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Triblock copolymer and poly(ethylene glycol) as templates for monolithic silica material with bimodal pore structure
Microporous and Mesoporous Materials 88 (2006) 31–37 www.elsevier.com/locate/micromeso
Triblock copolymer and poly(ethylene glycol) as templates for monolithic silica material with bimodal pore structure Y.W. Sun, Y.J. Wang, W. Guo, T. Wang, G.S. Luo
*
The State Key Lab of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Received 13 June 2005; received in revised form 24 August 2005; accepted 24 August 2005 Available online 4 October 2005
Abstract To synthesize monolithic silica materials with bimodal pore structures, a new dual-templating system of triblock copolymer and poly(ethylene glycol) was proposed and a subcritical water oxidation method was introduced to remove the organic templates. In this new system, triblock copolymer Pluronic P123 and poly(ethylene glycol) (PEG) mainly served as the templates for mesopores and macropores, respectively. The influence of PEG molecular weight, PEG concentration and TEOS concentration on the bimodal pore structure was investigated. The macro–mesoporous structure of synthesized monolithic silica materials with tetraethoxysilane (TEOS) as the silica source has been confirmed by a nitrogen adsorption–desorption measurement and SEM observations. The monolithic silica materials with macro–mesoporous structure were synthesized successfully using the new template system. The macropores size ranged from 0.6 lm to 4 lm and the silica skeleton size was from 0.5 lm to 1.5 lm. The mesopore size was decided by the composition of P123/PEG. PEG could enlarge the mesopore size from 3–4 nm (only P123 applied) to 8–10 nm. Using PEG with different molecular weights as template, two different macropore morphologies were observed and two possible mechanisms of macropores formation were hypothesized, named as the micelles mechanism and the network mechanism, respectively. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Silica material; Bimodal pore structure; Dual templates; Subcritical water oxidation method
1. Introduction Recently, silica materials with hierarchical pore structure have attracted much interest, for example, materials containing small (micro- or mesopores) and large (macropores) pores, since they combine the benefits of each pore size regime. The micro- or mesopores possess size- or shape-selectivity and high surface area. Usually, the surface area of mesoporous material is about 102–103 m2/g. The macropores provide easier access to the active sites and reduce the pressure drop over the material [1,2]. Therefore, materials with such structure have a profound promise in catalysis and separation processes, such as chromatogra-
*
Corresponding author. Tel.: +86 1062783870; fax: +86 1062770304. E-mail address: [email protected] (G.S. Luo).
1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.08.018
phy, separation for large molecules, drug release, etc. The advantages of producing hierarchical pore structure within one material are evident. There are two widely used approaches to produce the bimodal pore structure. One is based on phase separation strategies in combination with sol–gel process. Tanaka and coworkers [3–6] utilized the hydrolysis and polycondensation of tetramethoxysilane (TMOS) in the presence of PEG to produce the interconnected macropores and silica skeleton structure. Through the solvent-exchange in ammonia, mesopores were tailored on the silica skeleton. The macropore diameter can be controlled by adjusting the PEG concentration, which affects the relative rate of phase separation to the sol–gel transition. The mesopore size can be controlled independently of the macropore size through post-synthesis treatment in ammonia solution. These macroporous/mesoporous materials have
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already been applied in high performance liquid chromatography. However, in this approach, no or only weak regular arrangements of the mesopores are achieved with a rather bulky network structure in the micrometer range [7]. The other approach is to use dual or multiple templates in different ranges to produce hierarchical pore structure. For the mesopores, the supramolecular arrays of amphiphilic block copolymers or other surfactants are commonly used as templates, through which the materials exhibit a narrow pore size distribution. The templates producing the macropores are various. Holland et al. [1] used arrays of monodiperse polystyrene spheres as templates for macropores and tetrapropylammonium hydroxide for mesopores. Sen et al. [2] synthesized silica materials with macro–meso–microporous structure by using polystyrene spheres, F127 and P123 as templates, respectively. Sun et al. [8] presented a two-step route to produce bimodal pore structure. Firstly, MCM-41 nanoparticles were prepared, using CTAB as mesopore templates. Then the nanoparticles were cross-linked around assemblies of triblock copolymer to produce macroporous structure. Groenewolt et al. [9] prepared silica monoliths with bimodal pore structure by nanocasting mixtures of fluorinated nonionic surfactants and micelles of two hydrocarbon block copolymers. Micelles with different sizes formed by different surfactants acted as the templates for hierarchical pore structure. Amatani et al. [10] synthesized monolithic silica gel with macro–mesoporous structure via using triblock copolymer P123 and 1,3,5-trimethylbenzene as structuredirecting agent and micelle-swelling agent. Smatt et al. [11] designed a dual-templating route to synthesize silica monoliths. PEG and CTAB were utilized as templates for macropores and mesopores, respectively. The materials exhibited bimodal pore structure and arranged mesopores on the skeleton. Compared with ionic surfactants, triblock copolymers have more advantages, because they are capable to impart larger pores and thicker walls, besides being industrially available, hazard-free and easy to remove from the mineral framework (by thermal treatment or solvent extraction) [12]. According to the earlier research in our laboratory, the block copolymer is also superior to the ionic surfactant in hydrothermal stability and in easy adjustment of the pore size [13]. Can block copolymer together with PEG serve as template for meso- and macropores? What is the possible mechanism for the templates to produce pores in the silica framework? These questions aroused our interest in exploring the new dual-templating route. In this work, we explored this new dual-templating system, PEG together with block copolymer to prepare silica materials with bimodal pore structure. The relationship between the morphology and the preparation condition was studied in detail. Moreover, a subcritical water oxidation method was introduced to remove the templates in our preparation route.
