J. Phys. Chem. 1996, 100, 13725-13731
13725
Crystal Structures of Encapsulates within Zeolites. 2. Argon in Zeolite A Nam Ho Heo* and Woo Taik Lim Department of Industrial Chemistry, Kyungpook National UniVersity, Taegu, 702-701 Korea
Karl Seff* Department of Chemistry, UniVersity of Hawaii at Manoa, Honolulu, Hawaii 96822-2275 ReceiVed: January 22, 1996; In Final Form: March 22, 1996X
The positions of Ar atoms encapsulated in the cavities of fully dehydrated Cs3Na8H-A (Cs3-A) have been determined. Cs3-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 Cs3-A(5Ar) (a ) 12.253(2) Å, R1 ) 0.055, R2 ) 0.056) and Cs3-A(6Ar) (a ) 12.253(2) Å, R1 ) 0.064, R2 ) 0.065), have been determined by single-crystal X-ray diffraction techniques in the cubic space group Pm3hm 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 Cs3-A(5Ar) and Cs3-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 Cs3-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 Cs3-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.
Introduction Zeolites containing rare-gas atoms in their cavities are both ideal and realistic models for confined atoms.1-5 Atoms confined in zeolitic cavities behave as finite fluids and not as subvolumes in independent equilibrium with an infinite reservoir.4,5 A knowledge of the local structure adopted by such confined atoms is important in understanding the differences between the bulk and confinement-modified fluid properties, as they relate to transitions between phases, and to sorption and transport in such nanoporous materials. Large quantities of gas molecules can be encapsulated in zeolite cavities if their molecular kinetic diameters are somewhat larger than the effective diameters of the zeolite windows. This can be accomplished by heating the zeolite and gas at high pressure, followed by quenching to ambient temperature while the high pressure is maintained.6-10 These encapsulated gas molecules can sustain high-pressure concentrations without leakage at room temperature; in this way zeolites may be used as storage media. Controlled decapsulation can be achieved by relaxing the window blockage by reheating the zeolite, and/ or by exposing the zeolite to small polar molecules.11-14 The utilization of zeolites as a storage medium for small gas molecules was extensively examined, experimentally and theoretically, long ago by G. A. Cook,15 D. W. Breck,16 and R. M. Barrer et al.17-19 From their work on argon (kinetic diameter ) 3.40 Å) and krypton (3.60 Å),20 they concluded that the sodalite cages in zeolites A and X were acting as storage containers and that these atoms must enter through six-oxygenring windows of free diameter ca. 2.2 Å. It was subsequently shown that both cavities (the R- and β-cages) of zeolite A can be utilized as nanocontainers for small gas molecules if all eightX
Abstract published in AdVance ACS Abstracts, July 1, 1996.
S0022-3654(96)00217-1 CCC: $12.00
oxygen-ring windows are blocked by large monopositive cations such as Cs+, Rb+, or K+.9,10,21-25 Although the term encapsulation means a physical trapping of gas molecules within volumes such as zeolitic cavities, and is therefore different from sorption, some or all of the encapsulated molecules must interact by similar mechanisms, perhaps only weakly, with their host and with each other. The interactions involved must include those between the encapsulated molecules themselves in the electrostatic field of the zeolite as well as those between the encapsulated molecules and the inner surface of the zeolitic cavity. At high densities, the molecules are inevitably crowded within their limited volumes. Crowding may enhance induced dipoles, to give interaction energies among molecules which may therefore be greater than those of simply physisorbed molecules. From the point of view of adsorption, Derouane et al. have shown that the heat of adsorption in a zeolite cavity can be enhanced by the increased contact between a sorbed molecule and a wall with high curvature, giving rise to the concept of “confinement catalysis”.26 Recent Monte Carlo and molecular dynamics calculations have supported this view and have suggested that the confined guest species are even more densely packed than the bulk fluids.27 Moreover, the existence of such a “confinement effect,” emphasizing the intermolecular interactions between sorbed molecules, has been elucidated by a number of recent NMR studies with 129Xe atoms encapsulated in various zeolitic cavities.4,5,28 Recently, Kr4 clusters, formed by confinement in the cavities of Cs3Na8H-A (Cs3-A)29,30 were characterized crystallographically.31 In that structure, of Cs3-A(5Kr), four Kr atoms in the large cavity formed a rhombus (planar) with an inter-krypton distance of 4.67(3) Å and an angle of 95.6(5)°. In this rhombic ring of four Kr atoms, the charge dipoles induced on the Kr atoms by their interactions with the zeolite alternate around the © 1996 American Chemical Society
13726 J. Phys. Chem., Vol. 100, No. 32, 1996 ring, showing the effect of the electrostatic fields in the zeolite cavity. Since Fraissard’s pioneering work,32,33 xenon (kinetic diameter ) 3.96 Å)20 sorbed into various zeolites has been extensively studied by NMR spectroscopy to probe the confinement effect and the internal structure of cavities in zeolite A,5,28,34-36 zeolite Rho,33,37,38 and other zeolites.34,39-42 To further characterize the confinement effect, argon atoms were encapsulated in the cavities of fully dehydrated Cs3-A29-31 and their positions were observed crystallographically. Both the Cs+ and Na+ ions were expected to contribute to the encapsulation of Ar atoms by blocking the eight- and six-rings, respectively.7,9,10,21-23,30,31 Although only weak interactions were expected, the precise coordinates of encapsulated Ar atoms, sensitive to the electrostatic field in a