Journal of Molecular Structure 614 (2002) 281–287 www.elsevier.com/locate/molstruc
MAS NMR and FTIR spectra of framework aluminosilicates W. Mozgawaa,*, Z. Fojudb, M. Handkea, S. Jurgab a
Faculty of Materials Science and Ceramics, University of Mining and Metallurgy (AGH), al. Mickiewicza 30, 30-059 Krako´w, Poland b Adam Mickiewicz University in Poznan´, Faculty of Physics ul. Umultowska 85, 61-614 Poznan´, Poland Received 17 December 2001; revised 12 March 2002; accepted 12 March 2002
Abstract In the work, the results of 27Al MAS NMR studies carried out for structures of different framework aluminosilicates are presented. The spectra of natural clinoptilolite and its sodium and hydrogen forms were measured. The zeolite spectra were obtained before and after incorporation of heavy metal cations (Pb2þ, Cd2þ, Ni2þ and Cr3þ) into the zeolite structure. After decomposition of the spectra into component peaks differences caused by ion exchange were observed. Additionally, NMR spectra of kalsilite (framework aluminosilicate, not belonging to zeolite group) and other zeolites (chabazite, heulandite and analcime) were measured. 27 Al MAS NMR spectra of all the samples studied were compared with the FTIR spectra measured in the middle infrared region. Based on the results obtained it has been proved that the interpretation of spectra obtained using these two different techniques can give some information concerning changes occurring in various framework aluminosilicate structures. q 2002 Elsevier Science B.V. All rights reserved. Keywords: NMR spectra; IR spectra; Aluminosilicate
1. Introduction Ion exchange ability of zeolites is one of the most important features of this group of aluminosilicates. This ability decides on numerous possible applications of zeolites. Heavy metal cations immobilisation is used in one of very important applications of zeolites, namely as ion exchange agents. The process of different ion incorporation into the zeolite framework can be observed by structural studies based on measurement methods, which are sensitive to the short range order. Solid state NMR spectroscopy and * Corresponding author. Tel.: þ48-12-172232; fax: þ 48-12331593. E-mail address:
[email protected] (W. Mozgawa).
infrared spectroscopy belong to such methods. NMR spectra give information on changes of atomic nucleus surroundings, whereas infrared spectra provide information usually concerned with chemical bond vibrations occurring in molecular units. 27 Al MAS NMR spectra provide information on the environment of aluminium atoms in the structure [1]. When aluminium atoms occur in tetrahedral coordination and create the three-dimensional framework, the peak showing the chemical shift of 60 ppm [1 –3] in the spectra can be observed. If the extraframework aluminium occurs in the structure, then it is present in octahedral coordination and simultaneously, the peak at about 0 ppm in the spectra is observed. However, the appearance of peaks in the range of 30 – 50 ppm can be connected with the
0022-2860/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 8 6 0 ( 0 2 ) 0 0 2 6 2 - 4
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penta-coordinated aluminium atoms or distorted tetrahedrally coordinated aluminium [4]. The changes resulting from the ion-exchange in non-tetrahedral cations surrounding AlO4 tetrahedra, should cause changes in the NMR spectra [5]. The ion-exchange cations compensate the deficiency of lattice positive charge in the aluminosilicate framework, caused by the substitution of a part of SiO4 tetrahedra by AlO4 ones. This is why the Al/Si ratio directly influences the ion exchange properties. On the other hand, the presence of high amount of alumino-oxygen tetrahedra, substituting SiO4 terahedra, causes weakening of aluminosilicate structure—Al –O bonds are weaker than Si – O ones. This is the reason why the zeolites containing large amounts of Al atoms in tetrahedral positions show lower chemical stability and cannot be applied in all cases. Considering the above remarks, the applications of zeolites possessing the Al/Si ratio of 1:5 like in e.g. clinoptilolite (the zeolite of heulandite group), seems to be optimum. The Al/Si ratio value can be estimated with high precision based on 29Si MAS NMR spectra. IR and 27Al MAS NMR spectra can also be used to estimate this ratio.
