8946
J. Phys. Chem. B 2000, 104, 8946-8951
Cation Crowding in Zeolites. Reinvestigation of the Crystal Structure of Dehydrated Potassium-Exchanged Zeolite X Lin Zhu and Karl Seff* Department of Chemistry, UniVersity of Hawaii at Manoa, Honolulu, Hawaii 96822-2275 ReceiVed: February 22, 2000; In Final Form: June 9, 2000
The structure of a single crystal of fully dehydrated K+-exchanged zeolite X was determined by X-ray diffraction methods in the cubic space group Fd3hm at 21 °C; ao ) 25.083(5) Å. Ion-exchange of a crystal of Na92Si100Al92O384 was done at 80 °C using flowing aqueous 0.1 M KNO3 with pH ) 12; 90(3) K+ ions were found per unit cell. R1 ) 0.066 for 352 reflections for which Fo > 4 σ(Fo); wR2 based on F2 and all data is 0.193. The structure of dehydrated K-X is much more complex than that of dehydrated Na-X; K+ ions are found at nine crystallographic sites, a large number due to cation crowding. Two distinct site I and three site I′ positions are occupied by six, eight, four, four, and four K+ ions per unit cell, respectively. Ten of the 16 double six-rings each accommodate two K+ ions with short K+‚‚‚K+ distances of 3.79(4) and 3.92(8) Å, indicative of the degree of cation crowding. Site II is nearly fully occupied with 28.4(4) K+ ions. The remaining 36 K+ ions are found at one III and two III′ sites; these appear to be close in energy for K+ ions so that no single site is preferred.
1. Introduction Zeolite X has a wide range of industrial applications, partly because of its large pore size and void volume. Its sorptive and catalytic properties depend also on its exchangeable cations: their size, their charge, their chemical nature, and their placement within the zeolite. To determine their cation placements, alkali-metal cation exchanged zeolite X has been studied by crystallographic methods. Li+-exchanged zeolites Y and X were studied using powder1-4 and single-crystal5 diffraction data, respectively. The crystal structure of dehydrated Na-X has been reported four times in recent years.6-9 Several structural studies of K+-exchanged faujasite-type zeolites have been reported. Pluth et al. investigated a single crystal of a dehydrated K+-exchanged natural faujasite (Si/Al ) 2.3).10 The structures of four hydrated and dehydrated partially K+-exchanged X and Y zeolites with various Si/Al ratios were also studied using powder data;11,12 only K+ positions inside the sodalite units were reliably determined. Parise et al.13 prepared a hydrated fully K+-exchanged LSX sample at 70 °C and studied its structure by X-ray powder diffraction methods. ICP chemical analysis indicated that no Na+ ions remained present in that sample. It seems that high temperatures facilitate complete K+ exchange into zeolite X. Even at ambient temperatures, complete exchange of K+ for Na+ in powders of LSX has been accomplished.14 Recently, Kim et al.15 studied a dehydrated single crystal of zeolite X which had been K+-exchanged by flow methods at 24(1) °C for 5 days. At five positions, about 87.5 K+ ions were found per unit cell. This is less, although possibly not significantly less, than the 92 expected. That work was repeated in this laboratory at 20(1) °C for 24 h, and again incomplete K+ exchange was seen: 81.8(25) K+ ions and 5.3(15) Na+ ions were found. These structures were surprising in a number of * Author to whom correspondence should be addressed. E-mail: kseff@ gold.chem.hawaii.edu.
