354
J. Phys. Chem. 1988, 92, 354-357
Raman Spectroscopy of Zeolite A: Influence of SVAI Ratio Prabir K. Dutta* and Beatriz Del Barco Department of Chemistry, The Ohio State University, Columbus, Ohio 4321 0 (Received: June 15, 1987)
A series of zeolites, all having the same cubooctahedral structure of zeolite A, but with Si/A1 ratio varying from 1.O to 2.7, were examined by Raman spectroscopy. The dependence of the framework vibrational frequencies on the Si/AI ratio was examined. Previous infrared data and normal-mode calculations along with the present Raman data are used to provide a consistent set of vibrational assignments.
Introduction Zeolites are an important class of aluminosilicate materials of general composition M,.,[(A102)x(SiOz)y]oHz0. Its framework structure is made up of corner-linked A104 and S i 0 4 tetrahedra, and the aluminum atoms can be neutralized by mono-, di-, or trivalent cations. Many framework geometries can be formed and the synthesis of new zeolitic structures is a constant challenge for zeolite chemists.' The actual determination of the framework structure and bonding and how these influence the catalytic and other properties of these materials is of considerable interest. Single-crystal X-ray diffraction studies have usually been the source of definitive structural information on zeolites." However, since it is difficult to grow large enough crystals for X-ray studies, many spectroscopic probes, including EPR, NMR, EXAFS, and infrared spectroscopy, have been developed to provide information about framework structure and the siting of cation^.^-^ Infrared spectroscopy is probably the most extensively used and the data analysis is based on a set of empirical correlations between structural units and framework vibrational bands.9 In order to improve on this, exact band assignments corresponding to specific lattice normal modes is required. The complexity of these systems necessitates that vibrational data for isotopic and chemically modified zeolites be available before any reasonable calculations can be attempted. A complementary data base in the form of Raman spectroscopic data is also very valuable in the analysis of the vibrational motion of these solids. Raman spectroscopy has been extensively used to study the structure of aluminosilicate glasses and minerals,I0 but only recently has been used in studying zeolitic structure."JZ This paper reports on Raman spectroscopic studies of zeolite A, in which the Si/Al ratio is varied from 1.0 to 2.7. The variation of the Raman spectra as aluminum atoms are replaced by silicon atoms without any modification of the framework structure is examined. The data analysis is done with the help of previous assignments, literature data on aluminosilicate glasses and minerals, infrared data, and some recent lattice vibration calculations on the zeolite A structure. Experimental Section The synthesis of ZK-4 samples was carried out using the (1) Sand, L. B. Pure Appl. Chem. 1980, 52, 2105. (2) Gramlich, V.; Meier, W. M. 2.Kristallogr. 1971, 133, 134. (3) Pluth, J. J.; Smith, J. V. J. Phys. Chem. 1979, 83, 741. (4) McCurker, L.; Seff, K. J . A m . Chem. SOC.1981, 103, 3441. (5) Vedrine, J. C. Chnracierizotion of Heterogeneous Coralysrs; Delanny, F., Ed.; Mace1 Dekker: New York, 1984; p 161. (6) Fyfe, C. A,; Thomas, J. M.; Klinowski, J.; Gobbi, G. C. Angew. Chem. 1983, 22, 259. (7) Morrison, T. I.; Iton, L. E.; Shenay, G. K.; Stucky, G. D.; Suib, S. L. J . Chem. Phys. 1981, 75,4086. (8) Flanigen, E. M. Zeolite Chemistry and Catalysis; Rabo, J. A,, Ed.; ACS Monograph No. 171; American Chemical Society: Washington, DC, 1976; p 80. (9) Flanigen, E. M.; Khatami, J.; Szymanski, M. A. Adu. Chem. Ser. 1971, No. 101, 201. (IO) McMillan, P. Am. Mineral. 1984, 69, 622. (11) Dutta, P. K.; Del Barco, B. J . Phys. Chem. 1985, 89, 1861. (12) Dutta, P. K.; Del Barco, B. J . Chem. Soc., Chem. Commun. 1985, 1297.