2. Experimental The samples of natural zeolites were obtained from Mineralogical Museum of Wrocl⁄aw University and private collection of the author. Kalsilite samples (K[AlSiO4]) were prepared using the sol – gel method and by devitrification of the obtained glasses. 2.1. Preparation of clinoptilolite samples Mineral samples containing about 30% of clinoptilolite were concentrated by separation process which allowed to obtain the maximum content of zeolite material. The concentration procedure involved grinding and drying at 60 8C for several days, mixing with water (10 h, mechanical stirrer) and material decantation. The process was repeated 50 times. On the basis of X-ray phase analysis the approximate 90% clinoptilolite content was determined. The samples also contained some amount of quartz and montmorillonite. The obtained material was examined using
the scanning electron microscope. The analysis of zeolite composition by EDX (energy dispersion Xray) spectroscopy was carried out. 2.2. Ionic exchange Clinoptilolite was transformed into sodium or hydrogen forms in order to increase the efficiency of ionic exchange. The zeolite was activated by 1 M aqueous solutions of NaCl and HCl. Then the Pb2þ, Cd2þ, Ni2þ and Cr3þ ions were introduced into zeolite structure from 0.01 M water solution of Pb(NO3)2, Cd(NO3)2, Ni(NO3)2 and Cr(NO3)3. After ion-exchange process the zeolites were washed with distilled water. X-ray phase analysis showed that no crystalline phases containing nitrate anions (NO2 3 ) or chloride ones (Cl2) occured in the zeolite structure. This indicates that heavy metal cations introduced do not crystallise as separate salts but are incorporated into zeolite framework. 2.3. Measurements The 27Al chemical shifts are reported in ppm from external 1 M AlCl3 solution. NMR spectra were recorded using a Bruker DSX 400 MHz spectrometer operating at 104.3 MHz for 27Al. Samples were held in 4 mm zirconia rotors and spinned at 7 kHz. For 27Al we used p/4 pulses 1.2 ms in a one pulse sequence. Number of total acquisitions was equal to 16. Infrared spectra were measured on a Bio-Rad FTS60 spectrometer. Spectra were collected after 256 scans at 2 cm21 resolution. Samples were prepared by the standard KBr pellets method. The X-ray structural analysis was performed with FPM Seifert XRD7 equipment using Cu Ka radiation. EDX spectra were measured on a JEOL 5400 scanning microscope equipped with the microprobe analyser LINK ISIS (Oxford Instrument).
3. Results and discussion 3.1. IR spectra The ion exchange process can be followed using IR spectra. Changes in the IR spectra generated by ionexchange can be observed in the range of
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Fig. 1. MIR spectra of different clinoptilolite forms in 710 – 660 cm21 range.
pseudolattice vibrations. In Fig. 1, the comparison of MIR spectrum of the initial clinoptilolite with the spectra of heavy metal cations-containing samples is shown. The spectra range was limited to 710 – 660 cm21. In this range, small but systematic changes in the intensities of the bands can be observed. The band at 676 cm21 is a good example of such changes. In the spectrum of initial zeolite, its intensity is very small. The exchange of non-tetrahedral cations causes progressive increase in the intensity of this band. This increase is the smallest when Pb2þ cation was introduced into the structure, and it is successively growing for Cd2þ, Cr3þ and Ni2þ cations. This band can be assigned to the pseudolattice vibrations of rings built up of alumino and silicooxygen tetrahedra. Relatively high value of wavenumbers indicates that this band originates from the vibrations of 4membered rings. These rings contain the smallest number of members among all rings present in the zeolite structure. The results of theoretical calculations show the relatively small deformation in case of these rings [6]. The increase in the band intensity during ion-exchange can be connected with the higher degree of ring deformation caused by incorporation of ions of higher ionic radii and of higher charge. The
Fig. 2. 27Al MAS NMR spectra of different clinoptilolite forms.
