13720
J. Phys. Chem. 1996, 100, 13720-13724
Two Anhydrous Zeolite X Crystal Structures, Cd46Si100Al92O384 and Cd24.5Tl43Si100Al92O384 Jeong Hwa Kwon, Se Bok Jang, and Yang Kim* Department of Chemistry, Pusan National UniVersity, Pusan 609-735, Korea
Karl Seff* Department of Chemistry, UniVersity of Hawaii, Honolulu, Hawaii 96822 ReceiVed: February 7, 1996; In Final Form: April 25, 1996X
The structures of fully dehydrated, fully Cd2+-exchanged zeolite X, Cd46Si100Al92O384 (Cd46-X; a ) 24.935(8) Å), and that of fully dehydrated Cd2+- and Tl+-exchanged zeolite X, Cd24.5Tl43Si100Al92O384 (Cd24.5Tl43-X; a ) 24.858(9) Å), have been determined by single-crystal X-ray diffraction methods in the cubic space group Fd3h at 21(1) °C. Cd46-X was prepared by ion exchange in a flowing stream of 0.05 M aqueous Cd(NO3)2 for 2 days. Cd24.5Tl43-X was prepared similarly using a solution 0.025 M each in Cd(NO3)2 and TlNO3. Each crystal was then dehydrated at 450 °C and 2 × 10-6 Torr for 2 days. Their structures were refined to the final error indices R1 ) 0.055 and R2 ) 0.077 with 544 reflections for Cd46-X, and R1 ) 0.054 and R2 ) 0.051 with 272 reflections for Cd24.5Tl43-X; I > 3σ(I). In the structure of dehydrated Cd46-X, Cd2+ ions are located at two different crystallographic sites. Sixteen Cd2+ ions fill site I, at the centers of the double sixrings; each Cd2+ ion is octahedrally coordinated by framework oxygens, all at 2.35(1) Å. The remaining 30 Cd2+ ions nearly fill the 32-fold site II in the single six-rings; each is three-coordinate planar to framework oxygens at 2.16(1) Å. The fractional occupancies in dehydrated Cd24.5Tl43-X are most easily explained with two types of unit cell: half have 14 Cd2+ ions at site I and four Tl+ ions at site I′; the remaining half have 15 Cd2+ ions at site I and two Tl+ ions at site I′. The remaining ten Cd2+ ions occupy site II; 22 Tl+ ions extend 1.52 Å into the supercage from their three oxygen planes to complete the filling of site II. The remaining 18 Tl+ ions are statistically distributed over site III, a 48-fold equipoint in the supercages on twofold axes; Tl-O ) 2.79(2) Å. It appears that Cd2+ ions prefer sites I and II in that order, and that Tl+ ions occupy the remaining sites, except that they are too large to be stable at site I.
Introduction The exchangeable cations in zeolites have received a great deal of attention in the scientific literature. The thermal stability, sorption parameters, and catalytic properties of zeolites all depend upon the type and number of exchangeable cations and their distribution over the available sites. Cation distributions in faujasite-type zeolites have been widely studied by X-ray diffraction methods.1-3 Recent single-crystal X-ray studies of hydrated chabazite4-6 and heulandite,7 exchanged under controlled conditions with mono- and divalent cations, have shown that the extraframework structures, especially the cation distributions, may be rationalized to a large extent in terms of the sizes, charges, and electronic natures of the exchanged cations. Li+-exchanged zeolites X and Y were studied using powder8 and single-crystal9 diffraction data. The structures of four K+exchanged X and Y zeolites with various Si/Al ratios were also studied in their hydrated10 and dehydrated11 states using X-ray powder data. Smolin, Shepelev, and Anderson studied the crystal structures of hydrated and dehydrated Ca2+-exchanged zeolite X.12 They also investigated the migration of cations during dehydration by heating the crystal in a stream of hot N2 gas. In the initial hydrated form, Ca2+ ions are located in the sodalite cavities and supercages. Upon dehydration, the Ca2+ ions migrate into the hexagonal prism and to sites in the single six oxygen rings. Calligaris et al. studied the crystal structures of hydrated (Cd44Si104Al88O384‚233H2O) and partially dehydrated Cd2+exchanged zeolite X (Cd44Si104Al88O384‚138H2O).13 To prepare the latter, a hydrated single crystal was kept at a pressure of 2 X
Abstract published in AdVance ACS Abstracts, July 1, 1996.
