6952
Langmuir 2008, 24, 6952-6958
Synthesis of Nanometer-Sized Sodalite Without Adding Organic Additives Wei Fan,† Kazumasa Morozumi, Riichiro Kimura, Toshiyuki Yokoi, and Tatsuya Okubo* Department of Chemical System Engineering, The UniVersity of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan ReceiVed December 8, 2007. ReVised Manuscript ReceiVed February 9, 2008 Aggregates (80 nm) of sodalite nanocrystals with crystallite sizes ranging from 20 to 40 nm have been synthesized from a sodium aluminosilicate solution at low temperature, without adding any organic additives, while paying attention to the key factors for the synthesis of nanosized zeolite crystals. The physical properties of nanosized sodalite crystals were characterized by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, 29Si solid-state magic-angle spinning (MAS) NMR, and N2 adsorption. As expected, the external surface area of nanosized sodalite crystals is significantly increased compared with that of microsized sodalite crystals. The size of synthesized sodalite crystals can be controlled from 20 nm to 10 µm. It is found that the preparation of a homogeneous aluminosilicate solution followed by the formation of an aluminosilicate hard gel by adjusting the initial composition, for example, SiO2/Al2O3 and Na2O/H2O ratios, is critical for synthesis.
1. Introduction Zeolites are a series of microporous crystals with intricate pores and channels in the size range from 0.3 to 1 nm, and they have widely been used as catalysts, adsorbents, and ionexchangers.1,2 Interest in the synthesis of zeolite nanocrystals has continuously grown toward several novel applications, such as hosts of photochemically or optically active guests,3–5 seeds of thin films,6–8 low-k dielectrics in microelectronics,9,10 chemical sensors,11,12 and so on. Most zeolite nanocrystals are synthesized from homogeneous clear solutions using abundant amounts of organic structuredirecting agents (SDAs), especially tetraalkylammonium (TAA) cations.11 Zeolite A and zeolite Y crystals smaller than 100 nm have been prepared from a diluted clear solution or gel system containing tetramethylammonium (TMA) cations.13–17 A colloidal suspension of sodalite crystals with a size of 37 nm with a narrow particle size distribution has been produced in the presence of * To whom correspondence should be addressed. E-mail: okubo@ chemsys.t.u-tokyo.ac.jp. Telephone: +81-3-5841-7348. Fax: +81-3-58003806. † Present address: Department of Chemical Engineering and Materials Science Institute of Technology, University of Minnesota, 151 Amundson Hall, 421 Washington Avenue SE, Minneapolis, Minnesota 55455.
(1) Breck, D. W. Zeolite Molecular SieVes; Wiley: London, 1974. (2) Barrer, R. M. Hydrothermal Chemistry of Zeolites; Academic Press: London, 1982. (3) Castagnola, N. B.; Dutta, P. K. J. Phys. Chem. B 1998, 102, 1696–1702. (4) Ryo, M.; Wada, Y.; Okubo, T.; Nakazawa, T.; Hasegawa, Y.; Yanagida, S. J. Mater. Chem. 2002, 12, 1748–1753. (5) Platas-Iglesias, C.; Vander Elst, L.; Zhou, W. Z.; Muller, R. N.; Geraldes, C.; Maschmeyer, T.; Peters, J. A. Chem.sEur. J. 2002, 8, 5121–5131. (6) Lassinantti, M.; Hedlund, J.; Sterte, J. Microporous Mesoporous Mater. 2000, 38, 25–34. (7) Lin, J. C.; Yates, M. Z. Chem. Mater. 2006, 18, 4137–4141. (8) Lai, Z. P.; Bonilla, G.; Diaz, I.; Nery, J. G.; Sujaoti, K.; Amat, M. A.; Kokkoli, E.; Terasaki, O.; Thompson, R. W.; Tsapatsis, M.; Vlachos, D. G. Science 2003, 300, 456–460. (9) Wang, Z. B.; Wang, H. T.; Mitra, A.; Huang, L. M.; Yan, Y. S. AdV. Mater. 2001, 13, 746–749. (10) Wang, Z. B.; Mitra, A. P.; Wang, H. T.; Huang, L. M.; Yan, Y. S. AdV. Mater. 2001, 13, 1463–1466. (11) Tosheva, L.; Valtchev, V. P. Chem. Mater. 2005, 17, 2494–2513. (12) Mintova, S.; Mo, S. Y.; Bein, T. Chem. Mater. 2001, 13, 901–905. (13) Mintova, S.; Olson, N. H.; Bein, T. Angew. Chem., Int. Ed. 1999, 38, 3201. (14) Mintova, S.; Olson, N. H.; Valtchev, V.; Bein, T. Science 1999, 283, 958. (15) Mintova, S.; Valtchev, V. Stud. Surf. Sci. Catal. 1999, 125, 141–148.
