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The relation between structure and vibrational spectra of natural zeolites
Journal of Molecular Structure 596 (2001) 129±137
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The relation between structure and vibrational spectra of natural zeolites W. Mozgawa* Department of Materials Science and Ceramics, University of Mining and Metallurgy (AGH), al. Mickiewicza 30, 30-059, KrakoÂw, Poland Received 23 December 2000; revised 17 February 2001; accepted 17 February 2001
Abstract In this paper, the vibrational (IR and Raman) spectra of natural zeolites, belonging to six structural groups are presented. The band characteristics of the middle range order occur in the spectral region of 400±800 cm 21. In this range bands due to the vibrations of different rings of alumino- and silicooxygen tetrahedra, creating the secondary building units, are located. Increase in the number of ring members results in the shift of the characteristic band position towards lower wavenumbers in the IR spectra. Such a tendency is not so evident in the Raman spectra. It has been proved that vibrational spectra are useful for the identi®cation of a majority of zeolite structural groups, though unequivocal assignment of pseudolattice bands is dif®cult. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Infrared spectra; Raman spectra; Natural zeolites
1. Introduction According to the most common classi®cation, zeolites are divided into seven groups based on the socalled secondary building units (SBU) [1]. SBU are built of rings of SiO4 and AlO4 tetrahedra, containing various numbers of members which create the over-tetrahedral form of middle range order. SBU gives information on the environment of a given tetrahedron, the number of members in the ring and possibly the type of ring (single (S) or double (D)). For example, S4R implies that the single 4-membered rings dominate in the structure; D6R means that double connected 6-membered rings dominate; while 4±1 means that a complex built of 4membered rings joined with a single tetrahedron dominates, etc. [2]. * Tel.: 148-12-1722-32; fax: 148-12-3315-93. E-mail address: [email protected] (W. Mozgawa).
In the IR and Raman spectra, the assignment of bands to the ring vibrations can be a basis for the determination of the SBU type and for the identi®cation of a given zeolite structure. Theoretical group analysis performed on isolated rings of silicooxygen tetrahedra, occurring in cyclosilicate structures has shown that in the IR spectra, a single band located in the region of the so-called pseudolattice vibrations, (in the range of 400±800 cm 21), corresponds to the ring vibration. Increase in the number of ring members results in the shift of characteristic band positions towards lower wavenumbers [3]. A similar tendency should be expected in the IR spectra of zeolites of framework structure, in which the rings connect one another, creating the spacial framework. Additionally, in relation to the IR spectra of cyclosilicates, substitution of a part of silicooxygen tetrahedra by aluminooxygen considerably in¯uences the position and shape of pseudolattice bands.
0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0022-286 0(01)00741-4
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W. Mozgawa / Journal of Molecular Structure 596 (2001) 129±137
Table 1 Tab. I. List of used zeolites Name Analcime Pollucite Harmotome Phillipsite Gismondine Laumontite Offretite Levyne Cancrinite Chabazite Gmelinite Faujasite Natrolite Scolecite Mesolite Thomsonite Edingtonite Epistilbite Dachiardite Mordenite Heulandite Clinoptilolite Stilbite Brewsterite
Typical formula Na16[Al16Si32O96]´16H2O (Cs,Na)[AlSi2O6]´nH2O Ba(Ca0.5,Na)[Al5Si11O32]´ 12H2O K2(Ca0.5,Na)4[Al6Si10O32]´ 12H2O Ca4[Al8Si8O32]´ 16H2O Ca4[Al8Si16O48]´ 16H2O KCaMg[Al5Si13O36]´ 15H2O NaCa2.5[Al6Si12O36]´18H2O Na6Ca[CO3(AlSiO4)6]´2H2O Ca2[Al4Si8O24]´12H2O Na8[Al8Si16O48]´22H2O Na20Ca12Mg8[Al60Si132O384]´27H2O Na16[Al16Si24O80]´16H2O Ca8[Al16Si24O80]´24H2O Na16Ca16[Al48Si72O240]´64H2O Na4Ca8[Al20Si20O80]´24H2O Ba2[Al4Si6O20]´8H2O Ca3[Al6Si18O48]´16H2O (Na,K,Ca0.5)4[Al4Si20O48]´18H2O Na3KCa2[Al8Si40O96]´28H2O (Na,K)Ca4[Al9Si27O72]´24H2O (Na,K)6[Al6Si30O72]´20H2O NaCa4[Al9Si27O72]´30H2O Sr2[Al4Si12O32]´10H2O
SBU( a) S4R S4R S4R S4R S4R S4R S6R S6R S6R D6R D6R D6R 4±1 4±1 4±1 4±1 4±1 5±1 5±1 5±1 4±4±1 4±4±1 4±4±1 4±4±1
Locality Aussig (Bohemia.) Anburn Maine (USA) Andreasberg, (Germany) Capo di Bove (Italy) Capo di Bove (Italy) Grodziszcze (Poland) Mont Semiol (France) Stolpen (Germany) Ural Mountains (Russia) Nidda (Germany) Nova Scotia (Canada) Kaiserstuhl (Germany) Aussig (Bohemia) Berufjord (Iceland) NaalsoÈ (FaroÈes) Kilpatrick (Scotland) BoÈhlet Mine (Sweden) Berufjord (Iceland) Elba (Italy) FaroÈes Islands Paterson (USA) DylagoÂwka (Poland) Tertiary (Iceland) Strontian (Scotland)
Rings 4,6,8 4,6,8 4,8 4,8 4,8 4,6,10 4,6,8,12 4,6,8 6,12 4,6,8 4,6,8,12 4,6,12 4,8 4,8 4,8 4,8 4,8 4,5,8,10 5,8,10 4,5,8,12 4,5,8,10 4,5,8,10 4,5,6,8,10 4,5,6,8
a
In the SBU studied, the following structural units were dominating:S4R: single 4-membered rings, S6R - single 6-membered rings, D6R double 6-membered rings, 4±1: complex of 4 tetrahaedra unit joined with an additional tetrahedron, 5±1: complex of 5-membered ring joined with an additional tetrahedron, and 4±4±1: complex of two 4-membered rings joined with an additional tetrahedron.
