Characterization of Amorphized Zeolite A by Combining High-Energy

Nov 6, 2012 - ... and High-Resolution Transmission Electron. Microscopy. Kaku Sato,. †. Toru Wakihara,*. ,†. Shinji Kohara,. ‡. Koji Ohara,. ‡...
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Characterization of Amorphized Zeolite A by Combining HighEnergy X‑ray Diffraction and High-Resolution Transmission Electron Microscopy Kaku Sato,† Toru Wakihara,*,† Shinji Kohara,‡ Koji Ohara,‡ Junichi Tatami,† Akira Endo,§ Satoshi Inagaki,∥ Izuru Kawamura,∥ Akira Naito,∥ and Yoshihiro Kubota∥ †

Graduate School of Environmental and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogayaku, Yokohama 240-8501, Japan ‡ Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan § National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba Central 5, Tsukuba, Ibaraki 305-8565, Japan ∥ Division of Materials Science and Chemical Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan S Supporting Information *

ABSTRACT: Zeolites are often used under mechanical stress, and it is therefore useful to study their mechanical properties for strength design in a wide range of applications. We characterized zeolite A by combining high-energy X-ray diffraction and high-resolution transmission electron microscopy techniques to investigate changes in its atomic arrangement during mechanical amorphization. A decrease in the T− O−T angle and distortion of the zeolite A lattice framework by compressive stress were confirmed using these techniques. We showed for the first time that at least one of the zeolite amorphization processes is caused by distortion of the framework structure by a compressive stress to form a higher density noncrystalline material.

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INTRODUCTION Zeolites are periodic, microporous materials usually composed of aluminosilicate tetrahedral frameworks. They have channel and cage dimensions ranging between 0.2 and 2.0 nm1 and are widely applied in the catalysis, ion-exchange, and gas-separation industries. Zeolites are also used under mechanical stress, and it is therefore useful to study their mechanical properties for strength design in a wide range of applications. For example, heterogeneous catalysts are exposed to strong compression and shear stresses through pelletizing and/or extrusion during the formation of the final catalyst pellet.2 Smaller zeolite crystals are preferred in some applications, for example, ion exchange, adsorption, and catalysis, since higher catalytic effectiveness, lower coke formation, faster diffusion, easier cation exchange, and template extraction may be possible where smaller-sized zeolites are used.3 For this reason, the zeolite crystal size is tuned by milling as a means of enhancing zeolite properties.4 However, different from typical ceramic powders, such as alumina and silicon nitride, zeolite structures are known to be amorphized (exhibit an X-ray amorphous state) without forming dislocations. Therefore, understanding the amorphiza© 2012 American Chemical Society

tion of zeolites by mechanical stresses in particular is critical in evaluating the role of atomic-scale structures in applications where zeolites are used. Changes in the characteristics of different zeolites, such as Y, X, A, ZSM-5, and mordenite when subjected to ball milling have been investigated extensively.5−9 However, because of the difficulty in understanding 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 materials.10,11 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 hydrothermal conditions, prior to the onset of crystallization of microporous aluminosilicate zeolites, can be determined by high-energy X-ray diffraction (HEXRD).12 Additionally, highReceived: June 1, 2012 Revised: November 4, 2012 Published: November 6, 2012 25293

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transmission electron microscopy (FE-TEM, 2100F, JEOL Tokyo, Japan). The crystallinity of zeolite A was estimated according to the following equation:

resolution transmission electron microscopy (HRTEM) can provide direct information on the interfacial structures between zeolites and noncrystalline materials. Mintova et al., for example, reported direct evidence by HRTEM regarding the nucleation of zeolites A and Y in nanoscale amorphous aluminosilicate gel particles, followed by full conversion of the gel aggregates into nanocrystalline zeolite.13,14 A combination of HEXRD and HRTEM appears to provide detailed information on the amorphization of zeolite; however, to the best of our knowledge, the combined use of these techniques has not been reported to date. Zeolite A, one of the most representative zeolites, is the focus of this study. Figure 1 shows the structure of zeolite A obtained

α (%) =

peak area 2θ 20 − 30°for the product × 100 peak area 2θ 20 − 30°for as‐received (1)

