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Zeolite Y coatings on Al-2024-T3 substrate by a three-step synthesis method
Microporous and Mesoporous Materials 86 (2005) 243–248 www.elsevier.com/locate/micromeso
Zeolite Y coatings on Al-2024-T3 substrate by a three-step synthesis method Ronnie Munoz, Derek Beving, Yachun Mao, Yushan Yan
*
Department of Chemical and Environmental Engineering, University of California, 900 University Avenue, Riverside, CA 92521, United States Received 27 March 2005; received in revised form 8 June 2005; accepted 15 June 2005 Available online 1 September 2005
Abstract A zeolite Y coating has been formed on aluminum alloy 2024-T3 (Al 2024-T3) by a novel three-step synthesis method. In the first step, the bottom layer is formed by synthesizing a high-silica-zeolite (HSZ) ZSM-5 film on the Al 2024-T3 substrate. In the second step, the ZSM-5 film is seeded with zeolite Y followed by a short ZSM-5 synthesis to form the bridging layer. Finally, the low-silicazeolite (LSZ) Y film is synthesized on the bridging layer by seeded growth to form the top layer. The ZSM-5 film offers excellent corrosion protection for the aluminum under the high pH conditions required for LSZ Y synthesis. The bridging layer is critical in providing adhesion between the bottom and top layers. 2005 Elsevier Inc. All rights reserved. Keywords: Zeolite Y; Coating; Film; Porous; Aluminum
1. Introduction Low-silica-zeolites (LSZs) such as A (LTA), Y and X (FAU) in powder composite form (e.g., pellets) have proven useful in many industrial applications. As coatings on porous substrates they form membranes capable of separating mixtures on the molecular level [1–6]. Examples include the separation of carbon dioxide from oxygen and saturated from unsaturated hydrocarbons. Their utilization as coatings on non-porous substrates can extend and improve adsorption applications over powder based adsorbents. For these adsorption applications, LSZ coatings are applied to structured substrates (e.g., monoliths), and the benefits are manifested as higher heat and mass transfer, lower pressure drop, and no attrition [7]. *
Corresponding author. Tel.: +1 951 827 2068; fax: +1 951 827 5696. E-mail address: [email protected] (Y. Yan). 1387-1811/$ - see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.06.032
Aluminum is a metal with many attractive properties including light weight, high mechanical strength, high heat conductivity, and high fabricability [8]. It is no surprise to see that aluminum has made its way into many industrial applications. The combination of LSZ coatings and aluminum is very advantageous. One way of improving an air conditioning system is to increase the heat transfer efficiency of the exchanging surfaces by applying a coating that selectively adsorbs moisture from the air. Hydrophilic and antimicrobial LSZ coatings on heat exchangers are also useful for water separation in space [9]. LSZs have proved to be very effective and efficient in moisture adsorption. By combining the low weight and high thermal conductivity of aluminum (approximately 15 times that of stainless steel) with the ability of LSZ to remove contaminants and produce oxygen enriched air, desirable alternatives are offered for weight and size conscious environments. Unfortunately, all attempts to combine LSZ and aluminum have met with great difficulty. Much success has
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been made in synthesizing LSZ coatings on ceramic, steel and other corrosion resistant metal substrates [1– 6,10–23]. Problems arise when attempting to synthesize LSZ coatings on aluminum substrates. LSZ synthesis solutions are known to have a very high pH, often greater than 14. This high pH is very corrosive and dissolves the aluminum substrate during synthesis. Here we show a novel three-step synthesis method that for the first time allows for the union of LSZ coating with aluminum substrates.
