Crystal structures of fully zinc(II)-exchanged zeolite A, hydrated and

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J. Phys. Chem. 1981, 85, 405-410

verified that Cu2+-O-Cu2+ forms in Cu2+-Y upon dehydration; CuOCu had been found crystallographically in the sodalite unit of this zeolite.37 Such bridges are considered to be the active species in Cu2+-X and Cu2+-Y for the catalytic oxidation of H2 by air.38 A broad absorption edge at 15 500 cm-' and a dark brown color (like (36)Chao, C.CGLunsford, J. J. J. Chem,Phys. 1972,57,2890. (37)Gallezot, P.;Ben Taarit, Y.; Imelik, B. C. R. Acad. Sci., Ser. C 1971,272,261.

405

that seen in this work), both similar to that of Cu20, were seen upon the activated deammination of Cu+Nall-A. xNH3, a yellow materia1.l'

Supplementary Material Available: Listings of the observed and calculated structure factors for crystals 1-4 (4 pages). Ordering information is available on any current masthead page. (38)Mahoney, F.;Rudham, R.; Summers, J. V. J. Chem. SOC.,Faraday Trans. 1 1979,75,314.

Crystal Structures of Fully Zinc(I1)-Exchanged Zeolite A, Hydrated and Partially Dehydrated at 600 OC Lynne B. McCusker and Karl Seff Chemistry Department, University of Hawaii, Honolulu, Hawaii 96822 (Received: June 9, 7980; In Final Form: October 70, 7980)

The crystal structures of fully Zn(I1)-exchanged zeolite A, both hydrated (a = 12.163(1) A) and partially dehydrated by evacuation at 600 "C (a = 12.049(1)A), have been determined by single-crystal X-ray diffraction techniques in the cubic space group Pm3m. The structures were refined to final R (weighted)indices of 0.065 and 0.045, respectively. In the fully hydrated structure, the Zn2+ions are distributed over three sites. One cation per unit cell is located at the center of the sodalite unit where it is coordinated to four nonframework oxygens (perhaps as Zn(OH)2(HzO)z)which are within hydrogen-bonding distance of framework oxygens. Four more cations are in the large cavity where each coordinates to three oxygens of a 6-oxygen ring of the aluminosilicate framework and to one nonframework oxygen deeper in the large cavity in a near tetrahedral manner. The remaining site is also in the large cavity but is opposite a 4-oxygen ring of the zeolite framework and is a long distance (3.8 A) from the nearest framework oxygens. If the distant framework oxygens are considered to be a single weak ligand, this cation has approximate trigonal bipyramidal geometry. The axial oxygen ligands are probably OH- ions bridging between this cation and the 6-oxygen-ringZn2+ions, and the two equatorial ligands are HzO molecules which hydrogen bond to other nonframework oxygens. A total of 29 HzO, OH-, or H30+oxygens per unit cell were located. These combine with some framework oxygens to form a complex hydrogen-bonded network within the aluminosilicatecavities. A comparison of hydrated Zn6-A and hydrated Zn5Naz-A shows significant differences which can be explained in terms of the extent of ion exchange. The coordination numbers of the Zn2+ions in this structure are lower than those of Cd2+ions in the analogous Cd2+ zeolite,and, consequently,the structures are very different. After evacuation at 600 OC and lo4 torr, the structure becomes much simpler. Two Zn2+ions per unit cell are located in the sodalite unit, each associated with three oxygens of the zeolite framework and one nonframework oxygen deeper in the sodalite unit in a distorted tetrahedral arrangement. The other four Zn2+ions are located almost in the planes of 6-oxygen rings and are coordinated to three framework oxygens in a trigonal planar manner. The structure is very much like that of similarlytreated Cd6-A and, as in that structure, residual water molecules (probably dissociated)are present even after such an extreme thermal treatment.

Introduction Crystal structures of zeolite A partially exchanged with the first-row transition metal ions Mn2+,' C O ~ +Zn2+,4,5 ,~,~ Ni2+,6and Fe2+ in their hydrated and/or activated forms have been reported. However, with the exception of C U ~ - Awhose ,~ structure has been solved very recently: no structure of zeolite A fully exchanged with any of these

ions has been studied. The primary reason for this is simply that complete exchange with many of the first-row transition metal ions impairs the crystallinity of zeolite A and makes crystallographic studies difficult or impossible. In fact, complete exchange of dipositive ions into zeolite A has been achieved for only strontium,'O barium,'O zinc," cadmium," and copper.8 Of these, the crystal structures of Ca,-A,12J3 Sr6-A,13 Ba6-A,14 Cd6-A,16,16and

(1)Yanagida, R. Y.;Vance, T. B., Jr.; Seff, K. Inorg. Chem. 1974,13, 723. (2) Riley, P. E.; Seff, K. J. Chem. SOC.D 1972, 1287. (3)Riley, P.E.;Seff, K. J. Phys. Chem. 1975, 79,1594. (4)Kim, Y.; Seff, K. J. Phys. Chem. 1980,84,2823. (5)Raghavan, N.V.;Seff, K. J. Phys. Chem. 1976,80,2133. (6)Firor, R. L.;Seff, K. J. Phys. Chem. 1978,82, 1650. (7)A discussion of nomenclature is available: (a) Yanagida, R. Y.; Amaro, A. A.; Seff, K. J.Phys. Chem. 1973,77,806. (b) Broussard, L.; Shoemaker, D. P. J. Am. Chem. SOC.1960,82, 1041. (c) Seff, K. Acc. Chem. Res. 1976,9, 121. (8)Lee, H.S.;Seff, K., J. Phys. Chem., preceding paper this issue.