ways. They indicate that complete K+ exchange is not readily achieved for large crystals, that there are more K+ positions than there are Na+ positions in dehydrated Na92-X, and that some very close K+‚‚‚K+ contacts are present. It seemed likely that K+ is reluctant to exchange fully into Na-X because of cation crowding.17-21 The exchange limits of cations that have large size and small charge can be partially explained by cation crowding. While the complete exchange of Li+ (radius ) 0.59 Å22), of Ag+ (radius ) 1.26 Å22), of K+ (radius ) 1.33 Å22), and of Tl+ (radius ) 1.47 Å22) for 12 Na+ ions per unit cell into zeolite A is easily achieved, the apparent exchange limits for Rb+ (radius ) 1.47 Å 22)18 and Cs+ (radius ) 1.69 Å 22)19 are 8.4 and 7.0 per unit cell, respectively. When a Na-X single crystal was ionexchanged with 0.05 M aqueous RbOH solution, only 71 of the 92 Na+ ions per unit cell were replaced by Rb+ ions.21 This work was done with the hope that K+ exchange into single crystals of zeolite X large enough for single-crystal X-ray crystallography (> ca. 0.1 mm in diameter) would go to completion if done at an elevated temperature, and to determine the structure of fully dehydrated, fully K+-exchanged zeolite X, which promised to be interesting. These results could be used to optimize the parameters in a Monte Carlo calculation of the K+ positions in zeolite X as was done for Li92-X and Na92-X by Vitale et al.7 2. Experimental Section 2.1. Sample Preparation. Large single crystals of sodium zeolite X of stoichiometry Na92Si100Al92O384 per unit cell were prepared in Russia.23 One of these, a colorless octahedron about 0.14 mm in cross-section, was lodged in a fine Pyrex capillary. It was ion exchanged with flowing aqueous 0.1 M KNO3 (Alfa, 99.992% purity), pH adjusted to 12 by adding concentrated KOH solution (Alfa, ACS grade, 85% min, K2CO3 2% max, heavy metals as Ag 1 ppm, Fe 1 ppm, Ni 1 ppm, Na 0.05%), at 80 °C for 24 h (for additional experimental details, see Table
10.1021/jp000710r CCC: $19.00 © 2000 American Chemical Society Published on Web 09/02/2000
Cation Crowding in Zeolites
J. Phys. Chem. B, Vol. 104, No. 38, 2000 8947
TABLE 1: Summary of Experimental Data crystal cross-section (mm) ion exchange time (h) ion exchange T (°C) mean flow ratea (mm/s) data collection T (°C) scan technique scan rate (degree/min) radiation (Mo KR) λ1 (Å) λ2 (Å) space group unit cell constant, ao (Å) 2θ range for ao (deg) no. of reflections for ao 2θ range in data collection (deg) no. of reflections gathered no. of unique reflections (m) no. of reflections (Fo > 4σ(Fo)) no. of parameters (s) weighing parameters (a/b) merging R (intensities, all reflections) R1b wR2c goodness of fitd
0.14 24 80 8.0 21 θ-2θ 3.0 0.70930 0.71359 Fd3hm 25.083(5) 14-19 20 3-50 3715 709 352 56 0.089/0 0.17 0.066 0.193 0.97
Flow rate ) (volume of exchange solution)(time)-1(cross-section area of capillary - cross-section of crystal)-1. b R1 ) Σ|Fo - |Fc|/ΣFo is calculated using all reflections with Fo > 4σ(Fo). c wR2 ) {Σw(Fo2 - Fc2)2/Σw(Fo2)2}1/2 is calculated using all reflections. d Goodness of fit ) {Σw(Fo2 - Fc2)2/(m - s)}1/2 is calculated using all reflections. a
1). The crystal was then dehydrated at 400 °C and 1 × 10-5 Torr for 48 h. While these conditions were maintained, the hot contiguous downstream lengths of the vacuum system, including a sequential U-tube of zeolite 5A beads fully activated in situ, were allowed to cool to ambient temperature to prevent the movement of water molecules to the crystal from more distant parts of the vacuum system that had not been baked out. Still under dynamic vacuum in its capillary, the crystal was then allowed to cool, and was sealed in its capillary and removed from the vacuum line by torch. The crystal remained colorless throughout. 2.2. Space Group Selection. The reflection conditions (h + k, k + l, l + h ) 2n; 0kl: k + l ) 4n) indicate that the space group is either Fd3h or Fd3hm. Fd3h was initially chosen because most crystals from this synthesis batch, regardless of subsequent chemical treatment, have been refined successfully in Fd3h.24 Their diffraction data refines to error indexes lower in Fd3h than in Fd3hm, with mean Al-O distances correctly ca. 0.10(3) Å longer than mean Si-O distances.24 This is in agreement with Loewenstein’s rule25 which requires alternation of Si and Al atoms for crystals with Si/Al ) 1, and requires that also, at least in the short range, for this crystal whose Si/Al ratio is 1.09. However, Fd3h was rejected and Fd3hm