J. Phys. Chem. 1980, 3 4 , 2827-2831
(6) (7) (6) (9) (10) (11) (12) (13)
(14) (15) (16) (17)
Raghavan, N. V.; Seff, K. J . Phys. Chem. 1976, 80, 2133. Riley, P. E.; Seff, K. Inorg. Chem. 1975, 14, 714. Riley, P. E.; Seff, K. Inorg. Chem. 1974, 73, 1355. Chameii, J. F. J . Crysf. Growth 1971, 8 , 291. Crur, W. V.; Leung, P. C. W.; Seff, K. J . Am. Chem. Soc. 1978, 700, 6997. Seff, K. J. Phys. Chem. 1972, 76, 2601. Riley, P. E.; Seff, K.; Shoemaker, D. P. J . phys. Chem. 1972, 76, 2593. Principal computer progams used in this study: T. Ottersen, COIRAFE data reductlon program, Univmtly of HawaU, 1973 full-matrix least squares, P. K. Gantzel, R. A. Sparks, and K. N. Trueblood, UCLA LS4, American Crystallographic Assoclatbn Program Library (old) No. 317 (revised 1976); Fourier program, C. R. Hubbard, C. 0. Qulcksall, and R. A. Jacobson, Ames Laboratory Fast F w k , Iowa State Untversity, 1971; C. K. Johnson, OATEP, Repat No. oRNL-3794, Oak Ridge National Laboratory, Oak Ridge, TN, 1965. Kim, Y.; Sen, K. J . Am. Chem. Soc. 1978, 100, 175. Kim, Y.; Seff, K. J . Phys. Chem. 1978, 82, 925. Firor, R. L.; Seff, K. J . Am. Chem. Soc. 1977, 99, 1112. Vance, T. B., Jr.; Seff, K. J . Phys. Chem. 1975, 79, 2163.
(18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30)
(31)
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Kim, Y.; Seff, K. J . Am. Chem. Soc. 1978, 100, 6989. Subramanlan, V.; Seff, K. J . Phys. Chem. 1977, 81, 2249. Cruickshank, D. W. J. Acta Crystalbgr. 1949, 2 , 65. Doyle, P. A,; Tuner, P. S. Acta Crystalkgr., W . - A 1988, 24,390. "International Tables for X-ray Crystallography"; Kynoch Press: Birmingham, England, 1974; Vol. IV, pp 73-87. Cromer, D. T. Acta Crysfalbgr. 1965, 78, 17. Reference 22, pp 149-50. "Handbook of Chemistry and Physics", 56th ed.; CRC Press: Cleveland, OH, 1974; p F-209. Quinones, H.; Baggb, S. J . horg. Nucl. Chem. 1972, 34, 2153. Lloyd, D. J.; Gab, J. Crysf. Strucf. Commun. 1973, 2 , 209. Cameron, A. F.; Taylor, D. W.; Nuttail, R. H. J . Chem. Soc., Dalton Trans. 1972, 1603. Dunant, P. J.; Durrant, B. "Introduction to Advanced Inorganic Chemistry"; Longmans: London, 1962; p 373. Amaro, A. A.; Kovaciny, C. L.; Kunr, K. 8.;Riley, P. E.; Vance, T. B., Jr.; Yanagkia, R. Y.; %ff, K. Mol. Sbves, Inf. Conf., 3rd, 1975, 113-7. Breck, D. W. "Zeolite Molecular Sleves"; Wiley: New York, 1974; pp 462-3.
