Crystal Structure of Dehydrated Zeolite 4 A
805
e Crystal Structure of
rated Zeolite 4
ussell Y. Vanagida, Allen A. Amaro, and Karl Seff" Che,v.n;slrjDepartment, University of Hawaii. Honolulu. Hawaii 96822 (Received August 18. 1972,
The crystal structure of vacuum-dehydrated (activated) zeolite A , of apparent composition Nalz!d1~Si12048has been determined by single-crystal X-ray diffraction techniques. Least-square refinement in. the space group P m 3 m ( a = 12.263 A) has led to a conventional R index of 0,069. Sodium ions tire d.istributed among three equipoints in the structure: eight of the twelve ions occupy threefold axis positions 0.20 A from the plane of three nearest neighbors in the six-oxygen ring; three more lie in the planes of the eight-oxygen rings, displaced from the center by 1.23 A along a diagonal mirror line; and the twelfth ion i s found on a twofold axis opposite a four-oxygen window in the large cavity. The shortest r.ppiroach distances to framework oxygen atoms are 2.32, 2.4, and 2.6 A respectively. Small changes in cation positions and in the zeolite framework are observed upon dehydration. The changes due to dehytirat ion in framework angles a t the three nonequivalent oxygen atoms, corresponding to rotations of durninosilioate tetrahedra, are much less than those observed for the TI-exchanged form of zeolite A. An attempt t o sorb GzCl4, although unsuccessful, yielded a structure in which the twelfth sodium ion is about 1.: A further from its nearest neighbors in the zeolite framework, presumably because an impurity or decomposition product which associat,es with that sodium ion was sorbed to a small extent.
Introduction A s a test of the suitability of conventional crystallo-
graphic procedures a.pplied to the small X-ray diffraction data sets which are currently available for the determination of the structures of organic complexes of zeolite A, the structure of full,y dehydrated (activated) zeolite 4A was redetermined with greater precision than had been done bef0re.l ,2 In such occlusion complexes the sorbed molecules are usually comparatively weak scatterers and the adverse effects of disorder and thermal motion are both expected t o be great. In this test, the three-dimensional Fourier synthesis was studied and refinements were attempted on many small peaks. For the procedures3 to be substantiated, no iiuccessful least-squares refinement of positions; other than those which could reasonably be ascribed to cations and framework atoms, must occur, even at occupancies corresponding to as few as one or two carbon. atoms per unit cell, whose general equipoint is 48fold. Meaningful comparisons can be made between this structure and those of hydrated 4A4 and dehydrated Tl(I)exchanged zeolite A 5 in which the structural changes upon dehydration or ion exchange can be discussed. Similar comparisons with other variously ion-exchanged, dehydrated, or cornplexetl z,eolite A structures should logically be made as these latter structures are deterrnined. Presently available2 results are inadequate for detailed C O M parisons. Description o f the Structure The structure of zeolite A has been previously described when it was determined627 and in more recent work.4 A stereodrawing of' eight unit cells, clearly illustrating how a cubic arrangement of eight sodalite units generates the large cavity, is availabie.8 Zeolite A can be considered to be composed of three distinct components. The aluminosilicate framework is anionic, possesses the full symmetry of a cube if the A13+ arid S i 4 + ior,s are disordered, has large channels of two kinds and two kinds of approximate1.y spherical voids, and
is relatively rigid. The principal channels have eight-oxygen (sixteen-membered) rings as their minimum constriction; these eight-oxygen rings are shared by the large cavities and allow material to pass to the interior of a zeolite crystal. The minor cavity (the sodalite unit) can be accessed only through the minor (six-oxygen or twelve-membered] channels. If a simple cubic arrangement of equivalent hollow spheres (ping-pong balls) were assembled, the symmetry would be precisely that of zeolite A; the sphere cavities would correspond to the large zeolite cavities; the smaller cavity a t the center of each cube of eight spheres would be the sodalite cavity a t the center of the sodalite unit, and a six-oxygen window would face into each large cavity; the eight-oxygen windows would exist a t each point two spheres touched, normal to their line of centers and connecting them. The cations which balance the framework charge are exchangeable and occupy sites within the zeolite cavities and channels. They coordinate to the zeolite framework and/or to sorbed molecules. The number of ions will, in general, not be equal to the number of equivalent positions available a t the most favored site; accordingly, as some equipoints are filled, other less favorable sites will accept ions until. all ions are placed. For this reason zeolitic sodium ions, for example, may be nonequivalent and some many-fold sets of equivalent positions may be only partially (statistically) occupied. Often a site, meaning a set of equivalent positions, cannot be more than 1h or filled because a greater filling would require sterically unreasonable approaches. P. A . Howell, Acta Crysfaiiogr.. 