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Synthesis of mesoporous silica molecular sieves via a novel templating scheme
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. AH rights reserved.
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Synthesis of mesoporous silica molecular sieves via a novel templating scheme Xiaoming Zhang, Zhaorong Zhang, Jishuan Suo, and Shuben Li State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, The Chinese Academy of Sciences, Lanzhou 730000, China
A novel templating scheme was demonstrated for the synthesis of mesoporous silica molecular sieve under mild conditions in which N, N-dimethyldodecylamine oxide was in situ prepared from its precursors and used directly as the structure-directing agent. X-ray diffraction and transmission electron micrography revealed that samples thus obtained exhibited a large number of worm-like and interconnected channels, but lack of long range packing order, that is the structure was disordered. Nitrogen adsorption/desorption isotherms confirmed that the pore diameter was in the range of mesoporous region and the sample bore extremely high specific surface areas (>1000m^/g). The structure of the mesoporous silica molecular sieve was determined by various synthesis parameters (especially, pH and surfactant to silica ratio of the gel).
1. INTRODUCTION The discovery of M41S family mesoporous molecular sieves by Mobil researchers has attracted great attentions in the fields of material sciences and catalysis. These type materials have been proved to be of potentials in ion-exchanger, adsorbent, heterogeneous catalysts, as well as catalyst supports. ^'^ Several different synthesis strategies have been proposed and successfully used to prepare mesoporous materials with a unique pore size distribution. The original MCM-41 (Hexagonal phase) and MCM-48 (Cubic phase) materials were prepared by a self-assembly process based on the electrostatic interactions between positively charged quatemary ammonium micelles (S^) and inorganic anions (r).^"^ Most of the studies focused on the synthesis and catalytic application of one dimensional MCM-41 materials because of its ease of preparation. Although the synthesis of cubic MCM-48 was relative difficult, it was also important for catalytic application because it exhibits three dimensional framework structures, and a few research reports have been devoted to the preparation and application of this mesophase.^'^ The electrostatic assembly has been extended to include charge-reversed (S"f) and counter-ion mediated (S^XT^ and S'X^F) pathways.^'^ Huo et al^^ has reported the synthesis of ordered mesoporous molecular sieves SBA-n by using gemini surfactants as the template. Ryoo et al^^ has prepared a disordered mesoporous silica material (KIT) through the
24
polymerization of silicate anions surrounding surfactant micelles in the presence of organic salts. Pinnavaia's group prepared mesoporous silica molecular sieves HMS'^ and MSU'^ involving the hydrogen bonding interactions between neutral inorganic precursor (I^) and neutral alkylamine (S^) or polyethylene oxide (N^) surfactants. More recently, Zhao et al^"^ reported the synthesis of mesoporous molecular sieves with extra-large pore diameter (<30 nm) and wall thickness {<6 nm) by using amphiphilic triblock copolymers as the template. All these mesostructures afford extremely high surface areas, often exceeding 1000 m^/g, which are important in the terms of heterogeneous catalysis and catalyst supports. Moreover, these mesoporous materials can be easily modified by incorporation of different cations, thus leading to a large family of materials with acidic or redox properties. Various metal cations, including Al,^^ B,^^ Ti,^^ Zr,^^ V,^^ Cr, Mo, Mn, ^^ W,^^ Fe,^^ and Sn,^^have been incorporated into the hexagonal mesoporous silica frameworks thus far. These metal modified mesoporous composites are potential in the heavy oil refinery and the synthesis of fine petrochemicals. Tertiary amine oxide exhibits cationic properties under acidic conditions, whereas possess nonionic nature under neutral and basic conditions. It contains two different hydrogen-bonding centers (N and O). Similar to the reported alkylamine and polyethylene oxide, these unique surfactants might be of potentials in directing the mesoporous molecular sieves via hydrogen-bonding forces and/or electrostatic forces between the surfactant headgroup and the inorganic precursors. In this paper, we demonstrated that N, Ndimethyldodecylamine oxide (DAO) was a promising template for the synthesis of mesoporous silica molecular sieves. Furthermore, we found that N, N-dimethyldodecylamine oxide can be prepared and used directly (i.e., in situ) from its precursors N, Ndimethyldodecylamine and aqueous H2O2. The effect of various syntheses conditions, such as pH value, aging time, aging temperature, surfactant concentration, as well as surfactant to Si02 ratio have also been studied in detail.
