Triblock copolymer templated materials

Apparent advantage of the prefabricated nanoparticles hes in the opportunity to use well-developed procedures to control particle size and particle size distribution and to characterize the particles by all possible means. Prefabricated particles can be introduced in the mesoporous materials in two ways. One avenue is direct incorporation of particles in sol-gel mixture as was recently reported in [60]. Here mesostructured nanoparticle-sihca monoliths have been synthesized by dispersing prefabricated Au or zeohte (sihcate) nanoparticles in sol-gel precursors containing SiCl4 and a Pluronic triblock copolymer template. To improve the compatibility of the Au nanoparticles with the poly(ethylene oxide) block, the nanoparticle surface was modified with a SiOj layer. Therefore, this technique requires good compatibihty of nanoparticles with the components of the sol-gel reaction, as it influences the distribution of nanoparticles through the material. Another hmitation of this method is encapsulation of particles within mesoporous material, but not necessary location in the pores. As was discussed above, this limitation is especially crucial for catalytic applications. 

Figure 2.16 Typical nitrogen adsorption-desorption isotherms at 77K for (a) MCM-41 materials templated with alkyltrimethylammonium bromide surfactants with hydrophobic tails of different lengths as indicated (volumes adsorbed for C12, C14, C16 and C18 were incremented by 200, 400, 600 and 800 ml(STP) g, respectively) and (b) nonionic triblock copolymer templated SBA-15 silicas synthesised at different temperatures (volumes adsorbed for 353 K and 373 K were incremented by 200 and 400 ml(STP) respectively). The hysteresis loops observed in this case are typical of the larger mesopores in these materials. Desorption points are represented by closed symbols. Reprinted with permission from Morishige, K. Tateishi, M., Accurate relations between pore size and the pressure of capillary condensation and the evaporation of nitrogen in cylindrical pores, Langmuir, 22, 4165 169. Copyright (2006) American Chemical Society...

The second case study. This involves all silica micro- and mesoporous SBA-15 materials. SBA-15 materials are prepared using triblock copolymers as structure-directing templates. Typically, calcined SBA-15 displays pore sizes between 50 and 90 A and specific surface areas of 600-700 m g with pore volumes of 0.8-1.2cm g h Application of the Fenton concept to mesoporous materials looks simpler since mass transfer would be much less limited. However, it is not straightforward because hydrolysis can take place in the aqueous phase. 

Furthermore, aggregates of nonionic surfactants like polyoxyethylene alkyl/aryl ethers of the Tergitol, Triton, and Brij series were used as templates for the formation of mesoporous silica materials in neutral or acidic media. The pore diameters of the materials that can be obtained with these surfactants are restricted to around 5.5 nm. The advantages of these surfactants over triblock copolymers are that they are cheap, nontoxic, and biodegradable. An overview of the most commonly used SDAs is compiled in Table 3.1. 

In the meantime it has also become feasible to sjmthesize other meso-porous materials that differ from CPG in that ttiey consist of individual, disconnected cylindrical pores. These so-called SBA-15 or MCM-41 silica pores can be synthesized using a technical-grade triblock copolymer as the structure directing template in aequous H2SO4 solution and tetraethyl orthosilicate as the silica source [33, 34]. After calcination [35], one obtains a regular array of individual cylindrical pores as illustrated by the transmission electron micrographs (TEMs) shown in Fig. 4.2. 

Imperor-Clerc, M., Davidson, R, and Davidson, A., Existence of a microporous corona around the mesopores of silica-based SBA-15 materials templated by triblock copolymers, J. Am. Chem. Soc., 122, 11925, 2000. 

Silica walls of calcined mesostructured SBA-15 materials templated by triblock copolymers... 

Silica-based SBA-15 materials, synthetised using triblock copolymers as templates, have a 2-dimensional hexagonal symmetry. PEO chains are deeply occluded within silica walls of SBA-15 and therefore the density of these walls, after calcination and elimination of PEO chains, may not be uniform. Hydrothermal treatment of SBA-15 can be used to increase their main mesopore diameter and decrease their wall thickness. Unique informations provided by modelling of XRD data complemented by TEM and N2 sorption show that calcined SBA-15 solids cannot be considered as ideal arrays of mesopores imbedded in a uniform silica matrix. The silica walls structure is complex as mesopores appear to be surrounded by a microporous corona of silica. We will also describe how this corona is affected by hydrothermal treatment. 

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