2. Experiments 2.1. Chemicals Tetraethoxysilane (TEOS, Fluka, 99%), triblock copolymer Pluronic P123 (EO20PO70EO20, BASF analytical purity), HCl (Being Chemical Plant, 36 wt.%), poly(ethylene glycol) (PEG, Beijing Chemical Plant, analytical purity), ethanol (Beijing Chemical Plant, analytical purity), H2O2 (Beijing chemical company, 30 wt.%) were used as received and doubly de-ionized distilled water was used throughout. 2.2. Materials synthesis procedure Silica materials with bimodal pore structure were prepared as follows. First, 2.29 g of TEOS was mixed with 1.58 g of HCl solution (pH = 2) in the presence of 1.00 g of P123 (template for the mesopores) and 0.428 g of ethanol. The mixture was stirred vigorously for 30 min at room temperature to form a homogeneous sol. The sol was stirred for another 4 days, which led to partial hydrolysis and polycondensation of TEOS. Then, 0.53 g of PEG was added to the transparent sol and the mixture was stirred until PEG dissolved. The resultant mixture was charged into a capillary tube which had been treated with 1 M NaOH mixture for 1 h in advance. The mixture reacted at room temperature and gel formed. After aging under the same conditions for 5–10 days, the capillary with the gel was transferred to an autoclave and immersed in the 5% H2O2 solution. The solution in the autoclave was heated up to 300 °C gradually under a pressure of 16 MPa to reach a subcritical state. The state was maintained for 1 h to remove the templates. Quantity changes, if any, will be indicated in Section 3. 2.3. Characteristics of the prepared materials A scanning electron microscopy (S-450, Hitachi, Japan) was employed to observe the morphology of the prepared material. Characterization of the mesopore structure was performed by BET (ASAP 2010, Micromeritics). 3. Results and discussion 3.1. Bimodal pores structure The major objective of this research is to study the PEG and P123 dual-templating route to produce silica materials with bimodal pore structure. The SEM photographs in Fig. 1 show the cross-section of the prepared materials with PEG2000 and PEG6000. These results indicate that the interconnected macroporous morphology was formed very well after removing the templates. The macropores and the silica skeleton were uniform. The size of the skeleton and the macropore was around 1 lm. The morphology of the skeleton changed from aggregates of silica particles to a
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Fig. 1. SEM photographs of the macroporous morphology. (a) PEG2000 (aggregates of particles), (b) PEG6000 (smooth network).
smooth network, when PEG molecular weight increased from 2000 to 6000. Fig. 2(a) shows the BET results of the prepared materials. The materials exhibit well-defined mesopores in the range of 8–10 nm. When skipping the PEG adding step, contrasting results are shown in Fig. 2(b). The mesopores size decreased to 3–4 nm and the distribution narrowed. PEG was used as the macropores template in this new dual-templating system, but PEG affected the formation of the mesopore structure by changing the micelle size of
P123. The addition of PEG enlarged the mesopore size and widened the mesopore size distribution of the material. 3.2. Influence of PEG molecular weight and PEG concentration Further research results revealed that both the molecular weight and the concentration of PEG have great effects on the macroporous morphology. Fig. 3 shows the morphologies of the materials, which were prepared with
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Fig. 2. Effect of PEG on mesopore size and distribution. (a) BET results of the system with 0.53 g of PEG2000, (b) BET results of the system without PEG.
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Fig. 3. SEM photographs of monoliths under different PEG molecular weights. (a) PEG400, (b) PEG2000, (c) PEG6000, (d) PEG8000, (e) PEG10000, (f) PEG12000.
