2758
DAVID H. OLSON
A Reinvestigation of the Crystal Structure of the Zeolite Hydrated NaX by David H. Olson Mobil Research and Development Corporation, Central Research Division Laboratories, Princeton, New Jersey 08640 (Received December 11, 1969)
The crystal structure of hydrated XaX has been reinvestigated using single crystal X-ray techniques. Although the basic structural features agree with the earlier powder X-ray study of Broussard and Shoemaker, several, more detailed, features of the structure hav: been found. The probable space group and lattice parameter of the crystal studied are Fd3 and 25.028 A, respectively. The space group and the two average tetrahedral atom (Si, A1)-oxygen bond distances, 1.619 (4)and 1.729 (4)A, indicate that the Si and A1 atoms are ordered to a considerable extent. The nonframework atom site occupancies and assignments are as follows: site I, 9 Naf; site 1’, 8 Na+ and 12 HzO; near site 11‘,26 HzO; site 11, 24 Na+ and 8 HzO. In addition, seven low occupancy supercage scattering sites due to HzO and/or Na were found. The occupancy of site I1 gives direct evidence for the influence of aluminum content on cation siting in hydrated faujasites; Le., a supercage six-ring of tetrahedra must contain three A1 ions before ion siting is energetically favorable. This rule appears to apply to hydrated mono and polyvalent cation faujasites.
Introduction Hydrated NaX is the parent material for all zeolite X derivatives, i.e., catalysts, drying agents, etc. The crystal structure of this commercially important zeolite was reported nearly a decade ago by Broussard and Shoemaker,’ and elucidation of its crystal chemistry (primarily ion-exchange properties) has relied heavily upon the results of their powder diffraction study. The availability of large crystals of hydrated NaX and our preliminary observation of diffraction symmetry lower than that assumed in the early study prompted us to reinvestigate the structure of the material. The lower diffraction symmetry suggested the possibility of obtaining information on the ordering of Si and A1 ions; this information was subsequently obtained. The cation distribution found differs in many respects from that found earlier,’ although many of the differences found can be attributed to the higher accuracy obtained in our study. Evidence for an effect of A1 content on the siting of cations in supercage six-rings also was found.
Experimental Section The KaX single crystal used in this study was grown by CharnelL2 The crystal, which was a well-formed octahedron 60 p on an edge, was mounted in an open capillary tube for X-ray examination. The preliminary diffraction photographs exhibited m3 Laue symmetry, and the extinctions were consistent with the spacz group Fd3. The lattice parameter, a0 = 25.028 (5) A, was determined by double-scanning diff ractometrya on a Siemens goniometer equipped with a General Electric Eulerian Cradle. The intensity data were collected using the moving-crystal, movingcounter technique with nickel-filtered copper radiation. One-minute background counts were taken at each end of a three-degree, three-minute scan. The intensities The Journal of Physical Chemistry, Vol. 74, No. 14, 1970
of the 1014 unique reflections contained in ‘/a octant (120” 28 cutoff) were measured. The standard deviations of the structure factors were computed as described earlier.4 Only those structure factors (694) having values greater than twice their standard deviations were used in solving and refining the structure. The unit cell composition of the Eeolite Na88 [A~~sS~IMOWI .220H20 was determined from the relation between the lattice The sodium conparameter and aluminum ~ o n t e n t . ~ tent represented by this composition agrees with the value found for the parent preparation, i.e., 15.0 wt % on a dry basis.
Determination and Refinement of the Structure Least-squares6 and Fourier techniques were combined with trial faujasite frametvork parameters and estimated cation parameters to solve the structure. The structure factors were computed using scattering factors for Si2+,Al+, 0-, and Na+, based upon the HFS ionic scattering factors of Hanson and P ~ h l e r . ~ I n the last two least-squares cycles, the scattering factors were adjusted in accordance with the assumption that T1 is a pure Si site, and T2 is a site having 0.08 Si plus 0.92 AI. For this compositional assignment, the average TI-0 (1) L. Broussard and D. P. Shoemaker, J. Amer. Chem. SOC.,82,
1041 (1960). (2) J. Charnell, Mobil Research and Development Corp., Paulsboro, N. J. 08066, to be published. (3) H.W.King and L. F. Vassmillet, “Advances in X-Ray Analysis 6,”W. M.Mueller, G . R . Mallett, and M. J. Fox, Ed., Plenum Publishing Co., New York, N. Y., 1961, pp 78-79. (4) D. H. Olson, J. Phys. Chem., 72, 4366 (1968). (5) E. Dempsey, G. H. Kuehl, and D. €3. Olson, ibid., 73, 387 (1969). (6) W.R . Busing, K . 0 . Martin, and H. A. Levy, “ORFLS,” Oak Ridge National Laboratory, Oak Ridge, Tenn., 1962. (7) H. P. Hanson and R . F. Pohler, Acta Crystallogr., 21,435 (1966).
