Mesostructured monoliths

Keywords sol-gel chemistry, mesostructured monoliths, glycol-modified silanes... 

S.A. El-Safty, T. Hanaoka, and F. Mizukami, Design of Highly Stable, Ordered Cage Mesostructured Monoliths with Controllable Pore Geometric and Sizes. Chem. Mater., 2005, 17, 3137-3145. 

Non-electrostatic assembly is particularly well suited for the direct synthesis of derivatized mesostructures because the framework-forming surfactants can be removed by ambient-temperature solvent extraction and does not necessitate the potentially destructive acid leaching necessary in electrostatic direct assembly.17 Also, HMS and MSU-X materials typically have ultrafine (<1 pm) particle sizes, while electrostatically assembled MCM-type compounds usually have blocky and monolithic morphologies with comparatively large particle sizes (>10 pm). On this basis, non-electrostatically assembled mesostructures are expected to have improved diffusion properties compared to their electrostatic MCM analogs, as will be demonstrated in section 3.3 below. 

The particle texture of HMS silica depends critically on the polarity of the solvent used to assemble the mesostructure. Water-rich solvent mixtures afford highly textured particles (denoted HMS-HTx) formed by the intergrowth of nanoscale framework domains (cf., Figure IB), whereas less polar alcohol-rich solvents afford more monolithic particles (denoted HMS-LTx) with little or no textural mesoporosity... 

Others have verified that functionalized wormhole mesostructures are typically more reactive catalysts than well-ordered framework structures11,13 14 15,16 17,18 In some cases, wormhole frameworks have been found to be less active than hexagonal MCM-4119,20,21,22 gut jn these iatter cases the wormhole framework lacked the textural mesoporosity that is characteristic of a HMS mesostructure assembled from a water-rich solvent. If HMS particles are monolithic and comparable in size to a well-ordered... 

Polyol-Modified Silanes as Precursors for Mesostructured Silica Monoliths... 

Carbon (C)-aerogels have been investigated for one decade as a promising material for electrochemical applications in supercapacitors, fuel cells and waste water treatment [1,2], C-aerogels are nanoporous, electrically conducting and monolithic materials that provide the unique possibility to tailor the carbon properties on a molecular scale. The surface area and the degree of microporosity can be adjusted almost independently of the overall porosity for which mainly meso- and macropores are responsible. Whereas the mesostructure is determined by the stoichiometry of the reactants in the precursor solution, the pyrolysis conditions control the micropore structure of the material [3,4]. High pyrolysis temperatures will increase the electrical conduchvity [5], an important property for many electrochemical applications. 

Fig. 5. Illustration of the diffusion limited activation in monolithic aerogel slices with different mesostructures.

Mixed solutions of cationic surfactants and nonionic poly(ethylene glycol) or block copolymers were employed for the synthesis of monolithic trimodal porous silica.[176] Lyotropic mixtures of block copolymers of different lengths with hydrophilic linear PEO chains were also applied to their nanocasting into bimodal micro-mesoporous silica to formulate the dependence of the mesopore sizes and the microporosity on the lengths or sizes of the hydrophobic and the hydrophilic blocks, but the mesostructures were worm-type in morphology and several hundred nanometers or more in size. 

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. 

Mesoporous materials were originally synthesized in irregular bulk or powder forms, which could limit their applications in separation, optics, electronics, and so on. Thus, it is highly desirable to produce mesoporous materials with controllable macroscopic forms. So far, mesoporous materials have been synthesized in a variety of forms including thin films, spheres, fibers, monoliths, rods, single crystals, and nanoparticles. The acidic synthetic route (S+X I+) developed by Huo etal appears to be the most appropriate for the morphological control of mesostructures. 

There are some excellent review articles on different aspects of mesostructured materials, such as synthesis, properties, and applications. " Extensive research effort has been devoted to the exploitation of new phases (lamellar, cubic, hexagonal structures), expansion of the pore sizes (about 2-50 nm are accessible), and variable framework compositions (from pure silica, through mixed metal oxides to purely metal oxide-based frameworks, and inorganic-organic hybrid mesostructures). Another research focus is on the formation of mesostructured materials in other morphologies than powders, e.g. monolithic materials and films, which are required for a variety of applications including, but not limited to, sensors (based on piezoelectric mass balances or surface acoustic wave devices), catalyst supports, (size- and shape-selective) filtration membranes or (opto)electronic devices. The current article is focused... 

The morphology of ordered mesostructured carbons is another important factor with respect to their practical applications. Various macroscopic morphologies are required, for example, films (in sensor, separation and optical applications), uniformly sized spheres (in chromatography) or transparent monoliths. Using suitable synthesis strategies, it is possible to control the external shape of the templated mesoporous carbon materials to generate powders, films and membranes, spheres, hollow spheres, rods, fibres, nanowires, nanotubes and monoliths. 

In the case of soft-template self-assembly synthesis, mesostructure assembly and morphology growth can be controlled concurrently. Due to the versatility of the solvent-based soft-template self-assembly process, highly ordered mesoporous carbons can be produced relatively easily with different morphologies such as thin film and membrane, " monolith,fibre,sphere, rod, single-crystal, " and discus-like crystal. Initially, mesoporous carbons synthesised with soft templates were exclusively in the form of films. " ... 

Guo XZ, Li WY, Nakanishi K, Kanamori K, Zhu Y, Yang H (2013) Preparation of muUite monoliths with well-defined macropores and mesostructured skeletons via the sol-gel pocess accompanied by phase separation. J Eur Ceram Soc 33 1967-1974... 

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