J. Phys. Chem. 1986,90,2331-2334
2331
Crystallization of Zeolite A: A Spectroscopic Study Prabir K. Dutta* and D. C. Shieh Department of Chemistry, The Ohio State University, Columbus, Ohio 43210 (Received: August 30, 1985; In Final Form: January 27, 1986) The transformation of aluminosilicate gel to zeolite A was investigated by Raman spectroscopy, supported by X-ray diffraction and NMR measurements. The gel, even though amorphous, has a structure consisting of predominantly four-membered rings connected in a random fashion. It is considerably depolymerized, consisting of Si atoms with one and two nonbonded oxygen atoms. For the transformation of this gel to zeolite A to proceed, it is essential to have Al(OH), species in solution. During the nucleation period, the gel reorganizes its structure by interaction with these Al(OH)4- ions and forms nuclei of zeolite A. The crystallization curve obtained by Raman spectroscopy closely resembles that from X-ray diffraction.
Introduction A. It has been possible, for the first time, to obtain the Raman spectrum of the gel. Band assignments have been made by Even though the number of zeolites synthesized in the laboratory comparison with Raman spectra of aluminosilicate glasses and is over 100, crystallographers predict that only a few percent of supporting N M R spectroscopy. Spectroscopic changes due to the the possible zeolite frameworks have been synthesized.'q2 The transformation of the gel to zeolite A are presented and discussed. major impediment to creating new zeolitic species is the lack of a basic understanding of the crystallization mechanism. Therefore, Experimental Section most of the advances have occurred through a wide variety of The aluminosilicate gels were formed by mixing clear solutions experimental attempts. However, there have been a number of mechanistic and structural studies on zeolite crystal g r o ~ t h . ~ , ~ of S i 0 2 gel (100-200 mesh, Davison chemical) and aluminum powder (-40 mesh, Alfa) separately dissolved in appropriate From such studies, two major proposals for the transformation concentrations of sodium hydroxide. Precautions were taken to of gels to crystals have emerged. One proposal suggests that the exclude C02 from the system and all experiments were carried amorphous aluminosilicate gel dissolves to form soluble alumiout in Teflon bottles. nosilicate species, which act as precursors for zeolite nuclei.s*6 The Zeolite A synthesis was done either at 50 or 95 OC in a thersecond mechanism suggests that the solid gel undergoes polymmostatted water bath without aging or stirring. Samples removed erization-depolymerization reactions, with subsequent formation from the bath were centrifuged and separated into liquid and solid of nuclei and crystals.' phases. The solid phase, henceforth called gel, was washed with Spectroscopic studies have provided support for both these deionized water till neutrality and then dried in a vacuum oven mechanisms. McNicol et al. studied the synthesis of zeolite A at room temperature prior to the Raman experiments. and faujasite using phosphorescence and Raman spectroscopy.* The Raman spectra were obtained with a Spex 1403 specThe found no changes in the vibrational spectra of the liquid phase trometer controlled by a Spex Datamate computer. Excitation during crystallization but observed a change in the position of the of all samples was done with the 457.9-nm line of a Spectra Physics Raman band of the tetramethylammonium cation that was present 171 Ar-ion laser. The slit widths were typically 6 cm-', and the in the solid phase of the gel. They postulated that the crystalscattered light was detected with a RCA C31034 GaAs PM tube. lization took place in the solid gel phase. Angell and Flank? on Typical scan times were 1-3 s/wavenumber. the other hand, found a change in the concentration of the aluThe 27Al N M R spectra were recorded at 130.3 M H z on a minate species during crystallization of