J . Phys. Chem. 1988, 92, 6017-6024
6017
Probing Extra-Framework Cations in Alkali- and Alkaline-Earth-Metal Linde Type A Zeolites by Fourier Transform Far-Infrared Spectroscopy Mark D. Baker,? John Godber,: Kate Helwig,s and Geoffrey A. Ozin* Lash Miller Chemical Laboratories, Department of Chemistry, University of Toronto, 80 St. George St., Toronto, Ontario, Canada, M5S 1 A1 (Received: October 23, 1987; In Final Form: April 25, 1988)
The FT-far-IR spectra (350-30 cm-') of self-supportingwafers of alkali- and alkaline-earth-metalLinde type A (LTA) zeolites and some of their binary combinations are reported for the first time. The sensitivity of this region of the vibrational spectrum to metal cation type, location, coordination number, geometry, and population provides a convenient diagnostic for probing metal cation site distributions, the sequence of metal cation replacement resulting from selective ion exchange and metal cation mobility and reorganization during the removal of intrazeolitic water. Systematic trends in the far-IR spectra of the metal cations in conjunction with cation frequency, intensity, mass, charge, and bond length relationships provide a basis for an internally consistent set of vibrational signatures for different cations in specific sites of alkali- and alkaline-earth-metal LTA zeolites. Studies of this kind are likely to find utility in the identification of cations located in the zeolite pores, which will certainly assist in understanding the fine tuning of window sizes in LTA zeolites. This is important in their applications for gas separation and purification.
Introduction Locating the siting of extra-framework cations in Linde type A zeolites (LTA) is crucial in the design and understanding of experiments aimed at fine tuning the dimensions of the entrance windows to the supercages. The window sizes can be precisely adjusted by the controlled placement of cations at strategic locations in the A-zeolite lattice. Therefore only molecules with suitable dimensions can diffuse into the bulk of the material. This is the basis of the application of LTA zeolites in molecular sieving and gas purification.' In this context, a good example is afforded by a comparison of the pore sizes of K8Na4-A and Na12-A. These zeolites contain monovalent extra-framework cations that partially block the eight-ring windows between supercages (these are principally located at site E as shown in Figure l), thus restricting entrance to molecules with a maximum kinetic diameter of 3 and 4 A,2 respectively. In Ca4Na4-A, however, the cations occupy six-ring sites (for example, site A in Figure l), resulting in an expanded entrance to the supercage. Therefore, molecules with up to 4.2-A kinetic diameters can be adsorbed into the bulk of this zeolite. A judicious selection of cation type and loading is a prerequisite for precise adjustment of the blocking action at the 0.1-A 8, level. Once this has been achieved, these materials can be effectively exploited in gas separation and purification applications. The far-IR spectra (350-30 cm-l) of LTA zeolites can be divided into two regions on the basis of the origin of the observed vibrational modes, as discussed in previous publication^.^-^ Framework vibrations occur above about 230 cm-I and are essentially decoupled from the extra-framework cation modes, which are observed below about 230 cm-'. It was also shown that distinct cation translations can be considered to arise from the motions of the cations located at different extra-framework sites and that a local molecule (decoupled oscillator) model of this situation was ~ a l i d . ~In. ~this approach, the framework vibrations are essentially decoupled from those of the extra-framework cations, and the individual cation motions are largely not correlation coupled.5a It was also discovered for Na-LTA that the vibrational frequencies of the cations were essentially insensitive to the symmetry of the unit cell and the arrangement of surrounding cation^.^ Thus, the frequency and intensity expressions that have been developed3J9 for describing cation modes, site locations, and populations and for obtaining angular information at a particular cation site are valid for the LTA zeolites to be discussed here.3-5 These ex-
'
Department of Chemistry and Biochemistry, University of Guelph, Guelph, Ontario, Canada, N 1G 2W 1. *Albright and Wilson Americas, 2 Gibbs Road, Toronto, Ontario, Canada, M9B 1R1. Chemistry Department, Stanford University, Stanford, CA 94305.
0022-3654/88/2092-6017$01.50/0
pressions will be used extensively to establish consistent assignments of the translational vibrations of cations residing in extra-framework sites in the LTA unit cell. In particular the far-IR signatures of alkali- and alkaline-earth-metal-exchanged LTA zeolites will be examined in detail in a designed effort to address the question of cation site location and population. The framework structure and sitings of exchangeable cations in the A-zeolite lattice are depicted in Figure 1. This shows a three-dimensional perspective of the structure with the most important cation locations schematically represented, following the site nomenclature of Mortier.6 Note that in this representation, the extent of the projection of the cations into the cages has been exaggerated. This figure illustrates the pseudo-unit cell of LTA, since the full cell is composed of eight pseudo-cells joined in cubic symmetry.' The structure is formed by joining eight cubooctahedral units (sodalite cages) via the four-rings, to form a cubic lattice. This arrangement of sodalite cages (0-cages) creates a larger polyhedral cavity known as the supercage (a-cage). The eight-ring windows of this cage connect adjoining supercages in the lattice, and it is through these apertures that molecules must pass. The charge-balancing extra-framework cations tend to be located on the 3-fold symmetry axes of the sodalite six-rings and are bound to three 0 3 framework oxygens. Cations located at this general site may be in the plane of the six-ring (site A), displaced into the supercage (site B), or displaced into the sodalite cage (site C), but always lying on the C3axis as shown in Figure 1. Note that each six-ring site can support the presence of only one cation. The other important cation locations are at sites E, G, and H. Site E cations are situated in the plane of the eight-ring and are bound to two 01 and one 0 2 framework oxygens with a local symmetry of C,. Occupancy of this site is clearly central (1) (a) Dwyer, J.; Dyer, A. Chem. Ind. (London) 1984, 237. (b) Breck, D. W. Zeolite Molecular Sieves; Wiley: New York, 1974. (2) The kinetic diameter is defined as the intermolecular distance of closest approach for two molecules colliding with zero initial kinetic energy (see ref lb) and is calculated from a Lennard-Jones potential. The difference between the apparent pore diameter (from crystallography) and the kinetic diameter is explained on the basis of activated diffusion of adsorbates into the zeolite pores. (3) Baker, M. D.; Godber, J.; Ozin, G. A. J . Am. Chem. SOC.1985, 107, 3033. (4) Baker, M. D.; Ozin, G. A,; Gcdber, J. Catal. Rev. Sci.-Eng. 1985, 27, 591. ( 5 ) (a) Godber, J.; Ozin, G. A. J . Phys. Chem. 1988, 92, 2841. (b) Gcdber, J.; McIntosh, D. F.; Ozin, G. A. submitted for publication in J . Phys. Chem. (6) Mortier, W. J. Compilation of Extra Framework Sites in Zeolites; Butterworths: Surrey, 1982. (7) Bursill, L. A,; Lodge, E. A.; Thomas, J. M.; Cheetham, A. K. J . Phys. Chem. 1981,85, 2409.
