2332
J. Phys. Chem. 1987, 91, 2332-2336
of the metal-glass cavity was 4.5-5.5 pm; an improvement in the quality of the polished surfaces will decrease this thickness and upgrade the performance characteristics of our method even further. The instrument has been designed and built not to replace any known surface electrochemical spectroscopy but to complement such spectroscopies. We also expect that this method will be used
as in situ analogue of Auger electron spectroscopy (in the surface analytical aspects of this spectroscopy) by providing reliable packing density information for smooth single-crystal surfaces without system emersion. Acknowledgment. Financial support by Dow Chemical and the University of Illinois is gratefully acknowledged.
Raman Spectroscopic Study of the Synthesis of Zeolite Y Prabir K. Dutta,* D. C. Shieh, and M. Puri Department of Chemistry, The Ohio State University, Columbus, Ohio 43210 (Received: November 3, 1986)
The formation of zeolite Y from colloidal silica and soluble silicate species was investigated by Raman spectroscopy. The role of aging of the reactant mixture was studied. During the nucleation period, the solid amorphous phase consists of predominantly six-membered aluminosilicate rings, which act as building blocks for the formation of zeolite Y . It is essential to have polymeric, highly condensed silicate units as a reactant if zeolite Y crystallization is to take place.
Introduction There is considerable interest in the design and synthesis of novel zeolitic lattices.' However, because zeolite nucleation and growth is not well understood at a molecular level, most of the discoveries in zeolite synthesis have evolved in an empirical fashion. The technological importance of these materials has resulted in extensive studies on the influence of composition, temperature, and environment on the zeolitization process.* A basic understanding of the crystallization process a t a molecular level coupled with this enormous body of existing empirical knowledge will provide opportunities for synthesis of new zeolites. Spectroscopic studies have provided, to date, much of the molecular information on these systems. McNicol et al. studied the synthesis of zeolite A and faujasite using phosphorescence and Raman ~pectroscopy.~Infrared spectroscopy of the solid and solution phases have been reported.4 Other Raman spectroscopic studies have been reported by Angell and Flank,5 and more recently by Roozebom and co-workers.6 The latter authors proposed a solution transport mechanism, even though no Raman bands specific to any aluminosilicate precursors were observed. Solidstate N M R spectroscopy has also provided information on the connectivity of the Si and Al atoms in the amorphous solid phase?-* Melchior, based on solid-state N M R studies of a series of faujasites with varying SiJAl ratios, proposed that the zeolite lattice was formed by combination of six-membered aluminosilicate rings with Lowenstein's rule as the only ~ o n s t r a i n t . ~ We have pursued Raman spectroscopy to study both zeolite structure and formation and have recently reported on the crystallization of zeolite A.1s13 It was possible, for the first time, to obtain the Raman spectra of the amorphous phase, and the data (1) Sand, L. B. Pure AppI. Chem. 1980, 52, 2105. (2) Barrer, R. M. Zeolites 1981, I , 130. (3) McNicol, B. D.; Pott, G. T.; h s , K. R. J. Phys. Chem. 1972. 76,
3388. (4) Beakd, W. C. Adv. Chem. Ser. 1973, No. 21, 162. (5) Angell, C. L.; Flank, W. H.ACS Symp. Ser. 1977, No. 40, 194. (6) Roozebm, F.; Robson, H. E.; Chan, S. S. Zeolites 1983, 3, 321. (7) Engelhardt, G.; Fahlke, B.; Magi, M.; Lippmaa, E. Zeolires 1983,3, 292. (8) Engelhardt, G.; Fahlke, B.; Magi, M.; Lippmaa, E. Zeolites 1985,5, 49. (9) Melchior, M. T. ACS Symp. Ser. 1983, No. 228, 243. (10) Dutta, P. K.; Del Barco, B. J. Phys. Chem. 1985, 89, 1861. (11) Dutta, P. K.; Del Barco, B. J . Chem. Soc., Chem. Commun. 1985, 1297. (12) Dutta, P. K.; Shieh, D.C. J. Phys. Chem. 1986, 90, 2331. (13) Dutta, P. K.; Shieh, D. C.; Del Barco, B. Chem. Phys. Lett. 1986,127, 200.