Migration of Framework Oxide Ions. Crystal Structures of Fully Cadmium(I1)-Exchanged Zeolite A Evacuated at 600 and 700 O C Lynne 8. McCusker and Karl Sen Department of Chemlstry, Unlverstty of Hawaii, Honolulu, Hawaii 96822 (Received: April 18, 1980)
The crystal structures of fully Cd2+-exchangedzeolite A evacuated at lo4 torr and 600 "C (a = 12.258(2)A) and 700 "C (a = 12.237(2)A) have been determined by single-crystalX-ray diffraction techniques in the cubic space group Pm3m. The structures were refined to final R (weighted)indices of 0.043 and 0.037, respectively. All six Cd2+ions in each structure lie on threefold axes and are associated with &oxygen rings of the aluminosilicate framework. They are distributed over two sites: one is recessed deeply into the sodalite unit and approaches three framework oxides and one nonframework oxide in a distorted tetrahedral manner; the second is in the large cavity near a 6-ring plane and approaches three framework oxides only. These cation positions are very similar to those previously found in C&-A evacuated at 500 "C-in fact, the three structures differ only in the distribution of Cd2+ions over the two sites. Evacuation at 600 "C reduces the number of Cd2+ions in the sodalite unit from three per unit cell at 500 "C to approximately two, with a corresponding decrease in the number of nonframework oxides and increase in the number of Cd2+ions at the large cavity site. Further heating to 700 "C does not cause this process to continue but actually reverses it. This may have occurred because, at these elevated temperatures, some of the protons (from dissociated water molecules) in the structure may be able to extract oxides from the zeolite framework. Various other attempts to prepare fully dehydrated and otherwise uncomplexed C&-A, by chemical reaction with the residual water for example, were also unsuccessful,
Introduction Complete ion exchange of zeolite A with dipositive ions has been achieved for only calcium,' strontium, barium,2 zinc, and cadmium.3 Crystallographic results are available for hydrated C%-A+6 B%-A,B Zne-A,' and and for partially dehydrated B%-A: ZQ-A,' and Cd6-A,8,9 but only the Ca2+-and Sr2+-exchanged zeolites have been studied in their completely dehydrated forms.1° B%-A does not retain its crystallinity upon dehydration," Zn6-A ia not completely dehydrated even after evacuation at 600 "C,' and CG-A still contains three H20 molecules per unit cell after evacuation at 500 0C.9 In both the C%-A and the Sr6-A dehydrated structures, five of the six cations are located at conventional sites opposite 6-oxygen rings of the aluminosilicate framework, but the remaining cation approaches framework oxides of an 8-oxygen ring at relatively long distances. Since Cd2+ is more covalent than either Ca2+or Sr2+,the arrangement of Cd2+ions in fully dehydrated Cd6-A was expected to be different and worthy of examination. An 8-oxygen-ring Cd2+ion would be a particularly undercoordinated and reactive species. 0022-3654/80/2084-2827$01.00/0
Fully hydrated, completely Cd(I1)-exchangedzeolite A contains at least 31 H20 molecules per unit cell. Evacuation at 25 "C and lo4 torr removes all but 5.5 of these, but, surprisingly, the zeolite still is not completely dehydrated after treatment at 500 O C and lo4 torr. Even under these relatively extreme conditions, three water molecules (probably dissociated) remain within the aluminosilicate cavities, and all six Cd2+ions are associated with &oxygen rings. Cd6-A is stable up to 800 0C,3 so evacuation a t temperatures higher than 500 "C to complete the dehydration process, and, perhaps, to promote one of the Cd2+ ions to an 8-oxygen-ring position, was expected to be possible. Since this would involve temperatures higher than those employed in any previous crystallographic study of zeolite A, the behavior of the zeolite framework could be interesting. Framework oxide mobility in Na-Y zeolite is measurable (by studying the oxygen isotopic exchange kinetics) at temperatures exceeding 600 "C.12
Experimental Section Crystals of zeolite A were prepared by Charnell's method.13 Complete Cd2+exchange of each zeolite A crystal @ 1980 American Chemical Society
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The Journal of phvsical Chemlstry, Vol. 84, No. 21, 1980
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TABLE I : Positional, Thermal,a and Occupancy Parameters Wyckoff position
occupancy factorb fixed varied
