Silica frameworks

The shape-controlled nanoparticles were incorporated into the silica framework by NE by the procedure outlined in Scheme 2. These catalysts are denoted as Pt(X )/SBA-15-NE where X = cubes, cuboctahedra, and octahedra. 

This study has shown that typical coating biocides can be encapsulated within modified silica frameworks. These porous frameworks offer a means to inhibit the aqueous extraction of the biocide. In such combinations the biocides retain their anti-microbial properties, while controlled delivery facilitates a dynamic equilibrium to maintain a minimum inhibitory concentration at the coating interface, over an extended time period. There is evidence that biocide housed in such frameworks has a longer effective activity for a given initial concentration, since it is to some extent protected from the usual environmental degradation processes. 

Morphosynthesis of macroporous silica frameworks in bicontinuous microemulsions. Advanced Materials, 10, 151—154. 

Negative Charge Centers in High-Silica Frameworks. 201... 

MAS) method (to record 29 Si signals) provides complementary information on the chemical bonding (Si-O-Si bond angles), aluminum distribution in the silica framework and secondary phases. 

There is also rapid development in the domain of standard silica-based zeolites. Their versatility can be extended by imprinting. For instance, Davis and Katz [15] recently successfully carried out imprinting and obtained a silica framework with pore walls anchoring three aminopropyl groups in cavities. Another achievement was reported by Ramamurthy, Schefer and coworkers [16]. The latter authors were able to obtain 90% diastereomeric excess of a product of the photochemical reaction in a commercially available zeolite containing chiral tropolone ether 433 in its pores. 

The effective area of the anionic aluminosilicate framework in the pores of a zeolite is at least 100 times the external surface area, and it can be as high as 1000 m2 g-1. Consequently zeolites are unusually effective as catalysts for reactions that are favored by aluminosilicate surfaces. Substitution of Sj4+ Al3+ in a silica framework makes it acidic and, potentially, coordinatively unsaturated. Suppose, for example, that we heat the NH4+ form of a zeolite. Ammonia is driven off, and one H+ remains to counterbalance each Al3+ that has substituted for a silicon. The protons are attached to oxygens of the aluminosilicate framework ... 

Adsorption isotherms of methane in silicalite have also been predicted in a number of calculation studies (62, 155, 156). Goodbody et al. (62) predicted a heat of adsorption of 18 kJ/mol and simulated the adsorption isotherm up to 650 bar. From the adsorption isotherm, they found that the sinusoidal pore volume contains more methane molecules at all pressures. Snurr et al. (155) performed GC-MC and MD simulations over a wide range of occupancies at several temperatures. The intermolecular zeolite-methane potential parameters were taken from previous MD studies (11, 87) and the methane-methane parameters from MD simulations were adjusted to fit experimental results for liquid methane (157). Electrostatic contributions were neglected on account of the all-silica framework, and methane was represented by a rigid, five-center model. 

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. 

For catalytic application it is necessary to incorporate hetero-atoms into the silica framework. Several samples have been synthesised using different aluminia precursors. The metal content was determined by X-ray fluorescence analysis, UV-VIS spectra, IR spectra and solid state NMR spectroscopy, respectively. X-ray fluorescence analysis provides information about the metal content of the samples. By variation of the metallic precursor concentration the metal content of the product could be enhanced up to 10 % w/w. 

To further characterize the effect of the ammonia hydrothermal treatment, we compared elemental analysis data and 1R spectra before and after ammonia hydrothermal treatment to quantitatively disclose the role of counterion between the silica framework and surfactants. In Table 2, the N/C molar ratio of the mesoporous materials prior to the ammonia hydrothermal treatment is nearly twice of that after the treatment. Moreover, the IR band at 1383 cm 1, which arises from the N03 stretch bending mode, completely disappears after ammonia hydrothermal treatment [20], These results verify that the existence of nitrate counterion (the nitrate/surfactant 1) between surfactant molecules and silica framework in the acid-made mesoporous materials. The bridging counterion N03 was completely removed after ammonia hydrothermal treatment. 