2. EXPERIMENTAL 2.1 Synthesis The mesoporous silica molecular sieve (designated as LZC) was prepared from tetraethylorthosilicate (TEOS) and N, N-dimethyldodecylamine as the silicon and organic precursors, respectively. In a typical synthesis, 2.0ml N, N-dimethyldodecylamine was dispersed in 5.0ml H2O at 60 °C and formed a turbid mixture. With magnetic stirring, 1.0ml H2O2 (30%wt) was added dropwise into the above mixture. The reaction was carried out at 60 °C for 3.5h, then at 75 °C for 12h. This clear template solution was then cooled to room temperature. Under vigorous stirring, the designed amounts of TEOS were introduced dropwise. The fmal composition of the reaction mixture was ITEOS: xDAO: 25H20,where x=0.1, 0.2, 0.5, 1.0, and 1.5 respectively. Allowing the precursor to age under moderate
25
stirring at room temperature for 48h, then product was centrifuged, washed with de-ionized water, air-dried, and calcined in air at 873K for 5h to remove the organic templates. 2.2 Characterization X-ray powder diffraction (XRD) patterns of the sample were recorded on a Rigaku D/Max 2400 X-ray diffractometer with Cu-Ka radiation (?i=0.15418nm). The surface areas and pore diameters were measured by BET and BJH methods on a Micromeritics ASAP 2010 Sorptometer. Before analysis, the sample was degassed at 423K and 1.07x10"^ KPa for 12h. The TEM image was obtained on a JEM-IOOC transmission microscope. The TG analysis was carried out on a DuPont 1090 Thermal Analyzer.
3. RESULTS AND DISCUSSION 3.1 Template N, N-dimethyldodecylamine is unsoluble, while N, N-dimethyldodecylamine oxide has good solubility in water. So it was latter rather than N, N-dimethyldodecylamine that acts as the structure-directing agent for the formation of mesostructures in aqueous media. Figure 1 depicts the transition electron micrograph (TEM) image of the calcined mesporpous sample (designated as LZC-C). Analogous to the TEM images of MSU and HMS mesoporous materials, there were a large number of worm-like and interconnected channels, but lack of long range packing order, that is the structure of the calcined sample was disordered. This was further verified by the XRD patterns. It exhibited a single, broad reflection in low 20 angles; the high order reflections were not resolved. Similar to the structures of HMS and MSU, the absence of high order reflection might be due to the weak hydrogen bonding forces and the corresponding small scattering domain sizes. 3.2 The effect of pH The silicon precursor and N, Ndimethyldodecylamine oxide exhibit different electronic properties under different pH conditions, thus the driving forces and the corresponding sample structure may be different in various pH values. We performed the synthesis process at pH<0, pH=2, pH=7, and pH>10, respectively. The silicon precursor did not condense when pH<0. Figure 2 illustrates the XRD patterns of the sample prepared
Figure 1. TEM image of calcined LZC mesoporous silica molecular sieves.