z U,,or U b U,, U,, U,, U,, U,, A. Cd,-A Evacuated at 600 C and torr 0 1 8 2 6 ( 2 ) 368461) 0 0 3 (1) 24.V 20(1) 13(1) 15(1) 0 2005 ( 6 ) 5000 45(5) 32(5) 30(5) 0 0 0 12.0 0 2 9 4 3 ( 4 ) 2943(4) 0 1 2 ( 4 ) 12.0 4 5 ( 5 ) 23(3) 23(3) 0 1121 (3) 1121 (3) 3268 (4) 31 (2) 31 (2) 27 (3) 6 (2) - 1 (2) - 1 (2) 24.0 1552 (6) 1 5 5 2 ( 6 ) 1 5 5 2 ( 6 ) 29(2) 29(2) 29(2) 7 (2) 7 ( 2 ) 7 ( 2 ) 2.22 (8) 1948 (4) 1 9 4 8 ( 4 ) 1 9 4 8 ( 4 ) 31 (1) 31 (1) 3 1 ( 1 ) 7(1) 7(1) 7 ( 1 ) 3.64 (8) 463e 463e 80Oe 1267e 1.70 (12) B. Cd,-A Evacuated at 700 C and torr 0 1 8 2 9 ( 4 ) 368063) 30(4) 14(4) 28(3) 0 0 1 (2) 24.0' 0 1968 (11) 5000 57 (12) 38 (11) 41 (10) 0 0 0 12.0 0 2957(7) 2957(7) 5 5 ( 1 1 ) 31 ( 6 ) 31 ( 6 ) 0 0 1 3 ( 9 ) 12.0 1121 (5) 1121 ( 5 ) 3256 (7) 4 0 ( 4 ) 40 ( 4 ) 47 (8) 11 (6) - 3 (5) - 3 ( 5 ) 24.0 1545 (15) 1 5 4 5 ( 1 5 ) 1545(15) 39(5) 39(5) 39(5) 8(5) 8(5) 8(5) 2.6 (2) 1952 (11) 1952(11) 1 9 5 2 ( 1 1 ) 30(4) 30(4) 30(4) 4(3) 4(3) 4(3) 3.2 (2) 469 463e 80V 1267e 2.3 (2) Positional parameters are given X l o 4 and thermal parameters X l o 3 . Numbers in parentheses are the estimated standard deviations in the units of the least significant digit given for the corresponding parameter. See Figure 1 for the identities of the atoms. The anisotropic temperature factor is e ~ p [ - 2 n ~ ( h ~ ( a * ) ~+Uk2(b*)'U,, ,, + Zz(c*)'U3,+ 2hk(a*b*)U,, + 2hZ(a*c*)U,, + 2kl(b*c*)U,,)]. Occupancy factors are given as the number of atoms or ions per unit cell. Occupancy for (Si) = 12 and occupancy for (Al)= 12. Exactly I / , by symmetry. e This parameter w a s held fixed in least-squares refinement. See text for further details. x
Y
was achieved by lodging it in a fine quartz capillary and allowing 0.05 M aqueous Cd(CzH30z)zhalf-saturated with Cd(OHIz (pH ca. 6-7) to flow past at a rate of -1 cm/s for 48 h at 25 "C. The crystals were washed by continuing this procedure for another 48 h with distilled water at 80 "C. Each crystal was evacuated at 10" torr and T = 600 or 700 "C for 48 h and was then sealed in ita capillary, still under vacuum, by torch. Diffraction intensities were collected on each crystal at 23-26 "C. The cubic space group Pm3m (no systematic absences) A Syntex four-circle comappeared to be a~pr0priate.l~ puter-controlled diffractometer with a graphite monochromator and a pulse-height analyzer was used for preliminary experiments and for the collection of diffraction intensities. Molybdenum radiation (Kal, X 0.70930 A; KaZ, X 0.71359 A) was used throughout. In each case, the cell constant, a = 12.258(2) A for the 600 "C crystal and a = 12.237(2) A for the 700 "C crystal, was determined by a least-squares treatment of 15 intense reflections for which 20" < 28 < 24". For both crystals, reflections from two intensity equivalent regions of reciprocal space (hkl, h Ik I1, and Ihk, 1 Ih Ik; hkl, h Ik I1, and khl, k Ih I1, for the crystals evacuated at 600 and 700 "C, respectively) were examined by using the 8-28 scan technique. Each reflection was scanned at a constant rate of 1.0 deg min-' from 1" (in 28) below the calculated Kal peak to 1" above the Kaz maximum. Background intensity was counted at each end of a scan range for a time equal to half the scan time. The intensities of 3 reflections in diverse regions of reciprocal space were recorded after every 100 reflections to monitor crystal and instrument stability. Only small random fluctuations of these check reflections were noted during the course of data collection. All unique reciprocal lattice points for which 28 < 70" were examined for the 600 "C structure (877), and those for which 28 < 60" for the 700 "C one (604). The high upper limits for 28 were chosen to give more complete data sets, even though few reflections with large 28 values showed significant intensity. The raw data for each region were corrected for Lorentz and polarization effects including that due to incident beam monochromatization; the reduced intensities were merged; and the resultant estimated standard deviations
were assigned to each averaged reflection by the computer program COMPARE.^^ Other details regarding data reduction have been discussed previously.ls An absorption correction was expected to be unimportant (pR ca.0.12 for both crystals) and was not a~p1ied.l~ Only those reflections in each merged data set for which the net count exceeded three times its corresponding esd were used in structure solution and refinement. This amounted to 356 and 170 reflections for the 600 and 700 "C crystals, respectively. (See paragraph at end of text regarding supplementary material.)