In the Raman spectra, the arrangement of ring bands are much more complicated because the bands, due to isolated silicooxygen rings, can be accompanied by one (in the case of 3-membered rings) or more bands whose position changes slightly and unsystematically as the number of ring members changes [3]. Similar dif®culties can occur in the interpretation of the Raman spectra of zeolites. However, the intensity of the ring bands should be relatively high in the Raman spectra. This results from the fact that they originate from the symmetric `breathing' ring vibrations, which should give rise to the bands of considerable intensity [4]. Therefore, such bands can be easily identi®ed and assigned to the vibrations of different types of rings. Unequivocal assignment of the bands in the vibrational spectra of zeolites is dif®cult because of such factors as: occurrence of the same structural units (e.g. the same number of ring members) in the structures of
zeolites belonging to different structural groups, various contents of non-tetrahedral cations, possibility of different phase admixtures, and the dependence of the zeolite structure on the place of its origin. The aim of this work is to determine the correlation between the obtained vibrational spectra and the middle range order, typical of every group of zeolites. 2. Experimental The samples of natural zeolites were obtained from Geological Museum of Jagiellonian University, Mineralogical Museum of Warsaw University, Mineralogical Museum of Wrocøaw University and the private collection of the author. X-ray diffraction was applied to identify the phases present in the samples. Spectroscopic studies in the mid-infrared regions (MIR) were carried out using
W. Mozgawa / Journal of Molecular Structure 596 (2001) 129±137
131
Fig. 1. MIR spectra of clinptilolite measured at room temperature and at 10 K.
Fig. 3. Raman spectra of S4R zeolites group.
Fourier transform spectrometer FTS 60V Bio-Rad. Standard KBr pellet technique was used. Spectra were collected after 256 scans at 2 cm 21 resolution. Low temperature spectra were recorded using the cryogenic refrigeration system (APD Cryogenic Inc.). Raman spectra were collected after 1000 scans at 4 cm 21 resolution using FTS 6000 Bio-Rad Spectrometer with Raman section (with Nd:YAG Spectra Physic T10 106 4c laser). The laser power on the samples was maintained at 300 mW.
3. Results and discussion
Fig. 2. MIR spectra of S4R zeolites group.
All natural zeolites examined in the present work by vibrational spectroscopic methods are listed in Table 1. The typical chemical formulae, the kind of SBU, locality and the number of ring members created by alumino- and silicooxygen tetrahedra dominating a given structure, are also listed in the table. Selected zeolites representing six of seven main structural groups have been studied. Spectra of
132
W. Mozgawa / Journal of Molecular Structure 596 (2001) 129±137
Fig. 4. MIR spectra of S6R zeolites group.
Fig. 5. Raman spectra of S6R zeolites group.