0 1988 American Chemical Society
6018
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988
Baker et al.
w 0
z
a a
Figure 1. Representation of the pseudo-cell of LTA illustrating the
m
framework structure surrounding a large cage and the common cation positions denoted by using the formalism of Mortier.6 Note that the positions of cations in sites B, C, G, and H are exaggerated for clarity.
0 v)
m
a
to “tuning” the zeolite pore dimensions. Cations coordinated in site H project into the supercage and are located at the center of the square faces of the oxygen four-ring window bound to two 0 3 and two 01 oxygens in local C4, symmetry. The bonding of site G cations is analogous to site H; however, these ions locate within the sodalite cages. The local site symmetry and population of these cations are crucial in understanding the low-frequency vibrational spectra of these materials, as will be discussed in the rest of this paper. Experimental Section
High-crystallinity (from X-ray diffraction measurements) zeolites 3A, 4A, and 5A were obtained from Union Carbide. The starting materials were modified by ion exchange at room temperature by using 0.1 M solutions of the cation. After being stirred, typically overnight, the slurries were filtered, washed, and then dried at about 150 “ C . They were then stored at room temperature over a saturated NH&1 solution prior to spectroscopic examination. Dehydration of the zeolites was achieved by applying a slow-temperature ramp to the self-supporting wafers from room temperature to 400 OC, over about 6 h. Usually the zeolites were pumped overnight before heating. This minimizes self steaming. In this paper we use the term hydrated to describe the zeolites after pumping at room temperature. Elemental analyses of these materials yielded the following cation compositions for the pseudo-unit cell (zeolite, cation composition; where A = (Si02)12(A102)12):3A, K8.1Na3.9-A;4A, Na12-A; 5A, Ca3,5Na5-A; K-A; Kll,6Nao,4-A; Rb,Na-A, Rb6.7Na5,2-A; Ca6-A, Ca5,gNao,2-A; Sr,-A, Sr5.8Nao.4-A; Ca5,5Lil-A, Ca5,5Lil-A; Ca5Li2-A, Ca5Li2-A; Ca4,6Li2.8-A, Ca4,6Li2,8-A; Li4,7Na7.2-A, Li4.7Na7.2-A; Li5,9Na6,0-A, Li5,9Na6,0-A. The instrumentation, software, and vacuum hardware used in these studies has been described in previous publication^.^,^ The curve-resolved spectra shown in this paper were obtained by fitting the observed vibrational spectra to Gaussian bands. The number of Gaussian components used for a fit used was kept to a minimum, and the bandwidths were held constant as far as possible, within the constraint of the number of cation vibrational modes that were anticipated to show infrared activity.
WAVENUMBER
Figure 2. Far-IR spectra of dehydrated (recorded at room temperature) Na,Li-LTA showing the replacement of Nat(H) and Na+(A) by progressive ion exchange of Li’: (A) Na12-A; (B) Na7.2Li4.7-A;(C) Na6,,,Li5,9-A. The frequencies of the resolved bands of Na12-A are as follows: v(E(A,)) = 216 cm-l; v(B,(E)) = 176 cm-I; v(E(H)) = 110
cm-I. TABLE I: Crystallographically Identified Cation Sites and Poaulations for Na-LTA” A
deh deh
7.8 8.0 8.0 8.0 8.0 8.0 8.0
deh hYd
deh deh deh
site E 2.9
ref
H 0.8
8
4.0 4.0
10
4.0
11 12
3.0
3.0
9
1.o 1.o
13 14
‘The numbers in this table refer to the number of cations per pseudo-unit cell; deh, is dehydrated; hyd is hydrated. TABLE I 1 Sites and Symmetries of Extra-Framework Cations in NaA Together with the Expected IR-Active Vibrations in These Sites
site
symmetry
A, B, C
C3”
IR-active modes E, A I
H
c4u c 2 u
E, AI BI, 2Ai
E
Results and Discussion
Alkali-Metal-Ion-Exchanged LTA. Na-LTA and Li,Na-LTA. The far-IR spectrum of dehydrated Na12-A is shown in Figure 2A. The bands occurring above 250 cm-’ relate to framework type vibrations of the unit cell that are somewhat insensitive to the nature of the metal cation guest. It is the lower frequency cation motion bands that are the subject of this discussion. In an undertaking of the interpretation of this spectrum, the previous crystallographic determinations of cation site occupancies in this zeolite are invaluable. These are collated in Tables I and 11. The data set shows some scatter; however, the most consistent set in view of the far-IR spectra (vide infra) locate 7-8 cations
form
in site A, 3 in site E, and the remainder in site H.8 Cations located at site A (denoted Na+(A)) can be compared to those occupying site I1 in zeolite Y , since both are located at (8) Pluth, J. J.; Smith, J. V. J . Am. Chem. SOC.1980, 102, 4704. (9) Reed, T. B.; Breck, D. W. J . Am. Chem. Soc. 1956, 78, 5972. (10) Howell, P. A. Acta Crystallogr. 1956, 13, 737. ( 1 1 ) Broussard, L.; Shoemaker, D. P. J . Am. Chem. SOC.1960,82, 1041. (12) Smith, J. V.; Dowell, L. G . 2.Krisrallogr. 1968, 126, 1. (13) Yanagida, R. Y.; Amaro, A. A,; Seff, K. J . Phys. Chem. 1973, 77, 805.