2. Experimental 2.1. Preparation of substrates A large sheet of aluminum alloy 2024-T3 was cut into substrates measuring 3.5 cm · 2 cm. A cleaning solution was prepared by mixing 3 g of Alconox detergent (Alconox Inc.) with 400 ml of double de-ionized (DDI) water and heated to 60 C. The substrates were then placed into the cleaning solution for 1 h at 60 C. They were removed, rinsed under de-ionized (DI) water with rubbing, and dried with compressed air. 2.2. Synthesis of bottom layer The bottom layer was formed by in situ crystallization of the ZSM-5. The synthesis solution had a molar composition of 0.16TPAOH:0.64NaOH:1TEOS:92H2O: 0.0018Al. TPAOH is tetraproplyammonium hydroxide and TEOS is tetraethylorthosilicate. A typical solution preparation started with the addition of 0.0126 g aluminum powder (200 mesh, 99.95 + %, Aldrich) to 100 g DDI water. 6.33 g sodium hydroxide (pellets, 97 + %, Aldrich) was then added to the solution and stirred for 30 min. Then 297.03 g DDI water and 20.12 g TPAOH (40 wt.%, Sachem) were added to the solution and stirred for 30 min. Finally, 51.5 g TEOS (98 wt.%, Aldrich) was added to the solution and stirred for 4 h at room temperature. Then two vertically fixed substrates and 30 ml of ZSM-5 synthesis solution were loaded into a Teflon lined 45 mL autoclave (Parr Instrument Co., Moline, IL), sealed and heated in a convection oven at 175 C for 16 h. The autoclave was then cooled under running tap water and the coated substrates were removed from the autoclaves, rinsed under DDI water and dried with compressed air. 2.3. Synthesis of bridging layer Preparation of the bridging layer began with the synthesis of zeolite Y seeds [24]. The synthesis solution for zeolite Y seeds had a molar composition of 10SiO2:1Al2O3:14Na2O:800H2O. A typical solution preparation started with the addition of 0.8308 g
aluminum powder (200 mesh, 99.95 + %, Aldrich) to 197.26 g DDI water. 17.23 g sodium hydroxide (pellets, 97 + %, Aldrich) was added to the solution and stirred for 60 min. 30.82 g of Ludox LS30 colloidal silica (30 wt.%, silica, Aldrich) was then added to the solution and stirred for 8 h at room temperature. 25 mL of the solution was placed in a Teflon vessel for microwave synthesis (MARS 5, Model XP-1500, CEM Corp., Matthews, NC). The temperature was increased from room temperature to 120 C in 90 s, held isothermally for 30 s, cooled naturally to 100 C, and held isothermally for 2 h. For washing, the solution was placed in a centrifuge at 3000 rpm for 15 min, decanted, diluted with DDI, and dispersed using sonication. The washing procedure was repeated until the decant pH measured less than 8. The sample was dried and dispersed in ethanol at 40 wt.%. An aqueous solution of 1.0 wt.% hydroxypropyl cellulose (HPC, Avg. MW = 100,000, Aldrich) with a pH of 8 was prepared. The ZSM-5 coated aluminum substrate was submersed in the solution for 2 min, removed, air dried, and the process repeated once more. The sample was then submersed in the zeolite Y seed suspension in ethanol for 2 min, removed, air dried, and placed into a convection oven at 60 C for 15 min. The sample was then cooled and submersed in the HPC solution so that the seeding process could be repeated once more. In the final step, for the application of the bridging layer, the same solution and method used in the synthesis of the bottom layer (ZSM-5 layer) was used in the coating of the sample. The variation was a 4.5 h (instead of 16 h) heating in the convection oven. 2.4. Synthesis of top layer Application of the top layer was a two-step process that began with the seeding of the sample possessing the bridging layer. The sample possessing the bridging layer was seeded with the same solutions and method used for the bridging layer. No modifications were made and the final step (ZSM-5 synthesis), for the application of the bridging layer, was not carried out. The final zeolite Y synthesis solution [25,26] for seeded growth had a molar composition of 1SiO2: 0.2Al2O3:10Na2O:200H2O. A typical solution preparation began with the addition of 0.432 g aluminum powder (200 mesh, 99.95 + %, Aldrich) to 133.2 g double de-ionized water. 32.0 g sodium hydroxide (pellets, 97 + %, Aldrich) was added to the solution and stirred for 60 min. 8.0 g of Ludox LS30 colloidal silica (30 wt.%, silica, Aldrich) was then added to the solution and stirred for 4 h at room temperature. Then two samples possessing zeolite Y seeded bridging layers along with 50 mL of the zeolite Y solution were placed vertically into a Teflon vessel for microwave synthesis
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Fig. 1. (a) SEM image and (b) EDAX spectrum of ZSM-5 bottom layer on Al-2024-T3.
