Dissolution of Zeolite in Acidic and Alkaline Aqueous Solutions As

that for heulandite (a natural zeolite crystal) (i) NaOH attacked the uppermost layer of aluminosilicate of the. (100) surface, leaving isolated or ag...
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J. Phys. Chem. 1996, 100, 18474-18482

Dissolution of Zeolite in Acidic and Alkaline Aqueous Solutions As Revealed by AFM Imaging Sadaaki Yamamoto,*,† Shoko Sugiyama,† Osamu Matsuoka,† Kazuo Kohmura,† Tadatoshi Honda,† Yasuyuki Banno,‡ and Hisakazu Nozoye§ Central Research Institute, Mitsui Toatsu Chemicals, Inc., 1190 Kasama-cho, Sakae-ku, Yokohama 247, Japan, Geological SurVey of Japan, 1-1-3 Higashi, Tsukuba, Ibaraki 305, Japan, and National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305, Japan ReceiVed: May 30, 1996X

Atomic force microscopy (AFM) makes it possible to directly detect morphological changes on the surface of a zeolite that are due to dissolution when the crystal is immersed in either an alkaline (0.1 N NaOH) or acidic (0.2 N H2SO4) aqueous solution at room temperature. The AFM images revealed for the first time that for heulandite (a natural zeolite crystal) (i) NaOH attacked the uppermost layer of aluminosilicate of the (100) surface, leaving isolated or agglomerated islands, (ii) similarly, H2SO4 attacked the (010) surface, forming pits, and (iii) step retreat did not occur for either solution. This unique dissolution pattern, in which the aluminosilicate layers of heulandite dissolve from terraces layer-by-layer, results from the characteristic pore structure of heulandite. This microscopic-level analysis should prove vital in the further development of new zeolite structures, which are critical to the activity of this important class of materials.

Introduction To date, many studies exist on the growth and the dissolution kinetics of a solid in a liquid. Recently, these are expanding research fields because growth and dissolution are basic chemical and physical phenomena involved in numerous fields, ranging from natural processes, such as mineral dissolution, to industrial applications, such as etching, cleaning, or singlecrystal growth in semiconductor device fabrication. Thus far, studies on the dissolution of materials have relied mostly on conventional approaches that used solid particles, where quantities such as weight loss and surface area of the particles or the concentration of elements eluted into a contacting fluid are measured. However, such approaches suffer from several inherent drawbacks that originate from the use of solid particles, thus obscuring our understanding of the dissolution phenomenon.1 One method to overcome these drawbacks is direct microscopic-level observation of the surface subjected to dissolution. Such a technique is now available, namely, atomic force microscopy (AFM). Pioneering works have demonstrated the potential of AFM in microscopic-level monitoring of the morphological changes in the growing or dissolving surfaces of crystals.2-12 Due to this potential, AFM can not only validate classical dissolution theory but also discover new, more informative phenomena not detectable by conventional approaches. In this work, we used AFM to explore the dissolution of zeolites, which are used as catalysts in reactions, such as cracking, hydrocracking, disproportionation, and isomerization,13 and as ion-exchange materials, adsorbents, and detergent additives.14,15 Zeolites are microporous crystalline aluminosilicates with a framework structure composed of corner-sharing SiO4 and AlO4 tetrahedra.16 This structure determines the catalytic or ion-exchange performance.15 Growth and dissolu† Mitsui Toatsu Chemicals, Inc. FAX: 81-45-895-8237. E-mail: [email protected] ‡ Geological Survey of Japan. FAX: 81-298-54-3533. E-mail: ybanno@ gsj.go.jp § National Institute of Materials and Chemical Research. FAX: 81-29854-4504. E-mail: [email protected] X Abstract published in AdVance ACS Abstracts, November 1, 1996.

