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Post-synthesis pore expansion of mesoporous silica SBA-15 in the organic template removal via solvothermal treatment
Sci. Bull. (2015) 60(11):1019–1025 DOI 10.1007/s11434-015-0793-0
www.scibull.com www.springer.com/scp
Article
Materials Science
Post-synthesis pore expansion of mesoporous silica SBA-15 in the organic template removal via solvothermal treatment Nan Li • Yongsheng Li • Shengjue Lu • Xingdi Zhang • Jianzhuang Chen • Dechao Niu Jinlou Gu • Jianlin Shi
•
Received: 8 March 2015 / Accepted: 10 April 2015 / Published online: 21 May 2015 Ó Science China Press and Springer-Verlag Berlin Heidelberg 2015
Abstract The synthesis of mesoporous material SBA-15 has been extensively reported in the past decades, which possesses a pore diameter of 6–8 nm on average. Here, a simple post-synthesis procedure has been developed to synthesize SBA-15 with further expanded pore diameter to above 10 nm simply by a solvothermal treatment replacing traditional hydrothermal step for mesopore template removal, which results in efficient pore expansion and the significantly promoted condensation of silica framework as well. This facile approach is believed applicable for pore expansions of other kinds of mesoporous silica materials. Keywords Solvothermal treatment Template removal Pore size expanding Framework condensation SBA-15
1 Introduction Following the discovery of ordered mesoporous materials, more and more researchers have focused on the
N. Li Y. Li (&) S. Lu X. Zhang J. Chen D. Niu J. Gu J. Shi (&) Laboratory of Low-Dimensional Materials Chemistry, Key Laboratory for Ultrafine Materials of Ministry of Education, East China University of Science and Technology, Shanghai 200237, China e-mail: [email protected] J. Shi e-mail: [email protected] J. Shi State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China
optimization of pore properties, which would provide great opportunities in the field of catalysis, separation and drug delivery [1–3]. In 1998, Zhao et al. [4] reported an important kind of mesoporous materials SBA-15 using Pluronic P123 (EO20PO70EO20) as the mesopore template under acid conditions for the first time. This kind of mesoporous silica is featured with much larger pore size of 6–8 nm on average than that of M41S family (2–3 nm). By changing the aging temperature or acid concentration of synthesis process, the physicochemical properties of SBA15 could also be well tuned [5, 6]. Nevertheless, the mesopore size of 6–8 nm was still not large enough, which greatly hindered their applications, especially in the biomedicine for the encapsulation of biomacromolecules, such as proteins or DNA with high molecular weight. For synthesizing mesoporous silica with relatively large mesopores, many efforts had been made, and pore expansion is one of the most frequently used approaches. For example, choosing other kind of surfactants with larger sizes [4, 7] or introducing a swelling agent into the template [8, 9] has been the common practices on the enlargement of template micelles, which could result in the pore size expanding. Unfortunately, the correspondingly resultant mesostructure ordering, as well as the thermal stability, would be affected significantly. In addition, hydrothermal and alcothermal treatment have also been used to synthesize large-pore mesoporous materials [10, 11]. On the other hand, there have two general methods for organic template removal: calcination and solvent extraction [7, 12–14]. Besides, special approaches for template removal were also developed, such as microwave digestion, UV/ozone treatment, supercritical fluid extraction and liquid-phase calcination [15–18]. However, these methods suffer from the drawbacks of complexity and/or the need of necessary special equipment.
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In spite of all the above-mentioned efforts in removing the organic template, the pore diameter usually suffers from significant shrinkage by calcination or mostly remains unchanged during extraction. In this paper, we present the simultaneous pore expansion and complete template removal by solvothermal treatment of SBA-15 in a mixed solution for the first time. Importantly, it is different from the pore expansion by using large or enlarged micelles in most literatures that mesopores are effectively expanded beyond 10 nm in the post-synthesis template removal process, which benefits the further framework condensation, mesostructure stability enhancement and the retaining of surface Si–OH groups.
