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Hydrothermal synthesis of analcime without template
Journal of Crystal Growth 532 (2020) 125424
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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Hydrothermal synthesis of analcime without template Heloisa R. Bortolini, Dirléia S. Lima, Oscar W. Perez-Lopez
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Laboratory of Catalytic Processes-PROCAT, Department of Chemical Engineering, Federal University of Rio Grande do Sul (UFRGS), Ramiro Barcelos Street, 2777, CEP 90035-007 Porto Alegre, RS, Brazil
ARTICLE INFO
ABSTRACT
Communicated by Keshra Sangwal
Analcime is a highly crystalline material that is found in nature or can be produced artificially. In this work the hydrothermal synthesis of analcime zeolite without organic template was carried out by evaluating the influence of SiO2/Al2O3, NaOH/SiO2 and H2SO4/SiO2 ratios. The samples were characterized with SBET, XRD and SEM. The results show that there is a wide range of synthesis parameters for obtaining analcime zeolite without template. The most important parameter for material formation and characteristics was the H2SO4/SiO2 ratio. The variation of SiO2/Al2O3 ratio influences the crystallinity of the formed material and, mainly, the synthesis yield. The minimum SiO2/Al2O3 ratio for analcime formation was 10.
Keywords: B1. Zeolites A2. Hydrothermal crystal growth A1. Crystal morphology A1. Characterization
1. Introduction Zeolites are highly crystalline, porous and hydrated aluminosilicates that contain tetrahedral units of [SiO4]4 and [AlO4]5 which are bonded by oxygen atoms, forming a three-dimensional network that extends infinitely [1]. These compounds are utilized in different application fields as adsorbents, ion exchangers and catalyst [2–4]. There are different routes for zeolite obtaining with several silica and aluminum sources [5–17]. Sustainable routes are currently drawing the most attention for being environmentally friendly and using less resources. Among them can be mentioned the microwave assisted routes, synthesis with low cost templates, with recycled templates, organic synthesis without solvent and in the absence of organic templates [18]. Analcime (ANA) is a natural zeolite with low Si/Al ratio constituted by hydrated sodium silicate and aluminum that has a typical unit cell composition of Na16[(AlO2)16(SiO2)32].16H2O with a Si/Al molar ratio of 1.8–2.8 [19,20]. Natural ANA sources are present in limited regions of the world, therefore many attempts with different silica and alumina sources were recently made to synthesize ANA, mainly by sol-gel and hydrothermal process [10,12,13,16,21]. For synthetic ANA the Si/Al relation varies in a wider range (1,5–3,0) depending on the cations nature, Si and Al sources and synthesis conditions [22]. Analcime has been reported to be a cation adsorbent [23] and has been used as a molecular sieve in separation of light gas/hydrocarbon mixtures [24]. This narrow-pore zeolite can be used in nuclear waste burial as a sorptive barrier for radioactive elements because of its ability to bind actinide cations by sorption irreversibly providing the
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reliable immobilization of toxic metals [25]. Some researchers have different goals for the use of analcime than conventional applications of zeolites. For example, it was found that analcime has potential for being used as precursor at leucite synthesis (KAlSi2O6), which is a promisor material for tooth porcelain due to its compatible thermal expansion with metal and high fracture toughness [26]. In addition, for effluent treatment, the analcime zeolite showed more efficiency than its natural analogue due to its specific structural properties. Chuayjuljit et al. [27] studied the effects of analcime zeolite synthesized as nucleating agent on crystallization behaviors and mechanical properties of isotactic polypropylene (iPP). It was found that ANAzeolite can form a mass of a-crystal and only a little b-crystal in the iPP, accompanied by a significant increase in the nuclei content, crystallization rate, and crystallization temperature, but a substantial decrease in the spherulite size. A large number of papers about analcime synthesis are found in the literature using organic templates as tetrapropylammonium hydroxide [2,28,29], dibenzyltetramethylethylenediamine [30], o-phenylenediamine [31], among others. In addition, some studies with more complex synthesis methods [13,32] and use of crystallization seeds [33] were reported. Ghobarkar et al. [34] prepared analcime through hydrothermal treatment of artificial glass with appropriate composition in the temperature range between 80 and 630 °C at 100 MPa of H2O pressure. It was found that the crystalline symmetry of analcime varied systematically with the temperature, from orthorhombic symmetry at low synthesis temperature, tetragonal for medium temperatures leading to
Corresponding author. E-mail address: [email protected] (O.W. Perez-Lopez).
https://doi.org/10.1016/j.jcrysgro.2019.125424 Received 19 November 2019; Received in revised form 7 December 2019; Accepted 10 December 2019 Available online 11 December 2019 0022-0248/ © 2019 Elsevier B.V. All rights reserved.
