4980
J . Phys. Chem. 1988, 92, 4980-4987
Fourier Transform Far-Infrared Spectroscopic Study of Cation and Anion Dynamics in M,X-Sodalites, Where M = Li’, Na’, K’, Rb’, Ca2’; X = CI-, Br-, I-, C104-, OHJohn Godber and Geoffrey A. Ozin* Lash Miller Chemical Laboratories, University of Toronto, 80 St. George St., Toronto, Ontario, Canada MSS 1A1 (Received: December 29, 1987)
The extent of coupling of cation and anion vibrations in sodalite is revealed through an examination of the far-infraredspectroscopy of a series of cation- and anion-exchanged isomorphs. Two cation and two anion absorptions in the far-IR are observed, consistent with the predictions of a complete unit cell analysis. While the cation translational frequencies are essentially unperturbed on varying the anion, absorptions of the anion translations are dependent on the cation. The intra-&cage thermal decomposition of perchloratosodalite into chlorosodalite supports the assignment of cation and anion far-IR absorptions.
Introduction The aluminosilicate sodalite has been known since antiquity, occurring naturally as the pigment ultramarine and the gem lapis lazuli.] Sodalite (SOD) is a feldspathoid and is a member of the group of framework silicates that includes zeolites. While not exhibiting most zeolite characteristics, structurally sodalite is closely related to the zeolites LTA and FAU. Indeed sodalite may be regarded as the end member formed from the different ways of connecting cubo-octahedral cages. Joining these cages directly through each of the six-ring faces with one of eight other cuboctahedra generates a space-filling framework (Federov solid) with restricted access to the interior under conditions of standard temperature and pressure (Figure 1A). The structural relationship with zeolites was initially the impetus for investigating the vibrational properties of these systems. The aluminosilicate sodalites contain encapsulated salts and in this regard can be considered similar to the well-known salt-inclusion compounds. These “packaged salts’’ give rise to interesting vibrational features,2 and the observed vibrational modes of the extra-framework ions in SOD are strongly influenced by the symmetry properties of the unit celLZ By comparison, the effect of the crystal potential on cations in LTA and FAU (except when %/A1 = 1.0-1.25) was shownZto be considerably smaller, and it was demonstrated that a local oscillator approximation is sufficient to describe the vibrational properties of the extra-framework cations. Sodalites have attracted considerable attention because of their use in cathode-ray screens and information storage device^.^,^ These applications relate to the fact that halosodalites, following sensitization, exhibit reversible photochromism and cathodochromism, both of which are associated with the generation of F centers located at halide vacancy sites4 In this paper it will be demonstrated that a systematic and extensive variation in the extra-framework anions and cations (see Figure 1B for ion locations) while maintaining the sodalite structure constant allows for a straightforward assignment of the origin of the associated cation and anion translational modes. Clearly the results of such a study have direct relevance to any application of sodalite materials that involves modification of the framework or extra-framework guests for specific functions. ( I ) Jaegar, F. M. Trans. Faraday SOC.1929, 25, 320. (2) Godber, J.; Ozin, G. A. J . Phys. Chem. 1988, 92, 2841. (3) (a) van Doorn, C. Z . ; Shipper, D. J.; Bolwijn, P. T. J . Electrochem. SOC.1972, 119, 85. (b) Williams, E. F.; Hcdgson, W. G.; Brinen, J. S. J. Am. Ceram. SOC.1969, 52, 139. (c) Hassib, A.; Beckman, 0.;Annersten, H. J . Phys. D 1977, IO, 771. (d) Phillips, W. J. Electrochem. SOC.1970, 117, 1557. (e) Takeda, T.; Watanabe, A. J . Electrochem. SOC.1973, 120, 1414. (f) Chang, I. F. J . Electrochem. SOC.1974, 121, 815. (4) (a) McLaughlin, S.D.; Marshall, D. J. Phys. Lett. A 1970, 32, 343. (b) Alig, R. C. J . Phys. Chem. Solids 1974, 35, 53. (c) Alig, R. C.; Flory, A. T. J. Phys. Chem. Solids 1975, 36,695. (d) Tang Kai, A. H.; Calais, J.-L.; Hassib, A. J . Phys. Chem. Solids 1979,40, 803. (e) Hodgson, W. G.; Brinen, J. S.;Williams, E. F. J . Chem. Phys. 1967, 47, 3719. (0 Shidlovsky, I . ; Nowik, I . Solid State Commun. 1976, 18, 155.
0022-3654/88/2092-4980$01.50/0
Results and Discussion Vibrational Spectroscopy of Halosodalites. The mid-infrared absorptions of sodalites and other framework silicates arise solely from vibrations of the f r a m e w ~ r k , ~and , ~ the * ~ ~positions of these bands, while sensitive to changes in the framework with changing extra-framework ions, fall into well-defined “fingerprint” regions ascribed to particular lattice motions. The vibrational frequencies of the halosodalites studied here are listed in Table I along with I also contains the vibrational frequencies literature v a l ~ e s .Table ~ of kaolin, the starting material for the synthesis of most of these samples. Kaolin transformation to sodalite is shown to be complete by the absence of Kaolin bands in the mid-IR and X-ray diffraction (XRD) spectra of the sodalites. A diagnostic of complete reaction is the absence of both hydroxyl group vibrations and the strong vibrational mode at 914 cm-I. Comparison of the Na,Br-SOD sample to an authentic sample6 confirms the acceptability of the synthesis and establishes the spectroscopic signatures of the halosodalites. Detailed examination of the vibrational spectra reveals that the asymmetric (T-0) stretch at about 994 cm-’ is insensitive to variations in the anion, while the symmetric stretches in the 730-670-cm-l region show a progressive downward shift (ca. 20 cm-’ at most) with increasing mass of the anion or cation. The mid-infrared, modes have been d i ~ c u s s e and d ~ ~are ~ ~included ~ here as an invaluable diagnostic for sodalite framework structural integrity as well as an indicator of the substantially decoupled nature of the ion motions from those of the framework. The far-IR spectra of sodium halosodalites are shown in Figure 2. Detailed inspection reveals absorption bands that are sensitive to the mass of the anion and, by utilization of cation- and anion-exchange techniques, modes that are dependent on the mass of the cation. There are also sodalite framework bands above 200 cm-’ (labelled F in the figures) similar to those observed in LTA and FAU zeolites, which also contain sodalite cage^.^*^^'^^" Two anion-sensitive modes are found for chlorosodalite and three for bromo- and iodosodalite. This state of affairs can be compared to that of the kaolin starting material, shown in Figure 3a. This layered material, containing only trace quantities of cations, displays weak residual far-IR absorptions and strengthens our contention that the far-IR sodalite bands originate from the ( 5 ) Henderson, C. M. B.; Taylor, D. Spectrochim. Acta, Part A 1977, 33,
283. (6) Donated by Dr. J. Brinen, American Cyanamid Co., Stamford, CT. (7) Taylor, M. J.; Marshall, D. J.; Evans, H. J. Phys. Chem. Solids 1971,
32, 2021.