(14) Subramanian, Y . ;Seff, K. J . Phys. Chem. 1977, 81, 2249.
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988 6019
Alkali- and Alkaline-Earth-Metal LTA Zeolites
h Figure 4. Representation of the geometry of a cation in site E illustrating the internal coordinate definitions used in the frequency and intensity calculations.
A
B
C Wavenumber Figure 3. Curve-resolved spectrum of dehydrated Na-LTA: (A) observed; (B) simulated by the curve analysis; (C) curve-resolved spectrum. The symmetry type and site giving rise to the absorption are illustrated. F refers to a vibration of the framework.
the six-ring interface between the a- and &cages. The Na+(II) ions produce a band at 189 cm-’ in the far-IR spectrum of NaS6-Y. The higher framework charge in zeolite A, (Si/Al = 1 .O for A, %/A1 = 2.5 for Y), results in a blue-shifted vibrational frequency for all related cation vibrations. Thus the most intense far-IR band in Figure 2 at 216 cm-’ is assigned to the Na+(A) vibration. Confirmation of this assignment stems from the curve resolution of the Na12-A spectrum, shown in Figure 3. With use of intensity the relative intensity expressions for a C3,Na(O), qua~imolecule,~ of the asymmetric E to the symmetric A, Na-O stretching modes can be written (sin2 cu/2)(pL,+ pNa[l - cos a]) z(E)’z(A1) = (3
- 4 sin2 a/2)(p0
+ pNa[l + 2 cos a])
(1)
This expression relates the observed intensities of these two modes to the geometry of the cation site and the reciprocal masses of sodium and the framework oxygens. For a pyramidal type site, with a positive stretchstretch interaction force constant, one can reasonably associate the high-frequency shoulder occurring around 250 cm-’ with the Al symmetry vibration. The relative intensities of the asymmetric E and symmetric A, vibrations were obtained
from the curve resolution in Figure 3, and using expression 1 results in a value of 113’ for the 0-Na-0 bond angle (a). The crystallographically determined value is 119.2’.* Therefore, the most intense band occurring at 216 cm-’ is assigned to the most populous cation, namely, Na+(A). Additional support for this assignment is apparent in the far-IR spectra of progressively Li+-ion-exchanged Na-A. Crystallographic determination^'^ have shown that the location of Li’ in the LTA lattice is restricted to the six-ring sites. In Figure 2 the spectra of dehydrated Na12-A, Na7,2Li4,7-A,and Na6,&iS19-Aare displayed. Here it is observed, that upon progressive loading with Li+ ions, the Na+(A) vibration first decreases in intensity, followed by changes in the cation bands in the low-frequency region. It has been shownSb that Li+-ion-stretching modes occur in the mid-IR and related framework modes occur as very weak bands in the far-IR. None of the observed far-IR vibrations of Na7,2Li4,7-Aand Na6,0LiS,9-Aare therefore directly related to Li+ cation modes. The siting of Li+ ions in the LTA six-rings therefore proceeds at the expense of the Na+(A) ions, signalled by the decrease in the relative intensity of the 216-cm-’ band with respect to the other cation site modes. This is convincing confirmatory evidence for the Na+(A) assignment. In Na12-A, site E is the second most populous, accommodating about three ions per pseudo-unit cell. On the basis of this population, one would expect it to give rise to the second most intense band. It is therefore tempting to assign the absorption at 176 cm-’ to Na+(E) ions. However, the unusual coordination geometry of these ions requires a detailed consideration of the frequency and intensity of the vibrations expected for this kind of site. A cation bound at site E, as shown in Figure 4, is interacting with two 0 1 and one 0 2 lattice oxygens in local C2, symmetry. With the geometry shown in Figure 4, the following stretching vibrations are seen to be infrared active: FI(A1) = (1/21’2)(Arl + Ar2)
(VI)
Of these three vibrations, the asymmetric B1mode is anticipated to be the most intense, and therefore this vibration is assigned to the absorption at 176 cm-I. Of the remaining two modes of A I symmetry, it is likely that one is strong and the other weak (inphase and out-of-phase coupling). The band at 142 cm-’ in Na12-A (Figure 2A) is due to one of these modes. In the Li,Na-A spectra this band remains upon Li+ exchange, exhibiting only a slight frequency shift to higher energy. Further evidence for the Na+(E) assignment is presented in the Appendix, where a detailed analysis of the intensities of the two E site AI symmetry modes is carried out. This calculation results in the following intensity ratio for the two totally symmetric site E stretching modes: Z(vz)/Z(vl)
= 6.05
Therefore, of the two A, Na+ cation modes
(2) u2 is
expected to be
(15) Jirak, Z . ; Bosacek, V.; Vratislav, S.; Herden, H.; Schollner, R.; Mortier, W. J.; Gellens, L.; Uytterhoeven, J. B. Zeolites 1983, 3, 255. (16) Leung, P. C. W.; Kunz, K. B.; Seff, K.; Maxwell, I. E. J. Phys. Chem. 1975, 79, 2157.