Journal of Crystal Growth 532 (2020) 125424
H.R. Bortolini, et al.
Wang et al. [35] prepared a novel and innovative hierarchical analcime zeolite by adding a gemini surfactant (C18–2–8Br2) which acted as a dual‐functional template. Through a one-step hydrothermal process, a hierarchical analcime zeolite with abundant intracrystalline mesopores was synthesized. The characterization showed that the mesoporous composites possess both the presence of mesopores and analcime structure. The results suggest that the dual‐functional template can be effective in the synthesis of hierarchical analcime zeolites. Although there are many works in the literature on the synthesis of zeolite analcime with different starting materials and different methods, there is still a need for an evaluation of the influence of some synthesis parameters on the final characteristics of the materials obtained, especially in the methodologies performed in the absence of organic templates. The objective of this work is to investigate the synthesis of analcime without organic templates, using variables like the Si/Al ratio, the amount of acid and the amount of sodium.
Table 1 Samples nomenclature prepared by hydrothermal synthesis varying the molar ratios of its main components. Samples
NaOH/SiO2
H2SO4/SiO2
SiO2/Al2O3
NA200 NA250 NA300 AC375 AC465 AC560 AC650 AC750 SAR50 SAR40 SAR30 SAR20 SAR10 SAR5
2.0 2.5 3.0 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5
0.5 0.75 1.0 0.375 0.465 0.56 0.65 0.75 0.465 0.465 0.465 0.465 0.465 0.465
50 50 50 50 50 50 50 50 50 40 30 20 10 5
2. Experimental 2.1. Synthesis procedure The samples were synthesized from a mixture of a basic solution with an acid solution. The basic solution was prepared by dissolution of sodium hydroxide (NaOH) and silica Aerosil (Evonik-Degussa) in distilled water. The acid solution was obtained by mixing aluminum sulfate, distilled water and sulfuric acid. The acid solution was slowly dripped in the basic solution under vigorous stirring. Afterward, the mixture was maintained under constant stirring at room temperature during 1 h for hydrogel formation. The hydrogel was transferred to an autoclave and placed in an oven for 24 h at 190 °C. Afterwards, the material was filtered under vacuum and washed with deionized water until the measured conductivity was lower than 50 µS/m. The sample was dried at 80 °C during 12 h. The molar ratios SiO2/Al2O3, NaOH/SiO2 and H2SO4/SiO2 were varied in order to observe its influences on the formation and properties of the obtained material. 2.2. Characterization
Fig. 1. Diffractograms of the samples with different NaOH/SiO2 ratio.
The characterization of the samples was carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM) and specific surface area measurements. The X-ray diffraction technique was utilized to identify the structure of the prepared materials accordingly its crystalline planes. The diffractograms were obtained in a X-ray diffractometer (Bruker, D2 Phaser), using radiation Cu-kα [4]. Scanning electron microscopy images were obtained in a Phenom equipment, Pro-X model, with backscattered electrons and tension of 10 kV [4]. The specific surface area of the samples was obtained by nitrogen physisorption measurements. The samples were first submitted to a pretreatment at 300 °C for a period of 3 h under vacuum. The analysis was carried out in a Quantachrome equipment, NOVA 4200e model. The specific surface area was determined by the multi-point BET (Brunauer-Emmett-Teller) method [36].
cubic symmetry at higher synthesis temperatures. Azizi et al. [12] reported nanostructured analcime obtained by hydrothermal method, without organic matrix, using silica extracted from stem of sorghum ash. It was observed that with the increase of Si/Al ratio the crystallization also rises for a ratio up to 30. However, when this ratio declined below 18, the crystallization tends to form the P zeolite. The samples crystalized at temperatures above 120 °C showed high crystallinity. García-Villén et al. [16] evaluated the viability of use solids residues from bathroom fittings industry to produce zeolite, utilizing a conventional hydrothermal treatment with different times and temperatures. It was observed that the crystallization of analcime and cancrinite is favored by high temperatures (between 150 and 200 °C) and that the increase of aging time leads to the disappearance of analcime.
