Silica template specificity

The silicalite-alumina membrane was prepared after adding a solution containing the silicalite precursor (i e silica + template) to the above-mentioned porous tube (hereafter called support) and a specific hydrothermal treatment performed [8], under the chosen conditions no material is formed in the absence of the porous support. The tube is then calcined at 673 K for removing the template. 

Table 2. Characteristics of the silica templates and the corresponding carbon materials a unit cell parameter Sbet- specific surface area Vp total pore volume (at P/Po=0.95) Pore size determined according to the BJH method - Maximum value of the BJH pore size distribution peak calculated from the adsorption branch of the N2 isotherm.

This chapter deals with the selective preparation, TEM/EXAFS/XPS characterization and catalysis of mono- and bimetallic nanowires and nanoparticles highly ordered in silica FSM-16, organosilica HMM-1 and mesoporous silica thin films. The mechanism of nanowire formation is discussed with the specific surface-mediated reactions of metal precursors in the restraint of nanoscale void space of mesoporous silica templates. The unique catalytic performances of nanowires and particles occluded in mesoporous cavities are also reviewed in terms of their shape and size dependency in catalysis as well as their unique electronic and magnetic properties for the device application. 

The synthesis of OMC involves the use of ordered mesoporous silica (OMS) template with a specific pore topology [7]. As illustrated in Figure 3.1, the appropriate carbon precursor (carbon sources such as sucrose, furfuryl alcohol, acetylene gas, pyrrole, and acrylonitrile) is fed into the pores of the template via the infiltration approach, followed by its carbonization to achieve the siUca-carbon composite and template removal in ethanol-water solution of HF or NaOH to obtain the mesoporous carbon replica. The structure of the as-obtained OMC strongly depends on the structure of the used template. Chang et al. [7] have reviewed the synthesis of OMC as support materials for fuel cell applications. The rod- and tube-type mesoporous carbon structures can be realized by filling carbon precursors in the template pores and coating carbon precursors as a thin film on the pore walls of the template, respectively. In order to get the well-defined structure of OMC, the template should have three-dimensional interconnected pore structure. On the other hand, the carbonization of the carbon precursors should be confined exclusively within the mesopores of the ordered mesoporous silica templates with sufficient carbon precursor filling therefore, before the pyrolysis process, the carbon soiu-ce should be converted to a cross-linked polymer induced by the use of the acid polymerization catalysts [5,7]. 

Poly-L-Lysine Poly-L-lysine (PLL) has been implicated in silica formation because of its abiUty to adopt an a-helix, 3-sheet or random coil conformation. PLL is an excellent silica template due to its ability to create different sihca morphologies by simply changing the reaction conditions to afford a specific secondary structure. Hawkins and coworkers have shown that silica pore sizes could be modified by changing the secondary structure of the polyamine [82]. For example, silica composites that are formed by a-helix PLL under basic conditions (pH 11.2) produced 1.5 run pore sizes, whereas silica formed using PLL P-sheets (heated to -52 °C) resulted in larger pore sizes (-1.5-8nm). In both cases, silica formation was dependent on the PLL concentration and reaction conditions. 

The second case study. This involves all silica micro- and mesoporous SBA-15 materials. SBA-15 materials are prepared using triblock copolymers as structure-directing templates. Typically, calcined SBA-15 displays pore sizes between 50 and 90 A and specific surface areas of 600-700 m g with pore volumes of 0.8-1.2cm g h Application of the Fenton concept to mesoporous materials looks simpler since mass transfer would be much less limited. However, it is not straightforward because hydrolysis can take place in the aqueous phase. 

Pr4N+ cations were recognized to form complexes with silica-te (46) or aluminosilicate (23) species and subsequently to cause replication of the so formed framework structure via stereo-specific interactions (template effect). During this process, they are progressively incorporated and stabilized within the zeolite framework (1 1,47,48). Rollmann (37) has shown that initial Pr +/... 

Si02 gel DDAB/toluene/water (48.7% DDAB, 19.5% decane, 31.8% aqueous silica sol) TMOS (partially hydroiyzed)/H20 (+ 0.4-10 wt%HF) Bicontinuous p,Es used as templates for microporous silica gels monodisperse pores (2 nm pore radius) large specific surface area (—103 m2/g) (51)... 

A novel synthesis route was developed to produce spherical silica particles. The synthesis is based on a modified Stoeber method and the room-temperature synthesis of MCM 41S-materials applying tetraethoxysilane, alcohol, water, ammonia and homopolymers as template. The specific surface area, the specific pore volume and the average pore diameter were varied in the following ranges 5 - 1,000 m2/g 0.1 — 1.0 cm3/g and 2-50 nm. With respect to catalytic applications hetero-atoms e.g. Al were incorporated into the silica framework. 

Niobium and titanium incorporation in a molecular sieve can be achieved either by hydrothermal synthesis (direct synthesis) or by post-synthesis modification (secondary synthesis). The grafting method has shown promise for developing active oxidation catalyst in a simple and convenient way. Recently, the grafting of metallocene complexes onto mesoporous silica has been reported as alternate route to the synthesis of an active epoxidation catalyst [21]. Further the control of active sites, the specific removal of organic material (template or surfactant) occluded within mesoporous molecular sieves during synthesis can also be important and useful to develop an active epoxidation catalyst. Thermal method is quite often used to eliminate organic species from porous materials. However, several techniques such as supercritical fluid extraction (SFE) and plasma [22], ozone treatment [23], ion exchange [24-26] are also reported. 

The mechanical properties of Micelle-Templated Silicas (MTS) are very sensitive items for industrial process applications which might submit catalysts or adsorbents to relevant pressure levels, either in the shaping of the solid or in the working conditions of catalysis or separation vessels. First studies about compression of these highly porous materials have shown a very low stability against pressure. These results concern these specific materials tested. In this study, we show very stable MTS with only a loss of 25% of the pore volume at 3 kbar. The effects of several synthesis parameters on the mechanical strength are discussed. 

A great many functionalized styrenes, including carboxylic acids, amino acids. Sehtff bases, or specific compounds. e.g t.-DOPA. have successfully been applied as print templates. Moreover, it has also been shown that silica gel can be imprinted with similar templates, and that (lie resulting gel has specific recognition sites determined by the prim molecule. 

Of special interest is the eventuality of stabilizing transition states by imprinting their features into cavities or adsorption sites using stable transition state analogs as templates. Studies towards such TSA footprint catalysis have been performed by generating TSA complementary sites as marks on the surface [7.73a] or as cavities in the bulk [7.73b] of silica gel. These imprinted catalytic sites showed pronounced substrate specificity [7.74a,b] (namely in the case of cavities [7.73 b]) and chiral selectivity [7.74c,d]. 

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