THBCRYSTAL STRUCTURE OF THE ZEOLITEHYDRATED NaX Table I: Observed and Calculated Structure Factors for Hydrated NaX
.
K FORS
U '.*..L 0
FCAL
..'...O
0
145 31b 0 I139 -1832 0 14.92 3 4 1 7 0 b10 -(I2
12 Ib
20 24
o inm
o
21 b.
1113
5% 548 2 1Y33 1152 2 I l I b -1840 2 I 2 6 1 1244 2 511 355 k 2k45 - 2 4 3 1 4 1570 35ni 5 I639 I300 4 913 -nJn b 051 R2O b 2b5k 2bR8 6 bk9 163 b 3111. - 5 1 0 6 15hl -Ib21 b 1800 - 1 R O O b 0 2 2 -R05 n 3350 3439
14
22 26 4
n
lb 20 2 b
LO I 4 IR 22 ~~
2b 4 8
0 40112 B 526
I2 20 24
R 3bP1 R LOO .10 1 5 3 L O 10.4ORb l k ' 10 9 1 1 I 0 I O 551 2 2 10 25311 6
8
I 2 Il8k 12 5301
12
3442
516 0196 -109 516 -41Ob -991 -511 2550 1131 5k25 -b45 -11RY -1810
2 b
I2 145 12 1 l h 1 l k 1146 I4 1 5 1 $ - 1 h b l 14 n a i nbi 1 4 bS3 -bo3 Iii 16 i o n 1 l b 621 - 5 k 5 I 6 4b81 - b 4 1 l b 1 0 3 4 -1Ok I t 1 1 1 2 7 -1bRR 1 R 9 4 1 -162 ZC 3'04 -363 20 3 4 7 . 1 3 k h b 2 1 1014 i n i n Z2 1 0 7 1 - ? I 3 5 2 2 Zh'I2 2 6 1 0 14 m t r -830 2 4 15l.h -151k 2b 081 804 2 b > 7 9 -511
H 1
1 3 1939
lb 24 2
IO 14
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I2 lb 6 14 4 R
2 b 10
n
12
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9
13
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10
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11 13
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21 2Y 2 ..5 97. 5 1
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1 9
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20
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9
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450
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5 1212 -1159 5 1235 4211
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191 777
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5 1 1 1 2 IM 7 I I R 4 I145 7 l i h l -1343 7 IlbL I l l q 7 7131 2CJ5 I L32 454 7 71', 836 7 $29 -3bl 9 564 -728 9 1'107 I b 3 b 'i 5 8 7 502 1 1 3 b l -1281 9 1519 -1552
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F i v e Reflection8 Not Included In
The Refinement
hk_imFCAL 2
2
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1951
2722
1 1 . 1 3596 6237 3
1
1
5 7
1
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1297 1072
3 1 1951
1389 1254 2361
lk......
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1 4 1182 -120% 14 3 4 9 154 16 on2 121 In kHl 405
lb
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11 4 1 6 4 5 0 6 3 11 I k R - 1 2 5 I7 I1 430 I90 19 11 2 8 1 -+4 2 1 I 1 I l l 2 -1k45 2 1 11 I 3 6 1 - 1 k i O 15 1 3 4 h 0 -39L 17 11 5'13 560 1 9 I 3 640 b54 21 I> 142 -96 13 1 5 360 - 2 8 1 1 5 1 5 918 - 4 0 3 11 1S 1 6 0 8bl 19 1 5 9 P b 905 2 1 15 3 5 2 - 1 4 9 1 5 1 1 62% 577 19 I 7 2 b l - 3 2 5 I S I V 1119 nil 11 1 9 3 9 7 - 3 9 9 1 5 2 1 300 243 ..,,L 12.1111. 12 12 Z b l l -26OL l b I 2 A10 103 I R 12 301 IYk 20 I 2 315 LEI 2 2 1 2 336 201 1 4 I h 1 5 1 1 -1'191 P O 14 2 9 4 65 I b Ih 629 585 1 8 16 418 -452 r.a#L m 1 7 1 1 1 1 1 1 11 1:i 1 5 2 7 -1'86 IT 1 3 1061 - I O k l 19 I P 3 1 1 313 21 13 b 2 4 - 0 5 k 1 5 IS 2 7 1 - i n 1 17 l b bo? -495 19 15 ?M3 323 1 5 11 3 3 1 - 3 2 3 1 5 11 4 8 1 408
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3 1135 3'159H 3 'I15 5 1218 5 441 2 402 5 1182 5 641 5 1953 5 441
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L.... I1
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H I FnDS FCAL 11) 10 5 4 ) - b 7 ¶
20
nzs
in
lb
I
14
11 1 1 5 4 - 1 2 6 2 11 413 n35
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12 14
b
I2
U
K
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H 10 I2 lb 20 24
Ib..*@..
