Top-down Tuning of Nanosized Zeolites by Bead-milling and ...

1. Introduction

Zeolites are hydrated, crystalline tectoaluminosilicate structures, consisting of TO4/2 tetrahedra where T indi-cates a tetrahedral atom such as Si, Al, P, Zn or Ge. Their structures contain subnanometer-sized and well-ordered void spaces, and some examples of their uses are as catalysts, ion exchangers, adsorbents and molecular-sieving membranes1),2). Zeolites with high external surface area allow for greater diffusion and easier access of reactants via pore mouths to the active sites. This is one of the reasons for the higher catalytic activity of some zeolites, because diffusion through the zeolite pore structure is often the rate-determining step in a catalytic reaction3). Therefore, preparations of zeolites with high external surface area4) by post-synthetic treatment5)～8) and by synthesis of nanosized zeolites9)～12) have been the focus of much attention. In general, the fabrication of nanosized zeolite particles is achieved by a 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 homoge-neous clear solutions containing a large amount of an expensive organic structure-directing agent such as tetramethylammonium hydroxide to promote nucle-ation13),14) or are synthesized in a confined space using inert media15),16), such as starch or a gelling polymer. The use of organic compounds has resulted in many advances but suffers from a number of drawbacks, such as a high production cost, contamination of waste water and air pollution arising from thermal decomposi-tion17),18). Therefore, a new method for synthesizing nano-zeolites without organic structure-directing agents is highly sought after.

This study focuses on a top-down approach19),20) for the fabrication of fine zeolites. Conventional milling methods such as ball- and planetary ball-milling have been utilized to miniaturize zeolites; however, destruc-tion of the zeolite framework causes pore blocking, which reduces their desirable properties21). Therefore, a milder milling method is required to suppress the reduction in crystallinity. Bead-milling in particular prevents damage to the target powder, such as amor-phization and/or the formation of dislocations, by using 30-500 μm diameter beads, as shown in Fig. 1, and has been applied widely in the ceramic powder processing field. A characteristic trait of bead-milling is that it

206 Journal of the Japan Petroleum Institute, 56, (3), 206-213 (2013)

J. Jpn. Petrol. Inst., Vol. 56, No. 4, 2013

＊ To whom correspondence should be addressed. (Present) Dept. of Chemical System Engineering, School of

Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, JAPAN.

＊ E-mail: [email protected]

[Review Paper]

Top-down Tuning of Nanosized Zeolites by Bead-milling and Recrystallization

Toru WAKIHARA＊ and Junichi TATAMI

Graduate School of Environment and Information Sciences, Yokohama National University,79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, JAPAN

(Received April 10, 2013)

The fact that zeolites with high external surface area allow diffusing reactants greater access to active sites, has led to interest in the preparation of nanosized zeolites. The organic template-free synthesis of nanosized zeolite has been a subject of special importance for scientific and industrial applications. Thus far, most research has focused on a bottom-up approach for the fabrication of nano-zeolites, that is, control of zeolite nucleation and crystal growth during hydrothermal synthesis. This review summarizes a new method for the top-down produc-tion of nano-zeolite powder by first milling the zeolite to produce nanoparticles. This technique destroys the outer portion of the zeolite framework thereby decreasing its ion-exchange properties and catalytic activity. To remedy this, the damaged portions were recrystallized using a dilute aluminosilicate or silicate solution after bead-milling. The combined bead-milling and post-milling recrystallization yielded nano-zeolites with high crystallinity. The nanosized powders showed higher ion-exchange properties for zeolite A and higher catalytic activity for ZSM-5, respectively.

KeywordsBead-milling, Recrystallization, Zeolite, Hydrothermal treatment, Nanoparticle, Amorphization

can be used to prepare large-scale amounts of fine zeo-lite without the use of a specific organic compound to control zeol i te nucleat ion and crysta l growth. Previous studies have reported that pulverized zeolites provide improved catalytic properties22)～24); however, it is still difficult to prevent damage to the zeolite struc-ture even if bead-milling is used. In this study, there-fore, the damaged portion has been recrystallized after bead-milling using aluminosilicate or silicate solution as shown in Scheme 12).

