23º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais 04 a 08 de Novembro de 2018, Foz do Iguaçu, PR, Brasil

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Influence of Alkalinity on the Synthesis of Zeolite A and Hydroxysodalite from Metakaolin R. C. Andrades 1,a, R. F. Neves 2,b, F. R. V. Diaz 1,c, A. H. M. Júnior 3,d 1 Polytechnic School of University

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Influence of Alkalinity on the Synthesis of Zeolite A and Hydroxysodalite from Metakaolin R. C. Andrades 1,a, R. F. Neves 2,b, F. R. V. Diaz 1,c, A. H. M. Júnior 3,d 1 Polytechnic School of University of São Paulo, Materials Engineering Department, Avenida Prof. Mello Moraes, 2463, São Paulo - SP, Brazil 2 Federal University of Pará, Chemical Engineering Department, Rua Augusto Corrêa, Guamá, Belém - PA, Brazil 3 Mackenzie Presbyterian University, Materials Engineering Department, Rua da Consolação, 930, São Paulo - SP Brazil a b c d Keywords: zeolite A, hydroxysodalite, metakaolin, alkalinity, synthesis Abstract. Zeolite A and hydroxysodalite were synthesized from kaolin waste of processing industries for paper coating. Kaolin was calcined at 700 C for 3 hours to dehydroxylate kaolinite and obtain metakaolin, an amorphous material with Si/Al ratio equal to 1, being suitable for production of zeolite A. Zeolites were synthesized under static hydrothermal conditions by reacting metakaolin with NaOH solutions of different concentrations, namely: 1, 1.8, 3, 5 and 9 M. All the syntheses were performed at 110 C for 24 hours. The zeolitic products were characterized by means of X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM). The results showed that the higher the NaOH concentration in the reaction medium, the higher the proportion of hydroxysodalite in the zeolitic samples. Introduction Zeolites are microporous aluminosilicates with a tridimensional crystalline structure which have pores of uniform size. Given the ability of zeolites to selectively adsorb molecules that are smaller than their pores, they are called molecular sieves[1]. The structure of zeolites consists of tetrahedra of TO4 (T = Al or Si) with oxygen atoms joining the vertices. The SiO4 tetrahedra are electrically neutral, but the AlO4 ones are negatively charged, which requires the presence of compensating cations that keep the electroneutrality of the zeolitic framework[2]. The chemical composition of a zeolite may be represented by the following formula: Mx/n [AlxSi1-xO2] mh2o, where M is a compensating cation and n its valence. The physical and chemical properties of zeolites allow them to be used as adsorbents, catalysts and ion exchangers[3]. The hydrothermal method is an alternative for obtaining synthetic zeolites. In this method, precursors containing Si and Al and a source of cations react in basic aqueous medium and under high temperature (normally over 100 C) and high pressure (usually over 1 bar) inside an autoclave[4]. Among the most important factors affecting the synthesis of zeolites, are the source of Si and Al, Si/Al ratio, alkalinity (NaOH/H2O ratio), temperature and time of reaction[1]. Zeolite A is a synthetic zeolite of low silica, having Si/Al ratio of 1. Its crystalline structure is cubic (lattice parameter a = 24.6 Å and spatial group Fm3c) and its secondary building units are the sodalite unit and the double four-ring (DR4)[2]. In its sodic form, zeolite A has pores with diameter of 4.2 Å (being called 4A zeolite or zeolite NaA). However, by means of ionic exchange, the diameter of the pores of zeolite A may be altered depending on the exchanged cation: K (3 Å) and Ca (5 Å)[5]. The applications of zeolite A include catalysis in the chemical and oil industries, medicine and food industry[6]. It must be also emphasized the extensive use of zeolite A in 467 detergents, as a substitute for phosphates which, because of their use being associated with eutrophication, were banned in several countries[7]. The hydrothermal synthesis of zeolite A from kaolin usually involves two steps. First, kaolin is calcined at a temperature above 550 C, resulting in dehydroxylation of kaolinite and the formation of metakaolin. Afterwards, metakaolin reacts under hydrothermal condition in NaOH solution[8]. During the synthesis of zeolites, there is nucleation of metastable phases that are successively substituted by other phases of lower free energy, in a tendency called Ostwald rule[9]. The increase of time, temperature or alkalinity favors the formation of