and T2-0 distances obtained, 1.619 (4)and 1.729 (4).%, Our average T-0 distances fit Smith and Bailey's curve respectively, fall within 0.010 b of Smith and Bailey's exactly for T1 (0.93 Si, 0.07 Al) and T2 (0.15 Si, 0.85 Curve8 relating average T-0 diStmMes and A1 Content. (8) J. V. Smith and S. w. Bailey, Acta Chrystdlogr., 16,801 (1963). The Journal of P h y e k l Chemistry, Vol. 74, No. 14, 1970
2760
DAVIDHaOLSON
Table 11: Fractional Coordinatesa and Esd's
Atom
T1 T2 01 02 03 04 Nal Na2 Na3A Na3B
Set
P
No./unit cell
X
Y
Z
g g g
1.00 1.00 1.00 1.00 1.00 1.00 0.56 (4) 0.25 (4)b 0 . 3 8 (4)b 0.37 (4)a 0.36 (5)b 0.27 (2) 0 25 (4)b 0.13 (1) 0.31 (3) 0.29 (3) 0.18 (2) 0.17 (2) 0.10 (2) 0.14 (2)
96.0 96.0 96.0 96.0 96.0 96.0 9 . 0 (6) 8 . 0 (13) 12.2 (13) 11.8 (13) 11.5(16) 25.9 (19) 8 . 0 (13) 12.5 (10) 29.8 (29) 27.8(29) 1 7 . 3 (19) 16.3 (19) 9 . 6 (19) 13.4 (19)
-0.05291 (9) -0,05352 (9) -0,1099 (3) -0:0025 (3) -0.0321 (3) -0.0706 (3) 0.0 0.060 (2)b 0.230 (2)b 0.238 (2)a 0.074 (2)b 0.093 (2) 0.245 (2)a 0.281 (2)b 0.353 (2) 0.239 (2) 0.174 (2) 0.212 (3) 0 312 (4) 0.258 (3)
0.12457 (12) 0.03671 (9) 0. '0002 (3) -0.0041 (3) 0.0730 (3) 0.0772 (3) 0.0 0 060 (2)b 0.230 (2)b 0.238 (2)b 0.074 (2)b 0.086 (2) 0.245 (2)b 0.298 (2)b 0.345 (2) 0 240 (2) 0.204 (2) 0.387 (3) 0 381 (4) 0.412 (3)
0.03509 (9) 0.12309 (12) 0.1054 (3) 0.1445 (3) 0.0680 (3) 0.1761 (3) 0.0 0.060 (2)b 0 230 (2)b 0.238 (2)b 0.074 (2)b 0.176 (2) 0.245 (2)b 0.275 (2)b 0.186 (2) 0.392 (2) 0.422 (2) 0.288 (3) 0 200 (4) 0.204 (3)
g
g e; C
e e
e
ow1
e
OW2 OW3 OW4 OW5 OW6 OW7 OW8 OW9 OW10
g
e
I
g g g g g g g
Fd3 origin a t 5.
Atom
ow1 ow2
OW3 OW4 OW5 OW6 OW7 OW8 OW9 ow10 a
Isotropic U .
I
I
I
I
Not refined, error estimated.