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tion are therefore important in the synthesis and modification used to control the properties of the zeolite. One example is dealumination, a well-known modification technique that controls the zeolite’s acidic strength and resistance to acid. The most often used dealumination technique is leaching by mineral acids, which involves the dissolution of aluminum from the framework of the zeolite crystal. Applied studies on the structure of dissolved zeolite surfaces using conventional techniques, such as high-resolution transmission electron microscopy (HRTEM), secondary electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS), reveal that (i) the structure is intact during the dissolution process and (ii) the surface is covered by an amorphous silica layer about 10-30 nm thick.17-20 On the other hand, mechanistic studies, also using conventional approaches, have prompted the theory that the surface species play an important role in the dissolution of zeolites.20 Although these structural and kinetic results have spawned several theories on the dissolution mechanisms of zeolites, obscurity still remains. The obscurity arises because these results do not always reveal the real dissolution process, which is actually the consequence of several complex physical and chemical processes. This is particularly true for zeolites because they have a channel structure and can accommodate several chemical species within the structural framework. Thus, the real dissolution “scenario” may be completely different from that based on classical dissolution theory. Direct imaging of the morphological changes in the dissolving surfaces of zeolites by AFM, however, will provide much needed insight. To gain this new insight, in this work we used AFM to investigate the initial morphological changes on the surface of heulandite crystals subjected to dissolution in 0.2 N H2SO4 and 0.1 N NaOH aqueous solutions. Experimental Section We used natural heulandite crystal from Oogawara, Himosashi-machi, Hirado-shi, Nagasaki, Japan (a registered specimen of the Geological Survey of Japan, registration number M11053). The structural assignment was done with the X-ray diffraction measurement (Rigaku, RINT 1500). X-ray fluorescence spectroscopy (Seiko Instruments, SEA 2010L) confirmed that the © 1996 American Chemical Society

Dissolution of Zeolite in Acidic and Alkaline Solutions crystals contain an alkali metal ion (K+) and alkaline earth metal ions (Ca2+, Ba2+, Sr2+). The crystals, used without any chemical treatment, were fixed with epoxy resin (Araldite Rapid, CIBA-GEIGY) on a steel plate and placed in an AFM sample holder. The (010) surface was obtained by using a scalpel to remove the upper crystal layer parallel to the (010) plane. The contact AFM imaging was done at room temperature with a NanoScope IIIa equipped with a liquid cell unit (Digital Instruments, Santa Barbara, CA). Either a 0.1 N NaOH or 0.2 N H2SO4 aqueous solution (ca. 0.1 mL) was dropped on the crystals in the cell. For flexibility in choosing an imaging spot, we used the cell without an O-ring that was normally used for sealing. Heulandite has a monoclinic crystal symmetry in which each unit cell has dimensions of a ) 17.73 Å, b ) 17.82 Å, c ) 7.43 Å, and β ) 116° 20′.21 Figure 1 shows the framework of heulandite, namely, layers formed by four-, five-, or six-member SiO4 and AlO4 tetrahedral rings (aluminosilicate layer). This framework has open channels of eight- and 10-member tetrahedral rings in three directions: (i) parallel to the a-axis are channels of eight-membered rings (Figure 1b), (ii) parallel to the c-axis are channels composed of eight- and 10-member rings (Figure 1c), and (iii) at an angle of about 50° to the a-axis are channels of eight-member rings. These channels are held together by a complex layering of four-, five-, and six-member tetrahedral rings. The (010) plane is the cleavage plane and has pores of 10-member rings (Figure 1d). The (100) surface has ordered channel mouths of eight-member rings (Figure 1b). Results and Discussion Figure 2a shows a typical AFM image of a (100) surface of heulandite in a 0.1 N NaOH aqueous solution at room temperature. Figure 2b shows a cross-sectional profile along the line labeled A-B in Figure 2a. These figures reveal the crystal growth-induced features. Concurrent with our research, Binder et al. also reported successful AFM imaging of growthinduced features on the (010) outer surface of natural heulandite.22 The (100) surface exhibited very flat terraces with several steps. The step heights were about 9 Å, which agree well with the size of an aluminosilicate layer (aluminosilicate layer A in Figure 1c). The images in Figure 2 also reveal a macrostep composed of nine aluminosilicate layers (A). Figure 2c,d,e and Figure 3a,b show AFM images of a (100) surface of heulandite immersed in a 0.1 N NaOH aqueous solution at room temperature for about one month. Although a SEM is unable to resolve the surface structural changes that were due to the dissolution in an alkaline aqueous solution,20 the AFM image that we obtained clearly reveals these changes due to the better resolution of the AFM. Terraces are no longer seen as being solid but consist of several rectangular “islands” with an average area of 300 × 600 Å2 and a height of about 9 Å and seemingly oriented about 45° to the step faces (indicated by the arrow in Figure 2c). The AFM images further reveal that terraces also have isolated islands (indicated by A in Figure 3b) or agglomerates of several islands (indicated by B in Figure 3b), again oriented about 45° to the step faces. There was no evidence of step retreat. The aluminosilicate layers are chemically bonded through Si-O-Si and Si-O-Al bonds. These bonds should prevent the stylus (which has a contact force of several nanonewtons) from moving the aluminosilicate islands from step sites to lower terraces. Repeat scan gave the same morphology regardless of the scan direction. Therefore, the morphology seen in these images was due not to scratches caused by the AFM stylus during scanning but the dissolution caused by the interaction with the NaOH aqueous solution.

