SYNTHESIS OF NaA ZEOLITE

Transcription

1 Proceedings of a 12 th International Conference MTCM 2009 September 22-24, 2009, Varna, Bulgaria SYNTHESIS OF NaA ZEOLITE Dimitar Georgiev, Bogdan Bogdanov*, Yancho Hristov, Irena Markovska*, Krasimira Angelova* Department of Material Science and Technology, *Department of inorganic substances and silicates, Assen Zlatarov University, 1 Prof. Yakimov Str., 8010 Bourgas, Bulgaria, : dgeorgiev@btu.bq INTRODUCTION To prepare synthetic zeolites with low content of SiO 2 like NaA, NaX and NaY, rock materials, clays, kaolin and waste materials like ash and cinder are mainly used [1-4]. The studies in this direction are focused on the use of cheap and easily available natural materials or waste products. Kaolin is the preferred starting material due to its high content of SiO 2 and Al 2 O 3 and favorable ratio of their contents and it is irreplaceable for syntheses of zeolites of high crystallization degree and cation-exchange capacity. Kaolinite [Al 2 Si 2 O 5 (OH) 4 ] is well known and widely spread mineral of the kaolin group. It is dioctahedral 1:1 layered alumosilicate with two kinds of surface interim layers. One of them is composed of structures of quartz type where each silicon atom resides in the center of a tetrahedron with oxygen atoms at its vertices. The other consists of gibbsite-type structures where each aluminium atom resides in the center of an octahedron with oxygen atoms and hydroxyl groups at the vertices. Besides, the hydroxyl groups remain on one side of the gibbsite-type layer while the oxygen atoms are on the other side (quartz side). This structure of kaolinite stipulates its plastic properties [5-7]. The use of kaolin to synthesize zeolites is based on the fact that this mineral can be preliminarily activated. As it has been reported earlier, the kaolin properties can hardly be improved by chemical methods since its reactivity is very low. The changes observed after alkali or acidic treatment are scarce, even under extreme conditions. Therefore, kaolin is usually used after isothermal activation (calcination) at temperatures from 600 to 900 С to obtain meta-kaolin which is active reaction phase. This is accompanied by loss of the crystallization water and significant structural changes the major part of AlO 6 octahedrons is transformed into tetra- and penta-coordinated units. The best conditions for preparation of meta-kaolin have been reported by many authors, all of them showing optimal temperature in the range C. The hydrothermal method is the most often used one to prepare synthetic zeolites from kaolin and meta-kaolin. Using this method, many authors reported for the preparation of LTA, FAU, CHA, zeolite NA-P1, zeolite Barrer KF, ANA, CAN, JBW, PHI, etc. [1-3, 8-9]. Studies on the stabilization of kaolin at high ph were published in [10]. Studies on the preparation of zeolites by the hydrothermal method in fluoride medium have also been reported [11]. Keywords: Zeolite NaA, kaolin, metakaolin, hydrothermal synthesis. 41

2 The advantages of the synthetic products obtained from kaolin are based mainly on the low cost of the initial materials. Since pure kaolin is hard to obtain, many producers of certain zeolites (e.g. LTA) pose requirements towards the initial materials like phase purity, impurities concentrations, particle size, etc. [12]. The effect of impurities (mainly iron) on the zeolite crystallization kinetics of zeolite, as well as its nature was studied [13]. Another author reported that the coordination number of aluminium in raw materials is quite important for the preparation of high quality zeolites. MATERIALS AND METHODS As initial materials for zeolite NaA obtaining were used: kaolin from Kaolin Company (Bulgaria) and NaOH. NaOH was obtained from Merck Company with 99.9% analytical grade. Kaolin (grade BoExtra ) has the following chemical composition, according producer (Table 1). Table 1. Chemical analysis of kaolinite mineral. Component % 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. Zeolite was synthesized in two main stages: I. Thermal activation of kaolin to obtain meta-kaolin. II. Hydrothermal treatment f meta-kaolin in different alkali media. The meta-kaolinization of the initial material was carried out (after drying at 110 C for 1 h) at 600, 650 and 700 о С in a muffle oven with isothermal period of 2 h. The hydrothermal alkali treatment of meta-kaolin was performed using a series of sample mixtures as shown in Table 2 in the following sequence. Different amounts of meta-kaolin and aqueous alkali solutions of different contents of NaOH are intensely homogenized at 60 C for 30 min. The homogenized past-like mixture is the placed in an autoclave reactor and treated at 140 C and 6-8 atm pressure for 3 h. Product Table 2. Compositions of experimental products. Quantity of mixture at correlation metakaolin / alkaline solution (g/100ml) concentration of NaOH, (N) Z Z Z Z Z The product obtained is washed several times with water and ethyl alcohol (to ph 7-8) and then dried at 110 C for 1 h and activated at 620 C for 2 h. The synthesized products were analyzed by XRD, FTIR and DTA TGA techniques. 42

