2157
Crystal Structures of Potassium-Exchanged Zeolite A (11) (12) (13) (14)
H. Wahlquist. J. Chem. Phys., 35, 1708 (1961). G. W. Smith, J. Appl. Phys., 35, 1217 (1964). F. H. Spedding and M. J. Pikal, J. Phys. Chem., 70, 2430 (1966).
R. A. Robinson and R. H. Stokes, “Electrolyte Solutions”, 2nd ed (re-
vised). Butterworths, London, 1965, Chapter 11. (15) J. Reuben, Biochemistry, 10, 2834 (1971). (16) H. Hagnas, Suomen Kemi., 40,130 (1967). (17) R. H. Stokes in “The Structure of Electrolyte Solutions”, W. J. Hamer, Ed.. Wiley, New York, N.Y., 1959, p 298. (18) L. Gutierrez, W. C. Mundy, and F. H. Spedding, J. Chem. Phys., 61, 1953 (1974).
(19) The detailed results of the viscosities of La(C104)3 solutions became available to the author after the manuscript of this paper was submitted for publication, It can be shown that the relaxation rates of the lanthanum-139 In La(C104)3solutions conform to the macroscopic viscosities: however, the relaxation rate at infinite dilution, I / TIo, obtained from a fit to e 7 is 312 sec-l, which is considerably higher than the value of llr? =’ 277 sec-’ obtained for chloride solutions. Thus our conclusion regarding the formation of outer-sphere ion pairs with perchlorate ions receives further support. (20) F. H. Spedding. L. E. Shiers, and J. A. Rard. J. Chem. Eng. Data, 20, 66 (1975).
Crystal Structures of Hydrated and Dehydrated Potassium-Exchanged Zeolite A Peter C. W. Leung, Kevin B. Kunz, Karl Seff Chemistry Depa~ment,University of Hawaii, Honolulu, Hawaii 96822
and 1. E. Maxwell KoniklUkdShell-Laboratorium, Amsterdam, The Netherbnds (Received March 3, 1975)
Publication costs assisted by the U S . National Sclence foundation
The crystal structures of hydrated (a = 12.301 ( 2 ) A) and vacuum-dehydrated (a = 12.309 ( 2 ) A) potassium-exchanged zeolite A have been determined by single-crystal X-ray diffraction techniques in the cubic space group Pm3m. In each case all 12 potassium ions per unit cell were located, and the structures refined to final conventional R indices of 0.114 and 0.057, respectively. In the dehydrated structure, thrie equivalent K+ ions lie near the centers of the oxygen 8-rings. Another six equivalent K+ ions lie on three of the four threefold axes opposite 6-rings in the large cavity, in an eightfold equipoint. The three remaining K+ ions are nonequivalent, and lie along the remaining threefold axis, two inside the sodalite unit and one recessed far into the large cavity. I t is most unique that this latter K+ ion is not within contact distance of any atom or ion; it occupies a relatively shallow energy minimum in the electrostatic field of the remainder of the structure. In the hydrated structure, eight K+ ions are found inside the large cavity on the threefold axes near the 6-oxygen rings; three more lie near the centers of the 8-rings but somewhat off those planes; the twelfth is located near the center of the unit cell. Twelve water molecules bridge between the eight threefold axis K+ ions along the edges of the cube. Three of these twelve can also be close to the twelfth K+ ion near the center of the large cavity. The sodalite unit contains eight water molecules which hydrogen bond to each other and to framework oxygens.
