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Influence of the catalyst on the formation and structure of bimodal mesopore silica
Recent Progress in Mesostructured Mesostructured Materials D. Zhao, S. Qiu, Y. Tang and C. Yu (Editors) © 2007 Elsevier B.V. All rights reserved.
667 667
Influence of the catalyst on the formation and structure of bimodal mesopore silica Xiaozhong Wanga*, Wenhuai Lib, Bing Zhongb andKechang Xiea " Key Laboratory of Coal Science and Technology, Ministry of Education and Shanxi Province, Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan, 030024 China. State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, the Chinese Academy of Sciences, Taiyuan, 030001 China.
1. Introduction In earlier investigations [1], we found that through controlling gelation other than precipitation of a reaction system which was used usually to prepare MCM-41 materials [2], a hierarchically structured porous silica gel monolith with well-defined bimodal mesopore size distribution (i.e. BMS) can be formed at ambient conditions. The unique bimodal mesostructure and fair thermal stability of BMS silica may find broad potential application in catalyst, catalyst supports, electronic, optical or sensing devices. Since the key to BMS silica synthesis is the gelation control of the reaction system, thus, any variation of the synthesis parameters all may influence the reaction kinetics of sol-gel alkoxides and then influences the mesostructure of the resultant BMS silica. In various synthesis parameters, the catalysts used undoubtedly are of the very importance, as that has been seen not only in the synthesis of conventional silica gels [3, 4], but also in the preparation of MCM-41 materials [5, 6]. For the reason, a study was conducted in our BMS silica synthesis employing various organic amines as catalyst to investigate their direct influence on the gelation and the bimodal mesostructure of the BMS silica produced. At the same time a full account of the trend of pore size adjustment of BMS silica was also given. 2. Experimental Section The synthesis of BMS silica followed the same procedures as previously reported [l(a), l(b)], except that the initially used mineralization agent
668
ammonia was replaced with some small molecule organic amines such as ethylamine (EA), n-butylamine (BA), n-hexylamine (HA), 1,2-ethanediamine (EDA), 1,4-butanediamine (BDA), 1,6-hexanediamine (HAD) for the hydrolysis and condensation of tetraethoxysilane (TEOS). The molar composition of the starting solutions was: 1.0 TEOS: 0.18 Ci6TAB (cetyltrimethylammonium bromide): (0.016-0.64) amine: 75 H2O. Gelation was determined visually by the fact that the solution no longer exhibited bulk fluid behavior. The resultant silica samples were characterized using XRD, N2 adsorption isotherms, SEM and TEM. 3. Results and Discussion Our preliminary tests under various sol-gel conditions show that the amount of the catalysts used in the starting compositions plays a key role in determining the morphology and the textural properties of the resultant silica materials. Fig. 1 shows the physical appearance of the sol-gel products prepared by various amounts of different amines addition with a constant other composition concentration. It is clear that three different typed of behavior can be classified, although these regions were significantly different for different amines used. Following the increase of the amount of amines used, the resultant silica products all change gradually from an initial opaque gel monolith via a viscous liquids between gel and precipitation (i.e. intermediate) to a rapid formed precipitation, as that has been seen in previous work [l(b)]. By way of convenient for comparing, the amount of different catalysts used is selected in such a region, typical as 0.064mol, to ensure the formation of a silica gel rather than precipitation. The relationship between the catalysts used and the gelling rate was investigated by measuring the time necessary for the solution to lose fluidity. This time, which we call gelation time, expressed in minutes after the addition of catalysts, is listed in Table 1. It is clear that the gelation times are influenced by the catalysts used in the synthesis, steeply decreasing with a increase of the carbon atom number in organic amine molecules, indicating a 0.064