relatively unperturbed zeolite, would be seen. Perhaps interesting clusters of Ar atoms would be found. Experimental Section Colorless single crystals of zeolite 4A, Na12Si12Al12O48‚ 27H2O (Na12-A‚27H2O),29 were synthesized by Charnell’s method.43 Crystals of hydrated Cs3-A (approximate composition Cs3Na8H-A) were prepared by dynamic (flow) ionexchange with an aqueous solution (pH ) 5.7), 0.04 M in Cs+ and 0.06 M in Na+ made by using CsNO3 and NaNO3 (both Aldrich 99.99%). This solution composition was carefully chosen so that all eight- and six-ring sites of the zeolite would be fully occupied by Cs+ and Na+ ions with occupancies of 3.0 and 8.0 per unit cell, respectively.30,31 A single crystal of hydrated Cs3-A (crystal 1), a cube 80 µm on an edge, was lodged in a fine Pyrex capillary with both ends open. This capillary was transferred to a high-pressure line connected to the vacuum line. After cautious increases in temperature of 25 °C/h under vacuum, followed by complete dehydration at 400 °C and 1 × 10-3 Torr for 2 days, forced sorption of Ar into the crystal was carried out at 400 °C for 2 days with 660 atm of Ar (Special Gas Co., 99.99%). Encapsulation was accomplished by cooling at pressure to room temperature with an electric fan. The encapsulation of Ar into a second crystal of Cs3-A (crystal 2) was carried out similarly, except with 1000 atm of Ar for four days. Following release of Ar gas from the line, both tips of the capillary were presealed with vacuum grease under nitrogen before being completely sealed with a small torch. No changes were noted in the appearance of the crystals upon examination under the microscope. The cubic space group Pm3hm (no systematic absences) was used in this work for reasons discussed previously.44,45 For each crystal, the same cell constant, a ) 12.253(2) Å at 21(1) °C, was determined by a least-squares treatment of 15 intense reflections for which 20 < 2θ < 30°. Each reflection was scanned at a constant scan speed of 0.5 °/min in 2θ with a scan width of (0.82 + 0.63*tanθ) and (0.56 + 0.71*tanθ)°, respectively, for crystals 1 and 2. Background intensity was counted at each end of a scan range for a time equal to half the scan time. The intensities of all lattice points for which 2θ < 70° were recorded. Absorption corrections (µR ca. 0.22 and 0.26 for crystals 1 and 2,46 respectively) were judged to be negligible for both crystals, since semi-empirical ψ-scans showed only negligible fluctuations for several reflections. Only those reflections in each final data set for which the net count exceeded three times its standard deviation were used in structure solution and refinement. This amounted to 274 and 201 reflections for crystals 1 and 2, respectively. Other crystallographic details are the same as previously reported.31 Finally, in order to obtain independent qualitative confirmation of the encapsulation content of the two crystals studied, an
Heo et al.
Figure 1. An argon-encapsulation isotherm for Cs2.7-A at various encapsulation pressures. A correction was made for the clay binder (nonzeolitic component) in the zeolite pellets used. See below for details.
argon encapsulation isotherm into pellets of similarly prepared Cs2.7-A was obtained by volumetric measurements of decapsulated argon gas (see Figure 1) using procedures reported previously.23,24 These encapsulation experiments were carried out at 400 °C for 30 min at each pressure. Structure Determination Cs3-A(5Ar) (Crystal 1). Full-matrix least-squares refinement47 was initiated with the atomic parameters of all framework atoms [(Si,Al), O(1), O(2), and O(3)], Cs+ at Cs, and Na+ at Na in Cs3Na8H-A.30,31 A refinement with anisotropic thermal parameters for all atoms converged quickly to the error indices R1 ) ∑|Fo - |Fc||/∑Fo ) 0.074 and R2 ) (∑w (Fo - |Fc|)2/ ∑wFo2 )1/2 ) 0.094 with occupancies of 2.94(3) and 7.8(2) for Cs and Na, respectively. A difference Fourier function base on this model revealed several peaks deep in the large cavity. A subsequent refinement of a peak on the threefold axis opposite a six-ring in large cavity as Ar(3) converged at (0.349, 0.349, 0.349) with the error indices R1 ) 0.067 and R2 ) 0.075 and occupancies of 2.91(3), 8.1(1), and 3.8(3) for Cs, Na, and Ar(3), respectively. When this model is refined with fixed occupancies of 3.0 and 8.0 (their maximum values by symmetry) for Cs and Na, respectively, the occupancy of Ar(3) converged to 3.6(3) with error indices R1 ) 0.066 and R2 ) 0.076. Inclusion of another peak opposite a four-ring in the large cavity as Ar(2) further reduced the error indices to 0.061 and to 0.069 with resulting occupancies of 1.8(2) and 1.8(2) at Ar(2) and Ar(3), respectively. Similarly, from the subsequent difference Fourier function based on a model with fixed occupancies of 2.0 at Ar(2) and 2.0 at Ar(3), a peak at (0.112, 0.112, 0.112) was introduced as Ar(1). This model converged to R1 ) 0.055 and R2 ) 0.056 with an occupancy of 1.2(1) for Ar(1). The final cycles of refinement with all occupancies fixed at 3.0, 8.0, 1.0, 2.0, and 2.0, respectively, for Cs, Na, and Ar(i), i ) 1-3, converged with no further changes in the error indices. Extensive but unsuccessful efforts were made to locate the 12th cation necessary for electroneutrality at the usual position opposite a four-ring in the large cavity. A final difference Fourier function was featureless. Considering the moderate amount of H+ in the ion-exchange solution (pH ) 5.7), and the small deviation from unity which may be expected for Si/ Al (perhaps 1.04),48 the unit cell formula of the zeolite itself is taken to be Cs3Na8Hx-A, x ca. 1.29 If the H ions were lost as water during crystal dehydration, the formula would be Cs3Na8-A with 0.5 framework oxygen absences per unit cell. For simplicity, the notation Cs3-A(5Ar) will be used for this crystal.
