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Spectroscopic studies of the zeolite materials: interaction of extra-framework aluminum with acetylacetone and hydroxyl groups
Journal of Molecular Structure 645 (2003) 171–176 www.elsevier.com/locate/molstruc
Spectroscopic studies of the zeolite materials: interaction of extra-framework aluminum with acetylacetone and hydroxyl groups M.A. Zanjanchi*, E. Mohabbati Department of Chemistry, Faculty of Science, University of Guilan, P.O. Box 1914, Rasht, Iran Received 4 September 2002; accepted 11 October 2002
Abstract Diffuse reflectance spectroscopy has been used to investigate structural modification of different acidic and neutralized zeolites subjected to acetylacetone treatments. Extra-framework aluminum species, produced upon expulsion of aluminum atoms from the framework of the zeolite Y, mordenite, mazzite and ZSM-5 form complexes with acetylacetone. A distinct welldefined 285– 290 nm band produced is due to the transformation of aluminum atoms to structures with highly ordered environments. Washing the acetylacetone-treated acidic samples with ethanol leads to extraction of most of the complexed aluminum out of the channels of zeolite mazzite only. The residence of Al – acac complexes in zeolite HY with the same pore opening as mazzite was attributed to the huge network of hydrogen bonds which stabilizes the complexes and reduces extent of extraction. However, upon neutralization of zeolite HY with sodium hydroxide, nearly all of the Al – acac complexes are removed out with ethanol washing and the band disappears in the DR spectrum. The DR spectra obtained for the acac-treated acidic and neutralized mordenite and ZSM-5 zeolites did not show any difference between them before and after ethanol washing. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Diffuse reflectance spectroscopy; Acetylacetone; Zeolites; Hydrogen bonds; Hydroxyl groups
1. Introduction Secondary or post-synthesis modification of microporous materials encompasses a variety of techniques to further control of catalytic activity and/or the shape selectivity of a specific microporous structure [1,2]. Dealumination of zeolites to increase the Si/Al ratio
* Corresponding author. Fax: þ 98-131-322-0066. E-mail address: [email protected] (M.A. Zanjanchi).
of the crystals is one of the method that has been employed to change or modify the acidity of these materials. There are three methods to prepare dealuminated zeolites: (1) hydrothermal treatment, (2) chemical treatment and (3) a combination of hydrothermal and chemical treatments [3]. By hydrothermal dealumination, aluminum atoms are removed from the framework but are not removed from the crystal. The nature of extra-framework aluminum species is not fully clear but they may have forms of cationic species (Al3þ, AlOþ, Al(OH)2þ, Al(OH)þ 2)
0022-2860/03/$ - 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 5 7 4 - 4
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M.A. Zanjanchi, E. Mohabbati / Journal of Molecular Structure 645 (2003) 171–176
and neural or polymerized species (AlO(OH), Al(OH)3, Al2O3) [3]. Chelating agents such as acetylacetone can be used to wash these extra-framework aluminum species out of the crystalline structure [4]. Interaction of acetylacetone with extra-framework aluminum have been studied by 27Al MAS NMR method following impregnation of the sample with a solution of acac in ethanol [5 – 8]. By this treatment the extra-framework aluminum atoms are transferred into stable Al –acac complexes which posses a highly ordered environment. Recently, we reported that diffuse reflectance spectroscopy could successfully be used for identification and estimation of extra-framework aluminum in a thermally treated and acid leached mazzite zeolite [9]. We showed that acetylacetone impregnation of the sample containing extra-framework aluminum leads to transformation of all of the aluminum species with distorted symmetry to a highly ordered environment where is discovered by a distinct well-defined band around 285 nm in the DR spectrum. The washing of the acetylacetone treated samples in hot ethanol caused a decrease of the intensity of this band indicate that some of the Al –acac complexes is extracted out of the solid. The aim of the present contribution is to apply the recently developed DRS approach to study the removal of extra-framework aluminum in zeolite Y, Mordenite, Mazzite and ZSM-5 by acetylacetone complexation. We will show that the size of pore openning, Si/Al ratio and hydroxyl groups concentration have great influence on stabilization of aluminum species inside the channels and cages of the zeolites.