0.53 g of PEG with different molecular weights, respectively, ranging from 400 to 12,000. When the molecular weight was above 2000, the materials exhibited an obvious interconnected macropores structure. The size ratio of the macropores/the silica skeleton ranged from 1 to 5, while the void/particle size ratio ranged from 0.25 to 0.4 in a conventional packed column of 5 lm particles. This suggests the prepared materials will provide lower pressure drop and higher permeability than a conventional packed column. The macroporous and skeleton morphologies shown in Fig. 3 can be divided into three types. The first type can be named as the aggregate type which was formed by the application of PEG2000. The aggregates of silica particles could be obviously distinguished from the silica skeleton. The macropore size is decided by the voids among the aggregates. The morphologies with the application of PEG6000 and PEG8000 belong to the second type, named as the network type. The skeleton morphology turned out to be a smooth network. The macropores depended on the voids of the network. Both the silica skeleton size and the macropores size are smaller than those of the aggregate type. The third type, named as the mixed type, was formed when PEG10000 and PEG12000 were applied. In this type, the skeleton morphology was a network with some larger pores on it. We hypothesize that there are two corresponding mechanisms of the macropores formation. According to sol–gel theory, PEG chains in the sol form a network structure. Si–OH groups of the silicic acid combine with EO groups on PEG and polycondensation of Si–OH occurs to form silica skeleton. With the hydrolysis and polycondensation of the silicane, the system undergoes a spinodal decomposition and two continuous phases are formed. One is silica
rich phase which will be the silica skeleton at the end. The other is solvent rich phase which will become the interconnected macropores in the materials after drying. We name this mechanism as the network mechanism. The macropores of the network type form according to this mechanism. The second mechanism named as the micelles mechanism explains the formation of the macropores in the aggregate type and the larger pores in the mixed type. PEG chains form micelles in the sol, around which silica particles aggregate. After the condensation of silicane and removal of PEG, the macropores are formed according to PEG micelles template. Therefore, the size of macropores formed according to the micelles mechanism is much larger than that of the network mechanism. The former one is about 0.6 lm while the latter one is about 3–4 lm. In the situation of PEG2000, the PEG chains mainly form micelles in the sol so that the macropores mainly belong to the aggregate type. When the length of PEG chains increases, PEG chains will tend to a network structure. The macropores shown in Fig. 3(c) and (d) are mainly produced by the network mechanism. When PEG12000 is applied, these two mechanisms exist simultaneously and there are two types of macropores in the materials (shown in Fig. 3(f)). Fig. 4 shows the morphologies of materials prepared with different PEG concentrations (weight percentage of the 5.298 g of mixture). When the PEG concentration was low, such as 4%, the silica skeleton stacked densely, because there were too few PEG chains in the mixture to form a network which could support the silica skeleton. There were also few macropores in the materials. With the increase of PEG concentration, the silica skeleton turned into a loose network and the macropores appeared because of the same reason.
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Fig. 4. The effect of PEG concentration on the skeleton. (a) 4% PEG6000 (0.21 g), (b) 7% PEG6000 (0.37 g), (c) 10% PEG6000 (0.53 g).
3.3. Influence of TEOS concentration The TEOS concentration impacts on the structure of the silica skeleton. In Fig. 5, the materials were prepared using the PEG8000 system with different TEOS quantities (1.60 g, 2.29 g, 3.00 g). In Fig. 5(a), the skeleton is composed of silica particles and these particles can be distinguished obviously. Since the silica skeleton was from the hydrolysis and polycondensation of TEOS, with the amount of TEOS increasing, the size of skeleton increased and the skeleton became stronger. As a result, the skeleton size climbed from 0.7 lm to 1.7 m when the TEOS amount was changed from 2.29 g to 3.0 g. When the concentration of TEOS was high enough, such as that of Fig. 5(c), the silica particle could not be distinguished obviously. 3.4. Influence of heat treatment After drying, heat treatment was carried out to remove the organic templates and obtain porous materials. The slowly programmed calcination is a widely used method, since slow calefaction can reduce the capillary pressure gradient, which causes the mechanical damage of the materials. The capillary pressure gradient occurs due to the formation of the liquid–gas interface in the pores [14]. Therefore, avoiding the formation of the liquid–gas interface will be effective to reduce the mechanical damage, such as cracking and shrinkage. According to the previous research of our laboratory [15], the subcritical water oxidation method is useful to remove the templates in the mesoporous silica materials preparation and to shape the