Argon in Zeolite A
J. Phys. Chem., Vol. 100, No. 32, 1996 13727
TABLE 1. Positional, Thermal, and Occupancy Parametersa occupancyd Wyckoff Position
x
y
z
b
c
β11 or Biso
β22
β33
β12
β13
β23
fixed
varied
12(1) 13(5) 32(4) 52(5) 87(1) 78(3)
0 0 0 -1(8) 0 116(6)
0 0 0 -1(6) 0 116(6)
5(3) 0 63(10) -1(6) 0 116(6)
24e 12 12 24 3 8 1 2 2
2.91(3) 8.1(1) 1.2(1) 1.8(2) 1.8(2)
6(5) 0 59(17) 1(10) 0 88(11)
24e 12 12 24 3 8 1 3 2
2.93(3) 7.6(2) 1.5(1) 2.8(3) 1.9(3)
(a) Cs3Na8H-A(5Ar), crystal 1 (Si,Al) O(1) O(2) O(3) Cs Na Ar(1) Ar(2) Ar(3)
24(k) 12(h) 12(i) 24(m) 3(c) 8(g) 8(g) 12(j) 8(g)
0 0 0 1126(3) 0 2022(5) 558(31) 3093(46) 3391(54)
1835(2) 2221(8) 2929(6) 1126(3) 5000f 2022(5) 558(31) 3093(46) 3391(54)
3713(2) 5000f 2929(6) 3399(5) 5000f 2022(5) 558(31) 5000f 3391(54)
22(1) 62(8) 64(8) 37(3) 128(3) 78(3) 16(4) 22(3) 45(5)
17(1) 55(7) 32(4) 37(3) 87(1) 78(3)
(b) Cs3Na8H-A(6Ar), crystal 2 (Si,Al) O(1) O(2) O(3) Cs Na Ar(1) Ar(2) Ar(3)
24(k) 12(h) 12(i) 24(m) 3(c) 8(g) 12(i) 12(j) 8(g)
0 0 0 1119(5) 0 2037(7) 0 3069(44) 3494(64)
1833(3) 2214(12) 2940(9) 1119(5) 5000f 2037(7) 706(41) 3069(44) 3494(64)
3709(3) 5000f 2940(9) 3376(7) 5000f 2037(7) 706(41) 5000f 3494(64)
18(2) 36(11) 68(13) 37(5) 110(4) 68(4) 7(3) 24(3) 43(7)
15(2) 59(12) 21(6) 37(5) 80(2) 68(4)
13(2) 10(9) 21(6) 37(8) 80(2) 68(4)
0 0 0 9(14) 0 88(11)
0 0 0 1(10) 0 88(11)
a Positional and anisotropic thermal parameters are given ×104. Numbers in parentheses are the estimated standard deviations in the units of the least significant figure given for the corresponding parameter. The anisotropic temperature factor is exp[-(β11h2 + β22k2 + β33l2 + β12hk + β13hl + β23kl)]. b Rms displacements can be calculated from βii values using formula µi ) 0.225a(βii)1/2, where a ) 12.253 Å. c Isotropic thermal parameters in units of Å2. d Occupancy factors are given as the number of atoms or ions per unit cell. e Occupancy for (Si) ) 12, occupancy for (Al) ) 12. f Exactly 0.5 by symmetry.