( 8 ) Henderson, C. M. B.; Taylor, D. Spectrochim. Acta, Part A 1979, 35, 929. (9) Baker, M. D.; Godber, J.; Ozin, G. A. J . Am. Chem. SOC.1985, 107, 3033. (10) Baker, M. D.; Godber, J.; Ozin, G. A. Catal. Reo. Sci. Eng. 1985, 27, 591. ( 1 1 ) Godber, J.; McIntosh, D. F.; Ozin, G. A,, submitted for publication in J . Phys. Chem.
0 1988 American Chemical Society
Cation and Anion Dynamics in M,X-Sodalites
The Journal of Physical Chemistry, Vol. 92, No. 17, 1988 4981
TABLE I: Mid-IR Framework Vibrational Frequencies for Halosodalites (cm-I) and Starting Material Kaolin (Literature Values (Ref 5) Given in Parentheses below the Observed Values) sample Na,CI-SOD" Na,Br-SODa Na,Br-SODb Li,CI-SOD K,CI-SOD Rb,Cl-SODC NaJ-SODb Ca,Br-SOD kaolin
US
vdT-0) 994 (985) 994 (985) 993,994 (985) 1154, 1047, 1006 (950) 1000 (995) 1129, 1033, 973 (995) 1002 1012 1117, 1033, 1008
714 (712) 709 (708) 707 (708) 740 (742) 68 1 (679) 803
729
702 689 794
914
" Synthesized from kaolin; see Experimental Section. Rb6,3,Cl-SOD. TABLE 11: Far-IR Vibrational Frequencies for Obtained from Self-Supporting Disks sample cation modes Li,Cl-SOD Na,CI-SOD 200, 110 K,Cl-SOD 163, 115 92, 59 Rb,C1-SOD Na,Br-SOD 200, 105 Ca,Br-SOD 188, 119 200, 104 Na,I-SOD
734 (736) 734 (732) 734 (732) 760 (762) 705 (703) 896
(T-0)
6(0-T-O) 670
541
657
614 (630) 694
470 (465) 468 (465) 466 (465) 450 (454) 442 (440) 440 (441) 465 466 468
436 (438) 435 (437) (437) 434 (428) 414 (412) 39 1 (392) 38 1 428
Donated by Dr. J. Brinen, American Cyanamid Co. 'Literature composition Nao,3K,,,-
Halosodalites (em-') anion modes 251, 107 174, 97
NO" 122, 82 161, 134, 69
NO" 143, 135, 52
" N O represents not observed (band overlap).
B
A
350
250 I50 WAVENUM BER
50
Figure 2. Far-IR spectra of (a) Na,CI-SOD, (b) Na,Br-SOD, (c) NaJ-SOD. Spectra were recorded at room temperature.
C Figure 1. Different representations of the structure of sodalite (SOD): (A) Framework of SOD, solid circles represent Si or AI (T) atoms, the open circles, represent bridging oxygen. (B) Unit cell of SOD, the framework is now represented by lines, each vertex being a T atom, while the positions of Na' (open circles) and CI- (solid circles) are shown. (C) Packing of S O D unit cells in the crystal.
translatory motions of cations and anions within sodalite cages. A description of these vibrations has been outlined,2 where it was demonstrated that the dense packing of the ions within the sodalite framework gives rise to strongly correlation-coupled modes whose activity could be rationalized in terms of the symmetry properties of the unit cell. Ion-exchange of chloro- and bromosodalite dramatically affects the far-IR translatory modes as shown in Figure 3 and Table 11. Exchange of the sodium ions in Na,Cl-SOD by lithium ions produces a material whose mid-IR spectrum shows reasonable agreement with literature values.' The absence of sodium ions is demonstrated in Figure 3b by the (almost complete) removal of the characteristic band a t 200 cm-'. Indeed no cation modes can be observed. Furthermore the chloride anion translatory
modes are weak compared to the starting Na,Cl-SOD. Consistent with these observations, lithium ion modes are expected to absorb in the mid-IR on the basis of solution-phase studies'* and normal coordinate calculations." As seen in Table 11, the chloride translatory modes are found at 251 and 107 cm-' and clearly display a greater correlation splitting than in the parent Na,C1-SOD. This indicates a more pronounced interaction between the anions and could be related to the observed decrease in the size of the unit cell. The unit cell dimensions of sodalites have been observed to vary with changes of cation and anion whereby the framework adjusts to the size requirements of the ions within the sodalite cage.13-15 By analogy, this effect is anticipated as the smaller Li+ ions replace Na' in Na,Cl-SOD. T h e literature reports that Na+ ions can be replaced by K+ ions by using a KC1 melt t e ~ h n i q u e .This ~ approach led to a trans(12) Edgell, W. F.; Watts, A. T.; Lyford, J.; Risen, W. M. J . Am. Chem. SOC.1966, 88, 1815. Edgell, W. F.; Lyford, J.; Wright, R.; Risen, W. M.; Watts, R. J . Am. Chem. SOC.1970, 92, 2240. Maxey, B. N.; Popov, A. I.; J . Am. Chem. SOC.1967,89, 223. (13) Nyman, H.; Hyde, B. G. Acta Crystallogr.,Sect. A 1984, A37, 1 I . (14) Depmeier, W. Acta Crystallogr., Sect. B 1984, 40, 185. (15) Hassan, I.; Grundy, H. D. Acta Crystallogr., Sect. B 1984, 40, 6.
