Director structure

In the presence of minor amounts of Pr4N+, poorly crystalline ZSM-5 is obtained (6) while in complete absence of template, only limited amounts of ZSM-5 appear after several days of crystallization (7, 8). In that case, hydrated sodium ions constitute a poor replacement as template or pore filling agents for Pr4N+ (8). The crystallization rate of such systems can be considerably increased by addition of amines that then primarily act as pore fillers (5, 9, 10) but not necessarily as specific structure directors during nucleation (5). 

The present paper provides an overview of the physical properties of silica mesostructures with representative wormhole and lamellar framework structures assembled through S°P pathways. The wormhole framework silica, denoted HMS silica, was assembled using an alkylamine surfactant as the structure director and a silicon alkoxide as the inorganic precursor. The lamellar framework silica with a vesicular hierarchical structure, denoted MSU-G silica, was obtained from tetraethylorthosilicate (TEOS) as the silica precursor and a bi-functional gemini amine surfactant of the type RNH(CH2)2NH2. We then provide examples of the catalytic activity of these disordered mesostructures in comparison to more ordered framework mesostructures such as hexagonal MCM-41. 

There are some other methods that might be envisioned that could lead to OMS and OL materials. One obvious direction would be to use structure directors or templates that are similar to those used in zeolite synthesi such as tetraalkylammonium halides. Unfortunately, we have observed that such structure directors and templates react with KMn04 and get oxidized to C02- Another seemingly obvious route would be electrochemical syntheses. Some research has been done in this area, however, it is difficult to synthesize a sizeable amount of material such as with controlled potential electrolysis. In addition, some early work showed the generation of amorphous materials that after inital formation can be heated to form spinel phases without apparently going through the OMS/OL phases. 

M. Tiemann and M. Froba, Mesostructured Aluminophosphates Synthesized with Supra-molecular Structure Directors. Chem. Mater., 2001, 13, 3211-3217. 

Structure directing agent, a blend of the BCP with chemical precursors of functional materials, rel3ung on microphase separation as an in situ structure director. 

Using functional molecules as structural directors in the chemical polymerization bath can also produce polyaniline nanostructures. Such structural directors include surfactants [16-18], liquid crystals [19], polyelectrolytes (including DNA) [20,21], or complex bulky dopants [22-24]. It is believed that functional molecules can promote the formation of nanostructured soft condensed phase materials (e.g., micelles and emulsions) that can serve as soft templates for aniline polymerization (Figure 7.3). Polyelectrolytes such as polyacrylic acid, polystyrenesulfonic acid, and DNA can bind aniline monomer molecules, which can be polymerized in situ forming polyaniline nanowires along the polyelectrolyte molecules. Compared to templated syntheses, self-assembly routes are more scalable but they rely on the structural director molecules. It is also difficult to make nanostructures with small diameters (e.g., <50 nm). For example, in the dopant induced self-assembly route, very complex dopants with bulky side groups are needed to obtain nanotubes with diameters smaller than 100 nm, such as sulfonated naphthalene derivatives [23-25], fidlerenes [26], or dendrimers [27,28]. 

Studies of the solution ordering of guanosine -2 -monophosphate dianlons, with alkali metal ion as structure directors, support the stacked tetramer model for nucleoside ordering. X-ray structures of nucleosides are mentioned in Chapter 22. 

This theory is based on the modernistic perspective and views an organization s optimal division structure, directors, boar members, production strategies, links to external organizations, etc. The main focus is on the acquisition of external resources and the consequences of this (Pfeffer Salancik 1978). The theory can be characterized as a modemistic-oriented (early) version of the agent theory. 

Further sensitivity of the gel towards fluoride as a structure director has also been observed in another non-aqueous synthesis system. In the presence of dimethylformamide in triethyleneglycol the gismondine topology is formed in... 

The (S+I ) route In this case, cationic surfactants (S+) are used as structure directors for anionic inorganic species (I ). This is recognized as the pathway for the syntheses of M41S mesoporous materials. 

This classification is very useful especially for other types of surfactant-inorganic interactions such as those involving neutral surfactants as structure directors (templates). Such interactions can be denoted as follows ... 

Synthesis may also be carried out at a lower pH using fluoride-containing media, wherein F ions are thought to act as structure directors via strong interactions with framework Si atoms. Consequently, the nucleation rate is decreased, which yields larger crystals relative to standard alkaline hydrothermal routes.The fluoride route under neutral/acidic pH conditions is also extremely useful to synthesize zeolite-like materials called zeotypes, which contain elements other than silicon and aluminum (e.g., titanosilicates, zirconosilicates, etc.). Under alkaline conditions, the precursors would be preferentially precipitated as hydroxide species rather than ordered arrays. 

This procedure is generally used to prepare zeolites. A gel is formed by hydro-thermal treatment (<200 °C) of an aqueous solution of NaOH, NaAl(OH)4, and Na2Si03 (a structure-director agent (SDA) can be added). Depending on the reactants nature and ratio, the SDA used, and the specific experimental conditions (temperature, pH, time, etc.), different structures can be formed. 

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