13, 737 (1960) J, V . Smith and L. G. Dowell, 2.Kristaiiogr.. 126. 135 (1968). K . Seff and D. P. Shoemaker, Acta Crysfaiiogr , 22, 162 (4967). V . Gram!ich and W . M . Meier. Z. Kr,sla/!ogr,1 3 3 , 134 (1971). P. E . Riley, K . Seff. and D. P. Shoemaker. J . Phys Chem.. 76, 2593 (1972). T B. Reed and 0. W. Rreck, J A m e r . C h e m . S o c , 78. 5972 (1 956). L . Broussard and D. P. Shoemaker, J . Arne? Chem Soc.. 82. 1041 (1960). W , M . Meier and D. t i . Olson. Advan Chem Ser No. 101, 166 (1971 1 , The Journal of Physical Chernvtry. Voi. 77. No. 6. 1973
R. Y. Yanagida, A. A. Arnaro, and K. Seff
866
Sorbed molecules are appropriately considered last. They may interact with the zeolite framework, with the cations, and/or with each other. Usually they may be reversibly sorbed and desorbed by varying the temperature and pressure of the system. The sites available to the sorbed molecules are more complex than those found by ions for several reasons. The symmetry of the zeolite before sorption is much lower, usually, than it is before the ions are placed because of the partially occupied cation sites. Also small molecules cannbt have the Oh symmetry that: monatomic icins have; accordingly, their symmetry may be incompatible with that of the most favored sorption site, and a substantial degree of disorder may be introduced. Furthermore, nonequivalent ions may associate with sorbed molecules, making the latter nonequivalent also. Usually atoms in sorbed molecules occupy sites of low symmetry wi,th, due to the finite sorption volumes of the zeolite, small occupancy parameters, leading to greater crystallographic indeterminacies. Also sorbed molecules are likely to have smaller scattering factors and larger thermal parameter:;. Usually, then, sorbed molecul'es are more difficult to locate than ions within the zeolite framework. ~ x ~ e r i ~ eSection ~ t a l Crystals of zeo1i.te 4A were prepared by Charnell'sg method, modified ti3 include a second crystallization using seed crystals from the first preparation. This sample was found by repeated wet chemical analysis to have an approximate framework formula of AX11.3Si12.704811.3- per unit cell. Although the values in this stoichiometric formula have been rounded further to the nearest integer in previous reports,"1'Dx11 the precision of the analysis does not exclude the possibility that the correct framework formula might be A1&i1204812-, which i s more ideal and which has been used extensively in previous investigations. Such incons,istencies have been noted previously.6 One crystal, a cube approximately 67 p on an edge, was cautiously dehydrated by slowly raising its temperature to 200" over the course of a few hours at atmospheric pressure, and then maintaining it a t 350" and 10--5Torr for 24 hr. The capillary tube containing this crystal was then sealed off under va.cuum and mounted on a goniometer head. Diffraction intensities were collected a t 19.5" for 0 < 28 < 70". A Syntex four-circle computer-controlled diffractometer with graphite-monochromatized Mo K n radiati.on (Kcq, X 0.701926 A; K a 2 , X 0.71354 A) and a pulseheight analyzer was used throughout for preliminary experiments and for the collection of diffraction intensities. The space group Pm3m (no systematic absences) was used instead of Frn% because no significant counts could be observed for expected major superstructure ("b") reflections,4 and because Gramlich and Meier4 have shown that framework deviations from the former space group are small evexi when appropriat,e. Still, Pm3m has only one tetrahedral framework site and accordingly treats A1 and S i atoms and their immediate environments as entirely equivalent. Therefore some minor but extensive disorder is implicit in this choice of space group. The crystals synthesized her'e appear not to have long-range Si& ordering. The cubic cell constant for the Pm3m unit cell (a = 12.263(2) A) was determined by a least-squares treatment of 15 intense reflections with 28 vdues between 20 and 24". The 8--28 scan technique was employed at a scan rate which varied from 0.5 (in 28) to 24"/min in such a way The Journal of Physical Chemistry, Vol. 77.