TMA cations.18 Pure silica ZSM-5 nanocrystals have been synthesized in the presence of tetrapropylammonium (TPA) cations.19 However, removal of SDAs often leads to irreversible aggregation of the nanocrystals and a decrease of the crystallinity. In addition, the yield of zeolite nanocrystals by this approach is normally lower than ∼10%, calculated based on the silicon source used.11 Furthermore, the use of organic SDAs tends to change the Si/Al ratio of the final products, which could drastically affect their applications.20 Recently, syntheses of zeolite nanocrystals in confined space have been developed using carbon black and gelling polymer as synthesis media. BEA (7-30 nm) and MFI (20-75 nm) nanocrystals have been synthesized inside the pores of mesoporous carbon.21,22 However, recovery of the products is difficult because removal of the carbon matrices has to be conducted by calcination, which will lead to the irreversible aggregation of the nanocrystals. Moreover, zeolite A (20-180 nm) and zeolite X (10-100 nm) nanocrystals have been synthesized using thermoreversible polymer hydrogels,23 although the particle size distribution is relatively broad. Therefore, the rational syntheses of zeolite nanocrystals by a simple process without using any organic additives, resulting in high yield, still remain a challenging target in this research field. So far, zeolite L nanocrystals have been synthesized without using any organic additives.24 In this synthesis process, the homogeneous potassium aluminosilicate solution gradually changes to amorphous hard gel before nucleation. Zeolite L nanocrystals are produced via the transformation of the amorphous (16) Schoeman, B. J.; Sterte, J.; Otterstedt, J. E. Zeolites 1994, 14, 110–116. (17) Fan, W.; Shirato, S.; Gao, F.; Ogura, M.; Okubo, T. Microporous Mesoporous Mater. 2006, 89, 227–234. (18) Schoeman, B. J.; Sterte, J.; Otterstedt, J. E. Zeolites 1994, 14, 208–216. (19) Persson, A. E.; Schoeman, B. J.; Sterte, J.; Otterstedt, J. E. Zeolites 1995, 15, 611–619. (20) Zimmerman, C. M.; Singh, A.; Koros, W. J. J. Membr. Sci. 1997, 137, 145–154. (21) Jacobsen, C. J. H.; Madsen, C.; Janssens, T. V. W.; Jakobsen, H. J.; Skibsted, J. Microporous Mesoporous Mater. 2000, 39, 393–401. (22) Schmidt, I.; Madsen, C.; Jacobsen, C. J. H. Inorg. Chem. 2000, 39, 2279– 2283. (23) Wang, H. T.; Holmberg, B. A.; Yan, Y. S. J. Am. Chem. Soc. 2003, 125, 9928–9929. (24) Tsapatsis, M.; Lovallo, M.; Okubo, T.; Davis, M. E.; Sadakata, M. Chem. Mater. 1995, 7, 1734–1741. (25) Tsapatsis, M.; Lovallo, M.; Davis, M. E. Microporous Mater. 1996, 5, 381–388.