26
2.0 4.0
Figure 2. XRD patterns of as-synthesized LZC mesoporous silica molecular sieves with different pH. (A) 2, (B) 7, (C) 10
6.0 20/°
Figure 3. XRD panems of as-synthesized LZC mesoporous silica molecular sieves with different temperature. (A) 80°C,(B)50°C. (C)RT
4.0
6.0 8.0 10.0 29/°
Figure 4. XRD patterns of calcined LZC mesoporous silica molecular sieves with different surfactant'SiO:. (A) 0.1, (B) 0.2, (C) 0.5. (D) 1.0. (E) 1.5
under the other pH conditions. The sample prepared under iso-electropoint of silica !pH=2) and strong alkaline conditions (pH>10) were amorphous, while the sample prepared under neutral conditions (pH=7) had a broad reflection at low angles. The difference in the structures with various pH conditions might be the different interaction forces between template and silicon precursors. N, N-dimethyldodecylamine oxide exhibits cationic nature under acidic conditions (pH<7) and nonionic properties under neutral (pH=7) and alkaline conditions (pH>7). While TEOS was positive charged bellow the iso-electronic point of silica (pH<2) and negative charged above the iso-electronic point of silica (pH>2)." So the interaction between the template and the silicon precursor under acidic conditions was positive charged template verse positive charged silicon species, but there was not a mediating counter-ion (like CT or Br' in S^XT assembly), thus can not form stable structures in that cases. Under strong alkaline conditions, the silicon precursor was highly negative charged, the interaction between neutral template and negative charged silicon species was too weak to form stable structure. Under neutral condition (pH=7), however, the hydrogen bonding forces between N, N-dimethyldodecylamine oxide (N and O atoms) and the slightly negative charged silicon precursor leaded to the formation of mesoporous silica structures, but lack of long range packing order. 3.3 The effect of aging temperature
Table 1 Properties of LZC mesoporous silica molecular sieves with different aging time Sample Aging time d-spacing Pore volume Pore diameter SBET /h /nm /cm^/g /nm W/g A 24h 4.37 938.2 0.46 3.19 B 48h 4.12 841.4 2.62 0.24 C 72h 4.01 787.7 0.24 2.62 The traditional mesoporous silica molecular sieves MCM-41 was prepared under hydrothermal conditions, while HMS and MSU were synthesized at mild conditions. The aging temperature can affect the gel chemistry of silica and the interaction forces between the surfactant and the inorganic precursors. We performed the synthesis of mesoporous silica molecular sieves at room temperature, 50 °C, and 80 °C, respectively. The sample prepared 80 °C was amorphous, while the sample prepared at room temperature and 50 °C exhibited mesoporous properties (Figure 3). The BET surface areas for those three samples were 841 m /g, 707 m /g, and 344 m^/g, respectively. This further confirmed that the driving forces for the formation of LZC mesostructures was the hydrogen bonding forces between silica precursor and N, N-dimethyldodecylamine oxide head-group, and the hydrogen bonding forces was weakened by higher temperature, thus the mesoporous structures were destroyed. 3.4 The effect of aging time Table 1 lists the d-spacing, BET surface areas, pore volumes, and pore diameters of the sample synthesized with different aging time. The XRD patterns of all samples were similar, and the d-spacing, BET areas, pore volume, and pore diameters shifted to the lower values upon the longing of aging time. This suggested the further cross-linking and condensation of the framework structures. 3.5 The effect of surfactant to silica ratios The structures of mesoporous molecular sieves were closely related to the surfactants to silica ratios."^ Because of the absence of high order reflections on the XRD patterns of all LZC Table 2 Properties of LZC mesoporous silica molecular sieves with different surf/Si02 Sample Surf/Si02 d-spacing SBET Pore volume Pore diameter /nm W/g /cmVg /nm 0.26 3.18 4.01 828.3 A 0.1 2.62 0.24 841.4 4.12 B 0.2 3.92 1.24 1102.1 4.05 C 0.5 4.06 1.22 1127.7 1.0 3.84 D 4.36 0.89 3.87 991.1 E 1.5
28
0 0.2 0.4 0.6 0.8 1.0 (P/Po)
0
0 0.2 0.4 0.6 0.8 1.0 (P/Po)
0.2 0.4 0.6 0.8 1
0 0.2 0.4 0.6 0.8 1.0 (P/Po)
0
