One-pot synthesis of silica monoliths with hierarchically porous structure
Graphical abstract
Poly(furfuryl alcohol) (yellow) and Pluronic F127 (PEO = blue, PPO = red) are used as co-templates to produce hierarchically porous silica (black) monoliths. Control over the architecture is achieved by adjusting the molar ratios of HCl, F127 and PFA relative to Si. Right side: low [FA] and pH; Left side: higher [FA] and pH.
Highlights
► SiO2 monoliths were prepared using Pluronic F127 and furfuryl alcohol as co-templates. ► The SiO2 monoliths presented high surface area and hierarchical pore morphology. ► Different morphologies are obtained depending on the acidity. ► The templating mechanism was elucidated to gain control over the morphology.
Introduction
Simple and inexpensive procedures are needed to produce multi-scale structured silica monoliths [1]. Diverse morphologies are required due to the wide variety of applications for these materials, such as sorption/separation, biomaterials engineering, membrane-based reactors and scaffolds for the formation of hierarchical carbon materials [2], [3], [4]. Hierarchical pore structures are ideal for applications where a solid material is in contact with a fluid phase because the macropores allow the fluid to quickly reach the high internal surface area provided by meso- or micropores [5]. Hierarchical structures are especially useful in the case of macroscopic materials, in which fast transport of molecules (solvent, reactants, products, probes, analytes, etc.) throughout the whole material is critical, as diffusion pathways are comparatively long [6], [7]. The size and structure of the macropores effects diffusion rate and capacity; therefore material architecture must be constructed to meet the needs of each application [8], [9]. The human body gives two excellent examples of how effectively hierarchical systems work: the respiratory system [10], and the circulatory system, where the presence of increasingly smaller features maximizes flow and exchange rates. By following nature’s example, effective designs can be created for a host of structured materials.
Monoliths are more practical than thin films or compacted powders for many applications, due to their robustness and ease of handling. The hierarchical pore architecture is especially important in macroscopic materials, as macroporosity is needed to encourage rapid access to internal porosity [7], [8], [9]. In the traditional templating approach, monoliths, thick films or xerogels with hierarchical porosity are prepared by using colloidal particles or pre-organized structures as sacrificial scaffolds [6], [11], [12], [13], [14], [15], [16]. While these techniques are capable of producing very complex structures of highly controllable dimensions, it often requires separate steps for the template preparation and the structure transfer. In order for hierarchically porous monoliths to be produced on an industrial scale, they must be prepared using cheap starting materials and through a simple method that is relatively insensitive to the changes in ambient conditions.
An alternative to templating preformed structures is to use emulsions or foams to produce porous silica. While this approach permits the continuous preparation of material, particular attention must be paid to the synthesis conditions, for the precursor systems are very sensitive to small variations in the composition [17], [18]. Another option is to use glycol modified silanes as precursors to construct hybrid materials with well-defined hierarchical porosity [19].
Phase separation is an attractive technique for synthesizing multiscale structured materials inexpensively and reproducibly. However, the instigative force of phase separation is crucial and must be thoroughly studied to achieve tailorability of the morphology. Nakanishi has pioneered phase separation through spinodal decomposition of polymers in solution as a means to produce macroporous inorganic monoliths in a single-step reaction, followed by calcination [20]. Generally, a silica precursor, the structure directing agent(s) (polymers or polymeric surfactants) and a solvent are mixed together and after inorganic condensation, evaporation and calcination, macroporous oxide results. The macropore size and pore volume can be controlled independently by varying the quantities of polymer and solvent [20]. Due to the strength of the driving forces involved, the morphologies created by spinodal decomposition change greatly with small variations of reagent ratios [21], curing temperature or other conditions. Obtaining a homogeneous structural evolution of the phase separation relative to the inorganic gelation requires precise selection of reagents, molar ratios and processing temperature [22].
