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Synthesis of the zeolites on the lightweight aluminosilicate fillers
Materials Research Bulletin 49 (2014) 210–215
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Synthesis of the zeolites on the lightweight aluminosilicate fillers W. Picho´r, W. Mozgawa, M. Kro´l *, A. Adamczyk Faculty of Materials Science and Ceramic, AGH University of Science and Technology, Mickiewicza Av., 30-059 Krakow, Poland
A R T I C L E I N F O
A B S T R A C T
Article history: Received 4 June 2013 Received in revised form 20 August 2013 Accepted 25 August 2013 Available online 31 August 2013
This work presents the results of synthesis of zeolites from perlite and fly ash microspheres via treatment with sodium hydroxide solution, at low temperatures of 30–90 8C under atmospheric pressure. Obtained products could be active as low-cost agents for the removal of heavy metals from wastewater. The influences of reaction temperature and time as well as NaOH concentration, on the type of the solid products were investigated. The results showed that mixture of zeolite Y, A and Na-P1 could be synthesized at temperatures varying in the range of 60–90 8C with NaOH concentration of 3–4 M. Regardless of the type of substrate used, it is possible to obtain the product in a similar way. The synthesized products are characterized by low bulk density, 0.16 and 0.95 g/cm3 for perlite and microspheres respectively. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures B. Chemical synthesis C. Infrared spectroscopy C. X-ray diffraction D. Microstructure
1. Introduction Natural zeolites are a group of hydrated tectoaluminosilicates with a specific and highly differentiated structure containing voids filled with ions and water molecules having a high freedom of movement [1]. Due to this construction, zeolites have many of varied and extremely valuable physicochemical properties and practical applications. The high cation exchange capacity, as well as the possibility of sorption of molecules or ions, decides on the use of zeolites among others in the process of heavy metal cations immobilization [2,3], removal of ammonium ions [4] or radionuclides [5] as well in carbon dioxide capture [6] etc. Zeolites may be obtained by heating some aluminosilicate materials in the presence of alkaline solutions. Various media of silica are used for synthesis of zeolites, inter alia natural minerals such as: kaolinite [7], volcanic glaze (perlite [8,9], pumice [10]), rice shells [11], diatomite, as well as coal fly ash [12,13] or synthetic silicates [14]. Depending on the type of raw materials and process conditions (temperature, pressure), final product can be obtained in a few hours or days. A type of zeolite structure, which is formed at given temperature, depends to a large extent on the composition of the starting mixture. However, process conditions influence the course of crystallization in systems having the same chemical composition. These include: pH of reaction solution, temperature, pressure and treatment time as well as degree of fineness of reagents or mixing [15]. Zeolites are
* Corresponding author. Tel.: +48 12 617 25 30; fax: +48 12 633 71 61. E-mail address: [email protected] (M. Kro´l). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.08.055
also the main component of geopolymers as well as slag alkaline binders [16–19]. The aim of this work is to obtain of zeolitic products characterized by low bulk density (<1 g/cm3) from light aluminosilicate fillers. The synthesis should be carried out in a simple way, at low temperatures and/or with short reaction times. The results of syntheses carried out on a laboratory scale will allow to determine the impact of analyzed factors on the amount and nature of the obtained reaction products. The results will be useful in the selection of parameters for synthesis of a material with potential applications such as floating on water sorbents of heavy metal cations. Among several possible light aluminosilicate materials such as vermiculite, glass granules, expanded perlite or microspheres, only two latter were selected for synthesis in this work. Others were considered unsuitable for zeolitization process due to insufficient reactivity at low temperatures (crystalline vermiculite) or too small amount of aluminum in the structure (glass granules). Both the perlite and the microspheres are aluminosilicate materials characterized by the similar chemical composition (Table 1). The main difference is in the type of porosity–the perlite has a lamellar structure and open porosity, and porosity of the microspheres can be described as a closed. It was examined whether synthesis may be carried out in the same manner, regardless of the type of the starting material. 2. Experimental The expanded perlite (PEX100, Piotrowice II) and the fly ash microspheres (EKO EXPORT) were used as the starting materials. The grain fractions, 0.125–2.0 mm and 0.125–0.5 mm respectively,