0022-3654/87/2091-2332$01.50/0
were interpreted with the help of Raman spectra of aluminosilicate minerals and glasses. Geochemists and solid-state physicists have, over the past ten years, made considerable advances in understanding the vibrational spectra of amorphous solids, which has been of considerable help to us in understanding the zeolitic systems.14 In this paper, we report on the Raman spectra of the solution and solid phases during zeolite Y crystallization and have explored the effects of aging and influence of different forms of silicon reactants.
Experimental Section Chemicals. Ludox TM (Dupont) was used as the source of silicon and aluminum powder (-40 mesh) was obtained from Alfa Chemicals. Precautions were taken to exclude C02 from the reaction mixture and all experiments were carried out in Teflon bottles . Zeolite synthesis was done at 90-95 O C in a thermostatted water bath without any stirring. Samples removed from the bath at various times during the crystallization process were centrifuged and separated into liquid and solid phases. The latter was extensively washed with deionized water and dried in a vacuum oven a t room temperature prior to the X-ray diffraction and Raman experiments. The Raman spectra were obtained with a Spex 1403 spectrometer controlled by a Spex Datamate computer. Excitation of all samples was done with the 457.9-nm line of a SpectraPhysics Ar ion laser. The scattered light was detected with a RCA C31034 GaAs P M tube. Slit widths were typically 6 cm-l, unless indicated otherwise in the figure captions, and scan times varied between 1 and 3 s per wavenumber. The Raman spectra shown in Figure 4 were recorded with considerably narrower slit widths (- 1 cm-I) to reduce the strong background scattering usually observed for these samples at low frequencies. The source of this scattering arises mostly from the size of these particles, which lead to a strong Tyndall scattering effect and cannot be filtered out by the spectrometer. The contribution from the thermal excitation of low-lying lattice modes is considerably small compared to this background. To obtain the spectra shown in Figure 4, we fitted the background between 100 and 700 cm-' in the original data with an exponential function and subtracted it out from the Raman spectra. Even though the physical justification for such a mathematical function is unclear, our purpose here is to compare the band structure between two similar materials in the low-frequency region. To further aid in (14) McMillan, P. Am. Mineral 1984, 69, 622.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 9, 1987 2333
Zeolite Y Synthesis
A
I
5
10
20
30
35
20 Figure 1. Powder X-ray diffraction patterns of the solid phase present at various times during zeolite crystallization: (a) mixing of reactants for 35 min, Si/A1 = 26; (b) 24-h aging at room temperature, Si/A1 = 15; (c) aging with 6 h of heating, Si/AI = 2.5; (d) aging with 12 h of
heating, Si/Al = 1.7; (e) aging with 18 h of heating, Si/A1 = 1.7; (f) aging with 24 h of heating, Si/A1 = 1.8; (8) aging with 30 h of heating, Si/A1 = 1.8. this comparison, we fitted the “reduced” Raman data with a set of Gaussian peaks and these are also shown in the figure. Powder X-ray diffraction patterns were obtained with a Rigaku Geigerflex D/Max 2B diffractometer with a Ni-filtered Cu Kcu source. The elemental analysis (Na, Si, Al) was done with a JEOL JXA-35 electron microprobe using energy dispersive methods. Zeolite A was employed as standard. The 29Si N M R spectra were recorded on a Bruker AM-500 N M R spectrometer with tetramethylsilane as an external standard. The spectra were recorded over a 10,416-kHz spectral width, using a 6-hs pulse and an acquisition time of 0.786 s. Six-thousand transients were acquired. The broad structureless background from the probe and tube was subtracted out by comparing with a spectrum obtained from pure water.