Samples were also characterized by Raman spectroscopy (Renishaw invia). The wavelength of the laser was 532 nm, and the laser power was approximately 5−10 mW. The 27Al MAS NMR spectra were obtained at 14.0 T using a Bruker AVANCEIII 600 (Rheinstetten, Germany) at a frequency of 156.4 MHz. A high-speed MAS probe was used at 13.0 kHz with 1024 pulses and a recycle time of 0.5 s. The 27Al chemical shifts were measured with respect to a 1 M aqueous Al(NO3)3 solution. Multiple quantum (MQ) MAS NMR was also measured to clarify the effect of quadrupolar broadening, the results of which are given in the Supporting Information. Water adsorption−desorption isotherms (Belsorp-aqua, BEL Japan, Inc.) were measured at 298 K to investigate the microporous structure of the samples. The samples were degassed at 473 K for 5 h in vacuum before adsorption measurements. For HEXRD analysis, 250 mg of the sample was pressed into a disk, and the measurements were carried out at room temperature. HEXRD spectra were obtained on a horizontal two-axis diffractometer, dedicated for glass liquid and amorphous materials, built at the BL04B2 high-energy X-ray diffraction beamline 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) using a Si(220) crystal. Pelletized samples were fixed to the sample stage. The maximum Q (Q = (4πsin θ)/λ), Qmax, collected in this study was 25 Å−1. The collected data were subjected to well-established analysis procedures including absorption, background, and Compton scattering corrections followed by normalization to the Faber− Ziman total structure factor, S(Q)23,24 (see the Supporting Information). The pair distribution function, G(r), is derived from eq 2:

Figure 1. (a) Crystal structure of zeolite A. (b) Typical ring structure shape present in the three-dimensional configurations.

from a single crystal structure analysis. The zeolite A framework consists of four- and six-membered rings (4R and 6R) assembled in three dimensions to produce larger units such as eight-membered rings (8R), double four-membered rings (D4R), and sodalite cages. These basic units exist in many zeolite structures such as faujasite and sodalite. Greaves and others performed seminal contributions on zeolite A amorphization as well as other type of zeolites.15−18 Also, there are several contributions on the pressure-induced amorphization of zeolite A.19−22 However, the mechanical amorphization of zeolites has not yet been reported on. This study focuses on the amorphization of zeolite A by milling, and changes in the atomic arrangement during mechanical amorphization are investigated using HEXRD and HRTEM as well as several structural analyses.

G(r ) = 4πr[ρ(r ) − ρ0 ] =

2 π

∫Q

Q max min

{Q [S(Q ) − 1]sin(Qr )}dQ

(2)

where ρ(r) is the microscopic pair density and ρ0 is the average number density (0.0593505/Å3). Reverse Monte Carlo (RMC) simulation25 was also carried out for the milled sample to extract the 3-dimensional atomic configuration using the HEXRD data. Note that the milled sample is a mixture of crystalline and noncrystalline materials but an almost totally amorphized material; therefore, RMC simulation was applied. In the RMC simulation technique, the atoms of an initial configuration are moved so as to minimize the deviation from experimental structural data using a standard Metropolis Monte Carlo algorithm.26 The starting configurations were generated using hard sphere Monte Carlo simulations with constraints applied to avoid physically unrealistic structure. There are two kinds of constraint: closest atom−atom approach and connectivity. The closest distances of Si−Si, Si−Al, and Al−Al were constrained to be 3, 3, and 4 Å, respectively (since an Al−O−Al bond is prohibited in a zeolite framework27). The constraints on the aluminosilicate

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EXPERIMENTAL SECTION Commercial zeolite A (4A, LTA type zeolite, Na/Si/Al/O = 1:1:1:4, Tosoh Co., Japan) was milled using a bead milling apparatus (Minicer, Ashizawa Finetech Ltd., Tokyo, Japan). A 60 g amount of zeolite A was dispersed in 100 mL of ethanol using an ultrasonic vibrator (VCX 600, Sonic & Materials Inc., USA), and the slurry was pulverized for 120 min using 300 μm diameter zirconia beads followed by additional milling for 360 min using 100 μm diameter zirconia beads. An agitation speed of 3000 rpm was used to shear and exert force on the zeolite agglomerates. The phases present and sample morphology were identified by conventional X-ray diffractometry (XRD, Multiflex, Rigaku, Tokyo, Japan) and field emission scanning electron microscopy (FE-SEM, S-5200 and S-4800, Hitachi, Tokyo, Japan). Details of the lattice structure were observed by field emission 25294