0.064 Gel Intermediate
I ' ' I ' ' I 0. 192 0 . 3 3 4 0 . 5 7 6
I ' ' I ' ' I 0. 192 0. 384 0. 5 7 6
Precipitation
Molalkali/molTEOS Fig. 1 Appearance of sol-gel products prepared with various amount of different alkali catalysts
669 Table 1. Structure parameters of BMS silica prepared with different amines as catalyst
Catalysts
Gel
(2M)
Time (min)
Framework Mesopore A/BET 2
NH3
90
(m /g) 624.7
Textural Mesopore V,
(cmVg)
(nm)
(m /g)
(cm /g)
D, (nm)
0.64
2.90
337.3
0.85
12.60
vf
A,BET 2
3
EA
45
487.1
0.32
2.63
303.2
1.22
18.42
BA
40
500.6
0.33
2.74
344.4
1.28
18.52
HA
30
559.1
0.36
2.66
274.9
1.35
20.94
EDA
35
633.9
0.43
2.74
307.24
1.15
16.35
BDA
10
774.3
0.56
3.01
290.2
1.32
21.03
HDA
6
893.9
0.64
2.95
284.9
1.36
22.30
rapid increasing of condensation rate, which would accelerate the aggregates of micelle-encapsulated silica colloidal particles in the sol. The gelation times also imply a relative strength of each amine in acting as a catalyst for the condensation of the hydrolyzed alkoxide. Generally, the weaker the alkalinity strength of the catalysts used is, the wider the concentration range suitable to the formation of a homogeneous gelis. The characteristic results show that the qualitative form of the XRD patterns, N2 adsorption isotherms, SEM and TEM images of the calcined silica samples obtained with various mineralization agents all are similar to the previous reported BMS silica phase [1]. However, the positions of the XRD intense reflection, the adsorption steps at lower and higher p/p0 on N2 adsorption curves and thus the pore size distribution of the framework and textural mesopores along with the size and the packing geometry of primary silica particles vary with the catalysts used. The related structure parameters are listed in Table 1. It is found that the diamine catalysts are more favorable for the formation of a well-ordered framework mesopore than that of monoamine catalysts, especially in increasing the carbon atom numbers in amine molecules used. The reason causing this difference may be related not only to the alkalinity strength resulting from the intrinsic difference in structure and properties of these amine molecules, but also to their locus dissolved in the micelles. For the monoamine molecules, the increase of carbon atom numbers makes them easier to be solubilized in the palisade layer of micelles, which would make the surface charge density of micelles decease and thus weaken the interaction force between micelles and silicate species. On the contrary, the diamine molecules are better solubilized close to the surface in the palisade layer or by adsorption at the micelle-water interface, which would improve the surface charge density of micelles and thus strengthen the interaction between surfactant micelles and silicate species and finally result in a more well-defined ordered framework
670
mesopore to be formed. Compared with the previous synthesized BMS silicas [1], the decrease in the specific surface areas and the pore volumes of framework mesopores may be related to the lower alkalinity provided by the catalysts used. The present results also show that the textural mesopore size of resulting silicas is more sensitive to the variation of catalysts used than its framework mesopore size. Moreover, the textural mesopore volume can be up to 2.0 or more times as large as the framework mesopore volume, which is of great interest to catalysis because they greatly facilitate mass transport to the framework mesopore. The TEM images show that all samples consist of agglomerations or packing of approximately equal sized like-spherical nanometer silica particles with an average diameter of c.a.20-40nm, depending on the catalysts used, that are joined together to form agglomerates a few tens of microns in diameter. The textural mesoporosity originating from the cavities between close-packed particles increases gradually in the order of NH3-EDABDA-HDA and NH3-EA-BA-HA, which is consistance with the changes of the gelation times and the XRD and N2 adsorption results. The framework mesopores consist of randomly distributed hexagonal and wormhole-like mesoporous channels and there is no apparent long-range order to the pore arrangement. 