Results Synthesis of Zeolite Y from Silica Sol. Zeolite Y (Si/Al = 1.8) was synthesized by starting from the composition 10Si020.5A12035Na20560H20 ([Si] = 0.9 M,’[Al] = 0.09 M, [NaOH] = 0.9 M). The reaction mixture was prepared by mixing appropriate amounts of silica sol with a solution of aluminum dissolved in sodium hydroxide. These samples were then aged for 24 h at ambient temperature and heated to 90 OC in a thermostatted water bath. Multiple batches were made up and the reaction was terminated at various times and the solution and solid phases were separated by centrifugation. The powder X-ray diffraction patterns of the solid phase and the Raman spectra of the solution and solid phases at various times during the crystallization process are shown in Figures 1-3. The Si/Al ratio of the solid phases are indicated in the figure captions. These figures clearly show that there are significant changes in the Raman spectra of the solid and solution phases during crystallization. The first evidence of zeolite crystals in the diffraction patterns is observed after heating the aged reaction mixture for 6 h. This time limit defines the nucleation stage, during which zeolite nuclei and small zeolite crystals not detected by X-ray diffraction are formed from the reactants. The structural changes
aoo.00
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R a m a n Shift (cm-l) Figure 2. Raman spectra of solution during zeolite crystallization: (a) initial mixing of reactants; (b) 24-h aging; (c) aging with 6 h of heating; (d) aging with 18 h of heating; (e) aging with 30 h of heating.
F
1
ii
c
3 0 0 - 0 0
8 0 0 = 0 0 1 3 0 0 ’ 0 ~
R a m a n Shift (cm-’) Figure 3. Raman spectra of the solid phase during zeolite Y crystallization: (a) mixing of reactants for 35 min, %/AI = 26; (b) 24-h aging at room temperature, Si/A1 = 15; (c) aging with 6 h of heating, Si/A1 = 2.5; (d) aging with 12 h of heating, Si/AI = 1.7; (e) aging with 18 h of heating, Si/AI = 1.7; (f) aging with 25 h of heating, Si/A1 = 1.8; (8) aging with 30 h of heating, %/AI = 1.8.
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2334 The Journal of Physical Chemistry, Vol. 91, No. 9, 1987
ao0.00
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R a m a n Shift (cm-') Figure 5. Raman spectra of (a) aged sample heated for 3 h, Si/AI = 4, (b) unaged sample heated for 3 hours, Si/A1 = 2.2. 200.00
4 5 0 - 0 0
700.00
R a m a n S h i f t (cm-3 Figure 4. Raman spectra of (a) aged sample, Si/A1 = 15, (b) aged sample heated for 2 h, %/AI = 7 . Spectral slit width = 1 cm-', 5 s/scan.
that take place within this period are therefore of considerable importance. Only a single Raman band is observed in the solution phase upon initial mixing of the reactants and remains so during aging of this reaction mixture. This band at 620 cm-I has been assigned to A1-0 symmetric stretching of the AI(OH),- species." When the reaction mixture was heated for 6 h, a strong band at 780 cm-' and weaker bands at 44 1 and 9 19 cm-I are observed. These bands are characteristic of monomeric silicate species (Si02(0H)22-).'6 These spectral features remain dominant in the solution phase during the remainder of the crystallization process. Weaker bands at 601 and 1025 cm-I due to dimeric silicate species appear in solutions that have been heated over 18 h (Figure 2, d and e). These data indicate that, during the initial stages of synthesis, aluminate ions from solution are incorporated into the solid phase and silicate ions are released into solution. This observation is in general agreement with that of Rozeboom and co-workers.6 The Raman spectra of the solid phase of the gel undergoes considerable changes during crystallization. The silica sol reactant has a Raman spectrum similar to that of vitreous silica, with bands at 440, 490, 600, 800, and 1000 cm-'. When the reactants are initially mixed (35 min of stirring), the solid phase that forms still resembles amorphous silica, with bands at 460, 800, and 1000 cm-'. The sharp bands at 490 and 600 cm-l have disappeared. The Si/A1 ratio of this material is 2 6 , indicating that aluminum is quickly incorporated into the framework. After 24 h of aging at room temperature, the general features remain unchanged, except for the appearance of the band a t 570 cm-l. With 6 h of heating at 90 "C, the spectrum of the solid phase changes considerably, and Raman bands are observed at 504, 570, 707, 861, and 1020 cm-I. As zeolite crystallization progresses, the 504-cm-l band considerably sharpens and moves to 510 cm-', the bands at 570, 707, and 861 cm-I gradually decrease in intensity and disappear, and the broad band at 1020 cm-' narrows to a band at 990 cm-I and a new band grows at 1092 cm-I. Since the powder X-ray diffraction data and the Raman spectra of the solid phase both indicate that the first 6 h of heating plays a crucial role in the formation of zeolite nuclei, we carefully investigated the early stages of the reaction system at increments of 1 h of heating. The solution species show a gradual decrease in Al(OH), concentration and are replaced by monomeric silicate ions. No evidence of any
-
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(15) Moolenar, R. J.; Evans, J. C.; McKeever, L. D. J . Phys. Chem. 1970, 74, 3629. (16) Dutta, P. K.; Shieh, D. C. Appl. Spectrosc. 1985, 39, 343.
