5267
J. Phys. Chem. 1991, 95, 5267-5271 derneath the surface (condition q 22b), provided K . ~ = 1. It is different for model B, where in case a the p r w should be controlled by the electron-transfer reaction (cf. eq 25). In case b, the critical rate constants for electron transfer are somewhat lower. However, according to the rate constant calculated above with = 1, only for particles with large R could the reaction rate b m e controlled by oxygen transport. If, however, should be much smaller than 1, the process for model B will always be controlled by the electron-transfer reaction and not by oxygen diffusion. Experimental data indicate much slower electron-transfer rates to oxygen from semiconductors than this theoretical analysis predicts. Specifically, a heterogeneous rate constant of lo-' cm s-' for O2reduction on colloidal T i 0 2 has been reported.M This corres nds, according to eq 9 and for 6 = 3 X lo4 cm, to k,, = 3 s- , Measurements on compact TiOz electrodes also indicate a much slower electron-transfer rate to O2than calculated above with X = 1 eV and K , ~= 1. It appears therefore that K , ~is very small for this electron-transfer reaction and that the process is not adiabatic. If the electrons were in traps on the surface (Ei= ET), the activation energy of eq 29 would increase. With a trap depth of 0.4 eV, the energy difference E O , - ET would be 1.4 eV at pH 5.5 and the calculated rate constant with
Po
(50) Gritzcl, M. Now. J . Chim. 1980, I , 151.
would be kd EJ 3 X lo3 s-'. One sees that trapping can reduce the electron-transfer rate considerably for energetic reasons. Both effects, a weak overlap between the electronic orbitals of O2 and the electrons of the semiconductor, which makes K , ~small, or electron transfer from traps, can explain the slow rates observed for the reduction of oxygen on semiconductors. But a small K , ~can be overcome by the creation of electronic states on the surface that strongly interact with O2 This can be achieved with metal atoms deposited on the surface and explains their catalytic activity.
Conclusions Our models indicate that in the absence of catalytic activation of the surface the rate constant for electron transfer to oxygen may be the decisive factor, determining the efficiency of spontaneous photoelectrochemical processes on semiconductor particles where oxygen is the electron acceptor. Catalysis of electron transfer to dioxygen becomes mandatory if a high quantum yield is to be achieved. Oxygen will also be essential in stabilizing the primary reaction products of photooxidation prooesses like reaction XI. In such processes, even the photoreduction of O2could be beneficial for the net photooxidation. Finally, in reaction XIII, oxygen will also increase the oxidation yield. We shall apply this analysis in a later paper to the technological problem of solarassisted oxidation of organic material on Ti02 particles in aerated aqueous solutions. Acknowledgment. We are grateful to the Department of Energy for the support of this work. Registry No. TiO,, 13463-67-7; 02,7782-44-7.
Vibrational Spectroscopic Examination of the Formation of Mordenite Crystals Jen Twu, Prabir K. Dutta,* Department of Chemistry, The Ohio State University, Columbus, Ohio 43210
and Charles T. Kresge Paulsboro Research Laboratory, Mobil Research and Development Corporation, Paulsboro, New Jersey 08066 (Received: November 2, 1990)
This study focuses on Raman spectroscopic examination of the gel phases present during growth of mordenite from both completely inorganic and organic preparations containing tetraethylammoniumions. In the inorganic preparation, four-membered aluminosilicate rings are observed at the early stages followed by a 'mordenite-like" amorphous phase, with disordered fourand five-membered aluminosilicate rings, which then quickly connect to form zeolite crystals. In the organic preparation, such intermediates are also observed at the early stages of synthesis. However, these units do not readily condense to form zeolite crystals, thereby suggesting that the role of the quaternary ammonium cation may be to stabilize the mordenite nuclei for extended periods of time until the proper connectivity is established to form zeolite crystals.