Introduction The K+ positions in potassium-exchanged zeolite A, K-A,‘ are likely to be similar, except for the effect of ionic radius, to those of the Na+ ions in Na-A. The position of the twelfth K+ ion, if it could be determined, would be of interest in the hydrated structure because it would suggest which of the possible positions previously determined2 for a twelfth Na+ ion is correct and would serve to complete the structure of hydrated Na-A, the form in which zeolite A is synthesized. In the dehydrated case, a position opposite a 4-oxygen ring was noted in dehydrated Na-A;3 the larger X-ray scattering factor of K+ might have allowed a strong confirmation of the previous result. An ion populating such a site at high temperature in this or another zeolite would be particularly available to sorbed molecules, would be particularly coordinatively or associately unsaturated, and could be a site of high catalytic activity. Such a site has been shown to be sensitive to sorbate ~ o n t e n t . ~ Recent ir studies of hydrated monovalent cation-exchanged zeolite A have led, using large-ring vibrational as-
signments,“ to specific conclusions about the placement of K+ ions in the zeolite. The work reported herein was undertaken in part to test the validity of these assignments and the conclusions based upon them. Experimental Section Crystals of zeolite 4A were prepared by a modification of Charnell’s m e t h ~ d ,including ~ a second crystallization using seed crystals from the first synthesis. Ion exchange with 0.2 N aqueous KCl solution, conducted for 1 week at 2 8 O with daily agitation and renewal of solution, yielded clear and colorless crystals. A flame test of these crystals indicated, by the absence of the yellow sodium D line, that the exchange was complete. Later, a separate batch of crystals was similarly prepared with 0.2 N aqueous KOH solution to discourage any possible hydrogen ion exchange into the zeolite. Subsequent crystallographic analysis indicated the presence of 12 K+ ions per unit cell in each sample. A crystal 0.08 mm on an edge, exchanged in KC1 solution, was dehydrated by a procedure3 similar to that used The Journal of PhysicalChemistry, Vol. 79,No. 20, 1975
2158
P. C. W. Leung, K. B. Kunz, K. Seff, and I. E. Maxwell
TABLE I: Positional, Thermal,= and Occupancy Parameters for K-A Wyckoff position
x
j'
z
1311 OI' B i s o
@22
j333
312
j3zS
P13
occu PancY factor
(a) Dehydrated 1886(3) 3774(2) 2457(8) 112 2833(6) 2833(6) 1111(4) 1111(4) 3597(6) 2311(4) 2311(4) 2311(4) 0 4770(25) 4770(25) 3557(62) 3557(62) 3557(62) 1306(24) 1306(24) 1306(24) 1849(34) 1849(34) 1849(34) 0 0 0
14(2) 16(2) 41(9) 22(8) 44(9) 26(5) 41(4) 41(4) 49(3) 49(3) 61(13) 93(30) 833(183) 833(183 4(1) 7(2)
19(2) 0 13(7) 0 26(5) 0 46(4) 33(13) 49(3) 50(8) 93(30) 0 833(183) -611(247)
0 0 0
1' 1 31(15) 1 -9(8) 1 50(8) 3/4 4 5 ( 5 5 ) 1/4 -611(247) 1/8 1/8 1/8 2(4)
0
-9(8) 50(8) 0 -611(247 .
(b) Hydrated
1825(2) 3712(2) 2228(11) 1/2 2941(7) 2941(7) 1113 (6) 1113(6) 3437(11) 2683(21) 2683(21) 1/2 978(15) 978(15) 978(15) 2585(4) 2585(4) 2585(4) 795 (56) 4460(34) 4460(34) 3828(141) 4021(81) 4621(81)
lO(2) 0 0 5(3) 1* 16(2) 16(2) 151(20) 14(7) 0 0 0 1 16(8) 18(4) 0 0 19(10) 1 32(8) 18(4) 23(10) 36(10) 32(4) 92(11) 36(10) 1 32(4) 93(27) -599(156) 0 387(71) 387(71) 0 1 103(11) 103(11) 103(11) -26(25) -26(25) 1 -26(25) 26(5) 1 56(2) 56(2) 56(2) 26(5) 26(5) 8(1) 1/8 1/24 8(5) a Positional and anisotropic thermal parameters are given X lo4; isotropic thermal parameters are in A2. Numbers in parentheses are the 0 0 0
estimated standard deviations in the least significant digits. See Figure 1for the identities of the atoms. The anisotropic temperature factor = exp[- (Bl1hZ+ &2k2 + 8 3 3 P + P&k + P&l+ Pzakl)]. Occupancy for (Si) = 1/2; occupancy for (Al) = 1/2.