increase in cation charge has a particularly great influence on changes in the spectra caused by ring deformation. Thus, the band at 676 cm21 can be treated as the ‘indicator’ band, helpful in the estimation of ion-exchange process. The increase of this band intensity is due to the presence of heavy metal cation in the zeolite structure. 3.2. 27Al MAS NMR spectra In Fig. 2 the comparison of 27Al MAS NMR spectra of different forms of clinoptilolite, has been presented. Spectra corresponding to the initial zeolite samples, its sodium and hydrogen forms, and spectra of the samples after ion-exchange process where Pb2þ, Cd2þ, Cr3þ and Ni2þ cations were introduced are shown. In all these spectra, only one intensive band at about 57 ppm is present. According to the literature data, this peak can be assigned to the aluminium atoms in tetrahedral coordination. In the
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Fig. 3. Decomposition of 27Al MAS NMR spectra: Na-clinoptilolite (a), after Pb (b), Ni (c) and Cd (d) exchange.
NMR spectra, the cations substitution causes small but visible changes (similar to the IR spectra). In all spectra, peaks consist of several components which results in their asymmetric shape. Determination of the parameters of particular lines becomes possible after their decomposition into separate component peaks. In Fig. 3, the example of Naclinoptilolite spectrum decomposition (Fig. 3a) together with the decompositions of the spectra of clinoptilolite samples where Pb2þ (Fig. 3b), Ni2þ (Fig. 3c) and Cd2þ (Fig. 3d) were introduced, has been shown. Identical procedure, proposed in the work [7], for all spectra decomposition was applied. The
decomposition was carried out with the use of WINIRe software. As a result, two peaks, of different chemical shifts different intensities, were obtained. The main peak, much more intensive (in Fig. 3 marked as ‘2’) is located at about 56 ppm, whereas the second one is located at higher values of chemical shift i.e. at about 61 ppm (in Fig. 3 marked as ‘1’). The presence of two peaks revealed after the decomposition procedure, can indicate occurrence in zeolite structure of aluminium atoms in two nonequivalent positions [8]. Each position is connected with the line located at different value of chemical shift. The differences in peak positions are so small
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Fig. 4. 27Al MAS NMR spectra of different aluminosilicates (‘ p ’ denotes the spinning sideband).
that one can assume the tetrahedral coordination of aluminium atoms in both cases. However, the surroundings of these atoms are different. Those differences can arise from e.g. different length of Al– O bonds or different Al – O – Al angles in AlO4 tetrahedra. This hypothesis needs to be confirmed by further studies but based on NMR spectra obtained, one can assume that the presence of the second, less intensive band can be connected with the occurrence of small amounts of the deformed AlO4 tetrahedra. The positions of both peaks do not change distinctly in other spectra because of the nontetrahedral cations exchange. More distinct changes are observed in the areas of these peaks. When different spectra are compared, it should be remembered that the initial clinoptilolite and Hclinoptilolite structures can differ from Na-clinoptilolite structure and the structure of zeolite with incorporated heavy metal cations. The initial clin-
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optilolite contains several types of non-tetrahedral cations (Ca2þ, Kþ, Sr2þ, Ba2þ, Mg2þ [9]) which results in different surroundings of AlO4 tetrahedra, whereas in the case of H-clinoptilolite, one cannot exclude dealumination of framework upon treatment with HCl. Such a process causes the destruction of zeolite structure, which in turn can influence the dispersion of lattice parameters, which finally can influence the peak parameters in NMR spectra. On the other hand, comparing the MIR spectra of the initial zeolite and its hydrogen form, one does not observe changes in the position of band due to the stretching vibrations Si – O(Al) located at 1088 cm21 [9]. This band should shift to the lower wavenumbers because of dealumination process. Thus, if the dealumination has occured, its degree was very small. Besides these two cases, incorporation of heavy metal cations causes slight but visible and systematic increase in peak areas of the