Table 111: Vibration' Tensor Components and Esd's
T1 T2 01 02 03 04 Nal Na2 Na3A Na3B
I
u 1 1
0.0155 (13) 0.0155 (13) 0.026 (4) 0.025 (4) 0.033 (4) 0.027 (4) 0.032 (6) 0.019 ( 8 ) ~ t b 0,019 (S)a,b 0.019 (8)=7' 0.023 (lo)+ 0.063 (13). 0.033 (lO)arb 0.'036 (14). 0.136 (17). 0.136 (16)" 0,059 (14). 0.107 (20)a 0.073 (26)a 0.092 (20)"
(i2)
U11
Was
UlZ
Ult
0.0149 (12) 0.0122 (12) 0.029 (4) 0.024 (4) 0.023 (4) 0.031 (4) 0.032 (6)
0.0136 (12) 0.0117 (12) 0.029 (4) 0.018 (4) 0.027 (4) 0.022 (4) 0.032 (6)
-0.0027 (12) 0.0017 (10) - 0.006 (3) 0.011 (3) 0.001 (3) -0.004 (3) 0.018 (4)
0.0019 (10) -0.0018 (13) 0.004 (3) -0.003 (3) 0.003 (4) -0.002 (3) 0.018 (4)
-
u58
-0.0031 (13)
-0.0032 (13) -0.004 (3) - 0.005 (3) 0.002 (3) 0.016 (3) 0.018 (4)
Not refined, error estimated.
Al), suggesting some mixing of A1 in the T1 site. However, we are inclined to believe that this indication of mixing of A1 in the T1 site is a consequence of crystal imperfections rather than actual substitutional disorder (Si and A1 ion mixing) in both sites. Earlier single crystal studiesg of synthetic zeolite X crystals revealed m3m rather than the m3 diffraction symmetry found here for hydrated NaX. Other cationic forms prepared from the NaX of this study also show m3 symmetry, The Journal of Physical Chemistry, Vol. 74, No. 14, 1970
which clearly indicates that the lower symmetry is a result of Si and A1 ordering and is not peculiar to the hydrated sodium form. The external morphological features of the crystals used in the earlier studies were less perfect than those of the NaX crystal examined here. It is conceivable that lattice imperfections give rise to volume elements with Si and A1 ordered in the opposite (9) Referred to in D. H.Olson, J. Phys. Chem., 7 2 , 1400 (1968).
THECRYSTAL STRUCTURE OF THE ZEOLITE HYDRATED NaX sense with respect to adjacent volume elements, to produce apparent substitutional disorder in the T1 site, and even m3m diffraction symmetry. Difference maps revealed seven scattering sites in the supercage, which result from partial occupancy by N a + ions and water molecules. The occupancy factors of these sites were refined using oxygen scattering factors. The maps also showed that the scattering matter in sites I’ and I1 probably results from two and three slightly displaced species, respectively.’O The positional and occupancy parameters for these species were adjusted by examination of a series of difference maps. On the basis of interatomic distances, the scattering matter in site I’ was assigned to N a + ion and water oxygen. For site 11, it was assigned to two N a + ions and a water oxygen, again on the basis of interatomic distances. Having two N a + ion positions in the I1 site is consistent with the low occupancy for the water molecule coordinated to these ions: i.e., only about onehalf of the Na+ ions are coordinated to supercage water (W4) , and these form weaker (and longer) bonds to the framework oxygens (02’s)I* than do the Na+ ions not coordinated to water. Because of the strong interaction between population parameters and temperature factors, these were refined in alternate least-squares cycles. All of the weak reflections omitted from the refinement had calculated structure factors within 2.3 standard deviations of their observed values. Five low-order reflections appeared to be affected by extinction and were not included in the refinement. A structure factor listing is given in Table I. The final R value was 0.088.12 Final positional and thermal parameters are given in Tables I1 and 111. Interatomic distances and angles were computed using ORFFE‘) and are given in Tables IV, V, and VI.
2761
Table IV : Framework Interatomic Distances and Angles (deg)a Distances
T-0 1 T-02
T-03 T-04 01-02 01-03 01-04 02-03 02-04 03-04 Tl-T2 T1-T2 Tl-T2 Tl-T2
(across 01) (across 0 2 ) (across 0 3 ) (across 0 4 )
Angles
T1-01-T2 T1-02-T2 T1-03-T2 T1-04-T2 a
Tetrahedron 1
Tetrahedron 2
1.626 (7) 1.622 (7) 1.616 17) 1.612 (7) Av 1.619 (4)*
1.738 (7) 1.719 (7) 1.737 (8) 1.723 (7) Av 1.729 (4)b
2.681 (10) 2,672 (10) 2.585 (IO) 2.608 (10) 2.634(10) 2.681 (10) Av 2.643 (4)b
2.862 (10) 2.828 (10) 2.794 (10) 2.817 (10) 2.768 (10) 2.875 (10) Av 2.824(4)b
3.078 (3) 3.154 (3) 3.112 (4) 3.196 (4) Av 3.135 (2)b Tetrahedron 1
01-T-02 01-T-03 01-T-04 02-T-03 02-T-04 03-1’44
(a)
Tetrahedron 2
111.3 (4) 111.o (4) 105.9 (4) 107.3 (4) 109.1 (4) 112.3 (4) Av 109.5 (2)b
111.7 (4) 108.9 (4) 107 6 (4) 109.2 (3) 107.1 (4) 112.3 (4) 109.5 (2)b I
Av
132.4 (4) 141.5 (5) 136.3 (4) 146.8 ( 5 ) Av 139.2(2)b
Estimated standard deviations are in parentheses.