Structure Determination Cd6-A Evacuated at 600 "C. Full-matrix least-squares refinement was initiated with the atomic parameters of C&-A evacuated at -400 "C7 for the atoms of the aluminosilicate framework [(Si,Al), O(l), 0(2), and 0(3)] and for two threefold-axis Cd2+ ion positions. This model converged quickly with an Rlindex, (CIFo- IFcll)/CFo, of 0.058 and a weighted Rz index, (Cw(Fo- lFC1)'/ CwF,2)'12, of 0.055. A subsequent difference Fourier function showed only a diffuse region of electron density in the sodalite unit similar to that noted in previous Cd2+-exchangedzeolite A structures.BJ8 All attempts to refine a sodalite-unit oxide position of any symmetry by least squares were unsuccessful, yet the inclusion of such a position in the structure factor calculation lowered R1 to 0.056 and Rzto 0.043. The error indices did not appear to be sensitive to the symmetry of the O(4) position, but did increase sharply if no representation of this electron density was included in the model. Consequently, the O(4) positional and thermal parameters found in the structure of Cd,-A evacuated at 500 "C@were arbitrarily chosen to represent the sodalite-unit electron density, and only the occupancy parameter was allowed to refine. Anisotropic refinement of all but the O(4) position converged with the final error indices R1 = 0.056 and Rz = 0.043 and the occupancies for the nonframework atoms shown in the last column of Table IA. The goodness of fit, (Cw(FoIFc1)2/(m- s)'I2, is 2.25; m (356) is the number of observations, and s (30) is the number of variables in least squares. All shifts in the final cycle of refinement were less than 2% of their corresponding esd's.
The Journal of Physical Chemistry, Vol. 84, No. 21, 1880 2829
Mlgratlon of Framework Oxide Ions
Figure 1. A stereoview" of the sodalite unit of Cde-A
evacuated at 500 "C.
TABLE I1 : Selected Interatomic Distances (A ) and Angles (deg)" Cd,-A evacuated at 600 C
700 "C
1.628 (2) 1.624 (4) (Si,Al)-O( 1) 1.643 (3) 1.640 (5) (Si,Al)-O(2) 1.701 (2) 1.703 (4) (Si,Al)-O( 3) 3.071 (7) 3.09 (1) Cd(l)-0(2) 2.232 (6) 2.22 (1) Cd(1 k O ( 3 ) 2.1 2.1 Cd( 1)-O( 4)' W2)-0(2) 2.946 ( 4 ) 2.95 (1) Cd( 2)-O( 36 2.162 (5) 2.15 (1) 2.5 2.2 O(4 kO(4 1 115.8 (4) 116.6 ( 7 ) O(1)-(Si,Al)-O( 2) 111.4 (2) 110.9 (5) O(1)-(Si,Al)-O( 3) O(2)-(Si,Al)-O(3) 104.9 (3) 105.3 (5) 0(3)-(Si,Al)-0(3) 107.7 (3) 107.3 (6) (Si,Al)-O(1)-(Si&) 164.5 (4) 168.0 (7) (Si,Al)-O(2)-(Si,Al) 157.1 (5) 155.3 (9) (Si,Al)-O(3)-(Si,Al) 136.9 (3) 136.6 (6) O(3)-Cd( 1)-O( 3) 113.0 (5) 113 (1) O(3)-Cd(1)-0(4)' 97 97 110 110 O(3)-Cd( 1)-0(4)' O(3)-Cd( 2)-O( 3) 118.8 (5) 119 (1) The numbers in parentheses are the estimated standard deviations in the units of the least significant digit given for for the corresponding parameter. The O(4) position was was not refined, so distances and angles involving O(4) are only approximate.