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Table 1 Chemical compositions of expanded perlite and fly ash microspheres. Chemical composition
Expanded perlite
Microspheres
SiO2 Al2O3 SiO2/Al2O3 MgO + CaO Na2O + K2O
68–75% 10–13% 4.5–7.5 1–4% 6–9%
45–60% 20–30% 1.5–3.0 1–4% 0.5–2%
were separated. The bulk densities were about 0.11 and 0.83 g/ cm3, respectively. (Pycnometer was used to determine the density of microspheres. In the case of perlite, because of the open porosity, the value is the apparent density.) Perlite was X-ray amorphous and microspheres beyond the aluminosilicate glassy phase contained mullite and small amount of quartz. The chemical compositions (a wavelength dispersive X-ray fluorescence spectrometer (WD-XRF) (Axios mAX 4 kW, PANalytical, Netherland) equipped with Rh source) are collected in the Table 1. The synthesis was carried out by mixing the perlite or the microspheres with aqueous solution of sodium hydroxide (POCH) in the concentration range 0.5–5.0 mol/dm3. The solid-to-solution ratio was maintained at 1:15 (g/ml). The reactions were performed under atmospheric pressure. Three different temperatures (30, 60 and 90 8C) and various durations (24, 48 and 72 h) were used. The final solid products were recovered by filtration and washing with distilled water until the pH of the filtrate was below 10. The samples were dried at temperature not exceeding 80 8C. The alteration products were identified by means of Philips Xray powder diffraction X’Pert system (CuKa radiation). The morphology and the crystal size were studied by scanning electron microscope FEI Nova NanoSEM 200. The existence of zeolite frameworks was confirmed by measured on Bruker VERTEX 70 v vacuum FT-IR spectrometer using the standard KBr (Merck) pellets methods.
Fig. 1. XRD patterns of products formed by reactions of expanded perlite in 0.5– 5.0 M NaOH at 90 8C for 72 h.
3. Results and discussion Temperature, reaction time and concentration of sodium hydroxide, as parameters influencing the zeolitization products were investigated. Fig. 1 shows an exemplary series of XRD patterns illustrating the reaction product of perlite in contact with solution of 0.5–5.0 M NaOH at 90 8C for 72 h. Depending on NaOH concentration, identified phases are: zeolite X, zeolite Na-P1, zeolite A and hydroxysodalite. The results show, that at atmospheric pressure and over a range of temperatures and NaOH concentrations, the major product was zeolite Na-P1. This result corresponds to related literature [20]–at a crystallization temperature below 100 8C the most stable structure is gismondine-type zeolite, Na-P1. It is visible, that low sodium hydroxide solution concentrations, 1.0–3.0 mol/dm3, favored the formation of zeolite Na-P1–in this range the amount of a crystalline phase increases in proportion to the concentration of NaOH. At higher concentrations (>3.5 mol/dm3) zeolite X as coexisting phase appears, whereas the highest concentration (5.0 mol/dm3) led to the formation of significant amount of hydroxysodalite. Reduction in synthesis time or temperature results mainly in the quantity of products. Regardless those, the solution of 0.5 mol/dm3 did not give any crystalline product. The presence of aluminosilicate framework in the obtained samples was confirmed using IR spectroscopic technique. MIR spectra of the sample obtained at 90 8C for 72 h are presented in Fig. 2. Every zeolitized solids show similar IR spectra, which contain a number of vibration bands of the structural units present in the structure of the material.
Fig. 2. FT-IR specrta of products formed by reactions of expanded perlite in 0.5– 5.0 M NaOH at 90 8C for 72 h.
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Fig. 3. Wavenumber of the Si–O(Si,Al) vibration band as a function of NaOH concentration.
