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Aluminum containing periodic mesoporous organosilicas: synthesis and etherification
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. Allrightsreserved
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Aluminum containing periodic mesoporous organosilicas: synthesis and etherification Jin-Won Kim^, Hyung Ik Lee*', Ji Man Kim^, Xingdong Yuan^'^ and Jae Eui Yie^* ^Catalyst and Surface Laboratory, School of Chemical Engineering and Biotechnology, Ajou University, Suwon, 442-749, Korea ^Functional Materials Laboratory, Department of Molecular Science & Technology, Ajou University, Suwon, 442-749, Korea '^Department of Petrochemical Technology, Fushun Petroleum University, Fushun, 113001, China Aluminum has been successfully incorporated within the frameworks of periodic mesoporous organosilicas (Al-PMO) by co-condensation of bis(triethoxysilyl)ethane and dibutoxyaluminotriethoxysilane. The Al-PMO materials exhibit highly ordered 2-d hexagonal structures, high surface areas, and narrow pore size distribution in the mesoporous range. The Al-PMO catalysts result in excellent catalytic activity and selectivity for etherification reaction between 2-naphthol and ethanol, which is comparable with those of beta zeolite. 1. INTRODUCTION Recently, periodic mesoporous organosilicas (PMO) have been synthesized by condensation of bridged silsequioxane in the presence of structure-directing agents, and attracted much attention due to their well-ordered mesostructures and noble framework structures [1,2]. The presence of organic groups within the frameworks is expected to give hydrophobic character and hydrothermal stability to the mesoporous materials. These properties are very important for the applications under hydrothermal conditions and organophilic reactions systems, compared with those of normal mesoporous silicas such as MCM-41 and MCM-48. However, the PMO materials constructed with organosilica frameworks (Si-PMO) are of limited use in catalysis, due to the lack of acidity and ion exchange sites. Incorporating other elements such as Al, Ti, Mn, Fe, V, etc. into the organosilica frameworks can improve the properties, which are important for the applications as catalysts and adsorbents. So far, there are a few reports on the modification with organic functional groups and on their applications [3]. It seems to be difficult to incorporate heteroatoms within the PMO frameworks with conventional methods that have been generally used in preparation of metal containing MCM-41. Etherification reaction of 2-naphthol is very important because the products have been extensively used in the fine chemical industry [4]. For example, 2-naphthyl methyl ether has been used in perfumery, which is traditionally manufactured from 2-naphthol and methanol in the presence of sulfuric acid. However, the drawbacks of such a process include corrosion, safety hazards, separation procedures, and environmental problems due to the use of sulfuric
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acid. A PMO material with solid acid properties is expected to be an excellent heterogeneous catalyst for this reaction due to its mesoporosity and hydrothermal stability. In the present work, incorporation of aluminum into the PMO frameworks has been successfully carried out by co-condensation between bis(triethoxysilyl)ethane (BTSE) and dibutoxyaluminotriethoxysilane (DBATES) in the presence of structure-directing agents, and the possibility of the materials for catalytic applications to etherification are investigated. 2. EXPERIMENTAL Al-incorporated PMO materials (Al-PMO) were synthesized by modified procedures described elsewhere [1] using BTSE as the framework source, DBATES as the aluminum source and octadecyltrimethylammonium chloride (ODTMACl) as the structure-directing agent. A typical gel compositions was 1 BTSE : 0.067 DBATES : 0.57 ODTMACl : 2.4 NaOH : 350 H2O : 10 EtOH : 0.012 HCl. BTSE and DBATES were prehydrolyzed and oligomerized under acidic conditions before mixing with surfactant solution. To investigate the effect of aluminum source on the materials, the Al-PMO materials (Si/Al — 30) were synthesized by using various kinds of aluminum sources such as A1(N03)3, Al(i-OC3H7)3 and NaA102. The resulting mixture was magnetically stirred at room temperature for 20 hr, and subsequently heated in an oven at 368 K for 20 hr. The precipitate was recovered by filtration, washed with doubly distilled water and dried at 373 K for 6 h. The as-made products were refluxed in an excess acidified ethanol with HCl to remove the surfactant. The products were obtained by filtration, washed with ethanol and dried at 373 K for 10 h. The solvent-extraction procedures were repeated three times. The Al-PMO materials were characterized by powder X-ray diffraction (XRD), N2 adsorption, FT-IR, solid-state MAS ^^Al NMR spectroscopy, thermogravimetric analysis (TGA). Etherification reactions between 2-naphthol and ethanol were carried out in a down flow fixed bed reactor at 453 K. The reaction conditions were 0.1 g of catalysts, ethanol/2-naphthol = 10/1, and reactants flow rate = 1.0 cc/h. 3. RESULTS AND DISCUSSION Figure 1 shows XRD patterns for the Si-PMO and the Al-PMO obtained by using DBATES as the aluminum source, before and after surfactant extraction. The XRD patterns for PMO materials before extraction (Figure la and Ic) give a very intense diffraction peak and two or more weak 2 4 6 8 peaks, which are characteristic of 2-d hexagonal 20/ degree (P6mm) mesostructure [5]. There are no significant Fig. 1. X-ray diffraction patterns for (a) changes upon removal of surfactant except for the as-made, (b) washed Si-PMO, (c) as- expected increase in XRD peak intensity. The AlPMO after surfactant removal (Figure Id) gives made and (d) washed Al-PMO.