'
The largest peak on the final difference Fourier function, whose estimated standard deviation was 0.08 e A-3 at a general position, was 8.2 e A-3 in height and was located at the origin where the esd was 3.8 e A-3.19 The final structural parameters are presented in Table IA. Interatomic distances and angles are given in Table 11. Cd6-A Evacuated at 700 "C.When the atomic parameters of the 600 "C structure for all but the O(4) position are used, simultaneous positional, occupancy, and thermal parameter refinement was initiated. This least-squares refiement converged quickly with R1 = 0.063 and R2 = 0.057. As in the case of the 600 "C structure, the only significant feature on a difference Fourier function was a diffuse region of electron density in the sodalite unit. Once again difficulties were encountered in trying to refine a sodalihunit oxide position, even though inclusion of such a position in the model caused a dramatic decrease in R1 to 0.055 and in R2 to 0.037. So, as before, the 500 "C structure's positional and thermal parameters for O(4) were used, and only the occupancy parameter was allowed to refine. Anisotropic refinement of all but the O(4) position converged to the final error indices R1 = 0.055 and R2 = 0.037, and with the occupancies for the nonframework atoms shown in the last column of Table IB. The goodness
Ellipsoids of 20% pmbablllty are shown.
TABLE 111: Deviation of Atoms ( A ) from the [ l l l ] Planes at O(3P Cd.-A evacuated a t ~~~
600 C -0.60 0.24 0.27
~
700 " C - 0.61 0.25 0.29
Cd(1) 2) O(2) a A negative deviation indicates that the atom lies on the same side of the plane as the origin.
of fit is 1.81, the number of observations is 170, and the number of parameters is 30. All shifts in the final cycle of refinement were less than 1% of their corresponding esd's. The largest peak on the final difference Fourier function, whose estimated standard deviation was 0.08 e A-3 at a general position, was 4.5 e in height and was located at the origin where the esd was 3.8 e A-3.19 The final structural parameters are presented in Table IB. Interatomic distances and angles are given in Table 11. The full-matrix least-squares program15 used in all structure determinations minimized X W ( A ~ Fthe ~ ) ~weight ; (u)of an observation was the reciprocal square of u, its standard deviation. Atomic scattering factorsmJ1for Cd2+, 0-, and (Si,A1)1.75+were used. The function describing (Si,A1)1.76+is the mean of the Sio, Si'+, Alo, and A13+ and (Si,functions. The scattering factors for Cd2+,0-, Al)1.75+were modified to account for the real component (f? of the anomalous dispersion c ~ r r e c t i o n . ~ ~ ~ ~ ~
Discussion In both the 600 and the 700 "C structures, all six Cd2+ ions per unit cell are associated with 6-ring oxides and are distributed over two threefold-axis sites. The Cd(1) position (Table I, A and B) is located in the sodalite unit, recessed 0.6 A from the [ l l l ] planes at O(3) (Table 111). Each Cd2+ion at this site approaches three framework oxides at O(3) at 2.22(1) A with a near-to-tetrahedral 0(3)-Cd(l)-0(3) angle (Table TI), and one nonframework oxide at O(4) at a poorly determined distance and angle (see Structure Determination section). The second Cd2+ ion site at Cd(2) is on the large cavity side of the 6-ring, and each cation there is coordinated only to three framework O(3) oxides at 2.15(1) A with a near-to-trigonal 0(3)-Cd(2)-0(3) angle. These Cd2+ion positions are very similar to those found in Cd6-A evacuated at 500 "CD (Figure 1). The only significant differences between the 500, 600, and 700 "C structures are the occupancies at Cd(l), Cd(2), and O(4). After evacuation at 500 "C, three Cd2+ions are located in the sodalite unit at Cd(l), where each coordinates to an
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The Journal of Physical Chemistry, Vol. 84, No. 21, 1980
McCusker and Seff
TABLE IV: Comparison of Variously Dehydrated Cd,-A Structures dehyd temp, C 500" G 600b
6OOc
7 00 crystal lC
crystal
2d
12.242 (2) 12.261 ( 2 ) 12.258 (2) 12.237 (2) 12.253 (2) 187 362 366 170 3 24 32 30 30 30 30 2.2 ( 1 ) 3.0 ( 3 ) 2.2 (1) 2.6 (1) 2.6 (1) 3.8 (1) 3.6 (1) 3.2 (2) 2.9 (1) 2 8 (3) 2.5 (2) 1.8 (1) 1.7 (1) 2.3 (2) 2.1 (1) 0.057 0.056 0.056 0.055 0.056 R, 0.036 0.044 0.043 0.037 0.047 Rl Reference 9. 500 < T < 600 'C. See text for further details regarding this crystal. This work. Cd occupancies refine to give only 5.4 ( 2 ) Cd" ions per unit cell, so this crystal may have been incompletely exchanged. e Occupancy factors are given as the number of atoms or ions per unit cell.