(9)Breck, D.W.; Eversole, W. G.; Milton, R. M.;Reed, T. B.; Thomas, T. L. J. Am. Chem. SOC.1966,78,5963. (10)Barrer, R. M.; Meier, W. Trans, Faraday SOC.1958,54, 1074. (11)Gal, I. J.; JankoviE, 0.;MalciE, S.; Radavanov, P.; TodoroviE, M. Trans. Faraday SOC.1971,67,999. (12)Thbni, W.2. Kristallogr., Kristallgeom., Kristallphys., Kristallchem. 1971,142,142. (13)Firor, R. L.;Seff, K. J. Am. Chen. SOC.1978,100,3091. (14)Kim, Y.; Subramanian, V.; Firor, R. L.; Seff, K. ACS Sym. Ser. 1980,No. 135,137. (15)McCusker, L. B.; Seff, K. J. Am. Chen. SOC.1979, 101, 5235. (16)McCusker, L.B.; Seff, K. J. Phys. Chem., in press.

0022-3654/81/2085-0405$01 .OO/O

0 1981 American Chemlcal Society

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chromator and a pulse-height analyzer was used for preliminary experiments and for the collection of diffraction intensities. Molybdenum radiation ( K q , h = 0.70930 A; Ka2, X = 0.71359 A) was used throughout. In each case, the cell constant, a = 12.163(1) A for the fully hydrated crystal and a = 12.049(1)A for the evacuated crystal, was determined by a least-squares treatment of 15 intense reflections with 20° < 28 < 24O. Listings of observed and calculated structure factors are available as supplementary material. (See paragraph at end of text regarding supplementary material.) For the evacuated crystal, reflections from two intensity-equivalent regions of reciprocal space (hkl, h 5 k 5 1, and hkl, 1 Ih Ik) were examined with the 8-28 scan technique. For the hydrated crystal, only one set of data was collected. Each reflection was scanned at a constant rate of 1.0 deg min-l from lo(in 28) below the calculated Kal peak to 1' 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 3 reflections in diverse regions of reciprocal space were recorded after every 100 reflections to monitor crystal and instrument stability. Only random fluctuations of these check reflections were noted during the course of data collection. All unique reciprocal lattice points (859 for the hydrated crystal, and 842 for the evacuated one) for which 28 < 70" were examined. The high upper limit for 28 was chosen to give a more complete data set, even though few reflections with large 28 values showed significant intensity. For the hydrated crystal, the raw intensities were corrected for Lorentz and polarization effects including that due to incident beam monochromatization by the computer program LP-76.22 For the evacuated crystal, the intensities for each region were similarly treated and merged by the computer program COMPARE.^^ The unweighted R = 2Cl12-Ill/C(Il + Iz)value, which describes the agreement between the observed intensities in the two equivalent data sets, was 0.066. Other details regarding Experimental Section data reduction have been discussed p r e v i ~ u s l y . ~No ~ abCrystals of zeolite A were prepared by Charnell's mesorption correction (MRca 0.12 for both the hydrated and thod.lg A single crystal, a cube 0.08 mm on an edge, was the evacuated crystals) was applied.24 lodged in a fine Pyrex capillary. Ion exchange was perOnly those reflections for each crystal for which the net formed by using flow methods: 0.1 M aqueous Z ~ I ( N O ~ ) ~count exceeded 3 times its corresponding esd were used solution was allowed to flow past the crystal at a velocity in structure solution and refinement. This amounted to of -4 mm/s for 48 h at 25 "C. The crystal appeared white 211 reflections for the hydrated crystal, and 270 for the in color after this treatment. A washing step was not evacuated one. included in the ion-exchange procedure, because it had S t r u c t u r e Determination been found to cause crystal damage and because earlier experiments had shown that over ion exchange, like that Hydrated Crystal. Full-matrix least-squares refinement found in Cd2+-exchangeof zeolite A,20did not occur during was initiated with the atomic parameters of Zn6-A which Zn2+exchange. had been dehydrated at 600 "C and lo4 torr and then A second crystal was prepared by using the same exexposed to C12 gas,25for the atoms of the aluminosilicate change procedure and 0.05 M Zn(N03)2(flow velocity -2 framework ((Si,Al),O(l), 0(2), and O(3)) and for the major mm/s for a period of 72 h). This crystal was then evacu(3-fold-axis)Zn2+ion position. This model converged with ated for 48 h at 600 OC and 10* torr. After it had cooled an R1 index, (CIFo- lFcll)/CFo,of 0.281, and a weighted to room temperature, it was sealed in its capillary under R2 index, (Cw(Fo- 1Fc',1)2/CwF,P)1/z, of 0.333. An ensuing vacuum with a torch. Microscopic examination showed difference Fourier function revealed numerous peaks. The this crystal to be clear and colorless. largest of these, those at chemically plausible positions, Subsequent diffraction intensities for both crystals were collected at 25(2) "C. The cubic space group Pm3m ap(22) Principal computer programs used in this study: Data reduction, peared to be appropriate.21 A Syntex four-circle comOttersen, T. COMPARE, University of Hawaii, 1973 and Ottersen, T. LP-76, puter-controlled diffractometer with a graphite monoUniversity of Hawaii, 1976. Full-matrix least-squares, Gantzel, P. K.; Cu+-A8 in their hydrated and/or activated forms have been investigated, but none involving ZQ-A has. Since zinc and cadmium are both group 2B elements, the behavior of Zn2+ in a zeolitic environment might be expected to resemble that of Cd2+. The six Cd2+ions in hydrated Cd6-A are all associated with 6-ring oxygens of the zeolite framework, but only three coordinate to these oxygens at conventional distances.16 Each of the other three cations, located at two crystallographically distinct positions, approaches three framework oxygens at very long distances (2.6 A) so that three non-framework-oxygen ligands can also be included in its coordination sphere. The unyielding aluminosilicate ligand has forced these cations to adopt this rather unusual coordination geometry. Even after evacuation at 700 "C, C&-A is not completely dehydrated.l' Nonframework oxygens can still be found in the aluminosilicate cavities. It is probable that, in order to better satisfy the charge distribution requirements of the zeolite framework, some Cd2+ions have hydrolyzed some of their coordinated water molecules to form monopositive species, CdOH+ and H+, making the removal of the last few (dissociated) water molecules unusually difficult. To examine the effects of complete vs. partial exchange and to compare the intrazeolitic behaviors of Zn2+and Cd2+,we determined the crystal structures of hydrated Zn6-A and of Zn6-A after evacuation at 600 "C. It was hoped that comparisons of these structures with hydrated Zn5Na?-A and with evacuated Zn5K2-A, respectively, would increase our understanding of the cation exchange process and that the zeolitic environment would cause some of the differences between Cd2+and Zn2+ions to be highlighted. For example, Zn2+is a smaller ion than Cd2+, so its coordination numbers could be lower and it might hydrolyze water to a greater extent (pKh for Zn2+: 9.60; pKh for Cd2+: 11.7018).

