Langmuir 1992,8, 722-726
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Vibrational Spectroscopic Study of the Evolution of the Framework of the Zeolite Ferrierite Prabir K. Dutta,' K. Mohana Rao, and Jong Yul Park? Department of Chemistry, The Ohio State University, 120 West 18th Avenue, Columbus, Ohio 43210 Received August 6,1991. In Final Form: November 1 , 1991
Ferrierite is distinct from other zeolites in that it does not contain four-membered aluminosilicaterings, but is composed of five-, six-, eight-, and ten-membered rings. It is difficult to synthesize ferrierite from a completely inorganic system, whereas the addition of organic structure directing agents such as pyrrolidine greatly speeds up the process. Using Raman spectroscopy,we have investigated the evolution of the ferrierite structure from an inorganic and organic gel system. In the inorganic system, the gel structure is primarily made up of four-membered aluminosilicaterings, and addition of pyrrolidine results in transformation of the gel to a more ferrierite-like structure. The NH2+ rocking mode of the pyrrolidine associated with the gel undergoes significant changes during nucleation. These data are consistent with a zeolite growth process which proceeds from a global ordering of the aluminosilicategel at the initial stages to the subsequentbuildup of smaller domains and finallyto the assembly of the specific units characteristic of the zeolite itself.
Introduction The molecular transformations that occur during zeolite crystallization from aluminate and silicate reactants are very complex and poorly understood.' Though the synthesis of different frameworks from a wide reactant composition in the presence or absence of various structure directing agents has been successfullyexamined,2and even commercialized on a large scale, the inability to understand and control events at a molecular level has thwarted attempts to design new materials. This problem is being addressed with the development of spectroscopic probes, such as NMR and Raman spectroscopies and neutron scattering.3-5 Clearly, more molecular information is necessary to understand the synthetic process. Such information may also be useful in understanding the assembly of other solid-state systems. In this study, we examine the synthesis of ferrierite. Ferrierite is quite distinct from most zeolites in that it does not contain four-membered aluminosilicaterings. The structure is composed of chains of five-membered rings which are connected through six- and ten-membered rings leading to intersecting channels of eight- and tenmembered rings.6 Ferrierite can be synthesized with difficulty from an inorganic system with NaOH at high temperatures (613 K) and long reaction times (10 days).'
* Author to whom correspondence should be addressed.
+ Permament address:
Department of Chemistry,Pusan National University, Pusan 609-735, Korea. (1) Barrer, R. M. Hydrothermal Chemistry of Zeolites; Academic, London, 1982. (2) Szostak, R. Molecular Sieues, Principles of Synthesis and Identification; Van Nostrand: New York, 1989. (3) (a) Harris, R. K.; Knight, C. T. G.; Hull, W. E. J.Am. Chem. SOC. 1981,103,1577. (b) Engelhardt, G.; Fahllke, B.; Magi, M.; Lippmaa, E. Zeolites 1985,5,49; Zeolites 1983,3,321. (c) Groenen, E. J. J.; Kortbeek, A. G. T. G.; Mackay, M.; Sudmeijer, 0. Zeolites 1986,6, 403. (d) McCormick, A. V.; Bell, A. T. Catal. Rev. 1989, 3, 97. (4) (a) Dutta, P. K.; Shieh, D. C.; Del Barco, B. Chem. Phys. Lett. 1986, 127,200. (b) Dutta, P. K.; Shieh, D. C. J . Phys. Chem. 1986,90, 2331. (c) Dutta, P. K.; Shieh, D. C.; Puri, M. J.Phys. Chem. 1987,91,2332. (d) Dutta, P. K.; Puri, M. J. Phys. Chem. 1987,91,4329. (e) Dutta, P. K.; Puri, M.; Shieh, D. C. Mater. Res. SOC. Symp. Proc. 1988,111, 101.(f) Dutta, P. K.; Bowers, C.; Puri, M. ACS Symp. Ser. 1989, 398, 98. (9) Twu, J.; Dutta, P. K.; Kresge, C. J. Phys. Chem. 1991,95, 5267. (5) Iton, L. E.; Trouw, F.; Brun, T. 0.; Epperson, J. E.; White, J. W.; Henderson, S. J. Submitted for publication in Langmuir. (6) Vaughn, P. A. Acta Crystallogr. 1966,21, 983.