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network connectivity were that all oxygen atoms were coordinated to two Si atoms (or one Si and one Al atom) and that all Si and Al atoms were coordinated to four O atoms. RMC simulation was then performed for a system containing 3500 atoms using the structure factor S(Q). The box length was chosen to correspond to the number density. Program code RMC++ was used.28

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RESULTS AND DISCUSSION FE-SEM micrographs of the samples are shown in Figure 2. The as-received zeolite has smooth cubic morphological

Figure 2. FE-SEM micrographs of samples (a) as-received and (b) bead milled for 480 min. Figure 4. Water adsorption−desorption isotherms of samples at 298 K (a) as-received (●, ○) and (b) bead milled for 480 min (△, ▲). Closed and open symbols mean adsorption and desorption branches, respectively.

features. After bead milling, the zeolite A morphology changes significantly. The raw zeolite, with an average size of 3.5 μm, forms agglomerates of tiny particles approximately 30−200 nm in diameter with average size of 110 nm after bead milling. XRD spectra of the samples are shown in Figure 3. The

origin. This isotherm irreversibility most likely results from the surface hydroxylation of formed amorphous silica, namely, chemisorption. During desorption at room temperature (298 K), only water physically adsorbed was removed from the surface. The framework connectivity of milled zeolite is explained by the Raman spectra in Figure 5. It has been shown that bands in

Figure 3. XRD spectra of samples (a) as-received and (b) bead milled for 480 min. All Bragg peaks are from zeolite A.

diffraction peaks assigned to an LTA structure in the beadmilled samples showed that the sample crystallinity 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%. The FESEM and XRD results imply that it is possible to downsize the zeolite by a top-down approach, that is, by milling, however, destruction of the framework structure is unavoidable because of the shear and forces exerted on the zeolite agglomerates. Water adsorption−desorption isotherms are shown in Figure 4. Both the as-received and bead-milled isotherms are typical IUPAC type I isotherms, corresponding to the adsorption of water molecules on the hydrophilic micropores of zeolite A. The amount of adsorbed water on zeolite A decreased significantly by bead milling for 480 min, suggesting that the microporous structure was destroyed and that a nonporous amorphous material formed, which has little or no contribution to the water adsorption in the low relative pressure region. Furthermore, a large hysteresis loop was observed for the milled sample, and the desorption branch did not return to the

Figure 5. Raman spectra of samples (a) as-received and (b) bead milled for 480 min.

the 300−650 cm−1 region are sensitive to the T−O−T bond angle,29−31 framework connectivity, and size of the ring present in the framework. Previous Raman investigations on silica assigned the bands at approximately 490−500 cm−1 to be related to 4Rs as shown in Figure 1; these peaks were seen in both samples and were therefore assigned as 4Rs. The 4Rs (ca. 490 cm−1) peak broadens after bead milling, indicating that the 4Rs have become distorted and/or destroyed by shearing and/ or compressive forces. Because crystallographically all T atoms in the zeolite A structure are part of 4Rs, this result indicates that the crystal structure of zeolite A is distorted to form various T−O−T angles by bead milling. The 27Al MAS NMR spectra are shown in Figure 6. The 27Al MAS NMR spectrum of the as-received material has a strong, sharp peak at 59 ppm corresponding to AlO4 tetrahedra. Al 25295

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several features. 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 the Si−Al distance.27 In the G(r) of the as-received zeolite, peaks are visible at 3.7−4.0 and 4.2−4.6 Å, which correspond mainly to the distances from the Si(Al) to the second oxygen (second Si(Al)−O). The peaks at 3.7−4.0 Å in the raw zeolite in particular result mainly from the second Si(Al)−O distances in 4R. In a similar way, the peaks at 4.35 Å result mainly from the second Si(Al)−O in 6R and 8R.12 It should be noted that, in the milled zeolite, Si−Al (3.0−3.3 Å) shifts to a shorter distance while Si(Al) 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 Si(Al)−O bonds and the average Si−O−Al angles are decreased by bead milling. Further, distances in Si(Al) to the second oxygen (3.7−4.0 and 4.2−4.6 Å) also shift to shorter distances. This result clearly shows that the intermediate-range order has changed to form a denser amorphous aluminosilicate network by compressive stress through milling. In order to obtain more detailed intermediate-range information on the ring structure, we performed RMC simulations for milled zeolite A. As can be seen in Figure 8a,