0022-3654/88/2092-0354$01.50/0
TABLE I: Framework Vibrational Frequencies of Zeolite A as a Function of Si/AI Ratio Si/A1 1.0
1.2
1.4
337 410 489 703 738 972 1050 1103
336 412 490 715
338 412 493 727
990
998 1057 1115
1050 1110
2.0 335 412 495 751 1011 1047, 1087 1133
2.7 335 412 496 780 1011 1046, 1093 1139
band assign. double ring double ring v,(T-O-T) double ring T-O(3) T-O(1)
T-O(2)
procedures of Kacirek and Lechert13 and Jarman et al.I4 Cold aging of the parent gels was necessary to obtain zeolites with Si/Al ratio from 1.0 to 2.0. The tetramethylammonium ions trapped in these cages were removed by heating at 450 O C in O2for 40 h. The calcined samples were ion exchanged with 2 M NaC1/0.1 M NaOH solution at 60 OC. It is absolutely essential to remove the last traces of organic impurities from the zeolites, if Raman spectra are to be obtained. The Raman spectra were obtained from hydrated zeolite samples by using radiation at 457.9 nm from a Spectra Physics Ar ion laser (Model 171), a Spex 1403 spectrometer, and a RCA C 31034 GaAs photomultiplier. Slit widths were typically 6 cm-' and laser powers of 50-100 mW at the sample were used. Powder X-ray diffraction patterns were obtained with a Rigaku Geigerflex D/Max 2b diffractometer with Ni-filtered Cu Ka radiation. Refined unit cell constants were obtained by a leastsquares iterative procedure. The elemental analysis (Na, Si, Al) was done with a JEOL JXA-35 electron microprobe using energy dispersive methods. Zeolite A was employed as the standard.
Results and Discussion A series of five zeolites, all with the zeolite A cubooctahedral structure and with Si/A1 ratios of 1 .O, 1.2, 1.4, 2.0, and 2.7, were investigated in this study. The zeolites with Si/A1 > 1 will be referred to as ZK-4 in the present discussion. The powder X-ray diffraction patterns of the calcined and Na+-exchanged forms of these series of zeolites are shown in Figure 1. The unit cell size decreases monotonically from 12.296 A (Si/AI = 1.0) to 12.085 8, (Si/Al = 2.7) as the Si/A1 ratio of the framework increases. This is in agreement with the observations of Jarman et al. on a series of similar ZK-4 ~amp1es.I~ The two factors that contribute to the decrease of the unit cell size are the change in the T-0-T angle and the T-0 average bond length as the aluminum atoms are replaced by silicon. Figure 2 shows the Raman spectra of the series of zeolite samples in the region between 300 and 1300 cm-'. The tetramethylammonium ions trapped in the zeolite cages during synthesis of zeolites with Si/Al > 1 were removed by calcination and (13) Kacirek, H.; Lechert, H. ACS Symp. Ser. 1977, No. 40, 244. (14) Jarman, R. H.; Melchior, M. T.; Vaughan, D. E. ACS Symp. Ser. 1983, No. 218, 267.
0 1988 American Chemical Society
Raman Spectroscopy of Zeolite A ,-
The Journal of Physical Chemistry, Vol. 92, No. 2, 1988 355
-
e
_I
6W
b
=-
I
a
do
m
im
pD0
syu Roo
r b ~
Q& J
im
w
zw
uD
wu mm
Figure 3. Dependence of Raman frequenciesof zeolite A and ZK-4 on Si/AI ratios: (a) 337 cm-l; (b) 410 cm-I; (c) 489 cm-'; (d) 703 cm-I; (e) 972 cm-I; (f) 1050 cm-'; (9) 1103 cm-I.