zeolites of D4R group have not been shown because this group includes only synthetic zeolites. As has been mentioned already, in the IR spectra the band characteristics of rings and hence of the middle range order, occur in the pseudolattice region, (in the range of 400±800 cm 21). They are not typical lattice vibrations (connected with the long range order) since such bands are located in the FIR spectral region, (below 400 cm 21). These bands cannot be assigned to the intramolecular vibrations of AlO4 and SiO4 tetrahedra, either. The lattice character of the discussed bands is indicated by the low temperature spectra. In Fig. 1, two clinoptilolite (group 4±4± 1) spectra, measured at room temperature (about 290 K) and at liquid helium temperature (10 K) are presented. Comparison shows temperature effect (Rakow effect [5]) in the case of bands in the range of 400±800 cm 21 (as the temperature decreases, there is an increase in the intensity and a decrease in the bandwidth at half maximum). This makes it possible to classify the discussed bands as pseudolattice ones, (connected with the presence of the over-tetrahedral form of order). The position of the ring bands depend
on several factors: the number of ring members, Al:Si ratio, kind of non-tetrahedral cations, degree of ring deformation, degree of zeolite hydration and arrangement of structure. Therefore, accurate determination of band characteristic of a given structural group is very dif®cult. One can only show some similarities of the spectra within the group. The situation is even more complex since in the spectra, only the band characteristic of rings of lower number of members (up to eight) can be identi®ed. In addition, interpretation of the spectra in the examined range is complicated by the band coincidence and the possibility of other phase admixtures in natural zeolites, despite the X-ray diffraction studies con®rming the presence of proper zeolites. Spectra of individual groups of zeolites are described below. 3.1. S4R Single 4-membered rings are the basic structural elements of this group. These rings exhibit the least complicated structure among all SBU. In the MIR
W. Mozgawa / Journal of Molecular Structure 596 (2001) 129±137
133
Fig. 6. MIR spectra of D6R zeolites group.
Fig. 7. Raman spectra of D6R zeolites group.
spectra of zeolites belonging to this group, (Fig. 2) the `ring band' in the range of 720±790 cm 21 is repeated. This band should be assigned to the 4-membered ring vibrations. Such rings contain the lowest number of members of all rings occurring in the zeolite structure. Therefore, due to these rings, the bands occur at relatively high wavenumbers in the pseudolattice band range. Simultaneously, in the same range, the bands can occur due to the vibrations of internal oxygen bridges Si±O±Si (at about 780 cm21) and Si±O±Al. (at about 720 cm 21). In the spectrum of gismondine, this band is poorly visible and it can be revealed after the spectrum decomposition [6]. The similarity of analcime and pollucite spectra, resulting from the fact that both zeolites are isostructual, attracts attention [7]. The spectra of harmotome and phillipsite, which is caused by the similarity of their structures are also similar [8]. In the spectrum of laumontite, different band shapes, in comparison with other spectra of this group, can be explained by an almost ideal order in the arrangement of alumino- and silicooxygen tetrahedra, while in other cases, some kind of a disorder is always present. This order appears in the spectra as the splitting
of bands in the range of 950±1150 cm 21, due to the internal stretching vibrations of Si±O(Si) and Si± O(Al) [6]. The spectrum of laumontite con®rms the opinion that its structure differs distinctly from the structure of other zeolites, and its presence in this group results from the presence of 4- and 6-membered rings (the situation is similar to that in analcim) [2]. Similar conclusions can be drawn from the comparison of the Raman spectra (Fig. 3). In these spectra, the ring bands usually are of high intensity (they are often the most intensive bands in the spectrum). In the spectra of S4R group, the bands in the range of 470±520 cm21 predominate and they are most probably due to the `breathing' vibrations of the 4-membered rings. These bands appear in the similar wavenumber range in the spectra of other aluminosilicate structures containing 4membered rings not belonging to zeolites [4]. 3.2. S6R Single 6-membered rings are the basic structural units in the case of this group. In the IR spectra, vibrations of these rings should result in the bands
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W. Mozgawa / Journal of Molecular Structure 596 (2001) 129±137
3.3. D6R In the IR spectra (Fig. 6), intensive bands in the range of 570±635 cm 21 correspond to the structural units consisting of double 6-membered rings. It should be noted that in the spectra, except for the one of faujasite, these bands can be observed at almost the same position. It can result from identical framework structure of chabazite and gmelinite [2]. In all these spectra, the bands at about 720 cm 21 (which can be assigned to the vibrations of 4-membered rings) also occur. The fact that these bands are connected with the internal bridge vibrations cannot be ignored. However, in the Raman spectra (Fig. 7), very intensive bands in the range of 465±475 cm 21, which with great probability can be treated as ring bands are observed. Comparison of the spectra of this group with the spectra of the S4R group indicates that the increase in the number of ring members does not signi®cantly in¯uence the positions of the characteristic bands in the Raman spectra. 3.4. 4±1 Fig. 8. MIR spectra of 4±1 zeolites group.
shifting towards lower wavenumbers with respect to the bands due to 4-membered rings. Comparing the spectra of this group of zeolites (Fig. 4), one can observe that the pseudolattice bands occur in the range of 610±630 cm 21. They can be assigned to the 6-membered ring vibrations. In the spectrum of kalsilite, which is the aluminosilicate phase containing only 6-membered rings, the bands can be observed in the same range and thus assigned in the same way [5]. The spectrum of cancrinite is disturbed by the presence of bands, due to the carbonate groups occurring in the zeolites structure (intensive bands in the range of 1300±1500 cm 21 and single band at 857 cm 21). In the Raman spectra (Fig. 5), the bands in the range of 420±480 cm 21 repeat due to the presence of 6-membered rings. The intensity of discussed bands is quite low, which may result from the ring deformation, causing the lowering of the ideal hexagonal symmetry. Additionally, in the spectrum of offretite, the band at 481 cm 21 predominates. It can be probably assigned to the 4-membered ring vibration [3].
