Transmission Electron Microscopy Observation on Fine Structure of

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Transmission Electron Microscopy Observation on Fine Structure of Zeolite NaA Membrane Zheng Liu,†,‡,§ Tetsu Ohsuna,*,‡ Kiminori Sato,† Takehito Mizuno,† Tomohiro Kyotani,† Takashi Nakane,† and Osamu Terasaki*,‡ Bussan Nanotech Research Institute Inc., Mitsui & Co., Ltd., Nanotech Park, 2-1 Koyadai, Tsukuba, Ibaraki, 305-0074, Japan, Structural Chemistry, Arrhenius Laboratory, Stockholm UniVersity, 106 91 Stockholm, Sweden, and National Institute of AdVanced Industrial Science and Technology (AIST), Higashi 1-1-1, Tsukuba 305-8565, Japan ReceiVed July 20, 2005. ReVised Manuscript ReceiVed NoVember 10, 2005

Fine structure of a high-performance zeolite NaA membrane filter for water/ethanol separation was investigated by transmission electron microscopy (TEM); the samples were prepared by ion milling and focused ion beam (FIB) techniques. The NaA membrane consists of a top layer with a thickness of 2-3 µm and a composite layer where the thickness is larger than 5 µm. NaA crystallites in the top layer show a columnar morphology toward an external surface, and a high-density amorphous phase is observed at the grain boundary between NaA crystallites. In the composite layer, NaA crystallites form an uncertain shape with many voids of various sizes at the triple point of NaA crystals’ grain boundary.

Introduction Zeolite membranes have been attracting much attention for application in molecular sensors and gas or liquid separation filters as one of the important inorganic membranes with temperature stability and solvent resistance.1-3 To be used as a separation filter, zeolite membranes are required to have enough mechanical strength without losing porosity, so two types of zeolite membranes could be considered: (1) thick zeolite polycrystalline films (selfsupported); (2) thin zeolite films supported by some porous materials (alumina, silica, iron oxide, etc.). There has been success in forming MFI-, FAU-, and LTA-type zeolites as separation membranes,4-10 and the LTA membranes have been commercialized by Mitsui Engineering and Shipbuilding Co. Ltd. LTA zeolite with Na+ as charge balancing cations (NaA) has been used as a molecular sieve with an aperture size of 0.4 nm. Since the NaA is hydrophilic, it is possible to separate water from water/organic liquid mixtures by water permeation through the NaA zeolite. Separation of * To whom correspondence should be addressed. T.O.: tel., +41-8-162379; fax, +41-8-163118; e-mail, [email protected]. O.T.: [email protected]. † Mitsui & Co., Ltd. ‡ Stockholm University. § AIST.

(1) Tavolaro, A.; Drioli, E. AdV. Mater. 1999, 11, 975. (2) Bein, T. Chem. Mater. 1996, 8, 1636. (3) Caro, J.; Noack, M.; Ko¨lsch, P.; Scha¨fer, R. Microporous Mesoporous Mater. 2000, 38, 3. (4) Sano, T.; Yanagisaita, H.; Kiyozumi, Y.; Mizukami, F.; Haraya, K. J. Membr. Sci. 1994, 95, 221. (5) Vroon, Z. A. E. P.; Keizer, K.; Gilde, M. J.; Verweij, H.; Burggraaf, A. J. J. Membr. Sci. 1996, 113, 293. (6) Tuan, V. A.; Li, S.; Falconer, J. L.; Noble, R. D. J. Membr. Sci. 2002, 196, 111. (7) Kusakabe, K.; Kuroda, T.; Murata, A.; Morooka, S. Ind. Eng. Chem. Res. 1997, 36, 649. (8) Kondo, M.; Komori, M.; Kita, H.; Okamoto, K. J. Membr. Sci. 1997, 133, 133. (9) Kita, H.; Horii, K.; Ohtoshi, Y.; Tanaka, K.; Okamoto, K. J. Mater. Sci. Lett. 1995, 14, 206. (10) Braunbarth, C. M.; Boudreau, L. C.; Tsapatsis, M. J. Membr. Sci. 2000, 174, 31.

