Amorphous solids silica

According to these data, the heat of formation of quartz from amorphous solid silica is AH = -4.69 kcal mole", which is greater than the value -1.78 found bv W ct... 

Method B After combining 2,4,6-collidine (9mL), IV (10.25 mmol), dibromobenzene (24.2g, 102.5 mmol) and cuprous oxide (2.95g, 20.51 mmol), the reaction mixture was heated at reflux under nitrogen for 24h. When the reaction had cooled to approx. 80°C, the mixture was filtered, then diluted with chloroform. The organic phase was warmed to 60°C with lOOmL cone. HC1, washed with water and then was filtered through a short bed of silica gel. The product, a light amorphous solid, was obtained in 75-90% yield. 

The same principles that are valid for the surface of crystalline substances hold for the surface of amorphous solids. Crystals can be of the purely ionic type, e.g., NaF, or of the purely covalent type, e.g., diamond. Most substances, however, are somewhere in between these extremes [even in lithium fluoride, a slight tendency towards bond formation between cations and anions has been shown by precise determinations of the electron density distribution (/)]. Mostly, amorphous solids are found with predominantly covalent bonds. As with liquids, there is usually some close-range ordering of the atoms similar to the ordering in the corresponding crystalline structures. Obviously, this is caused by the tendency of the atoms to retain their normal electron configuration, such as the sp hybridization of silicon in silica. Here, too, transitions from crystalline to amorphous do occur. The microcrystalline forms of carbon which are structurally descended from graphite are an example. 

Amorphous silica in nature may originate from aquatic organisms, secreted as amorphous solid in the form of shells, plates, or skeletons. Amorphous silica also is found in volcanic ash or in precipitated material from the hot supersaturated waters of hot springs. 

Two alternative explanations have been suggested which are both quite speculative. First, portions of mineral surfaces of intermediate polarity (e.g., siloxane regions, -Si-O-Si-) may permit some exchange of polar water and nonpolar organic sorbates (Hundal et al., 2001). Such surfaces occur in minerals like the faces of aluminosilicates. However, amorphous solids like silica (-Si-OH) and alumina (-A1-OH) have very hydrophilic exteriors when these inorganic materials are suspended in water. Yet these amorphous materials still clearly show sorption of apolar substances (e.g., Mills and Biggar, 1969a Schwarzenbach and Westall, 1981 Estes et al., 1988 Szecsody and Bales, 1989 Farrell et al., 1999). 

O. 12 mL (1.1 Eq) of tri-n-butyltin methoxide (24), and the solution was cooled to 0°C. This was treated with a solution of bromine (1.1 Eq), 2% inCHjClj dropwise, over a period of 1 h. Residual bromine was destroyed with cyclohexene and tile crude product was isolated by precipitation with hexanes to give an amorphous solid (184 mg, 92%). Chromatography on silica gel (hexanes/EtOAc, 2 3) gave pure product, identical with a sample prepared from the antibiotic [a]D - 4.6° (c 0.28, CHClji IR VlMI (KBr) 3460 (OH), 1683 (Cbz) cm"1. 

Into a solution of residue 59 (101 mg, 0.1 mmol) in 20 mL of dry toluene, kept at 60°C, was syringed, during 18 h and under argon, a freshly prepared solution of samarium diiodide in benzene-HMPA (9 1, v/v 6.3 mL, 0.51 mmol) which has been diluted with 3.8 mL of dry benzene. The solvents were distilled off under reduced pressure, and the residue was taken up in 10 mL of diethyl ether. The ether solution was washed with 10% aqueous solution of sodium bisulfite, then water, dried (MgS04), and concentrated. The crude product was dissolved in 1.5 mL of tetrahydrofuran and treated during 30 min at room temperature with 1.5 mL of a 40% aqueous solution of HF. The solution was neutralized with solid sodium carbonate, and concentrated. Flash chromatography on silica gel (cyclohexane-ethyl acetate, 3 1 to 1 2) afforded the product 80 (40.6 mg, 50%), a single isomer, as an amorphous solid. It was characterized by its diacetate [a]D +36° (c 4.0, CHClj). 

Miophane and Imogolite. Allnphanc is an amorphous clay lhal is essentially an amorphous solid solution of silica, alumina, and water. Allophanc has been found most abundanlly in soils and altered volcanic ash. It usually occurs in spherical form but has also been observed in libers. 

The yield of the reaction (Table 1) clearly depends or the nature of the solid and on the experimental conditions (temperature, time). Thus, with silica and alumina, amorphous solids with a relatively low acidity, the acetophenone oxime molecule reacts with a very low yield, being the only reaction product the hydrolysis one, acetophenone. With the synthetic mixed oxide silica-alumina, that possesses simultaneously BrOnsted and Lewis acidic centres, the conversion is quantitative, being also the major product the hydrolysis one (3), when the reaction is carried out at 160°C. 