material morphology. Thus, the subcritical water oxidation method is used to remove templates in this work. Fig. 6 shows the contrast between the materials after two different heat treatment methods: the subcritical water oxidation treatment at 300 °C for 1 h or 200 °C for 0.5 h under a pressure of 16 MPa and the calcination at 300 °C for 12 h under atmospheric pressure. The subcritical water oxidation is superior to calcination in shape keeping and efficiency. In the subcritical water oxidation, the oxidant was the peroxide solution. The solvent in the materials dissolves in the liquid phase. The organic templates are also decomposed, partly oxidated and dissolved. Therefore, in the removal of templates, the liquid–gas interface is avoided. The silica skeleton as well as the interconnected macropores of the materials are well kept. On the other hand, oxygen in air is the oxidant in the calcination treatment. All the organic templates are totally oxidated into water (vapor) and carbon dioxide. The liquid–gas interface cannot be prevented, which results in the cracking of the materials. Therefore, the method of subcritical water oxidation is a high promising technique in the templating route to the porous materials preparation. The condition of the subcritical water oxidation impacts on the treatment result. The process of removing the templates includes an oxidation reaction. Therefore, the oxidability of the oxidant and the reaction time will determine the effect of the template removal. Enhancing the reaction temperature and prolonging the reaction time will avail the oxidation reaction. Fig. 7(a) indicates that the template can be well removed after subcritical water oxidation at 200 °C for 0.5 h in the PEG2000 system. However, when the same
Fig. 5. SEM photographs of monoliths prepared with different TEOS amounts. (a) 1.60 g of TEOS, (b) 2.29 g of TEOS, (c) 3.00 g of TEOS.
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Fig. 6. SEM photographs of the monoliths with different heat treatment methods P123:HCl:EtOH:TEOS:PEG = 1.00:1.58:0.428:2.29:0.53. (a) PEG2000 subcritical water oxidation 200 °C, 16 MPa, 0.5 h, (b) PEG6000 subcritical water oxidation 300 °C, 16 MPa, 1 h, (c) PEG8000 subcritical water oxidation 300 °C, 16 MPa, 1 h, (d) PEG2000 calcination 300 °C atmospheric pressure 12 h, (e) PEG6000 calcination 300 °C atmospheric pressure 12 h, (f) PEG8000 calcination 300 °C atmospheric pressure 12 h.
Fig. 7. SEM photographs of the silica materials after heat treatment. (a) PEG2000 subcritical water oxidation 200 °C, 0.5 h, (b) PEG8000 subcritical water oxidation 200 °C, 0.5 h, (c) PEG8000 subcritical water oxidation 300 °C, 1 h.
condition was applied into the PEG8000 case, the templates would not be removed effectively. The reason is that the chain length increases and PEG8000 is more difficult to decompose than PEG2000. To remove the template of PEG8000, the temperature should be higher than 300 °C and the reaction time should be longer than 1 h. The result is shown in Fig. 7(c). 4. Conclusions A new PEG and P123 dual-templating system was proved to be feasible to synthesize silica materials with bimodal pore structure. PEG and P123 were used as the macropore and mesopore templates, respectively. After removing the organic portion, the bimodal pore structure was well formed. The results show that PEG has interaction with P123 and enlarges the size of mesopores. Under different PEG molecular weights, the morphology of the
macropores and skeleton turned out to be of three types because of two possible formation mechanisms. With PEG concentration increasing, the size of the macropores became larger, while the size of the skeleton increased with the TEOS concentration increasing. Through control of those three factors, it is possible to get desirable skeleton and macropores morphology. Moreover, a subcritical water oxidation method could efficiently remove the organic templates and keep the well defined microsturcture from damage. The subcritical water oxidation method is a promising method to remove templates in synthesizing monolithic materials besides calcination and solvent extraction. Acknowledgement We wish to gratefully acknowledge the support of the National Science Foundation of China for this work.
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References [1] B.T. Holland, L. Abrams, A. Stein, J. Am. Chem. Soc. 121 (1999) 4308. [2] T. Sen, G.J.T. Tiddy, J.L. Casci, M.W. Anderson, Chem. Mater. 16 (2004) 2044. [3] N. Ishizuka, H. Minakuchi, K. Nakanishi, N. Soga, N. Tanaka, J. Chromatogr. A 797 (1998) 133. [4] N. Ishizuka, H. Minakuchi, K. Nakanishi, K. Hirao, N. Tanaka, Colloids Surf. A 187–188 (2001) 273. [5] M. Motokawa, H. Kobayashi, N. Ishizuka, H. Minakuchi, K. Nakanishi, H. Jinnai, K. Hosoya, T. Ikegami, N. Tanaka, J. Chromatogr. A 961 (2002) 53. [6] N. Tanaka, H. Kobayashi, N. Ishizuka, H. Minakuchi, K. Nakanishi, K. Hosoya, T. Ikegami, J. Chromatogr. A 965 (2002) 35.