J. Phys. Chem., Vol. 100, No. 47, 1996 18475 These isolated and agglomerated islands are part of the topmost aluminosilicate layer that was resistant to attack by the NaOH aqueous solution. The terrace surface around these islands remained flat, as shown in Figure 3b. This can be explained by assuming that the dissolution rate of the topmost aluminosilicate layer was larger than that of the underlying layer, and thus the topmost layer dissolved first, followed by the dissolution of the next layer, and so on. Consequently, the dissolution of heulandite proceeds via layer-by-layer dissolution of aluminosilicate layers. The dissolution of quartz, a typical silicate, in an alkaline aqueous solution involves first the migration of “β”-type SiO4 from ledges onto smooth regions (e.g., terraces), followed by their detachment into the solution.23 Here, step retreat was observed. Our AFM images revealing the drastic morphological changes in the terraces and the lack of retreat of the steps indicate that the dissolution mechanism of heulandite is completely different from that of quartz. Heulandite has a pore structure characterized by intersecting open channels of eightor 10-member tetrahedral rings (Figure 1). The (100) surface has well-ordered channel mouths of eight-member rings, which possibly explains the unique dissolution pattern of heulandite shown as Scheme 1 in Figure 7. According to the surface speciation model, the adsorption of OH- on surface functional species, >Si-OH and >Al-OH, leads to the formation of >Si-O- and >Al-O-, which then weakens the Si-O and Al-O bonds in the lattice of the surface and facilitates the detachment of >Si-OH and >Al-OH. Here, the dissolution rate is proportional to the surface concentration of >Si-O- and >Al-O-.20 Heulandite that we investigated contained alkali metal and alkaline earth metal ions in the channels (denoted by 1 in Figure 7). Therefore, the surface functional species that exist on the surface of both the channels (internal surface) and the terraces (external surface) are >Si-OH probably due to the framework defects. The channels can accommodate OH-, which attacks the surface >Si-OH, leading to the formation of >Si-O-. We suppose that the diffusion of OH- into the channels with the metal ions is slow due to a high activation energy. This means that the concentration of >Si-O- species at the entire surface of the uppermost aluminosilicate layers is much higher than at the surface of the underlying aluminosilicate layers. As a result, the dissolution leads to the drastic morphological change of the terrace. The aluminosilicate layers bridge each other by the Si-O-Si and Si-O-Al bonds (denoted by 2 in Figure 7). These bonds form the polyhedral framework of the aluminosilicate layers, too (denoted by 3 in Figure 7). These bonds are hydrolyzed catalytically by OHand H+ ions.24 This hydrolysis may enhance the dissolution of the uppermost layers. The higher concentration of OH- at the entrance of the channels due to the same reason as mentioned above results in the facial hydrolysis of the Si-O-Si and SiO-Al bonds of the uppermost aluminosilicate layers. When enough Si-O-Si and Si-O-Al bonds are hydrolyzed, that portion of the polymeric framework forming the uppermost aluminosilicate layers becomes unstable and eventually detaches from the surface into solution. Figure 4a shows an image of a (010) surface of heulandite in deionized water. The image shows that the morphology for the cleaved plane has several steps, similar to that reported by Scandella et al.25 The height of each step was about 9 Å, which again corresponds to the thickness of an aluminosilicate layer B (Figure 1c). Figure 4b shows an AFM image of a (010) surface of heulandite in a 0.2 N H2SO4 aqueous solution taken after the sample had been immersed in the solution for about

Figure 1. Schematic of the framework structure of heulandite: (a, top left) outline of one unit cell; (b, bottom left) (100) surface parallel to the a-axis (along the arrow indicated by A); (c, bottom middle) (001) surface parallel to the c-axis (along the arrow indicated by B); and (d, bottom right) (010) cleaved surface. The numbers 8 and 10 in parts b and c indicate the channels composed of eight- and 10-

member rings, respectively. The yellow and red lines are T (where T is either Si or Al) and oxygen, respectively. The white circles in part d are outlines of the pores on the (010) surface. The red balls in part d are coordinatively unsaturated surface oxygens, which react with water to form surface hydroxyl groups under atmospheric conditions.