0743-7463/92/2408-0722$03.00/0
The Si02/A1203ratio of the starting composition needs to be around 10. In contrast, the presence of organic molecules like pyrrolidine and diaminoalkanes speeds up the synthesis process and requires lower temperatures (350-450 K) and shorter time periods (few days).a This study examines the structural evolution of the zeolite in the organic and inorganic methods of preparation and the role of the organic species by Raman spectroscopy and diffraction. Based on the spectral changes, a model for zeolite growth is proposed. No previous mechanistic study has been reported for the synthesis of ferrierite. However, Raman spectroscopy has been used previously to examine the formation of zeolites A, X, and Y, mordenite, and ZSM-5.4
Experimental Section The synthesisof the zeolites followedthe procedures described in the literat~re.~J~ For the organic preparation, a solution of 1.8965g of NaAlOz (Nalco) and 0.1955 g of NaOH (Jenneile)was dissolved in 50 mL of water. To this solution was added 10.46 g of pyrrolidine (Aldrich)followed by 47.3565 g of silica sol (Ludox HS-30). The homogeneous gel thus formed was distributed in six Teflon-linedParr bombs and heated to 165"C. The bombs were periodically removed from the oven and centrifuged and the wet solid gel was examined by various techniques. In the inorganicpreparation, 0.0668 g of NaOH (Jenneile)was dissolved in 4.75 mL of water, to which 0.283 g of A1203 (Dispall20,Vista Chemical Co.) was added and shaken for 3 h. This mixture was then addedto 1.2505gof Si02 (Ludox40HS,Du Pont)suspended in 9.25 mL of water. The contents were poured in Parr Teflonlined acid digestion bombs and heated to 220 O C . X-ray diffraction patterns were obtained with a RigakuJGeigerflex D Max 2B diffractometer using Ni-filtered Cu K a as the source. Raman spectra were all recorded using 514.5-nm radiation from a Spectra Physics argon ion laser (Model 171). Typical laser power at the sample was 100 mW. Scanning times of 3 s/cm-l were used at a slit width of 6 cm-l, with photon counting electronics. All spectra were baseline corrected using Spectra Calc software (Galactic). X-ray fluorescence analysis using a Kevex 0700/7000 instrument with Rh as the anode was used for examining A1 incorporation.
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(7) (a) Hawkings, D. B. Mater. Res. Bull. 1967,2,961. (b) Barrer, R.
M.; Marshall, D. J. J. Chem. SOC. 1964,2296. (8) Araya, A.; Lowe, B. M. J. Chem. Res. 1985, 192. (9) Vaughn, D. E. W.; Edwards, G. C. US Patent 3966883, 1976. (10)Plank, C. J.; Rosinski, E. J.; Rubin, M. K. US Patent 4016245, 1977.
0 1992 American Chemical Society
Langmuir, Vol. 8, No. 2, 1992 723
Framework of the Zeolite Ferrierite
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Figure 1. Powder diffraction patterns in the course of ferrierite synthesis from an inorganic reactant composition: (A) 574 h; (B) 592 h; (C)648 h; (D) lo00 h.
Figure 2. Raman spectra of gel in the course of ferrierite synthesisfrom an inorganic reactant composition: (A) 574 h; (B) 592 h; (C)648 h; (D) lo00 h.
