10156
J. Phys. Chem. 1994,98, 10156-10159
Structure of the K32+Cluster in Zeolite A Tao Sun? and Karl SefF Department of Chemistry, University of Hawaii, Honolulu, Hawaii 96822 Received: July 13, 1993; In Final Form: June 20, 1994@
A single crystal of zeolite K12.5-A was prepared by exposing K-A at 350 "C to 0.3 Torr of potassium vapor, and its structure was determined by X-ray diffraction methods in the cubic space group Pmjm. At 350 "C, K32+ clusters form in at least 50% of the sodalite cavities. No excess potassium atoms (and therefore no clusters) are found in the large cavities. The triangular K32+cluster interacts primarily with framework oxygens, but the effect of the one delocalized electron can be seen in the longer K-0 bond lengths (2.78(2) as compared to 2.59(2) 8, in dehydrated K-A) and shorter K-K distances (4.35(6) as compared to 4.70(3) 8, in G3+). Due to considerations of partial occupancy, the K-K distance in K32+ is likely to be shorter than 4.35 8, (perhaps as little as 4.0 A), and its approach to framework oxygens is likely to be longer than 2.78 8, (perhaps as long as 2.9 8,).
Introduction
It has been known for about three decades that cationic alkali metal clusters can be synthesized in zeolite cavities by exposing zeolite powder to alkali metal vapor. Kasai and R a b ~ l - ~ exposed Na-Y zeolite powder to Na metal vapor and detected the tetrahedral alkali metal cluster N a 3 + by electron spin resonance (ESR). Na.55+ was found when Na-X was exposed to sodium metal vapor. Subsequently a series of ESR experiments were done by Edward~,~-lO Jacobs,11-13and K e ~ a n ' ~ . ' ~ on samples prepared by various synthetic methods. The new cationic alkali metal clusters Na32+,Na54+,K32+,and b3+, as well as Na, K, Rb, and Cs metal particles, were found in zeolites A, X, and Y. Barrer16 and Smeulders'','* found by ESR that N a 3 + clusters could form in synthetic sodalites; this indicated that the N Q ~ +clusters in zeolites A, X, and Y might also be in sodalite cavities. However, the actual positions of all of these cationic clusters within zeolites remained entirely unknown, and little could be said about the bonding within the clusters. Thomas found that Na2+ and Na32f could be produced in Na-A, Na,Li-A, Na-X, and Na,Li-X by y-irradiati~n.'~ He showed by time-resolved diffuse-reflectance spectroscopy that electrons ejected from photochemically excited arenes could induce the formation of N a 3 + clusters.20,21 Recently, linear Rb43+, C S ~ ~ and + , C S ~ ~clusters, + and triangular Rb32+ and Rb3+ clusters, were found in the sodalite cavities of zeolite A.22-29 Single crystals of K15-A (framework charge ca. -12 per 12.3 A unit cell) and K142-X (framework charge ca. -92 per 25.2 8, unit cell) were synthesized by exposing Na-A and Na-X to potassium metal vapor, and their structures were determined by X-ray diffraction methods.30It was found that each sodalite cavity of zeolite A contained 3.84(15) potassiums, indicating that at least most contained a tetrahedral Kqn+ ( n < 4) cluster, and that two-dimensional potassium continua has formed in its large-cavityleight-ringchannel system. At high potassium loadings, all potassium clusters in the sodalite cavities of zeolite X were linked by potassiums at the centers of double six-oxygen rings (D6R's) to form rings or chains (one-dimensional continua: zig-zag strands). A three-dimensional potassium continuum, similar to that found in C S ~ ~ * - X formed , ~ ~ in K142-X. Current address: Department of Chemical Engineering, M.I.T., 77 Massachusetts Avenue, Cambridge, MA 02139. Abstract published in Advance ACS Abstracts, September 1, 1994. @
In this work, K12.5-A was synthesized by exposing dehydrated K-A at 350 "C to 0.3 Torr of potassium metal vapor, and its structure was determined by single-crystal X-ray crystallography. At this low potassium loading, the simple but informative structure reported herein is seen. The approximate composition K14,,-A30532had been prepared by exposure of Na-A at 300 "C to 0.3 Torr of K(g). K17-A33 resulted when Na-A at 350 "C was treated at 1.2 Torr of K(g) for 48 h, followed by KO sorption (ca. 0.0005 Torr) at 150 "C for 24 h. Experimental Section
Single crystals of Na-A were prepared by Charnell's method.34 One of these, a cube -0.08 mm on an edge, was lodged in a Pyrex capillary. K+ ion-exchange was performed by flow methods: 0.1 M aqueous KNO3 (Fisher Scientific, 99.99%) solution was allowed to flow past the crystal at a velocity of -1 cm/s for 48 h at 21 "C. Then this capillary with its K-A crystal was connected to a vacuum line. After dehydration at 400 "C and 1 x Torr, the crystal at 350 "C was brought into vapor contact (0.3 Torr) with K(g) at 350 "C for 16 h. After the reaction, the crystal, now red-brown in color, was sealed off under vacuum. A Siemens four-circle computer-controlled P3 diffractometer with a graphite monochromator and a pulse-height analyzer was used for preliminary experiments and for the subsequent collection of diffraction intensities at 24 "C. Molybdenum radiation (Kal = 0.70930 A; Ka2 = 0.71359 A) was used throughout. The cell constant, a = 12.248(2) A, was determined by a least-squares treatment of 15 intense reflections for which 18" < 20 30". The space group Pm?m was used for this work. The b-reflections for its supergroup Fm?c were carefully checked; for only two reflections, (135), and (335) which violates the c-glide condition of Fm%, was I > 3 4 4 . The 8-28 scan technique was used for data collection. Each reflection for which 3" < 20 < 60" was scanned at a constant rate of 0.25 deg min-' in o from 0.6" below the calculated Kal peak to 0.6" above the Ka2 maximum. Background intensity was counted at each end of a scan range for a time equal to half the scan time. The intensities of three reflections in diverse regions of reciprocal space were recorded every 97 reflections to monitor crystal and instrument stability. Only small random fluctuations of these check reflections were observed.
