr for 8 days, sorption of Kr into the crystal was of Iraw.The value p = 0.04 was found to be appropriate for carried out at 400 "C with 635 atm of Kr (Union Carbide, the instrumentation used. Absorption corrections @R ca. 0.18 99.999%) for 5 days. Encapsulation was accomplished by and 0.42 for crystals 1 and 2,43 respectively) were judged to be negligible for both crystals, since semiempirical q-scans showed cooling at pressure to room temperature with an electric fan. only negligible fluctuations for several reflections. Only those Following release of Kr gas from the line, both tips of the reflections in each final data set for which the net count exceeded capillary were presealed with vacuum grease under nitrogen 3 times its standard deviation were used in structure solution before being completely sealed with a small torch. No changes were noted in the appearance of the crystals upon examination and refinement. This amounted to 283 and 236 reflections for under the microscope. crystals 1 and 2, respectively. The cubic space group Pm3m (no systematic absences) was Determination used throughout this work for reasons discussed p r e v i ~ u s l y . ~ ~ , ~Structure ~ A CAD4/Turbo diffractometer equipped with a rotating anode Dehydrated CsjNasH-A (Crystal 1). Full-matrix leastgenerator and a graphite monochromator was used for prelimisquares refinement44 was initiated with the atomic parameters nary experiments and for the subsequent collection of diffraction of all framework atoms [(Si, Al), 0(1), 0(2), and 0(3)], Cs+ at intensities, all at 21(2) "C. Mol bdenum radiation (Kal, 2 = Cs( l), and Na+ at Na( 1) in C~s.sNa3.5-A.0.5Cs.4~ The refinement 0.709 30 A; Ka2,L = 0.713 59 ) was used. In each case, the with anisotropic thermal parameters for all atoms converged cell constant, a = 12.256(1) and 12.260(3) A for crystals 1 and quickly to error indices of R1 = CIFo - lFcllEF,, = 0.0535 2, respectively, was determined by a least-squares treatment of and R2 = (cw(F, - IFcl)2/CwFo2)1n = 0.0510 with occupancies 15 intense reflections for which 20 < 20 < 30". The w - 20 of 2.869(14) and 8.26(9) for Cs and Na, respectively. Extensive scan technique was used for data collection. Each reflection but unsuccessful efforts were made to locate the twelfth cation was scanned at a constant scan speed of 0.5 deg/min in 20 with necessary for electroneutrality at the usual position opposite a a scan width of (0.48 0.68 (tan 0))'. Background intensity 4-ring in the large cavity. The final cycles of refinement with was counted at each end of a scan range for a time equal to a fixed occupancy of 8.0 (its maximum value by symmetry) half the scan time. The intensities of three reflections in diverse for Na converged to the error indices R1 = 0.0537 and R2 = regions of reciprocal space were recorded every three hours to 0.0514 with a resulting occupancy of 2.861(14) at Cs. Conmonitor crystal and instrumental stability. Only small random sidering the moderate amount of H+in the ion-exchange solution fluctuations of these check reflections were observed during the (pH = 5.7) and the small deviation from unity which may be course of data collection. The intensities of ail lattice points expected for Si/Al (perhaps 1 the unit cell formula of this for which 20 < 70" were recorded. crystd is taken to be Cs2.86Na8HX-Awith x ca. L3' For For both crystals, the raw intensities were calculated as Zraw simplicity, the notation Cs3NasH-A will be used. The number of Na+ ions may be somewhat greater than 8: the 0.14 8-rings = ATN(C - RB)/NPI, where C = total count, R = ratio of per unit cell not occupied by Cs+ may contain Na+, and an scan time to each background counting time (2.0), B = total back- ground count, NPI = ratio of fastest possible scan rate to additional unlocatable fraction may be present opposite 4-rings scan rate for each measurement (1.O), and ATN = attenuation in the large cavity. A final difference Fourier function was factor, respectively. The observed structure factor amplitude featureless except for a small peak of height 1.5 e/A3 at (0.25,
+
x
+
13330 J. Phys. Chem., Vol. 98, No. SO, 1994 TABLE 2: Selected Interatomic Distances (A) and Angles
Heo et al.