r
dard deviation computed using u = l / n
n
b
Stan-
71/2
li-1 1 biz
,
Discussion The rule forbidding adjacent A1 tetrahedra in zeolites requires perfect ordering of Si and AI ions in a Si/ A1 = 1zeolite X structure. Arguments favoring retention of the initial ordering scheme in X zeolites with Si:A1 ratios up to 1.4 have been presented by Dempsey, l 4 and indirect experimental evidence supporting this has been founds5 The average T-0 distances found for this hydrated NaX crystal indicate that the Si and AI ions are ordered, to a considerable extent. Although the average T1-0 distance of 1.619 8 indicates as much as 7% AI in the T1 site, this is believed to be an artifact, produced by crystal imperfections, such as out-of-step ordered domains, rather than evidence for substitutional disorder. The average T-0 distances found are taken to indicate that our crystal is near perfect, and that a perfect crystal would yield average T-0 distances consistent with a pure Si site (Tl) and a site containing both Si and AI ions (T2). This interpretation is consistent with
Dempsey’s ordered model,14although it is certainly riot as detailed (his ordered zeolite X model for Si/Al = 1.18 contains 24 unique Si and A1 atom sites per unit cell). (10) Sites I lie at centers of hexagonal prisms. Sites I1 lie on sixring faces of sodalite units, on the large caviby side. Sites I’ and 11’ lie on the other sides of the respective six-rings of the unprimed sites, within the sodalite cages. With the exception of the I sites, the sites are not defined by unique coordinates. (11) The framework oxygen atoms are numbered as follows: 01 is the bridging oxygen atom of the double six-membered ring (hexagonal prism), 0 2 is the oxygen atom that is in both the hexagonal prism six-ring and the supercage six-ring, 0 3 is the second oxygen atom of the hexagonal prism six-ring, and 0 4 is the second oxygen atom of the supercage six-ring (a threefold symmetry axis passing through the six-rings generates the remaining four oxygen atoms of each six-ring). (12) R = (ZIIFol - l F o l ~ ) / z i F o l . (13) W. R. Busing, K. 0. Martin, and H. A. Levy, ORFFE, Oak Ridge National Laboratory, Oak Ridge, Tenn., 1964. (14) E. Dempsey in “Molecular Sieves,” Society of Chemical Industry, London, 1968, p 293.
The Journal of Physical Chemistry, Vol. 74, No. 14, 1970
2762
DAVIDH. OLSON ~~
Table V : Interatomic Distances and Angles Involving Nonframework Atoms in and about the Small Pore System Distances,a A 2.623 (7) 0W2-OW2’c 3.618 (6) OW2-0W2’f 2.35 (2)“ OW2-0W2’0 3.09 (2)b Na3A-02 2.51 (7)b Na3A-04 2.65 (5)b Na3B-02 3.19 (5)b Na3B-04 2.64 ( 8 ) b OW3-02 2.96 (5) OW3-04 3.14 (6) Na3B-OW4 3.38 (4)
Nal-03 Nal-02 Na2-03 Na2-02 Na2-OW2 OW1-03 OW1-02 OW1-OW2 OW2-02 OW2’”02 OW2’“02 03-Ni~l-03’~ 03-Nal-03’h 03-Na2-03’c 03-Na2-OW2 03-0W1-03’c
Table VII: Nonframework Atom Distribution in Hydrated NaX Site
2.53 (8) 2.75 (8) 2.93 (8) 2.29 (3)* 3.00 (3)b 2.40 (3)b 3.06 (3)b 2 . 5 (2)b 3.2 (2) 2 . 1 (2)b
Angles,a deg 87.7 (2) 03-OW1-OW2 92.3 (2) 02-OW2-02‘ 101.6 (2)b 02-iYa3A-02’ 98.0 (2)b 02-Na3B-02’ 87.0 (2)b 02-OW3-02’
99.0 (4)b 78.4 (8) 114.0 (l)b 107.0 (l)b 99 0 (2)b I
a Unless noted to the contrary, the value in parentheses is the estimated standard deviation. Some parameters not refined by least squares, error estimated. Related to unprimed by yzx. d Related to unprimed by zxy. e Related to unprimed by ‘ / r x, - y, z. f Related to unprimed by z, I/, - x, 1/4 y. Related to unprimed by l/1 - y, z, 1 / 4 - x. h Related to unprimed by F y .