aluminosilicate ions were observed from the Raman spectra. The solid phase, however, undergoes major spectral changes. To illustrate this, we have compared in Figure 4 the Raman spectra of the solid phase immediately after aging and an aged sample heated for 2 h. Only the structurally sensitive region due to T-O-T bending modes (200-700 cm-I) is shown. The strong Rayleigh scattering background has been numerically subtracted out. Both these spectra have been fitted with a set of five Gaussian bands, positioned at 280,361,440,492, and 564 anT1.The dashed line running through the experimental data is a sum of these five bands. Band positions a t 280, 440, 492, and 564 cm-' were obtained from the shoulders observed in Figure 4b and the band at 361 cm-' was arbitarily chosen so as to provide the best fit. The necessity of deconvolution and the interpretation of these bands are discussed later. E'ects of Aging. A reaction mixture with composition identical with that discussed above for zeolite Y and prepared in a similar fashion leads to the formation of zeolite R (chabazite), if no aging is allowed to take place.I7 We also studied the Raman spectra of the solution and solid phases during this reaction. In comparison to zeolite Y , monomeric silicate ions are formed in solution more rapidly, within the first hour of heating. The solid phases also exhibit differences in spectra in these two crystallization pathways. Figure 5 compares the Raman spectrum of an aged and unaged gel, heated for 3 h at 90 O C . The major difference lies in the 400-500-cm-l region. In the unaged gel, the -500-cm-I band is considerably more prominent. Synthesis of Zeolite Y from Soluble Silicates. The most common source of silicon used in the synthesis of zeolite Y is silica sol, or colloidal s i 1 i ~ a . I We ~ examined the possibility of using soluble silicates as a source of silicon, and found that zeolite Y could be synthesized from such a source if the solution contains high concentration of silicon. Zeolite Y with an Si/Al ratio of 2.4 was synthesized by mixing equal volumes of 4 M silicate solution and 0.1 M aluminate solution, both prepared in 2 M NaOH, and heating the reaction mixture to 95 OC for 84 h. No aging was required. Figure 6 shows the Raman and 29SiNMR spectra of the starting silicate reactant. Five distinct peaks are observed in the N M R spectra. These have been assigned to monomeric silicate (-72 ppm), end groups (-80.2 ppm), middle groups of chains or rings (-88.6 ppm), silicon with one nonbonded oxygen atom (-97.6 ppm), and no nonbonded oxygen atoms (-108 ppm).I8 In comparison to the study by Engelhardt and mworkers on 1.65 M silicate solution,* these spectra show considerably greater quantities of silicon without any nonbonded oxygen atoms. This spectrum indicates that, a t these high concentrations of -
~~~~
(17) Breck, 216.
D.W. Zeolite Molecular Sieves; Wiley: New York, 1974; p
(18) Engelhardt, G.; Zeigan, D.; Jancke, H.; Hoebbel, D.; Wieker, W. Z. Anorg. Allg. Chem. 1915, 418, 17.
