Applications structure-directing agents

Zeolite synthesis around a metal complex was introduced by Balkus [40]. The metal complex is added to the zeolite synthesis mixture and is incorporated into the zeolite structure during the zeolite synthesis. Of course, this procedure is only applicable when the metal complex is soluble in the synthesis mixture and can withstand the hydrothermal synthesis conditions. Another requirement is that the zeolite structure-directing agent added must be removable by a milder... 

Many applications, especially in electrochemistry, rely on the use of expensive and rare metals, like Pt, Li, and rare-earth elements. In the synthesis of micro- and me-soporous materials, costly structure-directing agents are sometimes applied. The reduction of catalyst mass and the prevention of waste formation, for instance by recycling of synthesis additives, are therefore highly topical research issues. The practicability of future technologies based on catalysis will depend on the availability of efficient catalysts composed of abundant elements prepared by robust, preferentially aqueous-based synthesis methods and the reduction of environmental impacts arising from catalyst manufacture. 

L. Gomez-Hortiguela, F. Cora, C. R. A. Catlow, T. Blasco and J. Pariez-Pariente, Fluorinated dibenzyldimethylammonium as structure directing agent for the synthesis of ALPO and SAPO-5. Effect of fluorine, in Studies in Surface Science Molecular Sieves from basic research to industrial applications. A and B, 2005, pp. 327-334. 

Zeolites have micropores with ordered size and solid acidity, and there has been growing interest in the application of zeolites as catalysts as well as molecular sieves. We investigated the synthesis of novel zeolites by using ammonium salts as structure-directing agents (SDAs) [1, 2]. 

Since the phosphate-based, extra-large pore materials do not reveal stability that is likely sufficient for many commercial applications, I will limit my discussions below to silica-based materials. As I have discussed elsewhere, the lack of stability in phosphate-based materials may not be due to the presence of extra-large rings, but rather to the nature of the structural units [9], With silica-based, extra-large pore materials, it appears that bulky structure directing agents will be required for their preparation. I will make this assumption in further discussions below. 

For many applications, e.g. in the area of sensing, catalysis or separations, modifications of the inorganic backbone are required to provide a certain specific surface chemistry or active sites on the inner pore surface. Different routes, such as simple inclusion of molecules, or the covalent attachment of functional entities, are applied. Because of the specific synthesis strategy, the structure-directing agent can also be used for functionalization of the mesostructured material. Only in situ modifications are discussed postsynthesis modifications of the final porous material are, in principle, possible, but are beyond the scope of this article. 

The application of a 11-ferrocenylundecyl-ammonium bromide/hexa-decylammonium bromide surfactant mixture as structure-directing agent resulted in a lamellar mesostructured silica film, which showed electronic conductivity due to electron transport in the ferrocenyl chains. Lyotropic lithium triflate-silicate liquid crystals have been utilized as supramolecular templates in the synthesis of ionically conducting nanocomposite films. ... 

In another application, the work was focused on crystalline titania with ordered nanodimensional porous structures [368]. In this case, the mesoporous spherical aggregates of anatase nanocrystal were fabricated and cetyltrimethylammonium bromide (CTAB) was employed as the structure-directing agent. The interaction between cyclohexane microdroplets and cetyltrimethylammonium bromide self-assemblies was apphed to photodegrade a variety of organic dye pollutants in aqueous media such as methyl orange (see Table 6). 

PbS hollow nanospheres with diameters of 80-250 nm have been synthesized by a surfactant-assisted sonochemical route from Pb(CH3COO)2, thioacetamide, and sodium dodecylbenzenesulfonate [3]. 200-400 nm hollow silica spheres with mesoporous walls were prepared by the application of ultrasound to a mixture of nonionic polyoxyethylene surfactant and tetraethylorthosilicate. The presence of surface-active agents during ultrasonic synthesis was proved to be effective because surfactants can act as soft templates as well as structure directing agents for the assembly and subsequent... 

In this case surfactant micelles or liquid crystal templating were employed, which means that large assemblies of organic molecules were directing the formation of the final structure instead of individual molecules or cations. The initial application of cationic surfactants by Mobil like hexadecyltri-methylammonium bromide or hydroxide has been extended to neutral or even cationic structure-directing agents. Thus, in principle three synthetic procedures exist for preparation of mesoporous molecular sieves ... 

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