2 Experimental 2.1 Direct synthesis of pore-expanded SBA-15 with template removal The preparation process of pore-expanded SBA-15 was similar to that of the conventional SBA-15 [4, 19], and the difference was that the hydrothermal treating step was replaced with a solvothermal treating process and that no calcination step was involved. The process was described as following. In a typical synthesis, 4.0 g triblock copolymer Pluronic P123 was dissolved in a mixture of 30 g water and 120 g of 2 mol/L HCl in a Teflon-lined autoclave, and the mixture was stirred at 35 °C overnight. Then, 8.50 g tetraethyl orthosilicate (TEOS) was added into the solution under vigorous stirring. After 5 min of stirring, the mixture was kept under static conditions at 35 °C for 20 h and the precursor for SBA-15 was obtained. The precursor for SBA-15 was filtered, then washed with water and dried for further solvothermal treating, which was named as SBA-15-pre-as. In the meantime, the organic solvent was prepared by mixing 0.22 g sodium sulfate, 12 mL deionized water, 25 mL glycol, 7 mL n-butyl alcohol and 0.83 g polyethylene glycol (PEG, molecular weight 6,000) in a 50-mL Teflon-lined autoclave. Finally, 0.4 g SBA-15-pre-as was immersed into the mixed solution and stirred well. After 24 h of heating at different temperatures, the samples were collected by filtration, then washed with water and ethanol, respectively, and dried. The samples were named as SBA-15-st-T, respectively, where T represents the related temperature. For comparison, the SBA-15-pre-as was directly calcined at 550 °C in flowing air for 6 h at a heating rate of 1.5 °C/min, and the obtained material was named as SBA15-pre-cal. Moreover, samples prepared via conventional synthetic process of hydrothermal treatment (100 °C for
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24 h) and calcination (550 °C for 6 h) were named as SBA-15. 2.2 Characterization Fourier translation infrared spectroscopy (FT-IR) was conducted on a Nicolet 5700 Thermo FT-IR spectrometer (USA) using the KBr wafer technique. Thermogravimetric analysis (TGA) curves were obtained on a thermogravimetry–differential thermal analysis (TG–DTA) instrument (Netzcsh STA 409 PC, Germany) at a heating rate of 10 °C/min under nitrogen atmosphere from room temperature to 550 °C. Powder X-ray diffraction (XRD) data were collected on Bruker D8 Focus diffractometer (Germany) equipped with Cu Ka ra˚ ). XRD patterns were collected in the diation (k = 1.5405 A 2h ranges between 0.6° and 6° with a speed of 0.6°/min. N2 adsorption–desorption isotherms were measured at 77 K by using Quantachrome NOVA 4200e (USA). The Brunauer– Emmett–Teller (BET) method was utilized in the calculation of specific surface areas. The pore size distributions were derived from the adsorption branch of the isotherm by means of the Barrett–Joyner–Halenda (BJH) method. Field-emission scanning electron microscopy (FE-SEM) images were obtained by using a JEOL JSM-6700F field SEM (Japan). Transmission electron microscopy (TEM) observations were carried out on a JEOL-2100F electron microscope (Japan). Solid-state 29Si nuclear magnetic resonance (NMR) spectra were performed on a Bruker Avance III 300 spectrometer (Germany), and each spectrum was recorded after 128 acquisitions with 120-s repetition time as a compromise between nuclear relaxation and kinetics of silica polymerization.
3 Results and discussion 3.1 Effects of solvothermal treatment on the mesostructure of SBA-15-st-T Figure 1 shows the XRD patterns of the samples prepared under different conditions. As can be seen, all the samples, except for SBA-15-pre-cal, present three distinctive diffraction peaks in the small-angle region, which can be indexed as (100), (110) and (200), respectively, being attributed to the two-dimensional hexagonal (p6mm) mesostructure. This indicates that SBA-15-st-T samples possess similar mesostructures with the conventional SBA15. Notably, it can be found that the (100) diffraction peaks of SBA-15-st-T samples shifted toward lower diffraction angles, suggesting that the larger cell parameters and mesopores of SBA-15-st-T are obtained. Besides, little difference has been found between the SBA-15-st-T samples,
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demonstrating that solvothermal treating temperature has less effect on the resulted mesostructures. The N2 sorption isotherms and the corresponding pore size distribution curves for different materials are shown in Fig. 2. All the samples present type IV isotherms with H1 hysteresis loops, which are typical of ordered mesoporous materials with narrow pore size distributions. It is shown that the isotherms of the SBA-15-st-T samples are almost identical, possessing more distinct hysteresis loops than that of the SBA-15-pre-cal, and the step of capillary condensation in mesopores shifting to higher relative pressure. Thus, narrow pore size distributions and significant increment in mesopore sizes for SBA-15-st-T samples are achieved. Moreover, an additional type of mesopores of about 17 nm in diameter is observed for SBA-15-st-120, which can be ascribed to the secondary pores within the wall during the post-treatment.
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Table 1 summarizes the texture properties of the samples. Compared with SBA-15-pre-cal, the pore size and pore volume of SBA-15-st-T are substantially higher, reaching above 10 nm and 0.82 cm3/g, respectively. However, the specific surface areas of SBA-15-st-T are lower than those of SBA-15 and even SBA-15-pre-cal, which is believed to result from the merge and/or collapse of the some mesopores into larger ones (mesopores of 17 nm are observed) [4, 19–21]. This means that SBA-15 samples with rather larger mesopores could be facilely synthesized by adopting solvothermal treatment instead of traditional hydrothermal treatment. Typical SEM images of SBA-15-pre-cal and SBA-15-stT are shown in Fig. 3a–d. It can be seen clearly that all the samples present regular rod-like morphology. More interestingly, well-ordered mesopore channels are exhibited. It is worth noting that a certain amount of cavities can be found on the sample of SBA-15-st-120, corresponding to the results of pore size distribution curves presented in Fig. 2. This is probably due to the enlargement of the secondary pores within the pore wall, leading to mesopore collapse to some extent. High-resolution TEM (HR-TEM) images, as shown in Fig. 3e–h, reveal that all the samples of SBA-15-st-T are in highly ordered hexagonal arrangement. More importantly, an enlargement of mesopore channels, in comparison with SBA-15-pre-cal, is observed, which is in correspondence with the above results of XRD and N2 sorption analysis. In addition, obvious collapse of mesopore channels can also be found in the image of SBA15-st-120. 3.2 Exciting phenomena in the process of solvothermal treatment