The most intense band connected with internal Si–O(Si) and Si– O(Al) bonds’ vibrations occur in the range 1200–400 cm 1 [1]. A decreasing full width at half maximum of the most intense band at about 1057 cm 1 with increasing NaOH concentrations indicates a growing share of the crystalline phase. It is further evidence of the formation of the assumed reaction products. In the same time, this band shifts toward lower wavenumbers (from 1057 to 980 cm 1). This suggests increasing share of aluminum in tetrahedral positions compared to the starting material. Si/Al ratio affects the ion-exchange capacity of the synthesized zeolite. It should be noted, that the position of analyzed band does not change significantly for the synthesis products obtained under the influence of the NaOH concentration, which was higher than 3.0 mol/dm3 (Fig. 3). Additionally all samples show the band connected with O–H vibration in the range 3800–3000 cm 1 and at about 1640 cm 1. The presence of these bands is associated with adsorbed water molecules in zeolite framework. The range 800–500 cm 1 is pseudolattice vibrations region related to the bands originating from overtetrahedral structural units-rings made of silicaoxygen and aluminiumoxygen tetrahedral characteristic for zeolite structure type [1]. Starting from 1.0 M NaOH solution, several bands appear in this range of the analyzed spectra. Based on the position of the bands in this range, it is possible to identify a zeolite framework. At lower concentrations, two bands at about 717 and 598 cm 1 indicate the presence of zeolite Na-P1 in the samples, however their relationship with the presence of zeolite X can not be excluded [21]. Integral intensity of these bands is steadily increasing up to concentration of 3.0 mol/ dm3. At the same time, intensity of the band about 387 cm 1,
associated with the presence of aluminosilicate glassy phase, disappears almost completely. On the other hand, bands associated with the presence of sodalite, at about 731, 707, 666 and 562 cm 1, clearly dominate in the case of using 5.0 M NaOH, but their presence shall be indicated already at the concentration of 4.0 mol/ dm3. This agrees with the results obtained by XRD and corresponds to the earlier observation related with shifting of the Si–O(Si,Al) band (Fig. 3). 3.0 mol/dm3 is the threshold concentration, below which zeolitization occurs quantitatively. On the other hand, above this value, the crystallized phases are converted to a more compact structure, which should have a negative impact on the sorption properties of the material. Figs. 4 and 5 show the SEM micrographs of zeolityzation products obtained at 90 8C for 72 h. SEM images of various NaOH concentrations are slightly different in percent crystallization and crystal size. The expanded perlite has honeycomb structure and a large amount of pores. In lower concentration of NaOH (Fig. 4) the structure is fully preserved. Only locally clusters of crystalline phases which are the results of a process can be observed (Fig. 4a). Nucleating agents on the surface of perlite can be seen at large magnifications (Fig. 4b). Increasing NaOH concentration favors the formation of crystalline phases at the expense of the glassy phase (Fig. 5). SEM observations of the sample obtained at 60 8C are visible in the Fig. 6. As mentioned, lowering temperature of the process results in a significant reduction of the amount of products. Lengthening the time of synthesis from 24 to 72 h did not result in a significant increase in the amount of crystalline phases. The aim of this study was the synthesis of zeolites in the lowest possible temperature; however, keeping long synthesis for the temperature reduction is not a good direction of the optimization process. As mentioned, most desirable phases, due to the high sorption capacity, are zeolite X, A and Na-P1. To summarize the experiment of zeolite crystallization from perlite under the conditions studied, the results indicate that the largest amount of zeolites with high ion exchange capacity were obtained under the influence of 3.0– 4.0 M NaOH at 90 8C for 72 h. Bulk density of the obtained material was about 0.16 g/cm3. Fly ash microspheres were selected as the second material for the synthesis of zeolites. Compared to expanded perlite, microspheres are characterized by higher bulk density and lower SiO2/ Al2O3 ratio (Table 1). Using the materials with higher aluminum content should result in a product with a higher content of zeolite A [22] and thus increase the sorption capacity of the material. The phase compositions of the products obtained from the microspheres depending on different experimental conditions (NaOH
Fig. 4. SEM observations of the sample obtained from expanded perlite in 2.5 M NaOH at 90 8C for 72 h.
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Fig. 5. SEM observations of the sample obtained from expanded perlite in 4.0 M NaOH at 90 8C for 72 h.
Fig. 6. SEM observations of the sample obtained from expanded perlite: (a) in 3.0 M NaOH at 60 8C for 24 h; (b) in 4.0 M NaOH at 60 8C for 72 h.
Table 2 The phase compositions of the products obtained from the microspheres depending on the NaOH concentration, time and temperature. CNaOH [mol/dm3]
Temp. [8C]
Time [h]
X
A
1
75
48 72 48 72
+
+
48 72 48 72
+ + +
+ +
48 72 48 72
+ + + +
48 72 48 72
+ + + +
48 72 48 72
+ + + +
90
2
75 90
3
75 90
4
75 90
5
75 90
Identified zeolite phase Na-P1
Sodalite
+ + + +
+ +
+ +
+
+ + +
+ + + +
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Fig. 7. MIR spectra of zeolitic products obtained from microspheres after 72 h reaction time at: (a) 75 and (b) 90 8C.