Introduction Zeolites with the mordenite-type framework are important in various adsorption and catalytic applications.lu2 The crystal structure of mordenite has been examined by Meier, and consists of sheets of five- and four-membered aluminosilicate rings.3 The interconnection of these rings results in two parallel channels of twelve- and eight-membered rings of dimensions 6.7 X 7.0 A and 2.9 X 5.7 A, respectively. These channels are interconnected via (1) Jacobs, P. A. CorboniogenicAcrioiry ofZeolires; Elsevier: Amsterdam, 1977.
small side pockets of diameter 2.9 A. The synthesis of mordenites has been recently reviewed by Bajpaia4 Various silicon- and aluminum-containing reactants, including natural sources, have been used for the synthesis of mordenite type zeolites. The two important parameters are the Si02/A1203ratio and the pH of the starting composition, as is typical of most zeolite synthesis. The SiO2/AI2O3 ratios of mordenites usually range from 9 to 12.5 Higher ratios up to 20 can be obtained by carefully adjusting the Na20/Si02 ratio of the reactant mixture! Siliceous mordenite can also be synthesized ~
( 2 ) Barrer, R. M. Hydrorhermol Chemistry of Zeolites; Academic: London, 1982. ( 3 ) Meier, W. M.Z . Krisfollogr.,Krisrollgeom., Krisrollphys., Krisrallchem. 1961, l l S , 439.
0022-3654/91/2095-5267$02.50/0
(4) Bajpai, P. K. Zeolites 1986, 6, 2. ( 5 ) Culzaz, A.; Sand, L. B. Ado. Chem. Ser. 1973, 121, 140. Bodart, P.; Nagy,J. B.; Derouane, E. G.; Gabelica, Z. Srud. Surf Sci. Corol. 1984,18, 125.
0 1991 American Chemical Society
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in the presence of organic ions such as tetraethylammonium or diethylpiperidinium, with SiO2/AI2O3ratios reaching 40.' The other materials that form competitively along with mordenite are analcime, phillipsite, ZSM-5, and q ~ a r t z . ~ Only J a few studies have been reported on the mechanism of crystallization of mordenite. These were limited to the study of kinetics of the synthesis process, and the influence of reactant composition on the Si02/A1203ratio of the product.'*59 Bodart et al. have studied the liquid and solid precursors and intermediate species during mordenite synthesis by 27Al and 29SiNMR spectroscopy and scanning electron microscopy. They concluded that the formation of mordenite crystals proceeded through soluble aluminosilicate species obtained by dissolution of the amorphous phase.IO In this paper, we report the first Raman spectroscopic study of the mordenite crystal growth process for an inorganic and an organic preparation. Specifically, by using Raman spectroscopy and x-ray diffraction, we have focused on the following issues: the structure of the gel phase during nucleation, and its evolution to the zeolite: the nature of the interaction between the organic species and the gel framework during zeolite formation; and the assignment of the framework vibrations of mordenite. Experimental Section For the inorganic preparation,IIP a solution of sodium aluminate in NaOH was added to colloidal silica (Ludox AS 30 from DuPont), with extensive stirring. The gel composition was 3Na20:1A1203:13.1Si02:223H20and the pH was measured to be 11. The mixture was heated in Teflon-lined autoclaves to 190 OC. Different batches of identical reaction mixtures were heated for various times as indicated in the text. After the reaction was quenched, the mixture was centrifuged, and the wet solid phase (referred to as gel in the text) was directly examined by Raman spectroscopy and X-ray diffraction. For the organic preparation,IIb an alkaline silicate solution (N-Brand from PQ Corporation) was added to a mixture of A12(S04)3,H$04 and tetraethylammonium bromide (TEA+Br-). The resultant gel composition was 7.38Na20:1 AI2O3:24.7SiO2:3.41TEABr:597H20. The mixture was heated in Teflon-lined autoclaves to 120 "C. As in the inorganic preparation, a set of Teflon-lined Parr bombs with identical reactant compositions were heated for various times, quenched, centrifuged, and examined by spectroscopic methods. The powder diffraction patterns were obtained on a Rigaku Geigerflex D/Max 2B using a nickel-filtered Cu Ka source. The Raman spectra were obtained by using radiation of 457.9 nm from an argon ion laser (Spectra Physics 171). Power at the sample was -20 mW. A Spex double monochromator (Spex 1403) was used to discriminate against scattered radiation. The Raman light was detected with a GaAs PMT with photon counting. Typical slit widths were 6 cm-' and scanning times ranged from 1-3 s/cm-l. The background from the Rayleigh scattering in the low-frequency Raman spectra were removed with the help of Spectracalc programs.