0.2 0.4 0.6 0.8 1.0 (P/Po) Figure 5. N? adsorption/desorption isotherms of calcined LZC mesoporous silica molecular sieves with different surfactant/SiOi: (A) 0.1, (B) 0.2, (C) 0.5, (D) l.O, and (E) 1.5 silica samples, the XRD can not discriminate the samples synthesized at different surf./SiO: ratios. The nitrogen sorption techniques, however, can discern the minor difference in the pore structures of those samples. In this paper, we carried out the synthesis of mesoporous silica molecular sieves with different surfactant to silica ratios. Although the XRD patterns were similar to each other (Figure 4), the pore structures were different magnificently. Figure 5 shows the N2 adsorption/desorption isotherms of the mesoporous silica molecular sieves with different Surf./silica ratios. The corresponding BET surface areas, pore volumes, and BJH pore diameters are listed in Table 2. The isotherms of the samples with surf./silica <0.2 (Sample A and B) were type 4 in lUPAC classification, and there were not hysteresis loop, suggesting the existence of framework mesoporousity. In the case of those samples with surf/silica >0.2 (Smaple C, D and E), there appeared a large hysteresis loop at higher relative pressure. Furthermore, the BJH mean pore diameters of Sample D and E were larger than the corresponding d-spacings ( see Table 2).It was contradiction to the previous reported results for HMS and MSU mesoporous materials, in which the wall thickness was calculated from the difference between the pore diameter and d-spacing. These unusual data suggested that the pore structures of these mesoporous silica molecular sieves were magnificently different. Tlie increasing of Surf./SiO: from 0.2 to 1.5 lead to the decreasing of the packing orders of the
29 mesopore structures, and there were more textural mesopores in Sample D and E than in Sample A and B, thus a large hysteresis loop appeared in the N2 adsorption/desorption isotherms. The BJH pore diameter is a mean value of various type of pore structure. Sample D and E were more disordered than the other samples and possessed more textural pores, therefore mean values between the textural and framework pore diameter gave a relatively larger value.
4. CONCLUSION A disordered mesoporous silica molecular sieve has been synthesized successfully with N, N-dimethyldodecylamine oxide as the structure directing agent. TEM image and XRD patterns proved the presence of large number worm-like and interconnected channels. XRD and nitrogen adsorption techniques suggested that the pore structures of the samples were closely related to the pH value and the surfactant to silica ratios of the precursor gels, hi addition, we illustrated a novel templating scheme for the synthesis of mesoporous composites, that is, the combination of the synthesis of template and the synthesis of mesoporous materials. It offered a convenient, cost effective, and altemative assembly pathway for the synthesis of mesoporous molecular sieves, which was the highlight of catalysis and material sciences in recent years.
REFERENCES 1 A. Corma,Chem. Rev., 97(1997)2373. 2 S. Biz and M. L. Occelli, Catal. Rev. -Sci. Eng., 40(1998) 329. 3 C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992)710. 4 J. S. Beck, J. C. VartuU, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schimitt, C. T. -W. Chu, D. H. Olson, E. W. Sheppard, E. B. McCullen, J. B. Higgins and J. L. Schlenker, J. Am. Chem. Soc, 114 (1992) 10834. 5 W. Zhang, T. Pinnavaia, Catal. Lett., 38 (1996) 261. 6 M. Morey, A. Davidson, H. Ecker, G. D. Stucky, Chem. Mater., 8 (1996) 486. 7 M. Morey, G. D. Stucky, S. Schwarz, M. Froba, J. Phys. Chem. B, 103 (1999) 2037. 8 Q. S. Huo, D. L Margolese, U. Ciesla, D. G. Demuth, P. Feng, T. E. Gier, P. Sieger, A. Firouzi, B. F. Chmelka, F. Schuth and G. D. Stucky, Chem. Mater. 6 (1994) 1176. 9 Q. S. Huo, D. L margolese, U. Ciesla, P. Y. Feng, T. E. Gier, P. Sieger, R. Leon, P. M. Petroff, F. Schuth and G. D. Stucky, Nature, 368 (1994) 317. 10 Q. S. Huo, R. Leon, P. M. Petroff and G. D. Stucky, Science, 268 (1995) 1324. 11 R. Ryoo, J. M. Kim, C. H. Ko, C. H. Shin, J. Phys. Chem., 100 (1996) 17718. 12 P. T. Tanev and T. J. Pinnavaia, Science, 267 (1995) 865.