Zeolite Y Synthesis
The Journal of Physical Chemistry, Vol. 91, No. 9, 1987 2335
n
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are already aggregated in the solid phase, just by virtue of proximity and preorganization, it is highly likely that these units will join to form a zeolite nuclei in and around the solid phase. However, if zeolite synthesis is carried out in the absence of a solid phase, nuclei will have to form by reaction of the monomeric aluminosilicate building blocks in solution. But the concentrations of these species are considerably less in solution than in the solid phase, and it can be expected that the synthesis will be considerably slower. Therefore, obtaining structural information about these monomeric aluminosilicate building blocks is essential to gain an understanding of zeolite formation. However, no spectroscopic techniques have been able to detect these anions during zeolite synthesis, because of their low concentration. Our strategy has been to obtain spectral data on the amorphous solid phase, which provides structural information on the aluminosilicate building blocks that combine to form the zeolite. Structure of the Solid Phase during Zeolite Y Nucleation. The interpretation of the vibrational spectra of the amorphous solid phase present during zeolite synthesis is not straightforward because of the complexity of these systems and the lack of structural information. Only recently, solid-state 29Siand 27AlN M R data have been reported for aluminosilicate amorphous solids of varying Si/Al ratios. At S i / N ratios of 2-2.5, the chemical shifts indicate the presence of (SiO)2Si(OA1)2and (SiO)Si(OAl), units in the framework. In samples with lower Si/Al ratios ( l), Si(OA1)4 units appear to be pred~minant.'?~We have reported the Raman spectra of an aluminosilicate solid with Si/Al = 1, and the bands were assigned by amparison with spectra of aluminosilicate minerals and glasse~.'~J~ The same strategy is followed here. Over the past ten years, geophysicists and solid-state physicists have generated much experimental and theoretical data on silicate and aluminosilicate glasses and melts containing fully polymerized TO4 tetrahedra.22v23That these materials serve as good model systems for the amorphous phases present during zeolite synthesis is evident from the general similarity of their Raman spectra. In both these systems, bands are observed at 250-500 cm-I, assigned to motion of oxygen atoms in the plane perpendicular to the T-O-T bond, 550-600 cm-I, assigned to AI-0-Si stretches, 700-750 cm-', assigned to Al-0 stretches, and 950-1 100 cm-', assigned to Si-0 s t r e t c h e ~ . ~In~ .addition, ~~ the zeolitic intermediate phase also exhibit bands around 800-900 cm-I, which we have assigned to Si-0 stretches of nonbonded oxygen atoms,12and are not observed in the completely polymerized glass or melt systems. An inverse correlation between the prominent vibrational band in the region 300-600 cm-l and the sizes of the rings present in silicate minerals has been pointed For example, minerals such as benitoite, a-wollastonite and catapleite, which contain rings of three silicon atoms, exhibit a strong band at 600 cm-'. Aximite, with rings of four silicon atoms, has a band at 530 cm-' and quartz, cristobalite, and tridymite, which contain six-membered silicate rings, all exhibit bands below 450 cm-1.14,25 Sharma and coworkers have extended this correlation to aluminosilicate glasses and melts?628 They found that, if four-membered aluminosilicate rings are predominant, such as in anorthite glass, a band at 500-510 cm-' is observed, whereas, glasses such as orthoclase and nepheline, which contain six-membered aluminosilicate rings exhibit, bands below 470 cm-I. The Raman spectra of the solids formed upon immediate mixing of the reactants, as well as after aging, have broad asymmetric bands in the 200-600-cm-' region. From the correlation of ring sizes with vibrational bands that were discussed above, it is clear N
-40
-60
-80
-100
-120
(ppm)
figure 6. Raman and %i NMR spectra of a 4 M silicate solution. The relative areas of the peaks in the N M R spectra are: starting from left (-72 ppm), 0.03, 0.3, 2.1, 4.1, and 1.0.
silicon, there exists polymeric silicate anions containing an inner core of crosslinked silicon atoms and an outer core of branched silicon atoms with mainly one or two nonbonded oxygen atoms. The Raman spectrum also supports this view. In the frequency region between 300 and 800 cm-l, there is a close resemblance to amorphous silica with a broad band at 450 cm-I, a sharper band at 518 and 600 cm-l, and a band a t 780 cm-'. Bands between 800 and 900 cm-I are assigned to Si-0 stretches of silicon atoms bonded to two oxygen atoms,12 whereas the strong band at 1038 cm-I is assigned to S i 4 stretch of silicon atoms connected to one nonbonded oxygen atoms.Ig When the silicate and aluminate solutions are mixed, a faint turbidity appears, and no solid phase could be centrifuged out. Thus, we were unable to obtain any information on the aluminosilicate species that form. The Raman spectrum of the total aluminosilicate reaction mixture resembled that of the starting silicate solution, and no bands due to any aluminosilicate ions were observed.