tetramethylammonium hydroxide (Merck 8123) as a structure directing template according to the procedure described previously [9,11]. Zeolite ZSM-5 was synthesized according to the recipe given by Rubin et al. [12]. Tetraethylammonium hydroxide (Fluka 86631) was used as template for the synthesis of ZSM-5. The prepared gel was heated at 175 8C for 60 h. The oxide mole composition of the synthesized zeolites are given in Table 1. To remove occluded templates out of the channels of mazzite and ZSM-5 zeolites, these samples were heated in a muffle furnace in air at a rate of 2 8C/min up to 550 8C and kept at this temperature for 5 h. The acidic forms of the zeolites were prepared by NHþ 4 ion exchange followed by thermal treatments. Ammonium exchange forms of the mordenite, mazzite and ZSM-5 were prepared by treating the as-synthesized mordenite and calcined mazzite and ZSM-5 with 1 M solution of NH4NO3 under reflux at 80 8C for 5 –10 h, separately. For each of them, the solution was filtered and zeolite washed with deionized hot water several times and dried at 110 8C. The ammonium exchange for zeolite Y was performed three times at 25 8C with a 0.4 M solution of NH4Cl. The washing procedure was the same as others and the sample was dried at 110 8C. To convert the ammonium form of the zeolites to acidic ones, they were heated in a muffle furnace at a rate of 10 8C/min and holding at 500 8C for at least 7 h. For the complexation treatment, approximately 3 ml of a solution of acetylacetone in ethanol (38%) was added to 1.0 g of acidic or neutralized zeolite samples, separately. The samples were left in contact with stirred acetylacetone solution for 5 h. During this time the container was caped to prevent evaporation
2. Experimental 2.1. Sample preparation and treatments Zeolite Y was obtained from Union Carbid and has a Si/Al ratio of < 2.5. Mordenite was prepared by hydrothermal synthesis method described in the literature [10]. Sodium silicate solution (27% SiO2, 8% Na2O, 65% H2O, Merck) and aluminum sulfate (Fluka No. 6421) were used as sources of silicon and aluminum of the synthesis gel, respectively. The zeolite mazzite was synthesized in presence of
Table 1 Composition of the reacting gels for the synthesized zeolites Zeolite
Al2O3
SiO2
Na2O
R2Oa
H2O
MOR MAZ ZSM-5
1 1 1
15 15 80
8.5 4.8 24.0
– 1.2 20.0
1300 240 3000
a R is the templates tetramethylammonium hydroxide and tetraethylammonium hydroxide used for the synthesis of mazzite and ZSM-5 zeolites, respectively.
M.A. Zanjanchi, E. Mohabbati / Journal of Molecular Structure 645 (2003) 171–176
of the mixture. After that, samples were dried at room temperature in a stream of dry air. The samples were then washed by treating the solids in 30 ml ethanol for 3 h at 60 8C to perform extraction of Al – acac complexes. 2.2. Characterization X-ray diffraction measurements were performed on a Philips PW 1840 diffractometer with Cu Ka radiation at room temperature. XRD patterns were recorded using an automatic divergence slit system. Diffuse reflectance spectra were recorded by a UV2100 Shimadzu spectrophotometer, equipped with an integrating sphere assembly. A special cell, loaded with the solid sample and covered by a quartz window, was used in all measurements. All the spectra were recorded at room temperature against barium sulfate and plotted in terms of absorbance. Chemical analysis of the samples was done by energy dispersive X-ray analysis (EDX) joined to a Philips XL 30 scanning electron microscope. The template and water content of the samples were determined by thermogravimetry, using a Polymer Laboratories TG1500.