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13 S. A. Bagshaw, E. Prouzet and T. J. Pinnavaia, Science, 269 (1995) 1242. 14 D. Y. Zhao, J. L. Feng, Q. S. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka and G. D. Stucky, Science, 279 (1998) 548. 15 A. Sayari, C. Danumah, I. L. Moudrakovski, Chem. Mater., 7 (1995) 813. 16 A. Sayari, I. L. Moudrakovski, C. Danumah, C. I. Ratcliffe, J. A. Ripmeester, K. F. Preston, J. Phy. Chem., 99 (1995) 16373. 17 P. T. Tanev, M. Chibwe and T. J. Pinnavaia, Nature, 368 (1994) 321. 18 A. Tuel, S. Gontier and R. Teissier, Chem. Commun. (1996) 651. 19 J. S. Reddy and A. Sayari, Chem. Commun. (1995) 2231. 20 W. Z. Zhang, J. L. Wang, P. T. Tanev and T. J. Pinnavaia, Chem. Commun. (1996) 979. 21 Z. R. Zhang, J. S. Suo, X. M. Zhang and S. B. Li, Chem. Commun. (1998) 241 22 Z. Y. Yuan, S. Q. Liu, T. H. Chen, J. Z. Wang and H. X. Li, Chem. Commun. (1995) 973. 23 T. T. Abeld-Fattah and T. J. Pinnavaia, Chem. Commun. (1996) 665. 24 R. K. Her, The Chemistry of Silica, Wiley Press, New York, 1979.
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Synthesis of mesoporous silica molecular sieves via a novel templating scheme Xiaoming Zhang, Zhaorong Zhang, Jishuan Suo, and Shuben Li State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, The Chinese Academy of Sciences, Lanzhou 730000, China
A novel templating scheme was demonstrated for the synthesis of mesoporous silica molecular sieve under mild conditions in which N, N-dimethyldodecylamine oxide was in situ prepared from its precursors and used directly as the structure-directing agent. X-ray diffraction and transmission electron micrography revealed that samples thus obtained exhibited a large number of worm-like and interconnected channels, but lack of long range packing order, that is the structure was disordered. Nitrogen adsorption/desorption isotherms confirmed that the pore diameter was in the range of mesoporous region and the sample bore extremely high specific surface areas (>1000m^/g). The structure of the mesoporous silica molecular sieve was determined by various synthesis parameters (especially, pH and surfactant to silica ratio of the gel).
1. INTRODUCTION The discovery of M41S family mesoporous molecular sieves by Mobil researchers has attracted great attentions in the fields of material sciences and catalysis. These type materials have been proved to be of potentials in ion-exchanger, adsorbent, heterogeneous catalysts, as well as catalyst supports. ^'^ Several different synthesis strategies have been proposed and successfully used to prepare mesoporous materials with a unique pore size distribution. The original MCM-41 (Hexagonal phase) and MCM-48 (Cubic phase) materials were prepared by a self-assembly process based on the electrostatic interactions between positively charged quatemary ammonium micelles (S^) and inorganic anions (r).^"^ Most of the studies focused on the synthesis and catalytic application of one dimensional MCM-41 materials because of its ease of preparation. Although the synthesis of cubic MCM-48 was relative difficult, it was also important for catalytic application because it exhibits three dimensional framework structures, and a few research reports have been devoted to the preparation and application of this mesophase.^'^ The electrostatic assembly has been extended to include charge-reversed (S"f) and counter-ion mediated (S^XT^ and S'X^F) pathways.^'^ Huo et al^^ has reported the synthesis of ordered mesoporous molecular sieves SBA-n by using gemini surfactants as the template. Ryoo et al^^ has prepared a disordered mesoporous silica material (KIT) through the