Discussion There is considerable controversy on the mechanism of the formation of zeolites. Some investigators favor the formation of zeolites by polymerization of soluble aluminosilicate anions in the liquid phase," while others favor a solid-state mechanism involving reordering of the aluminosilicate gel to a crystalline state?] Much has been written and discussed regarding these theories. However, it appears to us that there is no fundamental difference between these two mechanisms-rather, it is a difference in degree. The complex gel equilibrium can be pictured as a highly insoluble solid phase in equilibrium with a small concentration of soluble aluminosilicate species, which in turn is in equilibrium with soluble silicate and aluminate species. This equilibrium can therefore be perturbed by altering any of these species. The solid material can be thought of as an polymeric unit built by condensation of smaller, monomeric aluminosilicate anions. These monomeric species act as building blocks for the zeolite. Since these units (19) Dutta, P. K.; Shieh, D. C. J . Raman Spectrosc. 1985, 16, 312. (20) Zhdanov, S. P. Adu. Chem. Ser. 1971, No. 101, 20. (21) Flanigen, E. M. Adu. Chem. Ser. 1973, No. 121, 119.
(22) Sharma, S. K.; Simons, B.; Yoder, Jr. H. S. Am. Mineral. 1983,68, 1113. (23) McKeown, D. A.; Galeener, F. L.; Brown, Jr., G. E. J . Non-Crysr. Solids 1984, 68, 361. (24) Galeener, F. L. Solid Stare Commun. 1982, 44, 1037. (25) Griffith, W. P. J . Chem. Soc. A 1969, 1372. (26) Sharma, S. K.; Maumone, J. F.;Nicol, M. F. Nature (London)1981, 292, 140. (27) Sharma, S. K.; Philpotts, J. A.; Matson, D. W. J . Non-Cryst. Solids 1985, 71, 403. (28) Matson, D. W.; Sharma, S . K.; Philpotts, J. A. Am. Mineral. 1986, 71, 694.
[a-
2336 The Journal of Physical Chemistry, Vol. 91, No. 9, 1987 SCHEME I
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that these amorphous aluminosilicate solids must contain ring structures. Because of the width of these bands, we resorted to curve deconvolution procedures to assist in understanding the ring structures that are present. The Raman bands shown in Figure 4 provide an indication of the ring structures that are present in these amorphous solid phases. We have previously assigned the 570-cm-I band to Si-O-A1 stretches of bonds interconnecting the rings.I2 The band at 492 cm-', based on Sharma's correlation, can be assigned to four-membered aluminosilicate rings. The bands at 440 and 361 cm-I are typical of six-membered aluminosilicate rings, e.g. nephelite and carnegieite crystals, which contain six-membered TO4 rings, have dominant Raman bands at 427 and 379 cm-I, re~pectively.~'It is also likely that lattice vibrations (mainly cation motion) contribute to the low-frequency region and could be the cause of the 280-cm-' band. The structure of the aged gel therefore appears to contain mainly six-membered and four-membered aluminosilicate rings. The relative proportions of these rings cannot be deduced from the Raman intensities. However, it is clear from Figure 4b that, upon heating the aged gel, there is a decrease in band intensity of the six-membered rings and increase in intensity of the four-membered rings. The type of correlation that we have discussed in associating ring structures with vibrational frequencies is empirical in nature. Another approach is to compare the Raman spectrum of the zeolitic intermediate phases with the spectrum of various aluminosilicate glasses that have been reported in the literature. The material shown in Figure 4b is probably the best representative of the structurally important intermediate aluminosilicate phase (2 h of heating), since it is present during the early formation of the zeolite Y nuclei. The most striking resemblance of the Raman spectrum of this material is with that of nepheline glass, as reported by Sharma et al.27 In the structure-sensitive 200-700-~m-~region (T-O-T bending), the spectra appear identical, with the major peak at -500 cm-' and shoulders at 280 and 440 cm-I (compare Figure 2 of ref 27). Taylor and Brown, based on X-ray radial distribution function analysis of nepheline glass, suggested that it is composed of predominantly six-membered rings, and this is also consistent with the crystallization behavior of this glass.29 Along with the correlation of the bands that we discussed above, and the striking similarity of the Raman spectrum between the solid phase and nepheline glass, we conclude that there is convincing evidence that six-membered rings are present in the amorphous aluminosilicate phase during zeolite nucleation. The formation of nuclei of zeolite Y will involve these rings as building blocks. Influence of Aging. Aging the reaction mixture before heat treatment has a significant impact on the structure of the zeolite framework that is formed. As is evident from Figure 5, the major difference between the nonaged and aged solid phase after 3 h of heating is in the frequency range between 400 and 550 cm-'. The aged material has considerable more intensity in the 400500-cm-l region, indicating the presence of six-membered alu-
-
(29) Taylor, M.; Brown, G.E.Geochim. Cosmochim. Acta 1979,43, 1467.