(17) McCusker, L. B.; Seff, K. J.Phys. Chem. 1980,84,2827. (18) Huheey, J. E. "Inorganic Chemistry", 2nd ed.; Harper and Row: New York, 1978; p 266. (19) Charnell, J. F. J. Cryst. Growth 1971,8, 291. (20) McCusker, L. B.; Seff, K. J. Am. Chem. SOC.1978, 100, 5052. (21) Cruz, W. V.; Leung, P. C. W.; Seff, K. J. Am. Chem. SOC.1978, 100, 6997.

Sparks, R. A.; Trueblood, K. N. UCLA LS4, American Crystallographic Association Program Library (old) No. 317 (modified). Fourier program, Hubbard, C. R.; Quicksall, C. 0.;Jacobson, R. A. Ames Laboratory Fast Fourier, Iowa State University, 1971. Johnson, C. K. ORTEP, Report No. ORNL-3794 Oak Ridge National Laboratory, Oak Ridge, TN, 1965. (23) Firor, R. L.; Seff, K.J. Am. Chem. SOC.1977, 99, 4039. (24) Vance, T. B., Jr.; Seff, K. J. Phys. Chem. 1975, 79, 2163. (25) McCusker, L. B., Ph.D. Thesis, University of Hawaii, Honolulu, HI, 1980.

Crystal Structures of

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Zinc(I1)-Exchanged Zeolite A

U

Figure 1. Stereoview** of the large cavity of hydrated Zn,-A.

Ellipsoids of 20% probability are shown.

Figure 2. Stereoview2' of the sodalite unit of hydrated Zn,-A.

Ellipsoids of 20% probability are shown.