Presence of a 4±1 complex, causes the appearance of the whole series of bands in the range of pseudolattice bands in the IR spectra (Fig. 8). Natrolite, scolecite and mesolite exhibit the same type of aluminosilicate framework, which explains similar shape and small differences in the band positions in the spectra. The structures of the remaining zeolites of this group are also similar, and therefore, they can be easily identi®ed by their spectra in the pseudolattice range. It seems that in the series of pseudolattice bands, the bands located in the range of 720±760 cm 21 can be assigned to the 4-membered rings. In all the structures of zeolites of this group, 8-membered rings also occur but the bands connected with such structural units are characterised by quite low intensity. Raman spectra of zeolites of this group are similar to each other with highly intensive bands at about 535 cm 21 and less intensive ones at about 440 cm 21 (Fig. 9). It can be considered that these two bands are useful for the 4±1 unit identi®cation. 3.5. 5±1 Unfortunately, the IR spectra of this group (Fig. 10) show the least similarity with each other. It can be
W. Mozgawa / Journal of Molecular Structure 596 (2001) 129±137
Fig. 9. Raman spectra of 4±1 zeolites group.
Fig. 10. MIR spectra of 5±1 zeolites group.
connected with the so-called `individual' structure of zeolites, despite the fact that characteristic 5membered rings occur in all zeolites of this group. However, most probably, the bands at 571 cm 21 in the epistilbite spectrum, at 530 cm 21 in the dachiardite one and at about 544 cm 21 in the mordenite spectrum can be assigned to the vibrations of 5membered rings. In the mordenite spectrum, a lot of bands occur in the pseudolattice range, which is caused by the presence of different types of rings in the structure. Unfortunately, distinct differences in the Raman spectra of zeolites belonging to this group (Fig. 11) in the band arrangement, make it impossible to assign the pseudolattice bands unequivocally, even though the literature data indicate the presence of the band due to 5-membered rings at 390 cm 21[9,10]. 3.6. 4±4±1 The 4±4±1 complex shows the most complicated SBU structure among all zeolites [2]. In this complex, both the 4-membered rings as well as 5-membered rings occur. Additionally, 8- and 10-membered rings
Fig. 11. Raman spectra of 5±1 zeolites group.
135
136
W. Mozgawa / Journal of Molecular Structure 596 (2001) 129±137
Fig. 12. MIR spectra of 4±4±1 zeolites group.
Fig. 13. Raman spectra of 4±4±1 zeolites group.
are possible to appear. In the IR spectra, the bands in the range of 590±610 cm 21 probably arise from the 5membered ring vibrations, whereas the bands in the range of 700±720 cm 21,, arise due to the 4-membered rings (Fig. 12). Weak bands in the range of 660± 670 cm 21 can result from vibrations of higher numbers of ring members. Raman spectra (Fig. 13) contains two groups of bands, one in the range of 390±415 cm 21, assigned to the 5-membered ring vibrations, the other in the range of 480±500 cm 21, connected with 4-membered ring vibrations (similar to S4R)[8].
the case of rings containing more than six members it is impossible to distinguish the characteristic `ring bands'. The given ranges can change slightly in the spectra of individual zeolites. In the Raman spectra, the bands connected with the middle range order occur in the range of 470±390 cm 21 and the number of ring members does not cause systematic changes in the band positions.
4. Summary In the IR spectra of natural zeolites, one can assign the band characteristics of ring vibration, built up of SiO4 and AlO4 tetrahedra. These bands occur in the range of 700±760 cm 21 for 4-membered rings, in the range of 560±610 cm 21 for 5-membered rings and in the range of 570±635 cm 21 for 6-membered rings. In
Acknowledgements Financial support was provided by the Polish Committee for Scienti®c Research (KBN) under grant no. 7 T08D 039 17. References [1] W.M. Meier, Molecular Sieves, Society of Chemical Industry, London, 1968. [2] D.W. Breck, Zeolite Molecular Sieves, Wiley, New York, 1974.
W. Mozgawa / Journal of Molecular Structure 596 (2001) 129±137 [3] M. Handke, M. Sitarz, W. Mozgawa, J. Mol. Struct. 450 (1998) 229. [4] L.W. Finger, R.M. Hazen, R.J. Hemley, Am. Mineral. 74 (1989) 952. [5] A.W. Rakow, Opt. Spiektrosk. 23 (1962) 203. [6] W. Mozgawa, M. Sitarz, M. Rokita, J. Mol. Struct. 511±512 (1999) 511.
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[7] A. Bolewski, A. Manecki, Mineralogia szczegoÂøowa, PAE, Warszawa, 1993 (in Polish). [8] G. Gottardi, E. Galli, Natural Zeolites, Springer, Berlin, 1985. [9] J. Twu, P.K. Duta, J. Phys. Chem. 95 (1991) 5267. [10] P.K. Dutta, K.M. Rao, J.Y. Park, J. Phys. Chem. 95 (1991) 6654.
