Iodide, silica surface

Unloaded silica does not recover HPA from aqueous solution. The surface of silica gel modified with quarternary ammonium salts (QAS) gets anion-exchange properties. The aim of the work is the elaboration of solid-phase reagents on the base of ion associate of HPA with QAS immobilized onto silica surface for the determination of phosphoms and organic reductants. Heterocyclic (safranine and lucigenine) and aliphatic (trinonyloctadecyl ammonium iodide and tetradecyl ammonium nitrate) compounds have been examined as QAS. 

Halides. Another treatment which can lower the hydroxyl population, or even eliminate it altogether, is halogenation of the silica surface.This removes hydroxyls, not by condensation as with CO and sulfur, but by replacement with halide, which prevents later attachment by Cr. The presence of halide probably also changes the electronic environment on the silica. Thus, fluoriding has long been used to increase activity but decrease RMip.i l Chloride also depresses RMIP, as well as the surface bromide and iodide of silica. However, these latter two have recently been studied, and it was possible to burn off most of the iodide or bromide with oxygen above 600 C, leaving a partially dehydroxylated surface. 

The nickel iodide-silica gel catalyst which was highly active at a given temperature and pressure became progressively deactivated when exposed to the same temperature at atmospheric pressure. Thus, in three successive experiments carried out at 230° and 6000-p.s.i.g. pressure, with the same sample of the catalyst the yields decreased in the order 41.0,24.6, and 16.4 % in the case of isobutyric acid, and in the order 38.4, 20.1, and 12.6% in the case of 2-methylbutyric acid, when the catalyst was exposed to the reaction temperature at atmospheric pressure for a few hours at the conclusion of each run. However, the addition of a few drops of cold water on the surface of the partly deactivated catalyst was found to restore its original activity completely. It was found possible to maintain the activity of the catalyst indefinitely in the presence of a small quantity of water, provided the... 

Thus, the peculiarities associated with the nickel iodide-silica gel catalyst in the reactions of carbon monoxide and alcohols appeared to be rather general and quite independent of the nature of the alcohol used in the system. It is thought that the activity of the nickel iodide-silica gel catalyst may be connected with a surface complex the formation of which requires the presence of traces of water. 

Results reported in Table I show that the presence of both a phosphonium salt and of a porous support like silica gel are important when 1-bromobutane is allowed to flow in the absence of a catalyst through ground potassium iodide, no 1-iodobutane is pratically obtained on the contrary, the conversion obtained in the presence of potassium iodide + silica gel clearly shows that the inorganic porous material plays the role of a solid solvent and actives iodide anion by interactions which diminish the bond strength of the K I ion pair. Furthermore catalytic amounts of the phosphonium salt lead to higher conversion while the presence of the anionic surfactant sodium laurylsulphate (NaLS) on the surface of silica gel does not yield different results from those obtained with silica gel alone. 

The amounts oi adsorption of the polymer on latex and silica particles were measured as follows. Three milliliters of the polymer solution containing a known concentration was introduced into an adsorption tube(lO ml volume) which contained 2 ml of latex (C = l+.O wt %) and silica(C = 2.0 wt %) suspensions. After being rotated(l0 rpm) end-over-end for 1 hr in a water bath at a constant temperature, the colloid particles were separated from the solution by centrifugation(25000 G, 30 min.) under a controlled temperature. The polymer concentration that remained in the supernatant was measured colorimetrically, using sulfuric acid and phenol for the cellulose derivatives(12), and potassium iodide, iodine and boric acid for PVA(13). From these measurements, the number of milligrams of adsorbed polymer per square meter of the adsorbent surface was calculated using a calibration curve. 

The calculation of the surface field of ionic solids at distances appropriate to an adsorbed molecule is highly dependent on this distance. No experimental method of measuring this distance has been developed as yet. If an assumed distance is used, a field value for argon adsorbed on caesium iodide is found to be 5.7 X 10 v./cm. [136) and for argon on KCl to be 4.3 X 10 v./cm. 137). As the surface of silica glass is not completely ionic, the above experimental value of the field (7 X 10 v./cm.) is in reasonable agreement with these theoretical figures. 

