Anhydrous Crystalline Silicas

Phosphoric acid does not react with anhydrous crystalline forms of Si02, AI2O3, Cr203, Z1O2 and Ti02 at room temperature. Some reaction occurs with the hydrated or colloidal forms of these oxides or with the anhydrous forms themselves if the temperature rises above about 200°C. Below this temperature silica glass is not appreciably attacked by the acid in moderate concentration. 

Inhalation of colloidal silicon dioxide dust may cause irritation to the respiratory tract but it is not associated with fibrosis of the lungs (silicosis), which can occur upon exposure to crystalline silica [5]. The main issue appears to be whether the silica is crystalline or not. Colloidal anhydrous silica Ph. Eur. is however amorphous. The irritation of the throat, eyes etc. is an issue though and because of lesser dust generation at weighing and processing the pressed anhydrous form (Aerosil 200 V ) is preferred. 

Certain anhydrous crystalline silicates yield layerlike crystalline hydrated silicas with the approximate formula (HjSiiO ),. These are cited to illustrate the complexity of such apparently simple compounds. 

Amorphous silica, ie, silicon dioxide [7631-86-9] Si02, does not have a crystalline stmcture as defined by x-ray diffraction measurements. Amorphous silica, which can be naturally occurring or synthetic, can be either surface-hydrated or anhydrous. Synthetic amorphous silica can be broadly divided into two categories of stable materials (1) vitreous silica or glass (qv), which is made by fusing quart2 at temperatures greater than approximately 1700°C (see Silica, vitreous silica), and microamorphous silica, which is discussed herein. 

Adsorption processes use a solid material (adsorbent) possessing a large surface area and the ability to selectively adsorb a gas or a liquid on its surface. Examples of adsorbents are silica (Si02), anhydrous alumina (AI2O3), and molecular sieves (crystalline silica/alumina). Adsorption processes may be used to remove acid gases from natural gas and gas streams. For example, molecular sieves are used to dehydrate natural gas and to reduce its acid gases. 

A solution of L-fucose (4.92 g, 30 mmol) in anhydrous DMF (50 mL) was stirred with a desiccant (Drierite or Sikkon, 1 g) at 0°C (ice bath), and 2-methaxypropene (2.16 g, 30 mmol) was added followed by p-toluenesulfonic acid (-20 ing). After 1 h at 0°C, an additional stoichiometric amount of reagent (2.16 g) was added, and stirring was continued for 2 h at 0°C. Sodium carbonate (-5 g) was added, and the mixture was stirred further 1 h at room temperature. The solids were filtered off, and the filtrate was evaporated under diminished pressure at 40°C to a syrup that contained (TLC, ethyl acetate) one major product plus minor, fast-migrating components. Rapid chromatography of the product on a column of silica gel gave pure, crystalline 3,4-O-isopropylidene-L-fucopyranose 31 yield 3.7 g (-60%) mp 110-111°C, [ ]D -90 - -70° (24 h, equil. e 0.2, water). 

Benzonitrile (1.520g, 14.8mmol) was added to the THF solution of the sodium salt generated from (1) (1.830g, 14.8 mmol) by the action of sodium hydride (1.24 eq) at room temperature, and the resulting solution was stirred at 50°C for 16h. After addition of water (2 ml) together with dichloromethane (70 ml), the mixture was dried over anhydrous sodium sulfate (without separation of the aqueous layer) and the solid phase was filtered off. Evaporation of the filtrate under reduced pressure and subsequent crystallization from dichloromethane/carbon tetrachloride/ cyclohexane (1 1 1) afforded (2) as a crystalline material (2.391 g, 10.5 mmol), m.p. 162-163°C (105 mg of (2) could be recovered by chromatography on silica gel of the mother liquor residue of evaporation the total yield of (2) was 75%). 

A mixture of ethyl l-cyclopropyl-6,7,8-trifluoro-l,4-dihydro-4-oxoquinoline-3-carboxylate (933 mg), 3-acetamidopiperidine (710 mg), triethylamine (400 mg) and dimethylsulfoxide (10 ml) was heated at 100°C for 2 hours with stirring. Thereafter the mixture was cooled down and ice water was added thereto. The resulting mixture was extracted with chloroform and the chloroform layer was washed with water three times before being dried over anhydrous sodium sulfate. Removal of the solvent in vacuum followed by purification by silica gel column chromatography (chloroform-ethanol) gave ethyl 7-(3-acetamidopiperidin-l-yl)-l-cyclopropyl-6,8-difluoro-l,4-dihydro-4-oxo quinoline-3-carboxylate (930 mg). Re-crystallization from ethanol-ether afforded a colorless crystalline substance (MP 217°-218°C). 

Under an atmosphere of dry nitrogen, titanium tetraethoxide (2.4 g, 10.4 mmol) in 10 ml of dry toluene was added over 10 min to 2-methyl-6-hydroxybenzothiazole (1.72 g, 10.4 mmol) in 40 ml of dry toluene. When the addition was complete, the reaction mixture was boiled for 15 min and then slowly distilled to remove the ethanol. A total of 20 ml of solvent was collected. The reaction mixture was allowed to cool to room temperature and P-phenylcinna-maldehyde (2.17 g, 10.4 mmol) in 50 ml of dry toluene was added dropwise to it. When the addition was complete, the reaction mixture was refluxed for 2 to 5 h, allowed to cool, and poured onto 100 ml of dilute aqueous ammonium chloride solution. The organic layer was separated, dried over anhydrous magnesium sulfate, and the solvent removed on a rotary evaporator. The residue was chromatographed on silica using 40% diethyl ether in pentane as eluent. The photochromic fractions were combined, the solvent removed, and the crystalline residue recrystallized from a heptane-benzene mixture. The product (1.6 g, 44%) had a melting point of 215°C. 

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