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The Journal of Physical Chemistry, Vol. 92, No. 21, 1988
Baker et al. TABLE 111: CrystallographicallyIdentified Cation Sites and Populations for K-LTA'
site form
hyd deh
A
C
E
G
3.0 3.0 2.0
0.1
1.0
6.0
2.0
1.4
4.4 Na
6.3 3.6
deh deh
B 8.0
H r e f 16 16 0.3 17b 0.6 17a
"The numbers in this table refer to the number of cations per pseudo-unit cell; deh is dehydrated; hyd is hydrated. The last entry in the table is for a K,,5Na, ,-LTA sample. W 0
z a
m LT 0
cn m
a
Wavenumber
>
3AJ
260
do
I
WAVENUMBER /cm-'
Figure 5. Far-IR spectra recorded at room temperature of dehydrated (A) Na,,-A, (B) Na4Kg-A, and (C) K12-A. more intense, and the far-IR band at 142 cm-l is assigned to this symmetric mode. The lower intensity symmetric counterpart vibration is found in the computer-resolved curves (see Figure 3) as a weak band at about 116 cm-'. Finally, the band at 110 cm-I in Na12-A is assigned to the minority cation in site H on the basis of its low population and its local environment. The local symmetry of Na+(H) resembles that of site 111' in zeolite Y. In this case, an absorption occurring at 90 cm-I is assigned to this cation. Therefore, the far-IR spectrum of sodium-zeolite A has been satisfactorily assigned in terms of sodium ions occupying A, E, and H sites in concert with the existing crystal structures of this material. This has been achieved by using a combination of selective ion exchange in conjunction with the frequency and intensity expressions developed p r e v i o ~ s l y . ~With ~ ~ J ~this information as a reference point the far-IR spectra of other alkali- and alkaline-earth-metal-exchanged LTA zeolites are now assigned. Potassium-Exchanged LTA. The locations of extra-framework potassium ions are now addressed through the eyes of the far-IR spectrometer following progressive replacement of Na+ ions with K+ ions, with the Na' ion assignments as a reference point in the vibrational analysis. The spectra of dehydrated Na12-A (4A), K8Na4-A (3A), and Kll.6Nao,4-Azeolites are shown in Figure 5, and the cation locations and populations of potassium ions in samples comparable to those studied here are collated in Table 111. The far-IR spectrum of the 3A sample (Figure 5B) exhibits a strong band at 213 cm-', which as described earlier is associated with Na+(A) ions. In concert with the crystal structure we assume that in our samples, all of the sodium ions occupy this site. Therefore, all of the other bands in the spectrum are assigned to potassium ion vibrations. Of these, the most intense at 179 cm-' is attributed to K+(B). Indeed, it is not only the intensity of this band that leads to this but also its frequency in comparison to the band observed for sodium ions in the six-ring site. Therefore, with use of the expressions developed p r e v i ~ u s l y the , ~ ~relative ~ frequencies of potassium and sodium ions residing in the same
Figure 6. Curve-resolved spectra of Na,K-LTA: (A) observed spectrum of Na4K8-A;(B) simulated spectrum of Na4K8-A; ( C ) curve-resolved spectrum of Na4K8-A;(D) observed spectrum of KI2-A; (E) simulated spectrum of KI2-A; (F) curve-resolved spectrum of KI2-A. The symmetry type and site giving rise to the absorptions are illustrated for the sodium and potassium ions. site can be determined. In this calculation, the crystallographic data used were cation-oxygen bond lengths (Na+(A) = 2.357 A, K+(B) = 2.495 h;) and 0-M-O angles (K+(B) = 1 loo, Na+(A) = 1 20°).'7a The result is to expect the K+(B) band at 173 cm-'. Comparison to the fully exchanged material K12-A (Figure 5C) confirms this assignment (see later). The weak low-frequency shoulder of this band is assigned to the symmetric A I infraredactive vibration. The remainder of the far-IR spectrum consists of a poorly resolved absorption below 100 cm-', which in the curve resolution (Figure 6) is shown to probably contain two bands. This observation, coupled with the fact that the pore opening of K8Na4-A is more constrained than in Na-A, suggests that this absorption originates from a pore-blocking K+ ion in site E. In the same manner described for K+(A), the frequency of this mode relative to Na+(E) can be calculated. The asymmetric BI symmetry vibration is therefore expected at 102 cm-', in good agreement with the observed absorption at 98 cm-I. The most intense symmetric mode of K+(E) is assigned to the band at about 72 cm-'. It is interesting to note that the intensity ratio in Na-A of the two most intense (asymmetric and symmetric) site E Na+ vibrations is Z(Bl)/Z(Al) = 2.1, while for KY in the KBNa4-A material this ratio is 1.7. This difference might be related to an angular difference between Na+(E) and K+(E) originating in the more constrained size of the 3A pore, although differences in bond transition dipole moments and force constants could also contribute to the observed effect. The far-infrared spectrum of essentially fully exchanged K-A is now discussed. The data (Figure 5C) clearly indicate that the Na+ ions have been replaced since the Na+(A) band at 21 3 cm-' (see Figure 5B) has vanished. The dominant feature of the K-A spectrum is a broad intense absorption at 172 cm-', assigned to (17) (a) Adams, J. M.; Haselden, D. A. J . Solid State Chem. 1983, 47, 123. (b) Pluth, J. J.; Smith, J. V. J . Phys. Chem. 1979, 83, 741.