major peak in the remaining NMR spectra. It means the increase in its relative intensity. This intensity depends on the type of incorporated cation and it increases in the order Cr ! Cd ! Ni ! Pb. In the second case, considerable less intensive peak the position of the peak is almost invariable. However, the integral intensity changes. The order of changes is different. Incorporation of Pb2þ causes decrease in peak intensity in comparison with the sodium form. The remaining cations increase the peak intensity in the following order: Cd ! Ni ! Cr. The heavy metal cations incorporation also causes the changes in peak maximum but these shifts are so small that it is very difficult to find any correlation between the peak position and ion introduced. Changes in the features of NMR spectra after heavy metal cations incorporation are caused by changes in the aluminium atom surrounding because these are AlO4 tetrahedra which introduce the negative charge making it possible to connect non-tetrahedral atoms. To summarise it can be concluded that shape and position of 27Al MAS NMR spectra are intensitive (within experimental accuracy) against the character of incorporated metal cations. NMR and IR spectroscopy can also be applied to study selected aluminosilicates structures. In Fig. 4, 27 Al MAS NMR spectra of four different zeolites (besides clinoptilolite, the spectra of heulandite, chabazite and analcime are shown) and kalsilite,
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Table 1 27 Al MAS NMR peak position and Al/Si ratio of used aluminosilicates Name
Typical formula
Al/Si
Peak position (ppm)
Kalsilite Analcime Chabazite Heulandite Clinoptilolite
K[AlSiO4] Na16[Al16Si32O96]·16H2O Ca2[Al4Si8O24]·12H2O (Na,K)Ca4[Al9Si27O72]·24H2O (Na2,K2,Ca)3[Al6Si30O72]·20H2O
1:1 1:2 1:2 1:3 1:5
59.4 58.2 58.0 56.9 56.7
which is not classified as zeolite. The selected structures differ in Al/Si ratio whose value changes from 1:5 in clinoptilolite, to 1:1 in kalsilite. Comparing the spectra, one can notice systematic shifts of maximum as the Al/Si ratio decreases (Table 1). The same tendency has been observed for levyne-type synthetic zeolities where with the increase of aluminium contents the line characteristic of the Al3þ cations in tetrahedral coordination shifts [10]. The position of this band can indicate the degree of substitution of silicon by aluminium in the aluminosilicate framework. The accurate determination of the Al/Si ratio was the subject of many a work [3]. The spectrum of kalsilite differs from the remaining spectra because of its complicated line shape. Besides the main maximum the shoulder at about 66 ppm is visible. The line asymmetry in the kalsilite spectrum is greater than that in clinoptilolite spectrum (described above). In this case it can be assumed that the asymmetry proves the presence of aluminium atoms in two different tetrahedral positions. The observed shoulder can be connected with deformation of AlO4 tetrahedra, since the studied kalsilite has been obtained by devitrification of the amorphous material. The devitrification could be incomplete and the part of amorphous phase could remain in the material. Greater half peak width, in comparison with the remaining spectra, may confirm this. NMR spectral line broadening is characteristic of the samples containing amorphous phase. The 29Si NMR measurements are planned as a continuation of the present work. The measurements should give some information on the Si surroundings, which can vary in aluminosilicate structures.
4. Summary Incorporation of heavy metal cations into zeolite structure causes slight but visible changes in the NMR as well as IR spectra. Decomposition of NMR spectra reveals two peaks appearing at different values of chemical shift and differing in intensity. These peaks can be connected in the presence of aluminium in two different tetrahedral positions. In the IR spectra the ionic exchange in zeolite causes the change in intensity of the band at about 767 cm21, connected with pseudolattice vibrations of rings formed by AlO4 and SiO4 tetrahedra. In the NMR spectra of different aluminosilicates the changes in line position for aluminium in tetrahedral coordination with the change of Si/Al ratio have been established.
Acknowledgment This work was supported by the Polish Committee for Scientific Research (KBN) under grant no. 7 T08D 039 17.
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