Results
sensitive to the average (T-0-T) angle. For a series of zeolites, this band is the most prominent band and has been found to vary from 380 to 530 cm-', with an inverse correlation between the frequency and the average (TO-T) ang1e.l' These studies also indicate that zeolites with four, six, eight, and higher membered rings such as present in zeolites A, X, and P, chabazite, and Cs-Dexhibit this band between 480 and 530 cm-l, whereas with introduction of five-membered rings such as in ZSM-5, mordenite, ZSM-48, and ferrierite, this band shifts to 380450 cm-'.ll The latter family of zeolites have higher average T-0-T angles. The frequency of the band in this region (400-600 cm-I) has been correlated also with the presence of discrete ring structures in amorphous aluminosilicate glasses and melts.lZ For example, in anorthite-like glasses, which have predominantly four-membered rings, the v,(T-O-T) band is at 500-510 cm-l, whereas for nepheline glass with sixmembered rings, this band is below 470 cm-'. We have examined the Raman spectra of the amorphous gel phase for zeolites A, X, and Y and mordenite.* All of these materials exhibit a prominent broad band between 490 and 505 cm-' and have been assigned as arising primarily from four-membered aluminosilicate rings. However,the width of these bands (-30-40 cm-') indicates considerable disorder and also the presence of a distribution of ring sizes. In analogy, the band observed at 485 cm-I in the early stages of ferrierite synthesis (the first 600 h) is assigned to structures containing disordered four-membered aluminosilicate rings. Restructuring of the gel, that must be
Ferrierite crystals, as determined by the X-ray diffraction patterns were formed in both the inorganic and organic (pyrrolidine) methods of preparation. Below we focus on the two systems separately followed by a discussion contrasting and comparing the two routes. Inorganic Synthesis. The composition of the reaction system was 15 Siod1.5 NazO/&Od1500 HzO. Figure 1 shows the diffraction patterns obtained as a function of time. No crystals are apparent until 648 h of heating. As has been recognized in the literature, the choice of the starting composition is critical for the formation of ferrierite. Increasing the OH-/Si02 ratio by 16% speeded up the crystallization process but also resulted in the crystallization of mordenite, whereas lowering the OH-/SiOz ratio by the same amount resulted in zeolite crystallization only after lo00 h of heating, as shown in Figure 1D. Raman spectra of the unwashed, centrifuged, wet solid phase during the crystallization period are shown in Figure 2. The pH of the reaction mixture throughout the crystallization process ranged between 11and 11.5. The solubilities of silicates and aluminosilicates at these pH ranges are lower than the detection level of spontaneous Raman spectroscopy (-0.05 M). Therefore, all the Raman bands observed in Figure 2 arise from the solid phase of the gel. Of particular interest is the prominent band between 400and 500 cm-', which acta as a structure marker. During the first 500 h of synthesis, this band appears at 485 cm-'. Prior to crystallization a broad shoulder at 440 cm-' is observed which then emerges as the band at 432 cm-', characteristic of ferrierite crystals. We have discussed the assignment of this band in several previous publication^.^ It involves the motion of the oxygen atom in the T-0-T (T = Si, Al) bond, and its frequency is
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(11) (a) Dutta, P. K.; Shieh, D. C.; Puri, M. Zeolites 1988,8,306. (b) Dutta, P. K.; Rao, K. M.; Park, J. Y. J.Phys. Chem. 1991,95, 6654. (12) Sharma, S.K.;Simons, B.; Yoder, H. S., Jr. Am. Mineral. 1983, 68,1113.
724 Langmuir, Vol. 8, No. 2, 1992
Dutta et al.