TABLE 3: Deviations of Atoms
(A) from the (111) Plane at
00)” (a) Cs3NasH-A lcrvstal 1’)
(b) Cs3NasH-A(5Kr)
(crvstal2’) (Si, Al)-0(1) 1.654(4) 1.657(4) (Si, A1)-O(2) 1.655(6) 1.65l(8) 1.675(4) (Si, A1)-O(3) 1.671(4) Na-O(3) 2.298(5) 2.288(6) 2.938(6) Na-O(2) 2.949(5 ) 3.397(9) cs-O( 1) 3.387(10) 3.563(5) 3.582(6) Cs-0(2) 3.87(2) ~r(1)-0(3) 4.41(1) Kr( 1)-0(2) 3.80(3) Kr(21-W) 3.81(2) ~2)-0(3) 4.47(2) Kr(2)-0(2) 4.16(2) KrW-00) 4.44(3) Kr(3)-0(2) 4.94(2) W3)-0(1) Kr( 1)-Na 3.39(4) Kr(2)-Na 4.06(1) 3.23(2) Kr(3)-Na Kr(2)-Cs 4.42(2) Kr(3)-Cs 5.02(2) 4.67(3) =(2)-~3) O(1)-(Si, A1)-O(2) 107.4(4) 107.8(4) O(1)-(Si, A1)-O(3) 112.0(2) 112.0(3) O(2)-(Si, A1)-O(3) 107.4(2) 106.9(2) O(3)-(Si, A1)-O(3) 110.2(3) 110.8(3) (Si, Al)-O(1)-(Si, Al) 145.8(6) 145.3(7) 159.1(3) 160.4(5) (Si, Al)-0(2)-(Si, Al) (Si, Al)-0(3)-(Si, Al) 143.5(3) 143.3(4) 0(3)-Na-0(3) 118.2(1) 118.8(1) Kr( 1)-Na-0(3) 83.6(5) Kr(3)-Na-0(3) 96.4(4) 95.6(5) Kr(2)-Kr(3)-Kr(2) 84.4(5)’ Kr( 3) -Kr(2)-Kr(3) The numbers in parentheses are the estimated standard deviations in the units of the least significant digit given for the corresponding parameters. Required to be the supplement of the Kr(2)-Kr(3)-Kr(2) angle.
’
0.25, 0.25). This position refined as less than one oxygen per unit cell and was too close (ca. 1.01 8,) to Na, so it was omitted. The final structural parameters are given in Table la. Selected interatomic distances and angles are given in Table 2. CssNasH-A(SKr) (Crystal 2). Full-matrix least-squares refinement began with the atomic parameters of dehydrated Cs3NagH-A (crystal 1). The refinement with anisotropic thermal parameters for all atomic types in this model converged to the error indices R1 = 0.103 and RZ = 0.171 with occupancies of 2.85(4) and 7.1(2) for Cs and Na, respectively. Introducing a Kr atom at a peak (0.023, 0.023, 0.023, inside the sodalite unit) found in a difference Fourier function prepared from this model caused convergence to R1 = 0.093 and R2 = 0.127 in the following refinement with a resulting occupancy of 1.56(9) at Kr(1). A peak (0.282, 0.282, 0.5) found in a subsequent difference Fourier function was introduced at Kr(2). This model converged to error indices of R1 = 0.079 and R2 = 0.1 12, with resulting occupancies of 1.58(8) and 1.19(4) for isotropically refined Kr(1) and Kr(2), respectively. Addition of another peak (0.361, 0.361, 0.361) at Kr(3) with an isotropic thermal parameter reduced the error indices to R1 = 0.0536 and RZ = 0.0515 with occupancies of 2.83(1), 8.02(10), 1.01(4), 2.01(7), and 2.09(9) for Cs, Na, Kr(l), Kr(2), and Kr(3), respectively. When anisotropic thermal parameters were used for all atoms, the refinement converged to R1 = 0.0496 and Rz = 0.0464 with occupancies of 2.863(14), 8.13(9), 1.02(4), 2.03(7), and 2.03(9), respectively. A refinement with extinction coefficient varied gave the same result with better thermal parameters. Final cycles of refinement with occupancies fixed at 2.86, 8.0, 1.0,
(a) Cs3NasH-A
(b) CssNasH-A(SKr)
(crystal 1) 0.31
(crystal 2) Na 0.26 -3.13’ Kr(1) 3.48 Kr(3) A negative deviation indicates that the ion lies on the same side of the plane as the origin i.e., inside the sodalite unit. 0.86 8, from the origin.