sodalite, a more stable phase than zeolite A. Since it is a more stable phase, sodalite usually appears as a synthesis byproduct together with zeolite A, being normally difficult to obtain zeolite A as the only reaction product of hydrothermal reaction from metakaolin. Sodalite is a zeolite of chemical formula M8[ABO4]6X2, being M a monovalent cation, A and B are atoms tetrahedrally coordinated to oxygens (usually silicon and aluminum) and X is either a monovalent or divalent anion[10]. Sodalites that have hydroxide ions as anions for charge compensation are called hydroxysodalites, having chemical formula Na8[AlSiO4]6[OH]2. Hydroxysodalite has pores with diameter of 2.8 Å, being capable of separating molecules of H2, H2O and NH3 from larger molecules[11]. Experimental Kaolin from the city of Ipixuna, Pará, Brazil, was calcined at 700 C for 3 hours in a muffle oven to be converted into metakaolin. Five zeolite samples were prepared by reacting 3.89 g of metakaolin with 35 ml of NaOH aqueous solutions of different concentrations. Five NaOH concentrations were used, in such a way that the molar Na/Al ratio of the reaction medium were 1, 1.8, 3, 5 and 9, and the reaction products were called, respectively, Z-1, Z-1.8, Z-3, Z-5 and Z-9. All the synthesis reactions took place inside teflon lined stainless steel autoclaves at 110 C for 24 hours. Characterization The X-ray diffraction analyses were performed in a Rigaku MiniFlex 600 diffractometer with CuKα (λ = 1, Å) radiation. Data collection was carried out in the 2θ range 3-60, with a scanning speed of 2 /second with X-ray tube operating at 30 kv and 10 ma. For the purpose of phase identification, the X Pert HighScore software, version 3.0e, was used. Fourier transform infrared (FTIR) spectroscopy analyses were performed in a Nicolet is5 TermoFisher Scientific spectrometer, in the cm -1 region. The morphology of the obtained materials was analyzed using a Cambridge Stereoscan 440 scanning electron microscope (SEM) operating with secondary electrons. The samples were covered with gold and the images were obtained by a backscattered electrons detector. Results and Discussion The diffraction curves for the synthesis products (zeolites) after hydrothermal reaction of metakaolin (Z-1, Z-1.8, Z-3, Z-5 and Z-9) are shown in Fig. 1. The obtained phases are zeolite A and hydroxysodalite. The peaks belonging only to the structure of zeolite A are labelled with ZA, while the peaks that belong only to the structure of hydroxysodalite are labelled with S. Characteristic peaks belonging to both structures are labelled with ZA/S. It is clearly noticeable that, the more the alkalinity of the reaction medium that originated the zeolitic products, the more sodalite is formed, while zeolite A tends to disappear. While 468 product Z-1 is mostly formed by zeolite A, only sodalite was observed in product Z-9. This data is consistent with the fact that increasing alkalinity promotes higher crystallization rate via nucleation and growing of crystals due to the increase in the solubility of silica and alumina precursors, which results in higher concentration of reactive aluminosilicate species. As a result, the formation rate of zeolite A is increased and thereafter the subsequent formation of sodalite[2]. An elevation of the baseline of the diffraction curves between 20 and 35, particularly in sample Z-1, was observed. Such elevation was attributed to the presence of metakolin that was not converted into zeolite. However, by increasing alkalinity, such elevation of the baseline tends to decrease and become smoother, indicating that metakaolin is more efficiently converted into zeolite[12]. Figure 1. Diffractograms of synthesized zeolites (a) Z-1, (b) Z-1.8, (c) Z-3, (d) Z-5 and (e) Z-9. The FT-IR spectra of the synthesized products (zeolites) after hydrothermal reaction of metakaolin (Z-1, Z-1.8, Z-3, Z-5 and Z-9) are depicted in Fig. 2. It is clear, in all spectra, that the band at 792 cm -1 of metakaolin, caused by stretching of Al-O bond, is no longer present. Furthermore, the band attributed to stretching of Si-O bonds, which appeared in metakaolin at 1060 cm -1, is now shifted to 963 cm -1 in the zeolitic products, indicating asymmetric stretching of T-O (T = Al or Si) bonds[13]. The bands at 547 and 443 cm -1 are typical of vibration of double four-rings (DR4), which is one of the secondary building units of zeolite A[14]. The bands at 773, 709 and 663 cm -1, related to the symmetric stretching of T-O bonds, are characteristic of sodalite[15]; the increase of intensity of these bands as NaOH concentration rises turns out to be a result of the growing amount of sodalite in the samples, while the quantity of zeolite A tends to decrease, which corroborates the XRD results. 