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The relation between structure and vibrational spectra of natural zeolites W. Mozgawa* Department of Materials Science and Ceramics, University of Mining and Metallurgy (AGH), al. Mickiewicza 30, 30-059, KrakoÂw, Poland Received 23 December 2000; revised 17 February 2001; accepted 17 February 2001
Abstract In this paper, the vibrational (IR and Raman) spectra of natural zeolites, belonging to six structural groups are presented. The band characteristics of the middle range order occur in the spectral region of 400±800 cm 21. In this range bands due to the vibrations of different rings of alumino- and silicooxygen tetrahedra, creating the secondary building units, are located. Increase in the number of ring members results in the shift of the characteristic band position towards lower wavenumbers in the IR spectra. Such a tendency is not so evident in the Raman spectra. It has been proved that vibrational spectra are useful for the identi®cation of a majority of zeolite structural groups, though unequivocal assignment of pseudolattice bands is dif®cult. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Infrared spectra; Raman spectra; Natural zeolites
1. Introduction According to the most common classi®cation, zeolites are divided into seven groups based on the socalled secondary building units (SBU) [1]. SBU are built of rings of SiO4 and AlO4 tetrahedra, containing various numbers of members which create the over-tetrahedral form of middle range order. SBU gives information on the environment of a given tetrahedron, the number of members in the ring and possibly the type of ring (single (S) or double (D)). For example, S4R implies that the single 4-membered rings dominate in the structure; D6R means that double connected 6-membered rings dominate; while 4±1 means that a complex built of 4membered rings joined with a single tetrahedron dominates, etc. [2]. * Tel.: 148-12-1722-32; fax: 148-12-3315-93. E-mail address: [email protected] (W. Mozgawa).
In the IR and Raman spectra, the assignment of bands to the ring vibrations can be a basis for the determination of the SBU type and for the identi®cation of a given zeolite structure. Theoretical group analysis performed on isolated rings of silicooxygen tetrahedra, occurring in cyclosilicate structures has shown that in the IR spectra, a single band located in the region of the so-called pseudolattice vibrations, (in the range of 400±800 cm 21), corresponds to the ring vibration. Increase in the number of ring members results in the shift of characteristic band positions towards lower wavenumbers [3]. A similar tendency should be expected in the IR spectra of zeolites of framework structure, in which the rings connect one another, creating the spacial framework. Additionally, in relation to the IR spectra of cyclosilicates, substitution of a part of silicooxygen tetrahedra by aluminooxygen considerably in¯uences the position and shape of pseudolattice bands.
0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0022-286 0(01)00741-4
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W. Mozgawa / Journal of Molecular Structure 596 (2001) 129±137
Table 1 Tab. I. List of used zeolites Name Analcime Pollucite Harmotome Phillipsite Gismondine Laumontite Offretite Levyne Cancrinite Chabazite Gmelinite Faujasite Natrolite Scolecite Mesolite Thomsonite Edingtonite Epistilbite Dachiardite Mordenite Heulandite Clinoptilolite Stilbite Brewsterite
Typical formula Na16[Al16Si32O96]´16H2O (Cs,Na)[AlSi2O6]´nH2O Ba(Ca0.5,Na)[Al5Si11O32]´ 12H2O K2(Ca0.5,Na)4[Al6Si10O32]´ 12H2O Ca4[Al8Si8O32]´ 16H2O Ca4[Al8Si16O48]´ 16H2O KCaMg[Al5Si13O36]´ 15H2O NaCa2.5[Al6Si12O36]´18H2O Na6Ca[CO3(AlSiO4)6]´2H2O Ca2[Al4Si8O24]´12H2O Na8[Al8Si16O48]´22H2O Na20Ca12Mg8[Al60Si132O384]´27H2O Na16[Al16Si24O80]´16H2O Ca8[Al16Si24O80]´24H2O Na16Ca16[Al48Si72O240]´64H2O Na4Ca8[Al20Si20O80]´24H2O Ba2[Al4Si6O20]´8H2O Ca3[Al6Si18O48]´16H2O (Na,K,Ca0.5)4[Al4Si20O48]´18H2O Na3KCa2[Al8Si40O96]´28H2O (Na,K)Ca4[Al9Si27O72]´24H2O (Na,K)6[Al6Si30O72]´20H2O NaCa4[Al9Si27O72]´30H2O Sr2[Al4Si12O32]´10H2O
SBU( a) S4R S4R S4R S4R S4R S4R S6R S6R S6R D6R D6R D6R 4±1 4±1 4±1 4±1 4±1 5±1 5±1 5±1 4±4±1 4±4±1 4±4±1 4±4±1
Locality Aussig (Bohemia.) Anburn Maine (USA) Andreasberg, (Germany) Capo di Bove (Italy) Capo di Bove (Italy) Grodziszcze (Poland) Mont Semiol (France) Stolpen (Germany) Ural Mountains (Russia) Nidda (Germany) Nova Scotia (Canada) Kaiserstuhl (Germany) Aussig (Bohemia) Berufjord (Iceland) NaalsoÈ (FaroÈes) Kilpatrick (Scotland) BoÈhlet Mine (Sweden) Berufjord (Iceland) Elba (Italy) FaroÈes Islands Paterson (USA) DylagoÂwka (Poland) Tertiary (Iceland) Strontian (Scotland)
Rings 4,6,8 4,6,8 4,8 4,8 4,8 4,6,10 4,6,8,12 4,6,8 6,12 4,6,8 4,6,8,12 4,6,12 4,8 4,8 4,8 4,8 4,8 4,5,8,10 5,8,10 4,5,8,12 4,5,8,10 4,5,8,10 4,5,6,8,10 4,5,6,8
a
In the SBU studied, the following structural units were dominating:S4R: single 4-membered rings, S6R - single 6-membered rings, D6R double 6-membered rings, 4±1: complex of 4 tetrahaedra unit joined with an additional tetrahedron, 5±1: complex of 5-membered ring joined with an additional tetrahedron, and 4±4±1: complex of two 4-membered rings joined with an additional tetrahedron.