water and ethanol from their mixture (water/ethanol separation) is one of the most important challenges in the chemical industry today. Therefore, the NaA membrane with high performance in both selectivity and flux has been highly desired in this field. From the requirement of mechanical strength of the membrane, commercial NaA membranes are synthesized on the porous tubular R-alumina. Pinholes and microcracks, which are nonseparation pathways, are sometimes found in the zeolite layer after synthesis of the zeolite thin layer on the substrate. Investigation of the zeolite membranes is then usually required for the existence of such pinholes or cracks by using a light microscope and scanning electron microscope (SEM). Since the zeolite layer of the membranes consists of polycrystals, there must be grain boundaries between the crystallites, and voids and/or other phases of the materials would probably exist at the grain boundary. Such unexpected structure between crystallites might act as a pathway or a barrier to permeate the molecule; therefore, it is important to characterize the fine structure of the membrane for controlling the permeation property as well as understanding the growth mechanism of the zeolite layer. Such fine structures are usually too small to be detected by SEM; thus, transmission electron microscopy (TEM) is the only method for studying the fine structure. There are only a few reports of TEM research on fine structures of zeolite membrane.11,12 Sasaki11 and co-workers reported excellent work on the cross-sectional TEM observation of the MFI membrane grown on a porous alumina substrate. They prepared the TEM specimen by an ionmilling method and took high-resolution transmission electron microscope (HREM) images from MFI crystals in the (11) Sasaki, Y.; Shimizu, W.; Ando, Y.; Saka, H. Microporous Mesoporous Mater. 2000, 40, 63. (12) Li, S.; Li, Z.; Bozhilov, K. N.; Chen, Z.; Yan, Y. J. Am. Chem. Soc. 2004, 126, 10732.

10.1021/cm051597q CCC: $33.50 © 2006 American Chemical Society Published on Web 01/26/2006

Fine Structure of Zeolite NaA Membrane

top and the composite layers. Whereas the HREM method can provide clearly the fine structures of the grain boundary between two neighboring zeolite crystals on a sub-nano scale as shown by Sasaki, the HREM images normally show too little contrast at low magnification to clarify the morphology of zeolite crystals in the membrane. One can obtain a TEM image with high contrast for defining the grain morphology of zeolite crystals by using a smaller objective lens aperture (OLA), which is called a bright field (BF) image. The BF image is usually obtained by using the smallest OLA (only the direct beam can pass through the aperture), but in a recent TEM system, such a small OLA might give a shadow overlapping with the observed area. Sometimes, we choose therefore the second smaller OLA to obtain a low-magnification TEM image with high contrast, referred to here as semiBF image. On the other hand, when a different density material exists between two crystallites having the same density, a high-angle annular dark field scanning transmission electron microscope (HAADF-STEM) image will demonstrate a good contrast corresponding to the density differences of the materials. These methods seem to be powerful for the grain boundary investigation of polycrystalline and/or composite materials. To obtain the necessary performance of a NaA membrane for water/ethanol separation, study on the fine structures of both zeolite crystallites and porous alumina substrates is essential. In general, it is difficult to obtain a thin specimen with a constant thickness through an area of more than several micrometers in width by the ion-milling method. At present, the focused ion beam (FIB) method has been available for preparing cross-sectional TEM specimen with a size larger than 10 µm, maintaining a constant thickness of ca. 100 nm.13,14 In the present work, two kinds of TEM specimens were prepared from a high-performance NaA membrane filter by FIB and ion-milling methods. TEM observations, which include HREM, semi-BF, HAADFSTEM methods, and elemental analysis by an energy dispersive spectrometer (EDS), were performed on the fine structures of NaA membrane including porous substrate and the grain boundary between NaA crystallites in the zeolite layer. Experimental Section The NaA membranes were synthesized hydrothermally on the surface of a porous alumina substrate in a alumino-silicate sol solution (Na2O:SiO2:Al2O3:H2O ) 2:2:1:150) at 373 K for 4 h. The porous alumina tube with outer and inner diameters of 10 and 6.5 mm, a mean pore diameter of ca. 1.3 µm, and a porosity of ca. 50% supplied by Noritake Co. was used as a substrate. Before hydrothermal synthesis of the membrane, the surface of the substrate was seeded by the dip-coating method with NaA crystalline particles less than 1 µm in size supplied by Mizusawa Co. Ltd. After hydrothermal treatment, the surface of the tubular substrate was washed with water and a brush to take accumulated gel or crystal away and dried in air. The separating performance of the synthesized membranes was measured by pervaporation dehydration experiments on an ethanol (90 wt %)/water (10 wt %) solution at 348 K,

