Silica acid treatment

All silicon compounds on oxidation yield silica or silicates these are difficult to detect but silica (given by silicates after acid treatment) is insoluble in all acids except hydrofluoric acid. 

Nitric acid treatment lowered the methane uptake by about ten percent. This could be due to oxygen occupying sites within pores, but may be the result of weaker interaction between methane and an oxide surface as is observed for silica. Reduction of these treated carbons with hydrogen restored their original methane uptake. Clearly for methane storage, there is no advantage in modifying the carbon surface by nitric acid treatment. 

The residue consists of the impurities, and the loss in weight of the crucible gives the amount of pure silica present, provided that the contaminants are in the same form before and after the hydrofluoric acid treatment and are not volatilised in the operation. Although silicon is not the only element that forms a volatile fluoride, it is by far the most abundant and most often encountered element consequently the volatilisation method of separation is generally satisfactory. 

Silica gel [11] or alumina [11a, 12] alone, or silica and alumina together modified by Lewis-acid treatment [13] and zeolites [14], have been widely used as catalysts in Diels-Alder reactions, and these solids have also been tested as catalysts in asymmetric Diels-Alder reactions [12,13b,14]. Activated silica gel and alumina at 140 °C were used [15] to catalyze the asymmetric cycloaddition of (-)-menthyl-N-acetyl-a, S-dehydroalaninate (3) (R = NHCOMe) with cyclopentadiene in the key step for synthesizing optically active cycloaliphatic a-amino acids. When the reactions were carried out in the absence of solvent, a higher conversion was obtained. Some results are reported in Table 4.5 and compared with those obtained by using silica and alumina modified by treatment with Lewis acids. Silica gel gives a reasonable percentage of conversion after 24 h with complete diastereofacial selectivity in exo addition. 

Silica gel. Silica gel is a porous amorphous form of silica (Si02) and is manufactured by acid treatment of sodium silicate solution and then dried. The silica gel surface has an affinity for water and organic material. It is primarily used to dehydrate gases and liquids. 

A modification of the above cyclic method has proved more effective in the dealumination of Y zeolites. An almost aluminum-free, Y-type structure was obtained by using a process involving the following steps a) calcination, under steam, of a low-soda (about 3 wt.% Na O), ammonium exchanged Y zeolite b) further ammonium exchange of the calcined zeolite c) high-temperature calcination of the zeolite, under steam d) acid treatment of the zeolite. Steps a) and c) lead to the formation of ultrastable zeolites USY-A and USY-B, respectively. Acid treatment of the USY-B zeolite can yield a series of aluminum-deficient Y zeolites with different degrees of dealumination, whose composition depends upon the conditions of the acid treatment. Under severe reaction conditions (5N HC1, 90°C) an almost aluminum-free Y-type structure can be obtained ("silica-faujasite") (28,29). 

Surface acidity and catalytic activity develop only after heat treatment of a coprecipitated mixture of amorphous silicon and aluminum oxides. Similar catalysts can be prepared by acid treatment of clay minerals, e.g., bentonite. The acidity is much stronger with silica-alumina than with either of the pure oxides. Maximum catalytic activity is usually observed after activation at 500-600°. At higher temperatures, the catalytic activity decreases again but can be restored by rehydration, as was shown by Holm et al. (347). The maximum of activity was repeatedly reported for compositions containing 20-40% of alumina. 

To increase the solubility of calcium carbonate and reduce its potential for precipitation and hence membrane fouling, the pH of the feed water is lowered through the addition of acid (Greenlee et al. 2009). Sulphuric acid and hydrochloric acid are most commonly used. Acid treatment can also help to improve the coagulation of colloids and slightly increase the solubility of silica (Bergman 2007). 

Add methanol and extract with hexane/ethyl ether. Clean-up on Florisil column. Acid treatment and clean-up on silica gel column. 

The phosphate concentration in the tailings is upgraded to a level adequate for commercial exploitation through removal of the nonphosphate sand particles by flotation [32], in which the silica solids are selectively coated with an amine and floated off following a slurry dewatering and sulfuric acid treatment step. The commercial quality, kiln-dried phosphate rock product is sold directly as fertilizer, processed to normal superphosphate or triple superphosphate, or burned in electric furnaces to produce elemental phosphorus or phosphoric acid, as described in Section 9.2. 

Two methods of bonding a spiropyran to the surface of silica gel have already been mentioned the reaction of silica-bound salicylaldehyde with Fischer s base, and the reaction of 6-lithioBIPS with silica. Silica-bonded spiropyrans exhibited properties (reverse photochromism, visible light photobleaching, and color changes upon acid treatment due to salt formation) very similar to those of spiropyrans merely adsorbed onto silica. The differences in behavior were attributed to the different extent of absorption and desorption of atmospheric water caused by the difference in the number of free SiOH groups available in the two cases.86... 

Choice of a dissolution method also depends on the range of substances to be analyzed, but in some specific instances the chemical form of the element is of even more importance. For example, particles formed at high temperature usually contain refractory oxides and silicates which may require fusion with either an acidic or basic flux, or hydrofluoric acid treatment to remove silica, or a combination of these techniques. 

If the framework structure of a zeolite remains constant, the cation exchange capacity is inversely related to thd Si/Al ratio. Furthermore, fine tuning of the adsorptive and catalytic properties can be achieved by adjustment of the size and valency of the exchangeable cations. Dealumination of certain silica-rich zeolites can be achieved by acid treatment and the resulting hydrophobic zeolites then become suitable for the removal of organic molecules from aqueous solutions or from moist gases. 

This dilemma can be resolved partly by treating the minerals with hydrochloric acid. Carbonates dissolve, whereas silica and combustible matter do not. But the presence of bitumen and even vegetal matter interferes more or less with the reaction with hydrochloric acid, so the cleanest way to carry out acid treatment is on ignition residues. The ratio of acid solubles to ignition loss will then be below 56 44 in proportion to the amount of combustibles present. But because some common minerals dissolve in hydrochloric acid whereas other do not, the conventional method of mineral analysis is approximate at best. 

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