zeolite A and suggested Bruker AM-500 N M R spectrometer using ~ / pulses 2 of 25 ps a solution transport mechanism. Recently, Roozeboom and coand collecting 4500 fids of 8K data points with a repetition rate workers reported on Raman spectroscopic studies of crystallization of 0.14 s. The chemical shifts were determined relative to an of zeolites A, X, and Y.lo For zeolite A, they found a decrease external standard of an aqueous A1C13 solution. in concentration of aluminate species in solution and evolution The X-ray diffraction data was obtained with a Siemens D5000 of the Raman spectra of zeolite A from a gel with no Raman powder diffractometer. peaks. They proposed a solution transport mechanism, even The elemental analysis was done by X-ray fluorescence. though they acknowledged that no Raman bands specific to any aluminosilicate precursor species were observed. Results Engelhardt and co-workers11J2have studied the 29Siand 27Al Influence of Starting Composition. Synthesis of zeolite A was N M R spectra of zeolite A gels and found that the initial gel had attempted with different starting compositions, the primary alternating Si and A1 atoms. The transformation of the gel phase variable being the Si/Al ratios of the starting mixture. The to crystal resulted in considerable narrowing of the single peak concentration of sodium hydroxide was kept constant. Raman observed in both the 29Si (-93.2 ppm) and 27Al (55.7) N M R spectra of the initial gel and solution species as well as spectra spectra, due to an increase in ordering. of the solid and liquid phases after heating at 95 O C for various This paper presents a systematic Raman spectroscopic investimes were examined. For ratios of Si/A1 varying from 2 to 0.5, tigation of the formation of gels and their transformation to zeolite the initial gels formed had very similar Raman spectra but the (1) Sand, L. B. Pure Appl. Chem. 1980, 52, 2105. solution species varied considerably. Figure 1 shows the Raman (2) Barrer, R. M. Zeolites 1981, 1, 130. spectra of the initial solution and gel for Si/A1 ratios of 0.5 and (3) Kerr, G. T. J . Phys., Chem. 1966, 70, 1047. 2. For the higher ratio, the gel is in contact with predominantly (4) Culfaz, A.; Sand, L. B. Ado. Chem. Ser. 1973, No. 121, 140. Si02(0H)22-species (bands at 772 and 920 cm-I), for Si/Al = (5) Barrer, R.M.; Baynham, J. W.; Bultitude, F. W.; Meier, J. Chem. Soc. 1959, 195. 0.5, the gel is in contact with only Al(OH), species (620 cm-I). (6) Zhdanov, S. P. Adu. Chem. Ser. 1971, No. 101, 20. When these solutions were heated to 95 'C, the material with the (7) Flanigen, E. M. Adu. Chem. Ser. 1973, No. 121, 119. starting composition of Si/A1 = 0.5 rapidly converted to zeolite (8) McNicol, B. D.; Pott, G. T.; Loos, K. R. J . Phys. Chem. 1972, 76, A (confirmed by Raman and XRD measurements), whereas for 3388. the Si/Al ratio of 2, the gel was unaffected even upon heating (9) Angell, C. L.; Flank, W. H. ACS Symp. Ser. 1977, 40, 194. (10) Roozeboom, F.; Robson, H. E.; Chan, S. S, Zeolites 1983, 3, 321. for much longer times (8 h). Figure 1 clearly shows these changes. (1 1) Engelhardt, G.; Fahlke, B.; Mazi, M.; Lippmaa, E. Zeolites 1983, 3, By examining the intermediate Si/Al ratios, we have confirmed 292. that zeolite A can readily be formed only when significant amounts (12) Engelhardt, G.; Fahlke, B.; Magi, M.; Lippmaa, E. Zeolites 1985.5, 49. of Al(OH)4- species are present in solution. Also, in agreement 0022-3654/86/2090-2331$01.50/0
0 1986 American Chemical Society
2332 The Journal of Physical Chemistry, Vol. 90, No. 11, 1986
Dutta and Shieh
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Figure 1. Top and Middle: Raman spectra of the solution and solid phase formed on mixing aluminate and silicate solutions in NaOH with the ultimate composition shown at the top of the figure. Bottom: Raman spectra of the solid phase after heating the composition indicated on the top of the figure at 95 O C .