24
polymerization of silicate anions surrounding surfactant micelles in the presence of organic salts. Pinnavaia's group prepared mesoporous silica molecular sieves HMS'^ and MSU'^ involving the hydrogen bonding interactions between neutral inorganic precursor (I^) and neutral alkylamine (S^) or polyethylene oxide (N^) surfactants. More recently, Zhao et al^"^ reported the synthesis of mesoporous molecular sieves with extra-large pore diameter (<30 nm) and wall thickness {<6 nm) by using amphiphilic triblock copolymers as the template. All these mesostructures afford extremely high surface areas, often exceeding 1000 m^/g, which are important in the terms of heterogeneous catalysis and catalyst supports. Moreover, these mesoporous materials can be easily modified by incorporation of different cations, thus leading to a large family of materials with acidic or redox properties. Various metal cations, including Al,^^ B,^^ Ti,^^ Zr,^^ V,^^ Cr, Mo, Mn, ^^ W,^^ Fe,^^ and Sn,^^have been incorporated into the hexagonal mesoporous silica frameworks thus far. These metal modified mesoporous composites are potential in the heavy oil refinery and the synthesis of fine petrochemicals. Tertiary amine oxide exhibits cationic properties under acidic conditions, whereas possess nonionic nature under neutral and basic conditions. It contains two different hydrogen-bonding centers (N and O). Similar to the reported alkylamine and polyethylene oxide, these unique surfactants might be of potentials in directing the mesoporous molecular sieves via hydrogen-bonding forces and/or electrostatic forces between the surfactant headgroup and the inorganic precursors. In this paper, we demonstrated that N, Ndimethyldodecylamine oxide (DAO) was a promising template for the synthesis of mesoporous silica molecular sieves. Furthermore, we found that N, N-dimethyldodecylamine oxide can be prepared and used directly (i.e., in situ) from its precursors N, Ndimethyldodecylamine and aqueous H2O2. The effect of various syntheses conditions, such as pH value, aging time, aging temperature, surfactant concentration, as well as surfactant to Si02 ratio have also been studied in detail.
2. EXPERIMENTAL 2.1 Synthesis The mesoporous silica molecular sieve (designated as LZC) was prepared from tetraethylorthosilicate (TEOS) and N, N-dimethyldodecylamine as the silicon and organic precursors, respectively. In a typical synthesis, 2.0ml N, N-dimethyldodecylamine was dispersed in 5.0ml H2O at 60 °C and formed a turbid mixture. With magnetic stirring, 1.0ml H2O2 (30%wt) was added dropwise into the above mixture. The reaction was carried out at 60 °C for 3.5h, then at 75 °C for 12h. This clear template solution was then cooled to room temperature. Under vigorous stirring, the designed amounts of TEOS were introduced dropwise. The fmal composition of the reaction mixture was ITEOS: xDAO: 25H20,where x=0.1, 0.2, 0.5, 1.0, and 1.5 respectively. Allowing the precursor to age under moderate
25
stirring at room temperature for 48h, then product was centrifuged, washed with de-ionized water, air-dried, and calcined in air at 873K for 5h to remove the organic templates. 2.2 Characterization X-ray powder diffraction (XRD) patterns of the sample were recorded on a Rigaku D/Max 2400 X-ray diffractometer with Cu-Ka radiation (?i=0.15418nm). The surface areas and pore diameters were measured by BET and BJH methods on a Micromeritics ASAP 2010 Sorptometer. Before analysis, the sample was degassed at 423K and 1.07x10"^ KPa for 12h. The TEM image was obtained on a JEM-IOOC transmission microscope. The TG analysis was carried out on a DuPont 1090 Thermal Analyzer.