Twu et al. 1030
wh 1030
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Figure 1. Raman spectra of Ludox and gels obtained during synthesis
of mordenite from an inorganic preparation: (a) Ludox; (b-e) reaction mixture heated for 0, 4, 12, and 20 h, respectively. 72H
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Results and Discussion The results of the inorganic mordenite preparation are discussed first, followed by that of the organic mordenite and the framework vibrational band analysis. Inorganic Preparation. In the inorganic preparation, Ludox colloidal Si02was reacted with AI(OH),- at a pH of 1 1. Figure 1 shows the room-temperature Raman spectra of the gel phase obtained during the crystallization process at 190 OC. Ludox is
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Itabashi, K.; Fukushima, T.; Igawa, K. Zeolites 1986, 6, 30. Ueda, S.; Murata, H.; Koizumi, M.Am. Mineral. 1980, 65, 1012. Shiralkar, V. P.; Clearfield, A. Zeolites 1989, 9, 363. Ueda. S.;Kageyama, N.; Koizumi, M.; Kobayashi, S.; Fujiwara. Y.; Kyogoku, Y. J . Phys. Chem. 1984,88, 2128. (IO) Bodart, P.; Nagy, J. B.;Gabelica. Z.; Derauane, E.G.; J . Chim. Phys. Phys.-Chim. Biol. 1986, 83, 777. (11) (a) Sand, L. B. US. Patent 3436174, 1969. (b) Chu, L. B. U S . Patent 3758593, 1973. Chang, C. E.: Lang, W. H.; Silvstri, A. J. U S . Patent (6) (7) (8) (9)
2894184, 1975.
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Figure 2. Raman spectra of gels obtained during synthesis of mordenite from an inorganic preparation after heating: (a) 27 h; (b) 40 h; (c) 67 h; (d) 72 h.
a colloidal suspension of Si02particles of diameter 30 nm. The Raman spectrum of this material shown in Figure l a is characterized by bands at 440, 490, and 795 cm-' and is typical of vitreous silica.I2 In addition, there is a band at 978 cm-' due to the Si-OH groups on the surface of the Si02 parti~1es.I~The spectrum of the gel formed upon immediate mixing of the reactants is shown in Figure 1b. A broad band centered a t 450 cm-l and arising from the Si02network is observed. Unreacted Al(OH),retained in the gel gives rise to the band at 620 cm-I.l4 The band at 978 cm-' due to the Si-OH groups of the Si02disappears upon interaction with aluminate, indicating that initial incorporation of aluminum occurs by reaction of surface Si-OH with Al(0H)c. The sharp band a t 1060 cm-' arises from the CO,*- ion. After the reaction mixture has been heated for 4 h, the 620-cm-I band due to Al(OH)4- is no longer observed, indicating that all the aluminum (as detected by Raman spectroscopy) has been incorporated into the aluminosilicate solid. Raman bands characteristic of soluble monomeric, dimeric, and other oligomeric silicate species (450, 500, 530,600,780 cm-I) are not obse~ed.'~ This is a reflection of the low pH of the reaction mixture. In a previous study of silicate solutions by Raman spectroscopy, we (12) McMillan, P. Am. Mineral. 1986, 69, 622. (1 3) Gottardi, V.; Guglielmi, M.; Bertoluzza. A.; Fagnano, C.; Morelli. M. A. J . Non-Cryst. Solids 1984, 63, 71.
(14) Moolenaar, R. J.; Evans, J. C.; McKeever, L. D. J. Phys. Chem. 1970, 74, 3629. (15) Dutta, P. K.; Shieh, D. C. Appl. Spectrosc. 1985, 39, 343.
Examination of the Formation of Mordenite Crystals
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Figure 3. X-ray powder diffraction patterns of samples obtained from the inorganic mordenite preparation after hating: (a) 4 h; (b) 20 h; (c) 40 h.
