Silica, acid-base properties

The pH optical fiber sensor without any pH-sensitive dye was also described70. Porous silica layer made by the sol-gel method was cladded onto optical fibre core and was exploited as the optical transducer. Acid-base properties of silica surface caused that the surface charge of silica changed with pH of the solution. For example saturation of the sol-gel layer with cations leads to an increase of the electron density of the film, hence, the refractive index of the film. Since the surface charge of silica depends on pH, the refractive index of silica film varies also with pH. Thus, changes of... 

Solid metal sulphates and phosphates also exhibit acid—base properties their acid strength is lower than that of silica—alumina but they are stronger acids than some oxide catalysts [5]. Correlation of activity with electronegativity of cations has been obtained for several reactions [9, 50,51],... 

The acid-base properties of polymers, fillers and silane additives, as described by Fowkes [14] can be used to predict the effect of silanes on the dispersion of fillers in polymer, and viscosity of the mix. In general, opposites attract (give good dispersion) while like materials repel (poor dispersion) [15]. The effect of cationic silane (Z-6032) on the dispersion of silica (acidic filler) in this particular unsaturated polyester resin (acidic polymer) is shown in Table 6. Addition of Z-6032 in increments to a silica-filled polyester resin lowered the viscosity of the mixture to a minimum at about 0.4% silane based on the filler. 

The acid-base properties of amorphous mixed metal oxides can be varied by choosing different metal oxide constituents at diflerent concentrations and by changing the treatment of the sample (44). Thus, it appears that, by properly choosing the aforementioned variables, mixed oxides could be used to develop new catalysts with desired acid-base properties. The use of micro-calorimetric adsorption measurements to quantify the acid-base properties of metal oxides and mixed metal oxides has been limited, to date, to a few systems. However, for some of these solids, for example, silica, alumina, and silica-alumina, several investigations have led to a satisfactory description of their acidity and acid strength. We present here a compendium of those measurements and discuss some of the important properties observed. 

Hydrogen-bonding, steric effects, and acid-base properties of phenols are involved in their retention on silica gel and alumina with benzene and isopropylether as eluents. 

Scheme 1 Comparison between charges and Bronsted acid-base properties of the oligopeptide angiotensin II and a PVFA-co-PVAm/silica interphase...

It has been reported that methanol is formed from C0-H2 reaction over silica-supported Pd, Pt, Ir catalysts (5). The behaviour of the metal was found to be influenced by the carrier (6,7,8, 9). The selectivity in methanol was discussed in terms of acid-base properties of the support which influenced the non dissociative adsorption of CO on the metal required for oxygenated hydrocarbon formation, in terms of electronic interaction between the metal and the support or in terms of stabilization by the carrier of oxidized metal cations which would adsorb CO non dissociatively. We have studied the C0-H2 reaction at 553 K, 30 atmospheres over Pt supported on a variety of oxides. The characteristics of the catalysts are given in table 2... 

EQUILIBRIA OF SILICA SURFACE SITES COMPLEXING WITH THE ONE-VALENCE CATIONS IN AQUEOUS SOLUTIONS (ACID-BASE PROPERTIES OF SURFACE ADSORPTION SITES)... 

One of earliest methods of obtaining information about the acid-base properties of the catalyst surface, in particular of the silica-alumina, was to adsorb from nonaqueous solvents on to them a series of known organic indicators and to notice the color change.J... 

J. Keranen, C. Guimon, A. Auroux, E.I. Iiskola, L. Niinisto, Gas-phase synthesis, structure and surface acid-base properties of highly dispersed vanadia/titania/silica catalysts, Phys. Chem. Chem. Phys. 5 (2003) 5333. 

Lopez, T Gomez. R Llanos. ME Lopez-Salinas, E. Acidic-base properties of silica-magnesia sol-gel mixed oxides use of 2-butanol as test reaction. Materials Letters, 1999 38, 283-288. 

The effect of Y-AI2O3 modifying with silica on acid-base properties of the surface... 

The catalysts based on silica fiberglass materials can be efficiently used in many gas-phase and Hquid-phase catalytic processes such as acid-catalyzed isopropanol dehydration reactions. To date, the acid-base properties of fiberglass materials, including the Brpnsted acidity, were not studied in detail. The lack of detailed studies could be due to the low specific surface area of fiber-... 

A 2000 study from Buffon [69] examined the reaction of Schrock-type, alkoxy-Mo-alkylidenes with the surface OH groups of silica, silica-alumina, and niobium oxide. The specific mode of attachment of the Mo-complex was found to be highly dependent upon the acid-base properties of the support. For silica, it appears to be a Lewis acid-base interaction between the Mo center and surface silanols, whereas in the case of silica-alumina, the attachment appears to occur via the addition of a surface OH group across the Mo-imido bond. In the case of niobium oxide, it is possible that the OH adds across the Mo-alkylidene, deactivating the complex, as the resulting material was completely inactive for metathesis. The activities of both immobilized catalysts were less than the parent homogeneous... 

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