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The Journal of Physical Chemistry, Vol. 92, No. 17, 1988
Godber and Ozin
W
0
z
a
m LT
w
I l l
W A V E N U M B E R (ern-') WAVENJMBER
(cm'li
Figure 3. (A) Far-IR spectrum of a self-supporting wafer of kaolin demonstratingthe absence of strong far-IR absorptions below 200 cm-I:
(b) Li,CI-SOD; (c) Rb,Cl-SOD. The translational absorptions of the ions are indicated for each sample. formation of the sodalite into an unidentified material (XRD and mid-IR). The far-IR was also irreconcilable for this material in terms of a sodalite lattice. However, ion exchange of an aqueous, saturated KCI solution under reflux conditions led to a progressive change in the far-IR spectrum that could be explained in terms of a topotactic replacement of Na' by K' ions (Figure 4 and Table 11). The mid-IR spectrum of this material is in excellent agreement with that reported in the literature. Repeated ion exchange of Na,Cl-SOD with KC1 led to the growth of bands in the far-IR at 163 and 115 cm-l attributable to K+ ions at the expense of sodium ion absorption intensity at 200 and 110 cm-l (Figure 4). Unfortunately the anion bands could not be resolved because of overlap with the intense cation modes. Sodium ions are replaced in Na,C1-SOD by Rb' by treatment in a RbCl melt. The mid-IR of this material (Table I) does not compare well with the literature. However, this is not really unexpected since the reported sample had a cation composition of Na0,3K1,4Rb6,3, whereas in the sample of the present study it is clear that the Na' ions have been fully replaced by Rb'. The Rb+ cation translatory modes are identified at 92 and 59 cm-' (Table 11, Figure 3c), while the chloride anion modes are observed as shoulders on the cation vibrations at 122 and 82 cm-'. It is important to note that the spectra of these halosodalite samples do not exhibit any changes upon thermal vacuum treatment, consistent with the absence of entrapped water from the synthesis. Replacement of sodium in all these cases is confirmed by the absence of the characteristic sodium vibration at about 200 cm-'. Factors Influencing the Translational Frequencies of Halosodalites. The far-infrared spectra of halosodalites reveal two interesting effects related to the tightly bound nature of the ions within the sodalite cage. First, the cation translations are essentially insensitive to changes in the anion. This phenomenom is especially apparent for the Na+-exchanged materials. This signals a fingerprint region in which distinct vibrational frequencies will be observed for a particular cation. Secondly, the anion translational modes are dependent upon the cation type with
Figure 4. Far-IR spectra of (Na,K),CI-SOD showing the progressive exchange of Na+ ions for K+ leading to K,CI-SOD: (a) Na,CI-SOD;(b)
(K,Na),Cl-SOD, (c) K,CI-SOD. respect to both the frequency of vibration and the degree of correlation coupling. This is most apparent for the far-IR anion modes in the chlorosodalite series. An explanation of these two observations will now be offered. The correlation-coupled sodium vibrational modes for the Na,X-SOD (X = C1, Br, I) are observed at 200 and 110-104 cm-I. Cation exchange of these materials demonstrates that the mass of the cation is important for determining this vibrational frequency, as heavier cations shift the frequency to lower energy. Furthermore, Li' exchange leads to an absence of cation absorptions in the far-IR, the light mass of Li' and the strong coupling of these modes to the framework placing these vibrations in the mid-IR.2 Another factor that is important is the charge/radius ratio of the cation. This is apparent in Ca,Br-SOD (Table 11), which exhibits a cation mode at 188 cm-', while solely on the basis of mass considerations, a frequency closer to that of K+ occurring around 163 cm-' would be expected (cf. Ca-LTA and Ca-FAU9,'0*'6). Thus the mass and the radius of the ion clearly have an inverse relationship to the frequency of the translatory vibration. Attempts to find an analytical connection between the cation mass and radius and the vibrational frequency on the basis of standard vibrational spectroscopic approximations failed. For example, the diatomic approximation of v a m-1/2or others involving consideration of the Td M,X (M = cation, X = anion) unit were quite nonlinear. This failure can be traced to the highly correlated motion of the ion modes whose frequencies require rigorous interpretation in terms of the full lattice dynamics of the unit ce1L2 The framework responds to cation and anion type by contracting or expanding with concomitant alterations in the force constants between the cations, anions, and framework. The dependence of the anion vibrational frequency on cation type is best assessed with reference to the chlorosodalite series. Here it is seen that the degree of correlation coupling between anions is cation dependent. Since the correlation coupling requires interaction between vibrating moieties, the distance over which the interaction occurs is important. For this reason it would be expected that the degree of correlation coupling between anions (16) Godber, J
P Ph D Thesis, University of Toronto,
1987.