10.1021/la703838j CCC: $40.75 2008 American Chemical Society Published on Web 05/29/2008
Hydrothermal Synthesis of Nanometer-Sized Sodalite
hard gel that has a finely divided hierarchical structure.25 The studies on the formation of zeolite A nanocrystals synthesized from a sodium aluminosilicate solution at room temperature revealed that sodium cations provoke abundant aggregation of polymerized aluminosilicate species, resulting in the formation of a fairly open gel.26,27 The formed gel plays an important role in the formation of zeolite A nanocrystals. The crystallization processes of zeolite L and zeolite A nanocrystals indicate that one key route to synthesize zeolite nanocrystals without using any organic additives is (1) to prepare a homogeneous aluminosilicate solution by mixing the completely dissolved Si and Al sources; (2) to prepare a homogeneous amorphous aluminosilicate gel with a finely divided hierarchical structure which favors homogeneous nucleation and impedes crystal growth; and (3) to synthesize products at low temperature which favors the nucleation process, since the activation energy of crystal growth is generally higher than that of nucleation.28,29 Our idea is to synthesize other types of zeolite nanocrystals while considering the key factors for the synthesis described above. Sodalite with a cubic Im3m crystal symmetry consists of a six-membered ring (6R) with a pore size of 0.28 nm, and the maximum diameter of the void included in the framework is 0.63 nm. So far, much attention has been paid to sodalite crystals owing to a wide range of applications in semiconductors,30,31 hydrogen separation,32,33 hydrogen storage,34 and pigment occlusion.35 Furthermore, we have found that sodalite crystals are a useful support for alkali metal catalysts to combust the soot emitted from diesel engines.36 Recently, sodalite nanocrystals were synthesized by phase transformation of MFI nanocrystals.37 However, TPA cations as an SDA have been used in the synthesis of MFI nanocrystals, which in turn leads to the indispensable post-treatment for the removal of the SDA by calcination. Also, sodalite nanocrystals have been synthesized through localized solid-solid transformation of an aluminosilicate matrix formed by Al2O3 pillared clay, although the aggregation of nanocrystals is difficult to avoid.38,39 Here, we report an efficient, organic-free synthesis of 80 nm aggregates of sodalite nanocrystals with crystallite sizes ranging from 20 to 40 nm from a sodium aluminosilicate solution at low temperature while paying attention to the key factors for the synthesis of zeolite nanocrystals as descried above. To the best of our knowledge, this is the first report on the hydrothermal synthesis of sodalite nanocrystals without using any organic additives during the whole crystallization process. (26) Smaihi, M.; Barida, O.; Valtchev, V. Eur. J. Inorg. Chem. 2003, 4370– 4377. (27) Valtchev, V. P.; Bozhilov, K. N. J. Am. Chem. Soc. 2005, 127, 16171– 16177. (28) Mintova, S.; Valtchev, V. Zeolites 1993, 13, 299–304. (29) Feoktistova, N. N.; Zhdanov, S. P.; Lutz, W.; Bullow, M. Zeolites 1989, 9, 136–139. (30) Stein, A.; Ozin, G. A.; Stucky, G. D. J. Am. Chem. Soc. 1990, 112, 904–905. (31) Stein, A.; Meszaros, M.; Macdonald, P. M.; Ozin, G. A.; Stucky, G. D. AdV. Mater. 1991, 3, 306–309. (32) Xu, X. C.; Bao, Y.; Song, C. S.; Yang, W. S.; Liu, J.; Lin, L. W. Microporous Mesoporous Mater. 2004, 75, 173–181. (33) Julbe, A.; Motuzas, J.; Cazevielle, F.; Volle, G.; Guizard, C. Sep. Purif. Technol. 2003, 32, 139–149. (34) Buhl, J. C.; Gesing, T. M.; Ruscher, C. H. Microporous Mesoporous Mater. 2005, 80, 57–63. (35) Arieli, D.; Vaughan, D. E. W.; Goldfarb, D. J. Am. Chem. Soc. 2004, 126, 5776–5788. (36) Ogura, M.; Morozumi, K.; Elangovan, S. P.; Okubo, T. Appl. Catal., B 2008, 77, 294–299. (37) Yao, J. F.; Wang, H. T.; Ratinac, K. R.; Ringer, S. P. Chem. Mater. 2006, 18, 1394–1396. (38) Lee, S. R.; Han, Y. S.; Park, M.; Park, G. S.; Choy, J. H. Chem. Mater. 2003, 15, 4841–4845. (39) Choy, J. H.; Lee, S. R.; Han, Y. S.; Park, M.; Park, G. S. Chem. Commun. 2003, 1922–1923.