2. Miniaturization of Zeolites by Bead-milling

In this paper, we focus on the miniaturization of zeo-lite A25) and ZSM-526),27). Commercial zeolite A (4A, LTA type zeolite, Si/Al＝1.0, cation: Na＋, Tosoh Corp., Japan) and ZSM-5 (MFI type zeolite, Si/Al＝19.7, cat-ion: NH4

＋, 840NHA, Tosoh Corp., Japan) were used as starting materials. These zeolites were milled using a bead-milling apparatus (Minicer, Ashizawa Finetech Ltd., Japan). Typically, 60 g of zeolite A or ZSM-5 were 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 and then 360 min using 300 and 100 μm diam-eter zirconia beads, respectively. An agitation speed of 3000 rpm was used to shear and exert a 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 %. The phases present, and the product morphol-ogies were identified by conventional X-ray diffracto-metry (XRD; Multiflex, Rigaku Corp., Japan) and field emission scanning electron microscopy (FE-SEM; S-5200, Hitachi, Ltd., Japan).

Typical field emission scanning electron microscope (FE-SEM) images of the samples are shown in Fig. 2. The raw zeolites have smooth morphological features. After bead-milling, the zeolite A and ZSM-5 morpholo-gies have changed considerably. Both raw zeolites, with average size of 3.5 and 3.0 μm for zeolite A and ZSM-5, respectively, formed agglomerates composed of tiny particles approximately 50-200 nm in size and with an average size of 100 nm after bead-milling. XRD spectra of the samples are shown in Fig. 3. The sample crystallinity was estimated from the XRD peak areas (also shown in Fig. 3). The diffraction peaks assigned to the LTA and MFI structures in the bead- milled samples show that the sample crystallinity per-

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Fig. 1 Schematic Drawing of Bead-milling Apparatus

Scheme 1 Combination of Bead-milling and Recrystallization

Fig. 2● FE-SEM Micrographs of (a) Raw Zeolite A, (b) Zeolite A after Bead-milling for 480 min, (c) Raw ZSM-5 and (d) ZSM-5 after Bead-milling for 480 min

All Bragg peaks are caused by zeolite A for (a) and ZSM-5 for (b).Crystallinities of the milled samples are estimated from XRD peak areas (also shown).

Fig. 3● XRD Spectra of (a) Rraw and Bead-milled Zeolite A and (b) Raw and Bead-milled ZSM-5

sisted but the peak intensities decreased, indicating a decrease in crystallinity. The relative percentage crys-tallinity of zeolite A and ZSM-5 varied from the origi-nal zeolite set at 100 % to the milled crystallinity at 9 % and 10 %, respectively. These results suggest that it is possible to decrease zeolite particle size to below 100 nm by bead-milling; however, destruction of the outer zeolite framework is unavoidable, and this reduces the desirable properties of the zeolite. In the follow-ing section, the zeolite amorphization mechanism is discussed in terms of the pair distribution function anal-ysis.

3. Zeolite Amorphization Mechanism

Thus far, changes in the characteristics of different zeolites (such as Y, X, A, ZSM-5 and mordenite) when subjected to ball milling have been investigated exten-sively28)～31). However, because of the difficulty in understanding their amorphous structure, atomic scale changes during mechanical amorphization have not yet been well clarified. Diffraction methods (either X-ray or neutron) are commonly used to assess the atomic arrangement of disordered materials32),33). Changes in the atomic arrangement of amorphous materials have been characterized by pair distribution function analysis. Wakihara et al. reported that the structure of amorphous precursor species formed under hydro-thermal conditions, prior to the onset of microporous aluminosilicate zeolite crystallization, can be deter-mined by high-energy X-ray diffraction (HEXRD)34). However, the mechanical amorphization of zeolites has not yet been reported on. Here we focus on the amor-phization of zeolite A by bead-milling and changes in the atomic arrangement during mechanical amorphiza-tion are investigated using HEXRD35).