Fig. 1 (Color online) Small-angle XRD patterns of different samples
Figure 4a shows the FT-IR spectra of samples prepared with different procedures. Due to the existence of organic
Fig. 2 (Color online) N2 adsorption isotherms (a) and the corresponding pore size distribution curves (b) of different samples, where V is the pore volume and D is the pore diameter
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Table 1 Texture properties of different samples Pore diameterb Dp (nm)
Samples
da(100) (nm)
Specific surface area (m2/g)
Pore volume (cm3/g)
SBA-15-pre-cal
7.38
430
0.47
5.3
3.22
SBA-15-st-100
9.24
353
0.84
10.2
0.47
SBA-15-st-110
9.20
358
0.87
10.1
0.52
SBA-15-st-120
9.22
324
0.82
10.2
SBA-15
8.69
578
0.97
a
7.80
Pore wall thicknessc Tw (nm)
0.45 2.23
d(100) was obtained from the small-angle XRD patterns using Bragg’s equation
b
Dp was calculated from the adsorption branches of the N2 sorption isotherms
c
Tw was calculated by using the equation, Tw = d(100)(2/31/2) -Dp
Fig. 3 SEM and TEM images for the surface and inner microstructures of different samples. a, e SBA-15-pre-cal; b, f SBA-15-st-100; c, g SBA15-st-110; d, h SBA-15-st-120
template, SBA-15-pre-as shows several absorption bands at around 2,860–2,976 and 1,350–1,460 cm-1, which are characteristic peaks of the stretching and bending vibrations of C–H bonds. However, the characteristic peaks of the corresponding C–H bonds in the samples of SBA-15-stT cannot be detected, which are similar to that of SBA-15pre-cal, indicating the complete removal of organic template from the mesopores during the solvothermal treatment process, which is equivalent to the high-temperature calcination step in removing the template molecules. The efficiency of template removal was further evaluated via TGA. As shown in Fig. 4b, a steep weight loss of about 55 % between 450 and 823 K for SBA-15-pre-as is present, which can be mainly attributed to the decomposition of organic template, and the loss of a small amount of structural water from the silica framework due to the condensation of
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silanol groups. It is known that the weight changes before 473 K are mainly attributed to the loss of the physical water adsorbed in the pore channels [5, 22]. In contrast, the samples of SBA-15-st-T exhibit remarkably lower weight losses. In the temperature range of 450–823 K, the weight losses for SBA-15-st-100, SBA-15-st-110 and SBA-15-st-120 are only 8.7 %, 6.5 % and 5.2 %, respectively, which can be mostly attributed to the loss of chemical water. These results clearly demonstrate that the organic template has been removed efficiently, and suggest that higher temperature is beneficial to the removal of organic template. Figure 4c shows the 29Si NMR spectra of different samples. Various Qn groups (Qn = Si(OSi)n(OH)4-n, n = 2–4, where 4 represents the fully condensed silica) are marked on the spectra. As to SBA-15-st-110, the Q4 peak at -111 ppm is greatly enhanced, and the Q2 and Q3
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Fig. 4 Effect of solvothermal treatment on template removal and silica condensation of SBA-15-st-T tested by FT-IR spectra (a), TGA curves (b) and 29Si NMR spectra (c)
peaks, which are in the position of -91 and -101 ppm, respectively, are correspondingly weakened in comparison with SBA-15-pre-as, leading to remarkably decreased Q3/ Q4 ratio, which clearly suggesting that the solvothermal treatment strongly facilitates the condensation reaction of the silanol groups in silica framework. As for the SBA-15, the process of calcination at 550 °C can lead to an even higher degree of condensation. However, this kind of sample has lost most silanol groups, unfavorable for further surface modification and its functions in actual application. 3.3 The proposed mechanism for SBA-15-st via solvothermal treatment As illustrated in Scheme 1, the possible mechanism for changes in structure and the organic template removal in the process of solvothermal treatment is postulated according to the
arguments in the literatures [4, 11, 23–28]. The process can be divided into two stages. In the initial stage, as the degree of condensation of the silica network in SBA-15-pre-as is relatively low, there is great potential for further condensation. It is known that polyethylene oxide (PEO) blocks become less hydrophilic at high temperature; however, polypropylene oxide (PPO) blocks turn to be more hydrophobic. PEO blocks shrink into the hydrophobic region of micelles; meanwhile, the long-chain alcohol penetrates into the hydrophobic region under high pressure and the elevated temperatures, which would render the hydrophobic region swell. On the other hand, the sodium ion (Na?) might replace some of the hydrogenions (H?) on the surface of pore channels, promoting hydrolysis and recondensation of siloxane bonds to a certain degree. Accompanying the gradual expansion of organic template micelles in original mesoporous channels, the resultant pore size and pore volume are enlarged (as shown in Scheme 1). On the
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Scheme 1 The proposed mechanism for SBA-15-st via solvothermal treatment. S.T.: solvothermal treatment, T.H.: hydrothermal treatment
second stage, the enlarged primary pore channels of mesoporous silica as well as the secondary pore within the walls would lead to the improved network connectivity and weakened influence of pore walls on the diffusion of solvent molecules, thus providing a favorable environment for template dissociation. As higher temperature leads to further enlargement of secondary pore sizes, better effect of solvothermal treatment on template removal has been observed for SBA-15-st-120. In addition, the adding of sodium sulfate will attenuate the hydrogen bonding between organic template and silica network and make the layer of alcohol easy to be formed at the organic–inorganic interface, which is beneficial to the removal of organic template. Nevertheless, in spite of our efforts in understanding the processes during the solvothermal treatments, the process of solvothermal treatment is very complicated and further investigation is necessary.
and completely removed during the solvothermal treatment process. More importantly, the pore expansion and a further significant silica condensation are concurrently achieved. The present approach of mesopore template removal is much more advantageous than the traditional high-temperature calcination or hydrothermal extraction in pore expansion, Si–OH group retaining and framework condensation.