concentration, time and temperature) are collected in Table 2. When the reactions are carried out under the influence of 2–3 M NaOH at 90 8C and under the influence of 3–4 M NaOH at 75 8C perlite is typically converted to a mixture of zeolite X, A and Na-P1 with the highest yield. The major phase is zeolite X. Extending the reaction time from 48 to 72 h mainly affects the number of products, and only to a lesser extent, on the type of zeolite structure. However, zeolite A disappears with increasing reaction time (e.g. reaction with 3 M NaOH at 90 8C). Detailed analysis of the crystallization process shown [23] that zeolite A is an intermediate product in the synthesis of stable zeolite X. The fly ash microspheres include in its composition a significant amount of crystalline mullite and small amount of quartz. It is known that mullite is a relatively stable phase, however, based on analyzed spectra, due to the high coincidence of vibration bands for Si–O(Si,Al), it is difficult to describe its behavior during the synthesis. However, its presence should not have any impact on the pseudolattice range of the spectra thought. The bands
characteristic for quartz are observed in the spectra only for the lowest concentrations of NaOH. No presence of doublet bands at 780–800 cm 1 indicates complete reaction of this phase during the synthesis in higher NaOH concentration. Comparison of IR spectra of the samples synthesized in different conditions shows that the higher integral intensity of the bands in pseudolattice range was noted for those obtained at higher temperature (Fig. 7). A similar situation occurs in the case of products obtained at the same temperature, but with different times of synthesis (results are not presented hear). It can be concluded that the increase in reaction time and temperature favors the quantitative crystallization. A similar conclusion can be drawn from the SEM observations (Figs. 8 and 9). At lower concentrations of NaOH and shorter synthesis time, as in the case of the expanded perlite, unreacted portions of the microspheres are visible (Fig. 8). In the conditions considered as optimal, i.e. in 3.0 M NaOH at 90 8C for 72 h (Fig. 9), the layer of reaction product tightly covers the surface of the microspheres. Residues of glassy
Fig. 8. SEM observations of the sample obtained from microspheres in 2.0 M NaOH at 60 8C for 48 h.
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Fig. 9. SEM observations of the sample obtained from microspheres in 3.0 M NaOH at 90 8C for 72 h. (b) Microsection image.
phase only occur in the interior of the microspheres (Fig. 9b) and create a lightweight framework which is a carrier for zeolites. Bulk density of the obtained material is about 0.95 g/cm3. 4. Conclusions Using simple synthesis technique, it is possible to obtain a coating of sodium zeolites on the light microfillers characterized by density lower than 1 g/cm3. The resulting values are 0.16 g/cm3 for perlite and 0.95 g/cm3 for microspheres. Regardless of the differences in the chemical composition similar technique can be used for synthesis of the zeolites on expanded perlite or microspheres. Changing of synthesis parameters (NaOH concentration, temperature, time) affects the nature and quantity of obtained crystalline phases. The synthesis of zeolite is possible using the NaOH solution having a concentration above 0.5 M, at temperature at least 60 8C and at time 24 h. The results suggest the possibility of selecting the optimal conditions of zeolites synthesis at a relatively low temperature. The largest amount of zeolite was obtained at 90 8C and 72 h. Reduction in synthesis time or temperature results mainly in the quantity of products. Regardless of the type of substrate, a mixture of zeolite Y and Na-P1 can be received by reaction with sodium hydroxide at a concentration in the range 2– 4 mol/dm3 – above this range sodalite crystallizes. Acknowledgment Financial support this work was provided by The National Centre for Research and Development under grant no. PBS I 177 206.
References [1] D.W. Breck, Zeolite Molecular Sieves, John Wiley & Sons, New York-LondonSydney-Toronto, 1974. [2] S. Wang, Y. Peng, Chem. Eng. J. 156 (2010) 11–24. [3] E. Erdema, N. Karapinar, R. Donat, J. Colloid Interface Sci. 280 (2004) 309–314. [4] W. Franus, M. Wdowin, Miner. Resour. Manage. 26 (4) (2010) 133–148. [5] S. Chałupnik, W. Franus, M. Wysocka, G. Gzyl, Environ. Sci. Pollut. Res. (2013), http://dx.doi.org/10.1007/s11356-013-1877-5. [6] M. Wdowin, W. Franus, R. Panek, Fresenius Environ. Bull. 21 (2012) 3726–3734. [7] C.A. Rı´os, C.D. Williams, M.A. Fullen, Appl. Clay Sci. 42 (2009) 446–454. [8] G.E. Christidis, I. Phapaliars, A. Kontopoulos, Appl. Clay Sci. 15 (1999) 305–324. [9] U. Barth-Wirsching, H. Ho¨ller, D. Klammer, B. Konrad, Mineral. Petrol. 