The Journal of Physical Chemistty, Vol. 83, NQ. 67, 1979 2263
Crystal Structures of Dehydrated H Chabazite
(8) K. Sq@, H. Imamura, and S. Ikeda, J. phys. Chem., 81, 1762 (1977). (9) K. Soga, H. Imamura, and S . Ikeda, Nlppon Kagaku Kaishi, 1299 (1977). (10) K. Soga, H. Imamura, and S. Ikeda, Nippon Kagaku Kaishi, 423 (1978). (11) K. Soga, H. Imamura, and S . Ikeda, J . Catal., in press.
F. A. Kuijpers, Res. Rep. Suppl., 2 (1973). T. Dambrauskas and H. Cornish, Ind. Hyg. J . , 191 (1962). For example, K. Klshi and M. W. Roberts, Surface Sci.,62,252 (1977). K. M. Minacher, Y. S.Khodakov, and V. S. Nokhshunov, J . Catal., 49, 207 (1977). (16) K. Tanaka, H. Nihira, and A. Ozaki, J. phys. Chem., 74, 4910 (1970).
(12) (13) (14) (15)
Crystal Structures of Dehydrated H Chabazite Pretreated at 320 'C, and at 600 "C after Steaming W. J. Morfier,* Katholleke Universiteit Leuven, Centrum voor oppervlaktescheikun~een Collddale Scheikunde, De Croylaan 42, 8-3030Leuven (Heverlee), Belgium
G. S. D. Klng, and L. Sengler Katholieke Unlversiteif Leuven, Laboratorium voor Kristallografie, Redingenstraat 76 6, 8-3000Leuven, Belglum (Received January 30, 7979) Publication costs assisted by the Katholieke Universiteit Leuven
Two crystal structures of dehydrated NH4-exchangedchabazite crystals were studied by use of single-crystal X-ray diffraction methods after dehydration at 320 "C, and at 600 "C following a steaming period. No significant differences are observed between the two structures; there is no preference of the protons for a single oxygen species nor any indication of A1 removal from the framework after steaming. A strong correlation between T-0-T bond angles and T-0 distances in adjacent tetrahedra shows an identical slope with that of dehydrated H mordenite. The framework oxygens, all pointing either to the hexagonal prisms or to the centers of the framework eight-rings, prohibit the free vibration of OH bonds in the large cavity. This explains the 30 cm-' decrease of the HF bands as compared to the value predicted from the framework composition.
Introduction A study of the influence of framework aluminum content and cation type on the vibration frequency of the surface silanol groups vibrating in the large pores of dehydrated hydrogen zeolites has shown an unusual behavior for H chabazite.l All the OH stretching frequencies of the H forms of the zeolites A, L, X, Y, and ZK4, clinoptilolite, stilbite, and mordenite varied linearly with zeolite electronegativity. The high-frequency band of H chabazite deviated by as much as 30 cm-l from the predicted value. It has been suggested that structural effects were causing this decrease. Beyer et a1.2 have investigated the thermal stability of NH4-exchanged chabazite. It was found that the dehydration, deammoniation, and dehydroxylation steps were to a large extent separable, therefore allowing the preparation of H chabazite without dehydroxylation. On the other hand, materials exhibiting discrete physicochemical properties were obtained when deammoniation was performed under steaming conditions (deep bed treatment) or under v a ~ u u m . The ~ removal of framework A1 tetrahedra might cause this effect. Both these effects led us to the study of vacuum dehydrated NH4 chabazite at 300 "C (dHChab) and NH4 chabazite dehydrated a t 600 "C (dHChabDB) after a preliminary steaming a t the same temperature. Experimental Section and Results Crystals from Dunabogdany, Hungary were crushed and exchanged by a 24-h treatment with a 1M NH4C1solution a t 100 "C. This treatment was repeated 5 times. A bulk chemical analysis of the samples showed a 90% exchange, the remaining cations being Ca. The unit cell composition is then given as