HEXRD data were obtained on a horizontal two-axis diffractometer, dedicated for glass liquid and amor-phous materials, built at the BL04B2 high-energy X-ray diffraction of SPring-8. A bent crystal mounted on the monochromator stage fixed at a Bragg angle of 3° in the horizontal plane, provides an incident photon energy of 61.63 keV (wavelength λ: 0.2012 Å, 1 Å＝10–10 m) using a Si (220) crystal. Pelletized samples were fixed to the sample stage. The maximum Q (Q＝4πsinθ/λ), Qmax, collected in this study was 25 Å–1. The collected data were subjected to well-established analytical proce-dures including absorption, background and Compton scattering corrections followed by normalization to the Faber-Ziman total structure factor, S(Q)36),37). The pair distribution function, G(r), is derived according to pre-vious studies34),35).

Figure 4 shows the pair distribution function, G(r), of the raw and bead-milled zeolite A (see Fig. 3(a)). From the G(r) curves, it is possible to identify various distances associated with several features34),38). The

first peak in the G(r) is related to Si_O (ca. 1.61 Å) and Al_O (1.71 Å) distances, although the Q range obtained here is insufficient to resolve the two distances. Peaks at 2.3-2.4, 2.6-2.7 and 3.0-3.3 Å are related to Na_O, O_O and Si_Al distances. Note that as the Al_O_Al bond is prohibited in this zeolite framework, and the Si/Al ratio of zeolite A is 1, the peak at 3.1-3.3 Å is only related to Si_Al distance34). In the G(r) of raw zeolite A, peaks are visible at 3.7-4.0 and 4.2-4.6 Å, which correspond mainly to the distances from the T atom (T＝Si or Al) to the second oxygen (2nd T_O). The peaks at 3.7-4.0 Å in the raw zeolite, in particular, result mainly from the 2nd T_O distances in the four-membered rings (4R). In a similar way, the peaks at 4.35 Å result mainly from the 2nd T_O in 6R and 8R34). It should be noted that in the milled zeolite, Si_Al (3.0-3.3 Å) shifts to a shorter distance while T atom to the first oxygen (1.65 Å) and first Na_O and O_O do not. This result implies that the milling power is insufficient to shorten the nearest T_O bonds, and the average Si_O_Al angles are decreased by bead-milling. Distances in T atom to the second oxygen (3.7-4.0 and 4.2-4.6 Å) also shift to shorter distances. This result shows that the intermediate-range order has changed to form a denser amorphous aluminosilicate network by compressive stress through milling. It is deduced that the inter-structural space in the porous and low density zeolite (compared with other aluminosilicate materials) has to be destroyed to form a denser structure. The crystal-line atomic position can be moved more easily (without breaking Si_O_Al bonds) to produce an amorphous X-ray product. Sato et al. reported that mechanically amorphized zeolite is easily transformed back to the original zeolite structure by the dry gel conversion tech-nique39). They suggested that the minimum movement in atomic arrangement to form an X-ray amorphous material is thought to be the main mechanism for mill-ing amorphization in zeolites, indicating that it must be easy to recrystallize amorphized zeolite back to the crystalline state by post-milling treatment. In the fol-

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Fig. 4● Pair Distribution Function, G(r)s (1-5.5 Å), of Raw and Bead-milled Zeolite A

lowing section, we propose a new method for the fabri-cation of nanosized zeolite by a combination of milling and post-milling recrystallization.

