Solid–solid transformation route to nanocrystalline sodalite from Al-PILC at room temperature

Solid–solid transformation route to nanocrystalline sodalite from Al-PILC at room temperature

Journal of Physics and Chemistry of Solids 65 (2004) 421–424 www.elsevier.com/locate/jpcs Solid –solid transformation route to nanocrystalline sodali...

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Journal of Physics and Chemistry of Solids 65 (2004) 421–424 www.elsevier.com/locate/jpcs

Solid –solid transformation route to nanocrystalline sodalite from Al-PILC at room temperature Sung-Reol Lee, Man Park, Yang-Su Han, Jin-Ho Choy* National Nanohybrid Materials Laboratory, School of Chemistry and Molecular Engineering, Seoul National University, San 56-1, Shinlim-Dong, Kwanak-Gu, Seoul 151-747, South Korea Accepted 12 September 2003

Abstract The present study describes a solid – solid transformation of nanocrystalline sodalite from the solid gel mixture of Al2O3-pillared montmorillonite (Al-PILC) with sodium hydroxide at room temperature (25 8C) under an ambient atmosphere. Powder X-ray diffraction (XRD) analysis confirms that the X-ray crystalline sodalite products are crystallized after 12 days, whereas infrared absorption (IR) spectra reveal that diagnostic IR absorption peaks due to single four-membered ring of sodalite framework is observed even after 1 day. Scanning electron microscopy (SEM) shows that Al-PILC is transformed into discrete nano-sized sodalite particles (,50 nm). Although the induction period, the time elapsing before nucleation, for the solid– solid transformation takes much longer (12 days), the nanocrystalline sodalite is successfully obtained at this extremely mild synthetic condition through solid – solid transformation. q 2003 Elsevier Ltd. All rights reserved. Keywords: B. Crystal growth

1. Introduction Zeolites and related molecular sieve materials with porous framework structure have a wide range of applications such as selective adsorbents, ion exchangers, catalysts and catalyst supports, and inorganic matrices for the encapsulation of functional guest molecules [1 – 3]. Sodalites, a class of microporous aluminosilicates with the general formula of M8[ABO4]6X2, are built up of cornersharing tetrahedra of A and B ¼ Si4þ, Ge4þ, Al3þ, Ga3þ, etc. Their framework structures are unique consisted of only one kind of polyhedral cavity, the [4668] truncated octahedron. This cavity enclathrates a large variety of different cationic (M ¼ Liþ, Naþ, Kþ, etc.) and anionic (X ¼ OH2, Cl2, etc.) guest species [3]. Zeolites including sodalite are generally crystallized from an amorphous aluminosilicate gel under hydrothermal condition [4,5]. Recently, a great deal of effort has been directed at the exploitation of zeolite synthesis by solid – solid transformation, since the hydrothermal method not only requires excess water for the reaction, but it also results * Corresponding author. Tel.: þ 82-2-880-6658; fax: þ82-2-872-9864. E-mail address: [email protected] (J.-H. Choy). 0022-3697/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2003.09.013

in production of an alkaline waste solution [6,7]. These approaches exhibited much higher product yield, lower elemental losses, and shorter nucleation and crystal growth, compared to the hydrothermal reaction method. In addition, the solid –solid transformation also present the advantage of obtaining nanocrystalline particles which exhibit fascinating potentials because a discrete small size allows faster diffusion of reactants into inner active surface as well as the fabrication of defect-minimized film [7]. The synthesis of zeolites by solid – solid transformation has been known as the dry-gel and molten-salt reaction methods [6 –9]. These methods could hardly facilitate the synthesis of zeolite at room temperature (RT) since their reactions work typically at the temperature in the range of 80 – 200 8C under close system or at high temperature beyond 200 8C under ambient atmosphere. However, we have recently reported that nanocrystalline sodalite has been synthesized by solid – solid transformation from the aluminosilicate clay such as kaolinite, montmorillonite and Al2O3-pillared montmorillonite (Al-PILC) at 100 8C under an ambient atmosphere [10]. Al-PILC is a suitable starting material for the sodalite crystallization by solid– solid transformation because of its well-developed microporous

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structure and an Al/Si mole ratio favorable to the formation of sodalite framework. In this paper, we report the solid – solid transformation route to nanocrystalline sodalite from Al-PILC at RT under ambient atmosphere. The resultant products are characterized as a function of the reaction time using several techniques in order to obtain a better understanding of local structure during the evolution of nanocrystalline sodalite.