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Raman shift (cm-I) Figure 2. Raman spectra of the solid phase during zeolite A synthesis a t 50 O C a t various times ( s t a r t i n g composition
8.6Na201AI2O3ISiO2556H2O). with McNicol and co-workers,* we found no major changes in the composition of the solution species during the crystallization process. Also, no evidence was found for any aluminosilicate species either by Raman spectroscopy or 27AlNMR in the solution phase during crystallization. Gel-to-Zeolite Transformation. The starting composition 8.6Na201A120,1Si02556H20was chosen for studying the gradual transformation of the gel to crystal. The crystallization was carried out at 50 O C to slow the process, thus facilitating spectroscopic examination. As a function of time, Raman and 27AlN M R spectra of the solution phase and Raman spectra of the solid phase were examined. No changes were observed in the solution spectra, suggesting that the solid phase is in contact with AI(OH),throughout the crystallization process. Figure 2 shows the Raman spectra of the solid phase at various times. The Si/AI ratio of the initial gel was 1.07 for the first 22 h and then rapidly decreased to 1.0 at h. The initial gel, which was completkly gmorphous as determined by X-ray diffraction, showed Raman bands at 450 (broad), 504. 575, 705, 847, 877, 960 (shoulder), 966, 1040
Figure 3. Zeolite A crystallization curve as determined from the Raman data of Figure 2 (oridinate represents the contribution of the Raman spectrum of zeolite A necessary to simulate the total spectrum of the solid
phase). (shoulder), and 1082 cm-I. During the first 20 h, the solid phase remains amorphous (as determined by XRD), and subtle changes are observed in the Raman spectrum. These include decrease in intensity of the broad 450-cm-] band, shift of the 504-cm-I band to 496 cm-', decrease in intensity of the 847-cm-' band, disappearance of the shoulder at 960 cm-], and sharpening of the 996-cm-I band. These changes can then be thought to be ocurring during the nucleation cycle. The bands at 491 and 1100 cm-I, characteristic of zeolite A, are first observed around 22 h. X-ray diffraction data also show evidence of zeolite crystals at this time. Over the next 10 h or so, zeolite crystallization proceeds very rapidly and then levels off at longer times. Figure 2 clearly shows the evolution of the Raman bands due to zeolite A at 336,407, 491, 700, 733, 745, 970, 1040, and 1102 cm-l as a function of time. Using the Raman spectrum of the 20-h gel as representing the complete amorphous phase, and the 60-h crystal Raman spectrum as pure zeolite A, we could simulate the Raman spectrum (300-1300 cm-I) obtained at the intermediate times by taking differing contributions of these two extreme spectra. This indicates that, at all times during crystallization, the Raman spectrum is a sum of the 20-h gel and the crystal Raman spectra. What this implies is that there is no Raman spectroscopic evidence for the gel transforming to any other intermediate state once crystallization to zeolite A begins. Figure 3 is a plot of the conversion of gel to zeolite A, as measured by Raman spectroscopy. The crystallization curve follows a sigmoidal pattern and resembles that obtained from X-ray diffraction measurements.
Discussion Gel Structure. N M R studies have shown that the initial aluminosilicate gels synthesized under the conditions discussed in this paper consist of alternating Si and A1 atoms." Our elemental analysis indicates a Si/AI ratio slightly greater than one (1.07). We will use this as a basic starting model for our gel in making the vibrational assignments. It is a good approximation to assume that the bands observed above 300 cm-' (450,504,575,705,847, 996, 1040, and 1082 cm-I) in the Raman spectrum of the gel arise from framework vibrations, i.e. motions of the Si, AI, and 0 atoms.13 It is seen from Figure 2 that most of the bands observed in the gel are fairly broad, which is typical in the Raman spectra of disordered solids. A good comparison is the Raman spectrum of vitreous silica, which has been extensively investigated.14 The broadness of bands in vitreous silica has been explained, in part, as arising from the wide distribution of the S i U S i angle (between 120 and 180°), which gives rise to variation of the force constant and, hence, in frequency. It is the narrower bands, however, that are a surprising feature in these amorphous solids. In the gel spectrum in Figures 1 and 2, there are two bands at 504 and 575 cm-I that are considerably narrow (-30-cm-' half-width), and (13) Dutta, P. K.; Del Barco, B. J . Phys. Chern. 1985, 89, 1861. (14) McMillan, P.Arner. Mineral. 1984, 69,622.