("4)3.24Cao.lsA13.6OSi8.40024'1P.7H20
Two crystals approximately 0.2 X 0.2 X 0.2 mm for dHChab and 0.2 X 0.1 X 0.1 mm for dHChabDB were mounted in glass capillaries and dehydrated as follows: dHChab: 3 h at 100 "C, torr; 12 h at 275 "C and 2 h at 320 "C at which temperature the capillary was sealed; dHChabDB: (DB = deep bed) after evacuation, 18 torr of HzO was introduced in the system. The temperature was subsequently raised to 600 "C over a 1-h period, kept) at that temperature 1h, and evacuated under torr for 1 h before the capillary was sealed a t the same temperature. Using graphite-monochromatized Mo Ka radiation (A = 0.71069 A), we measured 4823 reflections for dHChab (up to sin O/A = 0.70) and 2929 reflections for dHChabDB (up to sin O/A = 0.60) on a Syntex P21 single crystal diffractometer in the omega scan mode, yielding, respectively, 818 and 493 independent reflections with I > 30. in R3m after averaging. We assumed the highest symmetry (R3rn)consistent with the diffraction symmetry. This was subsequently justified by the refinement. The unit cell parameters were obtained by the least-squares refinement of 24 intense diffractions between 25 and 30" (28), yielding the values for dHChab of a = b = c = 9.395 (3) A, (Y = 0 = y = 93.15 (3)", and for dHChabDB a = b = c = 9.396 (1)A, (Y = @ = y = 93.33 (1)". Data reduction was performed with the XRAY -76 ~ y s t e m .Intensity ~ data were corrected for Lorentz and polarization effects but no absorption corrections (FM, = 5.66 cm-') were applied. For the least-squares refinement the program NUCLS (a modified version of ORFLS by Busing, Martin, and Levy6) was used. Atomic scattering factors for Ca2+,0-,and for the T atoms 0.3A12++ 0.7Si2+6were used. Anomalous scattering corrections' were applied for
0022-3054/79/2083-2263$01.00100 1979 American Chemical Society
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The Journal of Physical Chemistty, Vol. 83, No. 17, 1979
W. J. Mortier, G. S. D. King, and L. Sengier
TABLE I[ : Positional Parameters and Population Parameters atom T
type 12i
O(1)
6f
Q(2)
6f
O(3)
6h
Q(4)
6h
Ca(I)
la
structure
population
dHChab dHChabDB dHChab dHChabDB dHChab dHChabDB dHChab dHChabDB dHChab dHChabDB dHChab dHChabDB
12 12 6.29(3) 6.30(4) 6.36( 3) 6.37 ( 4 ) 6.30( 3) 6.20(4) 6.32(3) 6.33(4) 0.250(4) 0.182(6)
TABLE 11: Anisotropic Thermal Parametersa ( X atom T O(1)
Q(2) O(3) O(4)
Ca(1) a
structure dHChab dHChabDB dHChab dHChabDB dHChab dHChabDB dHChab dHChabDB dHChab dHChabDB dHChab dHChabDB
In the form of exp -
j=
13
011
40514) 420(8 j 900(15) 972(26) 1148(17) 1161(27) 665( 1 3 ) 67 6(23) 1052( 1 9 ) 1080(28) 364(23) 418(64)
x la
xlb
0.10024(4) 0.10099(7) 0.2732(3) 0.2707(3) 0.1 277( 3 ) 0.1296( 3) 0.2502(3) 0.2508(3) 0.0117(3) 0.0141(4) 0 0
XIC
0.87198(4) 0.87280(6) 0 0
0.32940(4) 0.32959(6) -0.2732 -0.2707 -0.1277 -0.1296 0.2502 0.2508 0.0117 0.0141 0
'12 '/2
0.877 9(2) 0.8797( 2) 0.2808( 2) 0.2848(3) 0 0
0
lo5) 033
0 2 2
28214) 317(8j
P12
P11
520(5) 546(9) 859(21) 849(35) 472(17) 632( 33) 1194(25) 1207(39) 1235(26) 1324(43)
PI1
PI 1
PI1
P11
011 P11
PI1 PI1 011 011 PI1
013
21(3) 26(5) - 215(16) - 187 (24) - 126(18) -139(27) 90(14) 17(23) 486(19) 445(28) 105(17) 96(47 1
0 2 3
34(3) 33(6) -121(11) - 97( 1 7 ) 19(10) 49(17) 57(11) 53(17) 308( 1 6 ) 250(22)
-11(3) -13(5)
PI2
PlZ
PI2
012
--
p13
p13 6 1 3
p13 p13 0 1 3 P13
p13
;r: i=I 73jjhihj,
