Crystal Structure of Dehydrated K-Exchanged Zeolite A
The Journal of Physical Chemistry, Vol, 83, No. 6, 1979 741
The thioketene radical anion 4 - e seems to constitute a special case. As suggested by the X-ray structural analysis of the parent thioketene 4,11 the thiocarbonyl group is jammed between four methyl groups of the neighboring equatorial 2,6 tert-butyl residues. Thus the bending out of the thioketene plane is hindered and a significantly lower 13C coupling constant acSCresults (Table 11). The isotropic g values (Table 11) are remarkably low compared with the data determined for dialkylthioketyls (about 2.006(3).12 However, low g values seem to be a typical trait of a radicals (Table 111). In conclusion it can be stated with some justification that the thioketene radical anions have to be classified as radicals with a CP: geometry and a 2A‘ ground state.
(9) (10)
Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft. S. K. thanks the Fonds der Chemischen Industrie for a graduation stipend.
(16)
Jr., and J. T. Laemmle, J . Am. Chem. SOC.,98, 6561 (1976), and references cited therein; E. C. Ashby and J. S. Bowers, Jr., ibid., 99, 8504 (1977); V. I. Savin, Zh. Org. Khim., 12, 1857 (1976); M. Okubo, Bull. Chem. SOC.Jpn., 50, 2379 (1977). J. Voss, K. Thimm, and L. Kistenbrugger, Tetrahedron,33, 259 (1977). J. Voss and W. Walter, Justus Liebigs Ann. Chem., 734, 1 (1970). A. J. Fry, “Synthetic Organic Electrochemistry”, Harper and Row, New York. 1972. R. S. Nicholson, Anal. Chem., 38, 1408 (1966). For the notion "quasi-reversible" cf. D. Moller, Z. Phys. Chem. (Leiprig), 257, 289 (1976). E. Schaumann, S.Harto, and G. Adiwidjaja, Angew. Chem. 88, 25 (1976); Angew. Chem., Int. Ed. Engl., 15, 40 (1976). C.-P. Klages and J. Voss, Angew. Chem., 89, 744 (1977); Angew. Chem., Int. Ed. Engi., 16, 726 (1977). J. E. Bennett, B. Mile, and A. Thomas, Trans. Faraday Soc., 61, 2357 (1965); D. W. Ovenall and D. H. Whiffen, Mol. Phys., 4, 135 (196 1). M. J. Lin and J. H. Lunsford, J . Mag. Reson., 29, 151 (1978). J. E. Bennett, B. Mile, and A. Thomas, Trans. Faraday Soc., 63, 262 (1967). P. W. Atkins and M. C. R. Symons, “The Structure of Inorganic Radicals. An Application of ESR to the Study of Molecular Structure”, Elsevier, Amsterdam, 1967. A. D. Walsh, J . Chem. Soc. London, 2266 (1953). R. J. Buenker and S. D. Peyerimhoff, Chem. Rev., 74, 127 (1974). P. B. Ayscough, “Electron Spin Resonance in Chemistry”, Methuen, London, 1967. M. T. Rogers and L. D.Kispert, J . Chem. Phys., 46, 221 (1967). J. W. Cooper, D. Griller, and K. U. Ingold, J . Am. Chem. SOC., 97, 233 (1975); D. Griller, J. W. Cooper, and K. U. Ingold, ibid., 97, 4269 (1975). R. W. Fessenden and R. H. Schuler, J. Chem. phys., 39, 2147 (1963). C. E. Dykstra and H. F. Schaefer, 111, J. Am. Chem. SOC., 98, 2689 (1976). J. E. DelBene, J. Am. Chem. Soc., 94, 3713 (1972).
(6) (7) (8)
(1 1) (12) (13) (14) (15)
(17) (18) (19)
References and Notes Part 2: W. Schmuser and J. Voss, J. Chem. Res., 149, 1901 (1978). As discussed in the text, the structure of the thioketene radical anions differs considerably from the usual thioketyls derived from noncumulated thiocarbonyl compounds. However, the comparison of these two classes of radical anions is particularly important and justifies the treatment in a common series. E. Schaurnann and W. Walter, Chem. Ber., 107, 3562 (1974). C.-P. Klages, W.-R. Klein, E. Schaumann, and J. Voss, unpublished results. E. C. Ashby, J. D. Buhler, I.G. Lopp, T. L. Wiesemann, J. S. Bowers,
(20) (21)
(22) (23) (24)
Crystal Structure of Dehydrated Potassium-Exchanged Zeolite A. Absence of Supposed Zero-Coordinated Potassium. Refinement of Si,ACOrdered Superstructure Joseph J. Pluth and Joseph V. Smith* Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois 60637 (Received August 28, 1978) Publication costs assisted by the National Science Foundation
Accurate X-ray structure analysis of two crystals of dehydrated potassium-exchanged A zeolite showed no electron density for the supposed zero-coordinated K atom proposed by Leung et al. Instead, two small electron-density peaks were observed opposite to four oxygen atoms. The peak assigned to 0.5 atom of K(6) at 0.5,0.24, 0.24 is displaced into the main cage at -2.8 A to two O(3) and -3.0 8, to two O(1). The peak assigned to 0.15 atom of K(7) at 0.175,0,0 is displaced into the sodalite cage at -3.0 A to four O(3). These distances of 2.8-3.0 A between electron-density peaks need not apply to interatomic distances because the electron density for the framework oxygens is determined almost entirely by oxygens not bonded to K(6) and K(7). A plausible crystal-chemical argument suggests downward adjustment by -0.1-0.2 A to give interatomic distances. The other potassium atoms are 6.3K(1) at 0.23,0.23,0.23; 3K(2) at 0,0.47,0.47; and 1.5K(4,5) in an elongated peak centered on 0.16, 0.16, 0.16. All K atoms lie within 3 8, of several framework oxygens, and, although their coordinations are unusual, there is no evidence for zero coordination. An electron microprobe analysis yielded 11.8A1 and 12.2 Si per cell. X-ray diffraction data collected with Cu Ka radiation yielded intensities sufficiently strong for refinement of the superstructure which has regular alternation of Si and A1 in the framework. The complex arrangement of K atoms is qualitatively consistent with minimization of electrostatic energy, but may be complicated by weak Si,Al disorder.