The final structural parameters are given in Table 1(a). Selected interatomic distances and angles are given in Table 2. Cs3-A(6Ar) (Crystal 2). Full-matrix least-squares refinement began with the atomic parameters of the zeolite framework and the cations in Cs3-A(5Ar) (crystal 1). Refinement with anisotropic thermal parameters for all of the atoms in this model converged to R1 ) 0.095 and R2 ) 0.123. A refinement with Ar(3) at a peak (0.347, 0.347, 0.347) in an ensuing difference Fourier function converged to R1 ) 0.083 and R2 ) 0.098, with resulting occupancies of 2.93(3), 7.6(2), and 4.8(4) for Cs, Na, and Ar(3), respectively. Another difference Fourier function based on a model with fixed occupancies of 3.0 and 8.0 (their maximum values by symmetry) for Cs and Na, respectively, revealed a peak (0.305, 0.305, 0.5) opposite a four-ring in the large cavity. The following refinement with this peak as Ar(2) converged to R1 ) 0.075 and R2 ) 0.087, resulting in occupancies of 2.8(3) and 2.0(3) for Ar(2) and Ar(3), respectively. A subsequent refinement including a peak found in the sodalite unit at (0.0, 0.067, 0.067) as Ar(1) with fixed occupancies of 3.0 and 2.0 for Ar(2) and Ar(3), respectively, further reduced the error indices to R1 ) 0.062 and R2 ) 0.062, with a refined occupancy of 1.5(1) at Ar(1). Considering the impossibly short Ar(1)-Ar(1) approach of 2.62(7) Å (as compared to the inter-argon distance of 3.84 Å in solid Ar49 and the kinetic diameter of 3.40 Å20) which would be necessary if there were more than one argon atom in a sodalite unit, the occupancy at Ar(1) was fixed at 1.0 in the final refinement, resulting in R1 ) 0.064 and R2 ) 0.065. The final difference Fourier function was featureless; the notation Cs3-A(6Ar) will be used for this crystal. The final structural parameters are given in Table 1(b). Selected interatomic distances and angles are given in Table 2. Attempts to resolve the sodium position, to distinguish between the two which associate with Ar(3) and the six which do not, were inconclusive for each structure. Therefore, because only the average Na position has been learned, the Na to Ar approach distances must be somewhat incorrect in both structures.
The values of the goodness-of-fit, (∑w(Fo - |Fc|)2/(m - s))1/2, are 1.66 and 1.98; the number of observations, m, are 274 and 201, respectively, for crystals 1 and 2. The number of parameters, s, is 32 for both crystals. All shifts in the final cycles of refinement for both crystals were less than 0.1% of their corresponding estimated standard deviations. The quantity minimized in least-squares is ∑w(Fo - |Fc|)2, and the weights (w) are the reciprocal squares of σ(Fo), the standard deviation of each observed structure factor. Atomic structure factors for Cs+, Ar, Na+, O-, and (Si,Al)1.75+ were used.50,51 The function describing (Si,Al)1.75+ is the mean of the Si4+, Si0, Al3+, and Al0 functions. All scattering factors were modified to account for anomalous dispersion.52,53 Discussion Zeolite A Framework and Cations. The structural parameters of the framework atoms and cations are remarkably almost identical in all of the following structures: Cs3-A(5Ar), Cs3A(6Ar), Cs3-A(5Kr), and empty Cs3-A.30,31 The occupancies of the Cs+ ions in the eight-rings of the two Ar encapsulates are slightly greater (closer to integral) than they were in Cs3-A and Cs3-A(5Kr), due to the (purposefully) higher Cs+/Na+ ratio in the ion-exchange solution used for the preparation of Cs3-A in this work. In both Ar-encapsulate structures, three Cs+ ions per unit cell fully occupy the centers of the eight-rings at equipoints of local symmetry C4h (D4h in Pm3hm), positions commonly found in partially or fully Cs+-exchanged zeolite A.30,31,54-57 Each Cs+ ion is 3.404(11) Å from four O(1) oxygens and 3.589(6) Å from four O(2) oxygens in Cs3-A(5Ar) and 3.414(17) and 3.570(9) Å for the corresponding bonds in Cs3-A(6Ar) (see Table 2). Although these distances are substantially longer than the sum, 2.99 Å, of the conventional ionic radii of O2- and Cs+, these positions are well established experimentally30,31,54-57 and theoretically.58,59 Eight Na+ ions per unit cell fully occupy the six-ring centers.60 Each Na+ ion is 2.293(7) Å from three O(3) oxygens
13728 J. Phys. Chem., Vol. 100, No. 32, 1996
Heo et al.
TABLE 2. Selected Interatomic Distances (Å) and Angles (deg)a (a) Cs3-A(5Ar) (crystal 1)
(b) Cs3-A(6Ar) (crystal 2)
(Si,Al)-O(1) (Si,Al)-O(2) (Si,Al)-O(3) Na-O(3) Na-O(2) Cs-O(1) Cs-O(2) Ar(3)-Na Ar(1)-Na Ar(2)-Na Ar(2)-Cs Ar(3)-Cs Ar(1)-O(3) Ar(1)-O(2) Ar(2)-O(1) Ar(2)-O(3) Ar(3)-O(3) Ar(3)-O(2) Ar(2)-Ar(3) Ar(2)-Ar(2)
1.646(4) 1.649(8) 1.675(4) 2.293(7) 2.934(6) 3.404(11) 3.589(6) 2.91(5) 3.11(3) 4.09(2) 4.45(6) 5.00(7) 3.62(4) 3.83(4) 3.94(7) 3.93(4) 3.93(6) 4.23(8) 4.75(8)
1.649(6) 1.652(9) 1.677(6) 2.285(9) 2.946(9) 3.414(17) 3.570(9) 3.09(6) 3.40(3) 4.05(3) 4.44(6) 5.01(9) 3.58(5) 3.87(4) 3.90(7) 3.92(5) 4.12(7) 4.39(2) 4.63(9) 5.80(9)
O(1)-(Si,Al)-O(2) O(1)-(Si,Al)-O(3) O(2)-(Si,Al)-O(3) O(3)-(Si,Al)-O(3) (Si,Al)-O(1)-(Si,Al) (Si,Al)-O(2)-(Si,Al) (Si,Al)-O(3)-(Si,Al) O(3)-Na-O(3) Ar(1)-Na-O(3) Ar(3)-Na-O(3) Ar(1)-Na-Ar(3) Ar(2)-Ar(3)-Ar(2) Ar(3)-Ar(2)-Ar(3)
109.0(4) 111.7(3) 106.7(2) 110.8(3) 146.6(7) 161.3(5) 143.3(2) 118.4(2) 82.7(4) 97.4(7) 180.0b 88(1) 92(1)b
108.4(6) 112.4(4) 106.8(3) 109.7(3) 147.1(12) 159.6(5) 142.5(5) 117.7(5) 79.4(7) 98.9(9) 172.5(8) 78(2) 87(2)
a The numbers in parentheses are the estimated standard deviations in the units of the least significant digit given for the corresponding parameter. b Required to be the supplement of the Ar(2)-Ar(3)-Ar(2) angle.