Alkali- and Alkaline-Earth-Metal LTA Zeolites
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988 6021
W V
z
a m a 0 v1
m CI
WAVENUMBER 3 b
200 103 WAVE NUMBER
Figure 7. Far-IR spectra of (A) hydrated and (B) dehydrated Rb,Na-A.
the majority occupied K+(B) ions (6.3 K+ ions, Table 111). Locating the absorptions due to site C potassium ions produces a dilemma. Either the site C absorption occurs within the broad intense 172-cm-l band or it gives rise to the band at 120 cm-'. In the former case, the 120-cm-I band must be ascribed to the site G ions. It seems too intense to be so, judging solely upon its population, and therefore we prefer to assign this band to K+(C). Further progress on this problem is anticipated from our current studies on K+-ZK4. Of the remaining cations in K12-A the most populous site is that of the important pore-blocking K+ ions in site E (see Table 111). In K8Na4-A bands occurring at 98 and 72 cm-I are assigned to the B1 and A I type vibrations, respectively, of K+(E). In K12-A absorptions at 77 and 61 cm-I are attributed to these same vibrations of K+(E). The red shift of these bands is indicative of either a force constant or a geometry change of this site in KI2-A relative to K8Na4-A. If the differences are purely angular in origin and do not involve changes in the stretching force constants, then the 01-K-01 angle for this site in K8Na4-A can be estimated by using the frequencies of the respective B, modes and the bond angle in K12-A, which is known from X-ray crystallography to be 102.5O.17b This calculation leads to a value of 110.5O for K+(E) in K8Na4-A, which suggests that the potassium ion in the fully exchanged material is held less strongly in site E and therefore projects further into the eight-ring window, implying that the pore size would be slightly smaller. Of the remaining cations in K12-A, there are 0.3 K+ ions per pseudo-unit cell in site H and 0.15 K+ ions per pseudo-cell in site G. As discussed earlier the assignment of site G is equivocal; however, we prefer to associate this with a very weak low-frequency absorption. The similarity of this ion to those in site H makes it difficult to unequivocally identify it. The bands due to K+(H) are either overlapped by the K+(E) bands around 77-60 cm-I or occur at still lower frequencies. Rubidium-Exchanged LTA. The far-IR spectra of hydrated and dehydrated rubidium-exchanged Na-A are shown in Figure 7, with the curve-resolved spectrum of the dehydrated sample shown in Figure 8. The absorption bands due to the two cocations are easily distinguished. The higher frequency bands between 220 and 150 cm-' indicate the presence of Na+ ions, whereas the lower frequency absorptions signify the Rb+ ions. The division of the spectrum is apparent in both the spectra of the hydrated and dehydrated forms and is a consequence of the large mass difference between the ions. In the spectra of the hydrated material, the absorption bands are probably due to the collective motion of the aquated cation in the a-cages. Upon dehydration
Figure 8. Curve-resolved spectra of Rb,Na-A: (A) observed; (B) simulated; (C) resolved. The symmetry type and site giving rise to the assigned absorptions for sodium and rubidium ions are illustrated.
of the zeolite, the bands sharpen and give rise to absorptions characteristic of Na+(A) and Na+(E) as seen earlier at 217 and 180 cm-l, respectively (see Figure 2). Concomitantly, the lower frequency bands undergo a similar transformation. The similarity of the Na+ and Rb+ band shapes indicates that both ions are bonded in similar locations. Indeed this is in general agreement with the published crystal structures of Rb+-ion-exchanged A type zeolites. The crystallographic datal8 have shown that Rb+ ions are located in sites B/C and an unusual version of site E, the nature of which has been vigorously debated. Firor and Seff18state that this ion located in the eight-ring is "zero-coordinate", where the closest approach to a framework oxygen was purported to be 4.35 A. More recent determinations have shown the cation to be normally coordinated,8 and in what follows, the Rb+(E) ions will be considered to be coordinated at more normal distances and similar to those discussed above for Na12-A, K8Na4-A, and KI2-A. In light of the far-IR spectra for RbNa5-A this approach appears to be quite reasonable. The strong absorption at 98 cm-l in Figure 7 can be compared to those observed in Rbs6-X and Rb56-Y, in which site 11 ions are assigned to far-IR bands at 97 and 94 cm-', respe~tive1y.l~ This indicates that Rb+(B) and Rb+(C) contribute to the 98-cm-' band. The Rb+(E) B1 symmetry mode is assigned to the broad band around 54 cm-l. This agrees well with the frequencies observed for the potassium and sodium ions in this site. The calculation of the expected Rb+ band position based upon those observed for potassium and sodium gives values of 63 and 54 cm-I, respectively. The origin of the slightly better agreement between the observed and calculated Rb+(E) B, frequency based on Na+(E) compared to K+(E) cannot unambiguously be ascribed to any particular force constant or angular change at the cation site. The cause is probably an admixture of both effects. In summary, the far-IR absorptions of dehydrated Rb,Na5-A are assigned to cation occupancy of 3-fold sites (Na+(A) and Rb+(B/C)) and to occupancy of site E by both ions. The frequencies of the Na+ and Rb+ modes fall in distinct regions. This observation indicates that monitoring intensity changes in an extensive series of Rb,Na-A samples with varying cation complements in the unit cell would not be difficult. A study of this type would then allow for a firm statement to be made regarding the populations of the different ions in their extra-framework (18) Firor, R. L.; Seff, K. J . Am. Chem. SOC.1976, 98, 5031. (19) Godber, J. Ph.D. Thesis, University of Toronto, 1987. (20) Olken, M. M.; Baker, M. D.; Godber, J.; Helwig, K.; Ozin, G. A., unpublished results.
