Adsorption on silica gel

Ozone is strongly endothermic (A(g) +142.2 kJ/mol, 2.96 kJ/g) and the pure solid or liquid materials are highly explosive. Evaporation of a solution of ozone in liquid oxygen causes ozone enrichment and ultimately explosion [1], Organic liquids and oxidisable materials dropped into liquid ozone will also cause explosion of the ozone [2], Ozone technology and hazards have been reviewed [3], a safe process to concentrate ozone by selective adsorption on silica gel at low... 

The /V-BOC group in 148 is readily removed on mild heating after adsorption on silica gel (92% yield). 

A separation of myoglobin and lysozyme has been presented by Nicoud [44]. This purification was performed on SMB containing 8 columns using ACA 54 (Biosepra, France) as support. Very pure extracts (>98%) and raffinates (>98%) were obtained from a 50-50 mixture. An internal profile is given in Fig. 7. Another recently presented example is the separation of cyclosporine A from cyclic oligopeptide and other impurities in the reversed phase mode or by adsorption on silica gel presented by Schulte et al. [10]. 

The formation of silicon-flvxyride bonds on the surface of silica after treatment with hydrogen fluoride was never proven directly. However, there is a pronounced change in the adsorption and wetting properties. The silica becomes hydrophobic as was mentioned in a patent to Kimberlin (279a). Neimark and collaborators (279b) found a type V isotherm in the methanol adsorption on silica gel which had been treated with a solution of SiF in absolute alcohol. Wilska (280) obtained a water-repellent silica when solutions of HaSiPg were precipitated with ammonia. The Si—F bond is hydrolyzed only slowly. A considerable fluorine content of 7-10% F was reported in an older patent (281) for a silica that had been prepared by hydrolysis of SiF. ... 

Cyclohexadienones. The perturbation of the electronic absorption spectra by adsorption on silica gel can have a significant effect on subsequent photochemical reactions. For example, Hart has shown that 2,4-cyclo-hexadienones photochemically degrade cleanly in nonpolar solvents to a ketene (1) but in highly polar solvents or adsorbed on silica gel bicyclic ketone (2) is the predominant product.33 The absorption spectra indicate that in nonpolar solvents the lowest singlet state is the n,n state, from which the formation of the ketene proceeds. This n -n band is obscured by the band in polar media, inversion of the energy levels of the n,n, and the first... 

Normal-phase adsorption on silica gel with a relatively less polar mobile phase is the most widely used mode in conventional TLC. To improve separations, silica gel may be impregnated with various solvents, buffers, and selective reagents. Other commercial precoated layers include alumina, florisil, polyamide, cellulose, and ion exchangers. 

Table I. Contributions to the Second Central Moment— Adsorption on Silica Gel at 50°C.

In general, there is a wide variety of chromatographic modes (types) that can be employed for the HPLC determination of food components, but only a few have been used for the determination of NOC. These include partition/adsorption on silica gel, liquid-liquid partition on polar-bonded phase (e.g., cyano, amino) or nonpolar hydrophobic-bonded phase (e.g., reversed-phase), and anion-exchange chromatography. Macrae (61) discussed the theories behind the various modes of chromatography. 

Inoue, U. and Tochiyama, O. (1977) Determination of the oxidation states of neptunium at tracer concentrations by adsorption on silica gel and barium sulphate./. Inorg. Nucl. Chem., 39, 1443-1447. 

Figure 12.1 Total and irreversible ammonia adsorption on silica gel, thermally pretreated at 473, 673 and 973 K.

The commonest gel for adsorption purposes is silica gel but measurement have also been carried out on ferric oxide, aluminium oxide, stannous oxide and titanium oxide. The characteristics of adsorption on silica gel differ somewhat from those for charcoal, particularly with respective Freundlich isotherm which holds fairly accurately. Moreover, deviation from Henry s Law even, seems to be much less than for charcoal. Water is somewhat exceptional. Silica and alumina gels are used as adsorbents for removing moisture and for controlling humidities of rooms. 

Breath Collection in bags, adsorption on silica gel desorption to headspace vial analysis of headspace gases GC/MS-SIM 0.1 ppb NR Gruenke et al. 1986... 

The high-pressure-promoted (MeCN or CH2CI2, lOkbar, 42 or 65 °C, 1-3 d) and silica gel-catalyzed (rt, 2-10 d) reactions of epoxides with pyrroles and indoles have been demonstrated to be effective for the alkylation of these heterocycles (Scheme 113) <1996JOC984>. Nucleophilic epoxide-opening reaction with indoles 561 by adsorption on silica gel gave 2-(3-indolyl)-2-phenylethanols 562 in yield up to 88%. Indoles 562 were obtained also under high-pressure conditions in 13-61% yields. 

In connection with quantifying possible changes in the structure of liquids due to confinement in pores, an old rule, known as Gurvitsch s rule ) may be mentioned. It states that for a variety of different liquids the total pore volume is within narrow limits identical, if computed on the basis of their bulk densities. For Instance, McKee l finds a maximum variation of 4% for a number of organic adsorptives on silica gel with a (Kelvin) radius of 1.7 nm. suggesting that only minor structural deviations take place. However, for more strongly associated liquids like water, and for narrower pores, the deviations may be larger. Then the liquid-solid interaction also starts to play its role. 

Zr (a) Adsorption on silica gel. Elution of other fission products with H2SO4/ HNO3. Elution of Zr with 0.5 M oxalic acid. (b) Extraction with thenoyltrifluoroacetone (TTA) in benzene. Back-extraction with 2 M HF and precipitation as BaZrFe- Separation of Ba as BaS04. 

Unsaturated 1,4-diketones are available from the corresponding nitro ketones by adsorption on silica gel," oxidation with peroxodisulfate or reduction with ascorbic acid (equation 38). 

Acetylation. Adsorption on silica gel enables selective monoacetylation of symmetrical diols. For primary/secondary diol pairs the less hindered alcohol group is protected, using EtOAc-NaHSO or HCfOMelj-LnClj as the reagent. 

Grigg has employed the intramolecular Heck reaction to craft a series of spiro-pyrrolidines (213) [128], and Gronowitz has prepared several thienopyrroles, such as 214, via a Heck sttategy [129]. The A-BOC group in 214 is readily removed on mild heating after adsorption on silica gel (92% yield). 

Figure 4. The isotherms of excess adsorption on silica gel of acetone from water (a), dioxane from water (b) and dioxane from acetone (c).

Ai r Adsorption on silica gel, extraction with methanol GC/FID 0.02 mg/ sample No data NIOSH 1984... 

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