Introduction For 2 decadles, it has been known that dehydration of
some varieties of zeolites leads to unusual coordination of cations as water of hydration is lost and the unhydrated cations are forced into rings or even onto surfaces of the aluminosilicate framework. Considerable interest has arisen from the claim that some zeolites have zero-coordinated cations as defined in ref 1: “These results [on 0022-3654/79/2083-0741$01.00/0
Rb-exchanged zeolite A] allow us to employ a stringent definition of noncoordination. If the distance between two ions exceeds the sum of their corresponding radii by more than 1.0 A, then these ions may be considered not bonded or uncoordinated.” Zero-coordinated cations2are supposed to occur only in a zeolite with high AI/% ratio, and “large monovalent cations are required which “coat” the inner surface of the zeolite, filling all possible coordination sites 0 1979 American
Chemical Society
742
The Journal of Physical Chemistry, Vol. 83, No. 6, 1979
J. J. Pluth and J. V. Smith
TABLE I: Electron Microprobe Analyses expected atomic contents for 8 oxygens sample
n
Ansoglass An,, glass Anso glass Asb microcline Amelia albite Na-A (Chicago) Na-A (Hawaii) K-A (Chicago) K-AU
5 5 5 5 5 6
Aniooglass Ca-A K-A
9 12 7
obsd atomic ratios for 8 oxygens
Na A1 Si Ca K Na A1 Analytical Conditions: 1 5 kV, 10 nA, 50 pm diameter, 3 0 s 0.5 1.5 2.5 0.5 0 0.50 1.46 0.3 1.7 2.3 0.7 0 0.33 1.65 0.1 1.9 2.1 0.9 0 0.13 1.84 0.01 1.00 3.00 0.00 0.98 0.00 0.97 0.99 1.00 3.00 0.00 0.01 1.02 0.98
Si
Ca
K
0.51 0.72 0.92
0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00 0.00
1.02 0.01
1.91 1.92 1.88 2.00
2.53 2.32 2.12 3.01 3.00 2.05 2.05 2.10 2.02
Analytical Conditions: 1 5 kV, 1 0 nA, 20 pm diameter, 30 s 0 2.0 2.0 1.0 0 0.00 1.99 0.00 1.95 0.00 1.95
2.01 2.04 2.05
1.01 0.99
0.00 0.00
0.00
1.92
2.05 2.03 0.02
8 5 6
0.01
0.00 0.00 1.95 1.90
Single crystal rehydrated after X-ray study. n is the number of spots.
before all negative charges of the framework have been balanced”. Dehydrated type A zeolite has eight 6-rings and three 8-rings, and, in several structures supposed to contain 12 alkali a t o m ~ , l -the ~ twelfth alkali atom was placed in a zero-coordinated site. All the X-ray structure refinements made by Seff and colleagues have technical problems: (a) there is no independent determination of the chemical composition of the crystal used for X-ray analysis, (b) the supposed zero-coordinated cations are represented by irregular or weak electron-density peaks similar in size to residual peaks not ascribed to atoms, and (c) refinement was made for the pseudostructure (a = 12 A, Pm3m) rather than the superstructure with ordered A1 and Si atomse6 We now report crystal structure determinations of both the pseudo- and superstructures of K-exchanged A zeolite in which the chemical composition was tested by electron microprobe analysis and the experimental error of the electron density was halved over that in the earlier determination of K-exchanged A eol lite.^
Electron Microprobe Analysis Crystals of zeolite 4A (i.e., the Na variety of Linde A zeolite) were prepared by a modification of Charnell’s method7 including a second crystallization using seed crystals from the first synthesis. Batches of crystals were exchanged with either KC1 or CaC1,l M solution for 5 days at 95 “C to give K-A and Ca-A varieties. Seff kindly sent three crystals from his aliquot of the original supply of 4A crystals. Electron microprobe analyses were made of grains mounted in epoxy and polished with 0.25-pm diamond paste before coating with a 200 A carbon film. Since alkalies and water escape from zeolites when bombarded with an intense electron beam, analyses were made with a weak beam and a sensitive Li-drifted silicon crystal whose response was interpreted by a multichannel analyzer on-line to a computer. The energy spectrum was analyzed by the Reed-Ware procedures programmed by I. M. Steele. The calibration factors were determined from synthetic and natural feldspars and glasses. After initial tests, a set of analyses was made at 15-keV beam voltage, 5-nA beam current, and 5-pm beam diameter in order to check for chemical zoning across the 20-100-pm diameter crystals. Feldspar crystals and glasses were analyzed under the same conditions. For 30-s analyses, Na-A crystals gave the following counts: Na peak 600, background 80; A1 peak 1600, background 120; Si peak 1900, background 100. Counting statistics give a relative precision: Na 5%; A1 3%; Si 2.5%. Within these limits, no evidence of zoning