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Figure 2. (a, top) Top-view AFM image for a (100) surface of heulandite in a 0.1 N NaOH aqueous solution; (b, middle) cross-sectional profile along the line labeled A-B in part a; (c, bottom) top-view AFM images for a (100) surface of heulandite immersed in a 0.1 N NaOH aqueous solution at room temperature for about one month.

30 min at room temperature. The immersion induced the formation of pits, whose size increased with immersion time. The image clearly shows shallow pits (depth about 9 Å) over the entire terrace, with several pits up to about 3000 Å in diameter. The terraces were composed of small rectangular particles having an average size of 300 Å × 600 Å. Similar to the dissolution in an alkaline solution (NaOH), there was no evidence of step retreat. These pits were also on step sites.

Figure 5a is a close-up image of a pit formed on the step site labeled by the arrow in Figure 4b. Figure 5b,c shows crosssectional profiles along the lines A-B and C-D in Figure 5a. The images clearly reveal the characteristics of the dissolution process. The step height was about 9 Å (the vertical distance between the two red arrowheads in Figure 5b). Figure 5c indicates that the pit depth corresponds well with the thickness of an aluminosilicate layer (the vertical distance between the

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Figure 3. Perspective-view AFM images for a (100) surface of heulandite immersed in a 0.1 N NaOH aqueous solution at room temperature for about one month.

green and white arrowheads). The bottom surface of the pit remained as flat as the surface before immersion (indicated by the arrow in Figure 5c). A step can be seen in the pit; again,

the step height of about 9 Å (the vertical distance between the two red arrowheads in Figure 5c) corresponds well with the thickness of an aluminosilicate layer. The depth of the pits and

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Figure 4. (a, top) Top-view AFM image of a (010) surface of heulandite in pure water; (b, bottom) top-view AFM image of a (010) surface of heulandite immersed in a 0.2 N H2SO4 aqueous solution for about 30 min at room temperature.

the difference in surface morphology between the inside and outside of the pits disclose the layer-by-layer dissolution of aluminosilicate layers. The formation of the pits on the terraces and no step retreat suggest that the dissolution mechanism operating in an acidic solution is the same as that in an alkaline solution, as shown in Scheme 1 (Figure 7). The pore structure enhances the effectiveness of H+ in attacking the surface species, >Si-OH, and either the Si-O-Al or Si-O-Si bonds of the uppermost aluminosilicate layers. The result is the formation of pits on the terraces.

Although the dissolution mechanism is the same, the morphological changes in the acidic aqueous solution occur at a significantly faster rate than those in the alkaline aqueous solution (compare the sample immersed for 30 min as shown in Figure 4a with the smaple immersed for one month as shown in Figure 2a). This can be ascribed to the rapid hydrolysis of Al-O-Si bonds in an acidic solution compared with that of Si-O-Si bonds in an alkaline solution.24 The weakening of the bonds forming the uppermost layers by acid treatment was investigated by using an AFM stylus to

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Figure 5. (a, top) Close-up top-view AFM image of the pit labeled by the arrow in Figure 4b; (b,c) cross-sectional profiles along the lines A-B and C-D in part a, respectively.

etch a treated sample. Figure 6 shows an AFM image (14 µm × 14 µm) of an area that was treated with 0.2 N H2SO4 and a small section etched by scanning with the AFM stylus. The etching was done by scanning the AFM stylus over a 10 µm × 10 µm area under relatively strong force (10-8 N). The sample was then rotated 45° and a 8.6 µm × 8.6 µm area was rescanned under strong force. Following the etching, the area was scanned over a wider area (14 µm × 14 µm), resulting in the image shown in Figure 6. The depth of the etched area is about 10 Å, which is the thickness of one aluminosilicate layer. This supports our model that the bonds forming the uppermost layers are weakened by the acid treatment. Under prolonged treatment,

sections of such surface layers become unstable and presumably detach from the bulk material, leaving the pits that have been seen. The position of the terrace edge can be used to understand whether or not the dissolution process involves horizontal edge retreat or vertical (depthwise) surface dissolution). In Figure 5a the line of an edge of a terrace can be seen running through areas where pits were formed (see broken line in Figure 5a). The fact that the line runs directly through the pits indicates that the terrace edges that existed on the surface before treatment have not retreated, but have instead been replicated vertically as the surface has been removed. This is in line with our model

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Figure 6. “Drawing” on the dissolving surface of heulandite in a 0.2 N H2SO4 aqueous solution that was obtained by the scanning of a stylus with a force greater than 10-8 N.