No. 6. 1973
that more time was spent on weaker reflections to improve their standard deviations. Most reflections were observed at the slowest scan rate. The scan range varied from 2.0" at 28 = 3" to 2.5" at 28 = 70". All 881 unique reciprocal lattice points below the maximum 28 .value C7V) were examined. (This high upper limit was chosen for 28 to maximize the size of the data set, even though few reflections with large 28 values showed significant intensity.) A time equal to one-half of the scan time for each reflection was spent counting background at each end of the scan range. Two check reflections which were measured periodically during the collection of each data set showed no significant trend in intensity. The effects of Renninger reflection were assumed to be absent because the previous examinations of a related material showed these to be entirely absent. Standard deviations were assigned according to the formula
o(I) = [CT -I-0.25(tc/tb)'(Bl
+ B2) + ( ~ 0 ~ 1 ~ ' ~
where CT is the total integrated count obtained in a scan time of tc, B1 and B2 are the background counts each obtained in time t b , and I = CT - 0.5(tc/tb)(& + &). A value of 0.02 was assigned to the empirical parameter p to account for instrument instability. The net counts were then corrected for Lorentz and polarization effects; an absorption correction (pR = 0.017) was unnecessary. All 154 unique reflections for which the net intensity exceeded three times its standard deviation were used throughout. Structure Determination Initial full-matrix, least-squares refinement of the structure was carried out using parameters for the framework and Na(1) and Na(2) positions which had been determined and refined10 for the 32 NH3 complex Qf zeolite 4A. Only the Na(1) position, near center of the six-oxygen window, was allowed to refine with anisotropic thermal parameters. This model, which allowed for eleven sodium ions and which put the three Na(2) ions a t positions different by symmetry from those previously reported,z reof 0.069 and a fined to an R1 index, (ZIFo- IFcl/)/ZFo, weighted R2 index, ( Z w ( F o - (Fc\)2/EwFo2)1'2,of0.088. A three-dimensional difference Fourier function was prepared using structure factors calculated from the justrefined parameters. From this synthesis, seven small but predominant peaks were selected for least-squares refinement. None were unreasonably close to the atomic positions already included in the calculations. Six of the new positions would not refine with convergent thermal parameters at or near their observed coordinates. The seventh position refined quickly to the fractional coordinates 0.204, 0.204, 0.500 with a n isotropic thermal parameter of 1.6 A2, and the error indices decreased slightly to R.t = 0.069 and Rz = 0.086. This position was judged to be appropriate for a sodium ion, but less so for one or more water molecules, which might be expected to be closer to either the Na(1) or the Na(2) position. The occupancy was limited to one ion per unit cell a t this, the Na(3), position because eleven ions had been previously placed and because Loewenstein's ruleI2 indicates that the maximum number of aluminum atoms, and therefore of sodium ions, (9) J. F. Charnell, J. Cryst. Growth. 8, 291 (1971). (10) R, Y . Yanagidaand K.Seff. J , Phys. Chem.. 76, 2597 (1972). (11) K.Seff, J. Phys. Chem.. 76, 2601 (1972). (12) W. Loewenstein, Amer. Mineral.. 39,92 (1954).
Crystal Structure of Dehyd9atedZeolite 4A
807
TABLE I: Positional, Thermal and Occupancy ParametersQ Wyckoff position
X
Y
Z
(Si,AI) O(1) O(2) Q(3) Na(1)
24(k) 12(h) 72(1) 24(m) 8(g)
0 0 0 0.114(1) 0.200(1)
0.185( 1 ) 0.225(1) 0.290( 1) 0.114(1) 0.200 (1)
0.372(1)
Na(2)
720)
0
0.429 (3)
0.429(3)
Na(3)
12(j)
0.204(7)
0.204 (7)
'12
E, A'or anisotropic b's
'12
0.290(1) 0.345(1) 0.200 (1 )
1.4(6) 2.3(4) 2.6(4) 2.7(3) 3.007(1) 0.002(1) 0.012 (6) 0.005(3) 2(2)
Occupancy factor
1 1 1 1 1 '14
'/12(0.03)
nStandard deviations are in the units of the least significant digit given for the corresponding parameter. See Figures 1 and 2 for the identities of the atoms. For N a i l ) , the anisotropic temperature factor = exp[-bll(h2 f k 2 /2) - blZ(hk 4-h / + k i ) ] . For N a ( 2 ) , it is exp[-bll(h2) - b 2 z ( k 2 4-/ 2 ) ] .
+
per unit cell is twelve. A refinement of the occupancy parameter of Na(3) converged a t 0.8 ions per unit cell, a value insignificantly different from 1.0. When Na(2) was allowed to refire anisqtropically, the error indices remained the same. Decreasing the occupancy of this Na(2) position to 2.0 ions per unit cell caused one of these thermal parameters to becorne significantly negative. Restoring the N a ( 2 ) occupancy parameter and decreasing the occupancy of Na(1) to 7.0 ions per unit cell caused each error index, R, and Rz, to increase by 0.002 a t convergence. Accordingly the sodium ion at Na(3) appears to be the twelfth ion in the unit cell, and the best integral representation of the formula of the unit cell is N a n A112Si12048. The goodness-of-fit, ( Z w ( F , - 1Fcl)2/(ms ) ) ~ /is~ ,1.14, where m is the number of observations (154), and s (19) is the number of variables in the leastsquares refinement A table of observed and calculated structure factors is avaiiable;l3 the final structural parameters are presented in Table I. The full-matrix, least-squares program used14 minimizes Z;lo(flF1)2; the weights were the reciprocal squares of u, the standard deviation of each observation. Atomic scattering factorsls for N a + , Si2+, All5+, and 0- were used. In the last cycle of least-squaires refinement all shifts were less than I% of their corresponding esd's except those involving the anisotropic thermal parameters of 1Va(2),whose shifts were less than 10% of their esd's.