of Na,A1 and Si was found in traverses of grains. All further analyses assumed that crystals were chemically homogeneous, and data for several spots were averaged to increase the precision. Table I shows mean analyses of feldspar standards and K-A and Ca-A crystals taken with the beam current increased to 10 nA and the spot diameter increased to either 20 or 50 pm. It should be emphasized that the A1 and Si peaks overlap in a solid-state-detector spectrum, and that a correct analysis depends on accurate calibration from standards. Furthermore it must be emphasized that alkali ions move more easily than multicharged ions. Although it proved possible to obtain analyses of Na and K which were reproducible within counting statistics when the spot diameter was 20-50 pm, some worry remained that an electric gradient might cause migration near the impact of the electron beam. Consequently greater reliance was placed on the analyses of Si,A1and Ca than of Na and K. All microprobe analyses of Na-, K-, and Ca-A showed Si/Al > 1.0 (atomic) with a mean value near 1.06 for all data and a value of 1.04 just for Ca-A. When normalized to the feldspar standards, the mean value becomes 1.04 and the Ca-A value becomes 1.03. The analyses of the exchangeable cations are hard to interpret. When normalized to Amelia albite, the Na-A specimens from both Chicago and Hawaii have slightly more Na than Al, in violation of charge balance. For K-A normalized to Asbestos microcline, both A1 and K are lower than Si by 7 and 1170,respectively. For Ca-A normalized to Anlv glass, Ca matches Al, and both elements run 3% below SI. It is obvious that objective interpretation is impossible, but subjective reliance on Ca-A suggests that the Si/A1 ratio is near 1.03, which corresponds to 11.8 A1 and 12.2 Si. The Seff group attempted to obtain chemical compositions by population refinements from X-ray diffraction data, but statistical coupling with displacement (“thermal”) factors causes grave difficulties, as indicated by their common resort to arbitrary rounding to supposed stoichiometric values. In various papers, the cation content was taken to be equivalent to either 11 or 12 charges; thus for TI-A, the first structure determination was based on TlllAlllS~130489 and the revised determination on T111NalAl12Si12048.10 Reference 11on the structure refinement of dehydrated Na-A states: “This sample was found by repeated wet chemical analysis to have an approximate framework formula of A111,3Si12,704s11~3per unit cell. Although the values in this stoichiometric formula have been rounded further to the nearest integer in previous reports, ... the precision of the analysis does not exclude
-
The Journal of Physical Chemistry, Vol. 83, No. 6, 1979 743
Crystal Structure of Dehydrated K-Exchanged Zeolite A
TABLE 11: X-rav Diffraction Data and Structure Refinement exchange solution, T crystal diameter dehydration T, P, time space group wavelength, A filter (F), monochromator (M) cell dimension, A diffractometer orientation scan technique speed range background total intensities unique data set significant data set sin e / h maiximurri absorption coefficient absorption correction R weighted iP S a
PSa
PSb
LKSM
1 N KCl; 95 " C -85pm 300 "C, torr, 3 days Pm3m 0.71069 M 12.317 ( 3 ) Picker FACS-1 -7" from a fixed e -28 2" lmin 1.2-1.6" fixed 20 s 4888 488 392 (30) 0.65 11 cm" no 0.034 0.034 2.5
1 N KCl; 95 " C -85 pm 300-'C, torr, 2 days Fm3c 1.5418 F 24.600 ( 6 ) Syntex P2, -6" from a variable e -2e 29 t o 1.0" 1.3-1.6" variable, scanlbkg = 1 3928 (2127)' 691 (438)' 472 (379)' (20) 0.64 1 0 1 cm-I Yes 0.053 (0.043)' 0.046 (0,044)' 4.0 (3.9)a
0.2 N KC1; KOH; 28 "C 8 0 pm 300_"C, torr, 2 days Pm3m 0.71069 M 12.309 ( 2 ) Syntex ?
fixed e -2e 0.5"lmin 2.0-2.5" variable, scanlbkg = 1 889 889 214 (30) 0.81 11 cm-' no 0.058 0.050 1.65
Bracketed numbers refer to pseudostructure.