Fig 1A shows the XRD pattern of the calcined MCM-41 sample synthesized from C,6TMAB-silica and aluminosilicate systems with the Si/Al = oo(sample I) and 37(sample II) by using the delayed neutralization process. In both materials, there exist at least 4 sharp XRD peaks, which indicate well-ordered hexagonal structure of MCM-41. It means that the incorporation of aluminum into silica framework could not have significant effect on the arrangement of MCM-41 mesostructure. When the nitrogen adsorption-desorption isotherms... 

As shown in Figure 2, the infrared vibration bands of sample A (1236, 1090, 965, 800, 564 and 465 cm-1) are close to those of sample B (1234, 1090, 964, 799, 568 and 465 cm-1). They show that the internal local structures of both hexagonal silica frameworks are almost identical. With the transformation of regular hexagonal structure into amorphous phase (amorphous-1), all of network vibration bands are shifted to... 

DTA is used in this study to understand the endothermic and exothermic phenomena resulting from desorption, decomposition and combustion of water and surfactant molecules occluded in the framework of samples A and B. Both samples have these common DTA features (Figure 3) an endothermic peak below 100 °C (apparently due to the evaporation of physically adsorbed water), an endothermic peak below 300 °C (attributed to the removal of lattice water and the decomposition of surfactant molecules), and a strong exothermic peak at around 335 °C (attributed to the combustion of surfactant molecules in air). The DTA results distinctly show that the surfactant molecules are occluded in almost identical positions within silica framework of samples A and B. 

The introduction of HF reduces the pH value of the final suspension and increases its viscosity this influences further the polymerization mode of silicate species, leading to the formation of regular hexagonal silica framework. The experimental results reveal that, for F7Si molar ratio of 0.20-0.39, the pH value of the suspension changes from 12.5 to 9.5 and the XRD patterns of the synthesized materials display distinct four-peak profiles. 

When the synthesis is carried out at room temperature for 3 days, a solid containing only disordered mesoporous phase with a BET surface area of over 1000 m2/g is obtained. Its XRD pattern displays only a single [100] peak at low 20 angle. But if the crystallization time is prolonged to about 22 days, [100], [110] and [200] peaks except the absence of [210] peak can be distinctly resolved. This result shows that the synthesis time is important in transforming the ill-defined disordered mesoporous silica framework into integrated hexagonal MCM-41 framework. 

IR spectroscopy also indicated qualitatively that organosiloxanes were incorporated into the silica framework. The appearance of peaks at 3050 cm1 and 1430 cm1 for Ti-MCM-41-Ph is due to the presence of phenyl groups (data not shown) while a peak at 2987 cm1 for Ti-MCM-41-Me (data not shown) corresponded the presence of aliphatic C-H bonds. 

The contents of the current volume presents a sampling of more than 150 oral and poster papers delivered at the Symposium on Access in Nanoporous Materials II held in Banff, Alberta on May 25-28, 2000. The selected papers cover the three main themes of the symposium (i) synthesis of mesoporous silicas, framework-modified mesoporous silicas, and surface-modified mesoporous silicas, (ii) synthesis of other nanoporous and nanostructured materials, and (iii) characterization and applications of nanoporous materials. About 70% of the papers are devoted to the synthesis of siliceous mesoporous molecular sieves, their modification, characterization and applications, which represent the current research trend in nanoporous materials. The remaining contributions provide some indications on the future developments in the area of non-siliceous molecular sieves and related materials. Although the present book does not cover all topics in the area of nanoporous materials, it reflects the current trends and advances in this area, which will certainly attract the attention of materials chemists in the 21st Century. 

Gastuche (1963) reviewed the recent work she and others have done on the synthesis of the octahedral sheet. It is generally thought that the octahedral sheet acts as a template on which the silica framework somehow develops. Al, Mg, and Fe octahedral hydroxides are readily formed and fairly well understood. A number of mixed or double hydroxides (Al-Mg, Al-Fe, etc.) have been formed (Feitchnecht, 1962) but relatively little work has been done on them. 

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