have shown that soluble silicate species are not detected in the Raman spectrum below a NaOH concentration of 0.1 M.I5 The solubility of silica at pH 11 corresponds to 0.05 M,I6 whereas the typical detection limits for spontaneous Raman spectroscopy are in the 0.05-0.1 M range. Therefore, the observed bands in Figure I C at 495,800, 895, and 1025 cm-’ are all characteristic of the solid part of the gel phase. This spectral pattern remains more or less unchanged until the synthesis has proceeded for 20 h. At this time, there appear broad bands at 402 and 465 cm-I, in addition to the 495- and the 1030-cm-’ bands. The broad bands at 402 and 465 cm-’ sharpen to 400 and 471 cm-I, upon 27 h of heating (Figure 2a). The bands at 495,900, and 1030 cm-l are still observed at this time. After 40 h of heating this initial set of bands disappears, and the bands at 400,450,471,817, 1047, and 1160 cm-I remain. All these bands become prominent with increasing times of crystallization. Figure 3a-c shows the powder X-ray diffraction pattern obtained from the gel phase after crystallization times of 4,20, and 40 h. Crystallization is complete after -70 h of heating. The first formation of zeolite crystals in the X-ray diffraction patterns are only apparent after 27 h of heating. Therefore, the time period involving the first 20 h defines the nucleation period. At the early stages of this period (4-12 h) the Raman spectrum of the gel is characterized by bands at 495, 800, 900, and 1030 cm-I. In earlier publications, we have discussed the assignment of the Raman spectra of amorphous aluminosilicate solids present during zeolite crystallization by comparison with published data on minerals.”J* The band at 495 cm-’ is characteristic of vIL(T-O-T) vibrations of fourmembered aluminosilicate rings in solids and was observed in the Raman spectrum during the early stages of synthesis of zeolites A, X, and P.” The difference between these systems and the present mordenite synthesis is the lower pH, which should result in an increased degree of connectivity of the Si chains attached to A1.I6 Thus, we can schematically represent the amorphous aluminosilicate in the mordenite preparation as
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The Journal of Physical Chemistry, Vol. 95, No. 13, 1991 5269 The bands at 900 and 1030 cm-I arise from S i 4 stretches of nonbonded oxygen atoms. After 12 h of heating, only minor changes are observed in the spectrum, including a narrowing of the 495-cm-I band by 10 an-’. The most significant changes occur after 20 h of heating, well before the appearance of crystals in the X-ray diffraction patterns (27 h). These changes include the appearance of broad Raman bands at 402 and 465 cm-l, which upon further heating emerge as sharp bands characteristic of the mordenite framework. Clearly, we are observing an intermediate “mordenite-like” structure in the evolution from the four-membered aluminosilicate gel to the crystal. The broadness of the 402- and 465-cm-I peaks indicates that the structures responsible for these bands are considerably disordered even though on average, mordenite-like units are present. As discussed later, bands at 470 and 400 cm-I are assigned to the v, (T-0-T) modes indicative of frameworks containing four- and five-membered rings.Ig This would indicate that the four-membered rings initially present in the amorphous gel are undergoing changes in structure as they accommodate five-membered rings around them. For example, structural changes such as that depicted below could cause these spectroscopic changes.20
+KEL man -1
470,400 an -1
The broadness of the peaks arises from the disordered environment around these building blocks, giving rise to a distribution of T-0-T angles. Also, since the sensitivity of spontaneous Raman spectroscopy is low, it is clear that a significant concentration of these units will have to exist to make it possible to observe the Raman bands. We propose that these “mordenite-like” building units are dispersed throughout the solid phase after heating for 20 h and the formation of crystals involves the proper orientation and connection between these units. This appears to be a rapid process since mordenite crystals are observed after 27 h of heating. The Raman spectra indicate that the initial gel consists of fourmembered aluminosilicate rings, which are ultimately incorporated into the crystal, as shown above. This is in agreement with the current understanding of the structure of mordenite, in which it is thought that the A1 atoms are preferentially located in the four-membered rings, with double occupancy of these rings being favored?’-u This results in the Si02/A1203ratio of the mordenite framework being 10, which is what we find for the preparation used in this study. Organic Preparation. The synthesis of mordenite using organic reagents has been reported in the literature as a route to make materials with SiOz/Al2O3ratios approaching ~ 4 0 We . ~have ~ examined the system formed by reacting sodium silicate (N-Brand) with aluminum sulfate and TEABr at 120 OC. The reaction was carried out for 192 h and the gel phase examined periodically during the crystallization process. Figure 4a-c and the corresponding inserts a‘%’, show the Raman spectra of a solution of TEABr (0.1 M), crystalline TEABr, and a fully crystalline mordenite sample, respectively.
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0-4
(16) Her, R. K. The Chemisrry of Silicu: John-Wiley: New York, 1979. (17) Dutta, P. K.; Shieh, D. C. J . Phys. Chem. 1986, 90,2331. (18) Dutta, P. K.; Shieh, D. C.; Puri, M. J . Phys. Chem. 19%7,91,2332.