was chosen because (a) in least-squares refinement with Fd3h, only an insignificant difference (ca. 0.01 Å) was seen between the mean Al-O and Si-O bond lengths; and (b) the diffraction data refined to lower error indexes in Fd3hm. The erasure of the ca. 0.10 Å difference between Al-O and Si-O indicates that the Si and Al composition at the Si position is essentially the same as that at the Al position, and therefore the same as that of the entire crystal: the long-range Si, Al ordering has been entirely or nearly entirely lost.24 This can occur most easily if antidomains have formed.26 Although unlikely for crystals from this synthesis batch,23,24 it remains possible that they were present in the initial Na-X crystal. 2.3. Diffraction Data Collection. Diffraction intensities were collected at 21(1) °C with an automated Siemens P3 four-circle
computer-controlled diffractometer equipped with a pulse-height analyzer and a graphite monochromator. All unique reflections in the positive octant of an F-centered unit cell for which 4° < 2θ < 50° were recorded. The intensities of three reflections in diverse regions of reciprocal space were measured every 97 reflections to monitor crystal and instrument stability. Only small random fluctuations of these check reflections were observed. Lorentz, polarization, and profile corrections were made. Absorption corrections were not made; empirical corrections are ineffective for these crystals (octahedra). The unit cell constant and additional data describing the diffraction work are given in Table 1. 2.4. Structure Solution. Full-matrix least-squares refinements27 were done on Fo2 using all data. It was initiated with the atomic positions of the framework atoms [(Si,Al), O(1), O(2), O(3), and O(4)] in fully dehydrated, partially Cu2+exchanged zeolite Y.28 Because the SiO4 and AlO4 tetrahedra are indistinguishable in the space group Fd3hm, only the average species, (Si,Al), is considered in this work. This minor but pervasive disorder introduces additional inaccuracy to the final bond lengths and angles. Isotropic refinement of the framework atoms yielded the error indexes R1 ) 0.33 and wR2 ) 0.70 (definitions are given in footnotes to Table 1). The three largest peaks on the initial Fourier electron-density difference function were at (0, 0, 0), (0.070, 0.070, 0.070), and (0.243, 0.243, 0.243). Isotropic refinement including these peaks as K(I), K(I′), and K(II), respectively, converged with R1 ) 0.13 and wR2 ) 0.34. Their occupancies refined to 14.0(3), 12.6(6), and 28.4(8) ions per unit cell, respectively. The two largest peaks on the ensuing difference Fourier function were at (0.065, 0.065, 0.421) and (0.125, 0.125, 0.405). They refined to 11(2) K+ ions at K(III′a) and 10.8(9) at K(III), respectively, with R1 ) 0.10 and wR2 ) 0.29. The largest peak on the next difference Fourier function appeared at (0.070, 0.070, 0.421), K(III′b). Isotropic refinement including it converged with R1 ) 0.089 and wR2 ) 0.27. Anisotropic refinement of all positions except K(III′a) and K(III′b), which were isotropically refined, converged with R1 ) 0.069 and wR2 ) 0.21. The thermal ellipsoids at K(I) and K(I′) were both unacceptably elongated with small esds, so each was refined as two positions. In a subsequent step, K(I′) similarly became three separate positions. All five of these positions were refined with fixed occupancies and isotropic temperature factors. One of them, K(I′c), is off a 3-fold axis to avoid a short K(I′a)‚‚‚K(I′c) contact. The final error indexes refined to R1 ) 0.066 and wR2 ) 0.19. Further refinement with occupancies varying converged with large esds and no reduction in the error indexes. Attempts to divide K(III) into two or more positions were unsuccessful. The largest maximum and deepest minimum on the final difference Fourier function were 0.73 and -0.58 e/Å. The highest maxima were either too far from or impossibly too close to other atoms. Fixed weights were used initially; the final weights were assigned using the formula w ) q/[σ2(Fo2) + (aP)2 + bP + d + e sin(θ)] where P ) (Fo2 + 2Fc2)/3, with a and b as refineable parameters. Atomic scattering factors for (Si,Al), O, and K were used29 and corrections for anomalous scattering were made.30 The form factor for (Si,Al) was that of Si diminished by a factor of 0.966 to give the correct number of electrons. Final values and additional details are given in Table 1. The final structural parameters are given in Table 2, and selected interatomic distances and angles are in Table 3.