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Figure 3. Powder diffraction patterns in the course of ferrierite synthesis in the presence of pyrrolidine. occurring during this period by attack of OH- ion, leaves the average structure unaltered. This must proceed in a localized manner, and the sensitivity of Raman spectroscopy makes it difficult to detect the dynamics of the process. However, after 600 h of heating, before the appearance of any crystals in the diffraction pattern, the Raman spectrum shows a broadening of the peak at 485 cm-l with a distinct shoulder a t 440 cm-l. This clearly indicates that the gel structure has changed and has begun to acquire a ferrierite-like structure. This is evident from the growth and sharpening of this band to form the 435-cm-l band characteristic of ferrierite. Since the sensitivity of Raman spectroscopy is low, this would indicate that the precrystalline ferrierite-like structure is widespread throughout the gel. Also, its breadth (-50 cm-l) indicates that it is considerably disordered. Just the presence of numerous small crystals is not expected to produce such a broadening. This observation is similar to what we have noted for mordenite and may be a general feature of crystal g r ~ w t h . ~ ~ Organic Synthesis. The addition of certain organic species speeds up the ferrierite synthesis process. Figure 3 shows the X-ray diffraction obtained with pyrrolidine as the organicstructure directing agent and with the overall reactant composition being 20 SiOd1.2 Na~O/A1203/398 H20/12.6 pyrrolidine. The SiOdAl203 ratio of this preparation is considerably higher than the inorganic synthesis. Based on the literature, the Si/Al ratio of the framework for similar preparations is of the order of 10. As is evident from Figure 3, the synthesis process is completed within 80 h, considerably faster than the inorganic preparation. Figure 4 follows the Raman spectra of the gel phase as a function of crystallization time. There are two regions of interest. The first is the T-0-T bending region between 450 and 500 cm-l due to the aluminosilicate matrix and
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Figure 4. Raman spectra of gel in the course of ferrierite synthesis in the presence of pyrrolidine.
the second is the region between 850 and 950 cm-1, where the prominent band of pyrrolidine occurs. After initial mixing of reactants, these bands are observed at 440 and 902 cm-l. The band at 440 cm-l is characteristic of SiOz, however, it is typically much broader. Addition of pyrrolidine to the Si02 sol particles leads to a sharpening of this band. Amines are known to adsorb strongly on Si02 surface, presumably by H bonding to the surface oxygen atoms.13 The band at 902 cm-l is the characteristic pprolidine ring breathing vibration, which is the most strongly allowed band in the Raman spectrum.14 This is seen by comparison with Figure 5A,which is the spectrum for neat pyrrolidine. However,in the initially prepared gel besides the 902-cm-l band, weaker bands at 915 cm-*and a shoulder around 875 cm-l are also observed. Reaction for 11 h brings about changes in both the v,(T-0-T) bending region and the pyrrolidine vibration. A band is observed at 485 cm-' indicative of aluminosilicate formation, as in the inorganic preparation. Along with aluminum incorporation, the pyrrolidine band at -900 cm-l splits into two bands at 913 and 881 cm-l. Figure 5B shows the Raman spectrum of pyrrolidine dissolved in 6 M HC1, and in comparison to free base pyrrolidine, it is clear that two bands appear a t 912 and 880 cm-l upon protonation of the amine. In order to assign these bands, proline (2-carboxylpyrrolidine) serves as a good model, since detailed vibrational studies have been reported on this molecule.ls The zwitterionic form of proline, which contains the NH2+ group, exhibits two strong bands at 920 and 898 cm-1, the latter band disappearingeither upon deprotonation (high pH) or deuteration of the molecule. (13) Iler, R. K. The Chemistry of Silica;John-Wiley: New York,1979; Chapter 6. (14) Evans,J. C.; Wahr, J. C. J . Chem. Phys. 1959,31,655. (15) (a) Garfinkel, D. J . Am. Chem. SOC.1958,80,3827. (b)Herlinger, A. W.; Long, T. V. J. Am. Chem. SOC.1970,92, 6481.
Langmuir, Vol. 8, No. 2, 1992 725
Framework of the Zeolite Ferrierite ~
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Figure 5. Raman spectra of (A) neat pyrrolidine and (B)1 M pyrrolidine in 1 M HCl.