Cation and Anion Dynamics in M,X-Sodalites
The Journal of Physical Chemistry, Vol. 92, No. 17, 1988 4983 n
TABLE 111: Mid-IR Spectral Frequencies (cm-I) for the Framework Region and Anion Internal Modes for Hydroxo- and Perchloratosodalites Framework sample" 8(0-T-O) vAT-0) vAT-0) 995 730, 705, 657 458, 427 Na,OH-SOD 979 732, 712, 669 LiOH-SOD 469, 430 1003 723, 696, 655 456, 425 Na,CI04-SOD 979 749, 725, 678 480, 442 Li,C104-SOD 982 733, 712, 665 467, 433 Na,CI04-SODb 994 734, 712,668 Na,CI-SODC 470, 436 976 754, 709, 669 467, 434 Na-SOD Internal Modes of EncaDsulated Anions sample" Y, 6 Na,OH-SOD 3400 e Na,OD-SOD 2509 e Li,OH-SOD 3650 e Li,OD-SOD 2531 e Na-SOD N d N d Na,C104-SOD 1116 633 Li,CIO,-SOD 1142 650 CIOid 1119 625 "Dehydrated, in a CsCl matrix. bHeated to 650 "C ex situ and then recorded in CsCl after dehydration. Included for comparison. dReference 20. CAlthough a band is observed around 1420-1440 cm-I, its failure to show a deuterium isotope shift argues that rather being assigned to the deformational mode of encapsulated Na-OH24 and associated with the weak broad v(0H) absorption centered around 3400 cm-l, instead it arises from small amounts of carbonate probably formed from air contamination in the original ~ y n t h e s i s . ~The ~ expected 6(NaOH) in this spectral region could be overlapped and/or be too weak to observe. 'NO represents not observed.
\
\ . /
'
/
U Figure 6. (a) Model of Na,OH-SOD (hydrated) showing the proposed orientation of the unit cell contents based on the vibrational observations. (b) Model of the proposed anion sitting near a single cation (or three cations; see text) in the tetrahedral arrangement of sodium ions in every cage. The arrow indicates that the hydroxo anion can librate about the N a - O bond. The dotted circle in the center represents the usual position adopted by anions in the center of the cage.
in the splitting of the correlated anion modes. These observations argue in favor of strongly correlated cation/anion dynamics in the sodalite cages (see later). Internal Vibrational Modes of Hydroxo- and Perchloratosodalites. Hydroxo- (or basic) sodalite is easily synthesized from kaolin and sodium hydroxide" and can be considered to contain encapsulated sodium hydroxide m o i e t i e ~ . ' ~ - *The ~ , ~attention ~ paid
i
"t
1
IO
I
1
I
2400
1
1
0
Wavenumber
Figure 5. Mid-IR spectra of Na,OH-SOD showing the progressive desorption of water by thermal vacuum treatment: (A) 100 "C, (B) 210 OC, (C) 330 O C , (D) 450 O C . Note the fine structure apparent on the OH stretches in A and B. By 450 "C the hydroxo anion stretch is broad and featureless. The sample is dispersed in a CsCl matrix.
should be dependent on their physical separation in the unit cell. For example, if the unit cell edge (aofrom XRD) of M,C1-SOD ( M = Li, Na, Rb) is plotted against the frequency separation of the two coupled anion modes, a monotonic trend is observed. Therefore as the unit cell expands to accommodate cations of increasing size, the distance between anions in adjacent cells must increase. This is manifest in the vibrational spectrum as a decrease
(17) Barrer, R. M.; Cole, J. F.; Sticher, H. J . Chem. SOC.A 1967; 1523. (18) Barrer, R. M., Vaughan, D. E. W. J . Phys. Chem. Solids 1971, 32, 731. (19) Hassan, I.; Grundy, H. D. Acta Crystallogr., Sect. C 1983, 39, 3. (20) (a) Colm, H. J. Chem. Soc. 1952,4282. (b) Ross, S. D. Spectrochim. Acta 1962, 18, 225. (21) Barrer, R. M.; Daniels, E. A.; Madigan, G. A. J. Chem. Soc., Dalton Trans. 1976, 1805. (22) (a) Cotton, F. A.; Wilkinson, G. Aduanced Inorganic Chemistry, 3rd ed.; Wiley-Interscience: New York, 1972; p 52. (b) Ibid.; p 488. (23) Barrer, R. M.; Cole, J. F. J . Chem. SOC.A 1970, 1516. (24) Nakamoto, K. Infrared Spectra of Inorganic and Coordination Compounds, 2nd ed.; Wiley-Interscience: New York, 1970; p 82. (25) Felsche, J.; Luger, S . Ber. Bunsen-Ges. Phys. Chem. 1986,90, 731. (26) Moller, K. D.;Rothschild, W. G. Far Infrared Spectroscopy; Wiley-Interscience: New York, 1971. (27) Godber,J.; Ozin, G. A., submitted for publication in J . Phys. Chem. (28) Stucky, G . , personal communication. (29) Some extra lines observed in the XRD powder data of Na,ClO? SOD(A) could possibly arise from the trapping of some of the evolved O2In the sodalite cages along with the encapsulated CI- anion. Alternatively, thermal conversion of Na,C104-SOD to Na,Cl-SOD at 650 OC could cause some conversion to another crystalline phase. The effect is, however, not detected in the mid- or far-IR.