No experiments with variation in particle size of the silica gel have been done to study intraparticle diffusion effects. In silica gel such diffusion would be only through the pores (analogous to the macropores of a polystyrene) since the active sites lie on the internal surface. The silica gel used by Tundo had a surface area of 500 m2/g and average pore diameter of 60 A.116). Phosphonium ion catalyst 28 gave rates of iodide displacements that decreased as the 1-bromoalkane chain length increased from C4 to Cg to C16, The selectivity of 28 was slightly less than that observed with soluble catalyst hexadecyltri-n-butylphosphonium bromide U8). Consequently the selectivity cannot be attributed to intraparticle diffusional limitations. 

Many examples of this type of reaction are known the decomposition of arsine the decomposition of phosphine on surfaces of glass, f porcelain, J silica the decomposition of formic acid vapour on a variety of different surfaces— glass, platinum, rhodium, titanium oxide, and others the decomposition of nitrous oxide on the surface of gold Tf the decomposition of sulphuryl chloride on the surface of glass the decomposition of hydrogen iodide on the surface of platinum ff the decomposition of hydrogen selenide on the surface of selenium. J J A general discussion... 

Alumina will also bind Cr03 and stabilize it to 900°C, and it can polymerize ethylene when reduced to Cr(II). High surface area y alumina can be made having the porosity necesssary for good activity. Besides the electronic differences between Si—O—Cr and A1—O—Cr bonds, such alumina catalysts typically have 50-100% more hydroxyl groups than silica at normal calcining temperatures. This is clear in Fig. 21, which shows the hydroxyl populations of three different supports. The hydroxyl concentration was measured by reaction with methylmagnesium iodide. 

The reaction of I2/A1203 with alkenes even in the presence of NaCl does not result in iodo chlorides but in alkyl iodides. Evidently HI is generated by reaction of I2 with surface hydroxyls. This reaction can even take place with nonactivated alumina, but yields are higher (30-85%) with activated A1203. This reaction does not occur on silica gel. 

Although the surface chloride of silica cannot be burned off as easily, it will react with H2 or CH3OH and these can then be burned off. This treatment sometimes also produced an increase in RMIP. However, the effect was minor by comparison to the bromide and iodide, probably because water was formed during the burn-off. 

If the transport limitation is significant, the catalysis occurs predominantly near the surface of the ionic liquid, and the [Rh(CO)2l2] dissolved in the bulk is not fully utilized. One attempt to address these issues was to use a supported ionic liquid phase (SILP) catalyst, as reported by Riisager et al. [Ill], In this system, the ionic liquid (l-butyl-3-methylimidazolium iodide) was supported as a thin film on solid silica (the thin film offers little mass-transport resistance) and used in a fixed-bed continuous reactor with gas-phase methanol. Rates were achieved that were comparable to those in Eastman s bubble column carbonylation reactor with gas-phase reactants [109], but using a much smaller amount of ionic liquid. 

Potassium fluoride on alumina is sometimes used as a base.23 It may be that the fluoride ion displaces some surface hydroxyl from the alumina to produce some potassium hydroxide. Lithium chloride, bromide, and iodide on silica... 

Additional chemical evidence for the assignment of the 58 ppm resonance to the methoxy species III was the observation that it also formed from methyl bromide and methyl chloride in relative yields consistent with the leaving group stability T > Br > Cl. Methyl iodide was adsorbed on several zeolites with different Si/Al ratios, and the intensity of the 58 ppm resonance correlated with the A1 content, as it must for a framework-bound alkoxy. The final example of chemical evidence for the assignment regards the expected chemistry of species such as III and VI upon exposure to moisture. The Si-O-C linkage is easily hydrolyzed on a silica gel surface to form alcohols and/or ether. As demonstrated in Fig. 16, the species assigned to III readily hydrolyzes to methanol and dimethyl ether, whereas the proposed ethoxy species formed from ethyl iodide- C hydrolyzed to ethanol upon exposure to atmospheric moisture. 

Formation of cluster anions on surfaces has been discussed in Section 8.2.1 further examples on hydrated alumina or silica or on hydroxylated magnesia can be found elsewhere.[ Organic and inorganic iodides can also favor the formation of anions. 

The electrical charge residing on the surfaces of calcium oxalate crystals exposed to aqueous solutions should be strongly modified by the adsorption of charged ionic macromolecules. This effect has been reported for other solid surfaces such as silica ( ), silver iodide S,l), latexes, calcium phosphate ( ) / etc. That certain macromolecules adsorb on calcium oxalate has been demonstrated (10)y but the effect on surface charge has not been examined in detail. 

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