Mesoporous silica structures, templating

C.Z. Yu, Y.H. Yu, L. Miao, and D.Y. Zhao, Highly Ordered Mesoporous Silica Structures Templated by Poly(butylene oxide) Segment Di- and Tri-block Copolymers. Microporous Mesoporous Mater., 2001, 44, 65-72. 

P-07 - A Study on the mesoporous silica structures templated by triblock copolymers... 

Yu C., Yu Y. and Zhao D., Highly ordered large caged cubic mesoporous silica structures templated by triblock PEO-PBO-PEO copolymer, Chem. Commun. (2000) pp. 575-576. 

Davidson (2002) Mesoporous silica Structure evolution, role of templates crystallization variables + + n.a. Synthesis of mesoporous silica with defined properties... 

Fig. 27.5. Mesoporous silica nanoparticles as novel drug delivery systems, (a) Cocondensation method to form functionalized mesoporous silica structures in a surfactant template synthesis, (b) TEM image of mesoporous silica nanoparticles and sketch of a novel drug delivery particle which contains functionalized pores, closed by a gate, and is decorated with ligands for cell targeting, (c) Cell targeting by ligand-receptor interaction at the cell membrane, endosomal uptake and controlled release after pH change from early to late endosome...

Ordered mesoporous silica seems to be an ideal hard template, which can be used as a mold for other mesostructures with various compositions, such as ordered mesoporous carbon and metal oxides. Mesoporous silicas with various different structures are available, and silica is relatively easily dissolved in HF or NaOH. Alternatively, mesoporous carbons with a solid skeleton structure are also suitable choices as hard templates due to their excellent structural stability on thermal or hydrothermal and chemical treatment. A pronounced advantage of carbon is the fact that it is much easier to remove than silica by simple combustion. The nanocasting synthesis of mesoporous carbon by using mesoporous silica as template will be discussed in detail in the section on mesoporous carbon. In many cases, silica is unsuitable for synthesizing framework compositions other than carbon, since the leaching of the silica typically affects the material which is filled into the silica pore system. 

Zeolites were already employed as templates in the synthesis of microporous carbon with ordered structures.[247] The discovery of ordered mesoporous silica materials opened new opportunities in the synthesis of periodic carbon structures using the templating approach. By employing mesoporous silica structures as hard templates, ordered mesoporous carbon replicas have been synthesized from a nanocasting strategy. The synthesis is quite tedious and involves two main steps (i) Preparation and calcination of the silica mesophase, and (ii) filling the silica pore system by a carbon precursor, followed by the carbonization and selective removal of the silica framework. 

A major difference between the two first synthes of ordered mesoporous carbons using MCM-48 mesoporous silica as template [106,107] was in the nature of the precursor. Thus, Hyeon and coworkers used phenol-fomialdehyde whereas Ryoo and coworkers used sucrose, which was impregnated twice in the presence of sulphuric acid. Carbonization and removal of the silica template led the latter team to obtain the so-called CMK-1 carbon [113], whose periodic ordered structure was ascertained by XRD and by TEM [Fig. 34a)]. CMK-1 has a high BET surface area (1200-1800 m g" ) and uniform mesopores about 3 nm in diameter. 

CMK-2 [114] is an ordered mesoporous carbon obtained from sucrose as a source of carbon and SBA-1 silica as template [Fig. 34b)]. Electron diffraction showed that this carbon is composed of c s iirtercoimected with two different types of pores (meso- and micropores). CMK-3 [115] is an ordered mesoporous carbon that was synthesized using SBA-15 mesoporous silica as template and sucrose as carbon source. The structure of CMK-3 was the faithful replica of the mesoporous silica template, as revealed by XRD and TEM. This carbon had a hi BET surface area (1500 n g % and a pore size around 4.5 nm. The systematic control of pore wall thickness of hexagonal mesoporous silica templates (by varying the ratio of surfactants CwTAB and CmEOg, Fig. 35) afforded a close control of the... 

The synthesis of OMC involves the use of ordered mesoporous silica (OMS) template with a specific pore topology [7]. As illustrated in Figure 3.1, the appropriate carbon precursor (carbon sources such as sucrose, furfuryl alcohol, acetylene gas, pyrrole, and acrylonitrile) is fed into the pores of the template via the infiltration approach, followed by its carbonization to achieve the siUca-carbon composite and template removal in ethanol-water solution of HF or NaOH to obtain the mesoporous carbon replica. The structure of the as-obtained OMC strongly depends on the structure of the used template. Chang et al. [7] have reviewed the synthesis of OMC as support materials for fuel cell applications. The rod- and tube-type mesoporous carbon structures can be realized by filling carbon precursors in the template pores and coating carbon precursors as a thin film on the pore walls of the template, respectively. In order to get the well-defined structure of OMC, the template should have three-dimensional interconnected pore structure. On the other hand, the carbonization of the carbon precursors should be confined exclusively within the mesopores of the ordered mesoporous silica templates with sufficient carbon precursor filling therefore, before the pyrolysis process, the carbon soiu-ce should be converted to a cross-linked polymer induced by the use of the acid polymerization catalysts [5,7]. 

Two types of organized mesoporous silica, MCM-48 and SBA-15, which display different crystallographic structure, size and shape of the pores, were selected as templates for carbons. 

Sol-gel processing forms the basis for various routes employed for the fabrication of a wide diversity of functional materials. To impart a structural organization at various length scales, the syntheses are performed using templates. Most consist of a self-organized ensemble of surfactants and co-polymers [1-10]. They have been successfully applied to control the geometry and dimensions of pores that are periodically arranged as in the initial structures. Mesoporous silica materials of the MCM family, which were first synthesized by a team from the Mobil oil company [11,12], are a well-known example. 

In either the transcriptive or synergistic strategy, removal of the organic template by extraction or calcination renders the inorganic mesoporous structure. For synthetic schemes that are not compatible with the formation of stable template assemblies, an alternative approach is to use a preformed, templated inorganic host, such as mesoporous silica, as a mold to nanocast the desired material as an inverse replica of the host, such as that seen in Figure 14. ... 

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