26 Figure 1. Powder X-ray diffraction patterns of calcined and Na+-exchanged zeolite samples: (a) %/AI = 1.0;(b) Si/Al = 1.2; (c) %/AI = 1.4; (d) Si/A1 = 2.0; (e) Si/AI = 2.7.
x
3-4
0
Raman shift
pdo
(CllT')
Figure 2. Raman spectra of calcined and Na+-exchangedzeolite samples: (a) Si/AI = 1.0; (b) Si/A1 = 1.2; (c) Si/AI = 1.4; (d) Si/A1 = 2.0; (e) Si/AI = 2.7. Laser line = 457.9 nm.
followed by ion exchange with Na+ ions. The bands in Figure 2 therefore arise only from framework modes. In order to ensure a proper comparison, the zeolite A sample (Si/Al = 1.O) was also calcined and ion exchanged under similar conditions, even though no organic cations were present during the synthesis. The Raman spectrum of calcined zeolite A is similar to the uncalcined material except for a shift in frequency of the 1039-cm-' band to 1051 cm-'.I' The X-ray diffraction pattern (Figure l a ) indicates no differences in structure between the calcined and noncalcined
forms. The origin of this shift is unclear. In the discussions below on band assignments, we have considered the calcined zeolite A spectrum (Figure 2a) as the standard for comparison. The observed Raman bands as a function of Si/A1 ratio are summarized in Table I. Framework Vibrational Frequencies as a Function of SiJA1 Ratio. The dependence of the framework infrared frequencies of zeolites on the Si/AI ratio has been recognized for many years. The almost linear dependence of these frequencies on Si/Al ratio has been used to determine the A1 content of the framework for faujasitic ze01ites.I~ The dependence of the Raman frequencies of zeolite A on the corresponding Si/A1 ratios of the framework is shown in Figure 3. The low-frequency bands at 337 and 410 cm-' do not change with the Si/Al ratio. The strong band at 489 cm-' shows a weak dependence on Si/Al ratio. The 700-cm-' band exhibits the most rapid and almost linear increase in frequency with Si/Al ratios. In some zeolite A preparations, two bands at 860 and 900 cm-' are observed (Figure 2c,e). These bands are due to Si-0- stretching modes of terminal Si atoms in the crystal and are not considered as part of the framework bands.25 The three bands in the 900-1 100-cm-l region show a complicated dependence on the %/A1 ratios. It appears, therefore, that the Raman frequencies in general, do not correlate linearly with the Si/AI ratio in any simple fashion, unlike infrared bands. However, it is important to point out that the correlation of infrared bands with %/A1 ratio has been shown to exist only for faujasitic zeolites. The infrared data on ZK-4 is not as exhaustive and this correlation is not as well developed. Band Assignments. It is helpful to consider the empirical band assignments for the mid-infrared zeolites bands as a convenient starting point for analysis of the Raman modes. These mid-infrared bands have been classified into two types of vibration: motion related to the internal vibration of the TO4 framework which are therefore insensitive to variations in framework structure and vibrations related to external linkages between tetrahedra, which are structure sen~itive.~ Even though such a classification may appear simplistic for a complicated solid like a zeolite, it is (15) Pichat, P.; Beaumont, R.; Bathoumeuf, D. J . Chem. SOC.,Furuday Trans. 1 1974, 70, 1402.