4984
The Journal of Physical Chemistry, Vol. 92, No. 17, 1988
to this material in the literature is not as extensive as that of the haloso#alites, probably because photochromic or cathodochromic behavior has not been observed. However, its use as a storage media for "packaged" gas has been proposed.I8 The mid-IR framework frequencies of Na,OH-SOD are collected in Table 111, and comparison with the above halosodalites shows the expected framework bands, confirming the identity of the synthesized materials as sodalite and ensuring that the framework is intact. Hydroxosodalite as synthesized contains encapsulated water, which serves to stabilize a very small anion such as OH- (see later). In this regard hydroxosodalite exhibits the zeolitic property of removal of intracrystalline water without disruption of the f r a m e w ~ r k . The ~ ~ .progressive ~~ removal of this water is monitored in the mid-IR spectra shown in Figure 5. As synthesized, the water content is variable but according to the literature X-ray structural determinations is probably about 1.5-2 molecules per sodalite cage.Ig Upon thermal vacuum treatment the broad hydroxyl stretching region in the mid-IR (Figure 5) decreases in intensity along with the water deformational mode at about 1665 cm-I. Following a 210 OC dehydration the mid-IR spectrum of hydroxosodalite displays structure in the hydroxyl stretching region (Figure 5). A model consistent with the vibrational data is shown in Figure 6a for the encapsulated [(OH)(H,O)]- complex anion. The presence of water is evidenced by the characteristic deformational absorption at 1650 cm-l. Simple group theoretical reasoning demonstrates that this type of complex would give rise to three infrared-active modes for the 0-H stretches. Two bands arise from a symmetric and asymmetric v(0H) motion of the water moiety, while a third u(0H) will arise from the hydroxide unit. Upon dehydration, the hydroxide anion moves into close coordination to the sodium cations, giving rise to a broad, weak v(0H) absorption in the mid-IR. The breadth of this absorption could arise from unresolved rotational structure from hindered rotor dynamics of the anion with respect to its neighboring cation(s) (Figure 6b).30 Exposure of dehydrated Na,OH-SOD to D 2 0 followed by dehydration results in proton exchange and the appearance of the u(0D) stretching mode centered around 2509 cm-' (Table 111). Exchange of the Na+ ions in Na,OH-SOD by Li' slightly shifts the framework bands (Table 111). Of particular interest is the upward shift of the OH- stretching frequency to 3650 cm-' compared to the parent Na,OH-SOD. Protondeuteron exchange sees the v(0D) stretching frequency at 2531 cm-I. The higher frequencies of the hydroxo anion internal stretching modes of Li,OH-SOD and Li,OD-SOD (Table 111) relative to the Na,OH-SOD and Na,OD-SOD parents are consistent with a stronger interaction of the anion with the cations (see later). The framework bands of Na,ClO,-SOD are shifted to lower frequencies compared to other monovalent anionic samples in keeping with the higher mass of this anion. Nevertheless, the framework modes exhibit the same pattern observed above for the halosodalites (Table 111). The internal modes of the encapsulated C10, anion are observed at 11 16 and 633 cm-l for the asymmetric stretch and deformation, respectively (Figure 7a). These can be compared to literature values, where v(C1-0) is observed at 1119 cm-l and the OCI-0 deformation at 625 cm-'.M Thermal treatment of the perchlorato sample (ex situ) at 650 "C leads to the known2' transformation of Na,ClO,-SOD into Na,CI-SOD with the concomitant release of oxygen. The mid-IR (30) Basic aluminosilicates Naa[AISi0,]6(0H)2.nH20 have been the subject of a number of structural studies, showing different extents of hydrogen bonding between the framework, sodalite-encaged H20, and OH-. At 8 K Na2[AlSi04]6(OH)2is cubic. The oxygen of the OH- anion is centered in each sodalite cavity with a random distribution of the hydrogen among four sites at 1.09 .& from the oxygen and 2.28 .& from a sodium. Hydroxo anion dynamics at room temperature probably account for the breadth of the IR u(OH) in hydroxdalite, Nae[A1Si04]6(0H)2.Temperature-dependent midand far-IR studies over the range 10-400 K will be required to probe details of this intrasodalite cage dynamical phenomenon. Luger, S.; Felsche, J.; Fischer, P., Acta Crystullogr.,Secr. C 1987, 43, 1 . Baerlocher, C.; Felsche, J.; Fischer, P.; Luger, S . Zeolites 1986,6, 367. Bonderava, 0.S.; Malinovskii, Yu,A. Sou. Phys., Crystallogr. 1983, 28, 213. (31) Stein, A. MSc. Thesis, University of Toronto, 1988.
Godber and Ozin I
'
" ' r
1
Figure 7. Mid-IR (CsCI matrix) of Na,CI04-SOD (a) before and (b) after heating ex situ to 650 OC. The deformational and stretching vibrations of the encapsulated perchlorate anion is shown to be removed as the sample is transformed into Na,Cl-SOD. (c) Li,C104-SOD. TABLE I V Far-IR Translational Frequencies (em-') for Hydroxoand Perchloratosodalites (All Spectra Were Recorded at Room Temperature) sample" Li,CI04-SOD Na,CI04-SOD Na,C104-SODb Na,CI04-SODC Na,OH-SODd Na,OH-SOD' Li,OH-SODd+
cation modes 205, 205, 202, 195, 208, NO
112 112 111 112 108
anion modes 141, 125, 92 145, 130, 63 145, 130, 63 175, 98 170, 140 NO
"Self-supporting wafer. bHeated in situ to 500 "C. cHeated ex situ to 650 " C . dHydrated sample. eDehydrated at 450 "C.
of this calcination product (Figure 7b) shows close agreement with the spectrum of authentic chlorosodalite in the framework region as well as the absence of the perchlorate modes at 1116 and 633 cm-1.29Replacement of Na+ ions by Li+ in Na,C104-SOD reveals the same vibrational frequency and intensity pattern in the mid-IR with all bands being shifted to higher energies (Table I11 and Figure 7c). Most noteworthy is the appearance of the internal perchlorate modes at substantially higher frequencies: u(C1-0) at 1142 cm-I and S(0-Cl-0) at 650 cm-I. This is indicative of an increase in the force constants of the associated C1-0 internal coordinates and is most probably a result of a change in the cation-anion distance. The smaller Li+ ions probably lie in or close to the plane of the six-membered ring compared to a pyramidal coordination site for the larger Na+. Because of this, weaker Li+-03C10- electrostatic interactions result in strengthening of the perchlorate C1-0 bond compared to that in the parent Na,CIO,-SOD. Ex situ thermal treatment of Li,C104-SOD at 650 OC leads to a loss of intensity of framework vibrational modes, indicative of a decomposition of the sodalite lattice. At this stage it is unclear a s to why Na,CIO,-SOD transforms cleanly to Na,CI-SOD while under the same conditions the Li+ congener decomposes. Nevertheless, as demonstrated above, the mid-IR region is an invaluable guide for checking whether the framework remains intact after synthesis and following thermal, vacuum, and/or chemical posttreatments. Translatory Dynamics of Hydroxc- and Perchloratosodalites. The translatory ion modes of dehydrated sodium hydroxosodalite
Cation and Anion Dynamics in M,X-Sodalites
The Journal of Physical Chemistry, Vol. 92, No. 17, 1988 4985 I
I
I
I
W
V
z a
m IY
0 v)
m
a
W A V E N u M BE R (ern-' )
Figure 9. Far-IR spectrum of dehydrated Na-SOD (Soxhlet-extracted
Na,OH-SOD).