4. Recrystallization of Amorphized Zeolite A

Recrystallization of milled zeolite A was performed using dilute aluminosilicate solution with composition 405 Na2O : 1 Al2O3 : 51 SiO2 : 29900 H2O to provide a solution that is nearly in equilibrium with zeolite A (almost the same composition as the supernatant liquid after hydrothermal synthesis of zeolite A). This means that zeolite A is in neither macroscopic growth nor dis-solution mode40). Under these conditions, the poorly crystalline portions of the milled zeolite A are more easily dissolved than the more crystalline parts. The former portions therefore tend to recrystallize back onto zeolite A providing a more ordered product. This is the key idea in this study25),41). First, 100 mL of aque-ous solution was heated to 363 K in an oil bath. Milled zeolite A (3 g) was then added to the heated solution with stirring. After 60 min, the slurry was centrifuged and the supernatant liquid decanted. The residual solid was washed with distilled water several times. XRD spectra of raw and recrystallized zeolite A are shown in Fig. 5. After recrystallization, the crystallinity returns close to its original level (crystal-linity: 98 %). It is noteworthy that there was almost no loss in mass during recrystallization with the recrys-tallized product having 97 % of the milled starting ma-terial mass. It appears that selective recrystallization

occurs at the poorly crystalline parts of the milled zeo-lite A, where damage has been caused by bead-milling. The Si/Al ratio of the samples measured by X-ray fluo-rescence spectrometry (XRF) shows no change, that is Si/Al＝1; indicating that the material balance was maintained during milling and recrystallization. FE-SEM images of the sample after post-milling recrystal-lization are shown in Fig. 6. The product has sharply-defined nanoparticles approximately 30-100 nm wide. The average particle size after post-milling recrystalli-zation was estimated to be 40 nm from the FE-SEM image. It appears that a large number of crystallites were formed by bead-milling and each of them grew uniformly during post-milling recrystallization, result-ing in nanoparticles with high crystallinity.

The ion-exchange properties of nanosized zeolite A were investigated using recrystallized zeolite after bead-milling for 120 min. Raw and nanosized zeolite A are in the Na form, and the ion exchange property from Na＋ to Ca2＋ was investigated by a conventional method. Zeoli te (1 g) was added to 100 mL of 0.035 mol/L calcium nitrate solution and stirred for a prescribed period at 293 K. The slurry was centri-fuged and the supernatant liquid decanted. The ion exchange zeolites thus prepared were filtered, washed thoroughly and dried before the Ca/Na sample ratios were measured using XRF. Note that no changes were observed in the Si/Al ratios throughout the treatments; indicating that the sample compositions are 0.5x Ca : (1－x) Na : 1.0 Si : 1.0 Al : 4.0 O, where x＝0-1. Figure 7 shows the Ca/Na ratio of raw zeolite, milled and recrystallized samples along with the ion exchange period. The time taken to reach the equilibrium state is shortened significantly in the samples after bead-milling, indicating that the nano-sized zeolites allow for the greater diffusion of ions and molecules and their easier access to internal pore sites. The Ca/Na ratio of the saturated value after bead-milling, however, is lower than that of raw zeolite A. It is assumed that the amor-phous portion formed by bead-milling has less or no contribution to the ion exchange. On the other hand, the Ca/Na ratio of zeolite after recrystallization showed the highest value, that is, the ion exchange property was

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All Bragg peaks are caused by zeolite A. Recrystallized sample crystallinity is estimated from XRD peak areas (also shown).

Fig. 5● XRD Spectra of (a) Raw and (b) Recrystallized Zeolite A

Fig. 6● FE-SEM Micrographs of Recrystallized Zeolite A at (a) Low and (b) High Magnification

improved from the combination of bead-milling and re-crystallization.

5. Recrystallization of Amorphized ZSM-5

Recrystallization of the milled ZSM-5 was performed using a dilute silicate solution of composition 0.0525 Na2O : 0.117 SiO2 : 10.0 H2O27). Amorphous silica (Reolosil, Tokuyama Corp., Japan) and sodium hydrox-ide (Wako Pure Chemical Industries, Ltd., Japan) were used. As with zeolite A, the importance of this partic-ular ratio is that it provides a solution nearly in equilib-rium with ZSM-5. Under these conditions, the remaining ZSM-5 crystallites act as seeds and the poorly crystalline portions of the milled ZSM-5 are more easily recrystallized back onto the ZSM-5, resulting in a more ordered product. The aqueous solution was preheated to 363 K using an oil bath before 0.6 g of milled zeolite were added to 6 mL of aqueous solution and heated at 453 K in an autoclave under autogenous pressure. The ZSM-5 after post-milling recrystallization was obtained in Na-exchanged form, and was ion-exchanged to the NH4