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[7] N. Huesing, C. Raab, V. Torma, A. Roig, H. Peterlik, Chem. Mater. 15 (2003) 2690. [8] J.H. Sun, Z.P. Shan, T. Maschmeyer, M.O. Coppens, Langmuir 19 (2003) 8395. [9] M. Groenewolt, M. Antoneitti, S. Ploarz, Langmuir 20 (2004) 7811. [10] T. Amatani, K. Nakanishi, K. Hirao, T. Kodaira, Chem. Mater. 17 (2005) 2114. [11] J. Smatt, S. Schunk, M. Linden, Chem. Mater. 15 (2003) 2354. [12] Galo J. de A.A. Soler-Illia, E.L. Crepaldi, D. Grosso, C. Sanchez, Curr. Opin. Colloid Interf. Sci. 8 (2003) 109. [13] W. Guo, G.S. Luo, Y.J. Wang, J. Colloid Interf. Sci. 271 (2004) 400. [14] A.-M. Siouffi, J. Chromatogr. A 1000 (2003) 801. [15] W. Guo, Master thesis, Tsinghua University, Beijing, 2004, pp. 45– 47.
Triblock copolymer and poly(ethylene glycol) as templates for monolithic silica material with bimodal pore structure Y.W. Sun, Y.J. Wang, W. Guo, T. Wang, G.S. Luo
*
The State Key Lab of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Received 13 June 2005; received in revised form 24 August 2005; accepted 24 August 2005 Available online 4 October 2005
Abstract To synthesize monolithic silica materials with bimodal pore structures, a new dual-templating system of triblock copolymer and poly(ethylene glycol) was proposed and a subcritical water oxidation method was introduced to remove the organic templates. In this new system, triblock copolymer Pluronic P123 and poly(ethylene glycol) (PEG) mainly served as the templates for mesopores and macropores, respectively. The influence of PEG molecular weight, PEG concentration and TEOS concentration on the bimodal pore structure was investigated. The macro–mesoporous structure of synthesized monolithic silica materials with tetraethoxysilane (TEOS) as the silica source has been confirmed by a nitrogen adsorption–desorption measurement and SEM observations. The monolithic silica materials with macro–mesoporous structure were synthesized successfully using the new template system. The macropores size ranged from 0.6 lm to 4 lm and the silica skeleton size was from 0.5 lm to 1.5 lm. The mesopore size was decided by the composition of P123/PEG. PEG could enlarge the mesopore size from 3–4 nm (only P123 applied) to 8–10 nm. Using PEG with different molecular weights as template, two different macropore morphologies were observed and two possible mechanisms of macropores formation were hypothesized, named as the micelles mechanism and the network mechanism, respectively. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Silica material; Bimodal pore structure; Dual templates; Subcritical water oxidation method
1. Introduction Recently, silica materials with hierarchical pore structure have attracted much interest, for example, materials containing small (micro- or mesopores) and large (macropores) pores, since they combine the benefits of each pore size regime. The micro- or mesopores possess size- or shape-selectivity and high surface area. Usually, the surface area of mesoporous material is about 102–103 m2/g. The macropores provide easier access to the active sites and reduce the pressure drop over the material [1,2]. Therefore, materials with such structure have a profound promise in catalysis and separation processes, such as chromatogra-
*
Corresponding author. Tel.: +86 1062783870; fax: +86 1062770304. E-mail address: [email protected] (G.S. Luo).