’
2.0, and 2.0 for Cs, Na (as for crystal l), and Kr(i), i = 1-3, respectively, converged to the error indices R1 = 0.0497 and Rz = 0.0465. A small peak appeared at the same position as in crystal 1 in an otherwise featureless difference Fourier function. It was examined and dismissed as before. The final structural parameters are given in Table lb. Selected interatomic distances and angles are given in Table 2. The values of the goodness-of-fit, (Cw(Fo- IFcl)2/(m- s))”’, are 1.45 and 1.51; the number of observations, m, is 283 and 236, and the number of Parameters, s, is 27 and 37 for crystals 1 and 2, respectively. All shifts in the final cycles of refinement for both crystals were less than 0.1% for their corresponding estimated standard deviations. The quantity minimized in leastsquares is Cw(F, - IFC1)*, and the weights (w)are the reciprocal squares of a(Fo),the standard deviation of each observed structure factor. Atomic structure factors for Cs+, Kr, Na+, 0-, and (Si, Al)1.75+were ~ s e d . 4 ~The 5 ~ ~function describing (Si, Al)1.75+is the mean of the Si4+, Sio, A13+, and A1° functions. All scattering factors were modified to account for anomalous disper~ion.4~~~~
Discussion Framework and Cations of Zeolite A. The structural parameters of the framework atoms and cations are almost identical in Cs3Na8H-A and Cs3NasH-A(5Kr) (see Tables 1 and 2). This indicates the absence of extraframework atoms or molecules with strong ligational properties in the second structure. This close correspondence had never been seen before in dozens of pairs of structures, one empty and the other containing sorbed species. For example, the structure of the framework and the Na+ position in hydrated Cs3NagH-A are distinctly different, indicative of distortion, from those reported herein.51 In both structures, 2.86 Cs+ ion per unit cell nearly fill the centers of the three 8-rings (at equipoints of local symmetry C4h [& in Pm3m]), positions commonly found in partially or fully Cs+-exchanged zeolite A.45*52-54Each Cs+ ion is 3.397(9) A from four 0(1) oxygens and 3.563(5) 8, from four O(2) oxygens in CssNasH-A and is 3.387(10) and 3.582(6) 8, for the corresponding bonds in Cs3NasH-A(5Kr) (see interatomic distances in Table 2). Although these distances are substantially longer than the sum, 2.99 A, of the conventional ionic radii of 02- and Cs+, these positions are well established experand t h e o r e t i ~ a l l y . ~It~remains - ~ ~ possible that up to 3.0 - 2.86 = 0.14 Na+ ions per unit cell are also present to complete the occupation of all 8-rings. As in the crystal structure of dehydrated N ~ ~ z - Aeight : ~ Na+ ions per unit cell are located near the centers of the eight 6-rings per unit cell. Each Na+ ion is 2.298(5) 8, from three O(3) oxygens in Cs3NasH-A and 2.288(6) 8,from three O(3) oxygens in Cs3NasH-A(Sk) (see Table 2). These Na+ ions extend 0.31 and 0.26 A, respectively, into the large cavity from the (111) planes at O(3) (see Table 3). The 0(3)-Na-0(3) angles are
Krypton in Zeolite A
J. Phys. Chem., Vol. 98, No. 50, 1994 13331
Figure 1. Stereoview of a sodalite unit in CssNa&I-A(SKr),showing an encapsulated Kr atom near its center on a threefold axis. The zeolite A framework is drawn with light bonds between tetrahedrally coordinated (Si, Al) and oxygen atoms. The bonds between the Na+ ions and framework oxygens are indicated by fine solid lines. Ellipsoids of 20% probability are shown.