-
-
Table VI : Possible Interatomic Distances Nonframework Atoms in the Supercagea OW4-Na3B 0W4-0W6’c 0W4-0 W8 OW4-OW9 OW4-OW5 OW4-OW8“ OW5-OW7’ OW5-OW7’d OW5-OW10 0W5-0W8’E OW5-OW6’f OW5-0 1‘ 0 0W6-0 W7 OW6-OW7” OW6-OW10” OW6-04‘i OW6-OW7’’
2 . 1 (2)b 2.7 (2)‘ 2 . 8 (2)b 2.9 (2)b 3 . 1 (2)a 3.1 (2)’ 2.19 (7) 2.84 (7) 2.94 (9) 3.00 (9) 3.13 (8) 3.14 (5) 2.01 (8) 2.35 ( 8 ) 2.50 (9) 2.93 (5) 3.12 (8)
(A) Involving 2.60 (5) 2.65 (5) 2.97 (11) 2.99 (9) 3.15 (10) 3.14 (6) 2.40 (13) 2.49 (IO) 2.39 (10) 2.91 (7) 2.93 (10) 2.99 (13) 2.82 (10) 3.14 (19) 2.75 (8) 2.85 (15) 3 14(8)
0W7-01’2 OW7-04’i OW7-OW9’j OW7-OW8’f OW7-OWlO’” OW7-OW7‘k OW8-OW9’n OW8-OW10 OW8-OWlO’” OW8-01” OW&OWlO’” OW8-OW9” OW9-01’P OW9-OW9’g OW10-01’1, OW10-OW10” 0W10-04’a
* Because all of these nonframework species have fractional occupancy factors (0.10-0.31) the distances listed represent possible, not definite, interactions in the range 2.00-3.20 A. Unless otherwise stated, the value in parentheses is the estimated standard deviation. b Some parameters not refined by least squares, error estimated. The following are transformations applied to the unprimed atom. yzx. ‘/z - x, l/4 y, e 3 / r - y, 3/4 - z, 2. f zxy. 0 1/4 2,‘ / 4 - y, z. -‘/r 2. ’/z - 2, -‘/z y, ‘ / 4 2. ‘/z - y, - 1 / 4 z, 1 / 4 2. i Y, z, 1 / 4 .c. -=/4 z, ‘/z 2, 1 / 4 Y. f , ‘ / r Y1 ’/r z. -‘/4 Y, 1/z 2, ’ / 4 2. z, 3 / 4 2,3/4 - Y. y, I/, 2, 2. -2 3/4 - 2, a -‘/4 y, 1 / 4 2, ’/z 5. p ‘/4 ‘/z - 2, 1 / r 2, -‘/4 y. z, ‘/4 - 2,1/4 - Y. 8 / 4 - y, z.
+
-
+
+ + + -+ ++ + + +
-
+
-
+
+
+
-
The Journal of Physical Chemistry, Vol. 74, No. 14, 1070
+ +
+
I I’ 11‘ I1 Additional supercage sites
B and S
This study
16Na+ 0 0 32Na‘ None located
9Na+ ( N a l ) 8Na+ 12H20 (Na2 O w l ) 26Hz0 (OW2) 24Naf 8 H ~ 0(Na3 OW3) 127H20 (and Na) (OW4-10)
+ +
+ +
The nonframework atom distribution found in this study is strikingly different from that reported by Broussard and Shoemaker (B and S,l) (see Table VII); our results are in better agreement with the general comments made by B a d 6 following reevaluation of Broussard and Shoemaker’s data. Baur found that a model with partial occupancies of Na+ ion in I, 1’,and 11, and oxygen (water molecule) in 11’,gave a better fit than the B and S model. We find that the N a + ions in the small pore system are distributed between sites I and 1’,with site occupancies of 9 and 8 Na+ ions per unit cell, respectively. The total number of Na+ ions in those two sites, 17.0 (14), is within experimental error of 16, the number normally assumed to be within the small pore system of synthetic X and Y zeolites. This value agrees with Sherry’s ionexchange work, which clearly indicates that there are 16 equivalents of Na+ ion per unit cell in the small pore system of both X and Y . ~ e o l i t e s . ~ ~ - ~ ~ Consideration of the sites 1 and I’ occupancies given in Table VI1 suggests the following idealized occupancies: 8 Na+ ions in both I and I’ sites and 8 water molecules also in I’ sites. These values are within three standard deviations of the values of Table VII, and only the differences in the values for the water approach this limit. The reasoning behind these idealized occupancies is based upon the following assumptions. (a) The chemical composition and center of symmetry requirement leads t o the conclusion that in each unit cell there are 12 hexagonal prisms with six aluminum ions (Ale prisms), and four prisms with four aluminum ions (A&prisms), Only Ale prisms will have Na+ ions in their I or I’ sites. (b) Although the energy differences may be small, the Na+ ion prefers site I’ t o site I because of the more favorable coordination to oxygens (see Table V). (c) Repulsive forces between Na+ ions limit site I’ occupancy to one per sodalite cage. These three assumptions and the idealized occupancies suggest the (15) W. H. Baur, Amer. Mineral., 49, 697 (1964). (16) H. S.Sherry, J . Phys. Chem., 70, 1158 (1966)). (17) H. 5. Shwry, ibid., 72, 4086 (1968). (18) H. S. Sherry, J . Colloid Interface Sci., 28, 288 (1968).