8948 J. Phys. Chem. B, Vol. 104, No. 38, 2000
Zhu and Seff
TABLE 2: Positional,a Thermal,b and Occupancy Parameters atom
Wyc. pos.
Si,Al O(1) O(2) O(3) O(4) K(Ia) K(Ib) K(I′a) K(I′b) K(I′c) K(II) K(III) K(III′a) K(III′b)
192(i) 96(g) 96(g) 96(g) 96(g) 16(c) 32(e) 32(e) 32(e) 96(g) 32(e) 96(g) 96(g) 96(g)
cation site
x
y
z
U11c or Uiso
I ca. I I′ I′ ca.I′ II III III′ III′
-555(1) -1105(2) -46(2) -706(3) 1770(3) 0 94(6) 616(9) 785(11) 806(35) 2451(1) 1250 586(14) 797(13)
1243(1) 1105(2) -46(2) -706(3) 1770(3) 0 94(6) 616(9) 785(11) 668(14) 2451(1) 1250 586(14) 797(13)
352(1) 0 1397(3) 380(3) 3176(3) 0 94(6) 616(9) 785(11) 668(14) 2451(1) 4070(9) 4219(11) 4219(11)
19(1) 30(3) 25(3) 43(3) 40(4) 10(5) 43(6) 23(12) 30 30 32(2) 188(24) 51(12) 49(13)
U22
U33
U12
U13
U23
19(1) 30(3) 25(3) 43(3) 40(4)
17(1) 37(5) 33(4) 38(5) 27(5)
-3(1) 4(3) 10(3) 21(4) 29(4)
2(1) -9(2) 6(3) -3(3) 0(3)
-4(1) -9(2) 6(3) -3(3) 0(3)
32(2) 188(24)
32(2) 29(13)
13(2) -53(25)
13(2) 0
13(2) 0
occupancyd varied fixed 192 96 96 96 96 6 4 4 4 4 28.4(4) 12.2(9) 10.7(18) 12.8(21) ΣK+) 90.1(29)
a Positional parameters are given ×104. b Thermal parameters have the units Å2 × 103. Numbers in parentheses are the esds in the units of the least significant digit given for the corresponding parameter. Parameters without esds were fixed in least-squares refinement; the positional and anisotropic thermal parameters without esds were fixed by symmetry. c The anisotropic temperature factor ) exp[(-2π2a-2)(h2U11 + k2U22 + l2U33 + 2hkU12 + 2hlU13 + 2klU23)]. d Occupancy factors are given as the number of atoms or ions per unit cell.