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Fig. 4. XRD pattern of prepared zeolite Y seeds.
Fig. 2. XRD pattern of ZSM-5 bottom layer on Al-2024-T3.
Fig. 3. SEM image of prepared zeolite Y seeds.
Fig. 5. (a) XRD of zeolite Y seeded bottom layer and (b) XRD of completed bridging Layer. Peaks indicate that ZSM-5 and zeolite Y are present.
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(MARS 5, Model XP-1500, CEM Corp., Matthews, NC). The temperature was increased from room temperature to 80 C in 90 s, held isothermally for 1 min, cooled naturally to 60 C, and held isothermally for 1 h. The entire synthesis for the top layer may be repeated multiple times to increase the thickness of the zeolite Y top layer. 2.5. Characterization A Siemens D-500 diffractometer using Cu Ka radiation was used to obtain X-ray diffraction (XRD) patterns of the films. A Philips XL-30 instrument operating a 20 kV was used to perform the scanning electron microscopy (SEM) and the energy dispersive analysis of X-rays (EDAX). The samples were prepared for use by hydrofluoric acid etching of the coating to expose a cross-section of the sample.
3. Results and discussion The ZSM-5 coating was prepared on the Al-2024-T3 substrate and characterized. The presence of a continuous and evenly inter-grown polycrystalline zeolite film onto an Al-2024-T3 substrate was verified by SEM (Fig. 1(a)). The EDAX spectrum of the ZSM-5 coating indicates a high Si/Al ratio (Fig. 1(b)). XRD was used to verify the formation of a ZSM-5 film onto the AA-2024T3 substrate (Fig. 2). The zeolite coated substrate has a standard ZSM-5 diffraction pattern. It is known that ZSM-5 coatings containing the above characteristics offer excellent protection against corrosive environments
[27]. By affording the aluminum substrate this benefit, the eventual coating of a LSZ is made possible despite the high pH of the synthesis solution. The zeolite Y seeds were prepared and characterized before their use in the seeding of the ZSM-5 bottom layer. The zeolite Y seeds are approximately 850 nm in diameters (Fig. 3). An XRD pattern confirms these are pure zeolite Y seeds (Fig. 4). An XRD pattern was taken of the bridging layer before and after the short ZSM-5 synthesis. Before the short ZSM-5 synthesis, the XRD pattern of the zeolite Y seeded bottom layer is a combination of the ZSM-5 XRD pattern and the zeolite Y seed XRD pattern (Fig. 5(a)). These two XRD patterns are present in the completed bridging layer as well (Fig. 5(b)). This demonstrates that the bridging layer is composed of zeolite Y and ZSM-5. The reason that the two XRD spectra have different intensity is a direct consequence of changing composition through the bridging layer. The XRD spectrum of the bridging layer before the ZSM-5 synthesis (Fig. 5(a)) more closely resembles that of the zeolite Y seeds (Fig. 4) because of the large amount of zeolite Y on the surface right after seeding. After the ZSM-5 synthesis to complete the bridging layer, the XRD spectrum of the coating (Fig. 5(b)) resembles the ZSM-5 spectrum (Fig. 2) more closely because much of the zeolite Y seed has been covered up by the ZSM-5 crystals. Additional evidence for the existence of two zeolites in one layer is provided by examination of the SEM image containing a cross-section of the bridging and bottom layers on an AA-2024-T3 substrate (Fig. 6(a)). The cross-section was formed by etching the sample with hydrofluoric acid. The top and side EDAX spectra
Fig. 6. (a) SEM of bridging and bottom layer on Al-2024-T3. EDAX spectra of the (b) top and (c) side of the bridging layer. EDAX spectra of the (d) top and (e) side of the bottom layer.
R. Munoz et al. / Microporous and Mesoporous Materials 86 (2005) 243–248
Fig. 7. (a) SEM image at low magnification and (b) EDAX spectrum of top layer surface.
of the ZSM-5 bottom layer indicate that the Si/Al ratio is relatively high in the bottom layer (Fig. 6(d) and (e)). In contrast, EDAX spectra of the top and side of the bridging layer show decreased Si/Al ratios (Fig. 6(b) and (c)). This is consistent with the presence of the aluminum containing zeolite Y seeds throughout the bridging layer. By having zeolite Y and ZSM-5 present in the same layer, a bridge is formed, allowing a connection to the top layer. The final top layer of zeolite Y was synthesized by seeded growth on the bridging layer. The presence of a continuous and evenly inter-grown zeolite Y film onto the bridging layer was verified by SEM at low and high magnifications (Figs. 7(a) and 8). The high magnification image shows that the coating has a rather high surface roughness, which is believed to beneficial for adsorption applications. The EDAX spectrum of the zeolite Y top layer possesses a low Si/Al ratio (Fig. 7(b)). XRD was used to verify the formation of a
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Fig. 9. XRD pattern of top layer surface.
zeolite Y layer onto the bridging layer (Fig. 9). The coated substrate had a standard zeolite Y diffraction pattern. Together, the known composition of our zeolite Y synthesis solution and the low Si/Al ratio can be used to successfully conclude that a LSZ was applied onto an aluminum substrate. 4. Conclusion A novel three-step synthesis method was used to successfully apply a LSZ coating to an aluminum substrate. The synthesis of the HSZ bottom layer was able to offer the aluminum substrate corrosion protection so that a LSZ top layer could be synthesized. The synthesis of the bridging layer allowed for the adhesion of the LSZ coating to the substrate. The final LSZ synthesis produced a top layer that was continuous and evenly inter-grown. This new composite coating has the corrosion resistance from the bottom ZSM-5 layer and hydrophilicity and antimicrobial capability from the top zeolite Y layer and thus can be potentially useful for space applications.
Acknowledgement We thank the Strategic Environmental Research and Technology Program (SERDP) of the Department of Defense for financial support.
References
Fig. 8. SEM image at high magnification of top layer surface.
[1] V. Nikolakis, G. Xomeritakis, A. Abibi, M. Dickson, M. Tsapatsis, D. Vlachos, Journal of Membrane Science 184 (2001) 209–219.
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[2] L. Boudreau, M. Tsapatsis, Chemistry of Materials 9 (1997) 1705– 1709. [3] S. Morooka, T. Kuroda, K. Kusakabe, Studies in Surface Science and Catalysis 114 (1998) 665–668. [4] K. Aoki, K. Kusakabe, S. Morooka, Journal of Membrane Science 141 (1998) 197–205. [5] B. Jeong, Y. Hasegawa, K. Sotowa, K. Kusakabe, S. Morooka, Industrial & Engineering Chemistry Research 41 (2002) 1768– 1773. [6] B. Jeong, Y. Hasegawa, K. Sotowa, K. Kusakabe, S. Morooka, Journal of Chemical Engineering of Japan 35 (2002) 167–172. [7] Y.S. Yan, H.T. Wang, Encyclopedia of Nanoscience and Nanotechnology 7 (2004) 763–781. [8] T.M. Totten, D.S. Mackenzie (Eds.), Handbook of Aluminum: Physical Metallurgy and Processes, Marcel Dekker, 2003. [9] A. McDonnell, D. Beving, A. Wang, W. Chen, Y. Yan, Advanced Functional Materials 15 (2005) 336–340. [10] S. Davis, E. Borgstedt, S. Suib, Chemistry of Materials 2 (1990) 712–719. [11] V. Valtchev, S. Mintova, I. Vasilev, Journal of the Chemical Society—Chemical Communications (1994) 979–980. [12] G. Clet, J. Peters, H. van Bekkum, Langmuir 16 (2000) 3993– 4000. [13] G. Clet, J. Jansen, H. van Bekkum, Chemistry of Materials 11 (1999) 1696–1702.