the possibility that the correct framework formula might be A112Si12048L2-, which is more ideal and which has been used extensively in previous investigations". The repeated wet chemical analysis has higher Si/A1 (1.12) than the electron microprobe analysis (1.03), but no information was given on the procedures used for the wet chemical analysis, and on the analytical errors. Because the Si and A1 atoms are stable to the electron beam, and because all measurements of the three varieties of A zeolite give Si/Al greater than unity, we prefer to conclude that the aclual Si/Al ratio is greater than unity, but we cannot assign a definitive error limit. X-Ray Diffraction Analysis Two cubes of K-A, 85 pm on edge, were lodged in silica capillaries, deh,ydrated under torr at 300 "C for 2 days, sealed at that temperature, and slowly cooled to room temperature. Rotation photographs taken for 3 days clearly revealed the superstructure diffractions for the true cell with a = 24.6 A. The first data set (labeled PSa) was collected with monochromatized Mo K a radiation and a Picker FACS-1 diffractometer, and only the intensities for the pseudocell (PmBm; a = 12.3 A) were collected. Diffractions were measured for a quadrant out to sin d/h = 0.65. Averaging of 4888 symmetry-related diffractions yielded 488 unique diffractions of which 392 were above background at the 3a level. The data set (labeled LKSM) obtained by the Seff group yielded only 214 diffractions above background a t the 3a level from the 889 intensities measured out to sin 8/h = 0.81. Although the angular range of the LKSM set was larger than for the PSa set, most of the additional diffractions had nonsignificant intensities. Furthermore, the PSa set had a larger fraction of significant intensities in its data set as a result of averaging of symmetry-related diffractions. A second data set (labeled PSb) was collected with filtered Cu K a radiation and a Syntex P21 diffractometer, and intensities for the true cell (FmBc;a = 24.6 A) were collected. Diffractions were measured for an octant out to sin O / h = 0 64. Averaging of 3928 symmetry-related diffractions yielded 691 unique diffractions of which 472 were above background at the 2a level. Of these 472 diffractions, 379 belonged to the pseudostructure and 93 to the superstructure. This data set compares with the
290 and 90 diffractions listed for hydrated Na-A in ref 6. Experimental conditions are compared in Table 11. The cell parameter was obtained by least-squares refinement using 28 values of Friedel pairs (PSa: 15 pairs, 16 < 28 C 31; PSb: 12 pairs, 35 < 28 < 71) and weighted wavelengths for Ka. Probably the a1 and a2 wavelengths for Cu radiation were beginning to resolve, thereby producing a systematic error in the observed cell dimension a = 24.600(6) A. The cell dimension obtained with Mo radiation (a = 12.317(3)A) lies within 26 of the LKSM value of 12.309(2) A. For the PSa and PSb data, the estimated errors in the intensities (uI)were calculated by: uI = P [ S + t2B (k1)2]1/2 where P is scan rate, S is peak scan counts, B is total background counts, t is the ratio of peak-to-background observation times, and k is an instability constant (0.02). Symmetry-equivalent diffractions were averaged using
+
where Iiand ai are the intensity and standard deviation of the ith equivalent diffraction. The intensity data were corrected for Lorentz and polarization effects assuming a monochromator crystal half-perfect and half-mosaic. An empirical absorption correction, part of the Syntex software, was applied to the PSb data using absorption curves obtained for 17 diffractions with 28 values between 7 and 87" remeasured every 5" rotation around the diffraction vector. The smallest empirical absorption factor was 0.82. For the PSb data, all diffractions obeyed the conditions for FmBc, except perhaps for four weak diffractions: 9,9,9, intensity of 932 f 90; 31,1,1, 168 f 17; 21,21,9; 190 f 13; 25,12,12, 171 f 17. Gramlich and Meier6 recorded only diffractions permitted by FmSc, and obtained satisfactory refinement of hydrated Na-A in this space group. A good refinement was obtained for the PSb data in FmBc (see later), and the four weak intensities were ignored. Nevertheless, the possibility of lower symmetry must be considered. Structure Refinement Gramlich and Meier6 found alternation of Si and A1 on tetrahedral sites when hydrated Na-A was refined in FmSc. Refinement in the pseudo-cell (Pmgm,a = 12.3 A)
744
J. J. Pluth and J. V. Smith
The Journal of Physical Chemistry, Voi. 83, No. 6, 1979
. .
Z =034
2.036
Z =0 3 8
PSbf
PSbp
LKSM
Flgure 1. Difference-Fourier syntheses for the region around the supposed K(3) site, which is shown as a heavy dot. For each of the four sets of experimental data, three sections are shown at heights z = 0.34, 0.36, and 0.38 which should pass through any peak for the supposed K(3) site. Positive, negative, and zero contours are shown respectively by continuous, dashed, and dotdashed lines. The contour levels are PSa, 0.06 e/A3; PSbp 0.09 e/A3; PSbf 0.07 e/A3; LKSM, 0.11 e/A3.