48 (1993) 275–294. [10] N. Burriesci, M.L. Crisafulli, L.M. Saija, Mater. Lett. 2 (1) (1983) 74–78. [11] A.M. Yusof, N.A. Nizam, N.A.A. Rashid, J. Porous Mater. 17 (2010) 39–47. [12] W. Franus, Pol. J. Environ. Stud. 21 (2) (2012) 337–343. [13] X. Querol, F. Plana, A. Alastuey, A. Lo´pez-Soler, Fuel 76 (8) (1997) 793–799. [14] C.D. Williams, in: R.B. King (Ed.), Encyclopedia of Inorganic Chemistry, John Wiley & Sons, Athens, USA, 1994, pp. 4363–4391. [15] S.P. Zhdanov, S.S. Khvoshchev, N.N. Feoktistova, Synthetic Zeolites, Gordon and Breach Science, New York, 1990, p. 1. [16] V.D. Glukhovsky, Soil Silicates, Gosstroyizdat, Kiev, 1959, p. 154. [17] M. Criado, A. Ferna´ndez-Jime´nez, A.G. de la Torre, M.A.G. Aranda, A. Palomo, Cem. Concr. Res. 37 (2007) 671–679. [18] P. Duxson, A. Ferna´ndez-Jime´nez, J.L. Provis, G.C. Lukey, A. Palomo, J.S.J. van Deventer, J. Mater. Sci. 42 (2007) 2917–2933. [19] H. Takeda, S. Hashimoto, H. Yokoyama, S. Honda, Y. Iwamoto, Materials 6 (2013) 1767–1778. [20] S. Khodabandeh, M.E. Davis, Microporous Mater. 9 (1997) 161–172. [21] W. Mozgawa, J. Mol. Struct. 596 (2001) 129–137. [22] R.M. Barrer, E.A.D. White, J. Chem. Soc. (1952) 1561–1571 (Resumed). [23] G.E. Christidis, K. Galani, T. Markopoulos, in: P.W. Scott, C.M. Bristow (Eds.), Industrial Minerals and Extractive Industry Geology, The Geological Society Publishing House, UK, 2002, pp. 345–350.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Synthesis of the zeolites on the lightweight aluminosilicate fillers W. Picho´r, W. Mozgawa, M. Kro´l *, A. Adamczyk Faculty of Materials Science and Ceramic, AGH University of Science and Technology, Mickiewicza Av., 30-059 Krakow, Poland
A R T I C L E I N F O
A B S T R A C T
Article history: Received 4 June 2013 Received in revised form 20 August 2013 Accepted 25 August 2013 Available online 31 August 2013
This work presents the results of synthesis of zeolites from perlite and fly ash microspheres via treatment with sodium hydroxide solution, at low temperatures of 30–90 8C under atmospheric pressure. Obtained products could be active as low-cost agents for the removal of heavy metals from wastewater. The influences of reaction temperature and time as well as NaOH concentration, on the type of the solid products were investigated. The results showed that mixture of zeolite Y, A and Na-P1 could be synthesized at temperatures varying in the range of 60–90 8C with NaOH concentration of 3–4 M. Regardless of the type of substrate used, it is possible to obtain the product in a similar way. The synthesized products are characterized by low bulk density, 0.16 and 0.95 g/cm3 for perlite and microspheres respectively. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures B. Chemical synthesis C. Infrared spectroscopy C. X-ray diffraction D. Microstructure
1. Introduction Natural zeolites are a group of hydrated tectoaluminosilicates with a specific and highly differentiated structure containing voids filled with ions and water molecules having a high freedom of movement [1]. Due to this construction, zeolites have many of varied and extremely valuable physicochemical properties and practical applications. The high cation exchange capacity, as well as the possibility of sorption of molecules or ions, decides on the use of zeolites among others in the process of heavy metal cations immobilization [2,3], removal of ammonium ions [4] or radionuclides [5] as well in carbon dioxide capture [6] etc. Zeolites may be obtained by heating some aluminosilicate materials in the presence of alkaline solutions. Various media of silica are used for synthesis of zeolites, inter alia natural minerals such as: kaolinite [7], volcanic glaze (perlite [8,9], pumice [10]), rice shells [11], diatomite, as well as coal fly ash [12,13] or synthetic silicates [14]. Depending on the type of raw materials and process conditions (temperature, pressure), final product can be obtained in a few hours or days. A type of zeolite structure, which is formed at given temperature, depends to a large extent on the composition of the starting mixture. However, process conditions influence the course of crystallization in systems having the same chemical composition. These include: pH of reaction solution, temperature, pressure and treatment time as well as degree of fineness of reagents or mixing [15]. Zeolites are
* Corresponding author. Tel.: +48 12 617 25 30; fax: +48 12 633 71 61. E-mail address: [email protected] (M. Kro´l). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.08.055
