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Bead-Milling and Postmilling Recrystallization: An Organic Template-free Methodology for the Production of Nano-zeolites Toru Wakihara,*,† Ryuma Ichikawa,† Junichi Tatami,† Akira Endo,‡ Kaname Yoshida,§ Yukichi Sasaki,§ Katsutoshi Komeya,† and Takeshi Meguro† †
Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan ‡ National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba Central 5, Tsukuba, Ibaraki 305-8565, Japan § Research and Development Laboratory, Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan
bS Supporting Information ABSTRACT: Organic template-free synthesis of nano-zeolite has been an important subject of both scientific and industrial applications. Most research has focused on the fabrication of nano-zeolite by a bottom up approach, that is, control of zeolite nucleation and crystal growth during the hydrothermal synthesis. This communication reports a new method for the production of nano-zeolite powder by a top-down approach. In this study, the zeolite was first milled to produce a nanopowder. This technique can cause destruction of the outer portion of the zeolite framework and hence cause pore blocking, which deactivates various properties of the zeolite. To remedy this, the damaged part was recrystallized using a dilute aluminosilicate solution after bead milling. As a result of the combination of bead milling and postmilling recrystallization, nano-zeolite A (LTA type zeolite) about 50 nm in size with high crystallinity was successfully obtained.
The study presented here focuses on a top-down approach12,13 for the fabrication of nano-zeolites. Conventional milling methods such as ball-milling and planetary ball-milling do downsize zeolites; however, destruction of the outer zeolite framework causes pore blocking, and this impedes the desirable properties of the zeolite.14 Therefore, a milder milling method is required to prevent the degradation of crystallinity. Bead milling, in particular, suppresses damage to the target powder, e.g. amorphization and/or formation of dislocations, by the use of small beads 30500 μm in diameter (see Supporting Information). Previous studies have reported that zeolites that have been properly pulverized give better catalytic properties.1518 However, it is still difficult to prevent damage to the zeolite structure even if bead milling is used. In this study, therefore, the damaged part was recrystallized after bead milling treatment using a dilute aluminosilicate solution as shown in Scheme 1. This resulted in nano-zeolite A with an average size of about 50 nm, with high crystallinity, without using an organic template. Commercial zeolite A (4A, LTA type zeolite, Si/Al = 1.0, cation: Naþ, Tosoh Co., Japan) was used in this study. The zeolite A was milled using a bead milling apparatus (Minicer,
Z
eolites are hydrated, crystalline tectoaluminosilicate structures, formed from TO4 tetrahedra, where T indicates a tetrahedral atom, e.g. Si, Al, P, Zn, Ge, etc. Their intricate pore and channel system at sizes similar to simple molecules is the source of their immense importance in catalysis, adsorption, and ion-exchange.1 There are massive ongoing efforts into the synthesis of nano-zeolites for use in these fields,25 since zeolites with nanometer size particles allow for greater diffusion of ions and molecules and allow them easier access to the internal pore sites due to their high external surface area. In general, the fabrication of nano-zeolites is achieved by the bottom-up approach, which involves controlling zeolite nucleation and crystal growth during the hydrothermal synthesis. For example, most zeolite A (LTA type structure) nanocrystals with an average size less than 100 nm are synthesized from homogeneous clear solutions containing a large amount of an expensive organic template such as tetramethylammonium hydroxide to promote nucleation6,7 or are synthesized in a confined space using inert media,8,9 such as starch or gelling polymer. The use of organic compounds has resulted in many advances but suffers from a number of drawbacks, such as high production cost, contamination of wastewater, and air pollution arising from thermal decomposition.10,11 Therefore, a new method of synthesizing nano-zeolite without organic templates is strongly desired. r 2011 American Chemical Society
Received: February 1, 2011 Revised: March 8, 2011 Published: March 08, 2011 955
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Scheme 1. Schematic Illustration Describing the Fabrication Process of Nano-zeolite by Bead Milling and Postmilling Recrystallization
Figure 2. FE-SEM micrographs of the samples (a) zeolite A, (b) zeolite A after bead milling, and (c and d) zeolite A after recrystallization.