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Figure 1. Schematic diagram of a procedure for preparing TEM specimen by the ion milling method. A tubular porous alumina support with NaA membrane inside the tube (a) was cut into two small pieces (b,c). The pieces were covered with 1 µm Au layer by magnetron sputtering (d), and then they were glued by epoxy resin (e). The glued piece was lapped to 20 µm in thickness (f) and glued to the support ring of Mo (g); finally, the piece was ion-milled by Ar+ (h).

and results showed a high selectivity (R ) 14600) and a high flux (Q ) 4.3 kg m-2 h-1) for water-ethanol separation. It shows a high performance in dehydration compared with those of the previously reported NaA membrane.8-10 For TEM observation, two kinds of sample were prepared from the NaA membrane filter after the pervaporation experiment. The first was prepared by the ion-milling method using Fishione-1010 as shown in Figure 1. The porous alumina tube with NaA membrane in the inner side was first cut into small rectangles (3 × 3 × 1 mm3) and sputtered with a gold (Au) layer of 1 µm in thickness on the NaA membrane side by a magnetron sputtering method. Two of these small rectangles were glued together with both the NaA membrane side face to face and then lapped to 20 µm in thickness. Finally, the piece was ion-milled to make a hole at the center by incident argon ion (Ar+) beams with an accelerating voltage of 3 kV, a current of 3 mA, and incident angles of (10° (irradiated on both the top and the bottom sides). The second TEM sample was prepared by the focused ion beam (FIB) method using Hitachi FB-2100 attached with a microsampling system.13,14 The porous alumina tube with a NaA membrane in the inner side was first cut into a small rectangle (4 × 4 × 1 mm3) and covered with a carbon layer (100 nm in thickness, by heating evaporation) followed by platinum deposition (20 nm in thickness, by magnetron sputtering) on the NaA membrane side to avoid discharge problem. In the FIB system, a layer of tungsten (W) with a thickness of ca. 2 µm was deposited above the Pt layer to protect the surface morphology and the sample was gradually thinned by a gallium ion (Ga+) beam at an accelerating voltage of 40 kV with the micro-sampling method as illustrated in Figure 2. Scanning ion microscope (SIM) images in Figure 2, which show intensity distributions of secondary electrons emitted from a specimen surface by Ga+ beam irradiation, were taken by the FB-2100. SEM and EDS mapping images were taken by using JSM-6700F. JEM-4000EX with an accelerating voltage of 400 kV was used for HREM observations. HAADF-STEM and nano-area EDS analyses were performed with a 200 kV TEM of HF-2200.

Results and Discussions (13) Ishitani, T.; Umemura, K.; Ohnishi, T.; Yaguchi, T.; Kamino, T. J. Electron Microsc. 2004, 53, 443. (14) Sugiyama, M.; Sigesato, G. J. Electron Microsc. 2004, 53, 527.

Figure 3a shows an SEM image taken from the crosssectional region of NaA membrane grown on the surface of

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Figure 2. Snapshot of scanning ion microscope (SIM) images in the procedure of preparing TEM specimen by FIB with the microsampling system. An external surface showing a clear facet of LTA zeolite (a). W deposition in order to protect surface structure (b). Regions around the target area were grinded to 20 µm in depth by a Ga+ ion beam (c), and then the bottom surface was cut (d). The piece including the target area was picked up by microsampling system (e). The piece was placed on a half mesh and glued by W deposition (f) and was gradually thinned by Ga+ ion with a wide beam (g) and a narrow beam (h). The final target thickness was ca. 100 nm.

Figure 3. Cross-sectional SEM image of the NaA membrane grown on the porous alumina substrate (a). Elemental mapping images of Si (b), Na (c), and Al (d) obtained by EDS. The scale bar is 10 µm.

tubular alumina. In the SEM image, rounded particles (right side) and a layer with a thickness of ca. 3 µm (left side) are corresponding to R-alumina crystals and a part of the NaA membrane (top layer), respectively. From the image contrast, it is hard to find the inner surface of the NaA membrane

Figure 4. TEM image taken from the FIB specimen.

toward the alumina side. Figures 3b-d show the elemental mapping images of Si (Figure 3b), Na (Figure 3c), and Al (Figure 3d) detected by the EDS system. The areas containing both Si and Na should be regarded as the NaA membrane, and the areas containing only Al are assigned to be alumina particles. From the mapping images, one can see that the

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Figure 5. Semi-BF-TEM images (a, b) and a HAADF-STEM image (c) taken from the top layer.