Figure 7. Schematic of the dissolution scenario of heulandite in either a 0.2 N H2SO4 or 0.1 N NaOH aqueous solution.

that dissolution does not involve edge retreat, but instead involves sequential removal of surface layers. Consequently, the surface morphology seems to be intact after dissolution. That the structure does not change during the dealumination process

has been confirmed by high-resolution TEM imaging of dealuminated FAU zeolite.17,18 Therefore, such an intact structure during dissolution appears to be common in zeolites and can be ascribed to their pore structure.

18482 J. Phys. Chem., Vol. 100, No. 47, 1996 Conclusion AFM allows imaging of the initial surfaces morphological changes of heulandite that result from dissolution in acidic and alkaline aqueous solutions. For the first time, we have visualized the unique dissolution process of heulandite in either solution, namely, a layer-by-layer process. These images provide evidence that this dissolution is due to the pore structure and thus may be common to a wide range of zeolite crystals. This work demonstrates the unique capabilities of AFM in giving insight into the dissolution mechanism of zeolite crystals. Acknowledgment. This work was performed at Mitsui Toatsu Chemicals as a part of the research and development joint project on the civil industrial technology supported by the New Energy and Industrial Technology Development Organization. References and Notes (1) Unwin, P. R.; Macpherson, J. V. Chem. Soc. ReV. 1995, 109. (2) Gratz, A. J.; Manne, S.; Hansma, P. K. Science 1991, 251, 1343. (3) Johnson, P. A.; Eggleston, C. M.; Hochella, M. F. Am. Mineral. 1991, 76, 1442. (4) Hillner, P. E.; Gratz, A. J.; Manne, S.; Hansma, P. K. Geology 1992, 20, 359. (5) Hillner, P. E.; Manne, S.; Gratz, A. J.; Hansma, P. K. Ultramicroscopy 1992, 42, 1387. (6) Durbin, S. O.; Carlson, W. E. J. Cryst. Growth 1992, 22, 71.

Yamamoto et al. (7) Manne, S.; Cleveland, J. P.; Stucky, G. D.; Hansma, P. K. J. Cryst. Growth 1993, 130, 333. (8) H. Shindo, H.; Nozoye, H. Sur. Sci. 1993, 287/288, 1030. (9) Gratz, A. J.; Hillner, P. E. J. Cryst. Growth 1993, 129, 789. (10) Hillner, P. E.; Lacmann, R.; Schneeweiss, M. A. J. Cryst. Growth 1994, 141, 291. (11) Boshach, D.; Rammensee, W. Geochim. Acta 1994, 58, 843. (12) Schmidt, W. V.; Alkire, R. C. J. Electrochem. Soc. 1994, 141, L85. (13) Whitingham, M. S.; Jacobson, A. J. Intercalation Chemistry; Academic Press: New York, 1982. (14) Weitkamp, J. Catalysis and Adsorption by Zeolites, Studies in Surface Science and Catalysis; Ohlmann, G., Pfeifer, H., Frick, R., Eds. Elsevier: Amsterdam, 1991; Vol. 65. (15) Breck, D. W. Zeolite Molecular SieVes; Wiley: New York, 1974. (16) Barrer, R. M.; Coughlan, B. Molecular SieVes; Society of Chemical Industry: London, 1968; p 141. (17) Alfredson, V.; Ohsuna, T.; Terasaki, O.; Bovin, J. O. Angew. Chem., Int. Ed. Engl. 1993, 32, 1210. (18) Ohsuna, T.; Terasaki, O.; Watanabe, D.; Anderson, M. W.; Carr, S. W. Chem. Mater. 1994, 6, 2201. (19) Delannay, F.; Ccekiewicz, S. J. Zeolite 1985, 5, 69. (20) Ragnarsdottir, K. V. Geochim. Cosmochim. Acta 1993, 57, 2439. (21) Merkle, B.; Slaughter, M. Am. Mineral. 1968, 53, 1120. (22) Binder, G.; Scandella, L.; Schumacher, A.; Kruse, N.; Prins, R. Zeolites 1996, 16, 2. (23) Gratz, A. J.; Bird, P.; Quiro, G. B. Geochim. Cosmochim. Acta 1990, 54, 2911. (24) Casy, W. H.; Wetrich, H. R.; Arnold, G. W. Geochim. Cosmochim. Acta 1988, 52, 2785. (25) Scandella, L.; Kruse, N.; Prins, R. Surf. Sci. Lett. 1993, 281, L331.

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