In the Raman spectra, the arrangement of ring bands are much more complicated because the bands, due to isolated silicooxygen rings, can be accompanied by one (in the case of 3-membered rings) or more bands whose position changes slightly and unsystematically as the number of ring members changes [3]. Similar dif®culties can occur in the interpretation of the Raman spectra of zeolites. However, the intensity of the ring bands should be relatively high in the Raman spectra. This results from the fact that they originate from the symmetric `breathing' ring vibrations, which should give rise to the bands of considerable intensity [4]. Therefore, such bands can be easily identi®ed and assigned to the vibrations of different types of rings. Unequivocal assignment of the bands in the vibrational spectra of zeolites is dif®cult because of such factors as: occurrence of the same structural units (e.g. the same number of ring members) in the structures of
zeolites belonging to different structural groups, various contents of non-tetrahedral cations, possibility of different phase admixtures, and the dependence of the zeolite structure on the place of its origin. The aim of this work is to determine the correlation between the obtained vibrational spectra and the middle range order, typical of every group of zeolites. 2. Experimental The samples of natural zeolites were obtained from Geological Museum of Jagiellonian University, Mineralogical Museum of Warsaw University, Mineralogical Museum of Wrocøaw University and the private collection of the author. X-ray diffraction was applied to identify the phases present in the samples. Spectroscopic studies in the mid-infrared regions (MIR) were carried out using
W. Mozgawa / Journal of Molecular Structure 596 (2001) 129±137
131
Fig. 1. MIR spectra of clinptilolite measured at room temperature and at 10 K.
Fig. 3. Raman spectra of S4R zeolites group.
Fourier transform spectrometer FTS 60V Bio-Rad. Standard KBr pellet technique was used. Spectra were collected after 256 scans at 2 cm 21 resolution. Low temperature spectra were recorded using the cryogenic refrigeration system (APD Cryogenic Inc.). Raman spectra were collected after 1000 scans at 4 cm 21 resolution using FTS 6000 Bio-Rad Spectrometer with Raman section (with Nd:YAG Spectra Physic T10 106 4c laser). The laser power on the samples was maintained at 300 mW.
3. Results and discussion
Fig. 2. MIR spectra of S4R zeolites group.
All natural zeolites examined in the present work by vibrational spectroscopic methods are listed in Table 1. The typical chemical formulae, the kind of SBU, locality and the number of ring members created by alumino- and silicooxygen tetrahedra dominating a given structure, are also listed in the table. Selected zeolites representing six of seven main structural groups have been studied. Spectra of
132
W. Mozgawa / Journal of Molecular Structure 596 (2001) 129±137
Fig. 4. MIR spectra of S6R zeolites group.
Fig. 5. Raman spectra of S6R zeolites group.