2763
THECRYSTAL STRUCTURE OF THE ZEOLITEHYDRATED NaX
Figure 1. Stereoscopic view of the sodalite cage showing a hypothetical arrangement of nonframework atoms consistent with the observed occupancy factors. The smallest circles are Si and A1 sites. Nal’s are in I sites, Na2’s and Wl’s are in I’ sites, R2’s are near 11’ sites and Na3’s and W3’s are in I1 sites.
following hypothetical model for the distribution of the Naf ions and water molecules about the 16 hexagonal prisms: 8 A16 prisms having Na+ ions in site I, 4 A16 prisms having N a + ions in both I’ sites, and the 4 A14 prisms having water molecules in both I’ sites. A sodium ion in the I site is octahedrally coordinated by six 0 3 oxygen ions a t a distance of 2.62 A. Each N a + ion in site I’ is coordinated to three framework oxygens (D(Na2-03) = 2.35 A) and three water oxygens located near the 11’ sites (D(Na2-OW2) = 2.5 A) (see Figure 1 and the data of Table V). The water molecule in site I’ can hydrogen bond to any of the three 0 3 framework oxygens (D(OW1-03) = 2.65 A) and to two of the three water molecules in site 11’. The water molecules in this latter site are disordered about the threefold. axes and partially occupy 12 equivalent positions of each sodalite cage (although, strictly speaking, they do not occupy 11’ sites, for the sake of simplicity these are referred to as site 11’ water molecules). In addition to their coordination to the Naf ion in site If, the site 11’ water molecules can hydrogen bond to each other (D(OW2-OW29 = 2.5-2.9), to a framework oxygen (D(OW2-02) = 2.96 A), and/or to the water molecule in site 1’, as mentioned above. The disordered OW2’s have distances of 2.10, 2.33, and 2.64 A from O w l . Since the first two distances are unreasonably short, the site occupancies must be such that these distances do not actually occur. As can be seen from the hypothetical model of Figure 1, only two OW2’s per sodalite cage hydrogen bond to O w l . Clearly the number of Na+ ions in site I’ determines the number of water molecules in 11’. Both overcrowding and limited hydrogen bonding possibilities may be important factors in controlling the number of water molecules in 1’. I n the I1 sites we find, on a unit cell basis, 24 N a + ions and 8 water molecules with Na3A-02, Na3B-02, and OW3-02 distances (threefold) of 2.29, 2.40, and 2.5 8, respectively. The number of Na+ ions in I1 sites agrees exactly with the number of site I1 six-rings that contain three A1 ions (assuming there are no sixrings with one or zero AI ions). This is taken as direct
evidence for the influence of the aluminum content of a six-ring on the siting of supercage cations and suggests the rule that three AI ions per six-ring (Alasix-rings) are required to make partial dehydration and ion siting energetically favorable. Table VII119-22 shows the Table VIII: Effect of ,41 Content on the Siting of Cations in the Six-Rings of Hydrated Faujasites No.