173
the calcined and acidic samples show a very lowintensity background indicating that there is no significant loss of crystallinity upon heating treatments. The conversion of the as-synthesized zeolites into their calcined and acidic forms results in extraframework aluminum formation due to dealumination. Calcination lowers the symmetry of the environment of the aluminum atoms expelled from their sites to form extra-framework aluminum species. This will produce a broadened signal which can be visualized by 27Al MAS NMR [14] and by diffuse reflectance spectroscopy [9]. Impregnation of the acidic samples containing extra-framework aluminum with ethanolic acetylacetone will convert the broadened band to
3. Results and discussion The unit cell composition of the as-synthesized samples are shown in Table 2. They were calculated from the EDX data. EDX analysis was made upon several crystalline parts of the samples observed by scanning microscopy to measure amount of silicon and aluminum. The mean of the separate analysis was taken. X-ray diffraction patterns of the as-synthesized samples reveals high crystallinity and high purity of the samples [13]. Inspection XRD patterns of Table 2 Unit cell composition of the as-synthesized zeolites Zeolite
Unit cell formula
Y MOR MAZ ZSM-5
H1Na52.5[Al35.5Si138.5O384]265 H2O Na6.7[Al6.7Si41.3O96]23.5 H2O Na3.2TMA1.9[Al5.1Si30.9O72]26.5 H2O Na0.3TEA3.2[Al3.5Si92.5O192]10 H2O
Fig. 1. Diffuse reflectance spectra of acetylacetone treated zeolite samples: (a) HY, (b) HMOR, (c) HMAZ and (d) HZSM-5.
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a distinct well-defined band [9]. The appearance of this band is due to the transformation of the aluminum atoms with distorted symmetry and different coordination number to structures with highly ordered octahedral symmetry. Fig. 1 shows DR spectra of the acac-treated samples. A distinct band at about 285 – 290 nm is the result of the treating acidic samples with acetylacetone. We found the shape and intensity of this band depends strongly on the type of zeolites. This band is broadened in HMOR and ZSM5 samples, whereas it is sharper and with higher intensity in HMAZ and HY samples. The washing of acac-treated samples with hot ethanol shows a quite different feature between HMAZ and other samples. Fig. 2 compares the initial intensity of the Al– acac complex band with that of the samples after washing. The intensity of 285 –290 nm band in HY, HMOR and HZSM-5 do not change notably upon washing, however, the intensity of this band decrease drastically for
Table 3 Pore openings of the zeolites Zeolite
˚) Pore opening (A
Y MOR MAZ ZSM-5
7.4 £ 7.4 6.5 £ 7.0 7.4 £ 7.4 5.1 £ 5.5
HMAZ sample. One hypothesis to explain this discrepancy is that the extraction of aluminum – acetylacetone complexes is facilitated because of larger pore opening of HMAZ. However, The pore opening of HY zeolite is exactly the same as HMAZ (see Table 3) but we observe no efficient extraction of the Al– acac complexes from cavities of this zeolite. Retention of these species inside the cages of zeolite Y could be attributed to the hydrogen bonds in two ways. It has been claimed that acetylacetone complexes may become involved
Fig. 2. Diffuse reflectance spectra of acac-treated acidic zeolites before (left part) and after (right part) ethanol washing: (a) HY, (b) HMOR, (c) HMAZ and (d) HZSM-5.
M.A. Zanjanchi, E. Mohabbati / Journal of Molecular Structure 645 (2003) 171–176
in a hydrogen bond interaction with the hydroxyl groups of zeolites [15]. Therefore, a high concentration of hydroxyl groups in zeolite Y and hence a huge network of hydrogen bonds, may stabilize the complexes and reduces extent of extraction. Hydrogen bonds may also be established according to recent Benco et al. report [16]. Upon their ab initio molecular dynamic calculation results, the network of the hydrogen bonds considerably suppresses the mobility of the species and strongly stabilizes them in the cages. The multiple contacts of the occluded species with the hydroxyl groups are much stronger in low silica (higher aluminum content and therefore higher hydroxyl group concentration) zeolite. The calculated adsorption energy of 311 kJ/mol for their sample with Si/Al ¼ 2 is more than three times higher compared with that of the high silica zeolite [16]. In an attempt to study influence of the hydrogen bonding of hydroxyl groups, we neutralized our acidic zeolite via a NaOH titration method [17]. We examined again our solid samples by XRD to ensure
175
that the crystallinity of the samples is preserved after neutralization. We have repeated the acetylacetone treatments on fresh and neutralized zeolite samples. The results are shown in Fig. 3. It contains DR spectra of the neutralized acac-treated zeolite samples and their DR spectra following elusion with hot ethanol. This time in contrast to the behavior of the acidic form of the zeolite Y, we observed that nearly all of the Al – acac complexes are washed out with ethanol. However, we could not extract Al – acac complexes out of the channels of the neutralized mordenite sample (see DR spectra of this sample in Fig. 3 after acac treatment and after wash up). In fact our observation for neutralized mordenite is the same as that of the acidic mordenite (HMOR) sample (Fig. 2). We obtained the same results for neutralized ZSM5 sample. Therefore it can be concluded that efficient extraction of extra-framework aluminum by acetylacetone requires a suitable pore opening as large as ˚ . Although, our studies showed < 7.4 £ 7.4 A that extensive hydrogen bond formation unfavours
Fig. 3. Diffuse reflectance spectra of acac-treated neutralized zeolites before (left part) and after (right part) ethanol washing: (a) Y, (b) MOR, (c) MAZ and (d) ZSM-5.