The band a t 898 cm-l has therefore been assigned to the NH2+rockingmode. On the basis of this analogy, we assign the bands at 913 and 881 cm-l in protonated pyrrolidine to ring breathing and NH2+ rocking vibrations. Appearance of the latter band during ferrierite synthesis coincides with the incorporation of A1 into the solid gel structure, as determined by X-ray fluorescence, thus implying that the protonated pyrrolidine is neutralizing the charge on aluminum. The pKa of pyrrolidine is 11.27,16 indicating that a t the pH = 12.3 of the reaction mixture, about 12 ?6 of the pyrrolidine is in the protonated form. The association of pyrrolidine with the Si02 sol at the initial stage of the reaction (Figure4A) represents its distribution a t the reaction pH and is mostly the neutral form of the molecule. It is only after Al incorporation that the protonated form of pyrrolidine emerges as the major species in the gel. As the zeolite crystallization proceeds, changes are continually observed in both the v,(T-O-T) and the NH2+ rocking modes. After 38 h of heating, the band at 485 cm-' is still present along with a band at -438 cm-l. The latter band is typical of ferrierite-like structures. It is hard to distinguish this band from that of the Si02 at 0 h, but it would be quite unexpected if Si02 still maintained its integrity at pH -12 after heating for 38 h at 165 "C. So, we assign this band to pre-ferrierite-like structure, as was also apparent in the inorganic preparation. Associated with this change is the splitting of the NH2+ rocking mode into several bands. This would suggest that pyrrolidine is located a t sites within the aluminosilicate framework with varying RNH2+.-Al- interactions and also somewhat constrained a t these sites. This process continues with crystallization. After -62 h of reaction, the NH2+rocking band is clearly split into bands at 886 and 877 cm-', and finally with the formation of well-definedferrieritecrystals, the band is broadened without a well-defined peak. We interpret this as indicating that as crystallization proceeds, the protonated pyrrolidine finds itself in different sites. The broadening must arise from a range of different interactions between the NH2+ group and the A1 of the aluminosilicate framework at the different constrained sites that it is held. This is also an indication about the heterogeneity of the A1 sites. The v,(T-0-T) bending (16)CRCHandbook of Chemistry andPhysics; CRC Press: Cleveland,
OH,1975.
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Figure 6. Infrared spectra of dried gel in the course of ferrierite synthesis in the presence of pyrrolidine.
mode shows the characteristic ferrierite band at -435 cm-l, a t 62 h, well before the diffraction pattern shows the formation of crystals (80 h). The infrared spectra of the gel samples are shown in Figure 6. There are no clear changes associated with the processof crystallization as in the Raman spectrum,though features due to the ferrierite crystal are apparent after 80 hof heating. No bands characteristicof the organicspecies were observed in this region, presumably due to the strong extinction of the aluminosilicate species.
Discussion There are several issues that emerge from the above spectroscopic studies that are addressed in this section. The first is the relative length of time required for crystallization in the inorganic versus the organic preparation. As mentioned before, ferrieriteframeworkis made up of five-, six-, eight-, and ten-membered rings. The Raman spectra of the aluminosilicate gel in the early stages of synthesis show the presence of four-membered aluminosilicate rings. The presence of A1 in such rings has been shown to confer stability on these rings.17 Both mordenite and ZSM-5 that crystallize quite rapidly in this field contain four-membered aluminosilicate ringsaleThus, the necessity of disruption of these four-membered rings in the nucleation of ferrierite must slow down this process. The role of the pyrrolidine in speeding up the process could be to stabilize a different gel structure. From Figure 4, it is clear that the band at 485 cm-' due to four-membered aluminosilicate rings is being formed with A1incorporation. (17)(a) Meier, W. M.; Meier, R.; Gramlich, V. 2 . Kristallogr. 1978,
147, 329. (b) Derouance, E. G.; Fripiat, T. G. Proceedings of the 6th InternotionolZeoliteConference; Olson, D.,Bisio,A., Eds.;Butterworth:
Guildford, U.K., 1984;p 717. (18)Shiralkar, V. P.;Clearfield, A. Zeolites 1989,9, 363.