were examined by least squares. This led to the location of a Zn2+ion at the center of the sodalite unit (Zn(l)), a water oxygen position within coordinating distance of that Zn2+ion (0(4)),a water oxygen position in the large cavity (O(5)) within coordinating distance of the 3-fold-axis Zn2+ ions (Zn(2)))and a third water oxygen position in the large cavity (O(6))within hydrogen-bonding distance of the O(5) oxygens. Occupancy refinements for the two Zn2+positions led to 1.1and 3.9 Zn2+ions per unit cell at Zn(1) and Zn(2), respectively. These were adjusted to 1.0 and 4.0, respectively, and were held fixed at those values in subsequent refinements. To prevent divergence while determining oxygen occupancy parameters during the course of refinement, we generally held the thermal parameters fixed until occupancy parameters had converged. With a thermal parameter U = 0.076 A2, the occupancy at O(4) refined to 3.4 oxygens per unit cell; with U = 0.127 A2, that at O(5) refined to 8.2; and with U = 0.063 A2, that of O(6) refined to 7.7. These occupancies were revised to yield a model with four oxygens at 0(4), eight at 0(5),and eight at O(6). Since only four of the eight oxygens at O(5) could be coordinated to Zn2+ions at Zn(2), an attempt was made to refine the O(5) position as two adjacent positions, but it was not successful. Refinement of this model (with a single O(5) position) converged with R1 = 0.123 and R2 = 0.148. A subsequent difference Fourier revealed the remainder of the structure. Five peaks (at Zn(3), 0(7), 0 ( 8 ) , 0(9), and O(10)) appeared very clearly on the z = 1/2 section. Thus far only five of the anticipated six Zn2+ions per unit cell had been located and no recognizable Na+ positions had appeared, so an additional Zn2+ion position remained to be identified. It was not obvious which of the five newly found positions should be zinc, so all were refined as oxygens with occupancies varying and Us fixed at 0.127 A2. This refinement converged with R1 = 0.080 and R2 = 0.067. The

Zn(3) position emerged as the one with the highest occupancy (3.4(5) oxygens or -0.9 Zn2+ions) and with a partial coordination sphere (including two unavoidable 2.3-A approaches to O(5) oxygens), so it was assigned a Zn2+scattering factor. The remaining positions then refined to occupancies of 3.4(5) at 0(7), 2.7(3) at 0(8),2.3(3) at 0(9), and 1.8(3)at O(l0). These were revised, by the assumption of stoichiometry and the requirement that the structure be plausible, to the values shown in the last column of Table IA. The O(9) position was somewhat unstable in refinement, but removing it caused the error indices to increase sharply to R1 = 0.087 and R2 = 0.082. Consequently, its position was fixed at the one observed on a difference function generated with all other atoms in the structure, and only its isotropic thermal parameter was allowed to refine. Isotropic refinement of all other positions, except Zn(2) and those of the framework atoms which were refined anisotropically, converged with R1 = 0.078 and R2 = 0.065. The goodness of fit, (Cw(F,, IF,J)*/(rn- s))'i2, is 2.23; m (211) is the number of observations, and s (41) 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 largest peak on the final difference function, whose esd is 0.094 e A-3 at a general position, was 3.8 e A-3 in height and was located at (1/2, 1/2, where the esd is 0.7 e A-3.26 Two smaller peaks, 1.4 and 1.3 e k3in height, were located on the 3-fold axis, where the esd is 0.2 e at Zn(2) and near 0(5), respectively. The final results are presented in Tables IA and IIA. Possible atomic arrangements in a particular unit cell and in a particular sodalite unit are shown in Figures 1and 2, respectively. Euacuated Crystal. Full-matrix least-squares refinement was initiated with the atomic parameters of Zn5K2-A (26) Cruickshank, D.W. J.; Rollett, J. S. Acta Crystallogr. 1953,6,705.

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TABLE I : Positional, Thermal,a and Occupancy Parameters Wyckoff occupancy u,, factorb position X Y 2 u,,or uiso uz, ‘$3 ‘12 ‘13 A. Hydrated Zn,-A 0 1817 (4) l(2) 24.OC 368164) 1 6 ( 3 ) 11 (2) 0 0 4 (2) 2050 (17) 5000 0 50 (13) 56 (15) 23 (11) 0 0 0 12.0 0 2999 (7) 2999 (7) 0 2(7) 12.0 41 (11) 3 (5) 0 3 (5) 1115 (8) 1115 (8) 3307(9) 24.0 54 (7) 5 4 ( 7 ) 4 2 ( 1 0 ) 4 7 ( 9 ) - 1 8 ( 6 ) - 1 8 ( 6 ) 21 57 ( 4 ) 2157 (4) 2157 (. 4 .) 3 5 ( 2 ) 3 5 (.2 .) 35(2) 15(3) 15(3) 15(3) 4.0 0 0 0 36 ( 4 j 1.0 2973 (74) 2973 (74) 5000d 236 (57) 1.0 926 (25) 926 (25) 926 (25) 57 ( i 6 j 4.0 8.0 3097 (19) 3097 (19) 3097619) 136 (17) 8.0 2148 (45) 3724 (48) 5000 123 (21) 882 (71) 4 2006 7 8) 5000d 103 (45) 4.0 2.0 241 8 (70) 5000 5000d 25 (21) 2.0 4300e 4300e 5000d 1 3 2 (94) 0 5000d 31 (35) 5000d 1.0 B. Zn,-A Evacuated at 600 “C and torr 0 1829 ( 3 ) 366062) 16(2) 9(2) 14(1) l(1) 24.OC 0 0 0 57(8) 37(8) 36(6) 1872 (8) 5000 0 0 0 12.0 0 3034 (5) 3034(5) 36(6) 18(3) 18(3) 13(5) 12.0 0 0 1145 ( 3 ) 1145 (3) 3160(5) 32(3) 32(3) 33(5) -2(3) 24.0 lO(3) -2(3) 1515 (5) 1515 ( 5 ) 1515(5) 24(2) 24(2) 24(2) 2(3) 2.0 2(3) 2(3) 1895 ( 2 ) 1895 ( 2 ) 1 8 9 5 ( 2 ) 1 9 ( 1 ) 19(1) 19(1) 6(1) 4.0 6(1) 6(1) 345 (62) 345 (62) 784 (69) 127e a Positional parameters are given X lo4 and thermal parameters X lo3. Numbers in parentheses are the esd’s in the units of the least significant digit given for the corresponding parameter. See Figures 1 and 3 for the identities of the atoms. The anisotropic temperature factor is exp[- 2nz(h2(a*)’U,,+ k z ( b * ) z U , , + P ( C * ) ~ U ,+, 2hk(a*b*)U,, + 2hZ(a*c*)U,, + 2kZ(b*c*)U,,)]. Occupancy factors are given as the number of atoms or ions per unit cell. Occupancy for (Si) = 1 2 and occupancy for (Al) = 12. Exactly I / , by symmetry. e This parameter was held fixed in least-squares refinement. See the text for further details.