356 The Journal of Physical Chemistry, Vol. 92, No. 2, 1988 not unreasonable from the point of view of the solid-state theories that are currently used to explain the vibrational spectrum of tetrahedral glasses such as S O 2 , BeF2, GeS2, and GeSe2.I6 The model, developed by Sen and Thorpe, considers a single-nearest-neighbor central force and predicts that the TXT (X = S, 0, Se, or F) bond angle determines the vibrational coupling between neighboring TX, tetrahedra. At TXT angles of 90°, the system can be described by molecular modes corresponding to those of TX4 tetrahedra, whereas as this angle increases to 180° the coupling between adjacent tetrahedra increases, and a band picture begins to develop. The T U T angle in zeolites varies Over a broad range. In zeolite A, for example, these angles are typically 145' and 160'. The degree of coupling between adjacent tetrahedra in zeolites and the corresponding vibrational spectra are therefore going to be influenced by the specific structure. The assignment of vibrations of zeolites will be aided considerably by normal-coordinate calculations of the lattice modes. However, the difficulty associated with obtaining good force constants and the paucity of data make this a difficult task. There is, however, considerable merit in such calculations, if only to provide an intuitive understanding. Recently, No and co-workers have reported a vibrational calculation of zeolite A based on a pseudolattice method,I7 which is an improvement over the cluster model calculations done earlier by Blackwell.'* We will use the results of these calculations, along with the published infrared data and the previous and present Raman data on zeolite A, to provide a comprehensive picture of the zeolite vibrations. 3 W 4 5 0 cm-l. In the infrared, a band at 378 cm-' is observed. The frequency of this band is sensitive to the Si/Al ratio and has been assigned to motion of the six-membered aluminosilicate ring.17 The Raman spectrum has two bands in this region at 337 and 410 cm-I, which are insensitive to the Si/Al ratio. These Raman bands are largely unperturbed by the presence of different cations, such as Na+, K', and TI', and therefore exclude any significant contribution from the M'-O lattice modes." However, in the presence of Li', which is known to distort the framework structure,12J9the 337-cm-' band splits into two bands at 356 and 381 cm-I and the 410-~m-~ band shifts to 440 an-'.No and co-workers have assigned the 410-cm-' band to the ring-breathing mode of the double ring." The 337-cm-' band can be assigned to the twisting motion of the double ring. Both these modes must primarily involve the motion of oxygen atoms and are therefore insensitive to the Si/Al ratio, but sensitive to the distortions of the double ring (or cube). 450-600 cm-I. In the infrared spectrum of zeolite A, two bands at 464 and 550 cm-I are observed. These bands are sensitive to the Si A1 ratio and have been assigned to vibrations of the double ring.' The strongest band in the Raman spectra of most zeolites is observed in this region and appears at 489 cm-' for zeolite A. The Raman bands in this region for aluminosilicate glasses and minerals have been assigned to motion of oxygen atom in the plane bisecting the T-0-T bond ( V , ( T - O - T ) ) . ~ ~ , ~ We ~ have pointed out that this band is sensitive to the size of the aluminosilicate rings, and appears at 480-520 cm-' for zeolites with predominantly four-membered rings and at 380-400 cm-' for zeolites with five-membered ring.22 No and co-workers have assigned the 489-cm-I band in zeolite A to the four-membered Si2A1204ring.I7 This band only shows a slight dependence on the %/A1 ratio and increases from 489 to 496 cm-I as the .%/A1 ratio reaches 2.7. This again indicates that the vibration involves primarily the motion of the oxygen atom. In faujasitic zeolites, the frequency dependence of the prominent band in this region is the reverse of what is found in the present study. In zeolite X with a Si/Al
4
~
~~
(16) Sen, P. N.; Thorpe, M. F. Phys. Rev. B. 1977, 15, 4030. (17) No, K. T.; Bae, D. H.; Jhon, M. S. J. Phys. Chem. 1986,909, 1772. (18) Blackwell, C. S. J . Phys. Chem. 1979, 83, 3251, 3257. (19) Melchior, M. T.; Vaughan, D. E. W.; Jacobson, A. J.; Pictroski, C. F . Proceedings of the Sixth International Zeolite Conference; Olson, D., Bisio, A., Eds.; Butterworths: London, 1984; p 684. (20) Matson, D. W.; Sharma, S. K.; Philpotts, J. A. Am. Mineral. 1986, 71, 694. (21) Galeener, F. L. Phys. Rev. B. 1979, 19, 4292. (22) Dutta, P. K.; Puri, M. J . Phys. Chem. 1987, 91, 4329
Dutta and Del Barco t
Figure 4. Curve deconvoluted Raman bands in the 900-1300-~m-~region: (a) Si/A1 = 1.4; (b) Si/A1 = 2.0; (c) %/A1 = 2.7.