4. Conclusion It is found that BMS silica can be synthesized using a series of organic amine as catalysts. However, due to the difference in the structure and properties of these catalysts used, not only is their concentration scope suitable to the BMS silica synthesis clearly different, but the gelation time, thus the surface area, pore volume and bimodal mesoporosity of the resultant BMS silica can be tuned by this variable over a suitable range. (NSFC Grant No.20371034) 5. References [1] (a) X. Z. Wang, T. Dou and Y. Z. Xiao, Chem. Commun., (1998) 1035. (b) X. Z. Wang, W. H. Li, G. S. Zhu, S. L. Qiu, D. Y. Zhao and B. Zhong, Micropor. Mesopor. Mater., 71 (2004) 87. (c) X. Z. Wang, T. Dou, Y. Z. Xaio and B. Zhong, Stud. Surf. Sci. Catal., 135 (2001) 199. (d) X. Z. Wang, T. Dou, D. Wu and B. Zhong, Stud. Surf. Sci. Catal., 141 (2002) 77. (e) X. Z. Wang, T. Dou and B. Zhong, Frontiers of Solid State Chemistry, World Sci Publ, (2002) 227. (f) X. Z. Wang, W. H. Li, T. Dou and B. Zhong, Stud. Surf. Sci. Catal., 146 (2003) 255. (g) X. Z. Wang, W. H. Li, B. Zhong and D. Y. Zhao, Stud. Surf. Sci. Catal., 154 (2004) 555. (h) X. Z. Wang, W. H. Li, J. Y. Lin, H. L. Fan, C. S. Tian, B. Zhong and K. C. Xie, Stud. Surf. Sci. Catal., 158 (2005) 257. [2] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature 359(1992)710. [3] S. M. Jones, J. Non-Cryst Solid., 291 (2001) 206. [4] E. Framery and P. H. Mutin, J. Sol-Gel. Sci. Tech., 24 (2002) 191. [5] X. Z. Wang, T. Dou, Y. Z. Xiao and B. Zhong, J. Natural Gas Chemistry., 8 (1999) 216. [6] W. Lin, Q. Cai, W. Pang, Y. Yue and B. Zou, Micropor. Mesopor. Mater., 33 (1999) 187.
667 667
Influence of the catalyst on the formation and structure of bimodal mesopore silica Xiaozhong Wanga*, Wenhuai Lib, Bing Zhongb andKechang Xiea " Key Laboratory of Coal Science and Technology, Ministry of Education and Shanxi Province, Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan, 030024 China. State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, the Chinese Academy of Sciences, Taiyuan, 030001 China.
1. Introduction In earlier investigations [1], we found that through controlling gelation other than precipitation of a reaction system which was used usually to prepare MCM-41 materials [2], a hierarchically structured porous silica gel monolith with well-defined bimodal mesopore size distribution (i.e. BMS) can be formed at ambient conditions. The unique bimodal mesostructure and fair thermal stability of BMS silica may find broad potential application in catalyst, catalyst supports, electronic, optical or sensing devices. Since the key to BMS silica synthesis is the gelation control of the reaction system, thus, any variation of the synthesis parameters all may influence the reaction kinetics of sol-gel alkoxides and then influences the mesostructure of the resultant BMS silica. In various synthesis parameters, the catalysts used undoubtedly are of the very importance, as that has been seen not only in the synthesis of conventional silica gels [3, 4], but also in the preparation of MCM-41 materials [5, 6]. For the reason, a study was conducted in our BMS silica synthesis employing various organic amines as catalyst to investigate their direct influence on the gelation and the bimodal mesostructure of the BMS silica produced. At the same time a full account of the trend of pore size adjustment of BMS silica was also given. 2. Experimental Section The synthesis of BMS silica followed the same procedures as previously reported [l(a), l(b)], except that the initially used mineralization agent