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(210) and (300) peaks, which indicates excellent textural uniformity of the material. TEM image also indicates that the material has a highly ordered 2-d hexagonal structure. Lattice parameters (a), calculated from dwo spacings, for the Si-PMO and Al-PMO materials are 4.67 nm and 4.96 nm, respectively. Line broadening and large lattice parameter of the Al-PMO material, compared with those of Si-PMO, may be due to the Al incorporation within the frameworks. Nitrogen adsorption isotherms indicate that the BET surface areas of the Si-PMO and Al-PMO materials are 1050 mVg and 1692 m^/g, respectively. The pore sizes for the materials obtained by BJH model are 2.6 nm and 2.9 nm. From the lattice parameters and pore sizes, framework thickness for the materials is very similar (2.1 nm). According to IR spectra, all the PMO materials after surfactant — I — — I — removal exhibit strong bands at 2920 and 2890 cm' 100 -100 200 -200 assigned to C-H stretching and deformation Chemical shifts(ppm) vibrations, 1410 and 1270 cm' corresponding to C-H Fig. 2. 'Al MAS NMR spectra for Al- deformation vibrations, which means the presence of PMO materials obtained with (a) organic bridging group within the frameworks. Figure 2 shows ^''AI MAS NMR spectra of the AlA1(N03)3, (b) NaA102 (c) Al(i-OC3H7)3 (d) DBATES (Si/Al = 30) and (c) PMO materials obtained with different aluminum DBATES (Si/Al = 8) sources. NMR peak around 50 ppm and 0 ppm can be assigned to a tetrahedrally coordinated aluminum species within the framework and an octahedrally coordinated extraframework aluminum species, respectively. The NMR results in Figure 2 clearly show that the extraframework aluminum species are present in the Al-PMO materials obtained with A1(N03)3 and NaA102. In case of DBATES and Al(i-OC3H7)3, there is no NMR peak around 0 ppm, indicating that all the aluminum species are incorporated within frameworks. However, a significant amount of octahedrally coordinated aluminum species appears as the Si/Al ratio decreases when Al(i-OC3H7)3 is used as the aluminum source, whereas DBATES results in only framework aluminum species till Si/Al = 8 (Figure 2e). The results show that aluminum incorporation into the PMO frameworks is highly dependent on the nature of aluminum source. Figure 3 shows TGA results under nitrogen atmosphere for the Al-PMO 100 200 300 400 500 600 material before and after surfactant TerriDerature / °C Fig. 3. TGA diagrams for the Al-PMO material (a) removal. Before solvent extraction, weight loss of 5 wt% below 120 °C is attributed before and (b) after surfactant extraction
668 to the loss of small amounts of residual water adsorbed to the materials. This is followed by a weight loss of 30 wt% from 120 to 250 °C due to surfactant decomposition. An additional weight loss of 5 - 7 wt% above 500 °C indicates decomposition of organic bridging group within the framework. Figure 3b shows that there is little weight loss in the temperature range for surfactant decomposition (120 - 250 °C), indicating that the surfactant within the mesopores can be removed completely through the solvent extraction. The weight loss above 500 °C also appears in Figure 3b. According to the TGA results, the Al-PMO materials synthesized in the present work may be used below 500 °C without loss of organic bridging group within the frameworks. Figure 4 shows catalytic activities of 80 the etherification reaction between 2naphthol and ethanol. All the 70 materials give 100 % selectivity for 2naphthylethylether. As shown in ^ 60 Figure 4, beta (Si/Al = 13.5), c 50 mordenite (Si/Al = 15), HY (Si/Al = g 3) and ZSM-5 (Si/Al = 30) zeolites 40 0) result in 66 %, 43 %, 4 % and 1 %, > c 30 respectively. The Al-PMO material o O (Si/Al = 30, DBATES) exhibits 58 % 20 conversion and 100% selectivity for 10 the etherification reaction. The catalytic activity and selectivity are comparable with those of beta zeolite h . ^KO ^ K)®^•N^' .