close to 120" (1 18.2(1) and 118.8(1)", respectively), showing formally too small, must exist at 400 "C and (likely) at that Na+ is nearly trigonal, quite different from its near somewhat lower temperatures. tetrahedral geometry in hydrated Cs3NagH-A and C S ~ N ~ ~ - A . ~ ' The closest approach of Kr(1) to a Na+ ion is 3.39(4) A, and to three O(3) oxygens, 3.87(2) A. These approach distances The twelfth cation per unit cell, because it could not be located crystallographically, is assumed to be, at least predomiare substantially longer than the sum of the atomic and ionic nantly, a H+ ion. It is Na+ or another alkali metal cation in radii of Kr and Na+ (2.02 0.97 = 2.99 A) and the sum of zeolite A ion-exchanged with alkali metal v a ~ o r or " ~with a basic those of Kr and 02-(2.02 1.32 = 3.34 A), respectively, solution of an alkali metal salt. Indeed, about 0.5 Na+ per unit indicating that Kr( 1) is weakly held. Kr( 1) is displaced 0.86 8, cell was found opposite a 4-ring in the large cavity when a from the center of the sodalite unit toward a Na+ ion, where it zeolite A crystal was ion-exchanged with a basic solution to can be polarized by the electrostatic field of the zeolite, avoiding prepare c~3Nas-A.~'However, in order to avoid the difficulties the center of the sodalite unit which by symmetry has no in locating it in the Kr-encapsulated crystal and to avoid the electrostatic field. Therefore, it can be concluded that there must possible perturbation that this cation might have on symmetric be an attractive force between the polarized atom at Kr( 1) and arrangements of Kr clusters in the large cavity (vide infra), Cs3the electrostatic field of the zeolite with an energy minimum at this position. NagH-A was used for this encapsulation study. The Cs+ ions in the 8-rings, the main windows to the large This agrees qualitatively with a theoretical calculation for Kr cavity, and Na+ ions near the centers of the 6-rings, the only in a sodalite-like cage.13 The contours of constant London windows to the sodalite unit, play important roles in controlling dispersion energy calculated in the presence of 6-ring oxygens the passage of molecules at high temperature through these and Na+ ions showed a deep minimum at (0.1, 0.1, 0.1). windows, making the encapsulation of gas molecules in these Because the center of the sodalite cage was the position of cavities p o s ~ i b l e . ~Percolation ~-~~ theory shows that fewer than minimum energy in similar calculations with no Na+ ions in 3.0 Cs+ ions, as few as 2.3 per unit cell, suffice to close the the 6-rings, this suggested an attractive force between Kr and a 8-windows to diffusion at ambient t e m p e r a t ~ r e . ~ ~ * ~ ~ 6-ring Na+ ion. Although the sodalite composition used in that work and that of CssNagH-A(SKr) are slightly the Krypton Atoms in Zeolite A. Kr atoms in Cs3NagH-A(Sk) Kr(1) position in this structure indicates that this Kr atom has are found at three crystallographically distinct positions. Each its primary interaction with a Na+ ion. unit cell contains one Kr atom at Kr(1) on a threefold axis in the sodalite unit, two at Kr(2) opposite 4-rings in the large Two Kr atoms at Kr(3) are located on threefold axes in the cavity, and two at Kr(3) on threefold axes in the large cavity. large cavity, at positions similar to those found in the lowThe closest approach distances of these Kr atoms to the temperature Kr sorption complex of c i ~ N a 4 - A .The ~ approach nonframework cations are 3.23(2) and 4.42(2) A to Na+ and distances of these Kr(3) atoms to 