TABLE 3. Deviations of Atoms (Å) from the (111) Plane at O(3)a
Na Ar(1) Ar(3)
(a) Cs3-A(5Ar) (crystal 1)
(b) Cs3-A(6Ar) (crystal 2)
0.29 -2.81b 3.20
0.35 -2.65c 3.44
a A negative deviation indicates that the atom lies on the same side of the plane as the origin, i.e., inside the sodalite unit. b 1.18 Å from the origin (center of the sodalite unit). c 1.22 Å from the origin (center of the sodalite unit).
in Cs3-A(5Ar); 2.285(9) Å in Cs3-A(6Ar). These Na+ ions extend 0.29 and 0.35 Å, respectively, into the large cavity from the (111) planes at O(3) (see Table 3). The O(3)-Na-O(3) angles are close to 120° (118.4(2) and 117.7(5)° for crystals 1 and 2, respectively), so the Na+ ions are nearly trigonal-planar. As in the previously reported crystal structure of Cs3-A(5Kr), the 12th cation per unit cell, because it could not be located crystallographically, is assumed to be, at least predominantly, a H+ ion.30,31 Alternatively, it may have been lost as water (Vide supra). Argon Atoms in Cs3-A(5Ar) and Cs3-A(6Ar). The five and six argon atoms per unit cell of Cs3-A(5Ar) and Cs3A(6Ar), respectively, are found at three crystallographically distinct positions. By itself, this diversity of positions indicates that the Ar atoms are not arranging themselves by simple packing within the highly symmetric zeolites, to form, for example, a tetrahedron in the large cavity. This is attribute to
dipolar interactions among the sorbed atoms (Vide infra). In Cs3-A(5Ar), each unit cell contains one Ar atom at Ar(1) on a threefold axis opposite a six-ring in the sodalite unit, two at Ar(2) opposite four-rings in the large cavity, and two at Ar(3) on threefold axes opposite six-rings in the large cavity (see Figures 2 and 3). Similarly, each unit cell of Cs3-A(6Ar) has one Ar atom in the sodalite unit at Ar(1) (but near the intersection of two threefold axes), three (rather than two) at Ar(2), and two at Ar(3) as above (see Figure 4). The closest approaches of these Ar atoms to nonframework cations are 2.91(5) and 3.09(6) Å to Na+ and 4.45(6) and 4.44(6) Å to Cs+ ions, while those to framework oxygens are 3.62(4) and 3.58(5) Å, respectively, in Cs3-A(5Ar) and Cs3-A(6Ar) (see Table 2). Considering the radii of the cations (rNa+ ) 0.97 Å and rCs+ ) 1.67 Å), framework oxygens (1.32 Å), and Ar atoms (1.92 Å as rmin/220 and as found in the solid48), some of the Ar atoms are sufficiently close to their neighbors to be considered as having relatively strong interactions. In particular, when the distances are compared to the sum of the above radii for Na+ and Ar, 0.97 + 1.92 ) 2.89 Å, the approach distances of the threefold-axis argon atoms, Ar(3) and Ar(1), to the sixring Na+ ions, 2.91(5) and 3.11(3) Å, respectively, in Cs3A(5Ar), and 3.09 Å for Ar(3) in Cs3-A(6Ar), indicate direct interactions between Na+ ions and Ar atoms. In contrast, interargon distances of 4.75(8) and 4.63(9) Å between Ar(2) and Ar(3) in the corresponding large cavities (Vide infra) are nearly an Ångstrom larger than those in solid Ar. In each structure, an isolated argon atom at Ar(1) is found inside each sodalite unit. The occupancies from least-squares suggest that a second Ar(1) might be placed in some fraction of the sodalite units, especially in crystal 2. However doing so would lead to impossibly short Ar(1)-Ar(1) distances of 2.36 and 2.44 Å for Cs3-A(5Ar) and Cs3-A(6Ar), respectively. In Cs3-A(5Ar), Ar(1) on a threefold axis is 1.18 Å from the center of the sodalite unit where it can be polarized by the electrostatic field of the zeolite. This agrees well with a theoretical calculation which showed a similar position, about 1.5 Å from the center, to be an energy minimum for an Ar atom in a sodalite-like cage.8 The argon atom at Ar(1) in Cs3-A(6Ar) is at a position of different symmetry, approximately 0.73 Å away from the Ar(1) position in Cs3-A(5Ar). This must be a result of the higher loading of argon atoms in the large cavities, which modifies (by a small amount) the atomic positions of Cs3-A and, in turn, alters the electrostatic field in the sodalite unit. The closest approach distances of this Ar(1) to two six-ring Na+ ions and to two O(3) oxygens are 3.40(3) and 3.58(5) Å, respectively. The two argon atoms at Ar(3) in the large cavities of both Cs3-A(5Ar) and Cs3-A(6Ar) appear to interact much more strongly with six-ring Na+ ions than do those at Ar(1). In particular, the Ar(3)-Na+ interactions in Cs3-A(5Ar) are the shortest: these 2.91(5) Å distances are essentially the same as the sum of ionic and atomic radii of Na+ and Ar (2.89 Å). The corresponding ones in Cs3-A(6Ar) are somewhat longer, 3.09(6) Å, probably due to the additional interaction of each Ar(3) with a third argon atom at Ar(2) in the large cavity (Vide supra). Nonetheless, these distances are both shorter than the Ar(1)-Na+ distances, 3.11(3) and 3.40(3) Å in Cs3-A(5Ar) and Cs3-A(6Ar), respectively. Similarly, the large-cavity Kr atoms in Cs3-A(5Kr)31 interact more strongly with Na+ ions than do the small-cavity Kr’s. In contrast, the Ar(3)-O(3) distances (3.93(6) and 4.12(7) Å) are somewhat longer than Ar(1)-O(3), 3.62(4) and 3.58 Å, respectively. The other two argon atoms at Ar(2) located opposite four-rings in the large cavity of Cs3-A(5Ar) are also
Argon in Zeolite A
J. Phys. Chem., Vol. 100, No. 32, 1996 13729
Figure 2. A stereoview of a sodalite unit in Cs3-A(5Ar), showing an encapsulated Ar atom near its center on a threefold axis. The zeolite A framework is drawn with open bonds between oxygens and tetrahedrally coordinated (Si,Al) atoms. The interaction between Ar and Na+ is indicated by a fine solid line. Ellipsoids of 20% probability are shown.
Figure 3. A stereoview of the large cavity of Cs3-A(5Ar) with the only reasonable (except for orientation) arrangement of four Ar atoms at Ar(2) and Ar(3). The four-argon rhombus (nearly a square) is planar. The most significant interactions among Ar atoms, and those between Ar and Na+ ions and framework oxygens, are indicated by fine solid lines. Ellipsoids of 20% probability are shown.
Figure 4. A stereoview of the large cavity of Cs3-A(6Ar) with the only reasonable (except for orientation) arrangement of three Ar atoms at Ar(2) and two at Ar(3). The five argon atoms have a trigonal-bipyramid arrangement. See the caption to Figure 3 for other details.
long, 3.94(7) and 3.93(4) Å from their nearest oxygen atoms, O(1) and O(3), respectively; the corresponding distances for the three argon atoms at Ar(2) in Cs3-A(6Ar) are 3.90(7) and 3.92(5) Å. These approach distances are all substantially longer than the sum of ionic and atomic radii of O2- and Ar (3.24 Å), as in Cs3-A(5Kr).31 When the 2.91(5) Å Ar(3)-Na+ distance in Cs3-A(5Ar) and the 3.23(2) Å Kr(3)-Na+ distance in Cs3-A(5Kr) are compared to the corresponding sums of atomic and ionic radii, 2.89 and 2.99 Å for Ar-Na+ and Kr-Na+, respectively, it is seen that the Ar(3) atoms in the large cavity of Cs3-A(5Ar) interact much more strongly with the six-ring Na+ ions than do the Kr(3) atoms in Cs3-A(5Kr).31 Similar comparisions of approach distances to four-ring oxygens, 3.93(3) vs 3.24 Å and 3.81(2) vs 3.34 Å for Ar(2)-O(3) and Kr(2)-O(3), respectively, reveal that Ar atoms in Cs3-A(5Ar) interact more weakly with the framework oxygens than do the Kr atoms in Cs3-A(5Kr) . These relatively
shorter argon approach distances to Na+ ions and longer distances to framework oxygens, as compared to krypton, are a consequence of two competing factors: atom size and atomic polarizability in the electrostatic field of the zeolite cavity. Because argon is smaller and less polarizable than krypton, the inter-argon interactions in the large cavity should be weaker than the corresponding inter-krypton interactions in Cs3A(5Kr).31 As a result, the smaller argon atoms at Ar(3) should be able to approach Na+ ions more closely than Kr. Those at Ar(2) are relatively further way from the negative framework oxygens, perhaps because they cannot be simply polarized by the large four-rings. The four argon atoms at Ar(2) and Ar(3) on the inner-surface wall of the large cavity in Cs3-A(5Ar) may be placed within their partially occupied equipoints in various ways. The shortest possible inter-argon distances Ar(2)-Ar(3) ) 2.04(7) Å and Ar(2)-Ar(2) ) 3.30(10) Å are impossibly short and are
13730 J. Phys. Chem., Vol. 100, No. 32, 1996
Heo et al. encapsulation isotherm is supportive of the encapsulation capacities observed crystallographically in the large crystals. It shows a capacity of ca. 50 L at STP of Ar per kg dry Cs2.7-A at 600 atm. This corresponds to ca. 4.5 Ar atoms per unit cell of the zeolite. Extrapolations of this curve to 660 and 1000 atm of encapsulation pressure are consistent with the crystallographic results. At 1000 atm, six Ar atoms per unit cell of Cs3-A corresponds to an encapsulation capacity of 66.8 L at STP (119 g) of Ar per kg dry Cs3-A.