Dutta et al,
726 Langmuir, Vol. 8, No. 2,1992
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Figure 7. Raman spectra of (A) reactant composition 20 SiOd 1.2 NazO/A1203/398HzOheated at 165 "Cfor 72hand (B)sample in (A) heated with 12.6 mol of pyrrolidine for 24 h.
But, after 38 h of heating, the prominent band is at 438 cm-', typical of a gel with a higher average T-0-T angle. This becomes quite obvious from the data shown in Figure 7. The lower trace (Figure 7A) shows the Raman spectrum of a gel prepared from a similar composition as that for organic ferrierite synthesis, but in the absence of pyrrolidine and heated a t 165 "C for 72 h. The broad band at 485 cm-' is characteristic of the aluminosilicate species observed in the inorganic preparation. Addition of pyrrolidine to this system followed by heating for another 24 h brings about a shift of this band to 440 cm-l (Figure 7B) without any crystal formation. This is indicative of the fact that pyrrolidine is indeed stabilizing a gel structure that is more readily transformed to the ferrierite framework, thereby speeding up this process. The second issue is that the sensitivity of Raman spectroscopy in detecting crystal formation in both the inorganic and organicpreparation is superior to diffraction data, in that the Raman bands characteristic of ferrierite are observedsignificantly ahead of the diffraction patterns. Finally, the vibrational changes of the pyrrolidine molecule during zeolite growth provide information about the mechanism of crystallization. It is clear that the protonated pyrrolidine is associated with the A1 in the aluminosilicate gel since it appears along with A1 incorporation. The splitting and broadening of the NH2+rocking mode as crystallization proceeds indicates that the pyrrolidine molecule is being increasingly constrained at a variety of Al-containing sites. However, this anchoring proceeds through specific stages. In the initial stages of crystallization, after 11 h of heating, the pyrrolidine molecules in the aluminosilicate gel resembles molecules in solution, indicating that enough mobility exists. Restructuring of the gel after 38 h of heating leads to a
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splitting of the NH2+ rocking mode, indicating that there is a distribution of these molecules at different sites. This band splits further with crystallization, suggesting a diversity of sites as well as localization of pyrrolidine. This observation can be explained by proposing a zeolite crystallization mechanism, which initially involves global restructuring of the aluminosilicate gel, followed by buildup of smaller domain structures and finally the local structure that is characteristic of the zeolite structure itself. This idea is different from the more commonly accepted view of zeolite synthesis, where it is thought that small subunits characteristic of the zeolite assemble to form the larger domains. In the view proposed here, the detailed subunit connections to form the zeolite are the last step in the evolution process. A pictorial schematic of this process is shown in Scheme I. We should make it clear that the assembly of ferrierite as proposed here is a hypothesis that is consistent with our data. More specific details about the gel structure cannot be deduced from the Raman data. However, this picture is consistent with the Raman spectral changesobservedin the v,(T-O-T) bendmgregion, where we observe the formation of bands characteristic of pre-ferrierite structure (which are represented in their simplest form as chains though the presence of sheets and other structures are possiblelg)well before long range order that is typical of zeolite crystals is observed. In conclusion, the following points emerge from this study: (a) The inorganic synthesis of ferrierite takes a considerably longer time period, because of the restructuring of the predominantly four-membered aluminosilicate ring amorphous gel to produce a framework that does not contain such rings. The pyrrolidine tends to reorder the gel structure so as to make it more favorable to ferrierite formation. (b) The stages of trapping of the pyrrolidine cation suggest that the zeolite formation proceeds from a global ordering of the aluminosilicate gel at the initial stages to the buildup of specific building units at the final stages. (19) Davis, M. E.;Montes, C.; Hathaway, P.E.;Garces, J. M. Stud. Surf. Sci. CataE. 1989,49, 199.