3. RESULTS AND DISCUSSION 3.1 Template N, N-dimethyldodecylamine is unsoluble, while N, N-dimethyldodecylamine oxide has good solubility in water. So it was latter rather than N, N-dimethyldodecylamine that acts as the structure-directing agent for the formation of mesostructures in aqueous media. Figure 1 depicts the transition electron micrograph (TEM) image of the calcined mesporpous sample (designated as LZC-C). Analogous to the TEM images of MSU and HMS mesoporous materials, there were a large number of worm-like and interconnected channels, but lack of long range packing order, that is the structure of the calcined sample was disordered. This was further verified by the XRD patterns. It exhibited a single, broad reflection in low 20 angles; the high order reflections were not resolved. Similar to the structures of HMS and MSU, the absence of high order reflection might be due to the weak hydrogen bonding forces and the corresponding small scattering domain sizes. 3.2 The effect of pH The silicon precursor and N, Ndimethyldodecylamine oxide exhibit different electronic properties under different pH conditions, thus the driving forces and the corresponding sample structure may be different in various pH values. We performed the synthesis process at pH<0, pH=2, pH=7, and pH>10, respectively. The silicon precursor did not condense when pH<0. Figure 2 illustrates the XRD patterns of the sample prepared
Figure 1. TEM image of calcined LZC mesoporous silica molecular sieves.
26
2.0 4.0
Figure 2. XRD patterns of as-synthesized LZC mesoporous silica molecular sieves with different pH. (A) 2, (B) 7, (C) 10
6.0 20/°
Figure 3. XRD panems of as-synthesized LZC mesoporous silica molecular sieves with different temperature. (A) 80°C,(B)50°C. (C)RT
4.0
6.0 8.0 10.0 29/°
Figure 4. XRD patterns of calcined LZC mesoporous silica molecular sieves with different surfactant'SiO:. (A) 0.1, (B) 0.2, (C) 0.5. (D) 1.0. (E) 1.5
under the other pH conditions. The sample prepared under iso-electropoint of silica !pH=2) and strong alkaline conditions (pH>10) were amorphous, while the sample prepared under neutral conditions (pH=7) had a broad reflection at low angles. The difference in the structures with various pH conditions might be the different interaction forces between template and silicon precursors. N, N-dimethyldodecylamine oxide exhibits cationic nature under acidic conditions (pH<7) and nonionic properties under neutral (pH=7) and alkaline conditions (pH>7). While TEOS was positive charged bellow the iso-electronic point of silica (pH<2) and negative charged above the iso-electronic point of silica (pH>2)." So the interaction between the template and the silicon precursor under acidic conditions was positive charged template verse positive charged silicon species, but there was not a mediating counter-ion (like CT or Br' in S^XT assembly), thus can not form stable structures in that cases. Under strong alkaline conditions, the silicon precursor was highly negative charged, the interaction between neutral template and negative charged silicon species was too weak to form stable structure. Under neutral condition (pH=7), however, the hydrogen bonding forces between N, N-dimethyldodecylamine oxide (N and O atoms) and the slightly negative charged silicon precursor leaded to the formation of mesoporous silica structures, but lack of long range packing order. 3.3 The effect of aging temperature
Table 1 Properties of LZC mesoporous silica molecular sieves with different aging time Sample Aging time d-spacing Pore volume Pore diameter SBET /h /nm /cm^/g /nm W/g A 24h 4.37 938.2 0.46 3.19 B 48h 4.12 841.4 2.62 0.24 C 72h 4.01 787.7 0.24 2.62 The traditional mesoporous silica molecular sieves MCM-41 was prepared under hydrothermal conditions, while HMS and MSU were synthesized at mild conditions. The aging temperature can affect the gel chemistry of silica and the interaction forces between the surfactant and the inorganic precursors. We performed the synthesis of mesoporous silica molecular sieves at room temperature, 50 °C, and 80 °C, respectively. The sample prepared 80 °C was amorphous, while the sample prepared at room temperature and 50 °C exhibited mesoporous properties (Figure 3). The BET surface areas for those three samples were 841 m /g, 707 m /g, and 344 m^/g, respectively. This further confirmed that the driving forces for the formation of LZC mesostructures was the hydrogen bonding forces between silica precursor and N, N-dimethyldodecylamine oxide head-group, and the hydrogen bonding forces was weakened by higher temperature, thus the mesoporous structures were destroyed. 