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Microwave synthesis of SBA-15 mesoporous silica material for beneficial effect on the hydrothermal stability
Recent Progress in Mesostructured Materials D. Zhao, S. Qiu, Y. Tang and C. Yu (Editors) © 2007 Elsevier B.V. All rights reserved.
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Microwave synthesis of SBA-15 mesoporous silica material for beneficial effect on the hydrothermal stability Sang-Cheol Han, Nanzhe Jiang, Sujandi, David Raju Burri, Kwang-Min Choi, Seung-Cheol Lee and Sang-Eon Park* Laboratory ofNano-Green Catalysis andNano Center for Fine Chemicals Fusion Technology, Department of Chemistry, Inha University, Incheon, 402-751, Korea
Hydrothermally robust mesoporous SBA-15 silica material was synthesized for the first time using microwave with out adding any other chemical ingredients. For the sake of stability comparison, mesoporous SBA-15 material was also prepared by conventional hydrothermal technique. The stability of the material was tested by boiling water method and 29Si-NMR Q4/Q3 ratios. It was observed that SBA-15 synthesized under microwave conditions exhibited higher stability than that of hydrothermally synthesized SBA-15. 1. Introduction Recently, microwave heating has been greatly applied in the synthesis of nanoporous materials. Our group including others have used microwave method for the synthesis of hexagonal and cubic mesoporous materials such as MCM41 [1], MCM-48 [2], SBA-15 [3] and SBA-16 [4]. It offers several advantages such as homogeneous heating throughout the reaction vessel, the possibility of selective heating according to the desirability of the materials, homogeneous nucleation, crystal growth processes [5] and short crystallization time so on and so forth [6,7]. We also have found that this technique could provide an efficient way to control particle size distribution and macroscopic morphology of nanostructured materials [8,9]. Besides morphology control and short crystallization time, improvement of the degree of silica condensation in the mesoporous walls could be expected in the microwave synthesis. In this study, microwave synthesis was used as the strategy for supplying higher crystallization conditions in the synthesis of mesoporous materials. Microwave synthesis is essentially different from conventional heating. In
26
microwave processes, heat is generated directly from the interaction between molecules in the heated material by the electromagnetic fields created in the microwave oven. Microwave processing can be beneficial in the processing of materials with high dielectric constants, because absorption of microwave irradiation onto substrates depends on its dielectric constants and dielectric loss factors [10]. Water absorbed microwave energy efficiently due to the high values of dielectric constant, which is a very good absorber of microwave energy at the frequency of 2.450 GHz. So, the microwave effect in the synthesis of SBA-15 silica could be expected to be significantly enhanced. Our results showed that this technique was a powerful tool to control the pore wall condensation as well as facile synthesis of SBA-15 having high hydrothermal stability. 2. Experimental Section 2.1. Synthesis of Materials The synthesis of SBA-15 was carried out as follows: 8 g of triblock copolymer PI23 (EO20PO70EO20; M.W. 5800, Aldrich) was dissolved in 205 ml of water, followed by the 17 g of tetraethylorthosilicate (TEOS, Aldrich). To this solution, 65 g of concentrated hydrochloric acid (37.6%) was quickly added with vigorous stirring to obtain gel. The mixture was stirred at 40°C for 4 h and then transferred to a microwave digestion system (CEM Corporation, MAR-5). SBA-15 microwave synthesis was performed for 1, 2, 3, 4 and 5h at 373 K, and also it was synthesized by conventional hydrothermal method for 24 and 48 h at 373 K. The surfactants were removed through the solvent extraction method using ethanol solution and then by calcination at 813 K. 2.2. Stability test The hydrothermal stability of samples was tested by treatment in boiling water (0.2 L of water per 0.4 g of Materials) for 80 hrs. 2.3. Characterization X-ray diffraction patterns (XRD) were recorded on a Rigaku multiflex diffractometer with a monochromated high-intensity Cu Ka radiation (A,=0.15418 nm). The diffraction data were collected by using a continuous scan mode with a scan speed of 0.1°/min (20 kv, 10 mA).29Si NMR spectra were recorded on a Varian unity-inova 400 spectrometer. The samples were fitted in a 7mm SiO2 rotor, spinning at 4 kHz.