ratio of 1.2, we find this band at 513 cm-', and with increase of silicon content, the band decreases to 505 cm-I for zeolite Y (Si/Al = 1.8). The nearest-neighbor central force model predicts that, as the average T-0-T angle increases, the frequency of the v,( T U T ) mode decreases.I6 In the case of faujasitic zeolites, the average T-0-T angle increases by 2.4 f 0.5' from hydrated zeolite X to hydrated f a u j a ~ i t e . * ~If- ~this ~ correlation is correct, then the increase in frequency of the zeolite A system as Si/Al ratio increases would indicate a decrease in the average T-0-T angle. No single-crystal diffraction data is available on ZK-4 to support or refute this correlation. 650-850 cm-l. A weak infrared band is observed at 660 cm-' in zeolite A along with a band at 750 cm-' in ZK-4 ~amp1es.l~ The 660-cm-' band has been assigned to a T-O symmetric stretch. In the Raman spectrum of zeolite A, a band at 703 cm-I along with a shoulder at 738 cm-I is observed. This band exhibits the most marked increase in frequency (a range of 80 cm-l) as the Si/AI ratio increases (Figure 3). This is also the only band in zeolite A whose frequency exhibits an almost linear increase with Si/Al ratio. We have previously assigned this band to an A1-0 stretch, based primarily on comparison with aluminosilicate minerals and glasses, which usually show a ,band in this region.'* We have also observed a band in this frequency range in the amorphous aluminosilicate gel present at the early stages of zeolite ~ynthesis.2~No and co-workers, on the basis of their calculations, have assigned this mode to a double ring mode.I7 Examination of their normal modes indicates considerable T-O( 1)-T and 0(3)-T-0(3) bending motion in these vibrations. Their assignment appears to be more accurate for two reasons. Each A1 atom in the zeolite is always surrounded by four silicon atoms and the A1-0 stretch, therefore, should not depend on the Si/Al ratio. Also, in low-silica faujasites, such as zeolite X (Si/Al = 1.2), we do not observe a band in this region. Therefore, it is likely that the 700-cm-I band is characteristic of the double ring present in the zeolite A structure. The presence of Li' as the neutralizing cation of the framework shifts this band to 730 cm-', presumably (23) Olson, D. H. J. Phys. Chem. 1970, 74, 2758. (24) Olson, D. H.; Kokotailo, G. T.; Charnell, J. F. J . Colloid Interface Sci. 1968, 28, 305. (25) Dutta, P. K.; Shieh, D. C. J . Phys. Chem. 1986, 90, 2331.
J . Phys. Chem. 1988, 92, 357-360 due to the distortion of the double ring." 900-1200 cm-l. The infrared spectrum of zeolite A shows a strong band at 995 cm-I, along with weak shoulders at 1050 and 1090 cm".'' These bands have been assigned to T-O asymmetric stretches and are sensitive to the Si/Al ratio. The Raman spectrum of zeolite A shows three well-defined bands in this region at 972, 1050, and 1103 cm-I. We have assigned these bands as arising from Si-0 stretches." On the basis of studies of Li+ partially exchanging into Na12Aand KI2A,we have proposed that these vibrations were localized on motions of the three different types of oxygen atoms in zeolite A. The bands at 972, 1050, and 1103 cm-' were assigned to T-0(3), T-0(1), and T-O(2) stretching motions, respectively.I2 From N o et al.'s calculations of vibrations of zeolite A,I7 it is indeed clear that such a localized motion of only one of the types of oxygen is indeed possible. They calculate a band at 1088 cm-' which is characterized by the asymmetric stretch of the Si2A1,(0(3)), ring with no motion from the other O(1) and O(2) oxygen atoms. Examination of their normal modes also show a calculated frequency at 1134 cm-I, characterized mostly by T-O(2) motion. No normal mode distinguished only by T-O( 1) motion could be discerned from their study. The change in Si/AI ratios have a complicated effect on these frequencies. At Si/Al = 1.2, only the bands at 972 and 1103 cm-' are perturbed. Both these bands exhibit considerable broadening and shifts in frequency to 990 and 1110 cm-I. At this stoichiometry, one of the AI atoms in each sodalite cage is replaced by a Si atom (Si-13, Al-11). The rearrangement of the Si and
357
A1 atoms in this ZK-4 structure has produced no effect on the T-O( 1) vibration. As the Si/Al ratio increases to 1.4, all three bands exhibit further increases in frequency (Figure 3). At Si/A1 = 2.0, the T-O(l) band splits into two bands at 1047 and 1087 cm-I. Figure 4 shows a curve deconvoluted pattern in the 9001200-cm-l region for zeolites with Si/Al of 1.4, 2.0, and 2.7. Band maxima were chosen directly from the observed spectra and the widths and the intensities were varied to get the best fit to the observed data. At Si/Al = 2.7, the 1090-cm-' band decreases considerably in intensity. Examination of the intensities of the bands in the 900-1200-cm-' region indicates that the high-frequency component at 1100-cm-' region gradually gains in intensity at the expense of the 1050-cm-l band as the Si/Al ratio increases. It is difficult, at present, to relate these spectral changes to structural features of zeolite A. However, it is clear that the Raman spectra are sensitive to the arrangement of the Si and A1 atoms and the coupling between them. Considerably more data in the form of isotopic replacements, chemical modification, and normal-mode calculations are required before an exact correlation of the vibrational spectra apd zeolitic structure can be made. However, as this study, along kith the calculations of No et al., shows,17 there is much to be gained from these continuing studies on the vibrational spectra of zeolitic crystals.