IDi seu ssi on The test of the adequacy of data sets as small as those reported here for the determination of the positions of smail atoms such as carbon, oxygen, or sodium even a t low occupancies, has been successful. Of the seven positions which were as much as weakly suggested by a Fourier difference function, six were entirely divergent in leastsquares refinement; the sewnth converged quickly; no ambiguity was encountered. This indicates that acceptable rcfinement, when it occurs, successfully describes the positions of previously unlocated atoms. The inverse is not considered demonstrated; unacceptable refinement does not necessarily indicate that the atoms under consideration are absent. The aluminosilicate framework of the zeolite i s quite similar to that found by other workers,f-7JOJl and is presented in Tables I1 and 111. The most meaningful comparisons, within structcres of comparable or greater precision, are with hydrated neolixe 4A,4 and with hydrated and dehydrated Tl(I)-excbanged zeolite A.5 Surprisingly, since changes in framework angles of up to 17" occurred upon
TABLE IS: interatomic Distances(a) and Angles (degreesIa (Si,Al)-O(1) (Si,AI)-0 (2) (Si,AI)-0(3) Na(1)-0 (3) Na(1 )-0(2) Na(2)-0(2) Na(2)-0 (1) Na(3) -0 (1) Na(3)-0(3) Na(3)-Na(1)
1.65(1) 1.63(1) 1.68(1) 2.32(1) 2.90(1) 2.40(6) 2.64(3) 2.51 (7) 2.47(7) 3.68(7)
0(1)- (Si,AI)-0(2) 0 (1)-(Si, AI) -0(3) 0(2)-(Si,AI)-0(3) 0(3)-(Si,Aij-0(3) (%,AI)-0 (1) - (Si,AI) (Si,Ai) -0 (2)- (Si,Al) (Si,AI)-O (3)- (S1,41) Na(3)-0(1 )-(Si,Ai) Na(3)-0 (3)- (Si,AI) O ( 1)-Na(3)-0(1) O(3) -Na (3) -0 (3) Na (1) -Na (3)-Na (1 )
'i10.4 110.3 106.8 112.1 145.1 1155.6 145.5 88.2 89 102 101 178
(8) (8) (9) (9) (7) (15) (10) (4) (2)
(2) (2) (2)
a Standard deviations are in the units of the least significant digit given for the corresponding parameter.
TABLE I II: Aluminosilicate Framework Angles (degrees)a
O(?)-(Si,Ai)-0(2) O(l)-(Si,Al)-O(3) 0(2)-(Si,AI)-0(3) 0(3)-(Si,AI)-0(3) (Si,AI)-O( 1)-(Si,Al) (Si,Al)-0(2)-(Si,AI) (Si,AI)-0(3)-(Si,AI)
4A hydratedb
4 A dehydrated
TI-exchanged A hydratedc
108(1) 111(1) 108(.1) 111(1) 146(0.5) 160(0.5) 144 (0.5)
110(1) 110(1) 197(1) 112(1) 145(1) 166(1.5) 146( 1 )
110(1) 110(1) l07(1) 111(1) 148(1) 161 (2) i 44 (2)
TI-exchanged A dehydratedC
l08(1) !10(2) 109(1) 111(2) 162 (2) 144(2) 138 (2)
aStandard deviations have the units of degrees. Reference 4 . The approximate standard deviations given here are larger than those reported in ref 4 because averages over Si and AI positions have been taken. Reference 5 .
dehydration in the Tl(I)-exchanged material, the only significant change in the sodium form is at O(2) and is only f6" (see Table 111). In fact, the sign of the change a t Q ( 2 ) (13) Listings of the observea and calculated structure factors for the dehydrated structure and the corresponding structure factor table for the second structure (neariy dehydrated) discussed above, as well as tables of its structural parameters, bond lengths and angies, will appear following these pages in the microfilm edition of this volume of the journal. Single copies may be obtained from the Business Operations Office, Books and Journals Division, American Chemical Society. 1155 Sixteenth St.. N.W., Washington, D. C. 20036. Remit check or money order for $3.00 for photocopy or $2.00 for microfiche, referring to code number JPC-73-805. (14) P. K. Gantzel, R. A. Sparks, and I