WAVENUMBER (cm")
Figure 8. Far-IR spectra of Na,OH-SOD (a) when hydrated and (b) after a 450 O C in situ thermal dehydration.
in the far-IR fits the pattern unveiled above for the halosodalites. When hydrated, far-IR fingerprint bands are evident at 195 and 112 cm-' (Figure 8a and Table IV). In situ removal of encapsulated water leads to a significant change in the far-IR translatory vibrations (cf. Figures 8a,b and Table IV). The sodium ion modes appear at about 208 and 108 cm-' with the hydroxide anion modes observed at about 170 and 140 cm-' (shoulders). At this point it is pertinent to note that Na,F-SOD cannot be ~ynthesized,'~ and as the fluoride and hydroxide anions are isoelectronic and expected to be of comparable size, the existence of Na,OH-SOD is probably related to the role played by the encapsulation of water during the synthesis.25 If we assume that the radius of OH- is then to maintain normal bonding about the same as 0" (1.32 distances between ions in the dehydrated form, the lattice must contract. Initially the sodium ions are hydrated, receiving some charge density from the water as in zeolites, where the water acts as mediator between the cations and the lattice (anion). Removal of the water forces the sodium ions to respond to this loss. Charge compensation is not achieved however by shrinkage of the cell in an attempt to reduce the Na+ cation-hydroxy anion distance. Instead, in the dehydrated state the cations move closer to the lattice and away from the a n i ~ n . ' ~This ? ~ ~is manifest in an observed E-type parent cation vibrational frequency at higher energy than, for example, in the sodium halosodalites and greater correlation coupling between the cations (Table 11). In the dehydrated form, X-ray data19,25show that the anion is not located at the center of the sodalite cage as in, for example, halosodalites, but is found in a site near one of the cations. This suggests that as the anion is so small (cf. r(Cl-) = 1.81 A") relative to the void that it must fill, bonding interactions to the sodium ions (at room temperature) are maximized by preferential interaction with one (or three) cations, rather than maintaining a central location and a spherical cation d i s t r i b ~ t i o n .Librational ~~ motion of the hydroxide anion may contribute either to the breadth of absorptions in the mid- (internal) and far- (translatory) infrared regions or to the genesis of libratory far-IR modes for anhydrous hydroxosodalite. (Note that libratory modes are expected to be less intense than their translatory counterparts.26) This is schematically represented in Figure 6b, showing off-center rotation of the hydroxide anion about one of the Na-0 bonds. The situation may be that of a hindered rotor. where Coulombic interactions between the proton and nearby Na+ ions could impede free rotati01-1.~~
The translatory modes of the Li,OH-SOD sample are not observed in the far-IR. It has been n ~ t e d ' that ~ . ~the ~ translatory Li+ ion modes are located at higher energies, in the mid-IR. The absence of any absorptions in the far-IR that could be attributed to the hydroxo anion suggest that these too are now found in the mid-IR spectral region above 350 cm-I. This proposal is in line with the observations discussed earlier regarding the internal modes of Li,OH-SOD and Li,OD-SOD. The small size of the Li+ ion and its high polarizing power result in a strong Li-OH interaction and consequently either an increase of the translational frequencies of the anion and/or a reduction in IR intensity due to a lower dipolar interaction. Above it has been implied that the observation of correlation splittings of the far-IR translatory cation vibrations intimately involves the effects of the extra-framework anion. In zeolites where the cations are dispersely packed and there are no extra-framework anions, the far-IR cation vibrational spectra can (with certain caveats16) be explained in terms of local modes. Hydroxosodalite allows one to examine this proposal as the hydroxo anion may be removed, along with a sodium cation, by Soxhlet extraction:18 Na,OH-SOD
-
Na-SOD
+ NaOH(aq)
The mid-IR spectrum of dehydrated Na-SOD exhibits diagnostic framework modes (Table 111) and in particular the absence of hydroxo internal modes in the 3500- and 1400-cm-' regions, consistent with a sodalite framework devoid of the hydroxo anion. The far-IR spectrum of dehydrated Na-SOD, depicted in Figure 9, exhibits essentially one broad absorption peaking at about 205 cm-'. No longer apparent are the hydroxo anion translatory modes or the correlation-coupled low-frequency Na+ ion mode around 100 cm-I. The observed cation translatory mode displays some structure, and this most probably encompasses contributions from the weak framework absorption in this region, residual Na+ ion correlation coupling, symmetric A, and asymmetric E-type local Na+ cation vibrations, and effects related to the fact that the Soxhlet-extracted material, Na-SOD, is no longer a regular structure. With respect to the latter point, in Na,OH-SOD, every cage contains four Na+ ions and one OH- oriented in the same way; however, in Na-SOD, the three remaining sodalite cage Na+ ions may sit at any of the four available sodalite cage sites and are not necessarily distributed regularly as in Na,OH-SOD. One can consider that the remaining 205-cm-' band is the analogue of the sodium ion translatory modes observed in site I1 of FAU and sites A, B, and C of LTA.9,'0-'6 Figure 10a shows the far-IR sDectrum of Derchloratosodalite as synthesized, with "fingerprint" sodium cation absorptions at
4986
The Journal of Physical Chemisiry. Vo/. 92, No. 17. 1988
.O.l Abs.
Godber and Ozin
n
Figure 11. Proposed model far the orientation 01the perchlorate anion in the sodalite cage. The dark circles represent Na+ ions, the lighter circles the oxygens of CIO;. Each oxygen of CIO; sits in a face formed from the tetrahedron of Na+ ions.