TABLE 3: Selected Interatomic Distances and Angles distances (Å) (Si,Al)-O(1) (Si,Al)-O(2) (Si,Al)-O(3) (Si,Al)-O(4) mean (Si,Al)-O K(Ia)-O(3) K(Ib)-O(3) K(I′a)‚‚‚K(I′a) K(I′a)‚‚‚K(I′b) K(I′a)‚‚‚K(I′c) K(Ib)‚‚‚K(I′b) K(I′b)‚‚‚K(I′b) K(Ib)‚‚‚K(I′c) K(I′b)‚‚‚K(I′c) K(I′c)‚‚‚K(I′c) K(I′a)-O(3) K(I′a)-O(2) K(I′b)-O(3) K(I′b)-O(2) K(I′c)-O(3) K(I′c)-O(2) K(II)-O(2) K(II)-O(4) K(III)-O(4) K(III)-O(1) K(III′a)-O(4) K(III′a)-O(1) K(III′b)-O(4) K(III′b)-O(1) a
angles (deg) 1.649(3) 1.662(3) 1.667(4) 1.648(3) 1.657 2.669(9) 2.468(15), 2.913(19) 4.48(7),a 5.34(4)b 3.91(6)c 4.06(8)b 3.79(4)b 3.29(8)c 3.92(8)b 3.29(8)c 3.63(14),c 3.89(9)c 2.510(21) 3.047(13) 2.92(3) 3.311(23) 2.63(4), 2.97(9) 3.311(23) 2.92(6), 3.32(6) 3.014(8) 2.894(18) 3.355(11) 2.66(3) 2.93(2), 2.94(2) 2.61(3) 2.92(2), 2.94(2)
O(1)-(Si,Al)-O(2) O(1)-(Si,Al)-O(3) O(1)-(Si,Al)-O(4) O(2)-(Si,Al)-O(3) O(2)-(Si,Al)-O(4) O(3)-(Si,Al)-O(4) O(3)-K(Ia)-O(3) O(3)-K(Ib)-O(3) O(3)-K(I′a)-O(3) O(3)-K(I′b)-O(3) O(2)-K(IΙ)-O(2) O(1)-K(IΙΙ)-O(4) O(4)-K(IΙΙ)-O(4) O(1)-K(IΙΙ′a)-O(1) O(1)-K(IΙΙ′a)-O(4) O(1)-K(IΙΙ′b)-O(1) O(1)-K(IΙΙ′b)-O(4)
111.3(3) 109.2(4) 108.5(4) 107.6(4) 107.2(4) 112.7(4) 88.1(2), 91.9(2) 82.0(6), 86.8(3) 93.2(7) 76.7(15) 99.7(2) 50.4(2) 78.9(6) 114.8(10) 57.4(5), 57.3(5) 114.8(9) 57.9(4), 57.7(4)
Readily avoidable by cation placement. b Unavoidable. c Avoidable if there are four K(I′a)‚‚‚K(I′c) contacts per unit cell.
3. Results and Discussion Zeolite X is an aluminum-rich synthetic analogue of the naturally occurring mineral faujasite (see Figure 1). Exchangeable cations, which balance the negative charge of the aluminosilicate framework, are found within the zeolite’s cavities. They are usually found at the following sites shown in Figure 1: site I at the center of the double six-ring (D6R), site I′ in the sodalite cavity on the opposite side of either of the D6R's six-rings from site I, site II′ inside the sodalite cavity near a single six-ring (S6R), site II at the center of the S6R or displaced from this point into a supercage, site III on a 2-fold axis in the supercage opposite a four-ring between two 12-rings, and various III′ sites somewhat or substantially distant from III but
otherwise near the inner walls of the supercages or the edges of 12-rings. Exchangeable cations select sites within the zeolite that best balance its charge with conventional cation-to-oxygen bond lengths. In addition, cations with specific coordination requirements will seek to satisfy them. In this crystal structure, K+ ions are located at nine different crystallographic sites, indicating a structure of unexpected complexity. About 14 K+ ions per unit cell are on 3-fold axes in the D6Rs, at or near site I. Six at K(Ia) are exactly at the centers of their D6Rs, but each of the remaining eight at K(Ib) is pushed off center by a K+ ion at K(I′b) or K(I′c). Three site I′ positions, K(I′a), K(I′b), and K(I′c), are each occupied by four K+ ions, respectively, per unit cell. About 28 K+ ions
Cation Crowding in Zeolites
J. Phys. Chem. B, Vol. 104, No. 38, 2000 8949
Figure 1. Stylized drawing of the framework structure of zeolite X. Near the center of the each line segment is an oxygen atom. The nonequivalent oxygen atoms are indicated by numbers 1-4. Silicon and aluminum atoms alternate at the tetrahedral intersections except that Si must substitute for Al at about 4% of the Al positions in the crystal studied. Further mixing, due perhaps to the presence of antidomains, has occurred. Extra framework cation positions are labeled with Roman numerals (see Section 3).