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Al –acac extraction out of a low-silica zeolite (i.e. zeolite HY), but in the HMOR and HZSM-5, the case is different. The extraction is mainly controlled by pore opening in these zeolites, because we did not noticed any different between the acidic and neutralized forms of these zeolite in Al –acac extraction. However, the extraction of Al– acac complexes is much more efficient in neutralized mazzite compared with that of the acidic one (see Figs. 2c and 3c).
4. Conclusion From our diffuse reflectance spectrophotometric studies of structurally modified acac-treated zeolites, the following conclusions can be drawn: 1. Acetylacetone treatment of the zeolite samples containing extra-framework aluminum leads to rise of a distinct well-defined band at 285 –290 nm depending on the zeolite type. 2. The washing of the acetylacetone treated acidic samples in hot ethanol caused a considerable decrease of the intensity of 285 nm band in mazzite sample only. This indicate that some of the Al– acac complexes are removed out of the solid. 3. The washing of the acetylacetone treated acidic zeolite Y did not alter the intensity of the 290 nm band. A huge network of hydrogen bonds may be involved in stabilizing of the complexes inside the pores of zeolite HY. 4. Neutralizing of the zeolite HY prevents formation of the hydrogen bonds between complexed aluminum species and hydroxyl groups of this zeolite. In this condition aluminum species are extracted out of the solid. 5. Neutralizing of mordenite and ZSM-5 samples did not alter their behavior toward releasing Al –acac complexes and no change in the intensity of 295 nm was observed. 6. The efficiency of elusion of the mazzite containing hydroxyl groups (acidic mazzit) is less than the sample without hydroxyl groups (neutralized
mazzite) most probably due to existence of hydrogen bonds in acidic samples.
Acknowledgements The authors express their appreciation to the University of Guilan Graduate Office for financial support of this work.