automatically results in one tetrahedral site (T) with apparent disorder of Si and Al. The LKSM data were refined in Pm3m, and the PSa and part of the PSb set (denoted PSbp) were also refined in this space group to give a direct comparison. Subsequently the full PSb set (denoted PSbf) was refined in Fm3c using starting parameters from the ordered framework in hydrated Na-A.6 Refinement was carried out by standard difference-Fourier and least-squares techniques. No difficulty was experienced in obtaining refinement of the PSa and PSbp sets with the coordinates given by LKSM for the framework atoms and the K(1), K(2), K(4), and K(5) sites. In order to check the K(3) site, which had 0 14
0.14
024
034 014
3 24
been placed on the threefold axis at x = y = z = 0.3557(62), difference-Fourier maps were calculated for the LKSM, PSa, PSbp, and the PSbf sets in which only the framework atoms and the K(1), K(2), K(4), and K(5) atoms were used for the calculated structure amplitude (actually one atom with a strongly anisotropic displacement ellipsoid was used to represent the close pair of K(4) and K(5) sites). Sections at z = 0.34, 0.36, and 0.38 (Figure 1)show no significant electron density at x = y = z = 0.3557 for all four sets of data. All peaks fall in the size range of peaks in other areas of the unit cell, except for two places to be described presently where new K atoms were assigned. Consequently it was concluded that the peaks in Figure 1are merely the result of random experimental error. There is a positive area near x = y = z = 0.38 in all the difference-Fourier maps but it changes shape greatly between the four maps. Moreover it is -0.5 A away from the supposed zero-coordinated K(3), and in none of the four maps is it the strongest peak in the unit cell. Because the peaks for the LKSM set of data are larger than for the P S sets of data, the experimental error must be greater for the LKSM data. Details of contour heights are listed in the legend to Figure 1. Examination of the difference-Fourier maps showed a significant peak near 0.245, 0.245, 0.5 for all three sets of PS data, whereas the corresponding region of the LKSM map showed random peaks (Figure 2). The significant peak is inside the large cavity about 2.9 A away from four oxygen atoms of a 4-ring, and was assigned to a site weakly occupied by potassium, denoted K(6). Another peak, weaker but still significant, was found for the PS sets of data near 0.00, 0.00, 0.18, but the LKSM map showed large apparently random peaks (Figure 3). The peak differs in detail between the three sets of PS data at the level expected for random experimental error. This second site lies at about 3.0 8, from four oxygen atoms of a 4-ring, and was assigned to a site weakly occupied by potassium, denoted K(7). Occupancy by univalent cations of sites opposite 4-rings had been invoked in early discussions of the Linde A zeolites (discussed in ref 12), and the occurrence of peaks at K(6) and K(7) was not surprising. Furthermore, Subramanian and Seff13proposed that, in dehydrated Na-A zeolite, a sodium atom lies in the large cavity opposite a 4-sing. 034 0.4
3.24
334 0.14
0.24
0.34
0.24
034
z= 2.48
PSa
PSbf
Z=0.5C
LKSM
Figure 2. Difference-Fourier syntheses from which the K(6) site was identified. The peak near (0.243, 0.243, 0.5) for the three sets of PS data is significantly greater than the random error peaks, but the peaks for the LKSM data are essentially random. Contour levels as in Figure 1.
LKSM
PSbf
Figure 3. Difference-Fourier syntheses from which the K(7) site was identified. The peak near (0.18,0,0) for the three sets of PS data is significantly greater than the random error peaks, but the peaks for the LKSM data are essentially random. Contour levels as in Figure 1.
Flgure 4. Stereoplot of atoms in the large cage of dK-A zeolite. Displacement ellipsoids at 30% probability level.
Structure refinement was completed by conventional least-squares techniques. In spite of the small deviation of the superstructure from the pseudostructure, satisfactory refinement was obtained, and indeed the resulting distances in the framework indicate essential alternation of Si and A1 on the tetrahedral nodes (next section). Refinement of tlhe weakly occupied K(6) and K(7) sites was difficult, but refinement of both population and isotropic displacement factors was obtained for all the PS data. The LKSM data had been refined with one K atom at each of two sites (x = y = z = 0.1306 and 0.1849) 1.2 A apart on the triad axis opposite a 6-ring. The PS refinements were made with 1.5K atoms placed in an ellipsoidal peak centered at x == y = z = 0.16, approximately half-way between the K(4) and K(5) sites. Because it is extremely difficult to distiinguish between a strong anisotropic displacement of a single atom and a split-atom model, especially for weakly occupied sites, it is assumed here that the two refinements are comparable mathematically. In the next section, arguments are given that split-atom models may be more plausible than models with strong anisotropic vibration. Final parameters are compared in Table 111, and conventional refinement indices are given in Table I1 where R = 1;IIFnI - IFcII/CIFot R w = [CwIIFnI - IFcI12/CwIFo1211’2 s = [Cwllr~o( - IFc112/(n, - nJl1’2
The final least-squares cycle minimized CwllF,,I - lFc112 with w = The number of diffractions is no and the number of independent parameters is n,. Atomic scattering factors and anomalous scattering corrections for Si2+,Al+, 0-,and K+ were taken from Vol. 4 of “International Tables for X-ray Crystallography”. Factors for Si2+and Al+ were interpolated between factors for Si, Si+,Al, and Al+. Refinements in Pm3m used (Si2+ A1+)/2. Final models were checked by difference-Fourier maps which showed only random noise at the atomic sites. Calculated and observed structure factors are available as supplementary material. Computer programs are listed in footnote 14. Selected bond distances and angles are given in Table IV. A stereoview of the structure is given in Figure 4 and the bonding of K atoms in Figure 5.
+
Discussion The simple but important conclusion is the lack of valid evidence for zero-coordinated K(3) in dehydrated K-A zeolite. The second conclusion is that all the cations lie adjacent to framework oxygen atoms, though the coordinations are quite different from those found in hydrated zeolites and other inorganic materials in thermodynamic equilibrium at low temperature. All the K atoms (or ions, if an ionic model is being used) have between two and six oxygen neighbors within 3 A, if the distances in Table IV are taken at face value. Readers should be aware, of course, that averaging occurs of all distances in individual
'he Journal of Physical Chemistry, Vol. 83,
J. J. Pluth and J. V. Smith
No. 6, 1979
E m
3 hn
hh
*m
3 m &
h
h
h
h l m
hl Y
a
P m &
m
h
h
E!