469 Figure 5: FT-IR spectra of synthesized zeolites (a) Z-1, (b) Z-1.8, (c) Z-3, (d) Z-5 and (e) Z-9. SEM micrographs of samples Z-1, Z-3 and Z-9 are presented in Fig. 3. The micrographs of sample Z-1 show that it consists of cubic particles, which is the characteristic geometry of zeolite A. It is also evident that some of the cubic particles of zeolite A are intergrown. Similar morphology for zeolite A was found by other authors[14]. Platelet-shaped particles were also observed in sample Z-1, which indicates that metakaolin was not totally converted into zeolite. Sample Z-3 exhibits formation of many agglomerates of spherical particles, which is the typical geometry of sodalite. Furthermore, there are also approximately cubic particles of irregular geometry, indicating the coexistence of zeolite A and sodalite. Such results corroborate the XRD experiments, which suggested the presence of both phases in sample Z- 3. Sample Z-9 consists of agglomerates of spherical particles, indicating the presence of sodalite. Furthermore, it is revealed that there are nearly cubic particles from which spherical particles grow. According to Breck[3], the formation of metastable sodium aluminosilicate phases occurs by successive substitution of less stable phases by more stable phases. Such nucleation of sodalite from zeolite A, noticeable in the micrographs, was also reported in the work of Hums[16], who verified that the transformation of zeolite A to sodalite proceeds from the surface of the particles to the interior. 470 Figure 3. SEM micrographs of synthesized zeolites: (a) and (b) Z-1; (c) and (d) Z-3; (e) and (f) Z-9. Conclusions Zeolite A and sodalite were successfully synthesized via hydrothermal treatment using a cheap industrial waste, kaolin, which was calcined and subsequently used as starting material, and NaOH as mineralizing agent. Kaolin turned out to be a material with great potential for the synthesis of zeolite A and sodalite, being a good alternative to synthetic conventional reactants. Alkalinity is a synthesis variable that directly affects the type of zeolitic phase. The 471 results obtained from X-ray diffraction and infrared spectroscopy, which were corroborated by scanning electron microscopy, show that the higher the NaOH concentration employed in the hydrothermal synthesis, the more the proportion of sodalite. Although products that contained only sodalite (sample Z-9) were successfully obtained, it was not possible to obtain samples consisting purely of zeolite A, because of the thermodynamic stability of sodalite, which occurred as a concurrent phase. Acknowledgments The authors thank the Polytechnic School at University of São Paulo, for providing all research facilities and staff needed for carrying out this work, and CAPES for the financial support. References [1] H. van. Bekkum, E.M. Flanigen, J.C. Jansen, Introduction to Zeolite Science and Practice, Elsevier, Amsterdam, [2] S. Auerbach, K. Carrado, P. Dutta, Handbook of Zeolite Science and Technology, CRC Press, [3] D.W. Breck, Zeolite Molecular Sieves : Structure, Chemistry, and Use, Wiley, New York, [4] C.S. Cundy, P.A. Cox, Microporous Mesoporous Mater. 82 (2005) [5] X. Zhang, D. Tang, G. Jiang, Adv. Powder Technol. 24 (2013) [6] Y. MENG, F.-X. LI, Y.-C. FENG, J.-W. XUE, Z.-P. LV, Asian J. Chem. 25 (2013) [7] L. Ayele, J. Pérez-Pariente, Y. Chebude, I. Díaz, Microporous Mesoporous Mater. 215 (2015) [8] E.B.G. Johnson, S.E. Arshad, Appl. Clay Sci (2014) [9] X. Liu, Y. Wang, X. Cui, Y. He, J. Mao, Powder Technol. 243 (2013) [10] V.A.A. de Freitas, J.S. V. Lima, P.R. da C. Couceiro, Cerâmica 57 (2011) [11] M.S. Nabavi, T. Mohammadi, M. Kazemimoghadam, Ceram. Int. 40 (2014) [12] S.H. da S. Filho, L. Bieseki, A.A.B. Maia, H. Treichel, R.S. Angelica, S.B.C. Pergher, S.H. da Silva Filho, L. Bieseki, A.A.B. Maia, H. Treichel, R.S. Angelica, S.B.C. Pergher, Mater. Res. 20 (2017) [13] A. Demortier, N. Gobeltz, J.P. Lelieur, C. Duhayon, Int. J. Inorg. Mater. 1 (1999) [14] A.A.B. Maia, E. Saldanha, R.S. Angélica, C.A.G. Souza, R.F. Neves, Cerâmica 53 (2007) [15] M.C. Barnes, O. Addai-Mensah, A.R. Gerson, Microporous Mesoporous Mater. 31 (1999) [16] E. Hums, J. Thermodyn. Catal. 8 (2017)
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