Figure 6. 27Al MAS NMR spectra of samples (a) as-received and (b) bead milled for 480 min. 27Al chemical shifts measured with respect to a 1 M aqueous Al(NO3)3 solution. Shoulder peak at lower magnetic field indicated by an arrow.

atoms may become detached from the zeolite framework to form AlO6 octahedra having resonance at 0−5 ppm,32 indicating that the as-received zeolite A is crystalline and does not contain AlO6 species. The AlO6 species are not confirmed to be present in the sample even after bead milling as shown in Figure 6. This result clearly shows that most of the Al atoms are located in the AlO4 framework structure instead of forming a random structure. It should be noted that a shoulder peak at lower magnetic fields is visible after bead milling as indicated by the arrow in Figure 6. This shoulder peak at lower magnetic field is not mainly due to quadrupolar broadening as shown in the Supporting Information. According to previous MAS NMR studies,33 the chemical shift is very sensitive to the T−O−T angle. The shoulder peak at lower magnetic field indicates a narrowing in the T−O−T angle. Therefore, this result indicates that the zeolite framework is partially distorted to form decreased T−O−T angles. To determine which of the atomic-pair correlations change during the milling process, we carried out HEXRD measurements. Figure 7 shows the pair distribution function, G(r), of the asreceived and bead-milled samples. The Qmax collected in this

Figure 8. (a) Total structure factor S(Q) of the milled sample obtained from HEXRD (solid curve) and the RMC simulation (dotted curve). Note that the Löwenstein rule27 was imposed in the RMC simulation for the milled sample. The corresponding three-dimensional atomic configurations are shown in (b). On the right of (b), typical shape of the ring structures present in the three-dimensional configurations is highlighted; indicating distorted rings are formed in the milled sample. Bond angle distribution of Si−O−Al in the milled sample is shown in (c).

Figure 7. Pair distribution function, G(r)s (1−5.5 Å), of as-received and bead milled for 480 min are also shown.

the experimental S(Q) values are well reproduced by RMC in a wide range of scattering vector Q. Three-dimensional atomic configuration extracted from the RMC modeling of the milled sample shows that disordered even numbered rings are present in the structure whereas ordered structure can be seen in crystalline zeolite A (see Figure 1). This model strongly suggests that distorted rings are formed during bead-milling

study was 25 Å−1, and the pair distribution function, G(r), is derived from the Faber−Ziman total structure factor, S(Q) (see the Supporting Information). Previous reports on the structural characterization of pressure-induced amorphized zeolite A is useful for the data analysis in this study.19−22 From the G(r) curves, it is possible to identify various distances associated with 25296

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in Figure 9b. To understand the changes in atomic arrangement of the zeolite structure by bead milling, a ground particle is shown in Figure 10a. This particle is a mixture of crystalline and

treatment. The Si−O−Al bond angle distribution of the milled sample calculated from the RMC model is shown in Figure 8c. Note that, since an Al−O−Al bond is prohibited in a zeolite framework27 and the Si/Al ratio is 1 for zeolite A, the T−O−T angle corresponds to the Si−O−Al angle. The Si−O−Al distribution has a maximum at around 140°. The average T− O−T angle is 138.8°. On the other hand, it is possible to calculate the Si−O−Al bond angle from the crystal structure of zeolite A for the crystalline system.34 An average T−O−T angle is 148.4°. This value is 9.6° larger than that of the milled sample, supporting magnified three-dimensional atomic configuration; that is, rings in the milled sample are distorted by the compressive stress through bead-milling treatment. As shown in the Supporting Information, the change in the Si−O−Al angle corresponds well to that in the shift of the Si−Al distance in G(r). Water adsorption, Raman spectroscopy, 27Al MAS NMR, and HEXRD provide information on average structural changes. To observe the local structure directly, HRTEM measurements were conducted. Figure 9 shows TEM micrographs of the beadmilled samples. Lattice fringes were observed in some areas as shown in Figure 9a, where some crystal structure still existed after bead milling. However, most of the particles observed were pulverized vigorously and became noncrystalline as shown

Figure 10. TEM images of zeolite A after bead milling. (a) Low magnification image. (b) Magnified image of (a). (c) Same image as (b) with grid line corresponding to the crystal lattice of zeolite A.