668
ammonia was replaced with some small molecule organic amines such as ethylamine (EA), n-butylamine (BA), n-hexylamine (HA), 1,2-ethanediamine (EDA), 1,4-butanediamine (BDA), 1,6-hexanediamine (HAD) for the hydrolysis and condensation of tetraethoxysilane (TEOS). The molar composition of the starting solutions was: 1.0 TEOS: 0.18 Ci6TAB (cetyltrimethylammonium bromide): (0.016-0.64) amine: 75 H2O. Gelation was determined visually by the fact that the solution no longer exhibited bulk fluid behavior. The resultant silica samples were characterized using XRD, N2 adsorption isotherms, SEM and TEM. 3. Results and Discussion Our preliminary tests under various sol-gel conditions show that the amount of the catalysts used in the starting compositions plays a key role in determining the morphology and the textural properties of the resultant silica materials. Fig. 1 shows the physical appearance of the sol-gel products prepared by various amounts of different amines addition with a constant other composition concentration. It is clear that three different typed of behavior can be classified, although these regions were significantly different for different amines used. Following the increase of the amount of amines used, the resultant silica products all change gradually from an initial opaque gel monolith via a viscous liquids between gel and precipitation (i.e. intermediate) to a rapid formed precipitation, as that has been seen in previous work [l(b)]. By way of convenient for comparing, the amount of different catalysts used is selected in such a region, typical as 0.064mol, to ensure the formation of a silica gel rather than precipitation. The relationship between the catalysts used and the gelling rate was investigated by measuring the time necessary for the solution to lose fluidity. This time, which we call gelation time, expressed in minutes after the addition of catalysts, is listed in Table 1. It is clear that the gelation times are influenced by the catalysts used in the synthesis, steeply decreasing with a increase of the carbon atom number in organic amine molecules, indicating a 0.064
0.064 Gel Intermediate
I ' ' I ' ' I 0. 192 0 . 3 3 4 0 . 5 7 6
I ' ' I ' ' I 0. 192 0. 384 0. 5 7 6
Precipitation
Molalkali/molTEOS Fig. 1 Appearance of sol-gel products prepared with various amount of different alkali catalysts
669 Table 1. Structure parameters of BMS silica prepared with different amines as catalyst
Catalysts
Gel
(2M)
Time (min)
Framework Mesopore A/BET 2
NH3
90
(m /g) 624.7
Textural Mesopore V,
(cmVg)
(nm)
(m /g)
(cm /g)
D, (nm)
0.64
2.90
337.3
0.85
12.60
vf
A,BET 2
3
EA
45
487.1
0.32
2.63
303.2
1.22
18.42
BA
40
500.6
0.33
2.74
344.4
1.28
18.52
HA
30
559.1
0.36
2.66
274.9
1.35
20.94
EDA
35
633.9
0.43
2.74
307.24
1.15
16.35
BDA
10
774.3
0.56
3.01
290.2
1.32
21.03
HDA
6
893.9
0.64
2.95
284.9
1.36
22.30
rapid increasing of condensation rate, which would accelerate the aggregates of micelle-encapsulated silica colloidal particles in the sol. The gelation times also imply a relative strength of each amine in acting as a catalyst for the condensation of the hydrolyzed alkoxide. Generally, the weaker the alkalinity strength of the catalysts used is, the wider the concentration range suitable to the formation of a homogeneous gelis. The characteristic results show that the qualitative form of the XRD patterns, N2 adsorption isotherms, SEM and TEM images of the calcined silica samples obtained with various mineralization agents all are similar to the previous reported BMS silica phase [1]. However, the positions of the XRD intense reflection, the adsorption steps at lower and higher p/p0 on N2 adsorption curves and thus the pore size distribution of the framework and textural mesopores along with the size and the packing geometry of primary silica particles vary with the catalysts used. The related structure parameters are listed in Table 1. It is found that the diamine catalysts are more