Strong Silica Monoliths with Large Mesopores Prepared Using

Feb 1, 2011 - Attard , G. S.; Glyde , J. C.; Göltner , C. G. Nature 1995, 378, 366– 368. [Crossref], [CAS]. 19. Liquid-crystalline phases as templa...
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LETTER pubs.acs.org/Langmuir

Strong Silica Monoliths with Large Mesopores Prepared Using Agarose Gel Templates Glenna L. Drisko,† Xingdong Wang,‡ and Rachel A. Caruso*,†,‡ † ‡

Particulate Fluids Processing Centre, School of Chemistry, The University of Melbourne, Melbourne VIC 3010, Australia CSIRO Materials Science and Engineering, Private Bag 33, Clayton South VIC 3169, Australia

bS Supporting Information ABSTRACT: Mesoporous silica pellets with controllable shape and pore size were prepared using agarose gel templates. Robust (compressive strength of 3.3-25.1 MPa), crack-free silica monoliths have been produced with large mesopores (14-23 nm), high surface areas (410-540 m2 g-1), and large pore volumes (1.1-1.2 cm3 g-1). The synthesis was achieved by infusing preformed agarose gels with tetraethyl orthosilicate and the nonpolar condensation catalyst tetrabutyl ammonium fluoride. The infiltrated gels were transferred to water to initiate hydrolysis and condensation of the silica precursor. Fluoride catalyzed the gelation of silica in a matter of minutes; hence, the oxide maintained the shape of the agarose pellet. The mesopore size could be modified by altering the weight percent of agarose gel used. The method employed here is simple and reproducible. As these materials have such large mesopore dimensions, they could be used as hard templates or could be specifically functionalized for use in environmental remediation, as environmentally responsive materials, biocatalysts, or catalysts.

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he advent of ordered mesoporous silica via surfactant templating in the early 1990s1-3 led to heavy investigation of mesostructured materials due to the large range of potential applications. For example, silica has been used as a hard template for carbon and other nonsilicate materials,4,5 as the stationary phase for liquid chromatography,6 and as the rigid component of hybrid materials (e.g., for use in biomedical applications or fuel cells).7-9 Additionally, functionalized silicas have demonstrated potential as environmentally responsive materials,10-12 as biocatalyst platforms,13 optical sensors,14 and adsorbents for organic and inorganic pollutants.15 Many of these applications would benefit from strong monolithic materials with large mesopore diameters, pore volumes, and surface areas, yet it is still currently a challenge to meet all of these criteria. The few methodologies available for producing mesoporous silica monoliths are discussed in reviews by El-Safty16 and by Wan and Zhao.17 Mesoporous architectures are often achieved using evaporation induced self-assembly (EISA) to generate block copolymer superstructures that can template inorganic materials. This technique can produce ordered pores of uniform size in a variety of geometries.17,18 By using high molecular weight block copolymers and swelling agents, silica with large mesopore diameters (10-27 nm) can be synthesized.17 However, there are few reports of monoliths produced from EISA, as the formation of the mesophase is dependent on evaporative forces, which are highly variable in bulk volumes. An alternative to the EISA approach is to engage true liquid crystal templating.19 While this has the advantages of producing materials with large mesopores in the form of monoliths,20 if conditions are not carefully controlled and optimized, the final materials will contain r 2011 American Chemical Society

inhomogeneities.21 A highly promising route to synthesize silica in bulk harnesses lyotropic and microemulsion copolymer phases, which can produce a variety of pore architectures with pore sizes ranging from 3.5 to 14.3 nm.16 A simple way to achieve large mesopores in silica monoliths would be to use preformed templates. Gelled agarose, a thermoresponsive biopolymer, is an excellent template because the shape of the gel is readily controlled, and it is easy to handle, inexpensive, and derived from a renewable resource. The material morphology can be tailored to the application, as the agarose gel can be formed as either a film or a bulk material of various shapes. Previously, agarose gels have been used as templates to produce porous oxides of titanium, zirconium, niobium, and tin,22 aluminum or vanadium doped titanium oxide,23,24 and zirconium titanium oxide.25,26 The templating process is unaffected by ambient humidity and temperature, allowing for highly reproducible materials. Although agarose gels have proven to work well as templates for porous monolithic metal oxides, the technique was previously ineffective for silica,27 due to the slow kinetics of the alkoxysilane-silica sol-gel transition at neutral pH. Alkoxysilanes are much less reactive than titanium or zirconium alkoxides because the electron density in Si-O bonds is relatively evenly distributed, and therefore, the substitution of an alkoxide by a water molecule is a comparatively slow process. However, fluorides can catalyze the formation of silica under Received: November 29, 2010 Revised: January 4, 2011 Published: February 01, 2011 2124