＋ form prior to the investigation of its catalytic per-formance6). Cumene cracking into benzene and pro-pylene was conducted in a pulse-injection flow reactor

connected to an on-line thermal conductivity detector (TCD) gas chromatograph (GC-8A, Shimadzu Corp., Japan). The zeolite sample (10 mg) was pretreated at 673 K for 1 h to remove NH3 and obtain H-ZSM-5. Then, the catalyst bed temperature was maintained at 523 K, and 1 μL of cumene was injected into the He carrier gas (25 cm3/min). The benzene yield was used as an estimate of apparent catalytic activity of each sample . Ni t rogen adsorpt ion and desorpt ion (Autosorb-1, Quantachrome, USA) were performed at liquid nitrogen temperature (77 K). The sample was degassed in vacuum at 573 K prior to measurement. The total surface area was calculated according to the Brunauer-Emmett-Teller (BET) isothermal equation42). The external surface area was evaluated by the t-plot method42). The sample was degassed in vacuum at 473 K for 5 h prior to the adsorption measurements.

XRD spectra of the sample before and after post-milling recrystallization are shown in Fig. 8. As with zeolite A, the crystallinity returned close to its original level (crystallinity: 95 %) after recrystallization. It is noteworthy that there was almost no mass loss during recrystallization with the recrystallized product having 94 % o f t h e m i l l e d s t a r t i n g m a t e r i a l m a s s . Furthermore, no additional phases were present after re-crystallization, indicating that the remaining ZSM-5 was sufficiently crystalline to act as a seed during re-crystallization. The Si/Al ratio, BET surface area, external surface area as evaluated by the t-plot method and the benzene yield of samples for cumene cracking are summarized in Table 1. Note that the external surface area of raw ZSM-5 is an estimated value based

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Fig. 7● (a) Ca/Na Ratio vs. Ion Exchange Period of Raw and Milled Zeolite A, (b) Ca/Na Ratio vs. Ion Exchange Period of Raw, Milled (120 min) and Recrystallized Zeolite A

All Bragg peaks caused by ZSM-5. Recrystallized sample crystal-linity estimated from XRD peak area (also shown).

Fig. 8● XRD Spectra of (a) Raw and (b) Recrystallized ZSM-5

on the particle size observed by FE-SEM. No signifi-cant changes were observed in the Si/Al ratios, indicat-ing that the material balance was maintained during milling and recrystallization. FE-SEM images of the sample after post-milling recrystallization are shown in Fig. 9. After bead-milling, the ZSM-5 morphology changed considerably. The raw zeolite, with average size of 3.0 μm (see Fig. 2), formed sharply-defined nanoparticles approximately 30-100 nm wide. The average particle size after recrystallization was estimated to be 60 nm from the FE-SEM image. It is noteworthy that the external surface area calculated from the N2 adsorption isotherm was 69 m2/g for the recrystallized sample as shown in Table 1. This high external sur-face area is supported by the fact that nanoparticles are formed through milling and recrystallization as shown in Fig. 9.

The benzene yields of the first pulses are also sum-marized in Table 1 as a measure of catalytic activity. Note that the test reactions were conducted so that the rate-determining step of the catalytic reaction is from diffusion through the zeolite pore structure and reaction at the active sites6). Under these conditions, the ben-zene yield of raw ZSM-5 was 70.2 %. On the other hand, the catalytic activity was improved significantly in the samples obtained from post-milling recrystalliza-tion (raw ZSM-5 (70.2 %)→ recrystallized ZSM-5 (95.6 %)). It can be concluded that the difference in benzene yield results from the difference in average particle size. In this study, by a combination of bead-milling and post-milling recrystallization treatments, we showed that the nanosized zeolite has a much higher catalytic activity than that of raw ZSM-5.