3.4 The effect of aging time Table 1 lists the d-spacing, BET surface areas, pore volumes, and pore diameters of the sample synthesized with different aging time. The XRD patterns of all samples were similar, and the d-spacing, BET areas, pore volume, and pore diameters shifted to the lower values upon the longing of aging time. This suggested the further cross-linking and condensation of the framework structures. 3.5 The effect of surfactant to silica ratios The structures of mesoporous molecular sieves were closely related to the surfactants to silica ratios."^ Because of the absence of high order reflections on the XRD patterns of all LZC Table 2 Properties of LZC mesoporous silica molecular sieves with different surf/Si02 Sample Surf/Si02 d-spacing SBET Pore volume Pore diameter /nm W/g /cmVg /nm 0.26 3.18 4.01 828.3 A 0.1 2.62 0.24 841.4 4.12 B 0.2 3.92 1.24 1102.1 4.05 C 0.5 4.06 1.22 1127.7 1.0 3.84 D 4.36 0.89 3.87 991.1 E 1.5
28
0 0.2 0.4 0.6 0.8 1.0 (P/Po)
0
0 0.2 0.4 0.6 0.8 1.0 (P/Po)
0.2 0.4 0.6 0.8 1
0 0.2 0.4 0.6 0.8 1.0 (P/Po)
0
0.2 0.4 0.6 0.8 1.0 (P/Po) Figure 5. N? adsorption/desorption isotherms of calcined LZC mesoporous silica molecular sieves with different surfactant/SiOi: (A) 0.1, (B) 0.2, (C) 0.5, (D) l.O, and (E) 1.5 silica samples, the XRD can not discriminate the samples synthesized at different surf./SiO: ratios. The nitrogen sorption techniques, however, can discern the minor difference in the pore structures of those samples. In this paper, we carried out the synthesis of mesoporous silica molecular sieves with different surfactant to silica ratios. Although the XRD patterns were similar to each other (Figure 4), the pore structures were different magnificently. Figure 5 shows the N2 adsorption/desorption isotherms of the mesoporous silica molecular sieves with different Surf./silica ratios. The corresponding BET surface areas, pore volumes, and BJH pore diameters are listed in Table 2. The isotherms of the samples with surf./silica <0.2 (Sample A and B) were type 4 in lUPAC classification, and there were not hysteresis loop, suggesting the existence of framework mesoporousity. In the case of those samples with surf/silica >0.2 (Smaple C, D and E), there appeared a large hysteresis loop at higher relative pressure. Furthermore, the BJH mean pore diameters of Sample D and E were larger than the corresponding d-spacings ( see Table 2).It was contradiction to the previous reported results for HMS and MSU mesoporous materials, in which the wall thickness was calculated from the difference between the pore diameter and d-spacing. These unusual data suggested that the pore structures of these mesoporous silica molecular sieves were magnificently different. Tlie increasing of Surf./SiO: from 0.2 to 1.5 lead to the decreasing of the packing orders of the
29 mesopore structures, and there were more textural mesopores in Sample D and E than in Sample A and B, thus a large hysteresis loop appeared in the N2 adsorption/desorption isotherms. The BJH pore diameter is a mean value of various type of pore structure. Sample D and E were more disordered than the other samples and possessed more textural pores, therefore mean values between the textural and framework pore diameter gave a relatively larger value.
4. CONCLUSION A disordered mesoporous silica molecular sieve has been synthesized successfully with N, N-dimethyldodecylamine oxide as the structure directing agent. TEM image and XRD patterns proved the presence of large number worm-like and interconnected channels. XRD and nitrogen adsorption techniques suggested that the pore structures of the samples were closely related to the pH value and the surfactant to silica ratios of the precursor gels, hi addition, we illustrated a novel templating scheme for the synthesis of mesoporous composites, that is, the combination of the synthesis of template and the synthesis of mesoporous materials. It offered a convenient, cost effective, and altemative assembly pathway for the synthesis of mesoporous molecular sieves, which was the highlight of catalysis and material sciences in recent years.
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