3 RESULTS AND DISCUSSION Meta-kaolinization of kaolin has been described in many publications but it is important since it refers to certain type of kaolin mined in Bulgaria (type BoExtra, Kaolin Co.) and synthesis of zeolite from it. a) b) Figure 1. DTA/TGA curves of: a) uncalcined kaolinite; b) 650 oc calcined kaolinite The calcinations of the initial material was carried out with isothermal periods at 600, 650 and 700 о С. The DTA/TGA analyses carried out showed (Fig.1a) that the dehydration of the initial kaolin was occurred at 565 C. A new crystalline spinel phase formed at 975 о С. The mass loss during kaolin dehydration was 17.7%. a) b) Figure 2. IR spectra of: a) original kaolinite; b) 650 о C calcined kaolinite The most suitable temperature for the meta-kaolinization turned out to be 650 о С, since crystallization water is still present at 600 C and the curves at 650 and 700 о С are almost identical. Fig. 1b shows the DTA/TGA curve for meta-kaolin treated at 650 о С. It can be seen that the crystallization water had disappeared and the spinel phase is formed also at 975 о С. The mass loss here was 2.5% (up to 1000 о С). Fig. 2 shows the IR spectra of the initial kaolin and the one calcinated at 650 о C. For comparison, the IR vibrational frequencies observed and the values reported in the literature are presented in Table 3. 43

4 Table 3. IR vibrational frequencies of the experimental and literature data of kaolinite. Vibrations Wave number (cm -1 ) Literature wave number (experimental) according [15, 16] (cm -1 ) Ν(OH) Ν(OH) Δ(OH) Ν(SiO) 1116, 1010 Al-O Si-O 469, , 428 ν s (Si-O-Si) 790, , 754 AlО-H 755, , 694 The IR spectrum of the initial kaolin clearly shows the presence of bonded hydroxyl groups at 3620 and 3696 cm -1, the peaks for Si-O at 469 and 430 cm -1, as well as the peaks for the Si-O-Si groups at 790 and 755 cm -1. Table 4 presents comparative data on IR vibrational frequencies of meta-kaolin observed and these reported in the literature. As can be seen in Fig. 2b, vibrational frequencies for bonded hydroxyl groups were not recorded. A wide peak for the vibrational frequencies of SiO groups can be seen at 1087 cm -1 and another one for Al-O at cm -1. Table 4. IR vibrational frequencies of the experimental and literature data of metakaolinite. Vibrations Wave number (cm -1 ) Literature wave number (experimental) according [15,16] (cm -1 ) Al-O Ν (SiO) Si-O-Si After the hydrothermal syntheses, the best results were obtained for the Z4 zeolite. It was studied further and compared to the commercial product Zeolite NaA. b) a) Figure 3. IR spectra of: a) Zeolite Z4; b) Commercial zeolite NaA The IR spectrum of Zeolite Z 4 and, for comparison, the IR spectrum of commercial zeolite NaA, are shown in Fig.3. Up to 798 cm -1, the vibrational frequencies of the zeolite-forming groups of meta-kaolin Al-O from Аl 2 O 3 are observed. Further, the frequencies observed at about 1000 cm -1 indicate for the Si-O-Al bonds in ТО 4 tetrahedrons (Т is Si or Al). On the left-hand side of the IR spectra, several well shaped peaks at about 3450 cm -1 и 1650 cm -1 can be observed, which indicate for the zeolite water. The experimentally obtained IR spectra prove the formation of zeolite NaA due to the coincidence of the IR waves at 1004, 670, 554 and 464 cm -1 with these of the IR spectrum of the commercial zeolite NaA ( Al drich). 44