50
250 150 5c WAVENUMBER (cm-‘)
Figure 10. Far-IR spectra of Na.CIOrSOD and its 650 ‘C decomposition product: (a) Li,CIO,-SOD (b) Na,ClO,-SOD; (c) Na,CIO,SOD after 650 ‘C ex situ treatment, showing the transformation into Na,CI-SOD, (d) Na,CI-SOD (authentic sample) included far comparison.
205 and I12 mi1.The translatory modes of the perchlorate anion are assigned to the split absorptions at 145, 130 (see later), and 63 cm-l (Table IV and Figure lob). This assignment is confirmed by an examination of the far-IR spectrum of Li,CIO,-SOD (Figure loa). The Lit ion translatory modes are found in the mid-IR,16~” leaving an unobscured far-IR spectrum exhibiting only anion translations. This spectrum is shown in Figure 10a and Table IV, where C104-modes are observed at 141, 125 (split, see later), and 92 cm-I. The breadth and splitting of the high-frequency translatory C104- anion mode cannot simply be attributed to coupled motions of the perchlorate in a tetrahedral lattice site. It is quite possible that LO-TO splittings ohserved2J6 on the anion modes of the halosodalites are also contributing to the perchloratosodalite. Two other possibilities should be considered. First, weak librational motions of the tetrahedral CIO; anion could be observed in the far-IR. Rotations under the site group Td (assuming a P&3nspace group) transform as T I and therefore are not infrared active. Under the influence of the site group T (in a P43m space group) these motions transform as T , T2, and hence one is infrared active.’ Second, the splitting of the high-frequency anion mode cannot be due to contributions from 3sCI/37CIin natural abundance. If this was the origin of the effect, it should also be observed on the low-frequency partner of the anion translation, and this is not observed. For this interesting example of a tetrahedral anion inside a tetrahedron of sodium ions, it seems most likely that the structure on the high-frequency perchlorate anion mode originates from either a LO-TO splitting or a librational contribution,
+
The thermally induced transformation of Na.CIO,-SOD monitored in the far-IR is most revealing in regard to the encapsulated anion. Heating the sample in situ to 500 “ C (the limit of the far-IR cell heater) has essentially no effect on the translatory ion motions. This is to be contrasted with the result of heating the sample ex situ to 650 OC as shown in Figure 1Oc. Complete transformation of Na,CIO,-SOD into Na,CI-SOD is confirmed by comparison to the far-IR spectrum of authentic cblorosodalite shown in Figure 10d.’9 This series of spectra, besides being interesting in their own right, are important as they provide strong supporting evidence for the assignment of (a) the cation modes based on their insensitivity to this thermal treatment and (b) the anion modes as the perchlorate vibrations are seen to be eliminated and replaced by the chloride vibrations. The cationtanion assignments of sodalites reported in this and a previous study’ can then he considered to be secure. At this point the orientation of the perchlorate anion in the sodalite cage will he discussed. In light of the C1-0 bond length of 1.45 the essentially unperturbed u(Cl-0) stretch and a ( O - C l 4 ) deformation in the mid-1R compared to free C104-, and unperturbed sodium translatory motions in the far-IR compared to Na,CI-SOD, it is unlikely that the oxygen atoms of CIO; are directed at the Nat ions. It is more reasonable to expect that the anion is oriented in such a way with respect to the sodium ions that the latter assumes a six-coordinate geometry in the sodalite cage as illustrated in Figure 11, We note that in an unpublished X-ray structure determination of Na,MnO,-SOD, the tetrahedral MnO; anion lies within the tetrahedral group of four Na+ ions in the configuration described above for Na,CI0,-SOD.’8 Furthermore, the far-IR cation and anion translatory modes of Na,CIO,-SOD and Na,Mn04-SOD are very similar?’ Hopefully, our work will stimulate a diffraction study of the orientation of the perchlorate ion within the sodalite framework. Summary This work represents the first report of a systematic and extensive study of the vibrational features of a wide range of cations and anions encapsulated in the sodalite lattice. In terms of a fuller understanding of the extra-framework ions in sodalites and their role in photo- or cathodcchromic development, the foundation for extensions to more challenging problems is in place. The main conclusions of this study can be summarized as follows: ( I ) Systematic variations of the cations and anions in M,X-SOD (where M = Li, Na, K, Rb, Ca; X = CI, Br, I, O H , CIO,) allow one to assign diagnostic frequencies to specific cation and anion translatory modes as well as to evaluate the extent and origin of their observed correlation coupling. (32) 0% G. A.; Stein, A.; Stucky, G.. unpublished obxrvations.