References [1] R. Szostak, Molecular Sieves: Principles of Synthesis and Identification, Second ed., Blackie Academic and Professional, London, 1998, p. 193. [2] R. Szostak, Stud. Surf. Sci. Catal. 137 (2001) 261. [3] J. Scherzer, in: T.E. White, et al. (Eds.), Catalytic Materials: Relationship between Structure and Activity, ACS Symposium Series 248, 1984, p. 157. [4] P.E. Eberly Jr., S.M. Laurent, H.E. Robson, US Patent 3 506 400, 1970. [5] J. Klinowski, C.A. Fyfe, C. Gobbi, J. Chem. Soc. Faraday Trans. I 81 (1985) 3003. [6] V. Bosacel, D. Freude, T. Frohlrch, U. Pfeifer, H. Schniedel, J. Colloid Interface Sci. 85 (1982) 502. [7] P.J. Grobet, H. Geerts, J.A. Martens, P.A. Jacobs, J. Chem. Soc. Chem. Commun. (1987) 1688. [8] W.A. Buckermann, C.B. Huong, F. Fajula, C. Gucguen, Zeolites 13 (1993) 448. [9] M.A. Zanjanchi, A. Razavi, Spectrochim. Acta A 57 (2001) 119. [10] A.A. Shikh, P.N. Joshi, N.E. Jacob, V.P. Shiralker, Zeolites 13 (1993) 511. [11] R.H. Jarman, A.J. Jacobson, M.T. Melchior, J. Phys. Chem. 88 (1984) 5748. [12] M.K. Rubin, E.J. Rosinski, C.J. Plank, Zeolite Technology and Application, Recent Advances, vol. 51, Noyes Data Corporation, 1980. [13] M.M.J. Treacy, J.B. Higgins, Collection of Simulated XRD Powder Patterns for Zeolites, Fourth ed., Elsevier, Amsterdam, 2001. [14] G. Engelhardt, Stud. Surf. Sci. Catal. 137 (2001) 387. [15] M. Baltes, P. van DerVoort, B.M. Weckhuysen, R.R. Rao, G. Catana, R.A. Schoonheydt, E.F. Vansant, Phys. Chem. Chem. Phys. 2 (2000) 2673. [16] L. Benco, T. Demuth, J. Hafner, F. Hutschka, H. Toulhoat, J. Catal. 209 (2002) 480. [17] M. Crocker, R.H.M. Herold, M.H.W. Sonnemans, C.A. Emeis, A.E. Wilson, J.N. van der Moolen, J. Phys. Chem. 97 (1993) 432.
Spectroscopic studies of the zeolite materials: interaction of extra-framework aluminum with acetylacetone and hydroxyl groups M.A. Zanjanchi*, E. Mohabbati Department of Chemistry, Faculty of Science, University of Guilan, P.O. Box 1914, Rasht, Iran Received 4 September 2002; accepted 11 October 2002
Abstract Diffuse reflectance spectroscopy has been used to investigate structural modification of different acidic and neutralized zeolites subjected to acetylacetone treatments. Extra-framework aluminum species, produced upon expulsion of aluminum atoms from the framework of the zeolite Y, mordenite, mazzite and ZSM-5 form complexes with acetylacetone. A distinct welldefined 285– 290 nm band produced is due to the transformation of aluminum atoms to structures with highly ordered environments. Washing the acetylacetone-treated acidic samples with ethanol leads to extraction of most of the complexed aluminum out of the channels of zeolite mazzite only. The residence of Al – acac complexes in zeolite HY with the same pore opening as mazzite was attributed to the huge network of hydrogen bonds which stabilizes the complexes and reduces extent of extraction. However, upon neutralization of zeolite HY with sodium hydroxide, nearly all of the Al – acac complexes are removed out with ethanol washing and the band disappears in the DR spectrum. The DR spectra obtained for the acac-treated acidic and neutralized mordenite and ZSM-5 zeolites did not show any difference between them before and after ethanol washing. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Diffuse reflectance spectroscopy; Acetylacetone; Zeolites; Hydrogen bonds; Hydroxyl groups
1. Introduction Secondary or post-synthesis modification of microporous materials encompasses a variety of techniques to further control of catalytic activity and/or the shape selectivity of a specific microporous structure [1,2]. Dealumination of zeolites to increase the Si/Al ratio
* Corresponding author. Fax: þ 98-131-322-0066. E-mail address: [email protected] (M.A. Zanjanchi).