4 Conclusions
References
In summary, a facile method of synthesizing SBA-15 with much enlarged mesopores has been developed based on the solvothermal treatment of the precursor of SBA-15. Surfactant molecules in the mesopores could be effectively
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Acknowledgments This work was supported by the National Basic Research Program of China (2012CB933602), the National Natural Science Foundation of China (51172070, 51132009, 51202068 and 51472085), Shu Guang Project (11SG30) and the Fundamental Research Funds for the Central Universities. Conflict of interest of interest.
The authors declare that they have no conflict
1. Kresge CT, Leonowicz ME, Roth WJ et al (1992) Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism. Nature 359:710–712 2. Hudson S, Cooney J, Magner E (2008) Proteins in mesoporous silicates. Angew Chem Int Ed 47:8549–8582
Sci. Bull. (2015) 60(11):1019–1025 3. Park C, Oh K, Lee SC et al (2007) Controlled release of guest molecules from mesoporous silica particles based on a pH-responsive polypseudorotaxane motif. Angew Chem Int Ed 46:1455–1457 4. Zhao DY, Feng JL, Huo QS et al (1998) Triblock copolymer syntheses of mesoporous silica with periodic 50 to 300 angstrom pores. Science 279:548–552 5. Kruk M, Jaroniec M, Ko CH et al (2000) Characterization of the porous structure of SBA-15. Chem Mater 12:1961–1968 6. Choi M, Heo W, Kleitz F et al (2003) Facile synthesis of high quality mesoporous SBA-15 with enhanced control of the porous network connectivity and wall thickness. Chem Commun 12:1340–1341 7. Zhao DY, Huo QS, Feng JL (1998) Nonionic triblock and star diblock copolymer and oligomeric surfactant syntheses of highly ordered, hydrothermally stable, mesoporous silica structures. J Am Chem Soc 120:6024–6036 8. Sayari A (2000) Unprecedented expansion of the pore size and volume of periodic mesoporous silica. Angew Chem Int Ed 39:2920–2922 9. Fan J, Yu CZ, Wang LM et al (2001) Mesotunnels on the silica wall of ordered SBA-15 to generate three-dimensional large-pore mesoporous networks. J Am Chem Soc 123:12113–12114 10. Mizutani M, Yamada Y, Yano K (2007) Pore-expansion of monodisperse mesoporous silica spheres by a novel surfactant exchange method. Chem Commun 11:1172–1174 11. Sun JH, Moulijn JA, Jansen KC et al (2001) Alcothermal synthesis under basic conditions of an SBA-15 with long-range order and stability. Adv Mater 13:327–331 12. Margolese D, Melero JA, Christiansen SC et al (2000) Direct syntheses of ordered SBA-15 mesoporous silica containing sulfonic acid groups. Chem Mater 12:2448–2459 13. Yang CM, Zibrowuis B, Schmidt W et al (2004) Stepwise removal of the copolymer template from mesopores and micropores in SBA-15. Chem Mater 16:2918–2925 14. Yang CM, Zibrowius B, Schmidt W et al (2003) Consecutive generation of mesopores and micropores in SBA-15. Chem Mater 15:3739–3741 15. Tian BZ, Liu XY, Yu CZ et al (2002) Microwave assisted template removal of siliceous porous materials. Chem Commun 11:1186–1187
1025 16. Xiao LP, Li JY, Jin HX et al (2006) Removal of organic templates from mesoprous SBA-15 at room temperature using UV/ dilute H2O2. Micropor Mesopor Mater 96:413–418 17. Van Grieken R, Calleja G, Stucky GD et al (2003) Supercritical fluid extraction of a nonionic surfactant template from SBA-15 materials and consequences on the porous structure. Langmuir 19:3966–3973 18. Cauda V, Argyo C, Piercey DG et al (2011) ‘‘Liquid-phase calcination’’ of colloidal mesoporous silica nanoparticles in highboiling solvents. J Am Chem Soc 133:6484–6486 19. Sayari A, Han BH, Yang Y (2004) Simple synthesis route to monodispersed SBA-15 silica rods. J Am Chem Soc 126:14348–14349 20. Deng YH, Yu T, Wan Y et al (2007) Ordered mesoporous silicas and carbons with large accessible pores templated from amphiphilic diblock copolymer poly(ethylene oxide)-b-polystyrene. J Am Chem Soc 129:1690–1697 21. Galarneau A, Cambon H, Di Renzo F et al (2003) Microporosity and connections between pores in SBA-15 mesostructured silicas as a function of the temperature of synthesis. New J Chem 27:73–79 22. Yan XX, Deng HX, Huang XH et al (2005) Mesoporous bioactive glasses. I. Synthesis and structural characterization. J NonCryst Solids 351:3209–3217 23. Lai XY, Wang D, Han N et al (2010) Ordered arrays of beadchain-like In2O3 nanorods and their enhanced sensing performance for formaldehyde. Chem Mater 22:3033–3042 24. Lai XY, Shen GX, Xue P et al (2015) Ordered mesoporous NiO with thin pore walls and its enhanced sensing performance for formaldehyde. Nanoscale 7:4005–4012 25. Rumplecker A, Kleitz F, Salabas EL et al (2007) Hard templating pathways for the synthesis of nanostructured porous Co3O4. Chem Mater 19:485–496 26. Soler-Illia GJAA, Crepaldi EL, Grosso D et al (2003) Block copolymer-templated mesoporous oxides. Curr Opin Colloid Interface Sci 8:109–126 27. Sayari A, Shee D, Al-Yassir N et al (2010) Catalysis over poreexpanded MCM-41 mesoporous materials. Top Catal 53:154–167 28. Gallis KW, Landry CC (1997) Synthesis of MCM-48 by a phase transformation process. Chem Mater 9:2035–2038
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Article
Materials Science
Post-synthesis pore expansion of mesoporous silica SBA-15 in the organic template removal via solvothermal treatment Nan Li • Yongsheng Li • Shengjue Lu • Xingdi Zhang • Jianzhuang Chen • Dechao Niu Jinlou Gu • Jianlin Shi
•
Received: 8 March 2015 / Accepted: 10 April 2015 / Published online: 21 May 2015 Ó Science China Press and Springer-Verlag Berlin Heidelberg 2015
Abstract The synthesis of mesoporous material SBA-15 has been extensively reported in the past decades, which possesses a pore diameter of 6–8 nm on average. Here, a simple post-synthesis procedure has been developed to synthesize SBA-15 with further expanded pore diameter to above 10 nm simply by a solvothermal treatment replacing traditional hydrothermal step for mesopore template removal, which results in efficient pore expansion and the significantly promoted condensation of silica framework as well. This facile approach is believed applicable for pore expansions of other kinds of mesoporous silica materials. Keywords Solvothermal treatment Template removal Pore size expanding Framework condensation SBA-15
1 Introduction Following the discovery of ordered mesoporous materials, more and more researchers have focused on the