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Figure 1. Schematic of synthesis protocol. The white areas indicate agarose fibers, the dark gray is void space, the black areas are pores, and the light gray represents silica. The infused agarose gel contains TEOS (Si-(OEt)4), TBAF, tetrahydrofuran, and residual ethanol.

neutral pH conditions.28 F- and Si(IV) form strong bonds very quickly, creating a polarized, 5-coordinate silane. This moiety is more receptive to the addition of a water molecule, and hence, hydrolysis and condensation occur at an accelerated rate.29,30 The sol-gel mechanism of this process is outlined in detail in ref 30. Tetrabutyl ammonium fluoride (TBAF) is a source of fluoride that can initiate the gelation of tetraethyl orthosilicate (TEOS) in nonpolar solvents. TBAF has been used to catalyze the condensation of silica in order to produce xerogels31,32 and aerogels,33 but it has not yet been used to facilitate condensation within a preformed template structure. Here we describe the synthesis of mesoporous monolithic silica using agarose gel templates, TEOS, and the oleophilic condensation catalyst TBAF. To the best of our knowledge, silica monoliths with controllable mesopore sizes of such a large diameter have not been previously reported. Materials with these properties and prepared in a facile synthesis offer an advance to the field as they can easily be integrated into an assortment of applications. To control the structure of the mesoporous silica monoliths, 3, 5, 7, and 8 wt % agarose gel template pellets were used. The maximum attainable agarose density was 8 wt % while still forming a homogeneous gel. The agarose gels were prepared as previously reported (see the Supporting Information for details).22-26 To make an x wt % gel, x g of agarose powder was dispersed in (100 - x) g of distilled water. The dispersion was stirred and heated to 80 °C to dissolve the powder. The clear solution was poured while hot into test tube molds and allowed to set overnight, and then the agarose gels were cut into pieces, roughly 0.25 cm3. Over a period of 36 h, the solvent within the gels was exchanged from pure water to ethanol. Fifty gel pellets were soaked overnight in 100 mL of solution, composed of TEOS (50 mL) and 1 M TBAF in tetrahydrofuran (50 mL). The infused agarose gel pellets were transferred to distilled water to initiate hydrolysis and condensation reactions (see Figure 1). The silica visibly began to gel within 1 min without perceptible TEOS leakage from the agarose template. After 6 h, the pellets were removed from solution and dried, followed by calcination in air at 600 °C for 5 h (ramp 1 °C min-1) to eliminate the organic matter. Reproducible, crack-free, robust silica pellets resulted. In previous reports of agarose gel-templated metal oxides,22-26 the samples were prepared by first infiltrating the template with the metal alkoxide precursor and then transferring the agarose gels to an aqueous solution to initiate hydrolysis and condensation reactions. Silica could not be successfully prepared using exactly the same technique. The slower hydrolysis rate of alkoxysilanes than metal alkoxides resulted in a large proportion of the TEOS exiting the template prior to gelation, leading to the formation of weak and fractured silica pellets after calcination.

LETTER

Figure 2. Photograph of porous silica monoliths prepared using 5 wt % agarose gel templates.