[14] G. Clet, L. Gora, N. Nishiyama, J. Jansen, H. van Bekkum, T. Maschmeyer, Chemical Communications (2001) 41–42. [15] Y. Lee, P. Dutta, Journal of Physical Chemistry B 106 (2002) 11898–11904. [16] H. Kita, T. Inoue, H. Asamura, K. Tanaka, K. Okamoto, Chemical Communications (1997) 45–46. [17] K. Kusakabe, T. Kuroda, S. Morooka, Journal of Membrane Science 148 (1998) 13–23. [18] K. Kusakabe, T. Kuroda, A. Murata, S. Morooka, Industrial & Engineering Chemistry Research 36 (1997) 649–655. [19] M. Lassinantti, J. Hedlund, J. Sterte, Microporous and Mesoporous Materials 38 (2000) 25–34. [20] K. Weh, M. Noack, I. Sieber, J. Caro, Microporous and Mesoporous Materials 54 (2002) 27–36. [21] T. Bein, K. Brown, G. Frye, C. Brinker, Journal of the American Chemical Society 111 (1989) 7640–7641. [22] S. Mintova, D. Radev, V. Valtchev, Metall 52 (1998) 447–450. [23] V. Valtchev, S. Mintova, Zeolites 15 (1995) 171–175. [24] H. Katsuki, S. Furuta, S. Komarneni, Journal of Porous Materials 8 (2001) 5–12. [25] I. Kumakiri, T. Yamaguchi, S. Nakao, Industrial & Engineering Chemistry Research 38 (1999) 4682–4688. [26] T. Cetin, M. Tather, A. Erdem-Senatalar, U. Demirler, M. Urgen, Microporous and Mesoporous Materials 47 (2001) 1–14. [27] X. Cheng, Z. Wang, Y. Yan, Electrochemical and Solid State Letters 4 (2001) B23–B26.
Zeolite Y coatings on Al-2024-T3 substrate by a three-step synthesis method Ronnie Munoz, Derek Beving, Yachun Mao, Yushan Yan
*
Department of Chemical and Environmental Engineering, University of California, 900 University Avenue, Riverside, CA 92521, United States Received 27 March 2005; received in revised form 8 June 2005; accepted 15 June 2005 Available online 1 September 2005
Abstract A zeolite Y coating has been formed on aluminum alloy 2024-T3 (Al 2024-T3) by a novel three-step synthesis method. In the first step, the bottom layer is formed by synthesizing a high-silica-zeolite (HSZ) ZSM-5 film on the Al 2024-T3 substrate. In the second step, the ZSM-5 film is seeded with zeolite Y followed by a short ZSM-5 synthesis to form the bridging layer. Finally, the low-silicazeolite (LSZ) Y film is synthesized on the bridging layer by seeded growth to form the top layer. The ZSM-5 film offers excellent corrosion protection for the aluminum under the high pH conditions required for LSZ Y synthesis. The bridging layer is critical in providing adhesion between the bottom and top layers. 2005 Elsevier Inc. All rights reserved. Keywords: Zeolite Y; Coating; Film; Porous; Aluminum
1. Introduction Low-silica-zeolites (LSZs) such as A (LTA), Y and X (FAU) in powder composite form (e.g., pellets) have proven useful in many industrial applications. As coatings on porous substrates they form membranes capable of separating mixtures on the molecular level [1–6]. Examples include the separation of carbon dioxide from oxygen and saturated from unsaturated hydrocarbons. Their utilization as coatings on non-porous substrates can extend and improve adsorption applications over powder based adsorbents. For these adsorption applications, LSZ coatings are applied to structured substrates (e.g., monoliths), and the benefits are manifested as higher heat and mass transfer, lower pressure drop, and no attrition [7]. *
Corresponding author. Tel.: +1 951 827 2068; fax: +1 951 827 5696. E-mail address: [email protected] (Y. Yan). 1387-1811/$ - see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.06.032