N. Li Y. Li (&) S. Lu X. Zhang J. Chen D. Niu J. Gu J. Shi (&) Laboratory of Low-Dimensional Materials Chemistry, Key Laboratory for Ultrafine Materials of Ministry of Education, East China University of Science and Technology, Shanghai 200237, China e-mail: [email protected] J. Shi e-mail: [email protected] J. Shi State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China
optimization of pore properties, which would provide great opportunities in the field of catalysis, separation and drug delivery [1–3]. In 1998, Zhao et al. [4] reported an important kind of mesoporous materials SBA-15 using Pluronic P123 (EO20PO70EO20) as the mesopore template under acid conditions for the first time. This kind of mesoporous silica is featured with much larger pore size of 6–8 nm on average than that of M41S family (2–3 nm). By changing the aging temperature or acid concentration of synthesis process, the physicochemical properties of SBA15 could also be well tuned [5, 6]. Nevertheless, the mesopore size of 6–8 nm was still not large enough, which greatly hindered their applications, especially in the biomedicine for the encapsulation of biomacromolecules, such as proteins or DNA with high molecular weight. For synthesizing mesoporous silica with relatively large mesopores, many efforts had been made, and pore expansion is one of the most frequently used approaches. For example, choosing other kind of surfactants with larger sizes [4, 7] or introducing a swelling agent into the template [8, 9] has been the common practices on the enlargement of template micelles, which could result in the pore size expanding. Unfortunately, the correspondingly resultant mesostructure ordering, as well as the thermal stability, would be affected significantly. In addition, hydrothermal and alcothermal treatment have also been used to synthesize large-pore mesoporous materials [10, 11]. On the other hand, there have two general methods for organic template removal: calcination and solvent extraction [7, 12–14]. Besides, special approaches for template removal were also developed, such as microwave digestion, UV/ozone treatment, supercritical fluid extraction and liquid-phase calcination [15–18]. However, these methods suffer from the drawbacks of complexity and/or the need of necessary special equipment.
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In spite of all the above-mentioned efforts in removing the organic template, the pore diameter usually suffers from significant shrinkage by calcination or mostly remains unchanged during extraction. In this paper, we present the simultaneous pore expansion and complete template removal by solvothermal treatment of SBA-15 in a mixed solution for the first time. Importantly, it is different from the pore expansion by using large or enlarged micelles in most literatures that mesopores are effectively expanded beyond 10 nm in the post-synthesis template removal process, which benefits the further framework condensation, mesostructure stability enhancement and the retaining of surface Si–OH groups.
2 Experimental 2.1 Direct synthesis of pore-expanded SBA-15 with template removal The preparation process of pore-expanded SBA-15 was similar to that of the conventional SBA-15 [4, 19], and the difference was that the hydrothermal treating step was replaced with a solvothermal treating process and that no calcination step was involved. The process was described as following. In a typical synthesis, 4.0 g triblock copolymer Pluronic P123 was dissolved in a mixture of 30 g water and 120 g of 2 mol/L HCl in a Teflon-lined autoclave, and the mixture was stirred at 35 °C overnight. Then, 8.50 g tetraethyl orthosilicate (TEOS) was added into the solution under vigorous stirring. After 5 min of stirring, the mixture was kept under static conditions at 35 °C for 20 h and the precursor for SBA-15 was obtained. The precursor for SBA-15 was filtered, then washed with water and dried for further solvothermal treating, which was named as SBA-15-pre-as. In the meantime, the organic solvent was prepared by mixing 0.22 g sodium sulfate, 12 mL deionized water, 25 mL glycol, 7 mL n-butyl alcohol and 0.83 g polyethylene glycol (PEG, molecular weight 6,000) in a 50-mL Teflon-lined autoclave. Finally, 0.4 g SBA-15-pre-as was immersed into the mixed solution and stirred well. After 24 h of heating at different temperatures, the samples were collected by filtration, then washed with water and ethanol, respectively, and dried. The samples were named as SBA-15-st-T, respectively, where T represents the related temperature. For comparison, the SBA-15-pre-as was directly calcined at 550 °C in flowing air for 6 h at a heating rate of 1.5 °C/min, and the obtained material was named as SBA15-pre-cal. Moreover, samples prepared via conventional synthetic process of hydrothermal treatment (100 °C for
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24 h) and calcination (550 °C for 6 h) were named as SBA-15. 2.2 Characterization Fourier translation infrared spectroscopy (FT-IR) was conducted on a Nicolet 5700 Thermo FT-IR spectrometer (USA) using the KBr wafer technique. Thermogravimetric analysis (TGA) curves were obtained on a thermogravimetry–differential thermal analysis (TG–DTA) instrument (Netzcsh STA 409 PC, Germany) at a heating rate of 10 °C/min under nitrogen atmosphere from room temperature to 550 °C. Powder X-ray diffraction (XRD) data were collected on Bruker D8 Focus diffractometer (Germany) equipped with Cu Ka ra˚ ). XRD patterns were collected in the diation (k = 1.5405 A 2h ranges between 0.6° and 6° with a speed of 0.6°/min. N2 adsorption–desorption isotherms were measured at 77 K by using Quantachrome NOVA 4200e (USA). The Brunauer– Emmett–Teller (BET) method was utilized in the calculation of specific surface areas. The pore size distributions were derived from the adsorption branch of the isotherm by means of the Barrett–Joyner–Halenda (BJH) method. Field-emission scanning electron microscopy (FE-SEM) images were obtained by using a JEOL JSM-6700F field SEM (Japan). Transmission electron microscopy (TEM) observations were carried out on a JEOL-2100F electron microscope (Japan). Solid-state 29Si nuclear magnetic resonance (NMR) spectra were performed on a Bruker Avance III 300 spectrometer (Germany), and each spectrum was recorded after 128 acquisitions with 120-s repetition time as a compromise between nuclear relaxation and kinetics of silica polymerization.