0022-365419412098-10156$04.50/0 0 1994 American Chemical Society
J. Phys. Chem., Vol. 98, No. 40, I994 10157
Structure of the K32+ Cluster in Zeolite A TABLE 1: Positional, Thermal? and Occupancy Parameted Wyckoff atom position x Y z UisdUIl‘ UZZ Si(A1) 24(k) 0.0000 0.1851(4) 0.3743(3) 194(24) 179(22) 0(1) 12(h) O.oo00 0.2377(14) 0.5000 632(148) 492(82) O(2) 12(i) O.oo00 0.2876(9) 0.2876(9) 483(111) 166(57) O(3) K(l) K(2) K(3) K(4)
24(m)
8(g) 8(g) 6(f) 12Q)
0.1 123(7) 0.1 123(7) 0.1256(16) 0.1256(16) 0.2426(8) 0.2426(8) O.oo00 0.4741(24) 0.2409(26) 0.2409(26)
0.3510(10) 0.1256(16) 0.2426(8) 0.4741(24)
0.5000
336(48) 571(105) 702(57) 741(162) 252(224)
336(48) 571(105) 702(57) 741(162) 252(224)
u33
UI 2
u13
u23
43(19) 159(23) 0 0 0 12(79) 0 0 166(70) 166(57) 0 0 -9(47) 420(82) 121(55) -9(47) 571(105) 284(107) 284(107) 284( 107) 502(58) 702(57) 502(58) 502(58) 0 0 -261(256) 741( 162) 0 1102(537) -120(218) 0
fixed varied occupancyd occupancyd 24‘ 12 12 24 3
2.4(ly 5.6(ly 3.1(2) 1.5(2)
* u = 12.248(2) 8, in the space group Pm3m. * Positional and thermal parameters are given x 104. The anisotropic temperature factor = exp{(&2/u2)(hZU~~kzU22 pV33 -2hkUl2 I -I-2hlUl3 2kZU23)). Occupancy factors are given as the number of atoms or ions per unit cell. Occupancy for (Si) = 12; occupancy for (Al) = 12. f The sum of the occupancies at K(l) and K(2) was constrained in least squares to be 8 to avoid a too-short 2.48 8, K(l)-K(2) distance.
+
+
+
TABLE 2: Selected Bond Distances (A) and Angles (deg) 107.5(8) O(l)-Si(Al)-O(2) 1.669(8) 111.0(5) O(1)-Si(A1)-0(3) 1.664(4) 1.662(4) 2.781(21) 2.623( 12) 2.91(3) 3.23(4) 2.87(4) 2.95(3) 4.35(6)“
O(2)- Si(Al)-0(3) 0(3)-Si(Al)-0(3) Si(A1)-O( 1)-Si(A1)
Si(Al)-0(2)-Si(Al) Si(Al)-0(3)-Si(Al) O(3)-K( 1)-0(3) 0(3)-K(2)-0(3) O(1)-K(3)-O(1) 0(3)-K(4)-0(3) O( l)-K(4)-0(1) K( 1)-K( 1)-K(1)
107.6(5) 111.8(8) 134.6(11) 170.4(12) 149.3(8) 92.2(3) 104.5(5) 102.5(13) 78.3(13) 88.5(14) 60’
Likely a mean of K32+and not-partially-reducedK+-K+ distances.