S0022-3654(96)00364-4 CCC: $12.00
× 10-6 Torr at room temperature for 1 day; it was then slowly heated to 60 °C; after 3 h, the capillary tube was slowly cooled to 25 °C. The positions and occupancy numbers of Cd2+ ions and H2O molecules in hydrated Cd44-X were then compared with those of partially hydrated Cd44-X. In hydrated Cd44-X, the Cd2+ ions are located at two sites of high occupancy along threefold axes, one in the sodalite cage (site I′) and the other in the supercage (site II). A third site of low occupancy was found in the supercage (site III′). Partial dehydration caused the movement of some cations from sites I′ and III′ to I and II. The coordination number of the Cd2+ ions generally decreased, and the water molecules lost were primarily from the supercage. In this report, the fully dehydrated structures of Cd46-X and of a Cd2+- and Tl+-exchanged zeolite X are presented. The occupancy numbers and locations of cations in the structure of dehydrated Cd24.5Tl43-X are compared with those in dehydrated Cd46-X. Experimental Section Large colorless crystals of Na-X (stoichiometry; Na92Si100Al92O384) were prepared in St. Petersburg, Russia.14 Each of two single crystals, octahedra ca. 0.2 mm in diameter, was lodged in a fine Pyrex capillary. Cd46-X (crystal 1) was prepared using a 0.05 M Cd(NO3)2‚ 4H2O exchange solution. Cd24.5Tl43-X (crystal 2) was prepared using an exchange solution whose Cd(NO3)2‚4H2O:TlNO3 mole ratio was 1:1 with a total concentration of 0.05 M. Ion exchange was accomplished by flow methods: each aqueous solution was allowed to flow past each crystal at a velocity of approximately 1 cm/s for 3 days at 24(1) °C. Each capillary with its crystal was then connected to a vacuum line. Each crystal was cautiously dehydrated under vacuum by gradually increasing its temperature (ca. 20 °C/h) to 450 °C at a constant pressure © 1996 American Chemical Society
Two Cadmium-Exchanged Zeolite X Crystal Structures
J. Phys. Chem., Vol. 100, No. 32, 1996 13721
TABLE 1: Positional, Thermal, and Occupancy Parametersa Occupancya atom
Wycoff site position
x
y
z
β11
b
β22
β33
β12
β13
β23
2(5) 3(5) 4(2) 6(2) 2(2) 1(2) 3(2) 9(2)
-2(1) -1(2) -5(5) 3(4) 4(4) -8(4) 1(1) 10(1)
-1(1) 2(2) 2(3) -1(4) 4(4) 5(4) 1(1) 10(1)
0(2) -3(2) -3(4) -2(4) 1(3) -5(3) 1(1) 10(1)
0(1)d 3(1) 9(4) 3(3) 5(4) 2(3) 3(1) 13(1) 10(2) 8(1) 61(4)
-1(3) -3(4) 5(2) -1(7) 12(7) -3(6) -2(1) 15(4) 15(5) -1(1) 0
3(3) 0(3) -12(6) -8(7) 13(7) 9(6) -2(1) 15(4) 15(5) -1(1) 0
2(3) 0(4) -6(8) -1(8) -3(6) 4(5) -2(1) 15(4) 15(5) -1(1) -31(5)
varied
fixed
16.6(2) 29.5(2)
96 96 96 96 96 96 16.0 30.0
14.1(2) 9.7(2) 3.0(1) 21.6(1) 18.4(2)
96 96 96 96 96 96 14.5 10.0 3.0 22.0 18.0
(a) Dehydrated Cd46-X (Crystal 1) Si Al O(1) O(2) O(3) O(4) Cd(1) Cd(2)
I II
96(g) 96(g) 96(g) 96(g) 96(g) 96(g) 16(c) 32(e)
-528(6) -551(9) -1049(4) -14(8) -309(2) -632(9) 0 2212(9)
342(5) 1211(9) -15(2) -25(8) 604(3) 739(4) 0 2212(9)
I II I′ II III
96(g) 96(g) 96(g) 96(g) 96(g) 96(g) 16(c) 32(e) 32(e) 32(e) 48(f)
-531(3) -532(3) -1056(8) -42(8) -303(7) -665(6) 0 2150(5) 711(7) 2534(1) 4099(3)
352(3) 1224(4) -0(10) -30(10) 647(8) 793(7) 0 2150(5) 711(7) 2534(1) 1250
1225(4) 372(6) 1109(4) 1463(6) 651(7) 1671(4) 0 2212(9)
3(8) 2(6) 4(2) 7(2) 5(2) 10(2) 3(2) 9(2)
