Ammonia on silica

Calculate the rotational contribution to the entropy of adsorption of ammonia on silica at -30°C, assuming (n) that the adsorbed ammonia retains one degree of rotational freedom and (b) that it retains none. In case (n) assume that the nitrogen is bonded to the surface. 

Fig. 4. Variation of isosteric heats of adsorption with coverage for ammonia on silica-alumina catalysts of various compositions 33).

Figure 12.5 Irreversibly adsorbed and/or chemisorbed ammonia on silica gel as a Junction of the reaction temperature, with and without TCS modification.

For NH3 adsorption, experimental evidence has shown that the main interaction mechanism is H bonding of Si-OH to the N atom in NH3 [124], Ammonia gas is a widely used chemical in industry, and it has to be removed to less than one ppm, for instance, from the gaseous effluents of ammonia fertilizer plants, urea plants, and other sources [127], It is evident that silica is an excellent adsorbent of NH3 [124-126], Also, adsorption of ammonia on silica gel has received considerable attention recently, owing to its potential use in solar energy cooling cycles [128],... 

T. Shikada, K. Fujimoto, T. Kuniigi, and H. Tominaga, "Reduction of Nitric Oxide with Ammonia on Silica Supported Vanadium Oxide Catalysts", J. Chem. Tech. Biotechnol. 1983, 22A, 446-454. 

Reactions can also occur during chromatographic development. These can either be undesired reactions or planned derivatizations. Thus, Weicker and Brossmer [11] have reported, for example, that hexoses, pentoses and disaccharides can be ammated when ammonia-containing mobile phases are employed on silica gel G layers. On the other hand, fluorescamine or ninhydrin have been added to the... 

Note The reagent can be employed for qualitative and quantitative analysis on silica gel and RP layers. Ammonia, amine and acid-containing mobile phases should be completely removed beforehand. Amino phases cannot be employed. The NBD-chloride reagent is not as sensitive as the DOOB reagent (qv.) on RP phases. 

Note The dipping solution can also be sprayed on. The detection of the aromatic acids is best performed on cellulose layers, if ammonia-containing mobile phases have been employed. The reagent can also be employed on silica gel, aluminium oxide, RP 18 and polyamide layers. 

Note Rhodamine B is a universal reagent that can be used on silica gel, talc, starch [5] and cellulose layers, just as on urea [1] or silver nitrate-impregnated [7] phases. Liquid paraffin-impregnated silica gel and RP layers are less suitable, since the background to the chromatographic zones is also intensely colored. It is often possible to increase the detection sensitivity by placing the plate in an atmosphere of ammonia after it has been sprayed or dipped, alternatively it can be oversprayed with sodium or potassium hydroxide solution. 

It is often possible to increase the detection sensitivity in visible light by exposing the dipped or sprayed chromatogram to ammonia vapors it can also be sprayed with caustic soda or potash solution. When this is done the fluorescence intensity is reduced on silica gel layers and increased on RP ones. 

When the reaction of S2CI2 with ammonia is carried out in a polar solvent, e.g., DMF, the hydrolysis of the reaction mixture with aqueous HCl produces a mixture of the cyclic sulfur imides S7NH, 1,3-, 1,4- and 1,5-S6(NH)2 and 1,3,5- and 1,3,6-S5(NH)3, which can be separated by chromatography on silica gel using CS2 as eluant (Section 6.2.1). °... 

A solution of 1.5 mol equiv of butyllithium in hexane is added to 1.5 mol equiv of a 1 M solution of hexabutylditin in THF at 0 C under nitrogen, and the mixture is stirred for 20 min. The solution is cooled to — 78 °C and a solution of 1.5 mol equiv of diethylaluminum chloride in toluene is added. After stirring for 1 h at — 78 °C, a solution of 0.05 mol equiv of [tetrakis(triphenyl)phosphine]palladium(0) in THF is added followed by a solution of the allyl acetate in THF. The mixture is warmed to r.t., and stirred until the allyl acetate has reacted (TLC). The solution is cooled to 0°C, and an excess of aq ammonia slowly added. After an aqueous workup, the products arc isolated and purified by flash chromatography on silica gel using 1 % triethylamine in the solvent to avoid acid-induced loss of stannane. 

Popova and colleagues47 carried out TLC of oxidation products of 4,4 -dinitrodiphenyl sulphide (the sulphoxide and sulphone) on silica gel + a fluorescent indicator, using hexane-acetone-benzene-methanol(60 36 10 l) as solvent mixture. Morris130 performed GLC and TLC of dimethyl sulphoxide. For the latter, he applied a 6% solution of the sample in methanol to silica gel and developed with methanol-ammonia solution(200 3), visualizing with 2% aqueous Co11 thiocyanate-methanol(2 1). HPLC separations of chiral mixtures of sulphoxides have been carried out. Thus Pirkle and coworkers131-132 reported separations of alkyl 2,4-dinitrophenyl sulphoxides and some others on a silica-gel (Porosil)-bonded chiral fluoroalcoholic stationary phase, with the structure ... 

Note When combined with thin-layer chromatographic separation the reagent provides a specific detection method for nitrate and nitrite. The color development is often completed within a few minutes on silica gel plates. In the absence of ammonia vapor traces of oxides of nitrogen in the laboratory atmosphere can slowly cause the background to become reddish-brown. The simultaneous presence of the following ions in the chromatogram zones interferes with the detection of nitrate/nitrite I , 10J, IO4, MoO and H2PO2. 

Table 1 Absorption maxima and detection limits for some 2,6-dinitroaniline herbicides on silica gel layers after treatment with the reagent sequence and after additional exposure to ammonia vapor.

Note The reagent can be just as successfully employed on silica gel, kieselguhr, aluminium oxide and polyamide layers as it can with RP and NH2 phases. The final treatment with ammonia vapor to decolorize the background can be omitted in the last case. 

Zheng and Sun [67] used a thin-layer chromatographic method for the analysis of primaquine and other quinoline derivatives. The drug and other compounds were chromatographed on silica gel GF254 plate, with methanol aqueous 25 28% ammonia (200 3) and chloroform dichloromethane diethylamine (4 3 1), as mobile phases. Spots were located under ultraviolet radiation. The detection limit was 12 pg/mL. Total separation could be achieved by the use of two plates and the respective mobile phase. 

The organic phase was evaporated and the residue dissolved in chloroform. A portion of the chloroform solution was spotted on silica gel plates which were then developed in chloroform methanol ammonia (89 10 1). After development, the plate was air dried and sprayed with a 2M aqueous solution of ammonium bisulfate. After the plate was air dried for 1 hour, the fluorescent spot representing triprolidine was quantitated with a spectrodensitometer in the reflectance mode with 300 nm excitation and emission above 405 nm. With... 

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. ... 

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