Mesostructure orientation

The resulting mesoporous layers don t usually exhibit extraporosity at a larger scale, but as smaller pores in the oxide walls (Figure 25.24). ° Due to limitations associated with intrinsic mesostructure characteristics, anisotropy resulting from preferential orientations, or boundaries between ordered domains, the tem-plated mesoporosity is usually not directly interconnected. " In such situations, it does directly define the selectivity of the membrane, which depends on the pore size of the oxide walls. However, the resulting hierarchical porosity (templated mesopores and smaller pores of the oxide walls) favors a decrease in layer permeability. The templated mesoporosity can also be used to functionalize the membrane. 

Well ordered mesoporous silicate films were prepared in supercritical carbon dioxide.[218] In the synthesis in aqueous or alcoholic solution, film morphology of preorganized surfactants on substrate cannot be fully prescribed before silica-framework formation, because structure evolution is coincident with precursor condensation. The rapid and efficient preparation of mesostructured metal oxides by the in situ condensation of metal oxides within preformed nonionic surfactants can be done in supercritical CCU- The synthesis procedure is as follows. A copolymer template is prepared by spin-coating from a solution containing a suitable acid catalyst. Upon drying and annealing to induce microphase separation and enhance order, the acid partitions into the hydrophilic domain of the template. The template is then exposed to a solution of metal alkoxide in humidified supercritical C02. The precursor diffuses into the template and condenses selectively within the acidic hydrophilic domain of the copolymer to form the incipient metal oxide network. The templates did not go into the C02 phase because their solubility is very low. The alcohol by-product of alkoxide condensation is extracted rapidly from the film into the C02 phase, which promotes rapid and extensive network condensation. Because the template and the metal oxide network form in discrete steps, it is possible to pattern the template via lithography or to orient the copolymer domains before the formation of the metal oxide network. 

This mechanism has been utilized in casting, spin-coating and dip-coating processes, and a variety of films differing in pore size and pore orientation has been synthesizedJ The principle has been investigated extensively for silica-based systems. Recently, it was shown that mesostructured transition metal oxide based systems can also be synthesized by using the EISA process. 

Electrodeposition or pulsed-laser deposition techniques are additional possibilities for forming thin mesostructured films. These techniques have mainly been applied to non-silica mesostructured films such as niobia or platinum, but also hexagonally oriented mesoporous silica films were prepared by pulsed-laser deposition. ... 

For the inclusion of functional moieties, control of the location and the orientation of the guest species within the host architecture is of considerable importance. The supramolecular templating approach allows the in situ controlled placement of molecules due to the formation of reservoirs of different polarity (the hydrophobic interior of the micelles, the hydrophilic compartment, the inorganic framework) during the synthesis.However, the synthesis conditions must be carefully chosen because the mesostructure formation involves a delicate interplay between micelle formation, condensation of the inorganic framework, and the formation of the film. 

Figure 11.18 Three-level structure of a zeo-lite/sintered metal fiber catalytic bed (a) microstructure of a zeolite film of oriented submicron crystals, (b) mesostructure of...

In similar concept, the particle-mediated crystallization theory was used to explain well the formation of solid-phase nucleation in the reaction system. This theory suggested that some mesoscopic transformation process takes place in the solid phase followed by multiple nucleation growth phenomena. At the initial phase, it reorganizes into mesoscopic crystals followed by an orderly beautifully mesostructure via self-assembly, of well-aligned size and shape. The crystallographic orientation of the particle is equal in all directions so that the mesoscopic crystals can be reorganized in such a way under the processes of self-assembly. On the basis of the above explanation. Scheme 3.1 represents nucleation mechanism. 

Features of liquid crystallinity (both lyo- and thermotropic) are also to be found in exclusively inorganic systems. For example, metal carbonates can be crystallized in gels to form dense pm-sized aggregates of many crystallites that are themselves orientationally ordered, and devoid of any translational ordering. So, while the material is (micro)crystalline, its mesostructure on the pm scale deserves to be recognized as liquid crystalline. 

Figure 24-2. ID X-ray diffraction patterns of different as-synthesized (bottom) and calcined (top) silica films approximately 200-500 nm in thickness with (a) lameUar, (b) hexagonal, and (c) cubic mesostructural ordering. The diffraction patterns are indexed according to the different symmetries (lamellar, 2D hexagonal p6mm, and body-centered-cubic ImSm). The absence of, e.g., the (1 1 0) reflection in the hexagonal film is due to preferential orientational ordering of the cylindrical mesostructures parallel to the substrate. (Reprinted with permission from Alberius (2002) 2002 American Chemical Society)...

---