The Journal of Physical Chemistry, Vol. 92, No. 17, 1988 4987
Cation and Anion Dynamics in M,X-Sodalites TABLE V: X-rav Diffraction Reflections Rb,CI-SOD K,CI-SOD 20 14.25 20.10 24.65 31.90 35.04 38.00 43.30 48.20 50.40
Ire1 0.23 0.05 1.00 0.11 0.24 0.23 0.38 0.04 0.05
Na,OHSOD
29 13.65 18.10 23.70 25.45 30.90 33.70 36.55 41.60 48.30
Ire, 0.20 0.07 1.00 0.07 0.28 0.16 0.14 0.50 0.06
Na,C104SOD
20 19.40 27.70 28.25 31.00 33.80 39.75 41.45 46.15
Ire1
0.07 0.26 1.00 0.05 0.32 0.11 0.08 0.07
Na,C104SOD(A)
Na,I-SOD 20 20.10 24.55 28.40 34.90 31.75 40.45 42.97 45.35 47.75 49.90
Ire,
0.10 1.00 0.07 0.05 0.29 0.05 0.29 0.05 0.09 0.04
kaolin
29
I,~
20
I,~
29
I,,
20
14.10 20.10 24.50 25.50 28.40 31.75 34.85 37.80 42.95
0.58 0.09 1.00 0.21 0.13 0.59 0.53 0.07 0.40
14.00 19.75 24.15 27.90 30.25 30.45 31.32 34.40 37.25 39.87 42.40 47.07 49.30 51.45 55.60 57.55
0.03 0.06 1.00 0.03 0.02 0.02 0.04 0.28 0.06 0.08 0.25 0.04 0.03 0.08 0.06 0.09
14.25 20.20 21.50 24.75 25.60 29.65 32.10 35.30 38.12 41.70 43.45 45.85 48.35
0.24 0.06 0.48 1.00 0.05 0.06 0.08 0.30 0.18 0.06 0.24 0.03 0.04
12.65 20.50 21.75 25.30 35.48 36.35 38.15 38.85 39.60 45.90
I,., 0.94
br 0.17 1.00 0.26 0.23 0.18 0.35 0.18 0.12
(2) The ion vibrational frequencies exhibit a red-shift with increasing mass, the form of which cannot be quantified in terms of a simple, localized oscillator model. This is indicative of the coupled dynamics of the ions within the tightly packed sodalite cages of the unit ce1L2 (3) The vibrational features of the lithium and sodium hydroxosodalites are distinct to the other sodalites discussed above and must be rationalized in terms of the degree of hydration and the size of the anion. (4) The vibrational spectrum of the hydrated sodium hydroxosodalite provides structural information on the encapsulated [OH,H20] complex. (5) Soxhlet-extracted Na,OH-SOD yields Na-SOD with no evidence of correlated Na+ ion modes, indicating that vibrational coupling between Na+ ions in Na,OH-SOD is mediated via the central anion. An extension of this concept to M,X-SOD appears reasonable for explaining the origin of the strong correlation coupling observed in these densely packed systems. (6) The decomposition of Na,C104-SOD into Na,Cl-SOD establishes the validity of the cation and anion translational mode assignments. (7) The vibrational spectrum of Na,C104-SOD provides information on the orientation of the anion within the sodalite cages.
Experimental Section The materials described in this paper use the following nomenclature: for example, to describe a sodium chlorosodalite sample with an elemental composition of Na6(Si02)6(A102)6. 2NaC1, the abbreviation Na,Cl-SOD is used. The general nomenclature then is cation,anion-framework.
Synthesis of Sodalites. The materials were synthesized at 80 "C, crystallizing from an aqueous mixture of kaolin (Aldrich), N a O H (BDH), and the sodium salt of the anion to be encapsulated.23 Experiments with cations other than sodium yielded products that were neither kaolin nor sodalite, as evidenced by their mid- and far-IR and were not investigated any further. Sodalites were synthesized from a mixture of 200 mL of about 4 M NaOH, 2.0 g of kaolin, and an excess of the salt to be occluded. This was heated for between 36 and 48 h at 80 "C in polypropylene bottles. As an illustration of the general method, the Na,C1-SOD sample was synthesized from 200 mL of 4.0 M NaOH, 2.1 g of kaolin, and 23.84 g of NaC1. This mixture was then filtered hot through a medium frit and washed with 2 L of hot distilled water to give about 2 g of Na,C1-SOD. The other samples were prepared in the same way except for the hydroxosodalite, which was synthesized in 6.73 M NaOH. Cation Exchange. Exchange of the cations was accomplished by melting a salt containing the encaged anion (or an anion too large to enter the sodalite cages, e.g., nitrate or sulfate) and the cation to be encapsulated with the sodalite or by exhaustive aqueous ion exchange. For example, 0.22 g of Na,Cl-SOD and 2.02 g of RbCl were mixed well in a mortar and then heated to 800 "C in a muffle furnace for 20 min. The solid mass after cooling was ground, dispersed in 100 mL of H 2 0 , heated to 80 "C, and then filtered hot, washed, and dried. Li,Cl-SOD was synthesized by melting L i N 0 3 into Na,Cl-SOD in a porcelain crucible at 240 "C for 2 h. Li,C104-SOD was prepared in a similar manner. Exhaustive aqueous ion exchange was accomplished by, for example, treating 0.5 g of Na,Cl-SOD with a saturated solution of KCl(aq) at 100 "C for about 12 h. This was performed four times to obtain the K,C1-SOD sample. Soxhlet extraction on 0.5 g of sample was performed twice for 5 days each to obtain the extracted material. Decomposition of Na,C104-SOD. Na,C104-SOD was decomposed in a porcelain crucible by heating in a muffle furnace. The sodalite (2.0 g) was heated rapidly over about an hour to 500 "C and then slowly over 2 h to 650 "C. This temperature was then maintained for another 2 h. The sample was then cooled to room temperature over 18 h. XRD of Sodalites. The crystallinity of the synthesized sodalites was ascertained by powder X-ray diffraction. Samples were distributed on a glass slide and examined with Cu K a radiation (A = 1.5418 A), scanning at 2O/min over the range 10" to about 50". The most intense reflections are tabulated in Table V. Spectroscopic Measurements. Details regarding the spectrometers, cells, and associated procedures for recording and analyzing the spectra reported in this paper have been documented else~here.~.'~ Acknowledgment. The generous financial assistance of the Natural Sciences and Engineering Research Council of Canada's Operating and Strategic Grants Programmes and the Connaught Foundation of the University of Toronto are all gratefully acknowledged. We also express our deepest gratitude to Dr. Jerry Brinen (American Cyanamid) for the supply of some high-quality sodalite samples and invaluable technical assistance. Registry No. Na,C1-SOD, 12336-79-7; Na,Br-SOD, 53238-76-9; Li,Cl-SOD, 115362-99-7; K,C1-SOD, 115362-98-6; Rb,CI-SOD, 115362-97-5; Na,I-SOD, 53238-78-1; Ca,Br-SOD, 115363-00-3; Na,OH-SOD, 12393-56-5; Li,OH-SOD, 115362-93-1; Na,CIO,-SOD, 115362-95-3: Li.CIO,-SOD. 115362-94-2: Na.OD-SOD. 115383-30-7: Li,OD-SOD, 115362-96-4;' kaolinite, 13 18-74-7; sodalite, 1302-90-5:
