A STUDY ON THE SYNTHESIS AND STRUCTURE OF ZEOLITE NAX

Transcription

1 Journal Journal of Chemical of Chemical Technology Technology and Metallurgy, and Metallurgy, 48, 2, 48, 2013, 2, A STUDY ON THE SYNTHESIS AND STRUCTURE OF ZEOLITE NAX D. Georgiev 1, B. Bogdanov 2, I. Markovska 2, Y. Hristov 1 1 Department of Material Science and Technology, 2 Department of Inorganic Substances and Silicates Assen Zlatarov University, Bourgas, Bulgaria dgeorgiev@btu.bg Received 09 November 2011 Accepted 20 December 2012 ABSTRACT The synthesis of zeolite with valuable properties is a serious task, requiring numerous laboratory studies. Zeolite NaX (also called zeolite X, NaX, Linde X and molecular sieve 13X) is an analogue of the natural zeolite faujasite. The aim of the present paper is to investigate the most important stage of synthetic zeolite production - the process of its granulation. The granulation is obtained on the basis of three mechanisms involving different types of granulators or installations. The subject matter of the work is zeolite granulate of type Zeolite NaX with or without a binding substance. For this purpose, a laboratory installation of fluidized bed was designed and manufactured. The method of fluidized bed is a very effective method by which the synthesis takes place in the micro-volumes of the material. The granules obtained have an average particle size within the range of about 1 to 4 mm and also good mechanical strength. Keywords: zeolite NaX, natural zeolite, faujasite, zeolite granulate, fluidized bed. INTRODUCTION The studies on synthetic zeolites nowadays are focused mainly on the use of cheap initial or waste materials, as well as on the implementation of modern highly effective methods for their preparation. There are reports for studies on the preparation of granulated synthetic zeolite type NaX using a fluidized bed installation [1-3]. The zeolites of class FAU (NaX, NaY, Linde X, Ultrastabl Y) are used mainly as ion exchangers, adsorbents or catalysts in chemical, oil refining and gas industries. Besides, they can be applied in purification of natural gas from sulfur compounds, for drying oils and gases (air), as cooling agents, in separation of hydrocarbon mixtures, sorption of radio nuclides, etc. [4-5]. Traditionally, the basic raw material used to obtain zeolites with low content of silicon dioxide containing (like NaX and NaY) kaolin. Kaolin belongs to the large group of minerals known as clays. Its structure is built of silicon layers (Si 2 O 5 ) bonded to similar aluminium hydroxide layers (Al(OH) 4 ), called Gibbsite layers. Gibbsite is an aluminium hydroxide mineral which is bonded to the silicate layer (Silicate/Gibbsite) in a common layer called S/G layer. The relatively weak bonding between these S/G layers explains the plastic properties of the mineral. A similar structure have also the minerals of the kaolin group like Halloyzite, Dickite and Nacrite [6-9]. EXPERIMENTAL Raw Material The following initial materials were used for the preparation of zeolite NaX: kaolin from the Kaolin Company (Bulgaria). NaOH, LiOH, SiO 2 and Al 2 O 3. NaOH, LiOH, SiO 2 and Al 2 O 3 used were of Laboratory Reagent (LR) grade. The kaolin (grade BoExtra ) has the following chemical composition, according its producer, shown in Table

2 D. Georgiev, B. Bogdanov, I. Markovska, Y. Hristov Table 1. Chemical analysis of kaolinite mineral. Component Mass.% SiO ± 2.00 Al 2 O ± 0.50 TiO ± 0.05 Fe 2 O ± 0.05 CaO 0.15 ± 0.05 MgO 0.25 ± 0.05 Na 2 O 0.15 ± 0.05 K 2 O 0.60 ± 0.20 LОI * ± 0.60 * Loss of ignition. Particle size distribution in % (Sedi- Graph 5100) 10 μm μm μm Metakaolinisation The kaolin was calcined in a muffle furnace at tem perature C with a uniform gradation of 50 C giving a soaking period of 3 h each to get the metakaolins MK600 to MK700 (Fig. 1). The metakaolinisation of the initial material was carried out at a temperature of 650 C for 3 h. As can be seen from the results of the DTA/TGA analysis in Fig. 1, the crystallization water (of the initial material at C) was eliminated after this thermal treatment but the newly formed spinel phase formed again at C. The mass loss was 0.59 % (the same as at 700 C) [10]. Synthesis of zeolite NaX The activated (metakaolinated) natural product was dry mixed with amorphous SiO 2 to prepare a homogeneous mixture. Then sodium-alumosilicate solution (SAS) was added to obtain a gel-like product. The sodium alumosilicate solution was prepared from fresh solutions of sodium silicate (SS) and sodium aluminate (SA). Sodium silicate (SS) was prepared to have the SiO 2 concentration in the solution in the range g/l, and that of Na 2 O g/l. In the (SA) solution, the Al 2 O 3 concentration was g/l, while that of Na 2 O g/l. The blend obtained was mixed for 1 h in a special mixer at a temperature of C. The next stage was granulation of the amorphous blend and thermal treatment of the granules. The granulation was carried out in a specially designed and manufactured fluidized-bed installation for Fig. 1. DTA/TGA curve: (a) non-calcined kaolinite; (b) - kaolinite calcined at 650 o C (МК650). Fig. 2. Photograph of granules prepared in fluidized bed. 169

3 Journal of Chemical Technology and Metallurgy, 48, 2, 2013 granulation of clayish products. It allows simultaneous mixing and drying (up to C) of the ceramic blends, preparation and separation of granules sized from 0.5 to 4 mm [11]. For the purpose of the present work, the sizes of the granules obtained were 1-3 mm. The binding agent in this case was water. Fig. 2 shows a photograph of granules prepared from ceramic blends in the fluidized-bed installation. Obviously, the granules obtained were of the Blackberry type, as illustrated in Fig. 2. These are granules of a geometrically irregular shape; they do not release powder;are easily dissolving and have compact structure; their size can be precisely controlled and they have low wear resistance. Besides their excellent technical characteristics, the main advantage of this type of granules is that they can be obtained comparatively easy from dispersed powders and formed with a low content of binder. This is of crucial importance, since the binder might deteriorate the sorption properties of the material. The granules obtained were dried at a temperature of 90 C under fluidized bed conditions. They were then subjected to thermal activation in a muffle furnace at temperature of 720 C for 1 h on order to achieve an initial strength of the granules (called also green strength) since they are to be further processed. Table 2 shows the exemplary compositions of the initial amorphous blends from which the granules were obtained. The next very important stage is the crystallization (zeolitization) of the formed and granulated amorphous blend. It was carried out using crystallization solutions containing: aqueous solutions of NaOH, LiOH and SS. The NaOH and LiOH solutions were 260 g/l with respect Table 2. Compositions of the initial amorphous blends. Zeolite, index MK(650), g Amorphous SiO 2, g SS, ml SA, ml Z Z Z Z Z Table 3. Compositions of the crystallization solutions. Solution index Aqueous solution of NaOH, ml to Na 2 O and 200 g/l with respect to Li 2 O. Exemplary compositions of the crystallization solutions are presented in Table 3. The ratio between the amorphous blend and the crystallization solution was 1 to 5. The reactive mixture prepared crystallized (zeolitized) for h at temperature of C. The granules were then washed, dried and thermally treated (720 C, 2 to full dehydration). RESULTS AND DISCUSSION Aqueous solution of LiOH, ml SS, ml R R R R R The present paper discusses problems concerning the use of a natural material as a basic raw material for synthesis of granulated zeolite, type NaX. One of the technical possibilities to produce such a zeolite involves the use of lithium hydroxide as an additive in the alkali solution for crystallization (zeolitization) of the amorphous granules. The crystallization was carried out under isothermal conditions (temperature 90±10! for more than 30 h) depending on the sodium hydroxide concentration in the crystallization solution, the amount of lithium hydroxide additive and granules sizes (the bigger the granules, the longer the treatment time). The essence of this solution is that the lithium hydroxide addition to the alkali solution allows creating optimal conditions for the crystallization of the faujasite zeolite NaX within the granule, under the crystallization in a hydrothermal medium. Lithium ions, having significantly smaller ion radius than sodium ions, penetrate easier into granule pores and, simultaneously, they transport with themselves 170

4 D. Georgiev, B. Bogdanov, I. Markovska, Y. Hristov water contained in the hydrate coating. It, in turn, serves as a transport agent for sodium ions. Thus, the additive in the alkali crystallization solution allows for obtaining zeolite NaX of high phase purity. Furthermore, it facilitates the realization of a chemical equilibrium with respect to the final product and helps ensuring decreased crystallization (zeolitization) times and decreased alkali concentration in the crystallization solutions. Fig. 3 shows the X-ray diffractogram of a zeolite NaX sample synthesized. JCPDS files for automatic recognition were used. It can be seen that the main crystalline phases were zeolite NaХ and a small quantity of quartz (SiO 2 ) and Andalusite (AI 2 (SiO 4 )O) [12]. Fig. 4 shows the results from the DTA/TGA studies of Zeolite Z3 (R2, 95 C, 36 h). It can be seen from the thermal curves that the material lost more than 20 % of its mass under heating to 500 C which is considered enough to deduce that it can absorb significant amounts of water. Above this temperature, mass losses were only 0.40 % which means that the zeolite obtained was structurally stable. This is an important property for a zeolite which is to be used as a sorbent and for catalysis. The zeolite synthesized was found to be structurally stable up to 900 C and a new spinel-like phase was formed above this temperature. Fig. 3. XRD pattern of Zeolite Z3 (zeolitization R2, 95 о С, 36 h). CONCLUSIONS The possibility to prepare synthetic zeolite NaX from Bulgarian kaolin as the basic raw material was studied. The synthesis of zeolite NaX involves preliminary formation of granules by the method of fluidized bed, followed by crystallization (zeolitization). Thus, the following experimental results were obtained: Fig. 4. DTA/TGA of Zeolite Z3 (zeolitization R2, 95 о С, 36 h). 171

5 Journal of Chemical Technology and Metallurgy, 48, 2, 2013 The conditions for thermal treatment of the initial material of Bulgarian origin (kaolin BoExtra ) were studied and the treatment was found to be suitable for the synthesis of zeolite NaX. Using the method of fluidized bed, the conditions for preparation of suitable granules ( Blackberry type) with a preset size (1-3 mm) were studied with five amorphous ceramic blends containing kaolin, amorphous SiO 2, sodium silicate and sodium aluminate at appropriate quantitative ratios. Five crystallizing reactive solutions containing sodium hydroxide and lithium hydroxide as additive were developed. The role of the additive in the crystallization process in the optimization and in the achievement of a high phase purity of the zeolite synthesized, was established. The conditions for hydrothermal crystallization (zeolitization) of granules were found to be: 36 h at temperature of 95 C. The structure of the synthetic product Zeolite Z3 with zeolitization R2, 95 C, 36 h was studied by the methods of XRD, DTA/TGA, SEM and IR spectroscopy. The synthetic product Zeolite Z3 (obtained by the method of preliminary formation of granules in a fluidized bed, followed by zeolitization with a suitable reactive solution from the initial material kaolin of Bulgarian origin) was proved to have the same structure as Zeolite NaX. Acknowledgements The financial support of the Ministry of Education and Sciences (National Science Fund), Bulgaria, contract DO /2008 is gratefully acknowledged. REFERENCES Fig. 5. An IR spectrum and a SEM micrograph of Zeolite Z3 with zeolitization parameters R2, 95 о С, 36 h shows that the product obtained had the structure of Zeolite NaX. 1. R.M. Barre, Hydrothermal Chemistry of Zeolites, Academic Press, London, 1982, p D.W. Breck, Zeolite Molecular Sieves: Structure, Chemistry and Use, 1st Ed., John Wiley, New York, 1974, p R. Lussier, A novel clay-based catalytic material preparation and properties, J. Catal., 129, 1991, D. Akolekar, A. Chaffee, R.F. Howe, The transformation of kaolin to low-silica X zeolite, Zeolites, 19, 1997, M. Murat, A. Amokrane, J.P. Bastide, L. Montanaro, Synthesis of zeolites from thermally activated kaolinite. Some observations on nucleation and growth, Clay Miner., 27, 1992, S. Chandrasekhar, P.N. Pramada, Investigation on the Synthesis of Zeolite NaX from Kerala Kaolin, Journal of Porous Materials, 6, 1999, A. De Lucas, M.A. Uguina, I.Covian, L.Rodriguez, Synthesis of 13X zeolite from calcined kaolins and sodium silicate for use in detergents, Ind. Eng. Chem. Res., 31, 1992, A. De Lucas, M.A. Uguina, I. Covian, L. Rodriguez, Use of Spanish natural clays as additional silica source to synthesize 13X zeolite from kaolin, Ind. Eng. Chem. Res., 32, 1993, V.H. Bekkum, E.M. Flanigen, P.A. Jacobs, J.C. Jansen, Introduction to Zeolite Science and Practice, 2nd Revised Edn., Elsevier, Amsterdam, 1991, D. Georgiev, B. Bogdanov, Y. Hristov, I. Markovska, I. Petrov, Synthesis of NaA zeolite from natural kaolinite, 8 th International conference on the occurrence, properties and utilization of natural zeolites, July 2010, Sofia, p

6 D. Georgiev, B. Bogdanov, I. Markovska, Y. Hristov 11. D. Georgiev, B. Bogdanov, Y. Hristov, I. Markovska, Iv. Petrov, NaA zeolite synthesized in fluidized bed reactor, 15th International Metallurgy & Materials Congress (IMMC 2010), Istambul, November 11-13, 2010, M.M.J. Treacy, J.B. Higgins, Collection of simulated XRD powder patterns for zeolites, Elsevier, Amsterdam, 2007, p