washed with distilled water several times. The phases present and the morphology of the products were identified by conventional X-ray diffractometry (XRD, Multiflex, Rigaku, Tokyo, Japan), field emission scanning electron microscopy (FE-SEM, S-5200, Hitachi, Tokyo, Japan), and field emission transmission electron microscopy (FE-TEM, 2100F, JEOL Tokyo, Japan, and H-9500, Hitachi Tokyo, Japan). H2O adsorption and desorption measurements (Belsorp-Aqua, BEL Japan, Inc.) were carried out at 273 K to investigate the microporous structure of the samples. N2 adsorption and desorption measurements (Belsorp-mini, BEL Japan, Inc.) were also performed to evaluate the external surface area of the milled and recrystallized samples. The external surface area was calculated from the N2 adsorption isotherms using the BrunauerEmmetTeller theory. The sample was degassed in vacuum at 473 K for 5 h prior to the adsorption measurements. XRD patterns of the samples are shown in Figure 1. The crystallinities of the samples were estimated from the XRD peak areas, which are also shown in Figure 1 (additional information given in the Supporting Information). The diffraction peaks assigned to an LTA structure in the bead milled samples showed the crystallinity of the sample persisted but the peak intensities decreased, indicating a decrease in crystallinity. The relative percentage crystallinity of the samples varied from the original zeolite set at 100% to the milled at 9% but returned almost to the original levels after recrystallization at 98%. There was no loss of mass during the recrystallization treatment, with the recrystallized product having 101% the mass of the milled starting material. It appears that the selective dissolution and recrystallization occurs at the poorly crystalline parts of the milled zeolite A, where damage has been caused by bead milling. This increases the fraction of crystalline zeolite after the recrystallization, as shown in Figure 1. Furthermore, no additional phases are present after the recrystallization, indicating that remaining zeolite A was sufficiently crystalline to act as a seed during the recrystallization. The framework connectivity was also improved in the recrystallized zeolite A, as confirmed by Raman spectroscopy (Supporting Information). Typical FE-SEM images of the samples are shown in Figure 2. The raw zeolite has smooth cubic morphological features. After bead milling the zeolite A morphology has changed dramatically.
Figure 1. XRD patterns of the products: (a) zeolite A, (b) zeolite A after bead milling, and (c) zeolite A after recrystallization. All Bragg peaks seen in the samples are due to zeolite A. The patterns in parts b and c are offset vertically by 2700 and 3300, respectively, for clarity. The sample crystallinity as estimated from the XRD peak areas is also shown.
Ashizawa Finetech Ltd., Tokyo, Japan). 60 g of zeolite A was dispersed in 350 mL of distilled water or ethanol using an ultrasonic vibrator (VCX 600, Sonic & Materials Inc., USA), and the slurry was pulverized for 120 min using zirconia beads 300 μm in diameter followed by an additional milling for 360 min using zirconia beads 100 μm in diameter. An agitation speed of 3000 rpm was used to shear and exert force on the zeolite agglomerates. After milling, the slurries were dried overnight in an oven at 373 K. The recovery rate of the zeolite powder after bead milling was nearly 100%. Recrystallization of the milled zeolite A was performed using dilute aluminosilicate solution with the composition of 405 Na2O/1 Al2O3/51 SiO2/29900 H2O. The importance of this particular ratio is that it gives a solution nearly in equilibrium with zeolite A. This means that zeolite A is in neither macroscopic growth nor dissolution mode.19,20 Under these conditions, the poorly crystalline parts of the milled zeolite A are more easily dissolved than the more crystalline parts and tend to be recrystallized back onto the zeolite A, giving a more ordered product. First, 100 mL of the aqueous solution was heated to 363 K using an oil bath. 3 g of milled zeolite was then added to the heated solution with stirring. After a period of 60 min, the slurry was centrifuged and the supernatant liquid was decanted off. The residual solid was 956