3 Results and discussion 3.1 Effects of solvothermal treatment on the mesostructure of SBA-15-st-T Figure 1 shows the XRD patterns of the samples prepared under different conditions. As can be seen, all the samples, except for SBA-15-pre-cal, present three distinctive diffraction peaks in the small-angle region, which can be indexed as (100), (110) and (200), respectively, being attributed to the two-dimensional hexagonal (p6mm) mesostructure. This indicates that SBA-15-st-T samples possess similar mesostructures with the conventional SBA15. Notably, it can be found that the (100) diffraction peaks of SBA-15-st-T samples shifted toward lower diffraction angles, suggesting that the larger cell parameters and mesopores of SBA-15-st-T are obtained. Besides, little difference has been found between the SBA-15-st-T samples,
Sci. Bull. (2015) 60(11):1019–1025
demonstrating that solvothermal treating temperature has less effect on the resulted mesostructures. The N2 sorption isotherms and the corresponding pore size distribution curves for different materials are shown in Fig. 2. All the samples present type IV isotherms with H1 hysteresis loops, which are typical of ordered mesoporous materials with narrow pore size distributions. It is shown that the isotherms of the SBA-15-st-T samples are almost identical, possessing more distinct hysteresis loops than that of the SBA-15-pre-cal, and the step of capillary condensation in mesopores shifting to higher relative pressure. Thus, narrow pore size distributions and significant increment in mesopore sizes for SBA-15-st-T samples are achieved. Moreover, an additional type of mesopores of about 17 nm in diameter is observed for SBA-15-st-120, which can be ascribed to the secondary pores within the wall during the post-treatment.
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Table 1 summarizes the texture properties of the samples. Compared with SBA-15-pre-cal, the pore size and pore volume of SBA-15-st-T are substantially higher, reaching above 10 nm and 0.82 cm3/g, respectively. However, the specific surface areas of SBA-15-st-T are lower than those of SBA-15 and even SBA-15-pre-cal, which is believed to result from the merge and/or collapse of the some mesopores into larger ones (mesopores of 17 nm are observed) [4, 19–21]. This means that SBA-15 samples with rather larger mesopores could be facilely synthesized by adopting solvothermal treatment instead of traditional hydrothermal treatment. Typical SEM images of SBA-15-pre-cal and SBA-15-stT are shown in Fig. 3a–d. It can be seen clearly that all the samples present regular rod-like morphology. More interestingly, well-ordered mesopore channels are exhibited. It is worth noting that a certain amount of cavities can be found on the sample of SBA-15-st-120, corresponding to the results of pore size distribution curves presented in Fig. 2. This is probably due to the enlargement of the secondary pores within the pore wall, leading to mesopore collapse to some extent. High-resolution TEM (HR-TEM) images, as shown in Fig. 3e–h, reveal that all the samples of SBA-15-st-T are in highly ordered hexagonal arrangement. More importantly, an enlargement of mesopore channels, in comparison with SBA-15-pre-cal, is observed, which is in correspondence with the above results of XRD and N2 sorption analysis. In addition, obvious collapse of mesopore channels can also be found in the image of SBA15-st-120. 3.2 Exciting phenomena in the process of solvothermal treatment
Fig. 1 (Color online) Small-angle XRD patterns of different samples
Figure 4a shows the FT-IR spectra of samples prepared with different procedures. Due to the existence of organic
Fig. 2 (Color online) N2 adsorption isotherms (a) and the corresponding pore size distribution curves (b) of different samples, where V is the pore volume and D is the pore diameter
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Table 1 Texture properties of different samples Pore diameterb Dp (nm)
Samples
da(100) (nm)
Specific surface area (m2/g)
Pore volume (cm3/g)
SBA-15-pre-cal
7.38
430
0.47
5.3
3.22
SBA-15-st-100
9.24
353
0.84
10.2
0.47
SBA-15-st-110
9.20
358
0.87
10.1
0.52
SBA-15-st-120
9.22
324
0.82
10.2
SBA-15
8.69
578
0.97
a
7.80
Pore wall thicknessc Tw (nm)
0.45 2.23
d(100) was obtained from the small-angle XRD patterns using Bragg’s equation
b
Dp was calculated from the adsorption branches of the N2 sorption isotherms
c
Tw was calculated by using the equation, Tw = d(100)(2/31/2) -Dp
Fig. 3 SEM and TEM images for the surface and inner microstructures of different samples. a, e SBA-15-pre-cal; b, f SBA-15-st-100; c, g SBA15-st-110; d, h SBA-15-st-120
template, SBA-15-pre-as shows several absorption bands at around 2,860–2,976 and 1,350–1,460 cm-1, which are characteristic peaks of the stretching and bending vibrations of C–H bonds. However, the characteristic peaks of the corresponding C–H bonds in the samples of SBA-15-stT cannot be detected, which are similar to that of SBA-15pre-cal, indicating the complete removal of organic template from the mesopores during the solvothermal treatment process, which is equivalent to the high-temperature calcination step in removing the template molecules. The efficiency of template removal was further evaluated via TGA. As shown in Fig. 4b, a steep weight loss of about 55 % between 450 and 823 K for SBA-15-pre-as is present, which can be mainly attributed to the decomposition of organic template, and the loss of a small amount of structural water from the silica framework due to the condensation of