NaX BaX SrX Lax Faujasite (Ca, Mg, Na) Ca faujasite Ce faujasite La faujasite a
Av. no. of AI (&ring)
of 413 6-ring
2.75 2.66 2.66 2.66 1.83 1.83 1.83 1.83
No. of cations in site I1 (u.c.)=
Ref
24 21 21 21 0
24 22
This work 16
15
17
17 0
18 12
0 0 0
0 0 0
19 18 19
(U.C.)~
Unit cell.
effect of A1 content on ion siting in hydrated faujasitetype zeolites. In the zeolites that contain Ala six-rings, ion siting occurs, whereas in those with no Ala six-rings, there is no supercage ion siting. Furthermore, the rule appears to apply to mono-, di-, and trivalent cation systems. It is also consistent with Sherry’s ion-exchange results, which he has interpreted in terms of partial ion siting of cations in the supercages of X zeolites and no corresponding ion siting in Y It seems probable that the rule will apply to all zeolites that are sufficiently open to provide a choice between (19) D. H. Olson, unpublished research. (20) D. H. Olson and H. S. Sherry, J . Phus. Chem., 7 2 , 4095 (1968). (21) D. H. Olson, G. T. Kokotailo, and J. F. Charnell, J . Colloid Interface Sci., 2 8 , 305 (1968).
(22) (a) J. V. Smith and J. M. Bennett, Abstract No. LL5, “American Crystallographic Association Meeting,” Buffalo, Aug 1968; (b) J. M. Bennett and J . V. Smith, Mat. Res. Bzdl., 3, 633 (1968).
The Journal of Physical Chemistry, Vol. 74,No. 1.4, 1970
2764
DAVID H. OLSON
Figure 2. Stereoscopic view of nonframework atoms in the supercage. Possible Na+ ion sites are labeled W7 and W8. The supercage extends toward the viewer from Na3 which is in site 11.
siting of partially dehydrated ions and nonlocalization of fully hydrated cations. Zeolite A is such a structure: in Linde 4A and 5A, where all six-rings have three A1 ions, B and S1 have found that all six-rings contain sited cations. From the above rule, we would predict that in a ZK-4 material (high-silica A-type zeolite) with Si/ A1 2 2, there will be no ion siting in the hydrated zeolite. There are no experimental data to confirm or refute this. In the case of sodium ions, the equilibrium appears to be overwhelmingly in favor of ion siting in A13six-rings. This implies that for all hydrated sodium faujasites with Si/Al between 1 and 2, the number of Na+ ions per unit cell not sited in six-rings has the constant value 48, and that for materials with Si/Al greater than 2, this numbel, is simply 16 less than the total Na+ ion content. Although seven scattering sites (in addition to the occupied sites 11) were found in the supercage, their low occupancy factors, 0.10-0.31, imply that a t 25" the water-Naf ion mixture is not highly ordered. This is not an unexpected result. Baur points out several experimental results that indicate that the mobility of the cation-water mixture in the supercages of faujasites approaches that in salt solutions.15 This is consistent which reveals high with nmr data of Lechert, et aZ.,23r24 mobility of Na ions and HzO molecules a t full water loading. Lechert, et aZ., find further that the mobility of both species decreases with decreasing water content and that there is a sharp decrease in Na+ ion mobility a t about one-half loading, a point at which the water-
The Journal of Phusical Chemistry, Vol. 74, No. 14, 1970
to-mobile Naf ion ratio drops below four. Although the situation has not been studied exhaustively, no completely satisfactory combination of occupancies of the seven scattering sites was found. Table VI gives the possible interactions of these scattering sites in the interatomic distance range 2.0-3.2 +&. OW4 is 2.1(2) +& from the Na+ ion in site 11, and its occupancy factor implies that, on a time-average basis, about 50% of these N a + ions coordinate to supercage water molecules. Equally favorable water structures, that do not involve bonding to Na+ ions in site 11,must also exist. It should be noted that OW7 has relatively shortrange interactions with framework oxygens 01 and 04, 2.60 and 2.65 A, respectively, and three possible shortrange interactions with species OW5 and OW6 (twice). This suggest that OW7 is a partially dehydrated Naf ion. Similarly, OW8 has three short-range interactions with species OW9, OW10, and OWlO', suggesting that OW8 may also be an Na+ ion; however, this is less certain than the assignment of OW7 as an N a + ion. The location of these species in the supercage is shown in Figure 2. Species OW5, OW6, OW9, and OW10 are probably water molecules.
Aclcnowledgments. The many helpful discussions with E. Dempsey and the careful experimental assistance of N. H. Goeke are gratefully acknowledged. (23) H. Lechert, W. Gunsser, and A. Knappwost, Ber. Bunsenges. Phys. Chem., 72, 84 (1968). (24) A. Knappwost, H. Lechert, and W. Gunsser, Z . Phys. Chem. (Frankfurt a m M a i n ) , 58, 278 (1968).