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Journal of Crystal Growth 532 (2020) 125424
H.R. Bortolini, et al.
Fig. 2. SEM images for different samples: (a, b) NA200, (c, d) NA250 and (e, f) NA300.
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Journal of Crystal Growth 532 (2020) 125424
H.R. Bortolini, et al.
Fig. 3. Diffractograms of the samples with different H2SO4/SiO2 ratio.
3. Results and discussion Table 1 shows the synthesized samples with the adopted nomenclature and the nominal molar NaOH/SiO2, H2SO4/SiO2 and SiO2/ Al2O3 ratios. The samples are presented in 3 groups denoted by NA, AC and SAR, representing the ratios of NaOH/SiO2, H2SO4/SiO2 and SiO2/ Al2O3, respectively, followed by numbers related to the ratio. 3.1. Variation of NaOH/SiO2 ratio Fig. 1 shows the diffractograms of the samples with different NaOH/ SiO2 ratio. The XRD patterns of the three samples are typical of the analcime zeolite crystal structure (2θ = 16, 26 and 30.5°), with highly crystallinity [12,13]. The highest crystallinity is observed for the sample with low NaOH/SiO2 ratio. In addition, these materials represent good purity for the analcime phase as was also observed by Atta et al. [10] for a sample with altered alkalinity. According to these authors, the addition of NaOH increases the formation of nucleation species that involve the silica depolymerization. Fig. 2 shows the micrographs of the samples prepared with different NaOH/SiO2 ratios. The SEM images of the samples reveal that the crystallites have similar trapezohedron morphology. This same kind of morphology has been reported before [10,13,37] for the analcime zeolite. Among the different samples, the NA250 sample with NaO/SiO2 ratio of 2.5 gave highly homogeneous particles. Therefore, this ratio was adopted for the following synthesis. 3.2. Variation of H2SO4/SiO2 ratio Fig. 3 shows the XRD pattern for the crystallized analcime zeolite samples from different sulfuric acid/silica ratios. A slight increase in crystallinity may be observed with an increase in the acid/silica ratio up to 0.65. However, increase in the ratio to 0.75 (AC750) leads to a decrease in the peak intensity. This suggests that there is a maximum limit of H2SO4/SiO2 ratio for the crystallization of analcime of high crystallinity. Fig. 4. SEM images for different samples: (a, b) AC750, (c, d) AC650, (e, f) AC560, (g, h) AC465 and (i, j) AC375.
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Journal of Crystal Growth 532 (2020) 125424
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trapezohedron. Moreover, it was observed that a decrease in the SiO2/ Al2O3 ratio results in a decrease of the crystallinity of analcime (see Fig. 5). Although the theoretical SiO2/Al2O3 ratio for analcime crystallization is about 2 [20], the sample with the ratio 5 did not result in the formation of analcime but formed an amorphous material, as illustrated in Fig. 6(e). This suggests that a minimum ratio of SiO2/ Al2O3 = 10 in the gel was necessary to obtain the analcime zeolite by the method and conditions used here. It was observed that the SiO2/Al2O3 ratio has a strong influence on the amount of crystallized analcime and that lower SiO2/Al2O3 ratios result in higher amounts of analcime. This observation means that a higher analcime yield is obtained when the experimental ratio approaches the theoretical ratio. The results of investigation of the effect of variation of silica/alumina ratio on the synthesis of analcime show an inverse relation between its crystallinity and yield. Table 2 shows the specific surface area of the crystallized samples at different SiO2/Al2O3 ratios. All samples have low specific surface area around 1 m2/g, which can be attributed to the high crystallinity of these materials. Low value of the specific surface area indicates the formation of a low porosity material. The value of about 1 m2/g of the specific surface area of the samples crystallized in this study is lower than 10.40 m2/g, reported by Sakizci [20] for natural analcime.
Fig. 5. Diffractograms of the samples with different SiO2/Al2O3 ratio.
4. Conclusion
Examples of the morphology of crystallized samples are shown in Fig. 4. The SEM images of Fig. 4 show that trapezohedron morphology is typical of all samples. Comparison of the SEM images of the samples also reveals that the AC465 sample represents more homogeneous particles, both in size and form. Moreover, as seen in Fig. 3, this sample has high crystallinity. Therefore, it may be inferred that the best H2SO4/SiO2 ratio for obtaining analcime zeolite is 0.465. In view of this, this ratio was adopted for its later synthesis.