Figure 3. StereoviewZ2of the sodalite unit of Zn,-A evacuated at 600

evacuated at 400 OC5 for the framework atoms and two 3-fold-axis Zn2+positions. This model converged with R1 = 0.065 and R2 = 0.063. A difference function revealed the O(4) position deep in the sodalite unit. Including this position in least-squares refinement with U for O(4) fixed at 0.076 A2 led to R1 = 0.060 and R2 = 0.047 with 1.8(2) ions at Zn(l), 4.1(2) at Zn(2), and 2.0(2) oxygens at O(4). These occupancies were revised, by the assumption of stoichiometry and the requirement that the structure be plausible, to the values shown in the last column of Table IB. Allowing U for O(4) to vary with or without the occupancy fixed led to divergence, yet inclusion of that position in the structure lowered the R values significantly. Similar behavior has been observed before for the sodalite-unit oxygen positions in various Cd6-A so U for O(4) was fixed at 0.127 A2 as it had been in those structures. This model converged with the final error indices R1 = 0.059, R2 = 0.045, and GOF = 2.16, with 270 observations and 29 variables. All shift/esd values in the final cycle of refinement were less than 2% except for 0(4), for which they were less than 15%. The largest peak on the final difference function, whose estimated standard deviation is 0.080 e A-3 at a general

OC

and

lo-‘

torr. Ellipsoids of 20% probability are shown.

position, was 4.9 e A-3 in hei h t and was located at the 26 origin where the esd is 0.6 e The final results are presented in Tables IB and IIB. A likely atomic arrangement in a particular sodalite unit is shown in Figure 3. The full-matrix least-squares program22used in both structure determinations minimized xu(A1Fl)2; the weight (w)of an observation was the reciprocal square of cr, its standard deviation. Scattering f a ~ t o rfor s ~Zn2+, ~ ~ 0-, ~ and (Si,Al)1.75+ were used. The function describing (Si,A1)1.75+ is the mean of the Sio, Si4+,AlO,and A13+functions. All scattering factors were modified to account for the real component of the anomalous dispersion c o r r e c t i ~ n . ~ ? ~ ~

v)

Discussion Hydrated Crystal. The six Zn2+ions per unit cell in hydrated ZQ-A are distributed over three sites (Figure 1). (27) Doyle, P. A.; Turner, P. S. Acta Crystallogr., Sect. A 1968,%, 390. (28) “International Tables for X-ray Crystallography”;Kynoch Press: Birmingham, England, 1974; Vol. IV, pp 73-87. (29) Cromer, D. T. Acta Crystallogr. 1965, 18, 17. (30) Reference 28, pp 149-50.