of the crystals is one of the method that has been employed to change or modify the acidity of these materials. There are three methods to prepare dealuminated zeolites: (1) hydrothermal treatment, (2) chemical treatment and (3) a combination of hydrothermal and chemical treatments [3]. By hydrothermal dealumination, aluminum atoms are removed from the framework but are not removed from the crystal. The nature of extra-framework aluminum species is not fully clear but they may have forms of cationic species (Al3þ, AlOþ, Al(OH)2þ, Al(OH)þ 2)
0022-2860/03/$ - 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 5 7 4 - 4
172
M.A. Zanjanchi, E. Mohabbati / Journal of Molecular Structure 645 (2003) 171–176
and neural or polymerized species (AlO(OH), Al(OH)3, Al2O3) [3]. Chelating agents such as acetylacetone can be used to wash these extra-framework aluminum species out of the crystalline structure [4]. Interaction of acetylacetone with extra-framework aluminum have been studied by 27Al MAS NMR method following impregnation of the sample with a solution of acac in ethanol [5 – 8]. By this treatment the extra-framework aluminum atoms are transferred into stable Al –acac complexes which posses a highly ordered environment. Recently, we reported that diffuse reflectance spectroscopy could successfully be used for identification and estimation of extra-framework aluminum in a thermally treated and acid leached mazzite zeolite [9]. We showed that acetylacetone impregnation of the sample containing extra-framework aluminum leads to transformation of all of the aluminum species with distorted symmetry to a highly ordered environment where is discovered by a distinct well-defined band around 285 nm in the DR spectrum. The washing of the acetylacetone treated samples in hot ethanol caused a decrease of the intensity of this band indicate that some of the Al –acac complexes is extracted out of the solid. The aim of the present contribution is to apply the recently developed DRS approach to study the removal of extra-framework aluminum in zeolite Y, Mordenite, Mazzite and ZSM-5 by acetylacetone complexation. We will show that the size of pore openning, Si/Al ratio and hydroxyl groups concentration have great influence on stabilization of aluminum species inside the channels and cages of the zeolites.
tetramethylammonium hydroxide (Merck 8123) as a structure directing template according to the procedure described previously [9,11]. Zeolite ZSM-5 was synthesized according to the recipe given by Rubin et al. [12]. Tetraethylammonium hydroxide (Fluka 86631) was used as template for the synthesis of ZSM-5. The prepared gel was heated at 175 8C for 60 h. The oxide mole composition of the synthesized zeolites are given in Table 1. To remove occluded templates out of the channels of mazzite and ZSM-5 zeolites, these samples were heated in a muffle furnace in air at a rate of 2 8C/min up to 550 8C and kept at this temperature for 5 h. The acidic forms of the zeolites were prepared by NHþ 4 ion exchange followed by thermal treatments. Ammonium exchange forms of the mordenite, mazzite and ZSM-5 were prepared by treating the as-synthesized mordenite and calcined mazzite and ZSM-5 with 1 M solution of NH4NO3 under reflux at 80 8C for 5 –10 h, separately. For each of them, the solution was filtered and zeolite washed with deionized hot water several times and dried at 110 8C. The ammonium exchange for zeolite Y was performed three times at 25 8C with a 0.4 M solution of NH4Cl. The washing procedure was the same as others and the sample was dried at 110 8C. To convert the ammonium form of the zeolites to acidic ones, they were heated in a muffle furnace at a rate of 10 8C/min and holding at 500 8C for at least 7 h. For the complexation treatment, approximately 3 ml of a solution of acetylacetone in ethanol (38%) was added to 1.0 g of acidic or neutralized zeolite samples, separately. The samples were left in contact with stirred acetylacetone solution for 5 h. During this time the container was caped to prevent evaporation
2. Experimental 2.1. Sample preparation and treatments Zeolite Y was obtained from Union Carbid and has a Si/Al ratio of < 2.5. Mordenite was prepared by hydrothermal synthesis method described in the literature [10]. Sodium silicate solution (27% SiO2, 8% Na2O, 65% H2O, Merck) and aluminum sulfate (Fluka No. 6421) were used as sources of silicon and aluminum of the synthesis gel, respectively. The zeolite mazzite was synthesized in presence of
Table 1 Composition of the reacting gels for the synthesized zeolites Zeolite
Al2O3
SiO2
Na2O
R2Oa
H2O
MOR MAZ ZSM-5
1 1 1
15 15 80
8.5 4.8 24.0
– 1.2 20.0
1300 240 3000
a R is the templates tetramethylammonium hydroxide and tetraethylammonium hydroxide used for the synthesis of mazzite and ZSM-5 zeolites, respectively.