zeolites of D4R group have not been shown because this group includes only synthetic zeolites. As has been mentioned already, in the IR spectra the band characteristics of rings and hence of the middle range order, occur in the pseudolattice region, (in the range of 400±800 cm 21). They are not typical lattice vibrations (connected with the long range order) since such bands are located in the FIR spectral region, (below 400 cm 21). These bands cannot be assigned to the intramolecular vibrations of AlO4 and SiO4 tetrahedra, either. The lattice character of the discussed bands is indicated by the low temperature spectra. In Fig. 1, two clinoptilolite (group 4±4± 1) spectra, measured at room temperature (about 290 K) and at liquid helium temperature (10 K) are presented. Comparison shows temperature effect (Rakow effect [5]) in the case of bands in the range of 400±800 cm 21 (as the temperature decreases, there is an increase in the intensity and a decrease in the bandwidth at half maximum). This makes it possible to classify the discussed bands as pseudolattice ones, (connected with the presence of the over-tetrahedral form of order). The position of the ring bands depend
on several factors: the number of ring members, Al:Si ratio, kind of non-tetrahedral cations, degree of ring deformation, degree of zeolite hydration and arrangement of structure. Therefore, accurate determination of band characteristic of a given structural group is very dif®cult. One can only show some similarities of the spectra within the group. The situation is even more complex since in the spectra, only the band characteristic of rings of lower number of members (up to eight) can be identi®ed. In addition, interpretation of the spectra in the examined range is complicated by the band coincidence and the possibility of other phase admixtures in natural zeolites, despite the X-ray diffraction studies con®rming the presence of proper zeolites. Spectra of individual groups of zeolites are described below. 3.1. S4R Single 4-membered rings are the basic structural elements of this group. These rings exhibit the least complicated structure among all SBU. In the MIR
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Fig. 6. MIR spectra of D6R zeolites group.
Fig. 7. Raman spectra of D6R zeolites group.
spectra of zeolites belonging to this group, (Fig. 2) the `ring band' in the range of 720±790 cm 21 is repeated. This band should be assigned to the 4-membered ring vibrations. Such rings contain the lowest number of members of all rings occurring in the zeolite structure. Therefore, due to these rings, the bands occur at relatively high wavenumbers in the pseudolattice band range. Simultaneously, in the same range, the bands can occur due to the vibrations of internal oxygen bridges Si±O±Si (at about 780 cm21) and Si±O±Al. (at about 720 cm 21). In the spectrum of gismondine, this band is poorly visible and it can be revealed after the spectrum decomposition [6]. The similarity of analcime and pollucite spectra, resulting from the fact that both zeolites are isostructual, attracts attention [7]. The spectra of harmotome and phillipsite, which is caused by the similarity of their structures are also similar [8]. In the spectrum of laumontite, different band shapes, in comparison with other spectra of this group, can be explained by an almost ideal order in the arrangement of alumino- and silicooxygen tetrahedra, while in other cases, some kind of a disorder is always present. This order appears in the spectra as the splitting
of bands in the range of 950±1150 cm 21, due to the internal stretching vibrations of Si±O(Si) and Si± O(Al) [6]. The spectrum of laumontite con®rms the opinion that its structure differs distinctly from the structure of other zeolites, and its presence in this group results from the presence of 4- and 6-membered rings (the situation is similar to that in analcim) [2]. Similar conclusions can be drawn from the comparison of the Raman spectra (Fig. 3). In these spectra, the ring bands usually are of high intensity (they are often the most intensive bands in the spectrum). In the spectra of S4R group, the bands in the range of 470±520 cm21 predominate and they are most probably due to the `breathing' vibrations of the 4-membered rings. These bands appear in the similar wavenumber range in the spectra of other aluminosilicate structures containing 4membered rings not belonging to zeolites [4]. 3.2. S6R Single 6-membered rings are the basic structural units in the case of this group. In the IR spectra, vibrations of these rings should result in the bands
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3.3. D6R In the IR spectra (Fig. 6), intensive bands in the range of 570±635 cm 21 correspond to the structural units consisting of double 6-membered rings. It should be noted that in the spectra, except for the one of faujasite, these bands can be observed at almost the same position. It can result from identical framework structure of chabazite and gmelinite [2]. In all these spectra, the bands at about 720 cm 21 (which can be assigned to the vibrations of 4-membered rings) also occur. The fact that these bands are connected with the internal bridge vibrations cannot be ignored. However, in the Raman spectra (Fig. 7), very intensive bands in the range of 465±475 cm 21, which with great probability can be treated as ring bands are observed. Comparison of the spectra of this group with the spectra of the S4R group indicates that the increase in the number of ring members does not signi®cantly in¯uence the positions of the characteristic bands in the Raman spectra. 3.4. 4±1 Fig. 8. MIR spectra of 4±1 zeolites group.
shifting towards lower wavenumbers with respect to the bands due to 4-membered rings. Comparing the spectra of this group of zeolites (Fig. 4), one can observe that the pseudolattice bands occur in the range of 610±630 cm 21. They can be assigned to the 6-membered ring vibrations. In the spectrum of kalsilite, which is the aluminosilicate phase containing only 6-membered rings, the bands can be observed in the same range and thus assigned in the same way [5]. The spectrum of cancrinite is disturbed by the presence of bands, due to the carbonate groups occurring in the zeolites structure (intensive bands in the range of 1300±1500 cm 21 and single band at 857 cm 21). In the Raman spectra (Fig. 5), the bands in the range of 420±480 cm 21 repeat due to the presence of 6-membered rings. The intensity of discussed bands is quite low, which may result from the ring deformation, causing the lowering of the ideal hexagonal symmetry. Additionally, in the spectrum of offretite, the band at 481 cm 21 predominates. It can be probably assigned to the 4-membered ring vibration [3].