a, A no. of reflections no. of variables Cd( 1 ) occupancye Cd( 2) occupancye O(4) occupancye
O(4) (nonframework) oxide deeper in the sodalite unit. The other three Cd2+ions are at Cd(2). This stoichiometry is supported by the structure of this zeolite after exposure to Cd metal vapor: the three 3-coordinate Cd2+ions at Cd(2) react to yield Cd(1) species, and the three 4-coordinate cations at Cd(1) do not react. As intended, evacuation at 600 "C allowed more H20 to be removed from the zeolite, and, as might have been expected, this caused both the O(4) and the Cd(1) occupancies to decrease as the Cd(2) occupancy increased correspondingly. As the number of nonframework oxides needed to hold Cd2+ions in the sodalite unit decreased, some of the cations moved to the less crowded large cavity site. It was expected that this dehydration process would continue, perhaps to completion, upon evacuation at 700 "C, decreasing the number of nonframework oxides in the zeolite and the number of Cd2+ions in the sodalite unit. However, instead of decreasing further, these occupancies, at O(4) and Cd(l), actually increased while that at Cd(2) decreased. This complete reversal (see Table IV, columns 1,3, and 4) was so unexpected that two more crystals were prepared and examined to verify the results. The structure of the second crystal evacuated at 600 "C is insignificantly different from the one reported here (Table IV, column 2). Unfortunately, it was discovered later that the experimental temperature may have been somewhat less than 600 "C, although the structure does indicate that this was not the case. The second crystal evacuated at 700 "C is very similar to the first, but the Cd2+ ion occupancies s u m to only 5.4 per unit cell (Table IV, column 5), suggesting incomplete exchange. Consequently, these two additional structures are not reported in detail but are included in Table IV, which compares some aspeds of the variously dehydrated Cd6-A structures. These additional structures support the occupancies given in Table I and underline the need for an explanation. Between 600 and 700 "C something happens to cause some Cd2+ions to move back into sodalite units. The occupancy refinements for 0(4), which parallel those of Cd(l), suggest that additional nonframework oxides are responsible. The only source of additional oxides, considering the experimental conditions, appears to be the framework. The H 2 0 molecules remaining in the zeolite This after evacuation at 500 "C are probably di~sociated.6~ means that three protons per unit cell are associated with framework oxides, and perhaps, at temperatures as high as 600 and 700 "C, some of these protons are able to remove oxides from the framework to form OH- groups which can then re-form the apparently very stable (CdOH+)3arrangement in the sodalite unit. Oxide ions in the Na-Y framework have measurable mobility at temperatures exceeding 600 OC,l2 and dehydroxylation (removal of framework oxides by protons) of various zeolites occurs in the 600-700 OC temperature range,24p25 so such
an explanation for the behavior of the Cd2+ions is not an unreasonable one. It is not clear from the data available whether the reversal occurs above or below 600 "C. It appears from this study that complete dehydration of Cd6-A (and probably Zn6-A also) by thermal methods will be difficult, if not impossible. Still, the preparation and structure of completely dehydrated zeolite A which has been fully exchanged with relatively covalent dipceitive cations remains of interest because such materials may have novel properties. For example, C&-A would be very powerful desiccant, even at high temperatures, with no vapor pressure of its own. Therefore some alternative approaches were investigated. It was reasoned that C&-A evacuated at 600 "C should be an excellent desiccant, so a bulk sample of Cd6-A was prepared and placed on a sidearm near a capillary containing a Cd6-A crystal. This assembly was sealed onto a vacuum line, and as the crystal was heated to 500 "C, the powder sample was heated to 600 "C. While the crystal was maintained at 500 "C and lo4 torr, the bulk C4-A was cooled to -196 "C. After 6 h the crystal oven was turned off, and the crystal