The agarose gel structure is not stable in acidic and basic solutions, which could otherwise be used to accelerate the hydrolysis of TEOS.29 Due to these complications, some have claimed that agarose gels cannot be used as a template for silica.27 However, these synthetic barriers could potentially be overcome by using a condensation catalyst that functions at neutral pH. Initial efforts to use agarose gel to template silica employed ammonium fluoride (AF) in the aqueous solution. AF could not be infused throughout the gel prior to contact with water because this reagent is highly polar and hence insoluble in dry ethanol and TEOS. This catalyst certainly accelerated the condensation of silica in the external shell (about 1 mm) of the template; however, the hollow and structurally weak silica pellets produced indicate that AF did not penetrate further into the agarose gel. Therefore, the less polar TBAF was added to the alcoholic precursor solution and infiltrated into the template prior to contact with water. When the saturated agarose gel was transferred to an aqueous solution, the TBAF condensation catalyst homogeneously increased the TEOS hydrolysis rate within the macroporous network of the gel; thus, the pellet morphology was retained in the calcined silica (Figure 2). The silica monoliths are of controllable macroscopic shape, are free-standing, and have good optical clarity, untainted by residual carbon matter. The metal oxides previously produced by templating agarose gel contained hierarchical porosity with macropores around 80 nm in size.26 The macroporosity resulted from a coating of metal oxide nanoparticles on the fibers of the template and hence produced a less dense structure compared to the silica. Silica has been observed to cast rigid porous structures.34 The final silica structure resulting from agarose templates did not contain the large macropores and hence was structurally more dense than that obtained by metal oxides. Scanning electron microscopy (SEM) images (Figure 3) show globular domains of silica, mostly filling the agarose gel fiber network. The presence of porosity in the silica/agarose gel hybrid (see for example Figure 3b) implies that the template was not completely cast with silica. The agarose template produced large mesopores and allowed for variation of the pore size within the silica monolith. Transmission electron microscopy (TEM) images of the silica monoliths display two types of pore formations: interparticle and interaggregate voids (Figure 4a). The silica particles were 5-10 nm in diameter. The nitrogen sorption isotherm (Figure 4b) is Type IV with a H1 hysteresis loop, characteristic of mesoporous glasses.35 Type H1 hysteresis curves are most often associated with agglomerations of roughly spherical particles.36 The pore size distribution of silica produced from 5 wt % agarose gel (Figure 4c) shows a peak centered at 16 nm that slowly tapers off, indicating that there is a range of interaggregate pore sizes. Interparticle spaces increase the pore volume of the pellets. By varying the agarose gel density between 3 and 8 wt %, the pore size of the final silica material could be adjusted from 2125

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LETTER

Figure 3. SEM images of the (a) 5 wt % agarose gel template and the resulting (b) agarose/silica hybrid and (c) silica after calcination. The scale is the same for all images.

Figure 6. Compressive strength of calcined silica monoliths as a function of the weight percent of the agarose gel template.

Figure 4. (a) TEM image, (b) nitrogen sorption isotherm, and (c) BJH pore size distribution using the adsorption branch of a calcined silica monolith prepared from a 5 wt % agarose gel.

Figure 5. Change in (a) peak pore diameter and (b) surface area as a function of the weight percent agarose gel template.

14 to 23 nm (Figure 5a). The surface area increased from 410 to 540 m2 g-1 when decreasing the weight percent of agarose used in the template (Figure 5b). The pore volume was consistently between 1.1 and 1.2 cm3 g-1. As it is easy to tailor the weight percent of agarose in the gel, tuning the mesopore diameter is straightforward. The presence of these large pores is highly desirable for some applications, as large mesopores have been shown to increase the macromolecule loading in porous oxide supports.37 The silica monoliths were pressed between two plates using an Instron 5848 MicroTester instrument to determine the amount of force that the pellets could resist (Figure 6). Compressive strength testing measures the ability of a material to withstand axial force before cracking. Compression tests were performed at 22 °C and 37% humidity on nonpolished samples. Upon failure, the monoliths cracked into several pieces but did not crush or