NaA membrane penetrates the space (pore in the alumina substrate) surrounded by alumina particles. Here, the penetrated layer is denoted as the composite layer. The thickness of the NaA membrane is different from place to place and the total thickness of the top layer and composite layer is in the range of 10-15 µm. Figure 4 shows a TEM image taken from a cross section of the NaA membrane made by the FIB method. The image consists of some different contrasts corresponding to a W deposition layer, a NaA top layer, alumina particles, and NaA crystals in the pores surrounded by alumina particles. The outer surface of the top layer covered with W deposition shows a clear facet corresponding to that shown in the SIM image (Figure 2a). The thickness of the top layer is 2-3 µm and that of the composite layer is larger than 5 µm. It can be seen that the morphology of the NaA grain boundaries in the top layer is different from that in the composite layer as shown in Figures 5 and 7. In the top layer, NaA crystallites are formed in column shape grains growing from the alumina support to the outer surface, and the morphology of the columnar crystals is similar to that of MFI membranes as previously reported.15-17 Since bend contours can be clearly seen in the semi-BF-TEM images in Figures 5a and 5b, which were taken by using a smaller objective lens aperture than that of Figure 4, one can easily recognize the grain (15) Bons, A.-J.; Bons, P. D. Microporous Mesoporous Mater. 2003, 62, 9. (16) Gouzinis, A.; Tsapatsis, M. Chem. Mater. 1998, 10, 2497. (17) Bonilla, G.; Vlachos, D. G.; Tsapatsis, M. Microporous Mesoporous Mater. 2001, 42, 191.

boundary of columnar NaA crystallites. A HAADF-STEM image is shown in Figure 5c. There are linear bright contrasts showing the positions of the grain boundaries (indicated by arrows). In general, a brighter contrast in the HAADF-STEM image corresponds to a higher density region when the specimen thickness is almost constant. The HAADF image suggests that an unknown phase might exist at the grain boundary. A thin layer with a width of ca. 5 nm in dark contrast can be seen in the HREM image taken from the top layer of the FIB specimen (Figure 6). Since no lattice fringe had been observed in this thin layer and the contrast is always darker than that of NaA crystals, the structure of the thin layer seems to be amorphous and to have a higher density than that of NaA crystal. Namely, the thin layer is not a NaA crystal but a grain boundary phase (GBP). This speculation is consistent with the result that the contrast of the GBP is brighter than that of NaA crystal in the HAADFSTEM image in Figure 5c. Furthermore, the results of EDS line analyses from the GBP and from the NaA crystal illustrate that the Si/Al ratio in the grain boundary is higher than that in the NaA crystal. Although a nanosized higher density electron beam made a hole at the irradiated point in both the GBP and NaA crystal in a few seconds, all of the EDS results of Si/Al ratio obtained from several regions show the same tendency. Therefore, qualitatively, we can believe that the GBP is more Si-rich than the NaA crystal (the Si/ Al ratio of the NaA crystals in the top layer is ca. 1.1 measured by EDS analysis with a large area electron beam irradiation). Although Sasaki had pointed out an unknown phase with a width of ca. 10 nm at the grain boundary in

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Figure 6. HREM image taken from the GBP in the top layer. An enlarged image of the area indicated by a rectangle is inserted.

Figure 7. TEM images (a, c) and HAADF-STEM images (b, d) taken from the composite layer. Arrows in the images (a) and (b) indicate voids.

the top layer of the zeolite MFI membrane,11 since the contrast of the unknown phase in his TEM image is brighter than that of MFI crystallites, the GBP in our NaA membrane seems not to be corresponding to the unknown phase at the grain boundary in the MFI membrane. In the composite layer, on the other hand, the NaA crystallites are formed in smaller uncertain shape grains as shown in Figure 7. Especially, there are small holes (voids) between the composite layers indicated by arrows in Figures 7a and 7b. The holes show the contrast in the BF-TEM and

HDDF-STEM images. Figures 8a and 8b show HREM images taken from the composite layer of the ion-milled specimen. From Figure 8a, it can be seen that the lattice fringes of the two NaA crystallites touched together on the grain boundary without another phase in between. On the other hand, a bright contrast indicated by an arrow in Figure 8b can be regarded as a void surrounded by three NaA crystallites (triple point). We frequently observed such voids (showing bright contrast in the TEM image and dark contrast in the HAADF-STEM image) with similar or larger size in

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Figure 8. HREM images taken from the composite layer. The grain boundary (indicated by an arrow) between two NaA crystallites (a). A bright contrast indicated by an arrow corresponding to a void was observed at a triple point surrounded by three NaA crystallites (b).