Crystal Structures of Zinc(I1)-Exchanged Zeolite A

TABLE 11: Selected Interatomic Distances ( A ) and Angles (deg)a A. Hydrated Zn,-A 110.0 (9) 1.629 (6) O(l)-(Si,d)-0(2) 111.0 (9) 1.660 ( 6 ) O( 1)-(Si,Al)-O( 3) 107.9 (6) 1.666 (5) O( 2)-(Si,Al)-O( 3) O( 3)-(Si,Al)-O( 3 ) 109.0 (6) Zn( 1)-O( 4) (Si,Al)-O(1)-(Si,Al) 160.0 (8) 1.95 ( 5 ) Zn( 2)-O( 2) 3.00 (1) (Si,Al)-O(2)-(Si,Al) 150.0 ( 9 ) Zn( 2)-O( 3 ) 2.27 (1) (Si,Al)-0(3)-(Si,Al) 139.5 (7) 1.98 (4) Zn( 2)-O( 5) 109 (2) Zn( 3)-O( 5) 2.32 ( 2 ) 0(4)-Zn(l)-0(4) 1 1 2 (1) Zn( 3)-O( 8) 2.56 ( 7 ) O( 3)-Zn( 2)-O( 3) Zn( 3)-O( 1) 3.79 (11) 0(3)-Zn( 2)-O( 5) 107 (1) Zn( 3)-O( 3) 169 ( 2 ) 3.80 (11) 0(5)-Zn(3)-0(5) O( 5)-Zn( 3)-O( 8) 87 (4) 121 (2) 2.83 (10) O( 8)-Zn( 3)-O( 8) O(1)-0(7) 2.91 (4) 0(3)-0(4) 1 3 2 (2) 2.70 (3) Zn(1)-O(4)-O(3) 0(5)-0(6) 1 2 0 (1) 2.39 (9) Zn( 2)-O( 5)-Zn(3) 0(6)-0(7) 114 (1) 2.71 Zn( 2)-O( 5)-O( 6 ) O( 6)-O( 9) 120 (3) 2.90 (12) Zn( 3)-O( 5)-O( 6 ) O( 7)-0(7) 104 ( 2 ) O(8)-O( 9)b 2.44 O(6)-O( 5)-0(6) O( 8)-O(10) 118(2) 2.94 (9) O( 5)-O(6)-O( 5) O( 9)-0( 9)b 2.41 O( 5)-O(6)-O( 7) 96 ( 2 ) 144 (3) 3.10 (8) O(5)-O(6)-0(7) 0(6)-0(6) 70 O( 5)-O( 6)-O( 9)b 117 O( 7)-O( 6)-O( 9)b 81 O(6)-O( 7)-O( 6) 102 O( 6)-O( 7)-O( 7) 128 O(6)-O( 7)-O( 1) 110 O( 1)-O( 7)-O( 7) 76 Zn(3)-0(8)-0(9)b 105 Zn( 3)-O( 8)-O( 102 O( 9)-O( 8)-O( 10) 160 150 O( 6)-O( 9)-O( 9)b 95 O( 6)-O( 9)-O( 8 ) b 115 O( 8)-O( 9)-O( 9)b B. Zn,-A Evacuat;ed at 600 "C and torr (Si,&)-O(1) 1.615 ( 2 ) O(1)-(Si,Al)-0(2) 115.6 ( 5 ) 111.5 (3) (Si,AI)-O(2) 1.636 (4) O( 1)-(Si,Al)-O( 3) (Si,&)-0(3) 1.716 (3) 0(2)-(Si,A1)-0(3) 105.3 ( 4 ) 0(3)-(Si,Al)-0(3) 107.0 ( 4 ) Z n ( l ) - 0 ( 2 ) 3.167 (8) (Si,Al)-O(1)-(Si,Al) 176.3 ( 5 ) Zn( 1)-O( 3 ) 2.080 ( 7 ) (Si,AI)-0(2)-(Si,Al) 144.9 (6) Zn(1)-O(4) 2.18 (8) (Si,Al)-O( 3)-( Si,Al) 130.5 (4) Zn( 2)-O( 2) 2.997 ( 6 ) Zn(2)-O(3) 1.989 (6) O( 3)-Zn( 1)-O( 3) 111.3 (3) O(4)-0(4) 2.23 (8) O( 3)-Zn( 1)-O( 4 ) 96 (2) O( 3)-Zn( 1)-O( 4) 113 ( 2) 0(3)-Zn(2)-0(3) 119.3 (4) Zn(1)-O(4)-O(4) 146 (4)

(Si,&)-O(1) (Si,Al)-O( 2) (Si,Al)-O( 3)

a The numbers in parentheses are the esd's in the units of the least significant digit given for the corresponding Since the O( 9) position was not allowed to parameter. refine, distances and angles involving that position are only approximate.

One cation, at Zn(1) (Table IA), is located at the center of the sodalite unit and is tetrahedrally coordinated at 1.95(5) 8, (Table IIA) to four nonframework oxygens (HzO molecules, some of which may have dissociated) at O(4) (Figure 2). These four oxygens can each hydrogen bond at 2.91(4) 8, via each hydrogen atom to an O(3) framework oxygen. Four Zn2+ions are located on 3-fold axes and extend 0.66 8, into the large cavity from the [lll]planes at O(3) (Table 111). Each of these cations is coordinated in a nearly tetrahedral manner to three framework oxygens at O(3) at 2.27 8, and one nonframework oxygen at O(5) at 1.98(4)A. Each O(5) oxygen makes a hydrogen-bonding approach of 2.70(3) 8,to one or more O(6) water oxygens. The sixth Zn2+ion, at Zn(3), is located deep in the large cavity on a 2-fold axis opposite a 4-ring of the aluminosilicate framework. It is coordinated to two O(5) and two O(8) nonframework oxygens at 2.32(2) and 2.56(7) A, respectively. If the 4-ring oxygens of the zeolite framework

The Journal of Physical Chemistty, Vol. 85,No. 4, 198 1 409

TABLE 111: Deviations of Atoms ( A ) from the [ 111] Plane at O(3)a hydrated Zn,-A

Zn,-A evacuated a t 600 "C

Zn( 1) -3.89 Zn( 1) - 0.64 Zn( 2) 0.66 Zn( 2) 0.16 0.43 0.33 O( 2) O( 2) - 2.79 -1.93 O(4) O(4) O( 5) 2.63 a A negative deviation indicates that the atom lies on the same side of the plane as the origin.