M.A. Zanjanchi, E. Mohabbati / Journal of Molecular Structure 645 (2003) 171–176
of the mixture. After that, samples were dried at room temperature in a stream of dry air. The samples were then washed by treating the solids in 30 ml ethanol for 3 h at 60 8C to perform extraction of Al – acac complexes. 2.2. Characterization X-ray diffraction measurements were performed on a Philips PW 1840 diffractometer with Cu Ka radiation at room temperature. XRD patterns were recorded using an automatic divergence slit system. Diffuse reflectance spectra were recorded by a UV2100 Shimadzu spectrophotometer, equipped with an integrating sphere assembly. A special cell, loaded with the solid sample and covered by a quartz window, was used in all measurements. All the spectra were recorded at room temperature against barium sulfate and plotted in terms of absorbance. Chemical analysis of the samples was done by energy dispersive X-ray analysis (EDX) joined to a Philips XL 30 scanning electron microscope. The template and water content of the samples were determined by thermogravimetry, using a Polymer Laboratories TG1500.
173
the calcined and acidic samples show a very lowintensity background indicating that there is no significant loss of crystallinity upon heating treatments. The conversion of the as-synthesized zeolites into their calcined and acidic forms results in extraframework aluminum formation due to dealumination. Calcination lowers the symmetry of the environment of the aluminum atoms expelled from their sites to form extra-framework aluminum species. This will produce a broadened signal which can be visualized by 27Al MAS NMR [14] and by diffuse reflectance spectroscopy [9]. Impregnation of the acidic samples containing extra-framework aluminum with ethanolic acetylacetone will convert the broadened band to
3. Results and discussion The unit cell composition of the as-synthesized samples are shown in Table 2. They were calculated from the EDX data. EDX analysis was made upon several crystalline parts of the samples observed by scanning microscopy to measure amount of silicon and aluminum. The mean of the separate analysis was taken. X-ray diffraction patterns of the as-synthesized samples reveals high crystallinity and high purity of the samples [13]. Inspection XRD patterns of Table 2 Unit cell composition of the as-synthesized zeolites Zeolite
Unit cell formula
Y MOR MAZ ZSM-5
H1Na52.5[Al35.5Si138.5O384]265 H2O Na6.7[Al6.7Si41.3O96]23.5 H2O Na3.2TMA1.9[Al5.1Si30.9O72]26.5 H2O Na0.3TEA3.2[Al3.5Si92.5O192]10 H2O
Fig. 1. Diffuse reflectance spectra of acetylacetone treated zeolite samples: (a) HY, (b) HMOR, (c) HMAZ and (d) HZSM-5.
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a distinct well-defined band [9]. The appearance of this band is due to the transformation of the aluminum atoms with distorted symmetry and different coordination number to structures with highly ordered octahedral symmetry. Fig. 1 shows DR spectra of the acac-treated samples. A distinct band at about 285 – 290 nm is the result of the treating acidic samples with acetylacetone. We found the shape and intensity of this band depends strongly on the type of zeolites. This band is broadened in HMOR and ZSM5 samples, whereas it is sharper and with higher intensity in HMAZ and HY samples. The washing of acac-treated samples with hot ethanol shows a quite different feature between HMAZ and other samples. Fig. 2 compares the initial intensity of the Al– acac complex band with that of the samples after washing. The intensity of 285 –290 nm band in HY, HMOR and HZSM-5 do not change notably upon washing, however, the intensity of this band decrease drastically for
Table 3 Pore openings of the zeolites Zeolite
˚) Pore opening (A
Y MOR MAZ ZSM-5
7.4 £ 7.4 6.5 £ 7.0 7.4 £ 7.4 5.1 £ 5.5
HMAZ sample. One hypothesis to explain this discrepancy is that the extraction of aluminum – acetylacetone complexes is facilitated because of larger pore opening of HMAZ. However, The pore opening of HY zeolite is exactly the same as HMAZ (see Table 3) but we observe no efficient extraction of the Al– acac complexes from cavities of this zeolite. Retention of these species inside the cages of zeolite Y could be attributed to the hydrogen bonds in two ways. It has been claimed that acetylacetone complexes may become involved
Fig. 2. Diffuse reflectance spectra of acac-treated acidic zeolites before (left part) and after (right part) ethanol washing: (a) HY, (b) HMOR, (c) HMAZ and (d) HZSM-5.