6022
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988
Baker et al.
w V
z 4
m E
0 Ln
m
a
A
2bo
I
W A V E N U M BER
W A V E N U M BER
Figure 9.
Far-IR spectra of (A) hydrated and (B) dehydrated
Ca4Na4-A. positions. In conjunction with adsorption experiments on these materials one would have a powerful method for investigating and optimizing the sieving properties of the samples with respect to a specific separation process. In this regard, one can state that the pore size of the dehydrated Rb,Na-A sample discussed above would be more constrained than in the parent Na12-A zeolite, on the basis of the observation of a substantial far-IR absorption due to Rb+(E). Alkaline-Earth-Metal-Ion-Exchanged LTA. Divalent ions are exclusively found in the six-ring sites in LTA zeolites6 In the divalent exchanged zeolites the absorption bands due to sites B and C display a larger frequency separation than typically seen in the monovalent forms. This is found to be the case for the Caz+ and Sr2+LTA samples described below. It is important to recognize that the presence of divalent ions in extra-framework positions of LTA (Si/Al = 1.0) has been shown by crystallographyz1 and z7Almagic-angle spinning (MAS) NMR22to lead to the formation of a nonframework aluminate species. The detection of electron density at the center of the sodalite cage in dehydrated CaZ+-and Sr2+-ion-exchanged LTA samples is assumed to be the result of partial occupancy by a disordered A104* species (the asterisk indicates that the exact nature is unknown2'). The absence of this moiety in monovalent exchanged samples has been demonstrated by the MAS-NMR techniquez2suggesting that the partial dealumination of the lattice occurs either during the introduction of the divalent ion or during the subsequent dehydration. Thus, the far-IR spectra of divalent exchanged LTA may well reflect this process. The partial removal of aluminum from the framework may result in the observation of an aluminate absorption in the far-IR similar in origin to that observed for the translatory modes of encapsulated anions such as C104- in sodalite.'9,z7 In addition, the presence of an aluminate ion in the vicinity of extra-framework cations might be expected to perturb the cation frequencies to some extent. Also, the presence of the aluminate species means that the number of cations needed to balance the framework charge is lowered somewhat. The above effects therefore may result in some intensity loss, frequency shifts, and broadening of the far-IR cation translational modes. Calcium-Exchanged LTA. This discussion first addresses the far-IR spectrum of the commercially important zeolite 5A (Ca4Na4-A), followed by an analysis of the fully exchanged Ca6-A. The crystallography of Ca4Na4-A demonstrates that the cations are located only in 3-fold sites, on the six-rings of the sodalite cage^.^^^^^ The divalent Ca2+ ions reside near the plane (21) (a) Pluth, J. J.; Smith, J. V. J. Am. Chem. Soc. 1983, 105, 1192. (b) Pluth, J. J.; Smith, J. V. J . Am. Chem. Sor. 1982, 104, 6977. (22) Corbin, D. R.; Farlee, R. D.; Stucky, G . D . Inorg. Chem. 1984, 23, 2920.
Figure 10. Far-IR spectra of (A) hydrated and (B) dehydrated Cas-A; (C) observed spectrum of dehydrated Ca6-A; (D) simulated spectrum of Ca6-A; (E) curve-resolved spectrum of Ca6-A. Note that the site B A, mode probably lies within the framework vibration.