all atoms. The starting positional framework parameters were those of dehydrated Ca chabazite.s The Ca ions were located by difference Fourier methods. No other electron density was found except in site I, at the center of the hexagonal prism. Using anisotropic full matrix refinement, we obtained reliability factors for dHChab and dHChabDB of R = 0.030 and 0.034 and W R = 0.031 and 0.030, respectively, where R = CIIF,I - IFcll/CIFol and wR = [(CwllFoi- IFc112)/CwIFo12]1/2. In view of possible A102removal from the framework, the population parameters of the oxygens were refined, the occupancy factors of the T atoms being left constant as a reference. Final residuals for dHChab of R = 0.027 and wR = 0.027 and for dHChabDB of R = 0.033 and wR = 0.027 were obtained. The highest residual electron density shown in the difference Fouriers was in the center of the eight-ring (0 ' 1 2 with 0.61 e/A3 for dHChab and 0.77 e/A3 for dHChabDB. The remaining residual electron density peaks were below 0.29 e/A3 for both structures. The positional parameters and the population parameters are given in Table I, the anisotropic thermal parameters in Table 11, and the interatomic distances and bond angles as calculated by the program ORFFE' in Table 111. A list of observed and calculated structure factors is available as supplementary material. See paragraph at end of text regarding supplementary material. A statistical comparison between both structures was made as described earlier.1° The two structures differ significantly. Discussion No ordering of the Si and A1 tetrahedra was observed. The average T-0 distance of 1.64 A is close to that expected on the basis of the chemical composition, i.e., 1.65 A (Si-0 = 1.61 A, A1-0 = 1.74 A).11 The number of residual cations was identical with that given by the chemical analysis for the dHChabDB sample, but exceeded this amount for the dHChab sample. This is probably due to a difference in exchange level between the two crystals, the larger crystal (dHChab) containing more residual
TABLE 111: Interatomic Distances ( A ) and Bond Angles (deg) dHChab dHChabDB
T-O(1) T-O( 2) T-O( 3) T-O(4) mean
1.659(2) 1.609(1) 1.629(1) 1.664(1) 1.640
1.652(2) 1.610(1) 1.628(1) 1.660(1) 1.638
O( 1)-T-O( 2)
108.86( 1 2 ) 110.68(8) 106.95(15) 111,20(1 4 ) 111.85(14) 107.22(1 2 ) 109.46
109.07(15) 110.73(12) 107.25(20) 110.88(19) 111.63(19) 107.21(17) 109.46
2.659(2) 2.7 04( 3) 2.670(4) 2.672(2) 2.7 11(3) 2.651(2)
2.657(1) 2.698( 3) 2.667( 5 ) 2.666(2) 2.7 05( 4) 2.647( 4)
137.05(23) 166.79(25) 147.45(20) 138.86(1 6 )
138.99(30) 164.91(33) 147.36(28) 139.85(25)
2.631(2)
2.657(1)
108.83 110.64 109.70 108.67
109.02 110.53 109.61 108.70
0(1)-T-O( 3) 0(1)-T-0(4) 0(2)-T-0( 3) O( 2)-T-O( 4) O( 3)-T-0( 4) mean 0(1)-0(2)
O( 1 )-O( 3 1 O( 1)-O( 4 1
0(2)-0(3) O(2 )-0(4) 0(3)-0(4) T-O( 1 )-T T-O(2)-T T-O( 3)-T T-O( 4)-T Ca(I)-60( 4)
W) O(2) O(3) O(4)
cations. All of these cations were located in the hexagonal prism. The Ca(I)-0(4) distance for dHChab is slightly smaller than for dHChabDB and is therefore in agreement with the difference in occupancies of site I. Since only 18 or 25% of the hexagonal prisms were occupied, the observed Ca(1)-O(4) distance will be intermediate between the Ca-0 distance for hexacoordinated Ca (2.36 A)ll and that for the fully relaxed structure. The exchangeable
Crystal Structures of Dehydrated H Chabazite
cations are preferentially located at site I, where a fully octahedral coordination is provided from the framework. With more than one Ca ion per hexagonal prism, the site I occupancy is gradually replaced by an occupancy of both sides of the hexagonal prism (site 11) as was observed for dehydrated Ca-exchanged chabazite.12 The refinement of the occupancy factors for the oxygens did not reveal preferential dehydroxylation after the steaming treatment except in the case of the O(3) oxygens. Indeed, since the occupancy factors for oxygens OW, 0(2), and 0(4), are equal, within the standard deviation, the occupancy difference for O(3) is significant, however small. This difference accounts for only one tenth of an oxygen, i.e., only a minor fraction of the 3.2 protons present in the structure. If, during the pretreatment, AlOOH is removed from the lattice there is either no preference for any type of OH or 0 or there is no breakdown of the lattice at all. No other information can be obtained from the refinement since a t least one reference atom must be taken when refining on occupancy factors. If A1 were removed from the framework it would be highly probable to find it as extra cationic electron density. Since no other extra framework electron density was found, other than a t site I, and since also the occupancy of site I for the steamed sample is lower than for the unsteamed sample, it is very doubtful whether any structural breakdown has taken place. Indeed, for samples prepared under comparable conditions, an IR study by Jacobs et aL3 revealed no significant difference in the band areas for the OH stretching frequencies. It is remarkable that the occupancy factors for the oxygens are slightly greater than unity. This would indicate that there are more electrons on the framework oxygens and fewer on the T atoms than was assumed in the choice of the atomic scattering factors (11e- on T; 9 e- on 0). Care should be taken, however, in interpreting the ionicity of the T-0 bond from these data since there is a strong correlation between occupancy factors and thermal parameters. Since the structure is almost free of cations, the influence of the T-0-T and 0-T-0 bond angles on the T-0 distances can easily be investigated. These influences are explained by covalent bonding effects.13 The variation of the deviation of the average T-0 distances, A(T-0) with sec (T-0-T), is given in Figure l a , together with the relation observed earlier for dehydrated m ~ r d e n i t e . ' ~ Since oxygens O(l), 0 ( 2 ) , and O(3) were free of the influence of the Ca ions in site I, they are supposed to show only the influence of the bond angles on the T-0 distances. There is a striking similarity in the slopes of the curves for dehydrated chabazite and dehydrated mordenite. The position of the correlation line depends on the structure type and is a function of the average T-0-T angles. Figure l a illustrates the lengthening of the T-0(4) bond because of the coordination to site I. All other T-0 distances follow the same relation and there is no other indication of a preferential location of the protons at one of the oxygens. Considering a bond lengthening of 0.09 A per positive charge for a Si-0 bond,15 we found that the location of all protons at one oxygen type would give rise to a deviation on the order of 0.05 A. There must therefore be a fair equilibrium distribution of the protons between all oxygens. The T-0 distances are also influenced by the 0-T-0 bond angles in the tetrahedron. The variation of A(T-0) with the average of the three adjacent tetrahedral bond angles ( ( 0-T-0 ) 3) of the considered T-0 distance is given in Figure lb. It is observed that all T-0 distances follow
The Journal of Physical Chemistry, Vol. 83, No. 17, 1979 2261 z1.0 z1.1 F I
p t: 0
2
G1.2
i1.3
-.03 -.02 -.01
0
1\9 .01 .02
.03
A (T-0) Figure 1. (a) Relation between T-0-T angles and perturbation in T - 0 distance for dehydrated rnordenite (0)and dehydrated chabazite (8). The numbers are the labels of the oxygens. (b) Relation between ( O-T-O)3 and perturbation in T-0 distance for dehydrated chabazite.
the same relations and that there is almost no deviation for T-O(4). The correlation between the T-0-T, (OT-0)3, and the T-0 distances was analyzed before for dehydrated mordenite by multiple regression methods.16 Because the effects were too closely correlated here, it is useless to compare the values. The slope of A(T-0) vs. sec (T-0-T) for dehydrated mordenite was used to determine the number of protons at the different framework oxygen types for dehydrated faujasite.16 Although there was perfect agreement with the IR spectroscopic result, there could be some deviation for a structure with a different A1 content. Indeed the molar ratio Al/(Al+ Si) is 0.29 for faujasite and 0.16 for mordenite. The present results for a molar fraction of 0.27 clearly indicate that a carry-over was allowed. As a comparison, the relation A(T-0) = m sec (T-0-T) b for dehydrated mordenite and dehydrated chabazite (both structures with 0(1), 0(2), and O(3)) yields dH mordenite d H chabazite m -0.13249 -0.14645 b -0.15238 -0.18186
+
The slope of this curve should provide a reliable method for the estimation of bond lengthening effects for cation or proton location within the range of Al/(Al + Si) of 0.16-0.27. Although structurally there is not much difference between dHChab and dHChabDB, Jacobs and Beyer3 have found significant differences in the IR spectra of these compounds. They showed that two different OH stretching bands (3635 and 3545 cm-l) at pretreatment temperatures comparable to the dHChab sample were shifted after steaming (conditions comparable to the dHChabDB sample) to a major band at 3660 cm-' with a small shoulder at 3730 cm-'. Steaming at 700 "C resulted in a single OH stretching band at 3740 cm-l. At the same time, the framework vibration frequencies all changed equally for both pretreatments. Among the factors influencing the OH stretching frequency are the T-8-T bonding angles, the chemical composition (conveniently quantified by t h e electronegativity1J7),and the proton environment (small or large cavities). It has clearly been shown for H-faujasite-type18zeolites that the protons vibrating in the large cavity have a considerably higher frequency than protons
2268
The Journal of Physical Chemistry, Vol. 83, No. 17, 7979
W, J. Mortier, G. S. D. King, and L. Sengier
Q Figure 2. Stereoplot of framework atoms and cation site for dHChab at 30% probability.
vibrating in the small cavities. Moreover, a reduction of the bond angle results in a decrease of the T-0 bond overlap population and a polarization of the bond.13 Since the oxygens are more negatively charged, the OH frequency increases. The changes in the T-0-T bonding angles from the dMCab to the dHChabDB structure are +1.94, -1.88, -0.09, and +Q.99’ for O(1) to 0(4), respectively. The difference of about lo for O(4) is consistent with the higher occupancy of CaU) for the dHChab sample (smaller T-0-T angle). The change for the O(3) oxygens is nonexistent while for O(l) and O(2) the difference of about 2’ is certainly significant. Considering that the TO, tetrahedra are more or less rigid entities and that framework adaptations are easily made by rotation of the tetrahedra, we can conclude from Figure 2 that the 2’ change can be the result of the difference in cation contents at site I. Indeed, the longer Ca(1)-O(4) distance should result in a increase of the T-0(4)-T angle. With a constant T-O(3)-T angle the TO4 tetrahedron can be rotated by increasing the T-O( 1)-T angle and decreasing the T-O(2)-T angle as is observed. Therefore, the change in the angles cannot be a result of the different pretreatments but is merely caused by differences in the cation contents of the two crystals. The increase of the frequencies for the OH stretching and framework vibrations are certainly in agreement with a deal~minationl~ but this was considered to be highly improbable. Since no structural reason is observed for the difference in IR spectrum, it must be accepted that the steaming of the single crystals is different from the “deep bed” steaming in an oven in open air. It was mentioned in the Introduction that the OH stretching frequencies of H chabazite behave differently from other zeolite types. The reason for this difference can be found in the cavities where the protons are vibrating. For chabazite, it can be observed from Figure 2 that all oxygens point toward a “small cavity”, i.e., the hexagonal prism (O(4)) or to the center of the eight-ring (O(l), 0(2), and O(3)). There is no OH group where the proton is vibrating freely in the large cavity. Since the vibration of protons in the direction of a small cavity results in a considerable reduction of the frequencies, the decrease of 30 cm-l (ref 1) from the predicted value is consistent with this observation. The effect of the eight-ring, however, is smaller than the effect of a six-ring or a cubooctahedron in, e.g., faujasite-type zeolites where the difference between the high-frequency and low-frequency bands is of the order of 100 cm-l. This also implies
that protons, assigned to H F bands of which the frequency can be predicted by the electronegativity, must vibrate freely in the direction of a large cavity or channel larger than the dimension of a framework eight-ring. On the other hand, since the 3660-cm-l band for the “deep bed” steamed powders coincides with the electronegativity relations, structural breakdown (allowing rotations of the TO4 tetrahedra) is highly probable.
Acknowledgment. W.J.M. thanks the Belgisch Nationaal Fonds voor Wetenschappelijk Onderzoek for a research grant as “aangesteld navorser”. The authors acknowledge financial assistance from the Belgian Government (dienst voor programmatie van het Wetenschapsbeleid), from the Fonds voor Kollektief Fundamenteel Onderzoek, and from the research fund of the K. U. Leuven, and discussions with Dr. P. A. Jacobs. Supplementary Material Available: A listing of the observed and calculated structure factors for dHChab and dHChabDB (9 pages). Ordering information is available on any current masthead page.
References and Nates (1) P. A. Jacobs, W. J. Mortier, and J. B. Uytterhoeven, J . Inorg. Nucl. Chem., 40, 1919 (1978). (2) H. K. Beyer, P. A. Jacobs, and J. B. Uytterhoeven, J . Chem. Soc., Faraday Trans, 7 , 1111 (1977). (3) P. A. Jacobs and H. K. Beyer, manuscript in preparation. (4) J. M. Stewart, P. A. Machin, C. W. Diiklnson, H. L. Ammon, H. Heck, and H. Flack, Ed., “The X-ray System”, Technical Report TR-446, University of Maryland, 1976. (5) W. R. Busing, K. 0. Martin, and H. A. Levy, ORFLS, Report ORNL-TM-305, Oak Ridge Natlonal Laboratory, Oak Rldge, Tenn., 1962. (6)C. H. Macgillavry and G. D. Rieck, Ed., “International Tables for X-ray Crystallography”, Vol. 111, The Kynoch Press, Birmingham, 1968, p
202. (7) D. T. Cromer and D. Liberman, J , Chem. Phys., 53, 1891 (1970). (8) W. J. Mortler, J. J. Pluth,and J. V. Smith, Mater. Res. Bull., 12, 97 (1977). (9) W. R. Busing, K. 0. Martin, and H. A. Levy, ORFFE, Report ORNL-TM-306, Oak Ridge National Laboratory, Oak Ridge, Tenn. (10) W. J. Mortier, Acta Crystallogr., Sect. A , 29, 473 (1973). (11) R. D. Shannon and C. T. Prewitt, Acta Ctystalbgr., Sect. 6,25, 925 (1969). (12) W. J. Mortier. J. J. Pluth, and J. V. Smith, Mater. Res. Bull., 12, 97 (1977). (13) J. A. Tossel and G. V. Gibbs, Phys. Chem. Min., 2, 21 (1977). (14) W. J. Mortier, J. J. Fluth, andJ. V. Smith, Mat8r. Res. Bull., 10, 1319 (1975). (15) W. H. Baur, Trans. Am. Crystallog. Ass., 6, 129 (1970). (16) W. J. Mortier, J. J. Pluth, and J. V. Smith, J . Catal., 45, 367 (1978). (17) W. J. Mortier, J . Cafal., 55, 138 (1978). (18) P. A. Jacobs and J. B. Uytterhoeven, J. Chem. Scc., Farahy Trans. 7 , 69, 359 (1973). \
- I