Langmuir, Vol. 24, No. 13, 2008 6953
2. Experimental Section 2.1. Materials and Preparation of the Synthesis Mixtures. Sodium aluminate (Al/NaOH ) 0.75, Wako Pure Chemical Industries Ltd.), fumed silica (Cab-O-Sil M-5, Cabot), sodium hydroxide (Wako Pure Chemical Industries Ltd.), and distilled water (Wako Pure Chemical Industries Ltd.) were used in the synthesis. A typical synthesis mixture with the following composition 19Na2O/4SiO2/ 1Al2O3/190H2O was prepared by mixing the freshly prepared homogeneous aluminate solution and silicate solution. In order to prepare the homogeneous aluminate solution and silicate solution, a large amount of NaOH was used in the synthesis as shown above. Typically, 8.04 g of NaOH was dissolved in 15 g of distilled water. A total of 0.72 g of sodium aluminate was completely dissolved in half of the NaOH solution at 100 °C. The silicate solution was prepared by mixing 1.05 g of fumed silica with another half of the NaOH solution at 100 °C until the solution was clear. After cooling the two solutions down to 5 °C, the silicate solution was slowly dropped into the aluminate solution with vigorous stirring, which resulted in a clear homogeneous solution. Thereafter, the synthesis solution was transferred to a polypropylene bottle and heated at 60 °C under static conditions for 5 h. The products were separated by centrifugation (20 000 rpm, 30 min), followed by redispersion in water for several times until the pH value of the solution was ∼8.0. In order to remove the amorphous phase, the samples were washed again with 2 M NaOH solution three times at room temperature followed by redispersion in water for several times until the pH value of the solution was ∼8.0. The washing process using 2 M NaOH was only performed on the sample of sodalite nanocrystals. Finally, the samples were dried at 60 °C for 24 h. 2.2. Characterization. X-ray powder diffraction (XRD) was carried out on an M03X-HF instrument (MacScience) with Cu KR radiation (λ ) 1.540561 Å) at 40 kV and 40 mA. Crystallite size was calculated by the Scherrer equation corrected with the instrument broadening using the peaks at 14.05° and 24.53°, corresponding to (110) and (211) of sodalite crystals, respectively. Scanning electron microscopic (SEM) images were recorded on an S-900 instrument (Hitachi). The specimens were coated with Pt for 10 s using an ion sputter system with a magnetron electrode (Hitachi E-1030). Transmission electron microscopic (TEM) images were recorded using a JEM-2000EX II microscope (JEOL) operated at 200 kV. Inductively coupled plasma atomic emission spectroscopy (ICPAES) was carried out for the analyses of chemical compositions of the products on a Hitachi ICP-AES P-4010 instrument. Infrared absorption spectra were obtained on a Magna-IR 560 instrument (Nicolet) using a KBr wafer technique and recorded with a resolution of 4 cm-1. The N2 adsorption/desorption isotherms were measured at 77 K on an Autosorb-1 instrument (Quantachrome). Before the measurement, the samples were degassed at 250 °C and 10 Pa for 12 h. 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra were measured on a CMX-300 instrument (JEOL, 300 MHz for protons) at room temperature, with a spinning rate of 3 kHz with a pulse length of 4 µs, a recycle time of 120 s, and a scan time of 1400. The chemical shift was referenced to an external standard of tetramethylsilane at 0 ppm and polymethylsilane at -34.6 ppm as a secondary standard. Dynamic light scattering (DLS) was carried out on a Zetasizer Nano ZS90 instrument (Malvern Instrument Ltd.) using a laser source with a wavelength of 633 nm. The samples were filtered with 200 nm Whatman filter paper before the measurements. Generally, the weight-based concentration in the suspension was on the order of 0.01 wt %, which allowed for the assumption that the viscosity of the suspension was equal to that of H2O.
3. Results and Discussion 3.1. Synthesis of Sodalite Nanocrystals. The synthesis mixture with the molar composition of 19Na2O/4SiO2/1 Al2O3/ 190H2O is used to synthesize sodalite nanocrystals, denoted as S1. Microsized sodalite crystals (S2) are synthesized using the
6954 Langmuir, Vol. 24, No. 13, 2008
Fan et al.
Table 1. Initial Composition and Heating Time for the Synthesis of Sodalite Crystalsa run
molar composition of initial solution
heating time (h)
S1 S2 S3 S4 S5 S6 S7 S8
19Na2O/4SiO2/1Al2O3/190 H2O 29Na2O/4SiO2/1Al2O3/190 H2O 15Na2O/4SiO2/1Al2O3/190 H2O 15Na2O/4SiO2/1Al2O3/190 H2O 15Na2O/4SiO2/1Al2O3/190 H2O 15Na2O/4SiO2/1Al2O3/190 H2O 15Na2O/4SiO2/1Al2O3/190 H2O 15Na2O/4SiO2/1Al2O3/190 H2O
5.0 5.0 0.5 1.0 1.5 2.0 3.0 5.0
a
The synthesis temperature is 60 °C.