also the main component of geopolymers as well as slag alkaline binders [16–19]. The aim of this work is to obtain of zeolitic products characterized by low bulk density (<1 g/cm3) from light aluminosilicate fillers. The synthesis should be carried out in a simple way, at low temperatures and/or with short reaction times. The results of syntheses carried out on a laboratory scale will allow to determine the impact of analyzed factors on the amount and nature of the obtained reaction products. The results will be useful in the selection of parameters for synthesis of a material with potential applications such as floating on water sorbents of heavy metal cations. Among several possible light aluminosilicate materials such as vermiculite, glass granules, expanded perlite or microspheres, only two latter were selected for synthesis in this work. Others were considered unsuitable for zeolitization process due to insufficient reactivity at low temperatures (crystalline vermiculite) or too small amount of aluminum in the structure (glass granules). Both the perlite and the microspheres are aluminosilicate materials characterized by the similar chemical composition (Table 1). The main difference is in the type of porosity–the perlite has a lamellar structure and open porosity, and porosity of the microspheres can be described as a closed. It was examined whether synthesis may be carried out in the same manner, regardless of the type of the starting material. 2. Experimental The expanded perlite (PEX100, Piotrowice II) and the fly ash microspheres (EKO EXPORT) were used as the starting materials. The grain fractions, 0.125–2.0 mm and 0.125–0.5 mm respectively,
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211
Table 1 Chemical compositions of expanded perlite and fly ash microspheres. Chemical composition
Expanded perlite
Microspheres
SiO2 Al2O3 SiO2/Al2O3 MgO + CaO Na2O + K2O
68–75% 10–13% 4.5–7.5 1–4% 6–9%
45–60% 20–30% 1.5–3.0 1–4% 0.5–2%
were separated. The bulk densities were about 0.11 and 0.83 g/ cm3, respectively. (Pycnometer was used to determine the density of microspheres. In the case of perlite, because of the open porosity, the value is the apparent density.) Perlite was X-ray amorphous and microspheres beyond the aluminosilicate glassy phase contained mullite and small amount of quartz. The chemical compositions (a wavelength dispersive X-ray fluorescence spectrometer (WD-XRF) (Axios mAX 4 kW, PANalytical, Netherland) equipped with Rh source) are collected in the Table 1. The synthesis was carried out by mixing the perlite or the microspheres with aqueous solution of sodium hydroxide (POCH) in the concentration range 0.5–5.0 mol/dm3. The solid-to-solution ratio was maintained at 1:15 (g/ml). The reactions were performed under atmospheric pressure. Three different temperatures (30, 60 and 90 8C) and various durations (24, 48 and 72 h) were used. The final solid products were recovered by filtration and washing with distilled water until the pH of the filtrate was below 10. The samples were dried at temperature not exceeding 80 8C. The alteration products were identified by means of Philips Xray powder diffraction X’Pert system (CuKa radiation). The morphology and the crystal size were studied by scanning electron microscope FEI Nova NanoSEM 200. The existence of zeolite frameworks was confirmed by measured on Bruker VERTEX 70 v vacuum FT-IR spectrometer using the standard KBr (Merck) pellets methods.
Fig. 1. XRD patterns of products formed by reactions of expanded perlite in 0.5– 5.0 M NaOH at 90 8C for 72 h.
3. Results and discussion Temperature, reaction time and concentration of sodium hydroxide, as parameters influencing the zeolitization products were investigated. Fig. 1 shows an exemplary series of XRD patterns illustrating the reaction product of perlite in contact with solution of 0.5–5.0 M NaOH at 90 8C for 72 h. Depending on NaOH concentration, identified phases are: zeolite X, zeolite Na-P1, zeolite A and hydroxysodalite. The results show, that at atmospheric pressure and over a range of temperatures and NaOH concentrations, the major product was zeolite Na-P1. This result corresponds to related literature [20]–at a crystallization temperature below 100 8C the most stable structure is gismondine-type zeolite, Na-P1. It is visible, that low sodium hydroxide solution concentrations, 1.0–3.0 mol/dm3, favored the formation of zeolite Na-P1–in this range the amount of a crystalline phase increases in proportion to the concentration of NaOH. At higher concentrations (>3.5 mol/dm3) zeolite X as coexisting phase appears, whereas the highest concentration (5.0 mol/dm3) led to the formation of significant amount of hydroxysodalite. Reduction in synthesis time or temperature results mainly in the quantity of products. Regardless those, the solution of 0.5 mol/dm3 did not give any crystalline product. The presence of aluminosilicate framework in the obtained samples was confirmed using IR spectroscopic technique. MIR spectra of the sample obtained at 90 8C for 72 h are presented in Fig. 2. Every zeolitized solids show similar IR spectra, which contain a number of vibration bands of the structural units present in the structure of the material.