1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.08.018
phy, separation for large molecules, drug release, etc. The advantages of producing hierarchical pore structure within one material are evident. There are two widely used approaches to produce the bimodal pore structure. One is based on phase separation strategies in combination with sol–gel process. Tanaka and coworkers [3–6] utilized the hydrolysis and polycondensation of tetramethoxysilane (TMOS) in the presence of PEG to produce the interconnected macropores and silica skeleton structure. Through the solvent-exchange in ammonia, mesopores were tailored on the silica skeleton. The macropore diameter can be controlled by adjusting the PEG concentration, which affects the relative rate of phase separation to the sol–gel transition. The mesopore size can be controlled independently of the macropore size through post-synthesis treatment in ammonia solution. These macroporous/mesoporous materials have
32
Y.W. Sun et al. / Microporous and Mesoporous Materials 88 (2006) 31–37
already been applied in high performance liquid chromatography. However, in this approach, no or only weak regular arrangements of the mesopores are achieved with a rather bulky network structure in the micrometer range [7]. The other approach is to use dual or multiple templates in different ranges to produce hierarchical pore structure. For the mesopores, the supramolecular arrays of amphiphilic block copolymers or other surfactants are commonly used as templates, through which the materials exhibit a narrow pore size distribution. The templates producing the macropores are various. Holland et al. [1] used arrays of monodiperse polystyrene spheres as templates for macropores and tetrapropylammonium hydroxide for mesopores. Sen et al. [2] synthesized silica materials with macro–meso–microporous structure by using polystyrene spheres, F127 and P123 as templates, respectively. Sun et al. [8] presented a two-step route to produce bimodal pore structure. Firstly, MCM-41 nanoparticles were prepared, using CTAB as mesopore templates. Then the nanoparticles were cross-linked around assemblies of triblock copolymer to produce macroporous structure. Groenewolt et al. [9] prepared silica monoliths with bimodal pore structure by nanocasting mixtures of fluorinated nonionic surfactants and micelles of two hydrocarbon block copolymers. Micelles with different sizes formed by different surfactants acted as the templates for hierarchical pore structure. Amatani et al. [10] synthesized monolithic silica gel with macro–mesoporous structure via using triblock copolymer P123 and 1,3,5-trimethylbenzene as structuredirecting agent and micelle-swelling agent. Smatt et al. [11] designed a dual-templating route to synthesize silica monoliths. PEG and CTAB were utilized as templates for macropores and mesopores, respectively. The materials exhibited bimodal pore structure and arranged mesopores on the skeleton. Compared with ionic surfactants, triblock copolymers have more advantages, because they are capable to impart larger pores and thicker walls, besides being industrially available, hazard-free and easy to remove from the mineral framework (by thermal treatment or solvent extraction) [12]. According to the earlier research in our laboratory, the block copolymer is also superior to the ionic surfactant in hydrothermal stability and in easy adjustment of the pore size [13]. Can block copolymer together with PEG serve as template for meso- and macropores? What is the possible mechanism for the templates to produce pores in the silica framework? These questions aroused our interest in exploring the new dual-templating route. In this work, we explored this new dual-templating system, PEG together with block copolymer to prepare silica materials with bimodal pore structure. The relationship between the morphology and the preparation condition was studied in detail. Moreover, a subcritical water oxidation method was introduced to remove the templates in our preparation route.
2. Experiments 2.1. Chemicals Tetraethoxysilane (TEOS, Fluka, 99%), triblock copolymer Pluronic P123 (EO20PO70EO20, BASF analytical purity), HCl (Being Chemical Plant, 36 wt.%), poly(ethylene glycol) (PEG, Beijing Chemical Plant, analytical purity), ethanol (Beijing Chemical Plant, analytical purity), H2O2 (Beijing chemical company, 30 wt.%) were used as received and doubly de-ionized distilled water was used throughout. 2.2. Materials synthesis procedure Silica materials with bimodal pore structure were prepared as follows. First, 2.29 g of TEOS was mixed with 1.58 g of HCl solution (pH = 2) in the presence of 1.00 g of P123 (template for the mesopores) and 0.428 g of ethanol. The mixture was stirred vigorously for 30 min at room temperature to form a homogeneous sol. The sol was stirred for another 4 days, which led to partial hydrolysis and polycondensation of TEOS. Then, 0.53 g of PEG was added to the transparent sol and the mixture was stirred until PEG dissolved. The resultant mixture was charged into a capillary tube which had been treated with 1 M NaOH mixture for 1 h in advance. The mixture reacted at room temperature and gel formed. After aging under the same conditions for 5–10 days, the capillary with the gel was transferred to an autoclave and immersed in the 5% H2O2 solution. The solution in the autoclave was heated up to 300 °C gradually under a pressure of 16 MPa to reach a subcritical state. The state was maintained for 1 h to remove the templates. Quantity changes, if any, will be indicated in Section 3. 2.3. Characteristics of the prepared materials A scanning electron microscopy (S-450, Hitachi, Japan) was employed to observe the morphology of the prepared material. Characterization of the mesopore structure was performed by BET (ASAP 2010, Micromeritics). 3. Results and discussion 3.1. Bimodal pores structure The major objective of this research is to study the PEG and P123 dual-templating route to produce silica materials with bimodal pore structure. The SEM photographs in Fig. 1 show the cross-section of the prepared materials with PEG2000 and PEG6000. These results indicate that the interconnected macroporous morphology was formed very well after removing the templates. The macropores and the silica skeleton were uniform. The size of the skeleton and the macropore was around 1 lm. The morphology of the skeleton changed from aggregates of silica particles to a
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Fig. 1. SEM photographs of the macroporous morphology. (a) PEG2000 (aggregates of particles), (b) PEG6000 (smooth network).