Figure 9. Schematic sketch of the NaA membrane structure on the porous alumina substrate.

the composite layer, but not in the top layer. The existence of voids at the triple points suggests that the small voids may be connected with each other along triple point networks of the NaA polycrystals (void networks). The structural features would be corresponding to a hypothesis of the growing process in the space surrounded by alumina particles as follows: some small NaA crystallites are nucleated in the solution independently, and they grow until they are touching each other, but sometimes uncrystallized triple points remain because it might be hard for the solution to diffuse through a narrow pathway into the triple point surrounded by growing crystallites. Whereas the major parts of grain boundaries in the composite layer show similar structure to Figure 8, sometimes the grain boundaries showing dark contrasts in the BF-TEM image (Figure 7c) and bright contrasts in the HAADF-STEM image (Figure 7d) similar to the GBP in the top layer are observed in the parts near the top layer. Figure 9 shows a schematic sketch of the structure of the NaA membrane. From the TEM observations, the secondary growth mechanism of NaA crystals in the top layer seems to be the van der Drifts type of evolutionary selection.18 According to the van der Drifts selection, the top layer should have certain preferred orientations along the normal direction to the substrate; however, no selected orientation was observed in our XRD experiments. This is because in our case the top layer is thin, about 2-3 µm, and the surfaces of the alumina crystallites in the substrate are not parallel to the surface of the cylinder NaA. Therefore, the selected orientation was not detected by XRD even if the van der Drifts selection was operating in the top layer. It seems that (18) Drift, A. Philips Res. Rep. 1967, 22, 267.

there are two possibilities to perform the water-ethanol separation in the NaA/alumina membrane system, i.e., the composite layer and the top layer. The composite layer has many voids (probably void networks), and the top layer has no void but the GBP. Since the TEM specimens were prepared from the NaA membrane after pervaporation experiments, the GBP seems to be stable under the heating treatment of 348 K. According to some reports for gas permeation passing through nonzeolistic pores in zeolite membranes,19-24 it is still uncertain that the GBP can separate water and ethanol even if the GBP has a higher density than that of NaA crystal. Recently, we succeeded in a primitive pervaporation measurement for a single NaA crystal larger than 10 µm in size and obtained results of similar selectivity and a higher flux than that of the NaA membrane for waterethanol separation (unpublished results). Therefore, we speculate that the separation effectively proceeds from the NaA crystals in the top layer. Conclusions Through TEM observation, the structure of NaA membrane is clarified. There exist two layers of NaA crystals. The top layer including a column shape of NaA grains together with an amorphous GBP with higher density than that of NaA crystal seems to play an important role in the separation of water-ethanol. By using HAADF-STEM combined with an HREM method, it is possible to investigate the grain boundary structure of the top layer as well as the composite layer. FIB is the most powerful technique to obtain a uniform thickness specimen for wide area TEM observation of thin (a few micrometers) zeolite membrane. Acknowledgment. We acknowledge the assistance with FIB sample preparation by Mr. T. Sato (Hitachi Science Systems Ltd.) and the assistance with SEM observation by Mr. E. Aoyagi and Mr. Y. Hayasaka (Tohoku University). CM051597Q (19) Aoki, K.; Kusakabe, K.; Morooka, S. Ind. Eng. Chem. Res. 2000, 39, 2245. (20) Jareman, F.; Hedlund, J.; Creaser, D.; Sterte, J. J. Membr. Sci. 2004, 236, 81. (21) Snyder, M. A.; Lai, Z.; Tsapatsis, M.; Vlachos, D. G. Microporous Mesoporous Mater. 2004, 76, 29. (22) Bonilla, G.; Tsapatsis, M.; Vlachos, D. G.; Xomeritakis, G. J. Membr. Sci. 2001, 182, 103. (23) Xomeritakis, G.; Lai, Z.; Tsapatsis, M. Ind. Eng. Chem. Res. 2001, 40, 544. (24) Bowen, T. C.; Noble, R. D.; Falconer, J. L. J. Membr. Sci. 2004, 245, 1.