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silanol groups. It is known that the weight changes before 473 K are mainly attributed to the loss of the physical water adsorbed in the pore channels [5, 22]. In contrast, the samples of SBA-15-st-T exhibit remarkably lower weight losses. In the temperature range of 450–823 K, the weight losses for SBA-15-st-100, SBA-15-st-110 and SBA-15-st-120 are only 8.7 %, 6.5 % and 5.2 %, respectively, which can be mostly attributed to the loss of chemical water. These results clearly demonstrate that the organic template has been removed efficiently, and suggest that higher temperature is beneficial to the removal of organic template. Figure 4c shows the 29Si NMR spectra of different samples. Various Qn groups (Qn = Si(OSi)n(OH)4-n, n = 2–4, where 4 represents the fully condensed silica) are marked on the spectra. As to SBA-15-st-110, the Q4 peak at -111 ppm is greatly enhanced, and the Q2 and Q3
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Fig. 4 Effect of solvothermal treatment on template removal and silica condensation of SBA-15-st-T tested by FT-IR spectra (a), TGA curves (b) and 29Si NMR spectra (c)
peaks, which are in the position of -91 and -101 ppm, respectively, are correspondingly weakened in comparison with SBA-15-pre-as, leading to remarkably decreased Q3/ Q4 ratio, which clearly suggesting that the solvothermal treatment strongly facilitates the condensation reaction of the silanol groups in silica framework. As for the SBA-15, the process of calcination at 550 °C can lead to an even higher degree of condensation. However, this kind of sample has lost most silanol groups, unfavorable for further surface modification and its functions in actual application. 3.3 The proposed mechanism for SBA-15-st via solvothermal treatment As illustrated in Scheme 1, the possible mechanism for changes in structure and the organic template removal in the process of solvothermal treatment is postulated according to the
arguments in the literatures [4, 11, 23–28]. The process can be divided into two stages. In the initial stage, as the degree of condensation of the silica network in SBA-15-pre-as is relatively low, there is great potential for further condensation. It is known that polyethylene oxide (PEO) blocks become less hydrophilic at high temperature; however, polypropylene oxide (PPO) blocks turn to be more hydrophobic. PEO blocks shrink into the hydrophobic region of micelles; meanwhile, the long-chain alcohol penetrates into the hydrophobic region under high pressure and the elevated temperatures, which would render the hydrophobic region swell. On the other hand, the sodium ion (Na?) might replace some of the hydrogenions (H?) on the surface of pore channels, promoting hydrolysis and recondensation of siloxane bonds to a certain degree. Accompanying the gradual expansion of organic template micelles in original mesoporous channels, the resultant pore size and pore volume are enlarged (as shown in Scheme 1). On the
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Scheme 1 The proposed mechanism for SBA-15-st via solvothermal treatment. S.T.: solvothermal treatment, T.H.: hydrothermal treatment
second stage, the enlarged primary pore channels of mesoporous silica as well as the secondary pore within the walls would lead to the improved network connectivity and weakened influence of pore walls on the diffusion of solvent molecules, thus providing a favorable environment for template dissociation. As higher temperature leads to further enlargement of secondary pore sizes, better effect of solvothermal treatment on template removal has been observed for SBA-15-st-120. In addition, the adding of sodium sulfate will attenuate the hydrogen bonding between organic template and silica network and make the layer of alcohol easy to be formed at the organic–inorganic interface, which is beneficial to the removal of organic template. Nevertheless, in spite of our efforts in understanding the processes during the solvothermal treatments, the process of solvothermal treatment is very complicated and further investigation is necessary.
and completely removed during the solvothermal treatment process. More importantly, the pore expansion and a further significant silica condensation are concurrently achieved. The present approach of mesopore template removal is much more advantageous than the traditional high-temperature calcination or hydrothermal extraction in pore expansion, Si–OH group retaining and framework condensation.
4 Conclusions
References
In summary, a facile method of synthesizing SBA-15 with much enlarged mesopores has been developed based on the solvothermal treatment of the precursor of SBA-15. Surfactant molecules in the mesopores could be effectively
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Acknowledgments This work was supported by the National Basic Research Program of China (2012CB933602), the National Natural Science Foundation of China (51172070, 51132009, 51202068 and 51472085), Shu Guang Project (11SG30) and the Fundamental Research Funds for the Central Universities. Conflict of interest of interest.