was sealed in ita capillary with a torch. Subsequent data collection and structure solution showed that the residual H 2 0 molecules in the Cd6-A crystal had not migrated to the C&-A bulk sample as hoped. Apparently the activation energy required for this to occur is too high. In addition to the physical approach discussed above, two chemical procedures were attempted. First, a C&-A crystal was evacuated at 500 "C and lo4 torr and then exposed to 500 torr of C2H4 at 500 "C. While the crystal was kept at 500 "C, the C2H4 was evacuated and replaced several times. Finally, the crystal was evacuated, cooled to room temperature, and sealed in its capillary by torch. It was hoped that ethylene would react with the dissociated water in the zeolite to form ethanol which could then be evacuated. Structure solution indicates that the zeolite contains a substantial amount of sorbed material (presumably organic) and that it is not the empty, completely dehydrated Cd6-A desired. The second chemical reaction attempted involved exposing C&-A evacuated a t 500 "C and lo4 torr to sulfur while maintaining the crystal at 500 OC and continuing to evacuate. After several hours, the sulfur was distilled away from the crystal, which was then sealed in ita capillary by torch. The reaction H 2 0 + S H2S and SO2 has AG < 0 at 227 "C (calculated from heats of formation%), so if this reaction occurred in the zeolite, the two gaseous products might have been removed upon evacuation. Structure solution indicates that sulfur species (perhaps chains bridging between Cd2+ions) remained in the zeolitic cavities, so, whether the water was removed or not, the product still contained unwanted occluded material. Perhaps heating this crystal in dry O2would give SO2
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J. PhyS. Chem. 1980, 84, 2831-2835
2831
(3) W, I. J.; Jankovlc, 0.;W,S.; Radovanov, P.; Tocbx~vE,M. T m . Farady Soc. 1871, 67, 999. (4) A dsarssbn of nomenclatueb avalable: (a) Yanaglda, R. Y.; Amaro, A. A.; Seff, K. J. phys. Chem. 1873, 77, 805. (b) Broussard, L.; Shoemaker, D. P. J . Am. Chem. SOC. lQ60, 82, 1041. (c) Seff, K. Acc. Chem. Res. 1870, 9 , 121. (5) MI,W. 2. KrlstaxOgr., KrlstaWgeom., Krlstamhys. k&takhem. 1871, 142, 142. (6) Ktn, Y.; sukamanian, V.; Ra,R. L.; Seff. K. ACS Synp. SCW. 1980, No. 135, In press. (7) McCusker, L B.; Seff, K., s&n&ted for publcatkn h J. Pnys. aWnn. (8) McCuskec, L B.; Seff, K., sukn#ted for puMcetkn h J. Pnys. (%em. (9) McCusker, L. B.; Seff, K. J . Am. Chem. SOC. 1878, 101, 5235. (10) Fkor, R. L.; Sen, K. J. Am. Chem. Soc. 1978, 100, 3091. (11) Sheny, H. S.; Walton, H. F. J . phys. Chem. 1867, 71, 1457. (12) Antoshin, 0. V.; Minachev, Kh. M.; Sevastjanov, E. N.; Kondratjev, D. A.; New, C. 2. A h . Chem. Ser. 1871, No. 101, 514. (13) Chamell, J. F. J . Cryst. Qrowth 1871, 8 , 291. (14) Crur, W. V.; Leung, P. C. W.; Sen, K. J . Am. Chem. Soc. 1878, 100, 6997. (15) Rhcpal COmprAer pogarm used h thts study: O t t m , T., CORAR data reductkn progam, University of Hawaii 1973. Gantzel, P. K.; -5, R. A.; Truebkod, K. N., fuknatrlx leaet-squareS, UCLA LS4, American ayStaksaphic Assodetkn Progarn Wry (old)No. 317 (modiRed). W d , C. R.; Quldtsal,C. 0.;Jacobgon, R. A., Fwler progam, Ames Labaetay Fast F w k r , Iowa State ullverslty, 1971. Johnson, C. K., ORTEP. Report No. ORNL-3794 Oak Ridge Natbnal Labofatory, Oak Rldge, TN. 1965. (16) Fkor, R. L.; Seff, K. J. Am. Chem. Soc. 1977, 99, 4039. (17) Vance. T. B., Jr.; Seff, K. J . phys. Chem. 1875, 79, 2163. (16) Mccusker, L. B.; Seff, K. J. Am. Chem. Soc. 1878, 100, 5052. (19) Cnrickshank, D. W. J. Acta Crystalbgr. 1848, 2 , 65. (20) Doyle, P. A.; Turner, P. S. Acta Crystallogr., Sect. A. 1968, 24, 390. (21) “Intematknal T a m for X-ray Crystallography”; Kynoch Press: Blrmkrgham, England, 1974; Vol. IV, pp 73-87. (22) C r m , D. 1.Acta Crystalbgr. 1865, 18, 17. (23) Reference 21, pp 149-150. (24) Ward, J. W. ACS Monogr. 1876, 171, 118. (25) Breck, D. W. “Zedke Molecular Sleves”; Wlley: New Yak, 1974; pp 460-83. (26) “Handbodc of Chemistry and Physics”, 5lst ed.; Chemlcal Rubber PuMlshing 0.: Cleveland, 1970; p D45. (27) Raghavan, N. V.; Sen, K. J . phys. Chem. 1878, 80, 2133 and references therein.