6-ring Na+ ions, 3.23(2) A, Cs+ ions, respectively, while that to framework oxygens is 3.80are somewhat shorter than the corresponding Kr(1) distance, (3) A (see Table 2). Considerin the radii60,61of the cations 3.39(4) A, as well as shorter than those found in the lowtemperature crystal structure of C m a - A ( l l K r ) , 3.48 A (Q~+ = 0.97 8, and res+ = 1.67 ), framework oxygens (1.32 A), and Kr atoms (2.02 A in solid Kr6* and 1.98 8, as r&2, (perhaps less accurate because powder data was used).6 To half of the distance for maximum attraction from the Lennardapproach a Na+ ion, a Kr atom at Kr(1) must simultaneously Jones potential"), all the Kr atoms are sufficiently far from their approach three O(3) oxygens of the 6-ring more closely than must an atom at Kr(3). This can be seen in the shorter Krneighbors to be considered as having no more than weak interactions with the cations and framework oxygens. Similarly, (1)-0(3) distance, 3.87(2) A, as compared to Kr(3)-0(3), inter-krypton distances of 4.67(3) A between Kr(2) and Kr(3) 4.16(2) A. These approaches may be repulsive at these short distances, and this may account for Kr(1)-Na+, 3.39(4) A, being atoms (vide inffa), somewhat larger than those in solid Kr, are slightly longer than Kr(3)-Na+, 3.23(2) A. Altematively, this found in the large cavity. may be described as being due to the stronger electrostatic field The location of an isolated Kr atom at Kr(1) on a threefold existing in the larger cavity, resulting in a shorter approach axis inside a sodalite unit is unambiguous (Figure 1). It is distance with stronger interactions between the polarized Kr impossible for some sodalite units to have zero Kr(1) atoms and others to have more than one at symmetry equivalent atoms and Na+ ions. The anisotropic thermal parameters at positions, to average to one, because impossibly short Kr( 1)Kr(3) are much larger than those at Kr(1) (see Table 1 and Figures 2 and 3), consistent with the more crowded environment Kr(1) distances would result. Therefore, every sodalite unit around Kr(1) because of its closer approaches to framework contains one (1.02(4)) Kr( 1) atom. A dynamic process for the passage of a Kr atom through a 6-ring, whose aperture is oxygens.
+ +
R
Heo et al.
13332 J. Phys. Chem., Vol. 98, No. 50, 1994
Figure 2. Stereoview of the large cavity of Cs3NasH-A(5Kr)with the most reasonable (except for orientation) arrangement of Kr atoms at Kr(2) and Kr(3). Inter-krypton contacts are indicated by fine lines. The four-krypton rhombus is planar. See the caption to Figure 1 for other details. 4-oxygen ring
Na’
4-oxygen ring
Figure 3. Schematic diagram of the rhombus of four Kr atoms in the large cavity of CssNasH-A(SKr). The immediate environment of each Kr atom and the dipole moment it induces on each Kr are shown. The interactions between the polarized Kr atoms are indicated by dashed
lines. Finally, two Kr(2) atoms, located opposite 4-rings in the large cavity, each approach two 0(1) oxygens and two O(3) oxygens with the same approach distances, 3.80(3) and 3.81(2) A, respectively. (Each Kr(2) must also interact with two Na+ i y s in adjacent 6-rings with interatomic distances of 4.06(1) A.) These distances are very similar to Kr(1)-0(3), 3.87(2) A, and are the shortest Kr-0 distances in the structure. This Kr(2) position is consistent with the results of Klinowski et al.,64 which indicate that the 4-ring is the most stable sorption site for xenon in dealuminated Nay. The Kr-Na+ interaction can be seen, on the basis of approach distances, to be somewhat stronger