at O(1) and 0(3), 3.8(1)-A distant, are collectively considered to be a weak fifth ligand, the Zn(3) coordination is approximately trigonal bipyramidal. The especially long Zn(3)-0(8) approach is probably an artifact of the symmetry of the O(8) position (it would not refine off the 4-fold axis, at x , l/z, 1/2, to x , y , 1/2). The two axial O(5) oxygen ligands (each probably OH-of a hydrolyzed H20 molecule) can each bridge to a Zn2+ion at Zn(2), and it is likely that this arrangement is responsible for the location of a Zn2+ ion at such an unusual position. The two equatorial O(8) oxygens are hydrogen-bonding distances from O(9) and O(l0) water oxygens. Except for a portion of the unit cell near Zn(3), the O(5) oxygens (some bridging between the Zn2+ions at Zn(2) and Zn(3), some coordinated to Zn2+ions at Zn(2) only, and some not coordinated to Zn2+ions at all) and the O(6) oxygens form a pentagonal dodecahedral, hydrogenbonding network, similar to that proposed to exist in hydrated and Na12-A,32 within the large cavity. Nonframework oxygens at O(7) and O(9) provide hydrogen-bonding links from O(1) oxygens of the framework and from O(8) oxygens of the Zn(3) coordination sphere, respectively, to the O(6) oxygens of this network. In all, 29 nonframework oxygen species per unit cell were located. The atomic arrangement shown in Figure 1is consistent with the structural parameters presented in Table IA and the description given above, but the specific arrangement of cations and nonframework oxygens is unlikely to be correct in all detail. It is shown to illustrate the Zn2+ coordination spheres and the hydrogen-bonding possibilities, to indicate the complexity of the structure, and to demonstrate that the zeolitic cavities can accommodate 29 H20 molecules per unit cell without requiring impossibly short interatomic distances. The differences between the structures of hydrated Zng-A and hydrated Zn5Na2-A4 are intriguing. Both structures have a Zn2+ion at the center of the sodalite unit, but its coordination changes from octahedral in Zn5Na2-A to tetrahedral in Zn6-A. The difference appears to be real. For example, changing the oxygen's positional and occupancy parameters to reflect a tetrahedral geometry about the origin Zn2+ion in Zn,Na2-A raised the error indices from R1 = 0.078 and R2 = 0.077 to R1 = 0.082 and R2 = 0.081. Similarly, changing the tetrahedral coordination geometry in Zn6-A to octahedral caused the R values to increase from R1 = 0.078 and R2 = 0.065 to R1 = 0.080 and R2 = 0.073. Some considerations in explanation of this apparent structural difference follow. In hydrated Zn5Na2-A, Zn2+ions are associated with oxygens of only three of the eight 6-rings per unit cell. This means that a hexaquo zinc ion at the center of the sodalite unit can be accommodated if just one of the coordinated water molecules can adjust its position enough to avoid (31) Seff, K. Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, MA, 1964. (32) Gramlich, V.; Meier, W. M. Z.Kristallogr., Kristallgeom., Kristallphys., Kristallchem. 1971, 133, 134.

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making too close an approach to a 6-ring Zn2+ion. However, in Zn6-A, oxygens of four of the 6-rings are coordinated to 3-fold-axis Zn2+ions, so the environment in the sodalite unit is even less suited for the octahedral Zn(HzO)62+ion. In Zn6-A there are only six cations per unit cell to balance the anionic charge of the aluminosilicate framework, whereas in Zn5Na2-A there are seven. Consequently, the tendency of Zn2+ions to hydrolyze coordinated H 2 0 molecules to produce ZnOH+ and H+ ions may be enhanced in the fully exchanged zeolite. This combination of an unfavorable environment in the sodalite unit for a hexaquo zinc ion and the increased likelihood that hydrolysis will occur (perhaps to form a neutral Zn(OH),(H20), species in the sodalite unit) may cause the coordination number of the Zn2+ion at the origin to decrease as the number of Zn2+ions in the zeolite is increased (the lower the coordination number, the greater the hydrolysis constant). An increased tendency of Zn2+ions to hydrolyze water in ZQ-A could also explain why the non8-ring, large-cavity Zn2+ion positions in Zn6-A and ZnjNaz-A are so different. Once again, that Zn2+ion in the partially exchanged zeolite is octahedrally coordinated. It lies on a 4-fold axis, and the four equatorial oxygen ligands are within hydrogenbonding distance of framework oxygens. However, the corresponding Zn2+ion in the fully exchanged zeolite is located on a 2-fold axis and has assumed approximate bipyramidal geometry. It is probable, considering the ability of Zn2+to hydrolyze water and the lower number of cations in Zn6-A, that the two nonframework oxygens bridging between two 6-ring Zn2+ions and this 2-fold-axis Zn2+ion are hydroxides of dissociated H20 molecules and that a Zn2+ion would prefer a site where it could coordinate to two bridging hydroxides over one where it could coordinate to water molecules only. It appears that the extent of cation exchange in zeolite A can be an important factor, not only in cation site selectivity but also in the chemistry of the cations themselves. All of the Cd2+ions in hydrated Cd6-Ala lie on 3-fold axes, are associated with framework oxygens, and are either 5- or 6-coordinate. This is in marked contrast to the Zn2+ ions in hydrated Zn6-A, which are either 4- or 5-coordinate (the fifth ligand being very weak) and are not all associated with framework oxygens. Coordination numbers of 4,5, and 6 are common for both ions, but, because of their difference in size, Cd2+is more likely to be octahedral and Zn2+ tetrahedral or trigonal b i ~ y r a m i d a l . ~ This ~ is probably why the 3-fold-axis Zn2+ions are satisfied with tetrahedral coordination, while Cd2+ions prefer to sacrifice close approaches to framework oxygens to attain 6-coordination. Unlike the Cd2+ions in hydrated Cd6-A,16 two Zn2+ions per unit cell are not close to framework oxygens, perhaps because the charges of each of these Zn2+ions are at least partially balanced by one or two hydroxides. The hydroxide bridging arrangement in the large cavity (Zn(2)-0(5)-Zn(3)-0(5)-Zn(2)) and the tetrahedrally coordinated Zn2+ion (possibly Zn(OH)2(HzO)2)at the origin may result in part from the greater ability of Zn2+ions (relative to Cd2+)to hydrolyze coordinated H20molecules. So, although Zn2+and Cd2+are both group 2B ions, differences in coordination number requirements and hydrolysis constants cause them to behave dissimilarly in the zeolite-A environment. (33) Cotton, F. A.; Wilkinson, G. "Advanced Inorganic Chemistry",3rd ed.; Wiley: New York, 1972; pp 504-5.