5 a) Figure 4. XRD patterns of: a) Zeolite Z4; b) Commercial zeolite NaA b) Figure 5. SEM of Zeolite Z4 The diffractograms of the zeolite Z4 synthesized and the commercial Zeolite A are shown in Fig. 4. The diffraction maxima can be clearly distinguished (at d = 12.3, 8.71, 3.71, 3.29, 2.90, 2.62 Ǻ ) to prove the zeolite structure of zeolite NaA A SEM micrograph of Zeolite Z4 is presented in Fig. 5 where the crystalline formations of the zeolite synthesized can be seen. They are cubic crystals of almost the same size (3-4 μm) characteristic for zeolite NaA. CONCLUSIONS The possibility to prepare synthetic zeolite NaA from natural kaolin mined in Bulgaria using the method of hydrothermal synthesis was studied and the following experimental results were obtained: The conditions of kaolin thermal treatment (650 C, 2 h) were studied and the transformation into meta-kaolin phase and its suitability for synthesis of Zeolite NaA were investigated by the methods of IR spectrophotometry and DTA/TGA analysis. Five kinds of zeolites of different compositions were synthesized with different amounts of NaOH using the method of hydrothermal synthesis (at 140оС, pressure 6-8 atm, 3 h in an autoclave reactor). The composition of Zeolite Z4 was found to be the most suitable. 45

6 The structure of the Zeolite Z 4 obtained was studied by IR spectroscopy, XRD and SEM and the results obtained were compared to those of the commercial Zeolite NaA. The synthetic product Zeolite Z 4 (prepared from Bulgarian kaolin as initial material by the method of hydrothermal synthesis) was proved to have the structure of Zeolite NaA. ACKNOWLEDGEMENT The financial support of the Ministry of Education and Sciences (National Science Fund), contract DO /2008 is gratefully acknowledged! REFERENCES 1. Barrer, R.M., Hydrothermal Chemistry of Zeolites, Academic Press, London (1982) Breck D.W., Zeolite Molecular Sieves: Structure, Chemistry and Use, John Wiley and Sons, London (1974) Bekkum V.H, Flanigen E.M, Jacobs P.A., Jansen J.C., Introduction to Zeolite Science and Practice, 2 nd Revised Edn., Elsevier, Amsterdam (1991) Meier W.M., Olson D.H., Atlas of Zeolites Structure Types, 2 nd Ed., Butterworth and Co (1987). 5. Thomson J.G., Cuff C., Clays and Clay Minerals (1985) Uwins P.J., Mackinnow I.D.R, Thompson J.G., Yago A.J.E., Clays and Clay Minerals (1993) Chandrasekhar S., Pramada P.N., Journal of Porous Materials (1997) Ruiz R, Blanco C., Pesquera C., Gonzalez F., Benito I.; Lopez J.L., Appl. Clay Sci. (1997) Kim W., Zhang Q., Saito F., J. Chem. Eng. Japan (2000) Carroll S.A., Walther J.V., Geochimica et Cosmochimica Acta (1988) Rees L.V.C., Chandrasekhar S., Zeolites (1993) Llenado R.A. The use of sodium A zeolite in detergents, D. Olson and Bisio A. (Eds.), In: Proc. 6 th International Zeolite Conference (1983) Murat M., Amorkrane A., Bastide J.P., Montanaro L., Clay Minerals (1992) Madani A., Aznar A., Sanza J., Serratosa J.M., Journal of Physical Chemistry (1990) Markovic S., Dondur V., Dimitrijevic R., Journal of Molecular Structure (2003) Johnston C.T., Bish D.L., Eckert J., Brown L.A., Journal of Physical Chemistry B (2000)