than the Kr-0 (framework) interaction. If the conventional 02-radius (1.32 A) is subtracted from the average of the twg Kr-0 distances (3.81 A), a “Kr interaction radius” of 2.49 A is found. If the Na+ radius (0.97 A) is subtracted from the mean (3.31 A) of the Kr(1)-Na+ and Kr(3)-Na+ distances (3.39 and 3.23 A, respectively), a smaller “Kr interaction radius”, 2.34 A, is found. (These “Kr radii” can not be compared directly with those calculated” and observed61 in solid Kr, 1.98 and 2.02 A, respectively.) This point may be made more simply but less reliably by noting that the mean Kr-Na+ distance is 0.5 A shorter than the mean Kr-0 distance (3.81 - 3.31 = 0.50 A). Kr(2) and Kr(3) atoms on the inner-surface wall of the large cavity may be placed within their partially occupied equipoints in various ways. The shortest possible inter-krypton distances Kr(2)-Kr(3) = 2.02 A, Kr(2)-Kr(2) = 3.46 A, and Kr(3)-
Kr(3) = 3.63 A are impossibly short and are not considered further. The next set of distances Kr(2)-Kr(3) = 4.67 A, Kr(2)-Kr(2) = 4.90 A, and Kr(3)-Kr(3) = 5.13 8, offer two solutions. (Longer distances, corresponding to atoms on opposite sides of the large cavity, require impossibly short distances to complete a model and are dismissed.) The four Kr atoms may form a rhombus [-Kr(2)-&(3)-Kr(2)-Kr(3)] with Kr(2)-Kr(3) = 4.67(3) A and Kr(2)-Kr(3)-Kr(2) = 95.6(5)”. In this planar four-Kr ring, Kr atoms alternately approach Na+ ions and four-oxygen rings and are polarized oppositely, allowing their inter-krypton approaches to be attractive (see Figure 3). Alternatively, the four Kr atoms may form an irregular puckered four-sided figure [-Kr(2)-Kr(3)Kr(3)-Kr(2)-] with bond lengths, beginning with Kr(2)-Kr(3), of 4.67, 5.13,4.67, and 4.90 A, respectively. This arrangement, with its lower symmetry, in which only half of the Kr-Kr interactions have their induced dipoles oriented favorably, is considered unlikely and is not discussed further. It is to achieve this arrangement of alternating dipoles that the four Kr atoms have selected two crystallographicallydistinct positions. Four Kr atoms arranged tetrahedrally at Kr(3) would have been further apart (ca. 5.13 A) with repulsive inter-krypton approaches among their induced dipoles. Four Kr atoms arranged in the form of a square at Kr(2) would have been ca. 4.89 8, apart, again with repulsive inter-krypton approaches. The existence of such clusterings of rare-gas atoms with their interatomic attractions, as well as those with the zeolite, has been suggested by N M R studies of 129Xeconfined in the cavities of zeolite A.19320For example, McCormick et al. interpreted their observed 129Xechemical shifts in terms of Xe-Xe and Xe-zeolite A interactions.20 Despite the change of rare-gas atom and the differences in the degree of rare-gas loading and in the cationic composition of zeolite A, both types of interactions, both Kr-Kr and zeolite-&, are seen in the structure of Cs3NagH-A(5Kr). Five Kr atoms per unit cell of Cs3NagH-A corresponds to an encapsulation capacity of 208 g (55.7 L at STP) of Kr per kilogram of dry Cs3Na8H-A.
Acknowledgment. N.H.H. gratefully acknowledges the support of the Central Laboratory of Kyungpook National University for the purchase of the diffractometer and computing facilities. We thank Professors H. K. Lee and J. R. Shon for their stimulating discussions. Supplementary Material Available: Observed and calculated structure factors for fully dehydrated Cs3NagH-A and Cs3NagH-A(SKr) (8 pages). Ordering information is given on any current masthead page.