2(9) 2(6) 8(2) 3(2) 4(2) 5(2) 3(2) 9(2)
(b) Dehydrated Cd24.5Tl43-X (Crystal 2) Si Al O(1) O(2) O(3) O(4) Cd(1) Cd(2) Tl(1) Tl(2) Tl(3)
1223(4) 353(3) 1070(8) 1476(6) 651(8) 1678(8) 0 2150(5) 711(7) 2534(1) 1250
4(1) 1(1) 14(5) 2(3) 8(4) 2(3) 3(1) 13(1) 10(2) 8(1) 6(1)
4(1) 3(1) 3(3) 9(3) 4(4) -5(2)d 3(1) 13(1) 10(2) 8(1) 66(4)
a Positional and anisotropic thermal parameters are given ×104. Numbers in parentheses are esd’s in the units of the least significant digit given for the corresponding parameter. b The anisotropic temperature factor ) exp[-(β11h2 + β22k2 + β33l2 + β12hk + β13hl + β23kl)]. c Occupancy factors are given as the number of atoms or ions per unit cell. d This physically unacceptable value was increased by 3σ in the Figures.
of 2 × 10-6 Torr. These conditions were maintained for 48 h. After each crystal had cooled to room temperature, it was sealed under vacuum in its capillary by torch. Each crystal had remained colorless. X-ray Data Collection Preliminary crystallographic experiments and subsequent data collection were performed with an automated four-circle EnrafNonius CAD-4 diffractometer equipped with a graphite monochromator, a pulse-height analyzer and a micro-Vax 3100 computer. Mo KR radiation (KR1, λ ) 0.70930 Å; KR2, λ ) 0.71359 Å) was used for all experiments. The cubic unit cell constants determined by a least-squares refinement of 24 intense reflections for which 22° < 2θ < 27° are 24.935(8) Å for Cd46-X and 24.858(9) Å for Cd24.5Tl43-X (18° < 2θ < 24°). For each crystal, the ω-2θ scan technique was used. The data were collected at variable scan speeds. Most reflections were observed at slow speeds from 0.24 to 0.52 deg/min in ω for Cd46-X and from 0.11 to 0.33 deg/min in ω for Cd24.5Tl43X. The intensities of three reflections in diverse regions of reciprocal space were recorded every 3 h to monitor crystal and instrument stability. Only small random fluctuations of these check reflections were noted during the course of data collection. All unique reflections for an F-centered unit cell for which 2θ < 70° for Cd46-X and 2θ < 60° for Cd24.5Tl43-X were examined by counter methods. The raw data were corrected for Lorentz and polarization effects including incident beam monochromatization, and the resultant estimated standard deviations were assigned to each reflection by the computer programs BEGIN and GENESIS.15 Of the 1683 unique reflections measured for Cd46-X and 1236 for Cd24.5Tl43-X, only the 544 and 272 reflections, respectively, for which I > 3σ(I) were used in subsequent structure determintions. Structure Determination The crystal structures were solved in the cubic space group Fd3h. This is established for zeolite X and is consistent with the systematic absences observed. (a) Cd46-X (Crystal 1). Full-matrix least-squares refinement was initiated using the atomic parameters of the framework