Presence of a 4±1 complex, causes the appearance of the whole series of bands in the range of pseudolattice bands in the IR spectra (Fig. 8). Natrolite, scolecite and mesolite exhibit the same type of aluminosilicate framework, which explains similar shape and small differences in the band positions in the spectra. The structures of the remaining zeolites of this group are also similar, and therefore, they can be easily identi®ed by their spectra in the pseudolattice range. It seems that in the series of pseudolattice bands, the bands located in the range of 720±760 cm 21 can be assigned to the 4-membered rings. In all the structures of zeolites of this group, 8-membered rings also occur but the bands connected with such structural units are characterised by quite low intensity. Raman spectra of zeolites of this group are similar to each other with highly intensive bands at about 535 cm 21 and less intensive ones at about 440 cm 21 (Fig. 9). It can be considered that these two bands are useful for the 4±1 unit identi®cation. 3.5. 5±1 Unfortunately, the IR spectra of this group (Fig. 10) show the least similarity with each other. It can be
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Fig. 9. Raman spectra of 4±1 zeolites group.
Fig. 10. MIR spectra of 5±1 zeolites group.
connected with the so-called `individual' structure of zeolites, despite the fact that characteristic 5membered rings occur in all zeolites of this group. However, most probably, the bands at 571 cm 21 in the epistilbite spectrum, at 530 cm 21 in the dachiardite one and at about 544 cm 21 in the mordenite spectrum can be assigned to the vibrations of 5membered rings. In the mordenite spectrum, a lot of bands occur in the pseudolattice range, which is caused by the presence of different types of rings in the structure. Unfortunately, distinct differences in the Raman spectra of zeolites belonging to this group (Fig. 11) in the band arrangement, make it impossible to assign the pseudolattice bands unequivocally, even though the literature data indicate the presence of the band due to 5-membered rings at 390 cm 21[9,10]. 3.6. 4±4±1 The 4±4±1 complex shows the most complicated SBU structure among all zeolites [2]. In this complex, both the 4-membered rings as well as 5-membered rings occur. Additionally, 8- and 10-membered rings
Fig. 11. Raman spectra of 5±1 zeolites group.
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Fig. 12. MIR spectra of 4±4±1 zeolites group.
Fig. 13. Raman spectra of 4±4±1 zeolites group.
are possible to appear. In the IR spectra, the bands in the range of 590±610 cm 21 probably arise from the 5membered ring vibrations, whereas the bands in the range of 700±720 cm 21,, arise due to the 4-membered rings (Fig. 12). Weak bands in the range of 660± 670 cm 21 can result from vibrations of higher numbers of ring members. Raman spectra (Fig. 13) contains two groups of bands, one in the range of 390±415 cm 21, assigned to the 5-membered ring vibrations, the other in the range of 480±500 cm 21, connected with 4-membered ring vibrations (similar to S4R)[8].
the case of rings containing more than six members it is impossible to distinguish the characteristic `ring bands'. The given ranges can change slightly in the spectra of individual zeolites. In the Raman spectra, the bands connected with the middle range order occur in the range of 470±390 cm 21 and the number of ring members does not cause systematic changes in the band positions.
4. Summary In the IR spectra of natural zeolites, one can assign the band characteristics of ring vibration, built up of SiO4 and AlO4 tetrahedra. These bands occur in the range of 700±760 cm 21 for 4-membered rings, in the range of 560±610 cm 21 for 5-membered rings and in the range of 570±635 cm 21 for 6-membered rings. In
Acknowledgements Financial support was provided by the Polish Committee for Scienti®c Research (KBN) under grant no. 7 T08D 039 17. References [1] W.M. Meier, Molecular Sieves, Society of Chemical Industry, London, 1968. [2] D.W. Breck, Zeolite Molecular Sieves, Wiley, New York, 1974.
W. Mozgawa / Journal of Molecular Structure 596 (2001) 129±137 [3] M. Handke, M. Sitarz, W. Mozgawa, J. Mol. Struct. 450 (1998) 229. [4] L.W. Finger, R.M. Hazen, R.J. Hemley, Am. Mineral. 74 (1989) 952. [5] A.W. Rakow, Opt. Spiektrosk. 23 (1962) 203. [6] W. Mozgawa, M. Sitarz, M. Rokita, J. Mol. Struct. 511±512 (1999) 511.
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[7] A. Bolewski, A. Manecki, Mineralogia szczegoÂøowa, PAE, Warszawa, 1993 (in Polish). [8] G. Gottardi, E. Galli, Natural Zeolites, Springer, Berlin, 1985. [9] J. Twu, P.K. Duta, J. Phys. Chem. 95 (1991) 5267. [10] P.K. Dutta, K.M. Rao, J.Y. Park, J. Phys. Chem. 95 (1991) 6654.