^^^^ ^ that is the best one among various 6®^ ^^' ^\'.?^ ^'P^' kinds of solid acids catalysts in the Fig. 4. Catalytic activities of the materials for present work. etherification reaction between 2-naphthol and In summary, the highly ordered Alethanol. PMO material with framework aluminum can be successfully prepared using DBATES as the aluminum source. The material thus obtained is an excellent solid acid catalyst. The present synthetic strategy may be very useful for the rational design and preparation of PMO materials containing other elements within the frameworks. The authors are grateful for financial support by the Research Initiation Program at Ajou University (20012010) and Department of Molecular Science & Technology through Brain Korea 21 Project. REFERENCES 1. S. Inagaki, S. Guan, Y. Fukushima, T Ohsuna and O. Terasaki, J. Am. Chem. Soc, 121 (1999)9611. 2. T. Asefa, M. J. MacLachlan, N. Coombs and G. A. Ozin, Nature, 402 (1999) 867. 3. C. B. Mark, A. M. Michael, S. S. Mark and P G. Bruce, Chem. Mater., 13 (2001) 4760. 4. G. D. Yadav and M. S. Krishnan, Ind. Eng. Chem. Res., 37 (1998) 3358 5. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vatuli and J. S. Beck, Nature, 359 (1992) 710.
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Aluminum containing periodic mesoporous organosilicas: synthesis and etherification Jin-Won Kim^, Hyung Ik Lee*', Ji Man Kim^, Xingdong Yuan^'^ and Jae Eui Yie^* ^Catalyst and Surface Laboratory, School of Chemical Engineering and Biotechnology, Ajou University, Suwon, 442-749, Korea ^Functional Materials Laboratory, Department of Molecular Science & Technology, Ajou University, Suwon, 442-749, Korea '^Department of Petrochemical Technology, Fushun Petroleum University, Fushun, 113001, China Aluminum has been successfully incorporated within the frameworks of periodic mesoporous organosilicas (Al-PMO) by co-condensation of bis(triethoxysilyl)ethane and dibutoxyaluminotriethoxysilane. The Al-PMO materials exhibit highly ordered 2-d hexagonal structures, high surface areas, and narrow pore size distribution in the mesoporous range. The Al-PMO catalysts result in excellent catalytic activity and selectivity for etherification reaction between 2-naphthol and ethanol, which is comparable with those of beta zeolite. 1. INTRODUCTION Recently, periodic mesoporous organosilicas (PMO) have been synthesized by condensation of bridged silsequioxane in the presence of structure-directing agents, and attracted much attention due to their well-ordered mesostructures and noble framework structures [1,2]. The presence of organic groups within the frameworks is expected to give hydrophobic character and hydrothermal stability to the mesoporous materials. These properties are very important for the applications under hydrothermal conditions and organophilic reactions systems, compared with those of normal mesoporous silicas such as MCM-41 and MCM-48. However, the PMO materials constructed with organosilica frameworks (Si-PMO) are of limited use in catalysis, due to the lack of acidity and ion exchange sites. Incorporating other elements such as Al, Ti, Mn, Fe, V, etc. into the organosilica frameworks can improve the properties, which