are located at site II. The remaining ca. 36 K+ ions occupy one site III and two site III′ positions: 12.2(9) at K(III), 10.7(18) at K(III′a), and 12.8(21) at K(III′b). The total number of K+ ions found per unit cell, 90.1(29), is not significantly different from 92, the number required to balance the negative charges of the zeolite framework. It is possible that K+ exchange has been incomplete as has been proposed for Tl-X,31 or that the zeolite has lost some aluminate in conjunction with its loss of long-range order.24 H+ exchange seems less likely because the pH of exchange was 12. Of the 14 K+ ions inside D6Rs, the six at K(Ia) (see Figure 2, part a) coordinate to six framework O(3)s at 2.669(9) Å, very close to the sum of the ionic radii of K+ and O2-, 1.33 + 1.32 ) 2.65 Å.17 The eight at K(Ib) are ca. 0.4 Å off center along a 3-fold axis (see Figure 2, part c), 2.468(15) Å from three O(3)s and 2.913(19) Å from the remaining three. About 12 K+ ions are located at three site I′ positions. Four at K(I′a) bond to three O(3)s at 2.51(2) Å (see Figure 2, part b). The four at K(I′b) bond to three O(3)s at 2.92(5) Å, significantly longer than the sum of the radii of K+ and O2- (see Figure 2, part c). The other four at K(I′c) are off 3-fold axes to avoid a close K(I′a)‚‚‚K(I′c) contact; each binds to three O(3)s, two at 2.63(4) Å and one at 2.97(9) Å(see Figure 2, part d). These unusual occupancies and positions can be understood as follows. Among the 16 D6Rs per unit cell, six have one K+ ion at K(Ia) at the center (nothing further) (see Figure 2, part a), two have two K+ ions at K(I′a), one on each side (see Figure 2, part b), and the remaining eight have one K+ ion at K(Ib) off center in the D6R, and another at K(I′b) or K(I′c) on the opposite side of the D6R (see Figure 2, parts c and d). The rule governing the occupancies at sites I and I′, nI + (nI′/2) ) 16, is violated in this structure; that sum, (6 + 8) + (4 + 4 + 4)/2 ) 20, indicates the presence of cation crowding.
Figure 2. Stereoviews of the four kinds of double six-rings. Of the 16 D6Rs per unit cell, six are occupied as shown in a, two as shown in b, four as shown in c, and four as shown in d. For the purposes of the drawing, names of the atoms have been shortened. Ellipsoids of 50% probability are shown.
Two kinds of close K+‚‚‚K+ contacts must exist, one between one site I and one site I′ K+ ion, and the other between two K+ ions at site I′. The four 3.79(4) Å K(Ib)‚‚‚K(I′b) and four 3.92(8) Å K(Ib)‚‚‚K(I′c) contacts can be seen in Figure 2, parts c and d. Because there are 12 K+ ions per unit cell at I′ sites, four sodalite units must accommodate two K+ ions each. Only one of the longest 4.48(7) Å K(I′a)‚‚‚K(I′a) distances can occur per unit cell. If that occurs, only two of the next longest 4.06(6) Å K(I′a)‚‚‚K(I′c) approaches can exist per unit cell.
