Surfactant-templated mesoporous silicates

The silicate network can also act as an efficient size exclusion membrane. The pore size distribution can be controlled, at least to some extent, by incorporation of water soluble polymers such as poly(ethyleneglycol) or by pH changes (Brinker, 1989). Carefully tailored silicate membranes, using low pH synthesis even exhibit Knudsen flow, facilitating the separation of air components (Brinker, 1993). Incorporation of zeolites, surfactants, templated mesoporous silicates, and leaving template materials can be used to generate tailored size excluding films. 

Solvent extraction is the most important technique for recovering surfactants from mesoporous materials. However, it is not very effective when applied to microporous compounds. Davis et al. [186] successfully extracted borosilicate and silicate BEA stractures with acetic acid while a small template fraction could be removed for the aluminosilicate. 

Zeolitic structures with pore sizes of 2000 to 10000 pm are known as mesoporous solids, and can be formed by a method known as liquid crystal templating (LCT). The combination of a suitable cationic surfactant together with silicate anions form arrays of rod-like surfactant micelles (Figure 3.7) surrounded by a polymeric siliceous framework. On calcination the mesoporous structure is formed. 

The general consensus is that mesoporous silica materials synthesized using surfactant templates can form by two distinct yet related reaction pathways. The first involves the hydrolytic poly-condensation of a silicatropic mesophase while the second works through co-assembly of silicate and surfactant micellar building blocks. For ionic surfactants, cooperative-assembly with silicate precursors is the result of charge density and geometry matching and multi-dentate... 

Several conclusions may be drawn from these data. (1) Weakly ordered mesoporous structures with high surface areas were obtained after the surfactant template was removed from TEOS-treated phosphomolybdate salts by solvent extraction. (2) IR and XRD data confirm that the silicate was indeed incorporated in the salt structures and changed these structures. Finally the Mo-O-Si bond was formed. (3) In contrast to the formation of the Mo-O-Si, the Mo-O-Ti bond is difficult to form in the same reaction conditions. So we only obtained a silica-containing mesoporous PMA. A significant breakthrough may be the appearance of good catalytic activity with the synthesis of the porous phosphomolybdic acid. The applications of the material in catalytic reaction are also being studied in our future work. 

Two-dimensional DQ H MAS-NMR spectra were used to investigate the local structure of a surfactant-templated silicate thin film.498 Silicalite-1 powders (20-1000 nm. crystals) were characterised by 29Si MAS-NMR spectroscopy.499 13C and 29Si solid-state NMR data were used to characterise organic-inorganic mesoporous silica-based materials.500... 

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. 

I. Honma, H.S. Zhou, D. Kundu, and A. Endo, Structural Control of Surfactant-templated Hexagonal Cubic, and Lamellar Mesoporous Silicate Thin Films Prepared by Spin-casting. Adv. Mater., 2000, 12, 1529-1533. 

The surfactant templated synthesis of mesoporous ceramics was first reported in 1992 [1], and since that time there has been a veritable explosion in the number of papers in the area. There has been particular interest in the functionalization of mesoporous ceramics [2-S]. As outlined in Figure 1, the original synthesis employed rod-shaped micelles composed of cationic surfactant molecules as the pore template (more recently, this methodology has been extended to a wide variety of other surfactant systems and reaction conditions). When exposed to routine sol-gel conditions, the cationic micelles undergo an anionic metathesis with silicate anions, resulting in a glass-coated log which... 

The preparation of mesoporous surfactant-templated silicates is in principle straightforward, requiring a silica source, a catalyst for silica polymerisation, the template and a solvent. The templating reaction can be done under acidic, alkaline or neutral conditions. The solvent for these syntheses is usually water, although some work in solvent mixtures has been reported, as well as a few syntheses in nonaqueous sol-... 

EISA. The mechanism in each case has been the subject of much research and the current understanding of each mechanism is described below. The other two methods to produce mesoporous templated materials are electrodeposition,which has been successfully used to produce surfactant templated porous metal films from high concentration surfactant solutions, and nanocasting, where a surfactant templated silicate is used as a sacrificial template to generate further porous materials by coating the silica structure in another oxide or carbon precursors. The second material is sintered or solidified, and the silicate removed by HE or high pH solvent wash. This is discussed further in Section 2.10. 

Tilley s group recently showed that this method of surfactant templating can be extended to high-temperature pyrolysis of organometallic silicates to produce mesoporous, high-surface-area, metal-silicon oxides with narrow pore size maxima. ... 

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