atoms [Si, Al, O(1), O(2), O(3), and O(4)] of dehydrated Rb78Na28-X.16 Anisotropic refinement of the framework atoms converged to an unweighted R1 index, (∑(|Fo - |Fc||/∑Fo), of 0.53 and a weighted R2 index (∑w(F0 - |Fc|)2/∑wFo2)1/2, of 0.60. The initial difference Fourier function revealed two large peaks at (0.0, 0.0, 0.0) with peak height 24 eÅ-3 and at (0.22, 0.22, 0.22) with peak height 15 eÅ-3. These two positions were stable in least-squares refinement. Anisotropic refinement including these peaks as Cd2+ ions converged to R1 ) 0.054 and R2 ) 0.077 with occupancies Cd(1) ) 16.6(2) and Cd(2) ) 29.5(2). These values were reset and fixed at 16.0 and 30.0 Cd2+ ions at Cd(1) and Cd(2), respectively, because the maximum occupancy at Cd(1) is by symmetry 16, and because the cationic charge should be +92 per Fd3h unit cell. The error indices then converged to R1 ) 0.055 and R2 ) 0.077. In the final cycle of least-squares refinement, all shifts in atomic parameters were less than 0.1% of their corresponding standard deviations. The final difference function was featureless except for a peak of height 0.91 eÅ-3 at (0.17, 0.17, 0.17). This peak was not stable in least-squares refinement. An absorption correction (µR ) 0.19, Fcal ) 1.78 g/cm3)17 was made empirically using a ψ scan. The adjusted transmission coefficients ranged from 0.987 to 0.999. This correction had little effect on the final R values. The final structural parameters and selected interatomic distances and angles are presented in Tables 1 and 2, respectively. (b) Cd24.5Tl43-X (Crystal 2). Full-matrix least-squares refinement was initiated as for crystal 1.16 Anisotropic refinement of the framework atoms converged to R1 ) 0.60 and R2 ) 0.68. A subsequent Fourier synthesis revealed two large peaks at (0.25, 0.25, 0.25) with height 21 eÅ-3 and (0.0, 0.0, 0.0) with height 18 eÅ-3. Anisotropic refinement of the framework atoms, Tl+ at Tl(2), and Cd2+ at (Cd(1) converged to R1 ) 0.24 and R2 ) 0.30 with occupancies of 21.6(1) at Tl(2) and 14.1(2) at Cd(1). It is not difficult to distinguish Cd2+ from Tl+ ions for several reasons. Firstly, their atomic scattering factors are quite different, 46 e- for Cd2+ vs 80 e- for Tl+. Secondly, their ionic radii are different, Cd2+ ) 0.97 Å and Tl+ ) 1.47 Å.18 Also, the approach distances between Cd2+ and zeolite oxygens
13722 J. Phys. Chem., Vol. 100, No. 32, 1996
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TABLE 2: Selected Bond Distances (Å) and Angles (deg)a Cd46-X Si-O(1) Si-O(2) Si-O(3) Si-O(4) Al-O(1) Al-O(2) Al-O(3) Al-O(4) Cd(1)-O(3) Cd(2)-O(2) Tl(1)-O(3) Tl(2)-O(2) Tl(3)-O(4) Tl(3)-O(1) O(1)-Si-O(2) O(1)-Si-O(3) O(1)-Si-O(4) O(2)-Si-O(3) O(2)-Si-O(4) O(3)-Si-O(4) O(1)-Al-O(2) O(1)-Al-O(3) O(1)-Al-O(4) O(2)-Al-O(3) O(2)-Al-O(4) O(3)-Al-O(4) Si-O(1)-Al Si-O(2)-Al Si-O(3)-Al Si-O(4)-Al O(3)-Cd(1)-O(3) O(2)-Cd(2)-O(2) O(3)-Tl(1)-O(3) O(2)-Tl(2)-O(2) O(4)-Tl(3)-O(4) O(1)-Tl(3)-O(1)
1.60(1) 1.68(2) 1.66(1) 1.60(1) 1.70(1) 1.75(2) 1.77(1) 1.70(1) 2.35(1) 2.16(1)
112.1(8) 109.3(7) 112.6(7) 105.5(6) 105.1(7) 112.0(7) 114.2(8) 106.3(6) 112.1(6) 105.5(6) 101.5(7) 119.0(4) 129.2(8) 134.7(7) 115.7(3) 163.7(9) 90.4(4) 89.6(4) 119.2(5)
Cd24.5Tl43-X 1.62(2) 1.66(2) 1.70(2) 1.61(2) 1.65(2) 1.71(2) 1.71(2) 1.71(2) 2.40(2) 2.15(2) 2.53(3) 2.63(1) 2.79(2) 3.42(3) 112(1) 108(1) 111(1) 109(1) 106(1) 111(1) 112(1) 107(1) 113(1) 108(1) 106.0(9) 118.6(5) 133(1) 134(1) 128(1) 157(1) 91.7(6) 88.3(6) 119.6(9) 82.7(7) 90.1(7) 67.7(5) 133.5(6)
a
Numbers in parentheses are estimated standard deviations in the units of the least significant digit given for the corresponding values.