Crystallization of Zeolite A we will focus on their assignment at first. Fused vitreous silica, which serves as a useful model, also shows two sharp peaks at 490 and 604 cm-' relative to the other bands. This sharpness in the Raman bands, considering that it is a noncrystalline solid, has generated considerable interest in the assignment of these bands. These bands were originally assigned to defect structures, including broken Si-0 bands and tricoordinated oxygen atom^.^^,'^ Galeener has persuasively argued that these bands arise from the oxygen-breathing motion of planar fourfold and threefold rings, present in small concentration^.'^^'^ Revesz and Walrafen have also suggested that these bands are due to nonrandom structural features but favored a three-membered ring for the 490-cm-I band and either rings of various sizes for the 604-cm-I band or elongated Si-0-Si bands.19 We suggest similar origins for the bands at 504 and 578 cm-I in the gel spectrum, i.e. these bands arise from specific ordered structures in the otherwise random network. Extensive experimental studies and calculations on silicates and aluminosilicates have suggested that bands around 450-550 cm-' arise from motion of the bridging oxygen atoms in a plane perpendicular to the T-0-T unit.14 Recently, Sharma et aLzo have shown that a correlation exists between the frequency of this band and the smallest rings of TO4 tetrahedra in aluminosilicate glasses. This idea is also supported by the theoretical calculations of Galeener for silica g l a ~ s . ' ~ The , ' ~ assignment for the Raman bands in the gel is facilitated by direct comparison with aluminosilicate glasses. The best comparison is made with anorthite (CaAlzSizOs),which has a similar framework composition as the gel.21 Upon comparing the Raman spectrum of the gel (Figure 1 or 2) with that of anorthite glass (Figure 1 of ref 21), one is impressed with the striking similarity, except for the lack of the 867-cm-' band in anorthite. X-ray radial distribution function analysis of anorthite glass indicates that it consists of four-membered rings of TO4 tetrahedra.22 Based on this fact, Sharma et al. have assigned the 503-cm-' band in anorthite glass to the oxygen-breathing motion of the four-membered rings.20 We therefore suggest that the band at 504 cm-I in the aluminosilicate gel indicates the presence of four-membered rings of alternating Si04and A104 tetrahedra. The broad band at 450 cm-' is assigned to the presence of higher-membered ring structures and/or disordered structures. The correlation by Sharma et al. between ring size and frequency shows an inverse relationship between themSz0So, the picture of the gel that emerges is that it predominantly consists of fourmembered rings with small amounts of other ring structures. The origin of the 575-cm-' band is more complicated. Guth et al. reported a band at 577 cm-' in a solution of aluminate and silicate species ([Si] = 4.5 M, [All = 0.75 M, [NaOH] = 9 M) and assigned it as arising from aluminosilicate anions.23 We extensively investigated the Raman spectra of mixtures of aluminates and silicates in NaOH and found that soluble aluminosilicate species as evidenced by the 575-cm-' band could only be observed at very high hydroxide ion concentrations. Figure 4 shows the Raman spectrum of a solution for which [Si] = 0.75 M, [AI] = 0.25 M, and [NaOH] = 9 M. Bands at 770 and 920 cm-I are due to Si02(OH)2,2-the band at 620 cm-I is due to A1(OH)4-, and the new band at 575 cm-' is due to aluminosilicate species. To get a better idea of the structure of the aluminosilicate species, we examined the 27AlN M R spectra of the above solution which is shown in Figure 4. A strong peak at a chemical shift
The Journal of Physical Chemistry, Vol. 90, No. 11, 1986 2333
27AI NMR 77 I
I
60
so
7b
5b
4b
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'300
800
1300
hl-9 Figure 4. 27A1N M R spectrum and Raman spectrum of solution of [Si] = 0.75 M, [AI] = 0.25 M, and [NaOH] = 9 M.
-t 380.88
800. 80
1308.08
Raman s h i f t [ cm-'l (15) Bates, J. B.; Hendricks, R. W.; Shaffer, L. B. J . Chem. Phys. 1976, 61, 4163.
(16) Lucovsky, G.Phil. Mag. B 1979, 39, 513. (17) Galeener, F. L. Solid State Commun. 1982, 44, 1037. (18) Galeener, F. L. J . Non-Cryst. Solids 1982, 49, 53. (19) Revesz, A. G.; Walrafen, G. E. J . Non-Cryst. Solids 1983,54, 323. (20) Sharma, S. K.; Philptts, J. A,; Matson, D. W. J . Non-Cryst. Solids 1985, 71, 403. (21) Sharma, S. K.; Simons, B.; Yoder, Jr., H. S.Amer. Mineral. 1983, 68, 1113. (22) Taylor, M.; Brown, Jr., G.E. Geochim. Cosmochim. Acta 1979,43, 61. (23) Guth, J. L.; Caullet, P.; Jacques, P.; Wey, R. Bull. SOC.Chim. Fr. 1980, 3-4, 121.
Figure 5. Top: Raman spectrum of the solid gel formed initially (composition 8.6Na201A120,1Si02556H20).Middle: Raman spectrum of gel formed in D 2 0 (composition 8.6Na201A12031Si02556D20).Bottom: Raman spectrum of aqueous gel heated at 550 "C for 12 h.
of 77 ppm was observed. Mueller and c o - ~ o r k e r reported s~~ on the 27AlN M R spectra of tetramethylammonium aluminosilicate solutions and found that the chemical shift of 27Alcorrelated with the number of Si atoms surrounding the AI nucleus. A1(OH)4~~
(24) Mueller, D. Hoebbel, D.; Gessner, W. Chem. Phys Lett. 1981, 84, 25.