noncrystalline parts with a crack in the center caused by bead milling. Figure 10b magnifies Figure 10a where the surface area is partially amorphized by milling. Figure 10c shows the same image as Figure 10b with grid lines corresponding to the zeolite A crystal lattice. It is worth noting that the lattice appears to be distorted to the inward direction near the interface between the crystalline and noncrystalline parts. This provides direct evidence that zeolite A is amorphized through distortion by the compressive stress. This result strongly supports the other data: smaller water adsorption capacity, distortion of the T−

Figure 9. TEM images of zeolite A after bead milling. (a) Partly crystalline particle. (b) Vigorously pulverized particle. 25297

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O−T angle by Raman and 27Al NMR spectroscopies, and shorter second T−O distances by HEXRD. In general, it is rare for typical inorganic materials to be amorphized by milling, but dislocations can be generated to absorb the milling energy. On the other hand, the interstructural space in the porous and low density zeolite (compared with other aluminosilicate materials) has to be destroyed to form a denser structure. The crystalline atomic position can be moved more easily (without breaking Si−O−Al bondings) 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 technique.35 They indicated that the minimum movement in atomic arrangement to form an X-ray amorphous material is thought to be the main mechanism for milling amorphization in zeolites. By using combined HEXRD and HRTEM as well as several analytical measurements, it can be concluded that at least one of the zeolite amorphization processes is caused by the distortion in framework structure by the compressive stress.

measurements. This work has been supported partially by a “Grant for Advanced Industrial Technology Development” in 2011 from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

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(1) Moloy, E. C.; Davila, L. P.; Shackelford, J. F.; Navrotsky, A. Microporous Mesoporous Mater. 2002, 54, 1−13. (2) Wang, Z.; Lobo, R. F.; Lambros, J. Microporous Mesoporous Mater. 2003, 57, 1−7. (3) Renzo, F. D. Catal. Today 1998, 41, 37−40. (4) Xie, J. H.; Kaliaguine, S. Appl. Catal., A 1997, 148, 415−423. (5) Zielinski, P. A.; Van Neste, A.; Akolekar, D. B. Microporous Mater. 1995, 5, 123−133. (6) Huang, M.; Auroux, A.; Kaliaguine, S. Microporous Mater. 1995, 5, 17−27. (7) Kosanovic, C.; Bronic, J.; Cizmek, A.; Subotic, B.; Smit, I.; Stubicar, M.; Tonejc, A. Zeolites 1995, 15, 247−252. (8) Kharitonov, A. S.; Fenelonov, V. B.; Voskresenskaya, T. P.; Rudina, N. A.; Molchanov, V. V.; Plyasova, L. M.; Panov, G. I. Zeolites 1995, 15, 253−258. (9) Tosheva, L.; Valtchev, V. P. Chem. Mater. 2005, 17, 2494−2513. (10) Kohara, S.; Suzuya, K.; Takeuchi, K.; Loong, C. K.; Grimsditch, M.; Weber, J. K. R.; Tangeman, J. A.; Key, T. S. Science 2004, 303, 1649−1652. (11) Petkov, V.; Billinge, S. J. L.; Shastri, S. D.; Himmel, B. J. NonCryst. Solids 2001, 293, 726−730. (12) Wakihara, T.; Kohara, S.; Sankar, G.; Saito, S.; Sanchez-Sanchez, M.; Overweg, A. R.; Fan, W.; Ogura, M.; Okubo, T. Phys. Chem. Chem. Phys. 2006, 8, 224−227. (13) Mintova, S.; Olson, N. H.; Valtchev, V.; Bein, T. Science 1999, 283, 958−960. (14) Mintova, S.; Olson, N. H.; Bein, T. Angew. Chem., Int. Ed. 1999, 38, 3201−3204. (15) Greaves, G. N.; Meneau, F.; Sankar, G. Nucl. Instrum. Methods Phys. Res., Sect. B 2003, 199, 98−105. (16) Meneaua, F.; Greaves, G. N. Nucl. Instrum. Methods Phys. Res., Sect. B 2005, 238, 70−74. (17) Greaves, G. N.; Meneau, F.; Sapelkin, A.; Colyer, L. M.; Gwynn, I. A.; Wade, S.; Sankar, G. Nat. Mater. 2003, 2, 622−629. (18) Greaves, G. N.; Meneau, F.; Kargl, F.; Ward, D.; Holliman, P.; Albergamo, F. J. Phys.: Condens. Mater 2007, 19, 415102/1−415102/ 17. (19) Haines, J.; Levelut, C.; Isambert, A.; Hérbert, P.; Kohara, S.; Keen, D. A.; Hammouda, T.; Andrault, D. J. Am. Chem. Soc. 2009, 131, 12333−12338. (20) Readman, J. E.; Forster, P. M.; Chapman, K. W.; Chupas, P. J.; Parise, J. B.; Hriljac, J. A. Chem. Commun. 2009, 3383−3385. (21) Huang, Y. J. Mater. Chem. 1998, 8, 1067−1071. (22) Huang, Y.; Havenga, E. A. Chem. Phys. Lett. 2001, 345, 65−71. (23) Faber, T. E.; Ziman, J. M. Philos. Mag. 1965, 11, 153−173. (24) Kohara, S.; Itou, M.; Suzuya, K.; Inamura, Y.; Sakurai, Y.; Ohishi, Y.; Takata, M. J. Phys.: Condens. Matter 2007, 19, 506101− 506115. (25) McGreevy, R. L.; Pusztai, L. Mol. Simul. 1988, 1, 359. (26) Metropolis, N.; Rosebluth, A. W.; Rosebluth, M. N.; Teller, A. H.; Teller, E. J. Phys. Chem. 1953, 21, 1087. (27) Löwenstein, W. Am. Mineral. 1952, 39, 92. (28) Gereben, O.; Jovari, P.; Temleitner, L.; Pusztai, L. J. Optoelectron. Adv. Mater. 2007, 9, 3021−3027. (29) Yu, Y.; Xiong, G.; Li, C.; Xiao, F. S. Microporous Mesoporous Mater. 2001, 46, 23−34. (30) Li, C.; Xiong, G.; Liu, J. K.; Ying, P. L.; Xin, Q.; Feng, Z. C. J. Phys. Chem. B 2001, 105, 2993−2997. (31) Dutta, P. K.; Shieh, D. C.; Puri, M. J. Phys. Chem. 1987, 91, 2332−2336. (32) MacKenzie, K. J. D.; Smith, M. E. Multinuclear Solid-State NMR of Inorganic Materials; Pergamon: New York, 2002; Vol. 6.