have been determined in the previous structure (Cd46-X) (see Table 2) and are indicative. Finally, the requirement that the cationic charges sum to +92 per unit cell does not allow the major positions to refine to acceptable occupancies with an alternative assignment of ionic identities. A subsequent difference Fourier function revealed two additional peaks at (0.21, 0.21, 0.21) with height 7.4 eÅ-3 and at (0.41, 0.125, 0.125) with height 6.6 eÅ-3. Inclusion of these peaks as ions at Cd(2) and Tl(3) lowered the error indices to R1 ) 0.093 and R2 ) 0.097. The occupancy numbers at Cd(2) and Tl(3) refined to 9.7(2) and 18.4(2), respectively. Anisotropic refinement converged to R1 ) 0.085 and R2 ) 0.088. The third Tl+ ion position was found on an ensuing Fourier function at (0.07, 0.07, 0.07) with height 4.2 eÅ-3. Anisotropic refinement of framework atoms and all cations converged to R1 ) 0.053 and R2 ) 0.050. The occupancies of Cd(1), Cd(2), Tl(1), Tl(2), and Tl(3) were fixed at the values shown in Table 1 considering the cationic charge per unit cell. The final error indices were R1 ) 0.054 and R2 ) 0.051. The shifts in the final cycle least-squares refinement were less than 0.1% of their corresponding standard deviations. The final difference function was featureless except for a peak of height 0.8 eÅ-3 at (0.085, 0.085, 0.143). This peak was not stable in least-squares refinement. An absorption correction (µR ) 1.18 Fcal ) 2.44 g/cm3)17 was made empirically using a ψ scan. The adjusted transmission coefficients ranged from 0.972 to 0.999. This correction had no effect on the final R indices. All crystallographic calculations were done using MolEN15 (a structure determination program package supplied by Enraf-
Figure 1. A stylized drawing of the framework structure of zeolite X. Near the center of the each line segment is an oxygen atom. The different oxygen atoms are indicated by the numbers 1-4. Silicon and aluminum atoms alternate at the tetrahedral intersections, except that Si substitutes for about 4% of the Al’s. Extraframework cation positions are labeled with Roman numerals.
Nonius). The full-matrix least-squares program used minimized ∑w(Fo - |Fc|)2; the weight (w) of an observation was the reciprocal square of σ(Fo), its standard deviation. Atomic scattering factors19,20 for Si, Al, O-, Cd2+, and Tl+ wer used. All scattering factors were modified to account for anomalous dispersion.21 The final structural parameters are listed in Table 1, and selected interatomic distances and angles are given in Table 2. Discussion Zeolite X is a synthetic counterpart of the naturally occurring mineral faujasite. The polyhedron with 14 vertices known as the sodalite cavity or β cage may be viewed as the principal building block of the aluminosilicate framework of the zeolite (see Figure 1). These β-cages are connected tetrahedrally at six-rings by bridging oxygens to give double six-rings (D6Rs, hexagonal prisms), and, concomitantly, an interconnected set of even larger cavities (supercages) accessible in three dimensions through 12-ring (24-membered windows). The Si and Al atoms occupy the vertices of these polyhedra. The oxygen atoms lie approximately halfway between each pair of Si and Al atoms but are displaced from those points to give near tetrahedral angles about Si and Al. The nomenclature of the cation sites is as follows: site I, at the center of a D6R; site II, at the center of the single six-ring (S6R, shared by a β- and a supercage), or displaced from this point into a supercage; sites I′ and II′ lie in the sodalite cavity, on opposite sides of the corresponding six-rings from sites I and II, respectively; and site III, on a twofold axis opposite a four-ring inside the supercage. (a) Cd46-X (Crystal 1). In this simple and straightforward structure, Cd2+ ions are found at two crystallographic sites. The Cd2+ ions at Cd(1) fill the 16-fold site I (Cd(1)-O(3) ) 2.35(1) Å, O(3)-Cd(1)-O(3) angles ) 90.4(4)° and 89.6(4)°) (see Figure 2). The remaining 30 Cd2+ ions, at Cd(2), almost fill the 32-fold site II in the supercage. The Cd(2)-O(2) distance, 2.16 Å, is shorter than the sum of the conventional radii18 of Cd2+ and O2-, 0.97 Å + 1.32 Å ) 2.29 Å, presumably because Cd(2) is only three-coordinate. These Cd2+ ions are slightly recessed, 0.19(1) Å, into the supercage from the plane of the three O(2) oxygens (see Tables 2 and 3). The O(2)-Cd(2)O(2) bond angle, 119.2(5)°, is nearly trigonal planar.