High crystallinity analcime was obtained in a broad range of synthesis parameters using NaOH/SiO2, H2SO4/SiO2 and SiO2/Al2O3 ratios. It was found that changes in the NaOH/SiO2 ratio have insignificant influence on analcime crystallinity, but the H2SO4/SiO2 ratio strongly influences the crystallinity, which increases with the acid content. Different SiO2/Al2O3 ratios result in samples with different crystallinity and yield. The crystallinity decreases as the SiO2/Al2O3 ratio decreases. A minimum SiO2/Al2O3 ratio of about 10 in the gel was found to be necessary for the formation of analcime.
3.3. Variation of silica/alumina ratio The last parameter to be evaluated was the SiO2/Al2O3 ratio. The diffractograms of the synthesized samples are showed in the Fig. 5. All samples, except for SAR5, show characteristics peaks of analcime zeolite. This observation demonstrates the possibility of crystallizing this material in a wider range of SiO2/Al2O3 ratio than the range for natural analcime lying between 1.8 and 2.8 [20]. Therefore, it may be argued that the crystallinity of analcime decreases with decreasing SAR. A similar result was reported by Azizi and Yousefpour [30], who crystallized analcime in a range of SiO2/Al2O3 ratio from 5 to 19. In a later work, Azizi et al. [12] showed that it was possible to obtain analcime in the range of SiO2/Al2O3 ratio between 18 and 45. The SEM images of the morphology of analcime crystallites are presented in Fig. 6. It may be seen that the morphology of the crystallites obtained at different ratios of SiO2/Al2O3 is in the form of
CRediT authorship contribution statement Heloisa R. Bortolini: Methodology, Validation, Investigation, Writing - original draft. Dirléia S. Lima: Formal analysis, Writing original draft. Oscar W. Perez-Lopez: Conceptualization, Supervision, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Fig. 6. SEM images for different samples (a) SAR50, (b) SAR40, (c) SAR30, (d) SAR20, (e) SAR10 and (f) SAR5.
Acknowledgments
Table 2 Specific surface area of different samples. Samples
SiO2/Al2O3 ratio
Specific surface area (m2/g)
SAR50 SAR40 SAR30 SAR20 SAR10
50 40 30 20 10
1.22 0.85 1.17 0.96 1.23
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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Hydrothermal synthesis of analcime without template Heloisa R. Bortolini, Dirléia S. Lima, Oscar W. Perez-Lopez
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T
Laboratory of Catalytic Processes-PROCAT, Department of Chemical Engineering, Federal University of Rio Grande do Sul (UFRGS), Ramiro Barcelos Street, 2777, CEP 90035-007 Porto Alegre, RS, Brazil
ARTICLE INFO
ABSTRACT
Communicated by Keshra Sangwal
Analcime is a highly crystalline material that is found in nature or can be produced artificially. In this work the hydrothermal synthesis of analcime zeolite without organic template was carried out by evaluating the influence of SiO2/Al2O3, NaOH/SiO2 and H2SO4/SiO2 ratios. The samples were characterized with SBET, XRD and SEM. The results show that there is a wide range of synthesis parameters for obtaining analcime zeolite without template. The most important parameter for material formation and characteristics was the H2SO4/SiO2 ratio. The variation of SiO2/Al2O3 ratio influences the crystallinity of the formed material and, mainly, the synthesis yield. The minimum SiO2/Al2O3 ratio for analcime formation was 10.