favorable for the formation of a well-ordered framework mesopore than that of monoamine catalysts, especially in increasing the carbon atom numbers in amine molecules used. The reason causing this difference may be related not only to the alkalinity strength resulting from the intrinsic difference in structure and properties of these amine molecules, but also to their locus dissolved in the micelles. For the monoamine molecules, the increase of carbon atom numbers makes them easier to be solubilized in the palisade layer of micelles, which would make the surface charge density of micelles decease and thus weaken the interaction force between micelles and silicate species. On the contrary, the diamine molecules are better solubilized close to the surface in the palisade layer or by adsorption at the micelle-water interface, which would improve the surface charge density of micelles and thus strengthen the interaction between surfactant micelles and silicate species and finally result in a more well-defined ordered framework
670
mesopore to be formed. Compared with the previous synthesized BMS silicas [1], the decrease in the specific surface areas and the pore volumes of framework mesopores may be related to the lower alkalinity provided by the catalysts used. The present results also show that the textural mesopore size of resulting silicas is more sensitive to the variation of catalysts used than its framework mesopore size. Moreover, the textural mesopore volume can be up to 2.0 or more times as large as the framework mesopore volume, which is of great interest to catalysis because they greatly facilitate mass transport to the framework mesopore. The TEM images show that all samples consist of agglomerations or packing of approximately equal sized like-spherical nanometer silica particles with an average diameter of c.a.20-40nm, depending on the catalysts used, that are joined together to form agglomerates a few tens of microns in diameter. The textural mesoporosity originating from the cavities between close-packed particles increases gradually in the order of NH3-EDABDA-HDA and NH3-EA-BA-HA, which is consistance with the changes of the gelation times and the XRD and N2 adsorption results. The framework mesopores consist of randomly distributed hexagonal and wormhole-like mesoporous channels and there is no apparent long-range order to the pore arrangement. 4. Conclusion It is found that BMS silica can be synthesized using a series of organic amine as catalysts. However, due to the difference in the structure and properties of these catalysts used, not only is their concentration scope suitable to the BMS silica synthesis clearly different, but the gelation time, thus the surface area, pore volume and bimodal mesoporosity of the resultant BMS silica can be tuned by this variable over a suitable range. (NSFC Grant No.20371034) 5. References [1] (a) X. Z. Wang, T. Dou and Y. Z. Xiao, Chem. Commun., (1998) 1035. (b) X. Z. Wang, W. H. Li, G. S. Zhu, S. L. Qiu, D. Y. Zhao and B. Zhong, Micropor. Mesopor. Mater., 71 (2004) 87. (c) X. Z. Wang, T. Dou, Y. Z. Xaio and B. Zhong, Stud. Surf. Sci. Catal., 135 (2001) 199. (d) X. Z. Wang, T. Dou, D. Wu and B. Zhong, Stud. Surf. Sci. Catal., 141 (2002) 77. (e) X. Z. Wang, T. Dou and B. Zhong, Frontiers of Solid State Chemistry, World Sci Publ, (2002) 227. (f) X. Z. Wang, W. H. Li, T. Dou and B. Zhong, Stud. Surf. Sci. Catal., 146 (2003) 255. (g) X. Z. Wang, W. H. Li, B. Zhong and D. Y. Zhao, Stud. Surf. Sci. Catal., 154 (2004) 555. (h) X. Z. Wang, W. H. Li, J. Y. Lin, H. L. Fan, C. S. Tian, B. Zhong and K. C. Xie, Stud. Surf. Sci. Catal., 158 (2005) 257. [2] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature 359(1992)710. [3] S. M. Jones, J. Non-Cryst Solid., 291 (2001) 206. [4] E. Framery and P. H. Mutin, J. Sol-Gel. Sci. Tech., 24 (2002) 191. [5] X. Z. Wang, T. Dou, Y. Z. Xiao and B. Zhong, J. Natural Gas Chemistry., 8 (1999) 216. [6] W. Lin, Q. Cai, W. Pang, Y. Yue and B. Zou, Micropor. Mesopor. Mater., 33 (1999) 187.