M.A. Zanjanchi, E. Mohabbati / Journal of Molecular Structure 645 (2003) 171–176
in a hydrogen bond interaction with the hydroxyl groups of zeolites [15]. Therefore, a high concentration of hydroxyl groups in zeolite Y and hence a huge network of hydrogen bonds, may stabilize the complexes and reduces extent of extraction. Hydrogen bonds may also be established according to recent Benco et al. report [16]. Upon their ab initio molecular dynamic calculation results, the network of the hydrogen bonds considerably suppresses the mobility of the species and strongly stabilizes them in the cages. The multiple contacts of the occluded species with the hydroxyl groups are much stronger in low silica (higher aluminum content and therefore higher hydroxyl group concentration) zeolite. The calculated adsorption energy of 311 kJ/mol for their sample with Si/Al ¼ 2 is more than three times higher compared with that of the high silica zeolite [16]. In an attempt to study influence of the hydrogen bonding of hydroxyl groups, we neutralized our acidic zeolite via a NaOH titration method [17]. We examined again our solid samples by XRD to ensure
175
that the crystallinity of the samples is preserved after neutralization. We have repeated the acetylacetone treatments on fresh and neutralized zeolite samples. The results are shown in Fig. 3. It contains DR spectra of the neutralized acac-treated zeolite samples and their DR spectra following elusion with hot ethanol. This time in contrast to the behavior of the acidic form of the zeolite Y, we observed that nearly all of the Al – acac complexes are washed out with ethanol. However, we could not extract Al – acac complexes out of the channels of the neutralized mordenite sample (see DR spectra of this sample in Fig. 3 after acac treatment and after wash up). In fact our observation for neutralized mordenite is the same as that of the acidic mordenite (HMOR) sample (Fig. 2). We obtained the same results for neutralized ZSM5 sample. Therefore it can be concluded that efficient extraction of extra-framework aluminum by acetylacetone requires a suitable pore opening as large as ˚ . Although, our studies showed < 7.4 £ 7.4 A that extensive hydrogen bond formation unfavours
Fig. 3. Diffuse reflectance spectra of acac-treated neutralized zeolites before (left part) and after (right part) ethanol washing: (a) Y, (b) MOR, (c) MAZ and (d) ZSM-5.
176
M.A. Zanjanchi, E. Mohabbati / Journal of Molecular Structure 645 (2003) 171–176
Al –acac extraction out of a low-silica zeolite (i.e. zeolite HY), but in the HMOR and HZSM-5, the case is different. The extraction is mainly controlled by pore opening in these zeolites, because we did not noticed any different between the acidic and neutralized forms of these zeolite in Al –acac extraction. However, the extraction of Al– acac complexes is much more efficient in neutralized mazzite compared with that of the acidic one (see Figs. 2c and 3c).
4. Conclusion From our diffuse reflectance spectrophotometric studies of structurally modified acac-treated zeolites, the following conclusions can be drawn: 1. Acetylacetone treatment of the zeolite samples containing extra-framework aluminum leads to rise of a distinct well-defined band at 285 –290 nm depending on the zeolite type. 2. The washing of the acetylacetone treated acidic samples in hot ethanol caused a considerable decrease of the intensity of 285 nm band in mazzite sample only. This indicate that some of the Al– acac complexes are removed out of the solid. 3. The washing of the acetylacetone treated acidic zeolite Y did not alter the intensity of the 290 nm band. A huge network of hydrogen bonds may be involved in stabilizing of the complexes inside the pores of zeolite HY. 4. Neutralizing of the zeolite HY prevents formation of the hydrogen bonds between complexed aluminum species and hydroxyl groups of this zeolite. In this condition aluminum species are extracted out of the solid. 5. Neutralizing of mordenite and ZSM-5 samples did not alter their behavior toward releasing Al –acac complexes and no change in the intensity of 295 nm was observed. 6. The efficiency of elusion of the mazzite containing hydroxyl groups (acidic mazzit) is less than the sample without hydroxyl groups (neutralized
mazzite) most probably due to existence of hydrogen bonds in acidic samples.
Acknowledgements The authors express their appreciation to the University of Guilan Graduate Office for financial support of this work.
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