8950 J. Phys. Chem. B, Vol. 104, No. 38, 2000
Figure 3. All site III′ positions (near the edges of 12-rings) in the structures, determined by single-crystal methods of (a) Kca.90-X (this work), (b) Na92-X (ref 9), and (c) T1ca90.8-X (ref 31). In all structures the 12-rings are sparsely occupied; no more than two cations need to occupy any 12-ring so all very close contacts are readily avoided. In (a) the (Si,Al) postion is labeled as alternating Si and Al atoms because of the short-range order that must exist;20 the K+ ions at site III are not shown. For the purposes of the drawing, the names of the atoms have been shortened. Ellipsoids of 20% probability are shown.
Because the shorter K(I′a)‚‚‚K(I′b), K(I′b)‚‚‚K(I′b), K(I′b)‚‚‚ K(I′c), and K(I′c)‚‚‚K(I′c) distances should be avoided, it is most
Zhu and Seff likely that there are no K(I′a)‚‚‚K(I′a) and four K(I′a)‚‚‚K(I′c) contacts per unit cell (see Table 3, footnote c). The thermal parameter at K(Ib) is noticeably large. Two K+ positions must exist near K(1b), one near K(I′b) and the other near K(I′c) (see Figure 2, parts c and d). However, they could not be distiguished crystallographically. Kim et al. found totals of 15.3(5) and 12.5(8) K+ ions at sites I and I′, respectively, in (apparently) incompletely K+exchanged zeolite X.13 Assuming the integers 15 and 12 for these two occupancies, respectively, that structure would have five, one, two, and eight of each of these types of D6Rs, respectively, as compared to six, two, four, and four in this work. In the supercages, only 28 K+ ions are found at site II. (Several unexchanged Na+ ions, or possibly even H+ ions, might also be there to complete the filling of site II.) The K(II)-O(2) distance, 2.656(8) Å, is same as the sum of the K+ and O2radii, 2.65 Å.17 About 12 K+ ions at K(III) are loosely held by two O(4)s at 2.894(18) Å and four O(1)s at 3.355(11) Å. Two site III′ positions are occupied by 10.7(18) and 12.8(21) K+ ions, respectively. Both positions are close to one O(4) and two O(1) framework oxygens (see Figure 3, part a). However, both are qualitatively different from the III′ positions occupied by Na+ ions in the structure of dehydrated Na92-X9 (Figure 3, part b), presumeably because K+ (radius ) 1.33 Å 17) is substantially larger than Na+ (radius ) 0.97 Å 17). K+ ions, like Tl+ (radius ) 1.47 Å,17 Figure 3, part c),32 fit well into longer arcs of the zeolite framework such as the 12-ring bay at O(1)-Si-O(4)Al-O(1) and into a cradle of six oxygen atoms at site III. The occupation of three positions rather than a single one of lowest energy at sites III/III′, is not easily explained. Large interpotassium distances are readily available among these three positions. The distribution might be due to their closeness in energy, the asymmetric environment generated by the partial occupancy of K+ ions at the I′ sites, and the Si/Al disorder among the T positions. The change of space group from Fd3h to Fd3hm might be due to the high temperature used for K+ exchange. (This was done to increase the extent of K+ exchange.) When a crystal from the same batch was ion exchanged at 21(1) °C with the same solution used in this work, the space group Fd3h was retained: the mean Al-O distance was ca. 0.07 Å longer than the mean Si-O distance.16 (A lesser degree of K+ exchange was observed in that work whose precision was similar to that reported here.) Because the pH and the temperature of ion exchange were not far from those at which zeolite LSX forms from the gel, and Na+ and K+ were both present initially as in LSX synthesis, recrystallization may have occurred to generate antidomain regions24 within the structure. Alternatively, but less likely,24 such disorder may have been present in the crystal studied when it was initially selected for this work. The relatively large number of reflections with Fo > 4σ(Fo) in the diffraction data set and the relatively small final error indexes (see Table 1) indicate that the zeolite framework has not been otherwise damaged by this treatment. Supporting Information Available: A table of observed and calculated structure factors squared with esds. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Herden, H.; Einicke, W. D.; Schollner, R.; Dyer, A. J. Inorg. Nucl. Chem. 1981, 43, 2533. (2) Forano, D.; Slade, R. C. T.; Krogh Andersen, E.; Krogh Andersen, I. G.; Price, E. J. Solid State Chem. 1989, 82, 95.