N
h
h
l
m PI
h
ri
is h
*m
h
3 .z
h
h
0
hh
rim
Nhl
h
h h l
hh
Wt-
h
The Journal
Crystal Structure of Dehydrated K-Exchanged Zeolite A
TABLE IV:
Interatomic Distances and Angles of Dehydrated K-Exchanged
of
Physical Chemistry, Vol. 83, No. 6, 7979 747
A
PSbf
.
T-O( 1 )
O(2) 2 o(3) mean K( 1)-3 O(3) K(1)-3 O(2) K(2)- O(1) K(2)- O(1) K(2)- O(2) K(2)- O(1) K(2)- O(1) K(3)-13 O( 3) K( 4)-3 O(3) K(4)-3 O(2) K( 5)43 O(3) K(5)-3 O(2) K(6)-2 O(3) K(6)-2 O(1) K(7)-4 O(3) o(i)-rr-o(2) o(1)-rr-o(3) o(2)-rr-o(3) O(3)-T-0( 3 ) T-O(1)-T T-O(2)-T T-O(3)-T a
PSa (Si,Al)
PSbp (Si,Al)
1.677(2) 1.664(1) 1.671(1) 1.671 2.590(3) 2.987(2) 2.837(6) 2.837(6) 3.295(8) 3.486(6) 3.486(6 )
1.675(2) 1.663(1) 1.669(1) 1.669 2.594(3) 2.985(2) 2.834(7) 2.834(7) 3.310(9) 3.461(8) 3.461(8)
2.61(2) 2.95(1)
2.57(2) 2.91(2)
Si
A1
1.577(11) 1.770(11) 1.627(12) 1.702(12) 1.736(8) 1.602(8) 1.602 1.736 2.593(4) 2.985(2) 2.78(6) 2.90(6) 3.315(10) 3.41(5) 3.51(5)
not present
2.57(2) 2.91(2)
values in two columns above are for K(4,5)
2.87(7) 2.99(6) 2.98(8) 107.9(2) 111.1(1) 107.5(1) 111.5(2) 129.0(2) 176.7(2) 151.9(2)
2.80(8) 2.94(7) 2.99(7 )
2.81(8) 2.94(7) 2.97(7) 107.9(2) 111.0(2) 107.5(2) 111.7(2) 128.5(3) 177.2(3) 152.1(2)
108.0(3) 1 1 0 4 2) l08.0(2) 111.1(3)
LKSM (Si,Al) 1.676(5) 1.660(3) 1.668(2) 1.669 2.620(7) 2.986(3) 2.86(3) 2.86(3) 3.37(5) 3.43(5) 3.43(5) 4.26(10)a 2.84(3) 3.11 2.51(9) 2.85(9) not present
107.7(3) 111.2(2) 106.9(2) 112.6(3)
128.5(3) 177.2(3) 151.9(2)
108.5(4) 111.0(4) 107.9(4) 110.2(4) 128.5(6) 178.4(5) 153.7(5)
Recalculated error: original value* of 0.01 A is too low.
Si
AI
0 3 03
03
03 AI
Flgure 5. Plot of
SI
01
nearest neighbors to K atoms in dK-A zeolite. Displacement ellipsoids at 50% probability level.
unit cells, and the distances to atoms of low fractional occupancy are subject to an unknown error. The coordinations and site occupancies need detailed discussion. Simultaneous least-squares refinement of population and displacement factors is not easy because of the strong correlation between the parameters. Refinement of K(1) in all three sets of PS data yielded 6.3K atoms within one standard deviation (Table 111), which compares with the value of 6 used by LKSM for this eightfold site. The K( 1) atom projects into the large cage
along the triad axis from the center of the 6-ring (Figure 4). Projecting along the triad axis in the opposite direction is a single ellipsoidal region of electron density which LKSM interpreted as two K atoms, one each a t x = y = z = 0.131 and 0.185, labeled respectively as K(4) and K(5). Using an elongated displacement ellipsoid, the PS refinements yielded 1.5K atoms a t a mean position K(4,5) a t x = y = z = 0.16. Both approaches to the refinement are mathematically valid, but for crystal-chemicalreasons it is plausible to argue that K atoms occur at different
740
The Journal of Physical Chemistry, Vol. 83, No. 6, 1979
positions in the elongated K(4,5) site and that the splitatom model of LKSM may be a reasonable approximation to a more complex atomic distribution. The K(2) atom is difficult to place because it is actually assigned statistically to four sites, each of quarter-occupancy displaced -0.45 A from the center of an %ring. Only a single broad peak occurs on an electron-density map, and it was necessary to fix the occupancy at the maximum value of 3 in order to obtain a refinement of the positional coordinates and the displacement ellipsoid. Figure 4 shows coordination only to the three closest oxygens, and the experimental data do not rule out models in which K(2) is displaced even further from the center of the 8-ring. Furthermore it is quite impossible to separate out thermal displacements from positional disorder of the center-of-motion of individual atoms. Both the K(6) and K(7) sites are weakly occupied, and it was possible to refine only a population factor and an isotropic displacement factor. Taken at face value, the P S refinements yield the following totals and bracketed errors of K atoms per pseudocell: PSa 11.08 (0.26); PSbp 11.43 (0.40); PSbf 11.38 (0.28). These totals are less than the 11.8 deduced from the electron microprobe analyses, but is the difference meaningful? If the LKSM value of 2K atoms for combined K(4) and K(5) were used instead of the PS values of 1.4 to 1.5 atoms, the apparent deficiency disappears completely. Even if this is not done, the apparent deficiency is only at the l a level for the PSbf data, if it is assumed that the standard deviations for occupancies are simply added. Actually the combined occupancies of K(1) and K(4,5) should not exceed 8 if pairs of K atoms are not to face each other across a 6-ring. If it is assumed that K(1) and K(4,5) together amount to 8 atoms, and K(2) adds on another 3 atoms, a total of 0.8 atoms would be needed in K(6) and K(7) to match the electron microprobe analysis. The actual sums for K(6) and K(7) are PSa 0.34(0.09); PSbp 0.65(0.15); PSbf 0.63(0.15). Probably the principal reason for the lower sum for the PSa data is the smaller B values obtained from the refinement (Table 111). It seems futile to press the matter further in view of experimental uncertainties which may be of a systematic as well as of a random nature. The full