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3. Results and Discussion SBA-15 synthesized by both microwave and hydrothermal methods exhibited highly ordered mesoporosity. The microwave synthesized SBA-15 exhibited extremely higher stability, which can be seen from the Fig.l. 29Sia NMR spectra exhibited three kinds of bands centered at chemical shifts of 8 = -92, -102, and -112 ppm, which can be ascribed to Si(OSi)x(OH)4.x framework units b where x=2 (Q2), x=3 (Q3), and x=4 4 (Q ), respectively. Notably, assynthesized SBA-15-4h was made Q Q up of almost fully condensed Q4 c silica units (8 = -112ppm). Q A small contribution was resulted from incompletely cross-linked Q3 (8 = -102 ppm), as deduc -ed -5 0 -1 0 0 -1 5 0 from the very high Q4/Q3 ratio of ppm 6:1, whereas no Q2 units were observed. And the higher irradiaFigure l.29Si MAS NMR spectra of microwave 4 3 tion times gave the higher Q /Q synthesized SBA-15 : a) SBA-15-MW-4h,b) ratios. It meant that microwave irra SBA-15-MW-3h, c) SBA-15-MW-2h. -diation might contribute the cond -ensation of silicas and the dehy- droxylation of the silica surface as well. On 3
4
2
a
b SBA-15-HT 24hr
SBA-15-MW 3hr
After
O Si
Before Before
Si
Si O Si
Si Si O Si O
Amorphous Pore wall Pore wall
High crystalline Pore wall
Si HO OH
Si HO Si
Si OH HO
Si
Si Si
OH HO
Si
OH Si
High Q4/Q3 ratio 0.5
1.0 1.0
1.5 1.5
22.0 .0
2.5 2.5
33.0 .0
3.5 3.5
4.0
4.5
5.0 5.0
0.5
1.0 1.0
1.5 1.5
Low Q4/Q3 ratio ratio
2 . 5 3.0 3.03 . 0 44.5 . 5 5 5.0 .0 2.02 . 02.5 3.5. 5 44.0
Figure 2. XRD patterns of as-synthesized a) SBA-15-MW-3hr, b) SBA-15-HT-48hr before and after at boiling water treatment for 80 hrs.
the other hand, mesoporous silica material (SBA-15) having a highly ordered hexagonal mesostructure was synthesized hydrothermally at 100°C and gave
28
much lower Q4/Q3 ratios below 1.0 [11]. These results indicated that microwave syntheses were indeed favorable for promoting silica condensation on the pore walls giving more hydrophobicity. The XRD patterns of microwave synthesized SBA-15 exhibited three distinct peaks which denoted the ordered hexagonal symmetry. After treatment of SBA15-MW-3h with boiling water for 80 h, it still gave clear peaks of 100 and 110 reflections of the hexagonally ordered mesostructure. In contrast, the boiling water treated SBA-HT-24h exhibited a very broad peak assigned to the (100) reflection. These results indicated that SBA-15-MW-3h had much better hydrothermal stability than that of the hydrothermally synthesized SBA-15. The materials prepared by microwave synthesis proven to be fully condensed mesopore walls, and exhibited higher hydrothermal stability at boiling water condition, than that hydrothermally prepared SBA-15 materials. 4. Conclusion Synthesis of SBA-15 molecular sieves was performed via hydrothermal and microwave approach to compare resulting hydrothermal stability properties of these materials. In case of SBA-15 prepared by microwave irradiation exhibited a higher hydrothermal stability. 5. Acknowledgement Authors are gratefully acknowledged advanced scientist supporting program (KRF-2005-041-C00238) and BK-21 program of the Korea Ministry of Education for the financial supported. 6. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
C. G. Wu and T. Bein, Chem. Commun. (1996) 925. S. -E. Park, D. S. Kim, J.-S. Chang and W. Y. Kim, Catal. Today 44(1998) 301. B. L. Newlkar, S. Komarneni and H. Katsuki, Chem. Commun. (2000) 2389. Y. K. Hwang, J. -S. Chang, Y. -U. Kwon and S. -E. Park, Micro. Meso. Mater. 68(2004)21. S. L. Burkett and M.E. Davis, In Comprehensive Supramolecular Chemistry, G. Alberti and T. Bein(eds), Vol. 7(1996) 465. C. S. Cundy, R.J. Plaisted and J. P. Zhao, Chem. Commun. (1998) 1465. Arafat, J.C. Jansen, A.R. Ebaid and H. Van Bekkum. Zeolites 13(1993) 162. S. -E. Park, J.-S. Chang, Y. K. Hwang, D. S. Kim, S. H. Jhung and J. -S. Hwang, Catal. Surveys from Asia 8(2004) 91. Y. K. Hwang, J.-S. Chang, S. -E. Park, D. S. Kim, Y. -U. Kwon, S. H. Jhung, J. -S. Hwang and M. S. Park, Angew. Chem. Int. Ed. 44(2005) 556. C. Gabriel, S. Gabriel, E.H. Grant B.S.J. Halsteed and D. P. Mingos, Chem.Soc.Rev. 27(1998)213. D. Li, Y. Han, J. Song, L. Zhao, X. Xu and F. -S. Xiao, Chem. Eur. J. 10(2004) 5911.