(19) Dutta, P. K.; Puri, M. J . Phys. Chem. 1987, 91, 4329. (20) Prof. H. Van Koningsfeld, Delft University of Technology, has also proposed that mordenite structures can be built from four-membered rings (personal communication). (21) Jambs, P. A.; Martens, J . A. Synthesis of High-Silica Aluminosilicare Zeolites; Elsevier: Amsterdam, 1987; p 321. (22) Meier, W. M.; Meier, R.; Gramlich, V. 2.Krisrallogr. 1978, 147, 329. (23) Derouane, E. G.; Fripiat, J. G. Proceedings of rhe 6rh International Zeolite Conference;Olson, D., Bisio, A., as.Butterworth, : Guildford, U.K., 1984; p 717. (24) Givens, E. N.; Plank, C. J.; Rosinski, U S . Patent 4052472, 1972, assigned to Mobil Oil Corp.
5270 The Journal of Physical Chemistry, Vol. 95, No. 13, 1991
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Figure 4. Raman spectra of (a) TEABr in solution (0.1 M) (b) TEABr crystals, and (c) mordenite crystals grown in the presence of TEABr. M
Figure 5. Raman spectra of gels obtained during crystallization of mordenite from an organic preparation: (a) 0 h; (b) 24 h; (c) 68 h.
cormponds to framework vibrations of mordenite. (a'+) Corresponding spectra in the 2800-3 100-cm-' region. The vibrations due to the aluminosilicate framework of mordenite are marked as 'M" on Figure 4c;the rest of the bands arise from TEA+. There are slight differences in intensities and splitting patterns between TEA+ in solution and in crystals (380-, 675-, 3000-cm-' bands), presumably due to crystal-packing effects. However, the spectrum of trapped TEA+ in mordenite is identical with that in solution, indicating that perturbation of the quaternary ammonium by the mordenite framework is minimal. This is quite distinct from the case of the tetrapropylammonium ion in ZSM5,19 which undergoes a major change in conformation in order to be accommodated in the zigzag channels of the zeolite. In the case of TEA+ mordenite, the size of the cation (6.5-7.5 A) would indicate that it is being held in the puckered 12-channels (6.7 X 7.0 A). The -CH3 end of the ethyl groups can protrude into the interconnecting channels of dimension 2.9 A, which connect the parallel 12- and 8-membered channels. There appears to be enough room around the TEA+ ion, and therefore the vibrational spectrum resembles that of free TEA+ during the zeolite synthesis and does not provide any information about the crystallization process, unlike that of ZSM-5.I9 Figures 5 and 6 show the Raman spectra of the gel during the zeolite crystallization process. The bands observed in the material upon initial mixing (Figure 5a) can all be assigned to the TEA+ cation (compare with Figure 4a) and the sulfate ion (980 cm-I). No bands characteristic of aluminosilicate species are observed. Figure 5b shows the spectrum from the gel after 24 h of heating at 192 OC. Besides the clear indication that the sulfate ion is being excluded from the gel, new bands are also observed at 360, 390, 460,490,600,780, and 1030 cm-' and are indicated on the figure. These bands could be arising from both solution and solid phases in the gel. The insert in Figure 6 (a') shows a difference spectrum between a solution of TEA+OH- and TEAsilicate ([Si] = 1 M). The bands a t 600, 780, and 1030 cm-' are characteristic of monomeric and dimeric silicate species. These same bands are appearing in the gel spectrum after 24 h of heating and can be assigned to solubilized silicate species trapped in the gel. This is in contrast to the inorganic preparation, in which no soluble silicate species were observed, even though the NaOH contents in both cases are similar. The remaining bands in the 300-
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Figure 6. Raman spectra of gels obtained during crystallization of mordenite from an organic preparation: (a) 144 h; (b) 168 h; (c) 192 h. (a') Difference spectrum of TEAOH and TEAsilicate solution.