of the six-ring (site A), while the sodium ions are displaced into the supercage (site B). Neutron diffraction studies of Ca4Na4-Az3 have also shown that the six-rings containing the Ca2+are severely distorted and that the Ca2+-03 bond length is quite short, at 2.287 A. The far-IR spectra of the hydrated and dehydrated Ca4Na4-A sample are shown in Figure 9. Apart from the framework absorption at 260 cm-', two other bands dominate the spectrum. Recalling that Na+(A) is identified in Na12-A by the presence of an absorption at 217 cm-' and that ions in sites B/C are found at lower frequencie~,~ one can then logically associate the band at 191 cm-I with Na+(B) ions. When the zeolite is hydrated, the Na+(B) vibration occurs at 184 cm-' and is accompanied by an absorption at 227 cm-I. Following dehydration, the latter band shifts to 237 cm-I and appears as a distinct shoulder on the framework mode at 260 cm-I. The shoulder at 237 cm-I is attributed to Ca2+(A)cations. Support for these assignments is based on the following arguments. The longer Na-03 bond length and lower 03-Na-03 bond angle in Ca4Na4-A compared to Na12-A both result in a lower vibrational frequency for the Na+ ion vibration in the former. With use of a local oscillator approximatior~,~-~J~ a frequency of about 207 cm-I is expected for the Na+(B) ions, consistent with the observations. The high Ca2+ frequency may be initially surprising since the mass of Ca2+ is nearly twice that of Na+. However, the 2+ charge appears in the cation frequency relat i o n s h i p ~ 'as ~ a 2'12 factor compared to 1+ monovalent counterparts. Furthermore, both the bond length of 2.28 A (cf. Na+(A) = 2.3 A) in Ca4Na4-A, and the distortion of the six-ring also result in a strong interaction of the Caz+ions with the lattice. Together, these effects will lead to a high vibrational frequency for CaZ+(A). In fact the distortion is such that the Ca2+ ion is nearly in the plane of the 0 3 oxygens of the six-ring. This distortion is such that it is very difficult to obtain frequency comparisons by using the local oscillator approximation. However, the Ca2+(A) assignment is in good agreement with the far-IR spectra of Ca-X and Ca-Y f a ~ j a s i t e s ~and . ~ ~is' ~supported by the observations for fully exchanged Ca6-A, which is described below. The far-IR spectra of Cab-A are shown in Figure 10 for the hydrated and dehydrated forms. Once again, the crystallographic dataz'= on samples similar to those described here are invaluable in reaching a consistent assignment of the far-IR spectra. In the dehydrated form, 4.4 Ca2+ions per pseudo-unit cell locate in site B, (Ca2+-03 = 2.27 A), almost in the plane of the 0 3 oxygens of the six-ring, and 1.2 Ca2+ions locate in site C within the sodalite cage (Ca2+-03 = 2.32 A). The far-IR band at 198 cm-' (Figure 10) is ascribed to the Ca2+ion with the longer bond length, namely, site C. Thus, assuming that the high-frequency shoulder apparent in the curve-resolved spectra (Figure 10E) is the symmetric counterpart vibration of CaZ+(C),one can calculate a 0-Ca-0 (23) Adams, J. M.; Haselden, D. A. J . Solid State Chem. 1984, 51, 8 3 .
Alkali- and Alkaline-Earth-Metal LTA Zeolites
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988 6023 1
'
1
'
1
'
T
10.1 A b s .
A
Ca"C
w
u
W
2
0
z
D
a
a
m
a
m iT
0
0
m
v)
(0
a
m CI
i
C
,i
E
'
'
I
3 b ' 2d0 100 WAVENUMBER
'
Figure 12. Far-IR spectra of (A) hydrated and (B) dehydrated Sr6-A. Figure 11. Far-IR spectra of dehydrated Ca,Li-A showing the progressive replacement of Ca2+(B)at ca. 230 cm-' by Li': (A) Ca6-A; (B) Ca5,5Lil-A;(C) Ca5,0Li2,0-A; (D) Ca4,6Li2.8-A.
bond angle of 97.7O for this pyramidal site. The estimate from the curve-resolved far-IR spectrum is probably in error due to pronounced band overlap in this region of the spectrum (Figure 10). The Ca2+(C)asymmetric stretching frequency and the bond length and pyramidal angle of Ca2+(B) from crystallography indicate that the CaZ+(B)asymmetric vibrational will occur at 250 cm-I. This fits quite well with the assignments arrived at for the Ca4Na4-A sample (vide supra). Note that also a band is evident in this region in the curve-resolved spectrum of Ca6-A (Figure 10E). The observation of a band at this frequency in the curve-resolved spectrum of Ca6A suggests that this site is indeed occupied in this sample. However, the framework vibration partially obscures the cation mode in question. Nonetheless, evidence in support of this assignment (Ca2+(B) = 245 cm-I) comes from modification of Ca6-A by ion exchange with Li+. Recall that Li+ ions occupy the six-ring sites and that their vibrational signatures are found above 350 Progressive exchange of Ca6-A with Li+ ions followed by dehydration yields a series of Ca6,.2Li,-A samples, the far-IR spectra of which are shown in Figure 11. Thus, the observation of a decrease in the intensity of the high-frequency vibration around 250 cm-l relative to that of Ca2+(C)indicates that a band due to Ca2+(B) exists within the bandwidth of the intense framework absorption. Following three Li+ exchanges, resulting in a sample with a cation complement of Ca4,6Li2,,-A, it is apparent that the framework band at 260 cm-l persists, with an intensity comparable to that of the Ca2+(C)vibration, the latter having shifted slightly to 181 cm-I. As a cautionary note, one should keep in mind that the framework vibrations between 350 and 230 cm-l do exhibit some cation-sensitive intensity and frequency effects. Thus, this assignment must remain equivocal. Strontium-Exchanged LTA. The far-IR spectra of hydrated and dehydrated Sr,-A are displayed in Figure 12. The form of the spectra is remarkably similar to those of Ca6-A. This is not surprising since the location and populations of Sr2+ ions are identical with Ca2+in Ca6-A.'lb The Sr2+ions are located in the six-rings, with 4.5 ions per pseudo-cell in site B and 1.2 ions per pseudo-cell in site C. The Sr6-A framework and cation modes can then be interpreted in terms of the Ca6-A data discussed above. Together the Ca2+- and Sr2+-LTA zeolites yield a consistent picture of the divalent cation far-IR vibrational frequencies
