Synthesis of Ordered Mesoporous Solids

Mesostructured organic-silica structures are formed with surfactant concentrations above the critical micelle concentration (CMC), indicating the importance of this first self-assembly step. The thermodynamically favoured shape of the micelles in this dilute regime (without inter-micelle interactions or presence of silicate ions) has been shown by 

Israelachvili to depend on the value of a dimensionless packing parameter, g, 

For example, whereas low area headgroup surfactants (high g values, g= 1) favour lamellar liquid crystal type phases or cubic phases with low curvature, such as the Ia-3d bicontinuous gyroidal structure, surfactants with higher area headgroups with lower g values favour strongly curved micellar shapes, such as cylindrical rods (MCM-41) or spheres (e.g. SBA-1, SBA-2). 

This bond-forming event may overlap with the hydrolysis step or may occur in a second, separate step. If the silicate-surfactant assembly is able to form under equilibrium conditions before the onset of crystallisation, very well-ordered solids can result, even showing crystalline morphologies. If the two steps overlap, the products are less likely to show long-range order. During condensation, the amount of Q silica increases and the silica becomes solid. 

2 Mechanisms of Silicate Mesophase Formation from Aqneons Solution 

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Recently Cabrera et al. [47] developed a new synthesis of ordered mesoporous solids involving the use of atrane complexes as precursors. This technique was tested to obtain silica, Al and Ti mesoporous oxides and simply and doubly doped oxides with a large list of metals [see Table 3 in reference 47]. Other dopants or chemical groups were added to MCM type materials. For... 

By contrast, the mechanisms of the synthesis of ordered mesoporous solids are very dilferent from those of silicate and phosphate frameworks. For control over the synthesis of mesoporous solids, the key is to understand the interactions of micellar surfactants with condensable inorganic framework-building units. Synthetic routes are also being developed to prepare mesoporous silicates made up of nanoparticles of zeolites, with the aim of combining advantages of microporous and mesoporous solids. 

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