Figure 5. Schematic diagram of the rhombus (a) and trigonal bipyramid (b) of four and five argon atoms in the large cavities of Cs3-A(5Ar) and Cs3-A(6Ar), respectively. The immediate environment of each Ar atom and the dipole moment it induces on each Ar are shown. The favorable interactions between the polarized Ar atoms are indicated by fine lines and the unfavorable ones by dashed lines.
dismissed. A distance found among the equipositions of Ar(3), 3.94(16) Å, suggests the possibility of an Ar(3)-Ar(3) interaction, but this is dismissed because the dipoles induced on this pair of Ar(3) atoms are oriented unfavorably. The next set of distances Ar(2)-Ar(2) ) 4.67(13) and Ar(2)-Ar(3) ) 4.75(8) Å offer two solutions. (Longer distances, corresponding to atoms on near opposite sides of the large cavity, require impossibly short distances to complete a model and are dismissed.) Among the two possible solutions, a planar fourAr ring arrangement, [-Ar(2)-Ar(3)-Ar(2)-Ar(3)-], with Ar(2)-Ar(3) ) 4.75(8) Å and Ar(2)-Ar(3)-Ar(2) ) 88(1)°, is selected as the most plausible due to its higher symmetry and favorably oriented induced dipoles as discussed previously.31 In this rhombus, Ar atoms alternately approach Na+ ions and four-oxygen rings and are polarized oppositely, allowing their inter-argon approaches to be attractive (see Figure 5a), although the inter-argon interactions in this case are substantially longer and therefore much weaker than the corresponding interactions in Cs3-A(5Kr). The five argon atoms in the large cavity of Cs3-A(6Ar), three at Ar(2) and two at Ar(3), may be placed within their partially occupied equipoints in various ways. The short distances among partially occupied equipoints of Ar(2) and Ar(3), such as 1.97(9) Å for Ar(2)-Ar(3) and 3.35(5) Å for Ar(2)-Ar(2), are impossibly short and are dismissed as above. The close interargon interactions between equivalent argon atoms (3.69(1) Å for Ar(3)-Ar(3)) are dismissed because their induced dipoles are oriented unfavorably, also as above. The next longest interargon distance, 4.63(9) for Ar(2)-Ar(3), is plausible. However, various arrangements remain possible. A trigonal bipyramidal arrangement is selected as most plausible because of its higher symmetry and by considerations regarding alternating polarizations of argon atoms as before with Cs3-A(5Ar) (see Figures 4 and 5b). Two Ar(3) atoms on a single threefold axis on opposite sides of the large cavity occupy axial positions and three Ar(2) atoms are at equatorial positions with inter-argon distances of 4.63(9) Å for Ar(2)-Ar(3) and 5.80(9) Å for Ar(2)-Ar(2). In this arrangement, - polarizations from all three Ar(2) atoms point toward the center of the large cavity where each can interact with each of the two δ+ polarizations from the threefold-axis Ar(3) atoms (see Figure 5b). The favorable Ar(2)-Ar(3) interactions, 4.63(9) Å, are nicely much shorter than the unfavorable equatorial Ar(2)-Ar(2) interactions, 5.80(9) Å. Although both the composition of the zeolite pellets used (see Figure 1) and the conditions of encapsulation are somewhat different from those of the above single crystals,23,24 the argon-
Acknowledgment. N. H. Heo gratefully acknowledges the support of the Korean Science and Engineering Foundation for research fund (KOSEF 941-0300-027-2) and from the Central Laboratory of Kyungpook National University for the diffractometer and computing facilities. Supporting Information Available: Observed and calculated structure factors for Cs3-A(5Kr) and Cs3-A(6Kr) (8 pages). Ordering information is given on any current masthead page. References and Notes (1) Guemez, J.; Velasco, S. Am. J. Phys. 1987, 55, 154-157. (2) Woods, G. B.; Rowlinson, J. S. J. Chem. Soc., Faraday Trans. 