Keywords: B1. Zeolites A2. Hydrothermal crystal growth A1. Crystal morphology A1. Characterization
1. Introduction Zeolites are highly crystalline, porous and hydrated aluminosilicates that contain tetrahedral units of [SiO4]4 and [AlO4]5 which are bonded by oxygen atoms, forming a three-dimensional network that extends infinitely [1]. These compounds are utilized in different application fields as adsorbents, ion exchangers and catalyst [2–4]. There are different routes for zeolite obtaining with several silica and aluminum sources [5–17]. Sustainable routes are currently drawing the most attention for being environmentally friendly and using less resources. Among them can be mentioned the microwave assisted routes, synthesis with low cost templates, with recycled templates, organic synthesis without solvent and in the absence of organic templates [18]. Analcime (ANA) is a natural zeolite with low Si/Al ratio constituted by hydrated sodium silicate and aluminum that has a typical unit cell composition of Na16[(AlO2)16(SiO2)32].16H2O with a Si/Al molar ratio of 1.8–2.8 [19,20]. Natural ANA sources are present in limited regions of the world, therefore many attempts with different silica and alumina sources were recently made to synthesize ANA, mainly by sol-gel and hydrothermal process [10,12,13,16,21]. For synthetic ANA the Si/Al relation varies in a wider range (1,5–3,0) depending on the cations nature, Si and Al sources and synthesis conditions [22]. Analcime has been reported to be a cation adsorbent [23] and has been used as a molecular sieve in separation of light gas/hydrocarbon mixtures [24]. This narrow-pore zeolite can be used in nuclear waste burial as a sorptive barrier for radioactive elements because of its ability to bind actinide cations by sorption irreversibly providing the
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reliable immobilization of toxic metals [25]. Some researchers have different goals for the use of analcime than conventional applications of zeolites. For example, it was found that analcime has potential for being used as precursor at leucite synthesis (KAlSi2O6), which is a promisor material for tooth porcelain due to its compatible thermal expansion with metal and high fracture toughness [26]. In addition, for effluent treatment, the analcime zeolite showed more efficiency than its natural analogue due to its specific structural properties. Chuayjuljit et al. [27] studied the effects of analcime zeolite synthesized as nucleating agent on crystallization behaviors and mechanical properties of isotactic polypropylene (iPP). It was found that ANAzeolite can form a mass of a-crystal and only a little b-crystal in the iPP, accompanied by a significant increase in the nuclei content, crystallization rate, and crystallization temperature, but a substantial decrease in the spherulite size. A large number of papers about analcime synthesis are found in the literature using organic templates as tetrapropylammonium hydroxide [2,28,29], dibenzyltetramethylethylenediamine [30], o-phenylenediamine [31], among others. In addition, some studies with more complex synthesis methods [13,32] and use of crystallization seeds [33] were reported. Ghobarkar et al. [34] prepared analcime through hydrothermal treatment of artificial glass with appropriate composition in the temperature range between 80 and 630 °C at 100 MPa of H2O pressure. It was found that the crystalline symmetry of analcime varied systematically with the temperature, from orthorhombic symmetry at low synthesis temperature, tetragonal for medium temperatures leading to
Corresponding author. E-mail address: [email protected] (O.W. Perez-Lopez).
https://doi.org/10.1016/j.jcrysgro.2019.125424 Received 19 November 2019; Received in revised form 7 December 2019; Accepted 10 December 2019 Available online 11 December 2019 0022-0248/ © 2019 Elsevier B.V. All rights reserved.
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Wang et al. [35] prepared a novel and innovative hierarchical analcime zeolite by adding a gemini surfactant (C18–2–8Br2) which acted as a dual‐functional template. Through a one-step hydrothermal process, a hierarchical analcime zeolite with abundant intracrystalline mesopores was synthesized. The characterization showed that the mesoporous composites possess both the presence of mesopores and analcime structure. The results suggest that the dual‐functional template can be effective in the synthesis of hierarchical analcime zeolites. Although there are many works in the literature on the synthesis of zeolite analcime with different starting materials and different methods, there is still a need for an evaluation of the influence of some synthesis parameters on the final characteristics of the materials obtained, especially in the methodologies performed in the absence of organic templates. The objective of this work is to investigate the synthesis of analcime without organic templates, using variables like the Si/Al ratio, the amount of acid and the amount of sodium.
Table 1 Samples nomenclature prepared by hydrothermal synthesis varying the molar ratios of its main components. Samples
NaOH/SiO2
H2SO4/SiO2
SiO2/Al2O3
NA200 NA250 NA300 AC375 AC465 AC560 AC650 AC750 SAR50 SAR40 SAR30 SAR20 SAR10 SAR5
2.0 2.5 3.0 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5
0.5 0.75 1.0 0.375 0.465 0.56 0.65 0.75 0.465 0.465 0.465 0.465 0.465 0.465
50 50 50 50 50 50 50 50 50 40 30 20 10 5
2. Experimental 2.1. Synthesis procedure The samples were synthesized from a mixture of a basic solution with an acid solution. The basic solution was prepared by dissolution of sodium hydroxide (NaOH) and silica Aerosil (Evonik-Degussa) in distilled water. The acid solution was obtained by mixing aluminum sulfate, distilled water and sulfuric acid. The acid solution was slowly dripped in the basic solution under vigorous stirring. Afterward, the mixture was maintained under constant stirring at room temperature during 1 h for hydrogel formation. The hydrogel was transferred to an autoclave and placed in an oven for 24 h at 190 °C. Afterwards, the material was filtered under vacuum and washed with deionized water until the measured conductivity was lower than 50 µS/m. The sample was dried at 80 °C during 12 h. The molar ratios SiO2/Al2O3, NaOH/SiO2 and H2SO4/SiO2 were varied in order to observe its influences on the formation and properties of the obtained material. 2.2. Characterization
Fig. 1. Diffractograms of the samples with different NaOH/SiO2 ratio.