McCusker and Seff

Evacuated Crystal. Evacuation at 600 "C and lo4 torr causes a significant rearrangement of the Zn2+ ions in Zn6-A (Figure 3). All of the cations are located on 3-fold axes and are distributed over two sites. Two Zn2+ions, at Zn(l), are recessed 0.64 A into the sodalite unit from the [111] planes at O(3) where each is coordinated to one nonframework oxygen deeper in the sodalite unit at O(4) at 2.18(8) A (Table IIB), and to three O(3) framework oxygens at 2.080(7) A,in a distorted tetrahedral manner. The O(4)-O(4) distance (2.23(8) A) is impossibly short, but the difficulties encountered in the refinement of this position indicate that its coordinates are poorly defined, so interatomic distances involving the O(4) position are unreliable. The remaining four Zn2+ions, at Zn(2), lie almost in the 6-ring planes, where each is coordinated in a trigonal planar fashion to three O(3) oxygens of the zeolite framework at 1.989(6) A. The structure itself and the difficulties encountered in the refinement of the sodalite-unit oxygen position are reminiscent of the structure of Cd6-A evacuated at 600 "C.17 The cation distribution and the number of nonframework oxygens in the sodalite unit are approximately the same in the two structures. In ZQ-A-~H~O the framework is even more warped than in Cd6-A-2Hz0,presumably because Zn2+is a smaller ion and does not fit a 6-ring as well as does Cd2+. The (Si,Al)-O(l)-(Si,Al) angle is almost linear (176.3(5)') and the (Si,A1)-0(3) distance is 1.716(3) A as compared with 1.615(2) and 1.636(4) A for (Si,Al)-O(1) and (Si,Al)-O(2). Since all six of the Zn2+ ions are associated with O(3) oxygens, the (Si,A1)-0(3) bond has been weakened and lengthened. This effect has been observed and discussed previously for other zeolites including Zn5Kz-A,5but the amount of distortion is particularly pronounced in Zn6-A. The 10.3' range in the angles at (Si,Al) is a further indication of the strain on the framework induced by the Zn2+ ions. Both the magnitude and the direction of each deviation from a tetrahedral angle are the same as those found in Cd6-A evacuated at 600 'C. There are no major differences or inconsistencies in the evacuated Zn6-A and Zn5K2-A structures. The Zn2+ions in both structures are located on 3-fold axes and have either trigonal planar or tetrahedral geometries. Partially exchanged Zn5K2-A contains 3.5 water oxygens per unit cell (2.5 in the large cavity and 1in the sodalite unit) while Zn6-A contains only 2 (both in the sodalite unit), but the Zn5K2-A crystal was evacuated at a lower temperature, 400 "C, so its higher water content is reasonable. Since both structures contain residual H20molecules after evacuation at such elevated temperatures, it is probable that those water molecules are dissociated. As suggested in the discussion of hydrated Zn6-A, some of the Zn2+ions are likely to have hydrolyzed coordinated water molecules, to form monopositive ZnOH+ and H+ species, to produce a larger number of cations, so the anionic charge of the aluminosilicate framework can be balanced more evenly. Acknowledgment. This work was supported by the National Science Foundation (Grant No. CHE77-12495). We are indebted to the University of Hawaii Computing Center. One of us (L.B.M.) gratefully acknowledges a research fellowship from the University of Hawaii Chemistry Department. Supplementary Material Available: Listings of the observed and calculated structure factors for both structures (Supplementary Tables 1and 2) (4 pages). Ordering information is given on any current masthead page.