are important for the applications as catalysts and adsorbents. So far, there are a few reports on the modification with organic functional groups and on their applications [3]. It seems to be difficult to incorporate heteroatoms within the PMO frameworks with conventional methods that have been generally used in preparation of metal containing MCM-41. Etherification reaction of 2-naphthol is very important because the products have been extensively used in the fine chemical industry [4]. For example, 2-naphthyl methyl ether has been used in perfumery, which is traditionally manufactured from 2-naphthol and methanol in the presence of sulfuric acid. However, the drawbacks of such a process include corrosion, safety hazards, separation procedures, and environmental problems due to the use of sulfuric
666
acid. A PMO material with solid acid properties is expected to be an excellent heterogeneous catalyst for this reaction due to its mesoporosity and hydrothermal stability. In the present work, incorporation of aluminum into the PMO frameworks has been successfully carried out by co-condensation between bis(triethoxysilyl)ethane (BTSE) and dibutoxyaluminotriethoxysilane (DBATES) in the presence of structure-directing agents, and the possibility of the materials for catalytic applications to etherification are investigated. 2. EXPERIMENTAL Al-incorporated PMO materials (Al-PMO) were synthesized by modified procedures described elsewhere [1] using BTSE as the framework source, DBATES as the aluminum source and octadecyltrimethylammonium chloride (ODTMACl) as the structure-directing agent. A typical gel compositions was 1 BTSE : 0.067 DBATES : 0.57 ODTMACl : 2.4 NaOH : 350 H2O : 10 EtOH : 0.012 HCl. BTSE and DBATES were prehydrolyzed and oligomerized under acidic conditions before mixing with surfactant solution. To investigate the effect of aluminum source on the materials, the Al-PMO materials (Si/Al — 30) were synthesized by using various kinds of aluminum sources such as A1(N03)3, Al(i-OC3H7)3 and NaA102. The resulting mixture was magnetically stirred at room temperature for 20 hr, and subsequently heated in an oven at 368 K for 20 hr. The precipitate was recovered by filtration, washed with doubly distilled water and dried at 373 K for 6 h. The as-made products were refluxed in an excess acidified ethanol with HCl to remove the surfactant. The products were obtained by filtration, washed with ethanol and dried at 373 K for 10 h. The solvent-extraction procedures were repeated three times. The Al-PMO materials were characterized by powder X-ray diffraction (XRD), N2 adsorption, FT-IR, solid-state MAS ^^Al NMR spectroscopy, thermogravimetric analysis (TGA). Etherification reactions between 2-naphthol and ethanol were carried out in a down flow fixed bed reactor at 453 K. The reaction conditions were 0.1 g of catalysts, ethanol/2-naphthol = 10/1, and reactants flow rate = 1.0 cc/h. 3. RESULTS AND DISCUSSION Figure 1 shows XRD patterns for the Si-PMO and the Al-PMO obtained by using DBATES as the aluminum source, before and after surfactant extraction. The XRD patterns for PMO materials before extraction (Figure la and Ic) give a very intense diffraction peak and two or more weak 2 4 6 8 peaks, which are characteristic of 2-d hexagonal 20/ degree (P6mm) mesostructure [5]. There are no significant Fig. 1. X-ray diffraction patterns for (a) changes upon removal of surfactant except for the as-made, (b) washed Si-PMO, (c) as- expected increase in XRD peak intensity. The AlPMO after surfactant removal (Figure Id) gives made and (d) washed Al-PMO.