smooth network, when PEG molecular weight increased from 2000 to 6000. Fig. 2(a) shows the BET results of the prepared materials. The materials exhibit well-defined mesopores in the range of 8–10 nm. When skipping the PEG adding step, contrasting results are shown in Fig. 2(b). The mesopores size decreased to 3–4 nm and the distribution narrowed. PEG was used as the macropores template in this new dual-templating system, but PEG affected the formation of the mesopore structure by changing the micelle size of
P123. The addition of PEG enlarged the mesopore size and widened the mesopore size distribution of the material. 3.2. Influence of PEG molecular weight and PEG concentration Further research results revealed that both the molecular weight and the concentration of PEG have great effects on the macroporous morphology. Fig. 3 shows the morphologies of the materials, which were prepared with
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Fig. 3. SEM photographs of monoliths under different PEG molecular weights. (a) PEG400, (b) PEG2000, (c) PEG6000, (d) PEG8000, (e) PEG10000, (f) PEG12000.
0.53 g of PEG with different molecular weights, respectively, ranging from 400 to 12,000. When the molecular weight was above 2000, the materials exhibited an obvious interconnected macropores structure. The size ratio of the macropores/the silica skeleton ranged from 1 to 5, while the void/particle size ratio ranged from 0.25 to 0.4 in a conventional packed column of 5 lm particles. This suggests the prepared materials will provide lower pressure drop and higher permeability than a conventional packed column. The macroporous and skeleton morphologies shown in Fig. 3 can be divided into three types. The first type can be named as the aggregate type which was formed by the application of PEG2000. The aggregates of silica particles could be obviously distinguished from the silica skeleton. The macropore size is decided by the voids among the aggregates. The morphologies with the application of PEG6000 and PEG8000 belong to the second type, named as the network type. The skeleton morphology turned out to be a smooth network. The macropores depended on the voids of the network. Both the silica skeleton size and the macropores size are smaller than those of the aggregate type. The third type, named as the mixed type, was formed when PEG10000 and PEG12000 were applied. In this type, the skeleton morphology was a network with some larger pores on it. We hypothesize that there are two corresponding mechanisms of the macropores formation. According to sol–gel theory, PEG chains in the sol form a network structure. Si–OH groups of the silicic acid combine with EO groups on PEG and polycondensation of Si–OH occurs to form silica skeleton. With the hydrolysis and polycondensation of the silicane, the system undergoes a spinodal decomposition and two continuous phases are formed. One is silica
rich phase which will be the silica skeleton at the end. The other is solvent rich phase which will become the interconnected macropores in the materials after drying. We name this mechanism as the network mechanism. The macropores of the network type form according to this mechanism. The second mechanism named as the micelles mechanism explains the formation of the macropores in the aggregate type and the larger pores in the mixed type. PEG chains form micelles in the sol, around which silica particles aggregate. After the condensation of silicane and removal of PEG, the macropores are formed according to PEG micelles template. Therefore, the size of macropores formed according to the micelles mechanism is much larger than that of the network mechanism. The former one is about 0.6 lm while the latter one is about 3–4 lm. In the situation of PEG2000, the PEG chains mainly form micelles in the sol so that the macropores mainly belong to the aggregate type. When the length of PEG chains increases, PEG chains will tend to a network structure. The macropores shown in Fig. 3(c) and (d) are mainly produced by the network mechanism. When PEG12000 is applied, these two mechanisms exist simultaneously and there are two types of macropores in the materials (shown in Fig. 3(f)). Fig. 4 shows the morphologies of materials prepared with different PEG concentrations (weight percentage of the 5.298 g of mixture). When the PEG concentration was low, such as 4%, the silica skeleton stacked densely, because there were too few PEG chains in the mixture to form a network which could support the silica skeleton. There were also few macropores in the materials. With the increase of PEG concentration, the silica skeleton turned into a loose network and the macropores appeared because of the same reason.
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Fig. 4. The effect of PEG concentration on the skeleton. (a) 4% PEG6000 (0.21 g), (b) 7% PEG6000 (0.37 g), (c) 10% PEG6000 (0.53 g).