The authors declare that they have no conflict
1. Kresge CT, Leonowicz ME, Roth WJ et al (1992) Ordered mesoporous molecular sieves synthesized by a liquid-crystal template mechanism. Nature 359:710–712 2. Hudson S, Cooney J, Magner E (2008) Proteins in mesoporous silicates. Angew Chem Int Ed 47:8549–8582
Sci. Bull. (2015) 60(11):1019–1025 3. Park C, Oh K, Lee SC et al (2007) Controlled release of guest molecules from mesoporous silica particles based on a pH-responsive polypseudorotaxane motif. Angew Chem Int Ed 46:1455–1457 4. Zhao DY, Feng JL, Huo QS et al (1998) Triblock copolymer syntheses of mesoporous silica with periodic 50 to 300 angstrom pores. Science 279:548–552 5. Kruk M, Jaroniec M, Ko CH et al (2000) Characterization of the porous structure of SBA-15. Chem Mater 12:1961–1968 6. Choi M, Heo W, Kleitz F et al (2003) Facile synthesis of high quality mesoporous SBA-15 with enhanced control of the porous network connectivity and wall thickness. Chem Commun 12:1340–1341 7. Zhao DY, Huo QS, Feng JL (1998) Nonionic triblock and star diblock copolymer and oligomeric surfactant syntheses of highly ordered, hydrothermally stable, mesoporous silica structures. J Am Chem Soc 120:6024–6036 8. Sayari A (2000) Unprecedented expansion of the pore size and volume of periodic mesoporous silica. Angew Chem Int Ed 39:2920–2922 9. Fan J, Yu CZ, Wang LM et al (2001) Mesotunnels on the silica wall of ordered SBA-15 to generate three-dimensional large-pore mesoporous networks. J Am Chem Soc 123:12113–12114 10. Mizutani M, Yamada Y, Yano K (2007) Pore-expansion of monodisperse mesoporous silica spheres by a novel surfactant exchange method. Chem Commun 11:1172–1174 11. Sun JH, Moulijn JA, Jansen KC et al (2001) Alcothermal synthesis under basic conditions of an SBA-15 with long-range order and stability. Adv Mater 13:327–331 12. Margolese D, Melero JA, Christiansen SC et al (2000) Direct syntheses of ordered SBA-15 mesoporous silica containing sulfonic acid groups. Chem Mater 12:2448–2459 13. Yang CM, Zibrowuis B, Schmidt W et al (2004) Stepwise removal of the copolymer template from mesopores and micropores in SBA-15. Chem Mater 16:2918–2925 14. Yang CM, Zibrowius B, Schmidt W et al (2003) Consecutive generation of mesopores and micropores in SBA-15. Chem Mater 15:3739–3741 15. Tian BZ, Liu XY, Yu CZ et al (2002) Microwave assisted template removal of siliceous porous materials. Chem Commun 11:1186–1187
1025 16. Xiao LP, Li JY, Jin HX et al (2006) Removal of organic templates from mesoprous SBA-15 at room temperature using UV/ dilute H2O2. Micropor Mesopor Mater 96:413–418 17. Van Grieken R, Calleja G, Stucky GD et al (2003) Supercritical fluid extraction of a nonionic surfactant template from SBA-15 materials and consequences on the porous structure. Langmuir 19:3966–3973 18. Cauda V, Argyo C, Piercey DG et al (2011) ‘‘Liquid-phase calcination’’ of colloidal mesoporous silica nanoparticles in highboiling solvents. J Am Chem Soc 133:6484–6486 19. Sayari A, Han BH, Yang Y (2004) Simple synthesis route to monodispersed SBA-15 silica rods. J Am Chem Soc 126:14348–14349 20. Deng YH, Yu T, Wan Y et al (2007) Ordered mesoporous silicas and carbons with large accessible pores templated from amphiphilic diblock copolymer poly(ethylene oxide)-b-polystyrene. J Am Chem Soc 129:1690–1697 21. Galarneau A, Cambon H, Di Renzo F et al (2003) Microporosity and connections between pores in SBA-15 mesostructured silicas as a function of the temperature of synthesis. New J Chem 27:73–79 22. Yan XX, Deng HX, Huang XH et al (2005) Mesoporous bioactive glasses. I. Synthesis and structural characterization. J NonCryst Solids 351:3209–3217 23. Lai XY, Wang D, Han N et al (2010) Ordered arrays of beadchain-like In2O3 nanorods and their enhanced sensing performance for formaldehyde. Chem Mater 22:3033–3042 24. Lai XY, Shen GX, Xue P et al (2015) Ordered mesoporous NiO with thin pore walls and its enhanced sensing performance for formaldehyde. Nanoscale 7:4005–4012 25. Rumplecker A, Kleitz F, Salabas EL et al (2007) Hard templating pathways for the synthesis of nanostructured porous Co3O4. Chem Mater 19:485–496 26. Soler-Illia GJAA, Crepaldi EL, Grosso D et al (2003) Block copolymer-templated mesoporous oxides. Curr Opin Colloid Interface Sci 8:109–126 27. Sayari A, Shee D, Al-Yassir N et al (2010) Catalysis over poreexpanded MCM-41 mesoporous materials. Top Catal 53:154–167 28. Gallis KW, Landry CC (1997) Synthesis of MCM-48 by a phase transformation process. Chem Mater 9:2035–2038
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