2 1989, 85, 765-781. (3) Cooper, D. W. Phys. ReV. A 1988, 38, 522-524. (4) Chmelka, B. F.; Raftery, D.; McCormick, A. V.; de Menorval, L. C.; Levine, R. D.; Pines, A. Phys. ReV. Lett. 1991, 66, 580-583. (5) McCormick, A. V.; Chmelka, B. F. Molecular Physics 1991, 73, 603-617. (6) Breck, D. W. Zeolite Molecular SieVes: Structure, Chemistry, and Uses; John Wiley & Sons: New York, 1974; pp 623-628. (7) Fraenkel, D. CHEMTECH 1981, 1, 60-65. (8) Barrer, R. M.; Vaughan, D. E. W. J. Phys. Chem. Solids 1971, 32, 731-743. (9) Fraenkel, D.; Shabtai, J. J. Am. Chem. Soc. 1977, 99, 7074-7076. (10) Kwon, J. H.; Cho, K. H.; Kim, H. W.; Suh, S. H.; Heo, N. H. Bull. Korean Chem. Soc. 1993, 14, 583-588. (11) Barrer, R. M.; Vansant, E. F.; Peeters, G. J. Chem. Soc., Faraday Trans. 1 1978, 74, 1871-1881. (12) Thijs, A.; Peeters, G.; Vansant, E. F.; Verhaert, I. J. Chem. Soc., Faraday Trans. 1 1986, 82, 963-975. (13) Niwa, M.; Kato, S.; Hattori, T.; Marakami, Y. J. Chem. Soc., Faraday Trans. 1 1984, 80, 3135-3145. (14) Kwon, J. H. M.E. Thesis, Kyungpook National University, 1993. (15) Cook, G. A. Argon, Helium, and the Rare Gases; Interscience: New York, 1961; Vol. 1, p 228. (16) Breck, D. W. J. Chem. Educ. 1964, 41, 678-689. (17) Barrer, R. M.; Gibbons, R. M. Trans. Faraday Soc. 1963, 59, 25692582. (18) Barrer, R. M.; Vaughan, D. E. W. Trans. Faraday Soc. 1967, 63, 2275-2290. (19) Barrer, R. M.; Vaughan, D. E. W. Surf. Sci. 1969, 14, 77-92. (20) Reference 6, pp 634-641. (21) Fraenkel, D. J. Chem. Soc., Faraday Trans. 1 1981, 77, 20292039. (22) Fraenkel, D.; Ittah, B.; Levy, M. J. Chem. Soc., Chem. Commun. 1984, 1391-1392. (23) Yoon, J. H.; Heo, N. H. J. Phys. Chem. 1992, 96, 4997-5000. (24) Rho, B. R.; Kim, D. H.; Kim, J. T.; Heo, N. H. Hwahak Konghak 1991, 29, 407-416. (25) Kim, D. H.; Kim, J. T.; Heo, N. H. Hwahak Konghak 1991, 29, 717-726. (26) Derouane, E. G.; Andre, J. M.; Lucas, A. A. Chem. Phys. Lett. 1987, 137, 336-340 and references therein. (27) Vanderlick, T. K.; Scriven, L. E.; Davis, H. T. J. Chem. Phys. 1989, 90, 2422-2436 and references therein. (28) Jameson, C. J.; Jameson, A. K.; Gerald II, R.; de Dios, A. C. J. Chem. Phys. 1992, 96, 1690-1697. (29) The nomenclature refers to the contents of the Pm3hm unit cell: e.g., Na12-A represents Na12Si12Al12O48, and Cs3Na8H-A and Cs3-A represent Cs3Na8HSi12Al12O48. (30) Cho, K. H.; Kwon, J. H.; Kim, H. W.; Park, C. S.; Heo, N. H. Bull. Korean Chem. Soc. 1994, 15, 297-304. (31) Heo, N. H.; Cho, K. H.; Kim, J. T.; Seff, K. J. Phys. Chem. 1994, 98, 13328-13333. (32) Fraissard, J.; Ito, T. Zeolites 1988, 8, 350-361. (33) Ito, T.; Fraissard, J. Zeolites 1987, 7, 554-558.
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J. Phys. Chem., Vol. 100, No. 32, 1996 13731 (47) Calculations were performed with Structure Determination System, MolEN; Enraf-Nonius: The Netherlands, 1990. (48) Blackwell, C. S.; Pluth, J. J.; Smith, J. V. J. Phys. Chem. 1985, 89, 4420-4423. (49) Merck Index, 11th ed.; Merck & Co.: Rahway, NJ, 1989; pp 123124. (50) Doyle, P. A.; Turner, P. S. Acta Crystallogr., Sect. A 1968, 24, 390-397. (51) Reference 46, pp 73-87. (52) Cromer, D. T. Acta Crystallogr. 1965, 18, 17-23. (53) Reference 46, pp 149-150. (54) Heo, N. H.; Seff, K. J. Am. Chem. Soc. 1987, 109, 7986-7992. (55) Vance, T. B., Jr.; Seff, K. J. Phys. Chem. 1975, 79, 2163-2167. (56) Firor, R. L.; Seff, K. J. Am. Chem. Soc. 1977, 99, 6249-6253. (57) Subramanian, V.; Seff, K. J. Phys. Chem. 1979, 83, 2166-2169. (58) Ogawa, K.; Nitta, M.; Aomura, K. J. Phys. Chem. 1978, 82, 16551660. (59) Takaishi, T.; Hosoi, H. J. Phys. Chem. 1982, 86, 2089-2094. (60) Yanagida, R. Y.; Amaro, A. A.; Seff, K. J. Phys. Chem. 1973, 77, 805-809.
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