’This an le is required by symmetry (assumes that a shorter distance, 3.18(2)
f, is too short and is not used).
Standard deviations were assigned to individual reflections by o(Z) = [w2(CT
+ B , + B,) + (pZ)z]’”
where CT is the total integrated count, B1 and BZ are the background counts, and Z is the intensity. The value of p was found to be 0.006 by least-squares analysis of the final structure. The intensities were corrected for Lorentz and polarization effects; the contribution of the monochromator crystal was calculated assuming it to be half-perfect and half-mosaic in character. Profile fitting was done. Of the 636 unique reflections, only the 188 for which the net count exceeded three times its standard deviation were used in subsequent structure determination and refinement. Full-matrix least-squares refinement was initiated with fixed positional and thermal parameters for the framework atoms [Si(Al), O( l), 0(2), 0(3)] and four potassium position^?^*^^ The final refinement converged nicely to
The final difference function was featureless. The goodnessof-fit, (xw(Fo - \FJ),/(m - s))’I2, is 1.11; the number of observations, m, is 188, and the number of parameters, s, is 37. The largest maximudminimum in the final difference function is 1.1 e/A3/-l.0 e/A3. The final structural parameters and selected interatomic distances and angles are presented in Tables 1 and 2, respectively. The chemical composition indicated crystallographically is K12.5-A per unit cell. Results and Discussion
About 2.5 potassium ions are found in each sodalite cavity at K(1), on threefold axes. This occupancy is larger than the
SI AI Figure 1. K32+cluster in the sodalite cavity of zeolite A. Ellipsoids of 20% probability are shown. 1.5 ions found in the sodalite cavity of anhydrous, simply K+exchanged, zeolite A.35,36This increase in occupancy, coupled with the potassium-rich zeolite composition, indicates that KO atom sorption has occurred. The ionic character of these K( 1) potassium ions is clearly indicated by their short distances to framework oxygens. However, the K-0 distance (see K10 3 in Figure l), 2.78(2) A, is substantially longer than the corresponding 2.57(2) or 2.61(2) 8, distances in dehydrated K-A,35-36indicative of partial Kf reduction. Furthermore, the potassium ions at K(l) (coordinates x = y = z = 0.1256(16)) are recessed cu. 0.64(4) A deeper into the sodalite cavity than those in dehydrated K-A (x = y = z = 0.1557(25)), indicative of bonding, or at least of diminished repulsion, among these K( 1) ions. It is apparent that the valence electrons of the sorbed K atoms have delocalized over a number of K+ ions to form cationic clusters in the sodalite cavities. To avoid an unreasonably short 3.18(2) 8, contact, these K(l) potassium ions must be alternately arranged among the 6-rings in the sodalite cavity. (An n-ring is a 2n-membered ring; its aperture is defined by the n oxygens of the ring.) The K-K bond length is then 4.35(6) A. (It might be somewhat less (vide infra).) This distance is much shorter than the 4.70(3) A distance found in Q3+ in zeolite A,30,32indicating that these K-K interactions in the sodalite cavity are stronger. (The sodalite units themselves are nearly the same in these two structures; the O(3) positions are 0.08 A closer to the 6-ring centers in this structure, and this can be attributed to the K+ ions at K(2).) The greater degree of bonding to be expected for K3,+ (one e- distributed over the three K+-K+ contacts of
10158 J. Phys. Chem., Vol. 98, No. 40, 1994
Figure 2. Electron density function in the (0, 0, 1) plane showing an
8-ring (a 16-membered,8-oxygen ring). Contour levels begin at 4 e-/%13 and have 4 e-/%13increments. The potassium peak at the center represents the sum of four off-center peaks, only one of which can be occupied in any particular 8-ring. The contouring at the very center is a shallow maximum.