Fig. 2. FT-IR specrta of products formed by reactions of expanded perlite in 0.5– 5.0 M NaOH at 90 8C for 72 h.
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Fig. 3. Wavenumber of the Si–O(Si,Al) vibration band as a function of NaOH concentration.
The most intense band connected with internal Si–O(Si) and Si– O(Al) bonds’ vibrations occur in the range 1200–400 cm 1 [1]. A decreasing full width at half maximum of the most intense band at about 1057 cm 1 with increasing NaOH concentrations indicates a growing share of the crystalline phase. It is further evidence of the formation of the assumed reaction products. In the same time, this band shifts toward lower wavenumbers (from 1057 to 980 cm 1). This suggests increasing share of aluminum in tetrahedral positions compared to the starting material. Si/Al ratio affects the ion-exchange capacity of the synthesized zeolite. It should be noted, that the position of analyzed band does not change significantly for the synthesis products obtained under the influence of the NaOH concentration, which was higher than 3.0 mol/dm3 (Fig. 3). Additionally all samples show the band connected with O–H vibration in the range 3800–3000 cm 1 and at about 1640 cm 1. The presence of these bands is associated with adsorbed water molecules in zeolite framework. The range 800–500 cm 1 is pseudolattice vibrations region related to the bands originating from overtetrahedral structural units-rings made of silicaoxygen and aluminiumoxygen tetrahedral characteristic for zeolite structure type [1]. Starting from 1.0 M NaOH solution, several bands appear in this range of the analyzed spectra. Based on the position of the bands in this range, it is possible to identify a zeolite framework. At lower concentrations, two bands at about 717 and 598 cm 1 indicate the presence of zeolite Na-P1 in the samples, however their relationship with the presence of zeolite X can not be excluded [21]. Integral intensity of these bands is steadily increasing up to concentration of 3.0 mol/ dm3. At the same time, intensity of the band about 387 cm 1,
associated with the presence of aluminosilicate glassy phase, disappears almost completely. On the other hand, bands associated with the presence of sodalite, at about 731, 707, 666 and 562 cm 1, clearly dominate in the case of using 5.0 M NaOH, but their presence shall be indicated already at the concentration of 4.0 mol/ dm3. This agrees with the results obtained by XRD and corresponds to the earlier observation related with shifting of the Si–O(Si,Al) band (Fig. 3). 3.0 mol/dm3 is the threshold concentration, below which zeolitization occurs quantitatively. On the other hand, above this value, the crystallized phases are converted to a more compact structure, which should have a negative impact on the sorption properties of the material. Figs. 4 and 5 show the SEM micrographs of zeolityzation products obtained at 90 8C for 72 h. SEM images of various NaOH concentrations are slightly different in percent crystallization and crystal size. The expanded perlite has honeycomb structure and a large amount of pores. In lower concentration of NaOH (Fig. 4) the structure is fully preserved. Only locally clusters of crystalline phases which are the results of a process can be observed (Fig. 4a). Nucleating agents on the surface of perlite can be seen at large magnifications (Fig. 4b). Increasing NaOH concentration favors the formation of crystalline phases at the expense of the glassy phase (Fig. 5). SEM observations of the sample obtained at 60 8C are visible in the Fig. 6. As mentioned, lowering temperature of the process results in a significant reduction of the amount of products. Lengthening the time of synthesis from 24 to 72 h did not result in a significant increase in the amount of crystalline phases. The aim of this study was the synthesis of zeolites in the lowest possible temperature; however, keeping long synthesis for the temperature reduction is not a good direction of the optimization process. As mentioned, most desirable phases, due to the high sorption capacity, are zeolite X, A and Na-P1. To summarize the experiment of zeolite crystallization from perlite under the conditions studied, the results indicate that the largest amount of zeolites with high ion exchange capacity were obtained under the influence of 3.0– 4.0 M NaOH at 90 8C for 72 h. Bulk density of the obtained material was about 0.16 g/cm3. Fly ash microspheres were selected as the second material for the synthesis of zeolites. Compared to expanded perlite, microspheres are characterized by higher bulk density and lower SiO2/ Al2O3 ratio (Table 1). Using the materials with higher aluminum content should result in a product with a higher content of zeolite A [22] and thus increase the sorption capacity of the material. The phase compositions of the products obtained from the microspheres depending on different experimental conditions (NaOH
Fig. 4. SEM observations of the sample obtained from expanded perlite in 2.5 M NaOH at 90 8C for 72 h.