Aluminum is a metal with many attractive properties including light weight, high mechanical strength, high heat conductivity, and high fabricability [8]. It is no surprise to see that aluminum has made its way into many industrial applications. The combination of LSZ coatings and aluminum is very advantageous. One way of improving an air conditioning system is to increase the heat transfer efficiency of the exchanging surfaces by applying a coating that selectively adsorbs moisture from the air. Hydrophilic and antimicrobial LSZ coatings on heat exchangers are also useful for water separation in space [9]. LSZs have proved to be very effective and efficient in moisture adsorption. By combining the low weight and high thermal conductivity of aluminum (approximately 15 times that of stainless steel) with the ability of LSZ to remove contaminants and produce oxygen enriched air, desirable alternatives are offered for weight and size conscious environments. Unfortunately, all attempts to combine LSZ and aluminum have met with great difficulty. Much success has
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R. Munoz et al. / Microporous and Mesoporous Materials 86 (2005) 243–248
been made in synthesizing LSZ coatings on ceramic, steel and other corrosion resistant metal substrates [1– 6,10–23]. Problems arise when attempting to synthesize LSZ coatings on aluminum substrates. LSZ synthesis solutions are known to have a very high pH, often greater than 14. This high pH is very corrosive and dissolves the aluminum substrate during synthesis. Here we show a novel three-step synthesis method that for the first time allows for the union of LSZ coating with aluminum substrates.
2. Experimental 2.1. Preparation of substrates A large sheet of aluminum alloy 2024-T3 was cut into substrates measuring 3.5 cm · 2 cm. A cleaning solution was prepared by mixing 3 g of Alconox detergent (Alconox Inc.) with 400 ml of double de-ionized (DDI) water and heated to 60 C. The substrates were then placed into the cleaning solution for 1 h at 60 C. They were removed, rinsed under de-ionized (DI) water with rubbing, and dried with compressed air. 2.2. Synthesis of bottom layer The bottom layer was formed by in situ crystallization of the ZSM-5. The synthesis solution had a molar composition of 0.16TPAOH:0.64NaOH:1TEOS:92H2O: 0.0018Al. TPAOH is tetraproplyammonium hydroxide and TEOS is tetraethylorthosilicate. A typical solution preparation started with the addition of 0.0126 g aluminum powder (200 mesh, 99.95 + %, Aldrich) to 100 g DDI water. 6.33 g sodium hydroxide (pellets, 97 + %, Aldrich) was then added to the solution and stirred for 30 min. Then 297.03 g DDI water and 20.12 g TPAOH (40 wt.%, Sachem) were added to the solution and stirred for 30 min. Finally, 51.5 g TEOS (98 wt.%, Aldrich) was added to the solution and stirred for 4 h at room temperature. Then two vertically fixed substrates and 30 ml of ZSM-5 synthesis solution were loaded into a Teflon lined 45 mL autoclave (Parr Instrument Co., Moline, IL), sealed and heated in a convection oven at 175 C for 16 h. The autoclave was then cooled under running tap water and the coated substrates were removed from the autoclaves, rinsed under DDI water and dried with compressed air. 2.3. Synthesis of bridging layer Preparation of the bridging layer began with the synthesis of zeolite Y seeds [24]. The synthesis solution for zeolite Y seeds had a molar composition of 10SiO2:1Al2O3:14Na2O:800H2O. A typical solution preparation started with the addition of 0.8308 g