a triangle), as compared to b3+ (one e- distributed over the six K+-K+ contacts of a tetrahedron) is consistent with this. This had been observed directly by Thomas. From hyperfine coupling constants of different sodium clusters in zeolite cavities, Thomas concluded that the interactions between the unpaired electron and the corresponding nuclei increased in the order N a 3 + < Na32f < Na2+.19 Therefore, the potassium ions in the sodalite cavities in this structure are most likely to have formed triangular K32+ clusters (Figure 1). Edwards exposed K-A to K metal vapor at ca. 250 "C and found, by ESR at 77 K, only K32+ clusters in the sodalite cavity of zeolite A.' No other ESR active species were detected, indicating that the K32+ cluster, rather than G3+, forms upon initial potassium metal sorption into K-A. If the ideal composition M12Si12Al12048 is assumed for zeolite A, there would be two types of unit cells. About half of the sodalite cavities would have a K32+cluster, and the large-cavity volume about it would have five K+ ions at K(2), three at K(3), and two at K(4). This sums to a charge of +12 per unit cell. Similarly, in the remaining half of the unit cells, the sodalite cavities would have two noninteracting K+ ions, surrounded by six K+ ions at K(2), three at K(3), and one at K(4). If fewer than two noninteracting K+ ions are present in these latter sodalite cavities, then, in order still to average to 2.5 potassium ions per unit cell at K( l), more than half of the sodalite cavities would contain K32f. If the slightly Si-enriched composition M11.8Si12.zA111.8048 reported by BlackwelP7 is correct, the percentage of K32+ clusters in the sodalite cavities would be higher. About 12.5 - 11.8 = 0.7 potassium atoms would be sorbed to form K32+ clusters in 70% of the sodalite cavities, leaving the remainder, which should be Si-rich, with only about one K+ ion. A strong interaction was seen in Kls-A30*32between K(3) and K(4). In that structure, the K+ ions at K(3) were found at the very centers of 8-rings, not at their conventional position (0,0.47,0.47). This does not occur in this structure: the 8-ring K+ ions refined nicely at (0, 0.47, 0.47); this is supported by the appropriate Fourier electron-density section (Figure 2). This indicates that the ions at K(3) are simply K+ (not partially reduced and not involved in clustering) in this structure.
Sun and Seff The 2.78(2) A K(1)-O(3) distance observed, which is too long for a simple K+ to framework-oxygen approach, must be understood to be an average of longer K32+-0(3) distances and the shorter K+-0(3) distances of normal (not partially reduced) K+ ions. The thermal ellipsoid at K(l) is appropriately elongated (see Figure 1). The actual K32+ approach to framework oxygens could be as long as 2.9 A, and the true edge length of the K32f clusters could be less than 4.35 A, perhaps as little as 4.0 A. It should be pointed out that two previous attempts to prepare this K32+cluster had failed. In the first experiment, a dehydrated K-A single crystal at 160 "C was exposed to ca. 0.0008 Torr of potassium metal vapor for 24 h. (The vapor pressure of K(I) at 160 "C is 0.0008 Torr.) A colorless transparent crystal was obtained whose structure, as determined by X-ray diffraction, was simply dehydrated K-A, the same as that reported by Smith;35no water molecules were located in this crystal. The second experiment was done at 300 "C with 0.3 Torr of potassium metal vapor, and again the resulting crystal was colorless, indicating again that no KO sorption had occurred. These results are inconsistent with reports using zeolite powder: Nozue et al. reported that KO atoms are easily sorbed by K-A to generate a dark brick-red powder at 160 0C,38and Edwards reported that K32+clusters formed in the sodalite cavity of zeolite A at 250 "C when K-A was exposed to KO vapor.7 The same discrepancy is established with sodium and zeolite A: Edwards reported that a temperature of 300 "C or above was required to synthesize a homogeneous powder sample of Na-A from a mixture of Na-A and Na metal.1° Again, in contrast, in a similar experiment using Na-A single crystals at 350 "C, McCusker found that no NaO sorption occurred.39 It might be expected that somewhat higher temperatures (and therefore vapor pressures), or longer sorption times, would be needed for metal atom sorption into single crystals as compared to powders because of the diffusion path difference. Nonetheless, it remains unclear why single crystals of K-A and Na-A have failed to sorb corresponding metal atoms at what appear to have been adequate conditions. The higher temperature used in this work, 350 "C, is apparently responsible for its success. It is interesting that excess KO atoms can be sorbed into zeolite A at 150 or 300 "C only if the preparation begins with KO atom reduction of the Na+ ions in a Na-A single whereas no such sorption is seen (vide supra) if the preparation begins with a single crystal of K-A. Two possible reasons are proposed for this result. Perhaps the reduction of Na+ by KO provides the energy (locally) for potassium atoms to enter and move within the zeolite. It is also possible that potassium clusters form only before the reduction reaction is complete, when the windows are "more open": in the preparation of Cs-A from Ca-A by reaction with CsO(g),linear Cs3*+ clusters and yet-unreacted CaZ+ions were seen in reaction intermediate^.^^ To summarize, K12.5-A was prepared by exposing K-A at 350 "C to potassium vapor, and its structure was determined by X-ray diffraction methods. At 350 "C, K32+ clusters form in the sodalite cavity. No excess potassium atoms (and therefore no clusters) are found in the large cavity. The K32f cluster interacts primarily with framework oxygens. The one delocalized electron binds the potassium ions in K32+ more strongly than it can bind those in a G3+cluster. Supplementary Material Available: Table of observed and calculated structure factors with estimated standard deviations (3 pages). Reflections for which I < 3 4 0 were not used in least squares and are indicated by a change of sign of their
Structure of the K32f Cluster in Zeolite A estimated standard deviations. Ordering information is given on any current masthead page.
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