An alternative to spinodal decomposition is to use polymerization-induced phase separation to produce the macroporous structure. We have recently reported a general method for the production of macroporous–mesoporous oxides, in which multi-scale templating can be achieved in a one-pot synthesis using furfuryl alcohol (FA), Pluronic F127 and an oxide precursor [23]. The versatility of this method arises from the in situ formation of a macropore template of varying hydrophobicity, morphology and dimensions. PFA can be generated from inexpensive FA via a cationic polymerization (Scheme 1), initiated by protic acids [24] or Lewis acids, such as titanium, tin and zinc cations [25], [26]. As PFA grows in molecular weight and forms crosslinks, it becomes more hydrophobic and phase separates from the reaction mixture [27]. The resulting polymerization-induced phase separation has been used to create composites of silica and carbon [28], [29]. Porous carbon materials were produced by treating the composite with hydrofluoric acid to dissolve the silica. Previous reports of inorganic monoliths prepared from tetraethyl orthosilicate (TEOS), FA and acid [30], did not make use of a block copolymer to control the formation of mesopores. High temperatures (900 °C) were needed to remove the carbonaceous template, yielding materials with relatively modest surface areas (200 m2 g−1) and disconnected macropores.
In the present report we describe the combination of two templates, PFA and Pluronic F127, to form hierarchically porous silica monoliths using polymerization-induced phase separation. The macropore architecture is adjustable and can be interconnected. Surfactant-templated mesoporosity results in nearly five times higher surface areas than previously reported PFA-templated silica monoliths [30]. A range of preparation conditions was explored to elucidate the formation mechanism of hierarchically porous silica monoliths, and to achieve control in the pore structure and size. Our results show that pH plays a critical role in the interaction between the mesoporogen (Pluronic F127) and macroporogen (PFA), and in the condensation kinetics of PFA and the silica matrix. These thermodynamic and kinetic aspects are essential to understand the co-assembly of the different building blocks (silica and FA oligomers, F127 micelles or other aggregates). When these interactions are understood, the properties of the material, e.g. pore size, morphology and surface area, can be controlled. This work is an important step towards the ultimate goal: silica with tailored porosity across multiple scales.
Section snippets
Materials
Ethanol was purchased from Merck. FA, TEOS and Pluronic F127 were obtained from Sigma Aldrich, while 37% hydrochloric acid (HCl) was purchased from Scharlau. To obtain a hydrophilic silica precursor, a prehydrolyzed solution was prepared in the following way: TEOS (20.8 g), ethanol (13.8 g) and 1.8 g of 0.14 M HCl were combined and refluxed for 2 h. The solution was allowed to cool and was then stored at 4 °C in an airtight container. The 0.14 M HCl solution was produced by combining 2.3 mL of 37% HCl
The synthesis of SiO2 monoliths
Using the PFA-F127 templating method reported here, we obtained intact porous SiO2 monoliths after gelation-drying. After a mild thermal treatment, a SiO2-PFA composite is obtained (Fig. 1a). PFA is compliant to the shrinkage produced upon SiO2 condensation: during the thermal treatment the excess PFA exudes from the monoliths, producing a black cracked shell around the monolith. Fig. 1b shows a photograph of a calcined sample held between fingers. During the gelation-drying stage, a phase
Discussion
Under the synthesis conditions explored in this study, the addition of F127 was crucial to the stability of the silica monoliths. Calcination of materials produced with the block copolymer resulted in intact monoliths, which could be handled with tweezers or fingers (Fig. 1b). However, in the absence of F127, the materials collapsed upon template removal. F127 has indeed an effect on the pore structure: templated samples present a more regular pore size and interconnectivity, as well as thicker
Conclusions
Monoliths with hierarchical macro–mesoporous structure were produced in the shape of the mold through a combination of block copolymer and PFA templating. Mechanically stable systems with high surface areas and well-defined tuneable pore size over two length scales have been produced. Interconnected macropores could be obtained by using highly acidic synthesis solutions with a minimum of 5 M equivalents of FA to Si, or by increasing the concentration of Pluronic F127. However, the high proton
Acknowledgements
Prof. Yi-Bing Cheng and his high temperature ceramics subgroup, particularly Dr. Kun Wang, are thanked for enlightening discussions about this research. Financial support for travel by GLD from The University of Melbourne to Comisión Nacional de Energía Atómica was generously provided by The University of Melbourne Postgraduate Overseas Research Experience Scholarships (PORES) and an Australian Research Council Nanotechnology Network Overseas Travel Fellowship. GLD was supported by the Albert
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