crumble. Silica pellets templated from 7 wt % agarose gel, which were 1.0-3.0 mm in height, length, and width, cracked under a force of 134 ( 28 N. To put this into perspective, 134 N is equivalent to the downward force of 13.7 kg, subjected to Earth’s gravity (9.8 m s-2). The silica templated from 5 wt % agarose gel templates had compressive strength of 22 MPa. These pellets are nearly as strong as the nonporous silica monoliths recently reported by Yang et al., which demonstrated compressive failure at 35 MPa.38 Silica aerogels have been reported to have compressive strength of 2.4 MPa.39 The pellet templated by 3 wt % agarose gel was the least strong (3.3 ( 1.1 MPa). The higher compressive yield points of the other samples can be attributed to a denser silica structure formed during the agarose templating process. In the 3 wt % agarose gel, the voids between the template fibers were much larger than those in the other templates,22 resulting in larger interaggregate voids in the SiO2. The robust silica pellets could be used to reinforce materials with a property of particular interest that is hindered by poor mechanical properties. For instance, poly(arylene ether)s and polyphosphazenes have been investigated for use as proton conducting membranes due to their good hydrolytic and oxidative resistance and conductivity.40 However, these polymer membranes tend to swell in water and therefore lose their mechanical integrity, which could be overcome by employing a rigid silica support.41 In conclusion, agarose gel templates were used to produce silica of controllable mesopore size as strong, crack-free monoliths. This is the first study using TBAF to catalyze the hydrolysis and condensation reactions of TEOS within a preformed template. TBAF can be used in nonpolar environments and can be substituted for AF when nonaqueous conditions are desired. By using this reagent, homogeneous silica pellets were formed with excellent resistance to force; compressive strength was as high as 25.1 MPa. Large mesopore sizes, pore volumes, and surface areas were obtained, and the mesopore size was easily adjusted 2126

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Langmuir between 14 and 23 nm by changing the weight percent of agarose in the template. We expect that this novel approach to producing strong silica monoliths with large mesopores will be used in host-guest chemistry and these structures will be integrated into advanced functional materials.

’ ASSOCIATED CONTENT

bS

Supporting Information. Experimental procedures. This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Mailing address: School of Chemistry, The University of Melbourne, Melbourne, VIC 3010, Australia. Fax: þ61 3 9347 5180. E-mail: [email protected].

’ ACKNOWLEDGMENT Drs. Dehong Chen, Cara Doherty, Simon Harrisson, and Andreas Ide are thanked for useful conversations about this research. Dr. Laura Miranda provided helpful feedback on the manuscript. Ms. Silvia Leo assisted with measurement of the pellets0 compressive strength, and Dr. Andrea O0 Connor, through the PFPC, provided access to infrastructure. G.L.D. conducted electron microscopy in the Electron Microscopy Unit of Bio21 at The University of Melbourne. The work was financially supported by the Australian Research Council through a Discovery Project (DP0877428). R.A.C. is the recipient of an Australian Research Council Future Fellowship (FT0990583). ’ REFERENCES (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710–712. (2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834–10843. (3) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 988–992. (4) Sch€uth, F. Chem. Mater. 2001, 13, 3184–3195. (5) Doherty, C. M.; Caruso, R. A.; Smarsly, B. M.; Adelhelm, P.; Drummond, C. J. Chem. Mater. 2009, 21, 5300–5306. (6) Giraldo, L. F.; Lopez, B. L.; Perez, L.; Urrego, S.; Sierra, L.; Mesa, M. Macromol. Symp. 2007, 258, 129–141. (7) Taylor-Pashow, K. M. L.; Rocca, J. D.; Huxford, R. C.; Lin, W. Chem. Commun. 2010, 46, 5832–5849. (8) Ladewig, B. P.; Knott, R. B.; Hill, A. J.; Riches, J. D.; White, J. W.; Martin, D.; Diniz da Costa, J. J. C.; Lu, G. Q. Chem. Mater. 2007, 19, 2372–2381. (9) Aparicio, M.; Mosa, J.; Duran J. Sol-Gel Sci. Technol. 2006, 40, 309–315. (10) Calvo, A.; Yameen, B.; Williams, F. J.; Azzaroni, O.; Soler-Illia, G. J. A. A. Chem. Commun. 2009, 2553–2555. (11) Calvo, A.; Yameen, B.; Williams, F. J.; Soler-Illia, G. J. A. A.; Azzaroni, O. J. Am. Chem. Soc. 2009, 131, 10866–10868. (12) Calvo, A.; Angelome, P. C.; Sanchez, V. M.; Scherlis, D. A.; Williams, F. J.; Soler-Illia, G. J. A. A. Chem. Mater. 2008, 20, 4661–4668. (13) Bellino, M. G.; Regazzoni, A. E.; Soler-Illia, G. J. A. A. ACS Appl. Mater. Interfaces 2010, 2, 360–365.

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