refinement of the PSbf data yields tetrahedral distances indicative of strong ordering of Si and A1 into alternate tetrahedra around the framework, as found in ref 6. The mean distances of 1.602 and 1.736 A agree within experimental error with the distances of 1.608 and 1.728 A found for hydrated Na-A zeoliten6 For feldspar minerals, the many structure analyses15indicate that the pure Si-0 and A1-0 distances are near 1.61 and 1.75 A, but there are subtle factors suggesting a variation of -0.01 A from one tetrahedron to another in various framework structures. The data for dK-A are consistent with one tetrahedron being occupied only by Si and the other mainly by A1 but with weak substitution by Si, but the experimental errors are too big to rule out complete order with a Si/A1 ratio of unity. At first sight, the distribution of K atoms into four sites [actually €ive if K(4,5) is split into K(4) and K(5)] is surprising. Why do not eight K occupy K(l), three occupy K(2), and the remaining K or fraction of a K atom occupy either K(6) or K(7)? It is qualitatively easy to see that such a simple distribution would not lead to the minimum electrostatic energy for a simple ionic model. Occupancy of K(6) would lead to repulsion with the two K(1) atoms in adjacent 6-rings (Figure 4). Furthermore if the Al/Si ratio is less than unity, the tetrahedral cations are not
J. J. Pluth and J. V. Smith
giving an exactly uniform contribution of charge to the framework; an occasional A1 site will be occupied by an Si atom. Unfortunately, the X-ray structural analysis yields only the average of all the unit cells, and the contents of individual ones are unknown. It is possible only to speculate that the K(4,5) site is occupied rather than the adjacent K(1) site in response to occupancy of an adjacent K(6) site, or perhaps even to occupancy by K(2) of the one out of the four possible sites which is closest to the 6-ring associated with the particular K(1) and K(4,5) sites. Occupancy of the K(7) site may similarly rule out occupancy of an adjacent K(4,5) site and enforce occupancy of the K(1)site. Perhaps the apparent splitting of the K(4,5) site depends on the electrostatic forces from the nearby ions. Thus occupancy of the adjacent K(6) site may force the K atom to move as far away as possible (Le., into the K(4) site of LKSM), while occupancy of the nearest choice of the K(2) sites may cause the K atom to move only into the K(5) site. These considerations will be explored in detail when further refinements have been completed of type A zeolite crystals exchanged with other monovalent cations. Although the K(6) and K(7) atoms are not zero coordinated, the distances to framework oxygens are apparently rather high at 2.8-3.0 A. Why should these distances not be near 2.6 A as for three-coordinated K(1)? We now suggest a plausible model that the observed distances of 2.8-3.0 A are falsely high by -0.2 A. The fractional occupancies of K(6) and K(7) sites are only -3 and -2%, respectively. Hence the positions of the O(1) and O(3) sites adjacent to K(6) and K(7) are determined almost entirely by oxygen atoms not bonded to K(6) and K(7). Oxygens bonded to K(6) or K(7) are more “satisfied” electrostatically than ones not bonded to K(6) or K(7) and should form weaker bonds with the Si and A1 atoms. Hence the bonded oxygens can be expected to be closer to the K(6) or K(7) site than the unbonded oxygens, and the apparent observed values of 2.8-3.0 A should be falsely high. For a T-0-T angle of 150°, increase of the T-0 distance by 0.05 A forces the oxygen outward by -0.16 A if the tetrahedral atoms stay in position. Such a displacement would be undetectable within the observed displacement ellipsoids of the O(1) and O(3) sites. Models of greater complexity can be envisaged, but are not worth pursuing at this stage of investigation. Further structural and electron microprobe analyses are planned of the dehydrated varieties of ion-exchanged A zeolite for which zero-coordinated cations have been proposed. In the meantime, it is suggested that the present study of dehydrated K-A zeolite has demonstrated the lack of valid evidence for zero-coordinated cations in one variety of A zeolite and has served as a warning against uncritical acceptance of present evidence for other varieties of A zeolite. Nevertheless, it is worth emphasizing that, although there is no evidence that cations float like Mahornet’s coffin in dehydrated K-A zeolite, all the K atoms have unusually low coordination for such a large univalent cation, This is reason enough for systematic structural study of this and other zeolites which have such important industrial applications. Acknowledgment. Supported by National Science Foundation Grant CHE75-22451plus general support from the Materials Research Laboratory and Union Carbide Corp. We thank G. T. Kokotailo for material assistance, and I. Baltuska for typing. D. W. Breck and K. Seff kindly made helpful criticisms and corrections. Supplementary Material Available: Listings of the observed and calculated structure factors (Supplementary
Ionic Association of Lithium Perchlorate
Tables 4) (6 pages). Ordering information is given on any current masthead page.