6. Conclusions

We demonstrated a new top-down approach for the preparation of nano-zeolite A and ZSM-5 using a com-bination of bead-milling and post-milling recrystalliza-tion. The combined bead-milling and post-milling recrystallization yielded nano-zeolite A and ZSM-5 approximately 40 and 60 nm in size with high crystal-linity, respectively. The nanosized zeolite can be ob-tained almost without loss of material through bead-milling and recrystallization. The effects of particle size and crystallinity on ion exchange and catalytic properties were investigated for zeolite A and ZSM-5. An improvement in the establishment of an equilibrium state and Ca/Na ratio of the saturated value was con-firmed in the nanosized zeolite A. Also, nanosized ZSM-5 powder yielded a larger amount of benzene compared with raw zeolite when cumene was cracked into benzene and propylene. The characteristic point of this method is that it is suitable for the large-scale production of nanosized zeolite, since specific organic compounds are not required to control zeolite nucle-ation and crystal growth. It is also possible to tune the average particle size by controlling the bead-milling period. From our investigations, this method is appli-cable to aluminosilicate zeolites with Si/Al＜20, and nanosized zeolites would be good candidates for several novel applications, e.g., catalyst, ion-exchanger, adsor-bent and seeds of thin films. Further studies are ongoing toward practical applications for the large-scale production of nano-zeolites.

AcknowledgmentThe authors would like to thank to Professors Y.

Kubota and S. Inagaki from Yokohama National University for their fruitful discussions. The authors would also like to thank Professors T. Tatsumi and T. Yokoi from the Tokyo Institute of Technology for the FE-SEM measurements. This work has been supported par t ia l ly by a “Grant for Advanced Indust r ia l Technology Development” in 2011 from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

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Table 1● Si/Al Ratio, BET Surface Area, External Surface Area as Evaluated by t-Plot Method and Benzene Yield of Samples for Cumene Cracking

Si/Al ratio BET surface area, SBET [m2/g] External surface area [m2/g] Benzene yield [%]

Raw 19.7 443 1 70.2Recrystallized 20.9 497 69 95.6

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Fig. 9● FE-SEM Micrographs of Recrystallized ZSM-5 at (a) Low and (b) High Magnification

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要　　　旨

粉砕および再結晶化法を組み合わせた微細ゼオライトの新規製造法

脇原　　徹†1)，多々見　純一

横浜国立大学大学院環境情報研究院，240-8501 横浜市保土ヶ谷区常盤台79-7†1)（現在）東京大学大学院工学系研究科，113-8656 東京都文京区本郷7-3-1

ミクロンサイズのゼオライト粒子に比べ，ナノゼオライトは外表面積が大きく，細孔内拡散が促進されることによって，イオン交換特性や吸着特性，触媒反応特性などが向上することが知られている。現在，各種特性の向上を目的としたゼオライトナノ粒子合成に関する研究が盛んに行われている。既往のゼオライトナノ粒子合成に関する研究の多くはボトムアップ法，すなわち4級アンモニウム塩や特殊な有機物を用い，核発生・結晶成長を制御することにより達成されている。しかし，コスト的な制約からゼオライト合成時に有機物を使用しない新規ナノゼオライト製造プロセスの確立が望まれている。そこで著者らはボトムアップ法に代わるナノゼオライトの調製法として，ゼ

オライト表面の非晶質化を最低限に抑えた粉砕が可能であるビーズミルを用いた，トップダウン手法による微細化に注目した。具体的には，ビーズミル粉砕処理したゼオライトにポスト処理（再結晶化処理）を施したところ，結晶性の高いナノゼオライトが調製できることを明らかにした。また，A型ゼオライトに関してはイオン交換特性，ZSM-5型ゼオライトに関しては触媒特性の向上を確認した。本稿で紹介した技術は，粉砕法と後処理を組み合わせることで高結晶性ナノゼオライトを調製することを可能とするものであり，有機テンプレートを使わない，大量生産に適したナノゼオライト作製プロセスとして有望であると考えている。

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