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data shown in Figure 1b. On the other hand, the zeolite nanoparticles were highly crystalline after the postmilling recrystallization (Figure 3bd). It appears that a large number of crystals were formed by bead milling and each of them grew uniformly during the postmilling recrystallization, resulting in nanoparticles with high crystallinity. H2O adsorption and desorption isotherms of the samples are shown in Figure 4. All are the typical IUPAC type I isotherms, corresponding to the adsorption of water molecules on hydrophilic micropores of the zeolite A. The amount of adsorbed H2O on the zeolite A was significantly lowered by the bead milling treatment for 480 min, suggesting the microporous structure was destroyed, resulting in nonporous amorphous material, which has little or no contribution of the H2O adsorption. Furthermore, a hysteresis loop was confirmed in the milled sample, indicating the strong aggregation of particles. After recrystallization, the nonporous, amorphous material was replaced with nano-zeolite particles with high capacity for H2O adsorption, equivalent to the original zeolite A. The external surface area calculated from N2 adsorption and desorption isotherms (see Supporting Information) was 50.4 m2/g for the milled sample and 58.0 m2/g for the recrystallized sample. This increase of the external surface area agreed well with the moderate increase of H2O adsorption isotherms at the middle P/P0 region. In this study, nano-zeolite A, about 50 nm in size, with high crystallinity, was obtained by a combination of bead milling and recrystallization. Although the particle size distribution of the products is relatively broad, the present method is suitable for the large-scale production of nano-zeolite, since the aluminosilicate solution for recrystallization is reusable and specific organic compounds are not needed to control zeolite nucleation and crystal growth. Further, nano-zeolite can be obtained without loss of material through bead milling and recrystallization treatments; that is, the yield of the zeolite nanocrystals by this method is almost 100%, hence making this method superior to desilication methods.2124 As far as we investigated, this method, a kind of seeded growth,25,26 is applicable to aluminosilicate zeolites with Si/Al < 20, and this nanosized zeolite would be a good candidate for several novel applications, e.g. ion-exchanger, adsorbent, and seeds of thin films. Further studies on the optimization of the conditions for bead milling and recrystallization of other zeolites will be published elsewhere.
Figure 3. TEM images of zeolite A after milling and postmilling recrystallization: (a) zeolite A after bead milling; (bd) zeolite A after postmilling recrystallization.
Figure 4. H2O adsorptiondesorption isotherms of (a) zeolite A, (b) zeolite A after bead milling, and (c) zeolite A after postmilling recrystallization. In the figure, external (Ext.) surface areas of the samples as evaluated from N2 adsorption isotherms are also shown. Note that the external surface area of the original zeolite A is an estimated value based on the particle size (closed symbols, adsorption; open symbols, desorption).
The raw zeolite, with an average size of 3.5 μm, has become agglomerates composed of tiny particles about 50200 nm and an average size of 100 nm after bead milling treatment. After recrystallization they have become sharply defined nanoparticles about 3060 nm across. The average particle size after recrystallization treatment was estimated to be 45 and 55 nm from the XRD peaks (see Supporting Information) and FE-SEM images, respectively. TEM images of milled and recrystallized samples are shown in Figure 3. As shown in Figure 3a, very little crystalline material was observed in the milled zeolite, with most of the sample appearing to be amorphous, as was seen in the XRD
’ ASSOCIATED CONTENT
bS
Supporting Information. Schematic drawing of bead milling, calculation conditions of average particle size from XRD peaks, results of Raman spectroscopy, and N2 adsorption/desorption isotherms. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: þ81-45-339-3957. Fax: þ81-45-339-3957. E-mail:
[email protected].
’ ACKNOWLEDGMENT We would like to thank Prof. T. Tatsumi and T. Yokoi at the Tokyo Institute of Technology for FE-SEM measurements. 957
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