Cation Crowding in Zeolites (3) Ple´vert, J.; Di Renzo, F.; Fajula, F. J. Phys. Chem. B 1997, 101, 10340. (4) Feuerstein, J.; Lobo, R. F. Chem. Mater. 1998, 10, 2197. (5) Shepelev, Yu. F.; Anderson, A. A.; Smolin, Yu. I. Zeolites 1990, 10, 61. (6) Olson, D. H. Zeolites 1995, 15, 439. (7) Vitale, G.; Mellot, C. F.; Bull, L. M.; Cheetham, A. K. J. Phys. Chem. B 1997, 101, 4559. (8) Porcher, F.; Souhassou, M.; Dusausoy, Y.; Lecomte, C. Eur. J. Mineral. 1999, 11, 333. (9) Zhu, L.; Seff, K. J. Phys. Chem. B 1999, 103, 9512. (10) Smith, J. V. Molecular SieVe Zeolites-I.; Flanigen, E. M., Sand, L. B., Ed.; Adv. Chem. Series, No.101, American Chemical Society: Washington, DC, 1971; p 171. (11) Mortier, W. J.; Bosmans, H. J. J. Phys. Chem. 1971, 75, 3327. (12) Mortier, W. J.; Bosmans, H. J.; Uytterhoeven, J. B. J. Phys. Chem. 1972, 76, 650. (13) Lee, Y.; Carr, S. W.; Parise, J. B. Chem. Mater. 1998, 10, 2561. (14) Ojo, A. Private communication. (15) Jang, S. B.; Kim, Y. Bull. Korean Chem. Soc. 1995, 16, 539. (16) Zhu, L. Ph. D. Dissertation, University of Hawai at Manoa, 2000. (17) Breck, D. W. Zeolite Molecular SieVes; John Wiley & Son: New York, 1973; p 579.
J. Phys. Chem. B, Vol. 104, No. 38, 2000 8951 (18) Barrer, R. M.; Rees, L. V. C.; Ward, D. J. Proc. R. Soc., Ser. A 1963, 273, 180. (19) Vance, T. B., Jr.; Seff, K. J. Phys. Chem. 1975, 79, 2163. (20) Jeong, M. S.; Kim, Y.; Seff, K. J. Phys. Chem. 1994, 98, 1878. (21) Lee, S. K.; Kim, Y.; Kim, D.; Seff, K. Bull. Korean Chem. Soc. 1998, 19, 98. (22) Handbook of Chemistry and Physics, 76th ed.; Lide, D. R. Ed.; CRC Press: Boca Raton, FL, 1995; Section 12, p 14. (23) Bogomolov, V. N.; Petranovskii, V. P. Zeolites 1986, 6, 418. (24) Bae, D.; Seff, K. Micro. Meso. Mater., accepted for publication. (25) Loewenstein, W. Am. Mineral. 1954, 39, 92. (26) Peterson, B. K. J. Phys. Chem. B 1999, 103, 3145. (27) Sheldrick, G. M. SHELX97, Program for the Refinement of Crystal Structures; University of Go¨ttingen: Germany, 1997. (28) Haniffa, R. M.; Seff, K. J. Phys. Chem. B 1998, 102, 2688. (29) International Tables for X-ray Crystallography; Kynoch Press: Birmingham, England, 1974; Vol. 4, p 73. (30) Reference 29, p 149. (31) Zhu, L.; Seff, K. Meso. Micro. Mater. 2000, 39, 187. (32) Kim, Y.; Han, Y. W.; Seff, K. Zeolites 1997, 18, 325.