The characterization of the samples was carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM) and specific surface area measurements. The X-ray diffraction technique was utilized to identify the structure of the prepared materials accordingly its crystalline planes. The diffractograms were obtained in a X-ray diffractometer (Bruker, D2 Phaser), using radiation Cu-kα [4]. Scanning electron microscopy images were obtained in a Phenom equipment, Pro-X model, with backscattered electrons and tension of 10 kV [4]. The specific surface area of the samples was obtained by nitrogen physisorption measurements. The samples were first submitted to a pretreatment at 300 °C for a period of 3 h under vacuum. The analysis was carried out in a Quantachrome equipment, NOVA 4200e model. The specific surface area was determined by the multi-point BET (Brunauer-Emmett-Teller) method [36].
cubic symmetry at higher synthesis temperatures. Azizi et al. [12] reported nanostructured analcime obtained by hydrothermal method, without organic matrix, using silica extracted from stem of sorghum ash. It was observed that with the increase of Si/Al ratio the crystallization also rises for a ratio up to 30. However, when this ratio declined below 18, the crystallization tends to form the P zeolite. The samples crystalized at temperatures above 120 °C showed high crystallinity. García-Villén et al. [16] evaluated the viability of use solids residues from bathroom fittings industry to produce zeolite, utilizing a conventional hydrothermal treatment with different times and temperatures. It was observed that the crystallization of analcime and cancrinite is favored by high temperatures (between 150 and 200 °C) and that the increase of aging time leads to the disappearance of analcime.
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Fig. 2. SEM images for different samples: (a, b) NA200, (c, d) NA250 and (e, f) NA300.
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Fig. 3. Diffractograms of the samples with different H2SO4/SiO2 ratio.
3. Results and discussion Table 1 shows the synthesized samples with the adopted nomenclature and the nominal molar NaOH/SiO2, H2SO4/SiO2 and SiO2/ Al2O3 ratios. The samples are presented in 3 groups denoted by NA, AC and SAR, representing the ratios of NaOH/SiO2, H2SO4/SiO2 and SiO2/ Al2O3, respectively, followed by numbers related to the ratio. 3.1. Variation of NaOH/SiO2 ratio Fig. 1 shows the diffractograms of the samples with different NaOH/ SiO2 ratio. The XRD patterns of the three samples are typical of the analcime zeolite crystal structure (2θ = 16, 26 and 30.5°), with highly crystallinity [12,13]. The highest crystallinity is observed for the sample with low NaOH/SiO2 ratio. In addition, these materials represent good purity for the analcime phase as was also observed by Atta et al. [10] for a sample with altered alkalinity. According to these authors, the addition of NaOH increases the formation of nucleation species that involve the silica depolymerization. Fig. 2 shows the micrographs of the samples prepared with different NaOH/SiO2 ratios. The SEM images of the samples reveal that the crystallites have similar trapezohedron morphology. This same kind of morphology has been reported before [10,13,37] for the analcime zeolite. Among the different samples, the NA250 sample with NaO/SiO2 ratio of 2.5 gave highly homogeneous particles. Therefore, this ratio was adopted for the following synthesis. 3.2. Variation of H2SO4/SiO2 ratio Fig. 3 shows the XRD pattern for the crystallized analcime zeolite samples from different sulfuric acid/silica ratios. A slight increase in crystallinity may be observed with an increase in the acid/silica ratio up to 0.65. However, increase in the ratio to 0.75 (AC750) leads to a decrease in the peak intensity. This suggests that there is a maximum limit of H2SO4/SiO2 ratio for the crystallization of analcime of high crystallinity. Fig. 4. SEM images for different samples: (a, b) AC750, (c, d) AC650, (e, f) AC560, (g, h) AC465 and (i, j) AC375.