and are useful models for the investigation of transition-metal (2+)-LTA systems. There is indeed internal consistency between the spectra of these two materials. The frequencies observed for the Sr2+and Ca2+ ions in sites B and C are well described by the frequency expressions described in ref 3, 4, and 19. In this approach, the differences in the spectra are a manifestation of the different masses and ionic radii of the ions that appear in the G matrix term of the calculation. Thus, the Sr2+modes are expected at 175 cm-I (B) and 145 cm-' (C), which agree well with the observed bands at 182 and 142 cm-I. It should be noted that an extension of these investigations to Ba-A is not possible due to the instability of this zeolite to dehydration.24 A Note on the Dealumination of Divalent-Ion-Exchanged LTA. Calcium- and strontium-exchanged LTA zeolites are known to undergo partial removal of aluminum ions from the framework to form an aluminate species, the exact nature of which is presently undefined. This transformation has been observed by X-ray crystallographic techniques as well as 27Aland 29SiMAS NMR. The origin of the effect is thought to be related to the hydrolysis reaction observed in divalent-cation-containing zeolite^.^^^^'+^^ In the far-IR spectra of the samples discussed above (Figures 9-12), it is perhaps noteworthy that a weak, broad unassigned absorption is observed at about 100 cm-I. Assuming that the translatory motions of a sodalite cage entrapped aluminate ion are similar to those observed for sodalite-encapsulated perchlorate anions (that is, 145 and 63 cm-I, assigned to a correlation split translatory m ~ d e ' ~ ,then ~ ~ )it, is possible that the low-frequency absorption around 100 cm-' is due to an extra-framework aluminate vibration. Alternatively, this far-IR absorption could be attributed to the ~ , ~ ~ in a cattranslational motion of a hydroxide a n i ~ n ' formed ion-induced hydrolysis reaction. Since this absorption is already present in the hydrated form, it is possible that dealumination (or hydrolysis to possibly form a Ca2+ OH- species) proceeds even upon ion exchange of the divalent cation into the lattice. Clearly, further work is required to clarify this aspect of the spectra. Conclusions The far-IR spectra of a series of ion-exchanged alkali- and alkaline-earth-metal LTA zeolites have been described. The (24) Kim, Y . ;Subramanian, V.; Firor, R. L.; Seff, K. ACS Symp. Ser. 1980, No. 135, 137.
6024
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988
assignments of the spectra have been addressed in terms of cation-related bands diagnostic of specific extra-framework binding sites. With utilization of a local oscillator approximation for describing the cation vibrational properties, in conjunction with selective ion-exchange techniques, a consistent set of vibrational assignments for the cation translatory modes has been compiled. The requirement of a reliable basis set from which to make comparisons is crucial in this type of study. In this regard, the full unit cell cation dynamics of Na-A has been determined and compared in detail to the observed far-IR ~ p e c t r u m .It~ is still difficult at this stage, however, to separate or gauge the effect of both geometry and force constant changes of the cation site on the far-IR spectra. Further progress in this regard is anticipated from complementary Raman data. 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 is greatly appreciated. We are also indebted to Dr. Edith Flanigen (Union Carbide) for supplying various high-crystallinity and ultrahigh-purity A-zeolites, as well as for invaluable technical assistance. We also acknowledge numerous enlightening discussions with Alex Kupperman during the preparation of this paper.
Appendix Intensity Analysis for Cations in Site E of LTA. The intensity analysis of the site E vibrational modes mentioned in the body of the paper is now outlined. With use of the intensity expressions25 in eq 3 and 4 simultaneous equations are assembled and solved for the two coupled A I symmetry modes:
Baker et al. and FKKt-Iare the corresponding matrix elements. The symmetry coordinates for the three infrared-active modes (see Figures 2-4) are
+
S, = ( 1 / 2 ' l 2 ) ( A r 1 Ar,)
(5)
S2 = AR
(6)
S3 = ( 1 / 2 ' l 2 ) ( A r I- Ar,)
(7)
Symmetrized G and F matrices are generated from their unsymmetrized counterparts by applying the U and U+ internal coordinate matrices,25yielding eq 8 and 9. Gym
=
+, + p I ( i + COS e) 21'2p1
2 1 / 2 ~cos , 8/2
cos 8 / 2
w1
(o
+ Pz
Faym=
2''tfiR
0 p2 + p,u - COS
0
( : + f i r
o
:'yrR
)
o0
e)
fR
1
(8)
(9)
fi - f i r A computer-aided force constant fitting routine26 applied to the three observed cation translations (2Al + B,) using the standard secular equation IGF,- XI = 0 and the crystallographically determined value of 0 = 129.8' yields the best fit M-0 bond stretching and stretch-stretch interaction force constants: fR = 0.1 lOO,f, = 0.129,fm = -0.O1245,LR = 0.01404 mdyn A-' and the predicted ordering of the M-0 stretching modes
q ( B , = 176 cm-I)
> v2(AI = 142 cm-I) > vl(Al = 116 cm-I)
The F1matrix can then be evaluated from Fsym,yielding 8.852
where I k is the intensity of the kth absorption band, hk = 0 . 5 8 9 1 5 ( ~ / 1 0 0 0(&/SF) )~, is the bond transition dipole moment derivative with respect to the kth symmetry coordinate, and GKKt (25) Wilson, E. B.; Decius, J. C.; Cross, P. C. Molecular Vibrations; McGraw-Hill: New York, 1955. (26) McIntosh, D. F.; Peterson, M. R. Programs No. 342, QCPE, Room 204, Department of Chemistry, Indiana University, Bloomington, IN 47401. (27) Ozin, G . A,; Godber, J. J . Phys. Chem., in press.
F-L( 1.598 0
1.598 9.319 0
0 0 8.337
1
(13)
The expressions shown in eq 3 and 4 can now be evaluated and yield Is,/Is, = 6.05 Registry No. Li, 7439-93-2; Na, 7440-23-5; K, 7440-09-7; Rb, 7440-17-7; Ca, 7440-70-2; Sr, 7440-24-6.