-
- -
Acknowledgment. We gratefully acknowledge the support provided by the National Science Foundation (CHE-8510614). We also thank Professor J. W. Downs for the cell refinement computer program.
Absorption Spectra of Intermolecular Charge-Transfer Transitions between Xenon and Halogen Molecules (F,, CI,, Br,) in Liquid Xenon Mario E. Fajardo, V. A. Apkarian,* Department of Chemistry, University of California, Imine, California 9271 7
Antonis Moustakas, Herman Krueger, and Eric Weitz* Department of Chemistry, Northwestern University, Evanston. Illinois 60201 (Received: June 16, 1987)
The intermolecular charge-transfer transitions between molecular halogens, X2, and Xe can be observed in UV spectra of X2:Xe liquid solutions (X, = F2,Cl,, Br,). In all cases the spectra occur near the UV cutoff of the spectrometer (187 nm). The absorptioncoefficients at 200 nm are 6.9 X 1.2 X 1O-Is, and 2.0 X lO-" an2for Xe:F,, Xe:CI,, and Xe:Brz, respectively. Implications with respect to one-photon- and two-photon-induced cooperative photoproduction of rare gas halides in gas and condensed phases are discussed.
Introduction Gas-phase laser-assisted reactions of halogens, X2, with rare gas atoms, Rg, to yield the rare gas halides kg
$:
X2
+ hv
-+
Rg'X2-
-+
Rg'X-
+X
(1)
(1) Yakovlenko, S.I. Sou. J . Quantum Electron. (Engl. Transl.) 1978,8, 151. Dubov, V. A,; Gudzenko, L. I.; Gurvich, L. V.; Iakovlenko, S . I. Chem. Phys. Lett. 1977, 45, 330; 1977, 46, 25. ( 2 ) Grieneisen, H. P.; Xue-Jing, Hu; Kompa, K. L. Chem. Phys. Lett. 1981, 82, 421. (3) Dubov, V. S.;Lapsker, Ya E.; Samoilova, A. N.; Gurvich, L. V. Chem. Phys. Lett. 1981, 83, 518.
0022-3654/88/2092-0357$01.50/0
xenon as the rare gas atom, to which all of the studies to date have been limited, the accepted for these photoinduced harpooning processes is in the vacuum-UV. The two-photon version of the same process Rg
+ X2 + 2hv
-+
Rg+X2-
-
Rg+X-
+X
(2)
(4) Wilcomb, B. E.; Burnham, R. J. Chem. Phys. 1981, 74,6784. (5) Yu, Y. C.; Setser, D. W.; Horiguchi, H. J. Phys. Chem.1983,87, 2209. Setser, D. W.; Ku, J. In Photophysics and Photochemistry above 6 eV; Elsevier: New York, 1985; p 621. (6) Boivineau, M.; Le Calve, J.; Castex, M. C.; Jouvet, C. Chem. Phys. Lett. 1986, 128, 528; J . Chem. Phys. 1986, 84, 4712.
0 1988 American Chemical Society