Dutta et al. minosilicate rings. The nonaged material, on the other hand, shows a sharp band at 500 cm-', indicative of four-membered aluminosilicate rings. As the nonaged reaction mixture is heated, the silica sol rapidly dissolves to form monomeric and dimeric species, which immediately react with Al(OH),- ions to form four-membered rings, as was observed during the crystallization of zeolite A.l2 So,aging provides a route for the polymeric silica framework to incorporate aluminum and form six-membered rings. Polymeric silicate units seem essential for formation of such rings. This also explains why it is essential to have condensed, polymeric units in silicate solution, if it is to be used as a starting reactant for synthesis of zeolite Y, without aging. Mechanism of Formation of Zeolite Y. The spectroscopic data discussed above indicates that six-membered aluminosilicate rings are dominant in the solid phase during zeolite Y nucleation. It is not obvious from these studies how these rings interconnect to form the zeolite framework. Melchior, based on the distribution of A1 atoms around Si atoms for a series of faujasite structures with varying Si/Al ratios, concluded that the pathway for zeolite crystallization involves ordering of six-membered rings into sodalite cages and subsequent formation of nuclei? Our experimental data provides support for this model. During the nucleation stage, which involves the first few hours of heating in this study, there is an increase in the intensity of the -500-cm-' band, which has been assigned to vibrations of four-membered aluminosilicate rings. It is important to recognize that, once an organized structure, such as a sodalite cage is formed, it is no longer possible to discuss vibrational frequencies in terms of specific ring structures because of the sharing of atoms by neighboring rings. The mineral sodalite, though it contains both six-membered and four-membered aluminosilicate rings, exhibits only a sharp band at 493 cm-'.13 Therefore, the concept of different vibrational frequencies for different ring sizes is true only if discrete ring structures are present. We propose a model for formation of zeolite Y (nominal Si/Al = 2), which involves six-membered aluminosilicate rings connecting to each other via four-membered silicate rings, and then two such units connecting to form a sodalite cage. Our idea is similar to that of M e l ~ h i o rbut , ~ the final arrangement of the Si,A1 atoms is slightly different. A schematic diagram is shown in Scheme I. The mechanism described above only applies to zeolite Y with Si/Al greater than 1.5. For zeolite X, with Si/A1 ratios less than 1.5, we have found that the crystallization process favors the formation of four-membered aluminosilicate rings as intermediate species.30 Conclusion Vibrational data on the solution and solid phases during formation of zeolite Y indicate the presence of six-membered aluminosilicate rings during the nucleation period. It is essential to have polymeric silicate units as the silicon reactant for these rings to form. Aging the sample provides a route for this to happen-if not, a high concentration of Si in solution is required to provide polymeric silica and aging is then not necessary. A model based on six-membered rings joining via four-membered rings to form sodalite cages is proposed as the mechanism for nucleation of zeolite Y. Acknowledgment. We gratefully acknowledge the support provided by the National Science Foundation (CHE-8510614). We also thank Mr. H. Hatfield for writing the deconvolution software. (30) Dutta, P. K.; Shieh, D. C., unpublished work