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trapezohedron. Moreover, it was observed that a decrease in the SiO2/ Al2O3 ratio results in a decrease of the crystallinity of analcime (see Fig. 5). Although the theoretical SiO2/Al2O3 ratio for analcime crystallization is about 2 [20], the sample with the ratio 5 did not result in the formation of analcime but formed an amorphous material, as illustrated in Fig. 6(e). This suggests that a minimum ratio of SiO2/ Al2O3 = 10 in the gel was necessary to obtain the analcime zeolite by the method and conditions used here. It was observed that the SiO2/Al2O3 ratio has a strong influence on the amount of crystallized analcime and that lower SiO2/Al2O3 ratios result in higher amounts of analcime. This observation means that a higher analcime yield is obtained when the experimental ratio approaches the theoretical ratio. The results of investigation of the effect of variation of silica/alumina ratio on the synthesis of analcime show an inverse relation between its crystallinity and yield. Table 2 shows the specific surface area of the crystallized samples at different SiO2/Al2O3 ratios. All samples have low specific surface area around 1 m2/g, which can be attributed to the high crystallinity of these materials. Low value of the specific surface area indicates the formation of a low porosity material. The value of about 1 m2/g of the specific surface area of the samples crystallized in this study is lower than 10.40 m2/g, reported by Sakizci [20] for natural analcime.
Fig. 5. Diffractograms of the samples with different SiO2/Al2O3 ratio.
4. Conclusion
Examples of the morphology of crystallized samples are shown in Fig. 4. The SEM images of Fig. 4 show that trapezohedron morphology is typical of all samples. Comparison of the SEM images of the samples also reveals that the AC465 sample represents more homogeneous particles, both in size and form. Moreover, as seen in Fig. 3, this sample has high crystallinity. Therefore, it may be inferred that the best H2SO4/SiO2 ratio for obtaining analcime zeolite is 0.465. In view of this, this ratio was adopted for its later synthesis.
High crystallinity analcime was obtained in a broad range of synthesis parameters using NaOH/SiO2, H2SO4/SiO2 and SiO2/Al2O3 ratios. It was found that changes in the NaOH/SiO2 ratio have insignificant influence on analcime crystallinity, but the H2SO4/SiO2 ratio strongly influences the crystallinity, which increases with the acid content. Different SiO2/Al2O3 ratios result in samples with different crystallinity and yield. The crystallinity decreases as the SiO2/Al2O3 ratio decreases. A minimum SiO2/Al2O3 ratio of about 10 in the gel was found to be necessary for the formation of analcime.
3.3. Variation of silica/alumina ratio The last parameter to be evaluated was the SiO2/Al2O3 ratio. The diffractograms of the synthesized samples are showed in the Fig. 5. All samples, except for SAR5, show characteristics peaks of analcime zeolite. This observation demonstrates the possibility of crystallizing this material in a wider range of SiO2/Al2O3 ratio than the range for natural analcime lying between 1.8 and 2.8 [20]. Therefore, it may be argued that the crystallinity of analcime decreases with decreasing SAR. A similar result was reported by Azizi and Yousefpour [30], who crystallized analcime in a range of SiO2/Al2O3 ratio from 5 to 19. In a later work, Azizi et al. [12] showed that it was possible to obtain analcime in the range of SiO2/Al2O3 ratio between 18 and 45. The SEM images of the morphology of analcime crystallites are presented in Fig. 6. It may be seen that the morphology of the crystallites obtained at different ratios of SiO2/Al2O3 is in the form of
CRediT authorship contribution statement Heloisa R. Bortolini: Methodology, Validation, Investigation, Writing - original draft. Dirléia S. Lima: Formal analysis, Writing original draft. Oscar W. Perez-Lopez: Conceptualization, Supervision, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Fig. 6. SEM images for different samples (a) SAR50, (b) SAR40, (c) SAR30, (d) SAR20, (e) SAR10 and (f) SAR5.
Acknowledgments
Table 2 Specific surface area of different samples. Samples
SiO2/Al2O3 ratio
Specific surface area (m2/g)
SAR50 SAR40 SAR30 SAR20 SAR10
50 40 30 20 10
1.22 0.85 1.17 0.96 1.23
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