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CONCLUSIONS This study has focused on the amorphization of zeolite A by milling, and changes in the atomic arrangement during mechanical amorphization were investigated by a combination of HEXRD and HRTEM in addition to other typical analyses. As a result, a decrease in the T−O−T angle and lattice distortion in the zeolite A framework by compressive stress were confirmed by HEXRD and HRTEM, respectively. Threedimensional atomic configuration extracted from the RMC modeling of the milled sample also supports that average the T−O−T angle was decreased in the bead-milled sample. We showed, for the first time, that at least one of the amorphization processes of zeolites is caused by the distortion of framework structure by the compressive stress to form a higher density noncrystalline material. Understanding the amorphization processes of zeolites is critical in evaluating the role of atomic-scale structures in performing zeolite applications. Current data show only changes in the zeolite A structure. Further studies on the amorphization of other zeolites are needed to provide conclusive evidence on the changes in atomic arrangement of a zeolite caused by mechanical stresses.

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ASSOCIATED CONTENT

S Supporting Information *

Multiple-quantum MAS NMR of milled sample, total structure factors S(Q) of as-received and bead milled for 480 min, and calculation of Si−Al distances. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone: +81-45-339-3957; fax: +81-45-339-3957; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The X-ray diffraction experiment at the SPring-8 was approved by the Japan Synchrotron Radiation Institute under proposal no.2010B1230. We would like to thank Prof. T. Tatsumi and T. Yokoi in Tokyo Institute of Technology for FE-SEM 25298

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(33) Johnson, G. M.; Mead, P. J.; Dann, S. E.; Weller, M. T. J. Phys. Chem. B 2000, 104, 1454−1463. (34) Gramlich, V.; Meier, W. M. Z. Kristallogr. 1971, 133, 134−149. (35) Sato, K.; Wakihara, T.; Kohara, S.; Tatami, J.; Inagaki, S.; Kubota, Y.; Komeya, K.; Meguro, T. J. Ceram. Soc. Jpn. 2011, 119, 605−608.

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