References and Notes
The Journal of Physical Chemistry, Vol. 83, No. 6, 1979 749 (8) S. J. B. Reed and N. G. Ware, J . Petrol., 18, 499 (1975). (9) P. E. Riley, K. Seff, and D.P. Shoemaker, J . Phys. Chem., 76, 2593 11973\ , -'-I'
(10) R. L. Firor and K. Seff, J . Am. Chem. SOC.,99, 4039 (1977). (11) R. Y. Yanagida, A. A. Amaro, and K. Seff, J. Phys. Chem., 77, 805 (19731. (12) D. W. Breck, "Zeolite Molecular Sieves", Wiley, New York, 1974. (13) V. Subrarnanian and K. Seff, J . Phys. Chem., 81, 2249 (1977). (14) In addition to local programs for data reduction, local variations of the following programs were used in this study: FORDAP Fourier programs of A. Zalkin, NUCLS a least-squares program resembling Busing-Levy ORFLS, ORFFE error function program of W. R. Busing and N. A. Levy, and ORTEP, a plotting program by C. Johnson. (15) J. V. Smith, "Feldspar Minerals", Springer, Heidelberg, 1975. 1
(1) R. L. Firor and K. Seff, J . Am. Chem. SOC.,98, 5031 (1976). (2) P. C. W. Leung, K. B. Kunz, K. Seff, and I. E. Maxwell, J. phys. Chem., 79, 2157 (1975). (3) T. B. Vanoe, Jr., and K. Seff, J . Phys. Chem., 79, 2163 (1975). (4) R. L. Firor and K. Seff, J . Am. Chem. Soc., 99, 1112 (1977). (5) R. L. Firor and K. Seff, J . Am. Chem. SOC, 99, 6249 (1977). (6) V. Gramlich and W. M. Meier, Z. Kristallogr., 133, 134 (1971). (7) J. F. Charnell, J . Cryst. Growth, 8, 291 (1971).
-I
Ionic Association of Lithium Perchlorate in Low Dielectric Constant Media from Very Dilute Soliutlons to Saturation at 293 and 198 K M. Nicolas" and R. Reich Laboratoife de Physique des So/ides,T85timent 510, Universif6 Paris-Sud, 9 1405-0rsay, Cedex, France (Received February 27, 1978; Revised Manuscript Received October 20, 1978) Publication costs assisted by Centre National de la Rechercbe Scientifique (France)
Electrical conductance and viscosity have been measured for solutions of LiC104 in methanol (MeOH), tetrahydrofuran (THF),ethyl acetate (EtAc), and in (1- r)MeOH + (x)THF mixtures at 293 and 198 K. The dielectric constant t varies from 33 (MeOH) to 6.094 (EtAc) at 293 K and from 57 (MeOH) to 8.57 (EtAc) at 198 K. As the solvent permittivity decreases or as the solute concentration increases, the solvent separated pairs become contact pairs, the contact pairs become triple ions, and the triple ions become quadrupoles. In very concentrated solutions, clusters are formed. The data for ion pairs and triple ions are interpreted by the Fuoss-Krauss theories and the parameters 'io,K,, a, KT,and have been determined. For calculation of KT, it is better to take AoT = 2 / 3 A o than -ioT = '/&. The product a ~ ist constant and KT is proportional to for 6.094 < t < 11.02. We show that, for a given concentration of 1M, the In A, vs. 6-l variation is a straight line, due to quadrupole formation. For very concentrated solutions, molar conductivity A, and fluidity @ ( 3 1/11) show the same exponential decrease vs. the solute molar fraction whatever the solvent nature. Both transport properties show transitions which may be interpreted as due to transformations of the electrolyte structure.
Introduction The conductance behavior of dilute electrolytes in media of high dielectric constant is now well understood and many experimental results verify the theory. It is known that when the solvent permittivity is high enough and the solution not too concentrated, an equilibrium exists between free ions and ion pairs. If the solvent dielectric constant decreases and becomes sufficiently low, the ion-ion interactions are more important than ion-solvent interactions and contact ion pairs, triple ions, quadrupoles, and even multiple clusters can be stable and numerous enough to affect the electrolyte transport properties. Few experiments are carried out in media of low dielectric on triple ions on the one hand,6-15or in very concent rated rnedial6-l8 on the other hand. In the first case, free ion$,in equilibrium with contact pairs give rise to conducting triple ions and the Debye-Onsager theory is no longer aldequate. In the second case, one has the problem of the concentrated solution structure as well as that of the sol id-liquid phase transition when saturation is reached. Therefore, in these two cases, theoretical and experimental problems are of great interest. With this in mind the transport properties of lithium perchlorate have been investigaited in methanol (MeOH), tetrahydrofuran (THF), ethyl acetate (EtAc), and in MeOH-THF mixtures at 293 and 1913 K. In these cases, the dielectric constant varies from 6.094 to 33 a t 293 K and from 8.57 to 57 at 198 'Laboratoire
