Surface chloride, silica

For example, Pt/Si02 catalysts are conveniently made by impregnating a silica support vith a basic solution (pH 8-9) of platinum tetraammine ions, Pt(NH3)4 (dissolved as chloride). As the silica surface is negatively charged, the Pt-containing ions attach to the SiO entities and disperse over the surface. The pH should be kept belo v 9 because other vise the silica surface starts to dissolve. 

Hydrosilylation of allyl chloride with HSiCl3 leads to Cl(CH2)3SiCl3 that can be used for many surface modifications after substitution of the chlorides by suitable functional groups [5], For instance it can be used to anchor soluble hydroformylation catalysts to a silica surface (Figure 18.3). Many examples have been reported and we present only one example of a selective Xantphos type ligand [6],... 

Modification Using a Halogenated Silica Surface. Silica gel is heated in 2 m HC1, filtered off, and washed with water the dried silica gel is then reacted with pure thionyl chloride. After removal of the excess of thionyl chloride and byproducts (S02 and HC1) under vacuum, chlorinated silica gel (=SiCl) is obtained. This is unstable in water and alcohol. The chlorinated silica gel can then be alkylated by one of a number of different reactions ... 

The influence of chemical modification of silica surfaces by treatment with trimethylsilyl chloride was studied also by A. V. Kiselev and collaborators (221), Babkin et al. (222), and Babkin and Kiselev (223). They reported that no change occurred in (CHglaSiCl-treated Aerosil even after storing for months underwater (221). Lowen and Broge (224) investigated the influence on the heat of wetting and methyl red adsorption. 

Organic groups are bound to the silica surface after grinding silica in organic liquids (277). A more controlled substitution of surface silanol groups was reported by Wartmann and Deuel (194). Silica gel which had been treated with thionyl chloride was allowed to react with phenyl lithium. Silicon-phenyl bonds could be detected by infrared spectroscopy. The phenyl content of Aerosil treated in this way as estimated from carbon analysis corresponded to 85% of the silanol groups (188). However, it is not certain whether the reaction... 

Desiccant. A substance with large surface area per pound that will adsorb or absorb water vapor from the air. Popular desiccants are activated alumina, calcium chloride, silica gel, and zinc chloride. 

Fused silica capillary tubes were variously coated for capillary electrophoresis. The chemical process involving a Grignard reaction is shown in equation 30. The silanol groups on the silica surface are treated with alkali, dried, convert to chlorosUanes with thionyl chloride and vinylmagnesium bromide replaces the chlorine atoms with vinyl... 

Fig. 23a. Hydration and dehydration reactions of the silica surface and the IR absorption frequencies of the surface species. Chemisorption of water produces surface silanols, which serve as adsorption sites for water, b Reaction of octadecyltrichlorosilane (OTS). Hydrolysis of the chloride group by trace amounts of water in solution to silanol is followed by condensation with surface silanols, resulting in covalent bond formation between the monolayer and the substrate. OTS molecules can also cross-link to form polymeric species during film curing [201]...

The nature of the support can have a very profound influence on the catalyst activity. Thus, phosphinated polyvinyl chloride supports are fairly inactive (75), and phosphinated polystyrene catalysts are considerably more active (57), but rather less active particularly when cyclic olefins are the substrates than phosphinated silica supports (76). Silica-supported catalysts may be more active because the rhodium(I) complexes are bound to the outside of the silica surface and are, therefore, more readily available to the reactants than in the polystyrene-based catalysts where the rhodium(I) complex may be deep inside the polymer beads. If this is so, the polystyrene-based catalysts should be more valuable when it is desired to hydrogenate selectively one olefin in a mixture of olefins, whereas the silica-based catalysts should be more valuable when a rapid hydrogenation of a pure substrate is required. 

Supported diallylrhodium complexes bound to oxide (silica) surface can be transformed to complex 8, which reacts with methane in a stoichiometric reaction to form methyl chloride 221 225 226... 

Besides some conventional supports, a new carrier material, poly-alumazane, was tested. Poly-alumazane, which was recently reported by Chinese researchers8, is a silica carrier, the surface of which is modified by subsequent treatments with aluminum chloride and ammonia, forming an Al-N phase on the silica surface. This coating is able to interact with two-valent noble metal ions (like Pd11) which is found to result -after reduction- in a catalyst with very high dispersion. 

The silylation of all glassware that contacts the plant extract has proven to effectively reduce adsorption losses. As diagrammed in Figure 8, the hydroxyl adsorption sites on the silica surface can be coated with dichlordimethyl silane. The unreacted chloride groups are then displaced with methanol in a substitution reaction. A secondary advantage of the silyation process is that water will not adhere to the glass surface. Aqueous residues bead together, which allows more efficient sample transfers. 

Figure 5.12 Illustration offorce curves between silica surfaces in aqueous solutions of sodium chloride at various molar concentrations. At short range there is a strong repulsion which is not accounted for in the standard DLVO theory, due to hydration forces. Drawn based on data in Horn [284].

X-ray structure analysis revealed a 7-coordinate rare-earth metal center with two asymmetrically / -coordinating tetramethylaluminate ligands, an asymmetrically / -coordinating siloxide ligand and one methyl group of a trimethylaluminum donor molecule (Fig. 28). Such heteroleptic complexes can be regarded as molecular models of covalently bonded alkylated silica surface species. Moreover, isoprene was polymerized in the presence of 1-3 equivalents of diethylaluminum chloride, with highest activities observed for (Cl) (Ln) ratios of 2 1 (Table 12) (Fischbach et al., 2006, personal communication) [150]. 

The use of a functionalized silica-supported salen-nickel complex has allowed Kumada cross-couplings to be performed in flow the corresponding polystyrene supported complex was shown to be inferior for a number of reasons. Catalyst 33 (Figure 4.7) with the longer tether was found to be more active than the benzyl ether tether used for catalyst 34. This was postulated to be due to the fact that catalyst 33 resided further away from the silica surface and hence was more available for reaction. Under the conditions used a maximum conversion of 65% was found for the 1 1 reaction of 4-bromoanisole and phenylmagnesium chloride, which was found to be comparable to that obtained in batch mode. However, during the reaction catalyst degradation was observed and the conversion reduced from 60% in the first hour to 30% in the fifth hour of the reaction [155,156]. 

Perspectives. Ordered mesostnictured and mesoporous silica has been known for little more than ten years. Tremendous progress has been made with respect to precise control of the structure, texture, and chemical fimctionality of the surface of these materials. His lecture surveyed the synthesis of such materials, with a focus on organically ordered mesoporous materials. Quite a number of contributions dealt with amorphous fumed silica, its Physical-Chemical Features and Related Hazard Risk Assessment (M. Heinemann), the description of fractal aggregates (C. Batz-Sohn), and the Characterization of Size and Structure of Fumed Silica Particles in Suspension (F. Babick). E. Brendle reported on Adsorption of Water on Fumed Silica, and in a second paper he summarized research on Methylene Chloride Adsorption on Pyrogenic Silica Surfaces. 

Methylene Chloride Adsorption on Pyrogenic Silica Surfaces... 

Water adsorption studies are obviously indicated for the evaluation of the hydrophilic character of silica. Besides, according to our earlier studies, methylene chloride (CH2CI2), used as a probe for Inverse Gas Chromatography (IGC) measurements at Unite concentration conditions, appears to be an alternative choice for determining the hydrophobicity of silica surfaces. Indeed, earlier indications suggest that this probe, when used under appropriate conditions, is not interacting with the hydrophilic silanol groups. The present work is complementary to the study of water adsorption on fumed silica samples, described in this book [1]. 

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