2. Experiment Al-PILC was synthesized by intercalation of Al-polyhydroxy cations ([AlO4Al12(OH)12(H2O)24]7þ, Keggin-type ions) into the interlayer of clay [11]. Sodalite synthesis was attempted with the pillared clay gel prepared by addition of 25 wt% water and solid NaOH ([OH2]/[Si] ¼ 10). The solid gel mixture was transferred into an open vessel, and then the reaction was extended up to 30 days. In order to investigate the transformation process, a part of the reaction product obtained periodically from the solid gel mixture was washed with water, centrifuged and dried at 80 8C for 12 h. Powder X-ray diffraction patterns of the materials were recorded using a Philips PW1830 automated powder X-ray ˚ ). diffractometer with Cu Ka radiation ðl ¼1.5418 A Elemental analyses of the reaction products were carried out by atomic absorption spectroscopy (ANALAB-9100). The samples were prepared by the dissolution 10% nitric acid solution after fusion with lithium metaborate (LiBO2) at 900 8C for 10 min. Infrared (IR) spectra were obtained at RT using a BRUKER IFS-88 FT-IR spectrometer by a standard KBr disk method. The morphology and particle size for the reaction products were observed by scanning electron microscopy (HITACHI S-4500). Prior to the SEM observation, the samples were coated with E-1030 ion sputter for 180 s.

Fig. 1. Powder X-ray diffraction patterns for the starting Al-PILC (a) and the reaction products obtained at room temperature (25 ^ 1 8C) with different reaction time. (b) 1 day, (c) 3 days, (d) 7 days, (e) 12 days, (f) 16 days, and (g) 30 days.

the homogeneous distribution of Si and Al and the rapid diffusion of hydrated NaOH into the interlayer space. As a consequence, the activated Si and Al atoms form a sodalite framework through in situ solid – solid transformation without any long-range diffusion. Further structure information during the solid – solid transformation can be provided by IR spectra, which are more sensitive to shorter-range order than X-ray diffraction. Fig. 2 represents IR spectra with the solid – solid transformation at RT. The vibrational frequencies of the Al-PILC (a) are well consistent with the literature values [13]. The absorption band at 3630 cm21 can be assigned to the stretching vibration of structural hydroxyl group (OH2) of silicate lattice and the strongest vibration at 1100 cm21 is due to the asymmetric stretching mode of the tetrahedrally coordinated Si in the clay sheets. The other ‘fingerprint’ absorptions are also observed at 920, 524 and 465 cm21 which correspond to the stretching

3. Results and discussion Fig. 1 represents the evolution of XRD patterns as a function of reaction period for the samples prepared at RT under an ambient atmosphere. The pristine Al2O3 pillared compound exhibits a characteristic (00l) diffraction series to suggest the formation of a regular intercalation compound ˚ ð2u ¼ 4:88Þ: After the with the basal spacing of 18.5 A reaction for 1 day, the (001) reflection of the Al-PILC shifts ˚ ), which indicates to higher 2u angle ð2u ¼ 6:98; d ¼ 12:8 A the collapse of interlayer Al2O3 pillars by NaOH. New diffraction peaks result from formation of crystalline sodalite after 12 days. This result clearly shows that AlPILC is successfully transformed into crystalline sodalite even at RT. All the diffraction peaks are comparable to those of highly crystalline sodalite synthesized by hydrothermal method [12]. Such a formation of sodalite is surely due to the well-developed micropores of Al-PILC, which lead to

Fig. 2. FT-IR spectra for the starting Al-PILC (a) and the reaction products obtained at room temperature (25 ^ 1 8C) with different reaction time. (b) 1 day, (c) 3 days, (d) 7 days, (e) 12 days, (f) 16 days, and (g) 30 days.