25 25
Microwave synthesis of SBA-15 mesoporous silica material for beneficial effect on the hydrothermal stability Sang-Cheol Han, Nanzhe Jiang, Sujandi, David Raju Burri, Kwang-Min Choi, Seung-Cheol Lee and Sang-Eon Park* Laboratory ofNano-Green Catalysis andNano Center for Fine Chemicals Fusion Technology, Department of Chemistry, Inha University, Incheon, 402-751, Korea
Hydrothermally robust mesoporous SBA-15 silica material was synthesized for the first time using microwave with out adding any other chemical ingredients. For the sake of stability comparison, mesoporous SBA-15 material was also prepared by conventional hydrothermal technique. The stability of the material was tested by boiling water method and 29Si-NMR Q4/Q3 ratios. It was observed that SBA-15 synthesized under microwave conditions exhibited higher stability than that of hydrothermally synthesized SBA-15. 1. Introduction Recently, microwave heating has been greatly applied in the synthesis of nanoporous materials. Our group including others have used microwave method for the synthesis of hexagonal and cubic mesoporous materials such as MCM41 [1], MCM-48 [2], SBA-15 [3] and SBA-16 [4]. It offers several advantages such as homogeneous heating throughout the reaction vessel, the possibility of selective heating according to the desirability of the materials, homogeneous nucleation, crystal growth processes [5] and short crystallization time so on and so forth [6,7]. We also have found that this technique could provide an efficient way to control particle size distribution and macroscopic morphology of nanostructured materials [8,9]. Besides morphology control and short crystallization time, improvement of the degree of silica condensation in the mesoporous walls could be expected in the microwave synthesis. In this study, microwave synthesis was used as the strategy for supplying higher crystallization conditions in the synthesis of mesoporous materials. Microwave synthesis is essentially different from conventional heating. In
26
microwave processes, heat is generated directly from the interaction between molecules in the heated material by the electromagnetic fields created in the microwave oven. Microwave processing can be beneficial in the processing of materials with high dielectric constants, because absorption of microwave irradiation onto substrates depends on its dielectric constants and dielectric loss factors [10]. Water absorbed microwave energy efficiently due to the high values of dielectric constant, which is a very good absorber of microwave energy at the frequency of 2.450 GHz. So, the microwave effect in the synthesis of SBA-15 silica could be expected to be significantly enhanced. Our results showed that this technique was a powerful tool to control the pore wall condensation as well as facile synthesis of SBA-15 having high hydrothermal stability. 2. Experimental Section 2.1. Synthesis of Materials The synthesis of SBA-15 was carried out as follows: 8 g of triblock copolymer PI23 (EO20PO70EO20; M.W. 5800, Aldrich) was dissolved in 205 ml of water, followed by the 17 g of tetraethylorthosilicate (TEOS, Aldrich). To this solution, 65 g of concentrated hydrochloric acid (37.6%) was quickly added with vigorous stirring to obtain gel. The mixture was stirred at 40°C for 4 h and then transferred to a microwave digestion system (CEM Corporation, MAR-5). SBA-15 microwave synthesis was performed for 1, 2, 3, 4 and 5h at 373 K, and also it was synthesized by conventional hydrothermal method for 24 and 48 h at 373 K. The surfactants were removed through the solvent extraction method using ethanol solution and then by calcination at 813 K. 2.2. Stability test The hydrothermal stability of samples was tested by treatment in boiling water (0.2 L of water per 0.4 g of Materials) for 80 hrs. 2.3. Characterization X-ray diffraction patterns (XRD) were recorded on a Rigaku multiflex diffractometer with a monochromated high-intensity Cu Ka radiation (A,=0.15418 nm). The diffraction data were collected by using a continuous scan mode with a scan speed of 0.1°/min (20 kv, 10 mA).29Si NMR spectra were recorded on a Varian unity-inova 400 spectrometer. The samples were fitted in a 7mm SiO2 rotor, spinning at 4 kHz.