2334 The Journal of Physical Chemistry, Vol. 90, No. 11, 1986 ions exhibit a sharp peak at 80.5 ppm. The width and position of the band at 77 ppm shown in Figure 3 suggests that the major aluminosilicate species in solution has a ,Si-0-Alf structure. The -OH groups on the Si atom are presumably deprotonated at these high hydroxide ion concentrations, preventing further polymerization. So, the presence of a Raman band at 575 cm-' in the gel indicates the presence of discrete Si-0-A1 units. The fact that this vibration shows up at a different frequency compared to the other Si-O-A1 units suggests that it is decoupled from the rest of the network. This would happen if either these Si-0-AI units had elongated bond lengths or very different Si0-AI angles.'' The broad band at 705 cm-l is assigned to A1-0 stretches and evolves to the corresponding bands in zeolite A (700, 733, and 745 cm-I) as crystallization pr0~eeds.I~ To assign the other bands, we carried out further experiments on the gel and the results are shown in Figure 5. The top spectrum is that of the initial aluminosilicate gel, the middle spectrum is that of the gel formed in NaOD/D20, and the bottom spectrum was obtained by heating the initial gel to 550 "C for 12 h. The experiment in D 2 0 confirmed that none of these high-frequency bands were due to Si-OH groups. When the solution was heated 550 OC,the band at 847 cm-l disappears and the 500-cm-I band considerably broadens. We assign the band at 847 cm-' to -S "'i
L O -
and its decrease in intensity upon heating shows that the gel is becoming more polymerized. This is also consistent with the broadening of the 500-cm-l band, indicating a considerably disordered structure on polymerization. The group of bands at 996 cm-' (960,996, 1040, and 1082 cm-', seen more clearly in D20) are all due to S i 4 stretches. The band at 996 cm-I is assigned to the Si-0 stretch of the four-membered rings. The band at 1040 cm-I arises from Si-0- groups with one nonbonded oxygen atom.25,26The weaker bands at 960 and 1082 cm-' must arise from the other ring and disordered structures present in the gel. Therefore, from Raman spectroscopy, the picture of the gel that emerges is that it consists predominantly of four-membered rings connected in a random fashion. The interconnecting Si-0-A1 bands are dissimilar from the ring Si-0-A1 bands. The gel is considerably depolymerized consisting of both one and two nonbonded oxygen atoms on the silicon atom. Also, the gel network contains a small fraction of rings of other sizes (other than four). Gel-to-Crystal Transformation. During the first 20 h, even though no zeolite A crystals are detectable by X-ray diffraction there are some changes in the Raman spectra of the aluminosilicate gel. These spectral changes include a decrease in intensity of the 450-, 847-, and 960-cm-' band and a shift in frequency of the 504-cm-' band to 496 cm-I. These changes indicate an alteration in the gel structure. There is an increase in polymerization as evidenced by the decrease in intensity of the Si-nonbonded oxygen (25) Dutta, P.K.;Shieh, D.C.J . Raman Spectrosc. 1985, 16, 312 ( 2 6 ) Brawer, S. A.: White, W. B. J . Chem. Phys. 1975, 63, 2421.
Dutta and Shieh SCHEME I
\ /
'"
stretch (847 cm-I). Also, there is increasing order in the system, since both the 450- and 960-cm-l bands decrease in intensity. The shift of the 504-cm-I band to lower frequencies suggest an alteration in the structure of the four-membered rings. We propose that during this nucleation period an important structural transformation is brought about by the reaction of nonbonded oxygen atoms on the Si atom with the Al(OH),- species in solution, which may give rise to structures that can then form eightmembered rings. A possible crystallization route is shown in Scheme I. Once eight-membered cubic rings are formed, they can join to form the nuclei for crystals of zeolite A. Once the crystallization process begins, we find that the Raman spectra of the solid phase is a sum of gel and crystal spectra, indicating that the gel does not undergo further structural changes. It must have the structural units in place. for zeolite A formation. As crystallization proceeds the bands at 575 cm-' due to interconnecting Si-O-A1 bands and the 847 cm-l due to nonbonded oxygen atoms decrease in intensity. Even after complete crystallization, there is a weak band at 846 cm-I, which must arise from surface Si atoms. In conclusion, we find that the gel that transforms to zeolite A has a well-defined structure consisting predominantly of four-membered rings. The conversion of gel to crystal requires the presence of AI(0H); ions in solution. During the nucleation period, the gel alters its structure by interacting with the Al(OH)4ions and may form cubic rings which act as the nuclei for zeolite A crystals.
Acknowledgment. We gratefully acknowledge the help of Dr. C. Cottrell in assisting with the N M R experiment and Dr. R . Kramer and Dr. R. Ross of Alcoa for recording the X-ray powder diffraction patterns and elemental analysis. We appreciate the financial help from the Office of Research and Graduate Studies of the Ohio State University.