Figure 1. XRD patterns of nanosized sodalite (S1) and microsized sodalite (S2), and simulated XRD pattern of sodalite crystals.
molar composition of 29Na2O/4SiO2/1Al2O3/190H2O. The synthesis conditions as well as the initial composition are summarized in Table 1. Figure 1 shows the power XRD patterns of S1 and S2 after heating at 60 °C for 5 h as well as the simulated one of sodalite crystals. The XRD patterns of both products match very well with the simulated one, indicating that pure sodalite crystals are successfully synthesized by the above approach. The diffraction peaks of S1 are less intense and much broader than those of S2, indicating the formation of smaller crystals. The crystallite size is calculated as ∼21 nm from the Scherrer equation. Figure 2 shows the SEM images of S1 before and after washing with 2 M NaOH solution. Sodalite nanocrystals embodied amorphous particles larger than 200 nm are observed in the SEM image of the sample before washing with 2 M NaOH solution, although the XRD pattern of the sample shows clear diffraction peaks assigned to sodalite crystals. This result suggests that the formed sodalite crystals are embedded in the amorphous phase. On the other hand, an SEM image after washing with 2 M NaOH shows distinct features of crystalline particles with a size of 20-40 nm, indicating that the amorphous phase surrounding the crystals is dissolved in the washing process with 2 M NaOH. The weight difference before and after washing with 2 M NaOH solution is ∼30%. The XRD patterns of them (shown in the Supporting Information) suggest that the amorphous phase declines after the washing process. The yield of the final product after washing with 2 M NaOH is 0.57 g zeolite/g SiO2. It was found that the yield of sodalite nanocrystals (∼37 nm) in the presence of organic SDA, TMA+, was 0.48 g zeolite/g SiO2.18 However, the Si/Al ratio in the product was determined as 27.8, which is much higher than that in our products. Although the different Si/Al ratios result in a difficulty to quantitatively compare the yield of these synthesis processes, it is clear that the yield
Figure 2. SEM images of nanosized sodalite crystals (S1) before (a) and after (b) washing with 2 M NaOH.
Figure 3. TEM image of nanosized sodalite crystals (S1). The inset shows the SAED pattern.
of synthesis without using organic SDAs is higher than that using organic SDAs. The TEM image in Figure 3 indicates that the products are 80 nm aggregates of sodalite nanocrystals with crystallite sizes ranging from 20 to 40 nm. DLS measurements also suggest that the average size of the aggregates is 80 nm. The corresponding particle size distribution from DLS measurements shows a good dispersibility of these nanocrystals (Figure 4).
Hydrothermal Synthesis of Nanometer-Sized Sodalite
Langmuir, Vol. 24, No. 13, 2008 6955
Figure 4. Number-based size distribution of the nanosized sodalite (S1) measured using DLS.
Figure 6. 29Si MAS NMR spectra of nanosized sodalite crystals (S1, a) and microsized sodalite crystals (S2, b).
Figure 5. FT-IR spectra of the nanosized sodalite crystals (S1) and microsized sodalite crystals (S2). Table 2. Lattice Constants and 29Si MAS NMR Chemical Shifts of the Hydrosodalites With Different Content of H2O43 lattice constant composition
a
R
chemical shift (ppm)
Na6[SiAlO4]6 Na6[SiAlO4]6xH2O (0 < x < 8) Na6[SiAlO4]68H2O sodalite nanocrystals
9.100 8.965
90° 90°
-90.5 -86.5 and -90.5
8.850
90°
-82.5 -83.0 and -85.5
The S1 and S2 samples were characterized using N2 adsorption/ desorption to determine their pore volume and surface area. It should be noticed that the maximum window in sodalite structure is 6R with a size of 0.28 nm. Therefore, N2 adsorption/desorption is not useful to characterize their internal micropore. The result shows that the Brunauer-Emmett-Teller (BET) surface of S1 is 93.2 m2/g. Compared with S1, S2 exhibits a very low BET surface area (