27
3. Results and Discussion SBA-15 synthesized by both microwave and hydrothermal methods exhibited highly ordered mesoporosity. The microwave synthesized SBA-15 exhibited extremely higher stability, which can be seen from the Fig.l. 29Sia NMR spectra exhibited three kinds of bands centered at chemical shifts of 8 = -92, -102, and -112 ppm, which can be ascribed to Si(OSi)x(OH)4.x framework units b where x=2 (Q2), x=3 (Q3), and x=4 4 (Q ), respectively. Notably, assynthesized SBA-15-4h was made Q Q up of almost fully condensed Q4 c silica units (8 = -112ppm). Q A small contribution was resulted from incompletely cross-linked Q3 (8 = -102 ppm), as deduc -ed -5 0 -1 0 0 -1 5 0 from the very high Q4/Q3 ratio of ppm 6:1, whereas no Q2 units were observed. And the higher irradiaFigure l.29Si MAS NMR spectra of microwave 4 3 tion times gave the higher Q /Q synthesized SBA-15 : a) SBA-15-MW-4h,b) ratios. It meant that microwave irra SBA-15-MW-3h, c) SBA-15-MW-2h. -diation might contribute the cond -ensation of silicas and the dehy- droxylation of the silica surface as well. On 3
4
2
a
b SBA-15-HT 24hr
SBA-15-MW 3hr
After
O Si
Before Before
Si
Si O Si
Si Si O Si O
Amorphous Pore wall Pore wall
High crystalline Pore wall
Si HO OH
Si HO Si
Si OH HO
Si
Si Si
OH HO
Si
OH Si
High Q4/Q3 ratio 0.5
1.0 1.0
1.5 1.5
22.0 .0
2.5 2.5
33.0 .0
3.5 3.5
4.0
4.5
5.0 5.0
0.5
1.0 1.0
1.5 1.5
Low Q4/Q3 ratio ratio
2 . 5 3.0 3.03 . 0 44.5 . 5 5 5.0 .0 2.02 . 02.5 3.5. 5 44.0
Figure 2. XRD patterns of as-synthesized a) SBA-15-MW-3hr, b) SBA-15-HT-48hr before and after at boiling water treatment for 80 hrs.
the other hand, mesoporous silica material (SBA-15) having a highly ordered hexagonal mesostructure was synthesized hydrothermally at 100°C and gave
28
much lower Q4/Q3 ratios below 1.0 [11]. These results indicated that microwave syntheses were indeed favorable for promoting silica condensation on the pore walls giving more hydrophobicity. The XRD patterns of microwave synthesized SBA-15 exhibited three distinct peaks which denoted the ordered hexagonal symmetry. After treatment of SBA15-MW-3h with boiling water for 80 h, it still gave clear peaks of 100 and 110 reflections of the hexagonally ordered mesostructure. In contrast, the boiling water treated SBA-HT-24h exhibited a very broad peak assigned to the (100) reflection. These results indicated that SBA-15-MW-3h had much better hydrothermal stability than that of the hydrothermally synthesized SBA-15. The materials prepared by microwave synthesis proven to be fully condensed mesopore walls, and exhibited higher hydrothermal stability at boiling water condition, than that hydrothermally prepared SBA-15 materials. 4. Conclusion Synthesis of SBA-15 molecular sieves was performed via hydrothermal and microwave approach to compare resulting hydrothermal stability properties of these materials. In case of SBA-15 prepared by microwave irradiation exhibited a higher hydrothermal stability. 5. Acknowledgement Authors are gratefully acknowledged advanced scientist supporting program (KRF-2005-041-C00238) and BK-21 program of the Korea Ministry of Education for the financial supported. 6. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
C. G. Wu and T. Bein, Chem. Commun. (1996) 925. S. -E. Park, D. S. Kim, J.-S. Chang and W. Y. Kim, Catal. Today 44(1998) 301. B. L. Newlkar, S. Komarneni and H. Katsuki, Chem. Commun. (2000) 2389. Y. K. Hwang, J. -S. Chang, Y. -U. Kwon and S. -E. Park, Micro. Meso. Mater. 68(2004)21. S. L. Burkett and M.E. Davis, In Comprehensive Supramolecular Chemistry, G. Alberti and T. Bein(eds), Vol. 7(1996) 465. C. S. Cundy, R.J. Plaisted and J. P. Zhao, Chem. Commun. (1998) 1465. Arafat, J.C. Jansen, A.R. Ebaid and H. Van Bekkum. Zeolites 13(1993) 162. S. -E. Park, J.-S. Chang, Y. K. Hwang, D. S. Kim, S. H. Jhung and J. -S. Hwang, Catal. Surveys from Asia 8(2004) 91. Y. K. Hwang, J.-S. Chang, S. -E. Park, D. S. Kim, Y. -U. Kwon, S. H. Jhung, J. -S. Hwang and M. S. Park, Angew. Chem. Int. Ed. 44(2005) 556. C. Gabriel, S. Gabriel, E.H. Grant B.S.J. Halsteed and D. P. Mingos, Chem.Soc.Rev. 27(1998)213. D. Li, Y. Han, J. Song, L. Zhao, X. Xu and F. -S. Xiao, Chem. Eur. J. 10(2004) 5911.