for dehydrating zeolite 4A, Na-A. The Pyrex capillary containing the crystal was sealed off by torch under vacuum Torr, and was after 48 hr of dehydration a t 300° and mounted on a goniometer head. The crystal remained clear and colorless. Another crystal from the same exchange, also 0.08 mm on an edge, was mounted on the tip of a fine glass fiber and was left exposed to the atmosphere (relative humidity 30%) during data collection. Subsequent diffraction intensities for both crystals were collected at 20°. Two crystals prepared from KOH solu$ion, also both 0.08 mm on an edge, were treated similarly. The cubic space group Pm3m (no systematic absences) appeared to be a p p r ~ p r i a t e . ~A . ~Syntex -~ four-circle computer-controlled diffractometer with a graphite monochromator and a pulse-height analyzer was used throughout for preliminary experiments and for the collection of diffraction intensities. Molybdenum radiation (Koa, X 0.70926 A; Ka2, X 0.71354 A) was used throughout. In each case, the cell constant, a = 12.309 (2) A for the dehydrated crystal (prepared from 0.2 N KOH solution) and a = 12.301 (2) bi for the hydrated crystal (prepared from 0.2 N KCl solution), was determined by a least-squares treatment of 15 intense reflections for which 28 < 24O. (As will be discussed later, only the results for these two of the four data sets are presented.) The 28-8 scan technique was employed at a constant scan rate of 0.5O/min (in 28). The scan range varied from 2.0' at 28 = 3O to 2 . 5 O at 28 = 70°. One-half of the total scan time was spent counting background a t each end of the scan range. All unique reciprocal lattice points (889 for each crystal) below a maximum 28 value of 70' were examined. The high upper limit for 28 was chosen to maximize the size of the data sets, even though few reflections with The Journal of phvsical Chemistry, Vol. 79, No. 20, 1975
large 28 values showed significant intensity. Three check reflections, monitored in each data collection after every 100 reflections, indicated no significant variation in intensity. Standard deviations were assigned according to the formula:
+
~ ( 1 =) [w2{CT 0.25(tc/tb)2(B1 B z ) )
+
where w is the scan rate, CT is the total integrated count obtained in a scan time, t,, B1, and B2 are the background counts each obtained in time tb, and I = w[CT - 0.5(tC/ tb)(B1 + Bz)]. A value of 0.02 was assigned to the empirical parameter p10 to account for instrumental instability. The net counts were then corrected1' for Lorentz and polarization effects, including that of the monochromator crystal which was assumed to be half perfect and half mosaic in character. An absorption correction was not necessary-the transmission coefficients are estimated to range within 0.6% of their average values. Only those reflections in each data set for which the net counts exceeded three times the corresponding esd's were used in structure solution and refinement. This amounted to 214 and 374 unique reflections for the dehydrated and hydrated crystals, respectively. S t r u c t u r e Determination Dehydrated K - A . Full-matrix least-squares refinement of dehydrated K[AlSi04]-A,1C or K-A (exchanged using KCl), began using the cation coordinates (adjusted to account for the differing ionic radii) and framework positions of thallium-exchanged zeolite A.8 Isotropic refinement converged quickly t o an R1 index, (21F , - IF, 11 )/ZF,, of 0.095 and an Rz weighted index, ( Z w ( F , - I F c 1 ) 2 / 2 ~ F , 2 ) 1 / 2of, 0.101. Anisotropic refinements of all framework atoms with
2159
Crystal Structures of Potassium-Exchanged Zeolite A
TABLE 11: Selected Interatomic Distances (8) and Angles (deg)a 1.676(5) 1.660(3) 1.668(2) 2.620(7) 2.986(3) 2.861(30) 3.372(45) 4.257(10) 2.840(25) 2.505(9)
(a) Dehydrated K-A O(l)--(Si, A1)-0(2) O(l)-(Si7 A1)-0(3) 0(2)-(Si, A1)-0(3) 0(3)-(Si, A1)-0(3) (Si,Al)-O(l)-(Si, All (Si, A1)-0(2)-(Si, Al) (Si, A1)-0(3)