aluminum powder (200 mesh, 99.95 + %, Aldrich) to 197.26 g DDI water. 17.23 g sodium hydroxide (pellets, 97 + %, Aldrich) was added to the solution and stirred for 60 min. 30.82 g of Ludox LS30 colloidal silica (30 wt.%, silica, Aldrich) was then added to the solution and stirred for 8 h at room temperature. 25 mL of the solution was placed in a Teflon vessel for microwave synthesis (MARS 5, Model XP-1500, CEM Corp., Matthews, NC). The temperature was increased from room temperature to 120 C in 90 s, held isothermally for 30 s, cooled naturally to 100 C, and held isothermally for 2 h. For washing, the solution was placed in a centrifuge at 3000 rpm for 15 min, decanted, diluted with DDI, and dispersed using sonication. The washing procedure was repeated until the decant pH measured less than 8. The sample was dried and dispersed in ethanol at 40 wt.%. An aqueous solution of 1.0 wt.% hydroxypropyl cellulose (HPC, Avg. MW = 100,000, Aldrich) with a pH of 8 was prepared. The ZSM-5 coated aluminum substrate was submersed in the solution for 2 min, removed, air dried, and the process repeated once more. The sample was then submersed in the zeolite Y seed suspension in ethanol for 2 min, removed, air dried, and placed into a convection oven at 60 C for 15 min. The sample was then cooled and submersed in the HPC solution so that the seeding process could be repeated once more. In the final step, for the application of the bridging layer, the same solution and method used in the synthesis of the bottom layer (ZSM-5 layer) was used in the coating of the sample. The variation was a 4.5 h (instead of 16 h) heating in the convection oven. 2.4. Synthesis of top layer Application of the top layer was a two-step process that began with the seeding of the sample possessing the bridging layer. The sample possessing the bridging layer was seeded with the same solutions and method used for the bridging layer. No modifications were made and the final step (ZSM-5 synthesis), for the application of the bridging layer, was not carried out. The final zeolite Y synthesis solution [25,26] for seeded growth had a molar composition of 1SiO2: 0.2Al2O3:10Na2O:200H2O. A typical solution preparation began with the addition of 0.432 g aluminum powder (200 mesh, 99.95 + %, Aldrich) to 133.2 g double de-ionized water. 32.0 g sodium hydroxide (pellets, 97 + %, Aldrich) was added to the solution and stirred for 60 min. 8.0 g of Ludox LS30 colloidal silica (30 wt.%, silica, Aldrich) was then added to the solution and stirred for 4 h at room temperature. Then two samples possessing zeolite Y seeded bridging layers along with 50 mL of the zeolite Y solution were placed vertically into a Teflon vessel for microwave synthesis
R. Munoz et al. / Microporous and Mesoporous Materials 86 (2005) 243–248
Fig. 1. (a) SEM image and (b) EDAX spectrum of ZSM-5 bottom layer on Al-2024-T3.
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Fig. 4. XRD pattern of prepared zeolite Y seeds.
Fig. 2. XRD pattern of ZSM-5 bottom layer on Al-2024-T3.
Fig. 3. SEM image of prepared zeolite Y seeds.
Fig. 5. (a) XRD of zeolite Y seeded bottom layer and (b) XRD of completed bridging Layer. Peaks indicate that ZSM-5 and zeolite Y are present.
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(MARS 5, Model XP-1500, CEM Corp., Matthews, NC). The temperature was increased from room temperature to 80 C in 90 s, held isothermally for 1 min, cooled naturally to 60 C, and held isothermally for 1 h. The entire synthesis for the top layer may be repeated multiple times to increase the thickness of the zeolite Y top layer. 2.5. Characterization A Siemens D-500 diffractometer using Cu Ka radiation was used to obtain X-ray diffraction (XRD) patterns of the films. A Philips XL-30 instrument operating a 20 kV was used to perform the scanning electron microscopy (SEM) and the energy dispersive analysis of X-rays (EDAX). The samples were prepared for use by hydrofluoric acid etching of the coating to expose a cross-section of the sample.