Krypton in Zeolite A References and Notes (1) Breck, D. W. Zeolite Molecular Sieves: Structure, Chemistry, and Uses; John Wiley & Sons: New York, 1974; pp 623-628. (2) Fraenkel, D. CHEMTECH 1981, 11,60-65. (3) Barrer, R. M.; Vaughan, D. E. W. Trans. Faraday SOC.1971,67, 2129-2136. (4) Vansant, E. F. In Innovation in Zeolite Material Science; Grobet, P. J., et al., Eds.; Elsevier Science Publishers: Amsterdam, The Netherlands, 1980; pp 143-153. (5) Barrer, R. M.; Vaughan, D. E. W. Surf. Sei. 1969, 14, 77-92. (6) Seff, K. Ph.D. Thesis, M.I.T., 1964 and references therein. (7) Barrer, R. M.; Vansant, E. F.; Peeters, G. J . Chem. Soc., Faraday Trans. I 1978, 74, 1871-1881. (8) Thijs, A.; Peeters, G.; Vansant, E. F.; Verhaert, I. J . Chem. SOC., Faraday Trans. 1 1986, 82, 963-975. (9) Niwa, M.; Kato, S.; Hattori, T.; Marakami, Y. J . Chem. SOC., Faraday Trans. 11984, 80, 3135-3145. (10) Kwon, J. H. ME Thesis, Kyungpook National University, 1993. (11) Reference 1, PD 634-641. (12) Cook, G. A. Aigon Helium and the Rare Gases; Interscience: New York, 1961; Vol. 1, p 228. (13) Barrer, R. M:; Vaughan, D. E. W. J . Phys. Chem. Solids 1971,32, 731-743. (14) Barrer, R. M.; Vaughan, D. E. W. Trans. Faraday SOC.1967, 63, 2275-2290. (15) Barrer, R. M.; Gibbons, R. M. Trans. Faraday SOC.1%3,59,25692582. (16) Barrer, R. M.; Gibbons, R. M. Trans. Faraday SOC. 1%5,61,948961. (17) Fraissard, J.; Ito, T. Zeolites 1988, 8, 350-361. (18) Ito, T.; Fraissard, J. Zeolites 1987, 7, 554-558. (19) Jameson, C. J.; Jameson, A. K.; Gerald, R., II; de Dios, A. C. J . Chem. Phys. 1992, 96, 1690-1697. (20) McCormick, A. V.; Chmelka, B. F. Mol. Phys. 1991, 73, 603617. (21) Ito, T.; Fraissard, J. J . Chem. Phys. 1982, 76, 5225-5229. (22) Tsiao, C.; Corbin, D. R.; Dybowski, C. R. J . Phys. Chem. 1990, 94, 867-869. (23) Tsiao, C.; Kauffman, J. S.; Corbin, D. R.; Abrams, L.; Carroll, E. E., Jr.; Dybowski, C. R. J . Phys. Chem. 1991, 95, 5586-5591. (24) Gameson, I.; Wright, P. A.; Rayment, T.; Thomas, J. M. Chem. Phys. Lett. 1986, 123, 145-149. (25) Chmelka, B. F.; Pearson, J. G.; Liu, S. B.; deMenorval, L. C.; Pines, A. J . Phys. Chem. 1991, 95, 303-310. (26) Fraissard, J.; Ito, T.; Springuel-Huet, M.; Demarquay, J. In New Developments in Zeolite Science and Technology; Murakami, Y., Ed.; Elsevier: Amsterdam, The Netherlands, 1986; p 393. (27) Fomkin, A. A. In Zeolites: Facts, Figures, Future; Jacobs, P. A., van Santen, R. A., Eds.; Elsevier: Amsterdam, The Netherlands, 1989; p 953. (28) Cheung, T. T. P.; Fu, C. M.; Whany, S. J . Phys. Chem. 1988,92, 5 170-5 180. (29) Fraenkel, D.; Shabtai, J. J . Am. Chem. SOC.1977,99,7074-7076. (30) Fraenkel, D. J. Chem. SOC., Faraday Trans. I 1981, 77, 20292039. (31) Fraenkel, D.; Ittah, B.; Levy, M. J. Chem. Soc., Chem. Commun. 1984, 1391-1392.