Two Cadmium-Exchanged Zeolite X Crystal Structures
J. Phys. Chem., Vol. 100, No. 32, 1996 13723
Figure 2. Stereoview of a representative sodalite cavity and D6R in dehydrated Cd46-X. All D6Rs are filled as shown with Cd2+ ions at Cd(1). About 75% of the sodalite cavities have four Cd2+ ions at Cd(2) in single six-rings as shown; the remainder have only three. Ellipsoids of 20% probability are shown.
TABLE 3: Deviations (Å) of Cations from Six-Ring Planes crystal 1 at
crystal 2
O(3)a
-1.36(1)
Cd(1) Tl(1)
-1.43(1) 1.63(1)
at O(2)b Cd(2) Tl(2)
0.19(1)
0.14(1) 1.52(1)
a A positive deviation indicates that the atom lies in the sodalite unit. b A positive deviation indicates that the atom lies in the supercage.
TABLE 4: Placement of Cations in Anhydrous Zeolite Xa site (order) structure
I (16)
Mg46-Xb Ca46-Xc Sr46-Xb Ba46-Xe Cd46-Xf Cd24.5Tl43-Xf
14.7(7) 16.3(5)d 16.4(2)d 13.6(1) 16.6(2)d 14.1(2) Cd
I′ (32) 4.2(11) 1.6(2) 3.0(1) Tl
II (32) 28.2(11) 31.3(7) 29.7(3) 28.9(2) 29.5(2) 9.7(2) Cd 21.1(4) Tl
III (48)
Figure 3. Stereoview of a representative sodalite cavity and D6R in dehydrated Cd24.5Tl43-X. About 90% of the D6Rs contain a Cd2+ ion at Cd(1). (If a Cd2+ ion is not present, the two adjacent I′ sites are occupied by Tl+ ions at Tl(1)). About three-fourths of the sodalite cavities have one Cd2+ ion at Cd(2) and three Tl+ ions at Tl(1); the remainder have two Cd2+ ions at Cd(2) and two Tl+ ions at Tl(1). Ellipsoids of 20% probability are shown.
18.4(2) Tl
a These occupancies were not constrained in least-squares refinement. Reference 24. c Reference 23. d The maximum occupancy at site I, 16, is not exceeded significantly. e Reference 22. f This work. b
Recently the crystal structures of Ba46-X,22 Ca46-X,23 Mg46and Sr46-X24 were determined (see Table 4). From these it appears that site I (at the center of the D6R) is generally the lowest energy cation site, unless the cation is simply too large or too small. Ca2+ ions in Ca46-X23 and Sr2+ ions in Sr46-X24 fill the site I position, with the remainder going to site II in the supercage, nearly filling it as in Cd46-X. Only about 13 Ba2+ ions (relatively large) in Ba46-X22 and 14 Mg2+ (relatively small) in Mg46-X24 occupy the 16-fold site I positions. (b) Cd24.5Tl43-X (Crystal 2). A structure with rational occupancies and filled rings is consistent with the occupancies found. It is described by the fixed occupancies given in the last column of Table 1(b). In it, every D6R hosts either a Cd(1) ion or two Tl(1) ions, and every S6R hosts either a Cd(2) or a Tl(2) ion. Rational occupancies of 14 at Cd(1) and 19 at Tl(3) might also have been chosen (consistent with the total cationic charge of 92+ required per unit cell), but these are a bit less consistent with the refined occupancies and leave the D6Rs incompletely filled; the unit cell formula would then be Cd24Tl44X. About 14.5 Cd2+ ions at Cd(1) occupy the 16-fold site I.