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Fig. 5. SEM observations of the sample obtained from expanded perlite in 4.0 M NaOH at 90 8C for 72 h.
Fig. 6. SEM observations of the sample obtained from expanded perlite: (a) in 3.0 M NaOH at 60 8C for 24 h; (b) in 4.0 M NaOH at 60 8C for 72 h.
Table 2 The phase compositions of the products obtained from the microspheres depending on the NaOH concentration, time and temperature. CNaOH [mol/dm3]
Temp. [8C]
Time [h]
X
A
1
75
48 72 48 72
+
+
48 72 48 72
+ + +
+ +
48 72 48 72
+ + + +
48 72 48 72
+ + + +
48 72 48 72
+ + + +
90
2
75 90
3
75 90
4
75 90
5
75 90
Identified zeolite phase Na-P1
Sodalite
+ + + +
+ +
+ +
+
+ + +
+ + + +
214
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Fig. 7. MIR spectra of zeolitic products obtained from microspheres after 72 h reaction time at: (a) 75 and (b) 90 8C.
concentration, time and temperature) are collected in Table 2. When the reactions are carried out under the influence of 2–3 M NaOH at 90 8C and under the influence of 3–4 M NaOH at 75 8C perlite is typically converted to a mixture of zeolite X, A and Na-P1 with the highest yield. The major phase is zeolite X. Extending the reaction time from 48 to 72 h mainly affects the number of products, and only to a lesser extent, on the type of zeolite structure. However, zeolite A disappears with increasing reaction time (e.g. reaction with 3 M NaOH at 90 8C). Detailed analysis of the crystallization process shown [23] that zeolite A is an intermediate product in the synthesis of stable zeolite X. The fly ash microspheres include in its composition a significant amount of crystalline mullite and small amount of quartz. It is known that mullite is a relatively stable phase, however, based on analyzed spectra, due to the high coincidence of vibration bands for Si–O(Si,Al), it is difficult to describe its behavior during the synthesis. However, its presence should not have any impact on the pseudolattice range of the spectra thought. The bands
characteristic for quartz are observed in the spectra only for the lowest concentrations of NaOH. No presence of doublet bands at 780–800 cm 1 indicates complete reaction of this phase during the synthesis in higher NaOH concentration. Comparison of IR spectra of the samples synthesized in different conditions shows that the higher integral intensity of the bands in pseudolattice range was noted for those obtained at higher temperature (Fig. 7). A similar situation occurs in the case of products obtained at the same temperature, but with different times of synthesis (results are not presented hear). It can be concluded that the increase in reaction time and temperature favors the quantitative crystallization. A similar conclusion can be drawn from the SEM observations (Figs. 8 and 9). At lower concentrations of NaOH and shorter synthesis time, as in the case of the expanded perlite, unreacted portions of the microspheres are visible (Fig. 8). In the conditions considered as optimal, i.e. in 3.0 M NaOH at 90 8C for 72 h (Fig. 9), the layer of reaction product tightly covers the surface of the microspheres. Residues of glassy
Fig. 8. SEM observations of the sample obtained from microspheres in 2.0 M NaOH at 60 8C for 48 h.
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Fig. 9. SEM observations of the sample obtained from microspheres in 3.0 M NaOH at 90 8C for 72 h. (b) Microsection image.
phase only occur in the interior of the microspheres (Fig. 9b) and create a lightweight framework which is a carrier for zeolites. Bulk density of the obtained material is about 0.95 g/cm3. 4. Conclusions Using simple synthesis technique, it is possible to obtain a coating of sodium zeolites on the light microfillers characterized by density lower than 1 g/cm3. The resulting values are 0.16 g/cm3 for perlite and 0.95 g/cm3 for microspheres. Regardless of the differences in the chemical composition similar technique can be used for synthesis of the zeolites on expanded perlite or microspheres. Changing of synthesis parameters (NaOH concentration, temperature, time) affects the nature and quantity of obtained crystalline phases. The synthesis of zeolite is possible using the NaOH solution having a concentration above 0.5 M, at temperature at least 60 8C and at time 24 h. The results suggest the possibility of selecting the optimal conditions of zeolites synthesis at a relatively low temperature. The largest amount of zeolite was obtained at 90 8C and 72 h. Reduction in synthesis time or temperature results mainly in the quantity of products. Regardless of the type of substrate, a mixture of zeolite Y and Na-P1 can be received by reaction with sodium hydroxide at a concentration in the range 2– 4 mol/dm3 – above this range sodalite crystallizes. Acknowledgment Financial support this work was provided by The National Centre for Research and Development under grant no. PBS I 177 206.
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