S.-R. Lee et al. / Journal of Physics and Chemistry of Solids 65 (2004) 421–424

vibration of Al – (OH) –Al(Mg), the symmetric stretching mode of internal SiO4 tetrahedra, and the structureinsensitive T –O bending modes of tetrahedral TO4 units (T ¼ Al and Si), respectively. The solid –solid transformation is clearly reflected on the gradual shift of Si – O stretching band from 1100 to 986 cm21 and the disappearing of the absorption bands associated with the structural hydroxyl groups in octahedral layers of clay (3630 and 920 cm21). The red-shift of the nas (Si – O) (ca.120 cm21) indicates the incorporation of Al atom into tetrahedral SiO4 network to form zeolitic (Al, Si)O4 tetrahedra with an alternating ordering of Al and Si tetrahedron [14]. A diagnostic feature of the sodalite formation is the appearance of a new absorption peak at 430 cm21 due to the formation of single four-membered ring (S4R) of sodalite unit. As the reaction period is extended, the absorption bands related to the structural hydroxyl group of clay gradually disappear while the absorption due to the S4R is progressively enhanced. Surprisingly the diagnostic absorption peaks due to the formation Si – O – Al zeolitic framework become clearly noticeable even after the reaction for 1 day, although the Xray crystalline sodalite products appear after the 12 days reaction (Fig. 1). This result reflects that the local environments are rapidly changed into sodalite framework. At the same time the symmetric stretching vibrations ðns (T – O)) in the 670 – 730 cm21 region are also enhanced. Since the symmetric vibrations of TO4 are sensitive to the mass of the anion or cation included in the sodalite cage, the peak positions give an invaluable diagnostic for encapsulated species. Comparing the observed peak positions with those of literature values [12], it seems to contain sodium hydroxide moieties. The IR spectra of the samples measured as a function of reaction time also show the gradual evolution of the absorption band at 524 cm21 which can be assigned to the Si – O – Al bending vibration of the clay and a clear decrease in its intensity reflects the reduced content of the octahedral cations as reaction proceeds. This band, which is the most sensitive indicator of the presence/absence of octahedral aluminum, is barely discernible after 30 days, indicating the complete transformation of aluminosilicate layer into sodalite. The evolution of morphology and particle size during the solid – solid transformation is investigated by scanning electron microscopy. As shown in Fig. 3(a), the Al-PILC is mainly composed of lamellar-type particles. After the solid –solid transformation for 12 days (b), the agglomerated particles appear at the surface of lamellar-type flakes, indicating that sodalite crystallizes at the expense of the AlPILC. After 30 days (c), the agglomerated particles composed of extremely nanocrystalline sodalites (, 50 nm) are dominant instead of the lamellar-type flakes. It is worthy to note here that the resultant sodalite particles show the lamellar-type morphology resembling the original clay. Therefore, this result suggests that the solid – solid

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Fig. 3. Scanning electron micrographs of the pristine Al-PILC (a) and resultant products obtained from the Al-PILC at room temperature (25 ^ 1 8C). (b) 12 days and (c) 30 days.

transformation into nanocrystalline sodalite takes place in the overall interlayer space of Al-PILC to result in the finely cracked particles.

4. Conclusion Three-dimensional aluminosilicates with sodalite structure are derived from Al-PILC by solid – solid transformation with NaOH(s) under an ambient condition. It is found that the X-ray crystalline sodalite product appears after the reaction for 12 days, although the local environments are rapidly changed into sodalite framework. The resulting nanocrystalline sodalites with the average size of , 50 nm are found to be agglomerated into polycrystalline particles, suggesting that the present solid – solid transformation route provide a new way of preparing nano-sized sodalite particles.

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Acknowledgements This work was supported by National Research Laboratory (NRL) project’99 and National R&D Project for Nano Science and Technology. Authors thank to the Ministry of Education for the Brain Korea 21 fellowship. References [1] D.W. Breck, Zeolite Molecular Sieves, Wiley, New York, 1972. [2] G.A. Ozin, A. Kuperman, A. Stein, Angew. Chem. Int. Ed. Engl 28 (1989) 359. [3] K.L. Moran, W.T.A. Harrison, I. Kamber, T.E. Gier, X. Bu, D. Herren, H. Eckert, G.D. Stucky, Chem. Mater. 8 (1996) 1930. [4] R.M. Barrer, Hydrothermal Chemistry of Zeolites, Academic Press, London, 1972.

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