X,24
Figure 4. Stereoview of another (minor) kind of sodalite cavity and D6R in dehydrated Cd24.5Tl43-X. If site I is not occupied by a Cd2+ ion at Cd(1), the two adjacent I′ sites are occupied by Tl+ ions at Tl(1). (About 10% of the D6Rs do not contain a Cd2+ ion at Cd(1)). About one-fourth of the sodalite cavities have two Cd2+ ions at Cd(2) and two Tl+ ions at Tl(1). The remainder have one Cd2+ ion at Cd(2) and three Tl+ ions at Tl(1). Ellipsoids of 20% probability are shown.
Cations with Pauling radii much greater than 1.25 Å should not be able to replace the Na+ ions in the D6Rs;26 the approximate value of 1.17 Å is suitable for comparison with the non-Pauling radii18 used throughout this report. The radii of the Cd2+ and Tl+ ions are 0.97 and 1.47 Å, respectively,18 so site I should be accessible only to Cd2+. Proof of this comes from the crystal structure of anhydrous Tl92-X in which site I
13724 J. Phys. Chem., Vol. 100, No. 32, 1996
Kwon et al.
Figure 5. A stereoview of the large cavity of dehydrated Cd24.5Tl43-X. One Cd2+ ion at Cd(2), three Tl+ ions at Tl(2) and two Tl+ ions at Tl(3) are shown. Most of the supercages can have this arrangement. The remainder may have two Cd2+ ions at Cd(2), two Tl+ ions at Tl(2) and three Tl+ ions at Tl(3). Ellipsoids of 20% probability are shown.
remains entirely unoccupied.25 Each Cd(1) is octahedrally coordinated by O(3) framework oxygens at 2.42(3) Å (see Figure 3). The Tl(1) position is at site I′, on a threefold axis in the sodalite unit opposite D6Rs (see Figure 4). This is 32-fold position, but it is occupied by only three Tl+ ions. Each Tl+ ion lies relatively far inside the sodalite cavity, 1.64 Å from the plane of the three O(3) framework oxygens of the D6R to which it is bound. The Tl(1)-O(3) distances are 2.53(3) Å, shorter than the sum of the corresponding ionic radii, 1.47 + 1.32 ) 2.79 Å.18 This indicates that each Tl+ ion coordinates strongly to its three O(3) oxygens as would be expected by the low coordination number. The Tl+ ion at site I′ is only 3.06 Å from site I. By the low occupancy at I′, it appears that this short intercation distance is being avoided. If a D6R has a Cd2+ ion in it, then the two adjacent sites I′ are unoccupied. If a D6R does not contain a Cd2+ ion, then two Tl+ ions lie outside at I′ sites. The fractional occupancies in dehydrated Cd24.5Tl43-X are most easily explained with two types of unit cells: half have 14 Cd2+ ions at site I and four Tl+ ions outside at site I′; the remaining half have 15 Cd2+ ions at site I and two Tl+ ions at site I′. The Cd(2) and Tl(2) ions occupy site II in the supercage with occupancies of 10 and 22, respectively, filling this 32-fold equipoint. Cd(2)-O(2) is 2.15(3) Å and O(2)-Cd(2)-O(2) ) 120.0(2)°; Tl(2)-O(2) is 2.63(1) Å and O(2)-Tl(2)-O(2) is 90.1(7)°. The 10 Cd2+ ions at Cd(2) are only 0.04 Å from the plane of the single six-ring; the 22 Tl+ ions at Tl(2) are much further, 1.70 Å, from the corresponding plane. Plausible ionic arrangements for a sodalite unit and a supercage are shown in Figures 2-4, respectively. The remaining 18 Tl+ ions occupy the 48-fold Tl(3) position at site III (see Figure 5). The Tl(2)-O(4) approach distance, 2.79(2) Å, is the same as the sum of the ionic radii of Tl+ and O2-, 1.47 Å + 1.32 Å.18 Conclusions This work indicates that all of the Na+ ions in zeolite X can readily be replaced by Cd2+ ions or by Tl+ and Cd2+ ions. It indicates also that full dehydration to yield relatively simple structures is possible. Considerations of ionic size and charge govern the competition for sites in Cd24.5Tl43-X. The smaller and more highly charged Cd2+ ions nearly fill site I, with the remainder going to site II as in Cd46-X, affirming that Cd2+ ions prefer site I. (Perhaps Cd2+ ions occupy only those D6Rs which contain six framework Al3+ ions, leaving those lower in Al3+ content (and therefore lower in charge) for pairs of Tl+ ions at site I′.) The
larger Tl+ ions, which are less able to balance the anionic charge of the zeolite framework because of their size, finish satisfying the D6Rs with some occupancy at I′, and finish filling site II, with the remainder going to the least suitable cation site in the structure, site III. Acknowledgment. This work was supported in part by the Basic Research Institute Program, Ministry of Education, Korea, 1994, Project No. BSRI-94-3409. Supporting Information Available: Tables of calculated and observed structure factors (11 pages). Ordering information is given on any current masthead page. References and Notes (1) Mortier, W. J. Compilation of Extra-framework Sites in Zeolites; Butterworth Scientific Ltd.: Guildford, UK, 1982. (2) Schollner, R.; Broddack, R.; Kuhlmann, B.; Nozel, P.; Herden, H. Z. Phys. Chem. (Leipzig) 1981, 262, 17. (3) Egerton, T. S.; Stone, F. S. J. Chem. Soc., Faraday Trans. I 1970, 66, 2364. (4) Calligaris, M.; Mezzetti, A.; Nardin, G.; Randaccio, L. Zeolites 1986, 6, 137. (5) Calligaris, M.; Mezzetti, A.; Nardin, G.; Randaccio, L. Zeolites 1985, 5, 317. (6) Pluth, J. J.; Smith, J. V.; Mortier, W. J. Mater. Res. Bull. 1977, 12, 1001. (7) Bresciani-Pahor, N.; Calligaris, M.; Nardin, G.; Randaccio, L.; Russo, E.; Comin-Chiaramonti, P. J. Chem. Soc., Dalton Trans. 1980, 1511. (8) Herden, H.; Einicke, W. D.; Schollner, R.; Dyer, A. J. Inorg. Nucl. Chem. 1981, 43, 2533. (9) Shepelev, Y. F.; Anderson, A. A.; Smolin, Y. I. Zeolites 1990, 10, 61. (10) Mortier, W. J.; Bosmans, H. J. J. Phys. Chem. 1971, 75, 3327. (11) Mortier, W. J.; Bosmans, H. J.; Uytterhoeven, J. B. J. Phys. Chem. 1972, 76, 650. (12) Smolin, Y. I.; Shepelev, Y. F.; Anderson, A. A. Acta Crystallogr., Sect. B 1989, 45, 124. (13) Calligaris, M.; Nardin, G.; Randaccio, L.; Zangrando, E. Zeolites 1986, 6, 439. (14) Bogomolov, V. N.; Petranovskii, V. P. Zeolites 1986, 6, 418. (15) Calculations were performed with Structure Determination Package Programs, MolEN; Enraf-Nonius, Netherlands, 1990. (16) Kim, Y.; Han, Y. W.; Seff, K. J. Phys. Chem. 1993, 97, 12663. (17) International Tables for X-ray Crystallography; Kynoch Press: Birmingham, England, 1974; Vol. II, p 302. (18) Handbook of Chemistry and Physics, 70th ed.; The Chemical Rubber Co.: Cleveland, OH, 1989/1990; p F-187. (19) Cromer, D. T. Acta Crystallogr. 1965, 18, 17. (20) International Tables for X-ray Crystallography; Kynoch Press: Birmingham, England, 1974; Vol. IV, pp 73-87. (21) Reference, 20, pp 149-150. (22) Jang, S. B.; Kim, Y. Bull. Korean Chem. Soc. 1995, 16, 248. (23) Jang, S. B.; Song, S. H.; Kim, Y. J. Korean Chem. Soc. 1995, 39, 7. (24) Yeom, Y. H.; Jang, S. B.; Kim, M. J.; Han, Y. W.; Kim, Y.; Seff, K. Unpublished work. (25) Kim, Y.; Han, Y. W.; Seff, K. Submitted to Zeolites, 1996. (26) Sherry, H. S. J. Phys. Chem. 1968, 72, 12.
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