600-cm-' region are assigned to the solid phase and, for the sake of clarity, have been replotted in Figure 7. The prominent band at 418 cm-I is due to TEA+, which also has a weak band at 387 cm-I, as seen for the 0-h unheated-gel spectrum in Figure 7a. The mordenite crystals exhibiuramework bands a t 397,445, and 467 cm-I as shown for the 192-h sample in Figure 7g. Upon heating the reaction mixture for 24 h (Figure 7b), bands characteristic
Examination of the Formation of Mordenite Crystals 387
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Figure 7. Raman spectra in the 300-600-cm~~ region for gels obtained
at various times during the crystallization of mordenite from organic preparation. (a-g) correspond to crystallization times of 0, 24, 68, 96, 144, 168, and 192 h, respectively.
of the mordenite framework appear at -390 and -470 cm-'. There is also considerable intensity at -490 cm-'due to disordered four-membered aluminosilicate rings (as was observed in the inorganic synthesis). The Raman spectrum in the 300-600-cm-' region remains unchanged up to 168 h of heating, and for the 192-h sample, no intensity at -490 cm-I due to disordered aluminosilicate rings is observed. The XRD patterns show the presence of zeolite crystals after 192 h of heating, as shown in Figure 8. What is most interesting about the spectra of the samples between the 24- and 168-h treatments is the appearance of bands a t 390 and 470 cm-' characteristic of the framework of mordenite, though actual crystals are only observed after 192 h of heating. In the inorganic preparation, it was noted that zeolite crystals are formed subsequent to the appearance of the 'mordenite-like" units in the gel phase. This raises the question as to why, in the organic preparation, do the mordenite crystals take so long to form, though the building blocks are apparent after 24 h of heating. It is difficult to unambiguously answer this question on the basis of this study. However, it is a general observation for many zeolite systems, that the synthesis of the higher Si/AI ratio material takes longer times as in ZK-4 compared to A or in zeolite Y compared to X and other systems.25 The Si02/A1203 ratio in the organic mordenite preparation was found to be 16. It will be important in these cases to preserve the building units of the particular zeolites long enough so that crystals can be formed. A role for the organic molecule could be to stabilize these units over extended period of times. Stabilization could be promoted by the prevention of OH- attack on the nuclei by means of an organic envelope created by the TEA ions associated with the structure. This organic cladding may also serve a role in promotion of spatial ordering of these 'mordenite-like" building units via organic-organic interactions. Further experiments will be needed to confirm these hypotheses.
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(25) Breck, D.W.Zeolite Molecular Sieves; Wiley: New York, 1976.
2-theta (degree)
Figure 8. X-ray diffraction patterns of gels obtained from mordenite in an organic preparation at (a) 168 and (b) 192 h.
Mordenite Framework Vibrations. The vibrations of the mordenite aluminosilicate framework are similar for both the inorganic and organic preparations and are characterized by bands at 395,445,470,817, 1047, and 1160 cm-I. The vibrations above lo00 cm-' are characteristic of S i 4 stretching modes, as observed in Si02.** In the frequency range of 300-600-cm-', we have established empirical correlations between the frequencies of the According Raman bands and the ring sizes and T-0-T to these correlations, four-membered rings exhibit bands in the 450-550-cm-' region, and five-membered rings in the 350-450cm-' region. On the basis of this model, the four- and fivemembered rings in the mordenite framework are responsible for the pattern of Raman bands at 390 and 470 cm-I. For a series of even-membered-ring zeolites, we reported that as the average T-0-T angle increases, the Raman band characteristic of the T-O-T bend decreases in frequency.26 On the basis of this correlation, the average T-O-T angle of the rings of mordenite would correspond to larger angles, as compared to typical zeolites such as zeolites A and faujasite.26 In conclusion, we have established the following characteristics of mordenite synthesis and structure: (a) in the inorganic preparation, four membered aluminosilicate rings formed in the initial stages evolve to produce disordered mordenite-like building units consisting of five and four-membered rings, which connect rapidly to form mordenite crystals; (b) in the organic preparation, mordenite-like structures are formed in the early stages of nucleation and remain so for extended periods of time before zeolite crystals appear; (c) the Raman spectrum of the aluminosilicate framework of mordenite shows bands characteristic of five- and four-membered rings. Further studies are continuing on the differences in crystallite size, distribution of A1 atoms across the crystal, and their siting in the inorganic versus organic mordenite syntheses. Acknowledgment. We thank Dr.Geoffrey Woolery for helpful discussions. (26) Dutta, P.K.;Shieh,
D.C.; Puri, M.Zeolites 1988, 8. 306.