3. Results and discussion The ZSM-5 coating was prepared on the Al-2024-T3 substrate and characterized. The presence of a continuous and evenly inter-grown polycrystalline zeolite film onto an Al-2024-T3 substrate was verified by SEM (Fig. 1(a)). The EDAX spectrum of the ZSM-5 coating indicates a high Si/Al ratio (Fig. 1(b)). XRD was used to verify the formation of a ZSM-5 film onto the AA-2024T3 substrate (Fig. 2). The zeolite coated substrate has a standard ZSM-5 diffraction pattern. It is known that ZSM-5 coatings containing the above characteristics offer excellent protection against corrosive environments
[27]. By affording the aluminum substrate this benefit, the eventual coating of a LSZ is made possible despite the high pH of the synthesis solution. The zeolite Y seeds were prepared and characterized before their use in the seeding of the ZSM-5 bottom layer. The zeolite Y seeds are approximately 850 nm in diameters (Fig. 3). An XRD pattern confirms these are pure zeolite Y seeds (Fig. 4). An XRD pattern was taken of the bridging layer before and after the short ZSM-5 synthesis. Before the short ZSM-5 synthesis, the XRD pattern of the zeolite Y seeded bottom layer is a combination of the ZSM-5 XRD pattern and the zeolite Y seed XRD pattern (Fig. 5(a)). These two XRD patterns are present in the completed bridging layer as well (Fig. 5(b)). This demonstrates that the bridging layer is composed of zeolite Y and ZSM-5. The reason that the two XRD spectra have different intensity is a direct consequence of changing composition through the bridging layer. The XRD spectrum of the bridging layer before the ZSM-5 synthesis (Fig. 5(a)) more closely resembles that of the zeolite Y seeds (Fig. 4) because of the large amount of zeolite Y on the surface right after seeding. After the ZSM-5 synthesis to complete the bridging layer, the XRD spectrum of the coating (Fig. 5(b)) resembles the ZSM-5 spectrum (Fig. 2) more closely because much of the zeolite Y seed has been covered up by the ZSM-5 crystals. Additional evidence for the existence of two zeolites in one layer is provided by examination of the SEM image containing a cross-section of the bridging and bottom layers on an AA-2024-T3 substrate (Fig. 6(a)). The cross-section was formed by etching the sample with hydrofluoric acid. The top and side EDAX spectra
Fig. 6. (a) SEM of bridging and bottom layer on Al-2024-T3. EDAX spectra of the (b) top and (c) side of the bridging layer. EDAX spectra of the (d) top and (e) side of the bottom layer.
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Fig. 7. (a) SEM image at low magnification and (b) EDAX spectrum of top layer surface.
of the ZSM-5 bottom layer indicate that the Si/Al ratio is relatively high in the bottom layer (Fig. 6(d) and (e)). In contrast, EDAX spectra of the top and side of the bridging layer show decreased Si/Al ratios (Fig. 6(b) and (c)). This is consistent with the presence of the aluminum containing zeolite Y seeds throughout the bridging layer. By having zeolite Y and ZSM-5 present in the same layer, a bridge is formed, allowing a connection to the top layer. The final top layer of zeolite Y was synthesized by seeded growth on the bridging layer. The presence of a continuous and evenly inter-grown zeolite Y film onto the bridging layer was verified by SEM at low and high magnifications (Figs. 7(a) and 8). The high magnification image shows that the coating has a rather high surface roughness, which is believed to beneficial for adsorption applications. The EDAX spectrum of the zeolite Y top layer possesses a low Si/Al ratio (Fig. 7(b)). XRD was used to verify the formation of a
247
Fig. 9. XRD pattern of top layer surface.
zeolite Y layer onto the bridging layer (Fig. 9). The coated substrate had a standard zeolite Y diffraction pattern. Together, the known composition of our zeolite Y synthesis solution and the low Si/Al ratio can be used to successfully conclude that a LSZ was applied onto an aluminum substrate. 4. Conclusion A novel three-step synthesis method was used to successfully apply a LSZ coating to an aluminum substrate. The synthesis of the HSZ bottom layer was able to offer the aluminum substrate corrosion protection so that a LSZ top layer could be synthesized. The synthesis of the bridging layer allowed for the adhesion of the LSZ coating to the substrate. The final LSZ synthesis produced a top layer that was continuous and evenly inter-grown. This new composite coating has the corrosion resistance from the bottom ZSM-5 layer and hydrophilicity and antimicrobial capability from the top zeolite Y layer and thus can be potentially useful for space applications.
Acknowledgement We thank the Strategic Environmental Research and Technology Program (SERDP) of the Department of Defense for financial support.
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Fig. 8. SEM image at high magnification of top layer surface.
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