J. Phys. Chem., Vol. 98, No. 50, 1994 13333 (32) Heo, N. H.; Yoon, J. H. J. Phys. Chem. 1992, 96, 4997-5000. (33) Heo, N. H.; Rho, B. R.; Kim, D. H.; Kim, J. T. Hwahak Konghak, 1991,29,407-416. (34) Heo, N. H.; Kim, D. H.; Kim, J. T. Hwahak Konghak 1991, 29, 717-726. (35) Heo, N. H.; Kwon, J. H.; Cho, K. H.; Kim, H. W.; Suh, S. H. Bull. Korean Chem. SOC.1993, 14, 583-588. (36) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular Sieves; Academic Ress: New York, 1978; Chapter 4 and references therein. (37) Barrer, R. M.; Peterson, D. L. Proc. R. SOC.1964, A280, 466485. (38) Barrer, R. M.; Klinowski, J. J. Chem. SOC.,Faraday Trans. I 1975, 71, 690-698. (39) The nomenclature refers to the contents of the Pm3m unit cell: e.g., Nal2-A represents NalzSil2A112048 and Cs3NaH-A represents Cs3NasHSi1z.42048. (40) Charnell, J. F. J . Cryst. Growth 1971, 8, 291-294. (41) Cruz, W. V.; Leung, P. C. W.; Seff, K. J . Am. Chem. SOC.1978, 100, 6997-7003. (42) Mellum, M. D.; Seff, K. J . Phys. Chem. 1984, 88, 3560-3563. (43) Intemational Tables for X-ray Crystallography; Kynoch Press: Birmingham, England, 1974; Vol. IV,pp 61-66. (44) Calculations were performed with:Structure Determination System, MolEN, Enrd-Nonius: The Netherlands, 1990. (45) Heo, N. H.; Seff, K. J . Am. Chem. SOC.1987, 109, 7986-7992. (46) Blackwell, C. S.; Pluth, J. J.; Smith, J. V. J . Phys. Chem. 1985, 89, 4420-4423. (47) Doyle, P. A.; Tumer, P. S. Acta Crystallogr., Sect. A 1968, 24, 390-397. (48) Reference 43, pp 73-87. (49) Cromer, D. T. Acta Crystallogr. 1965, 18, 17-23. (50) Reference 43, pp 149-150. (51) Cho, K. H.; Kwon, J. H.; Kim, H. W.; Park, C. S.; Heo, N. H. Bull. Korean Chem. SOC. 1994, 15,297-304. (52) Vance, T. B., Jr.; Seff, K. J. Phys. Chem. 1975, 79, 2163-2167. (53) Firor, R. L.; Seff, K. J . Am. Chem. SOC.1977, 99, 6249-6253. (54) Subramanian, V.; Seff, K. J . Phys. Chem. 1979, 83, 2166-2169. (55) Ogawa, K.; Nitta, M.; Aomura, K. J. Phys. Chem. 1978,82, 16551660. (56) Takaishi, T.; Hosoi, H. J . Phys. Chem. 1982, 86, 2089-2094. (57) Yanagida, R. Y.; Amaro, A. A.; Seff, K. J. Phys. Chem. 1973, 77, 805-809. (58) Fraenkel, D. J. Chem. SOC.,Faraday Trans. 1 1981, 77, 20412052. (59) Takaishi, T.; Yatsurugi, Y.; Yusa, A.; Kuratomi, T. J. Chem. Soc., Faraday Trans. I 1975, 71, 97-105. (60) Handbook of Chemistry and Physics, 64th ed; Chemical Rubber Co.: Cleveland, OH, 1983; p F-187. (61) Shannon, R. D.; Prewitt, C. T. Acta Crystallogr., Sect. B 1969, 825, 925-946. (62) Figgins, B. F.; Smith, B. L. Philos. Mag. 1960, 5, 186-188. (63) Barrer, R. M.; White, E. A. D. J. Chem. SOC. 1951, 1267-1278. (64) Anderson, M. W.; Klinowski, J.; Thomas, J. M. J . Chem. SOC., Faraday Trans. I 1986, 82, 2851-2862. ~~