3.3. Influence of TEOS concentration The TEOS concentration impacts on the structure of the silica skeleton. In Fig. 5, the materials were prepared using the PEG8000 system with different TEOS quantities (1.60 g, 2.29 g, 3.00 g). In Fig. 5(a), the skeleton is composed of silica particles and these particles can be distinguished obviously. Since the silica skeleton was from the hydrolysis and polycondensation of TEOS, with the amount of TEOS increasing, the size of skeleton increased and the skeleton became stronger. As a result, the skeleton size climbed from 0.7 lm to 1.7 m when the TEOS amount was changed from 2.29 g to 3.0 g. When the concentration of TEOS was high enough, such as that of Fig. 5(c), the silica particle could not be distinguished obviously. 3.4. Influence of heat treatment After drying, heat treatment was carried out to remove the organic templates and obtain porous materials. The slowly programmed calcination is a widely used method, since slow calefaction can reduce the capillary pressure gradient, which causes the mechanical damage of the materials. The capillary pressure gradient occurs due to the formation of the liquid–gas interface in the pores [14]. Therefore, avoiding the formation of the liquid–gas interface will be effective to reduce the mechanical damage, such as cracking and shrinkage. According to the previous research of our laboratory [15], the subcritical water oxidation method is useful to remove the templates in the mesoporous silica materials preparation and to shape the
material morphology. Thus, the subcritical water oxidation method is used to remove templates in this work. Fig. 6 shows the contrast between the materials after two different heat treatment methods: the subcritical water oxidation treatment at 300 °C for 1 h or 200 °C for 0.5 h under a pressure of 16 MPa and the calcination at 300 °C for 12 h under atmospheric pressure. The subcritical water oxidation is superior to calcination in shape keeping and efficiency. In the subcritical water oxidation, the oxidant was the peroxide solution. The solvent in the materials dissolves in the liquid phase. The organic templates are also decomposed, partly oxidated and dissolved. Therefore, in the removal of templates, the liquid–gas interface is avoided. The silica skeleton as well as the interconnected macropores of the materials are well kept. On the other hand, oxygen in air is the oxidant in the calcination treatment. All the organic templates are totally oxidated into water (vapor) and carbon dioxide. The liquid–gas interface cannot be prevented, which results in the cracking of the materials. Therefore, the method of subcritical water oxidation is a high promising technique in the templating route to the porous materials preparation. The condition of the subcritical water oxidation impacts on the treatment result. The process of removing the templates includes an oxidation reaction. Therefore, the oxidability of the oxidant and the reaction time will determine the effect of the template removal. Enhancing the reaction temperature and prolonging the reaction time will avail the oxidation reaction. Fig. 7(a) indicates that the template can be well removed after subcritical water oxidation at 200 °C for 0.5 h in the PEG2000 system. However, when the same
Fig. 5. SEM photographs of monoliths prepared with different TEOS amounts. (a) 1.60 g of TEOS, (b) 2.29 g of TEOS, (c) 3.00 g of TEOS.
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Fig. 6. SEM photographs of the monoliths with different heat treatment methods P123:HCl:EtOH:TEOS:PEG = 1.00:1.58:0.428:2.29:0.53. (a) PEG2000 subcritical water oxidation 200 °C, 16 MPa, 0.5 h, (b) PEG6000 subcritical water oxidation 300 °C, 16 MPa, 1 h, (c) PEG8000 subcritical water oxidation 300 °C, 16 MPa, 1 h, (d) PEG2000 calcination 300 °C atmospheric pressure 12 h, (e) PEG6000 calcination 300 °C atmospheric pressure 12 h, (f) PEG8000 calcination 300 °C atmospheric pressure 12 h.
Fig. 7. SEM photographs of the silica materials after heat treatment. (a) PEG2000 subcritical water oxidation 200 °C, 0.5 h, (b) PEG8000 subcritical water oxidation 200 °C, 0.5 h, (c) PEG8000 subcritical water oxidation 300 °C, 1 h.
condition was applied into the PEG8000 case, the templates would not be removed effectively. The reason is that the chain length increases and PEG8000 is more difficult to decompose than PEG2000. To remove the template of PEG8000, the temperature should be higher than 300 °C and the reaction time should be longer than 1 h. The result is shown in Fig. 7(c). 4. Conclusions A new PEG and P123 dual-templating system was proved to be feasible to synthesize silica materials with bimodal pore structure. PEG and P123 were used as the macropore and mesopore templates, respectively. After removing the organic portion, the bimodal pore structure was well formed. The results show that PEG has interaction with P123 and enlarges the size of mesopores. Under different PEG molecular weights, the morphology of the
macropores and skeleton turned out to be of three types because of two possible formation mechanisms. With PEG concentration increasing, the size of the macropores became larger, while the size of the skeleton increased with the TEOS concentration increasing. Through control of those three factors, it is possible to get desirable skeleton and macropores morphology. Moreover, a subcritical water oxidation method could efficiently remove the organic templates and keep the well defined microsturcture from damage. The subcritical water oxidation method is a promising method to remove templates in synthesizing monolithic materials besides calcination and solvent extraction. Acknowledgement We wish to gratefully acknowledge the support of the National Science Foundation of China for this work.
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