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(210) and (300) peaks, which indicates excellent textural uniformity of the material. TEM image also indicates that the material has a highly ordered 2-d hexagonal structure. Lattice parameters (a), calculated from dwo spacings, for the Si-PMO and Al-PMO materials are 4.67 nm and 4.96 nm, respectively. Line broadening and large lattice parameter of the Al-PMO material, compared with those of Si-PMO, may be due to the Al incorporation within the frameworks. Nitrogen adsorption isotherms indicate that the BET surface areas of the Si-PMO and Al-PMO materials are 1050 mVg and 1692 m^/g, respectively. The pore sizes for the materials obtained by BJH model are 2.6 nm and 2.9 nm. From the lattice parameters and pore sizes, framework thickness for the materials is very similar (2.1 nm). According to IR spectra, all the PMO materials after surfactant — I — — I — removal exhibit strong bands at 2920 and 2890 cm' 100 -100 200 -200 assigned to C-H stretching and deformation Chemical shifts(ppm) vibrations, 1410 and 1270 cm' corresponding to C-H Fig. 2. 'Al MAS NMR spectra for Al- deformation vibrations, which means the presence of PMO materials obtained with (a) organic bridging group within the frameworks. Figure 2 shows ^''AI MAS NMR spectra of the AlA1(N03)3, (b) NaA102 (c) Al(i-OC3H7)3 (d) DBATES (Si/Al = 30) and (c) PMO materials obtained with different aluminum DBATES (Si/Al = 8) sources. NMR peak around 50 ppm and 0 ppm can be assigned to a tetrahedrally coordinated aluminum species within the framework and an octahedrally coordinated extraframework aluminum species, respectively. The NMR results in Figure 2 clearly show that the extraframework aluminum species are present in the Al-PMO materials obtained with A1(N03)3 and NaA102. In case of DBATES and Al(i-OC3H7)3, there is no NMR peak around 0 ppm, indicating that all the aluminum species are incorporated within frameworks. However, a significant amount of octahedrally coordinated aluminum species appears as the Si/Al ratio decreases when Al(i-OC3H7)3 is used as the aluminum source, whereas DBATES results in only framework aluminum species till Si/Al = 8 (Figure 2e). The results show that aluminum incorporation into the PMO frameworks is highly dependent on the nature of aluminum source. Figure 3 shows TGA results under nitrogen atmosphere for the Al-PMO 100 200 300 400 500 600 material before and after surfactant TerriDerature / °C Fig. 3. TGA diagrams for the Al-PMO material (a) removal. Before solvent extraction, weight loss of 5 wt% below 120 °C is attributed before and (b) after surfactant extraction
668 to the loss of small amounts of residual water adsorbed to the materials. This is followed by a weight loss of 30 wt% from 120 to 250 °C due to surfactant decomposition. An additional weight loss of 5 - 7 wt% above 500 °C indicates decomposition of organic bridging group within the framework. Figure 3b shows that there is little weight loss in the temperature range for surfactant decomposition (120 - 250 °C), indicating that the surfactant within the mesopores can be removed completely through the solvent extraction. The weight loss above 500 °C also appears in Figure 3b. According to the TGA results, the Al-PMO materials synthesized in the present work may be used below 500 °C without loss of organic bridging group within the frameworks. Figure 4 shows catalytic activities of 80 the etherification reaction between 2naphthol and ethanol. All the 70 materials give 100 % selectivity for 2naphthylethylether. As shown in ^ 60 Figure 4, beta (Si/Al = 13.5), c 50 mordenite (Si/Al = 15), HY (Si/Al = g 3) and ZSM-5 (Si/Al = 30) zeolites 40 0) result in 66 %, 43 %, 4 % and 1 %, > c 30 respectively. The Al-PMO material o O (Si/Al = 30, DBATES) exhibits 58 % 20 conversion and 100% selectivity for 10 the etherification reaction. The catalytic activity and selectivity are comparable with those of beta zeolite h . ^KO ^ K)®^•N^' .^^^^ ^ that is the best one among various 6®^ ^^' ^\'.?^ ^'P^' kinds of solid acids catalysts in the Fig. 4. Catalytic activities of the materials for present work. etherification reaction between 2-naphthol and In summary, the highly ordered Alethanol. PMO material with framework aluminum can be successfully prepared using DBATES as the aluminum source. The material thus obtained is an excellent solid acid catalyst. The present synthetic strategy may be very useful for the rational design and preparation of PMO materials containing other elements within the frameworks. The authors are grateful for financial support by the Research Initiation Program at Ajou University (20012010) and Department of Molecular Science & Technology through Brain Korea 21 Project. REFERENCES 1. S. Inagaki, S. Guan, Y. Fukushima, T Ohsuna and O. Terasaki, J. Am. Chem. Soc, 121 (1999)9611. 2. T. Asefa, M. J. MacLachlan, N. Coombs and G. A. Ozin, Nature, 402 (1999) 867. 3. C. B. Mark, A. M. Michael, S. S. Mark and P G. Bruce, Chem. Mater., 13 (2001) 4760. 4. G. D. Yadav and M. S. Krishnan, Ind. Eng. Chem. Res., 37 (1998) 3358 5. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vatuli and J. S. Beck, Nature, 359 (1992) 710.