and/or SO3 gas and the desired Cd6-A. If C4-A can be prepared by ion-exchanging dehydrated Na12-A with Cd2+ in an anhydrous solvent, perhaps evacuation of this material at a suitable temperature would yield the completely dehydrated, uncomplexed, Cd(1I) form of zeolite A. There are at least three chemically different &rings in each of the two C4-A structures described here, yet only a single average O(3) position is used to describe the framework oxides at O(3) in each case. Consequently, actual cadmium to framework oxide approaches may be somewhat longer or shorter than reported. Although only average O(3) positions have been found, the (Si,Al)-0(3) distances (also averages) in both structures are significantly longer than the (Si,Al)-O(1) and (Si,Al)-0(2) distances. All six of the Cd2+ions per unit cell, in each structure, are associated with O(3) oxides, and consequently the (Si,A1)-0(3) bond has been weakened and lengthened. This effect has been observed and discussed previously in other zeolite systems including Zn6K2-A.27
Acknowledgment. This work was supported by the National Science Foundation (Grant No. CHE77-12495). We are indebted to the University of Hawaii Computing Center. Supplementary Material Available: Listings of the observed and calculated structure factors for both structures (SupplementaryTables 1and 2) (3 pages). Ordering information is given on any current masthead page. References and Notes (1) Breck, D. W.; Eversole, W. 0.; Milton, R. M.; Reed, T. B.; Thomas, T. L. J. Am. Chem. Soc. 1856, 78, 5963. (2) Baner, R. M.; Mebar. W. Trans. Farahy Soc. 1958. 54, 1074.
Use of Stabilized Zirconia Electrodes to Transport Oxide Ion into and out of Molten Salts M. L. D e a n h a d and K. H. Stern’ ChemrStrV M k n , Naval Rewrch b b a t w , WaShbwkm, D.
C. 20375 (Recelvd: Octotmr 18, 1979; In Final Form: May 27, 1~380)
Oxide ion (W-) can be electrolyzed into and out of molten salts through stabilized zirconia (SZ), using the electrode in the form 02(Pt)(SZ102-in melt. Equations relating the emf measured with an indicating SZ electrode to the charge Q passed have been derived and experimentally verified.
Introduction Oxide-ion-conductingsolid electrolytes are finding increasing use in a variety of applications as electrodes a t high temperatures.’ The solid electrolyte of this type which has been most extensively studied is Z r O 2 stabilized with several percent CaO, MgO, or Y203.Electrochemical applications of stabilized zirconia (SZ) electrodes can be divided into two areas: (a) equilibrium methods, in which the SZ acta as an indicator electrode, and (b) nonequilibrium methods, where the SZ acts as an oxygen pump. Some important uses of the SZ electrode are as follows: for equilibrium methods, (1) measure the O2 partial t Chemistry Department, George Mason University, Fairfax, VA 22030.
pressures in gases;2 (2) measure the solubility of O2 in metals;2 (3) measure the dissociation pressures of solid oxides;2 (4) measure the oxide activity in molten and glasses;s for nonequilibrium methods, (1) operate an oxygen fuel cell: (2) change the O2content of (3) measure the diffusivity of O2in metals? (4) titrate O2into metals? A preliminary studylo has indicated that the SZ electrode can be used to transport 02-ion into and out of molten salts. The intent of this paper is to present a detailed study of the SZ electrode as an 02-pump in molten NaCl and Na2SOI. Theoretical relationships between the oxide activity in the melt and the charge passed through the zirconia have been derived and compared with experimental results. One application of this technique is coulometric titrations using the SZ electrode as an 02-
This article not subject to US. Copyrlght. Published 1980 by the American chemical Society