Ozone was introduced into a soln of 25 (300 mg, 0.66 mmol) in CH2C12 (50 mL) at -78 °C. Upon the appearance of a blue color, 02 was bubbled into the mixture to remove excess 03 followed by the addition of DMS (0.5 mL) and pyridine (0.5 mL). After being stirred for 5 min, the mixture was poured into H20 and extracted with CH2C12 (2 x 50 mL). The combined extracts were dried (MgS04) and filtered. The solvent was removed from the filtrate and the crude product was purified by chromatography (silica gel, EtOAc/hexane 55 45) to give an amorphous solid yield 160mg (53%) HRMS calcd for [M + H]+, 455.2182 found, 455.2162. 

The acid catalysts first used in catalytic cracking were amorphous solids composed of approximately 87% silica (Si02) and 13% alumina (A1203) and were... 

Glass is an amorphous solid made mostly of silica molecules (Si02), the main ingredient in sand. Glass has long been used to construct beverage and food containers because it is sturdy, transparent, and fairly easy to make. 

To a stirred solution of ethyl l-(NG-nitro-L-arginyl)-4-methyl-2-piperidinecarboxylate hydrochloride in 200 ml of chloroform were added in turn 18.5 g of triethylamine, and 14.7 g of 3-methyl-8-quinolinesulfonyl chloride at 5°C, and stirring was continued for 3 hours at room temperature. At the end of this period, the solution was washed twice with 50 ml of water. The chloroform solution was dried over anhydrous sodium sulfate. Upon evaporation of the solvent, the residue was chromatographed on 50 g of silica gel packed in chloroform, washed with chloroform and eluted with 3% methanol-chloroform. The fraction eluted from 3% methanol-chloroform was evaporated to give 32.1 g (91%) of ethyl l-[NG-nitro-N2-(3-methyl-8-quinolinesulfonyl)-L-arginyl]-4-methyl-2-piperidinecarboxylate in the form of an amorphous solid. 

The S groups stabilized at the solid surface can differ in their chemical nature. In the disordered (amorphous) solids, to which silica refers and which are of most practical and theoretical interest, the variations in the spatial structure are inevitable. These variations cause variations in the properties of even those S groups having chemically identical composition of their coordination spheres and give rise to the inhomogeneous line broadening in the spectra of these groups and to their kinetic inequivalence. In this work, special emphasis is placed on the questions associated with these problems. 

In disordered solids, of which silica is an example, chemically similar groups may dilfer in their spectral characteristics. As a rule, a difference in spectral characteristics results in the difference in reactivities. Similar effects appear due to the difference in the spatial arrangement of atoms in the composition of these groups. This is characteristic of amorphous solids. Silylene centers on the silica surface are a convenient object for this experimental study [91]. 

When talking about difficulties in interpreting experimental evidences for disordered (amorphous) solids, the researchers often plead the lack of the theory of their matter structure. To what questions about the surface structure of amorphous silica might the theory answer ... 

The previous example took inspiration from earlier work of Lin et al.18 In their seminal work, Lin and coworkers functionalized the inner pores of MCM-41 with o-phthalic hemithioacetal moieties that are able to react with amines to produce a highly fluorescent isoindole derivative. In order to enhance selectivity in the sense we have discussed above, the solids were also hydrophobized with different groups such as propyl, phenyl, and pentafluorophenyl in a second step. Interestingly, some of these solids displayed a remarkably selective and differentiable response to dopamine versus the less lipophilic glucosamine. The authors also demonstrated that this selectivity was not observed when amorphous (nonporous) silica functionalized with the same organic groups was used, stressing the importance of the 3D... 

Among the supports that have been used in the preparation of supported transition metal nanoparticles are carbon, silica, alumina, titanium dioxide, and polymeric supports [57], and the most frequently used support is alumina [56], These supports normally produce an effect on the catalytic activity of the metallic nanoparticles supported on the amorphous material [60], In Chapter 3, different methods for the preparation of metallic catalysts supported on amorphous solids were described [61-71],... 

This section shows, for four examples of increasing complexity, how precipitates are formed and how the properties of the precipitates are controlled to produce a material suitable for catalytic applications. The first two examples comprise silica, which is primarily used as support material and is usually formed as an amorphous solid, and alumina, which is also used as a catalytically active material, and which can be formed in various modifications with widely varying properties as pure precipitated compounds. The other examples are the results of coprecipitation processes, namely Ni/ AI2O3 which can be prepared by several pathways and for which the precipitation of a certain phase determines the reduction behavior and the later catalytic properties, and the precipitation of (VOjHPCU 0.5 H2O which is the precursor of the V/P/O catalyst for butane oxidation to maleic anhydride, where even the formation of a specific crystallographic face with high catalytic activity has to be controlled. 

Perfluoroalkanedisulphonic acids (PFAS) are solid and possess strong acid properties both in solid state and in solution. To our knowledge, they have never been used in the alkylation of isobutane. They were obtained as dihydrate and as such were not acidic enough to be active in isobutane alkylation. Furthermore, they possessed low surface areas. The surface area can be increased by supporting PFAS on an amorphous solid, but it is critical that the solid does not attenuate the acid strength, We have found that a method for dehydrating PFAS and for supporting it on silica. The method allows to obtain an new catalyst, PFAS-Si(>2, which is active in the alkylation of isobutane. 

In their bulk properties, oxide adsorbents range from amorphous solids (e.g. silica)... 

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