ANALOGIES WITH SILICA

Figure 14 indicates that iron(II) reacts analogously with silica upon precipitation from a homogeneous solution kept at 363 K [24]. Reaction to a new solid phase is also apparent from the maximum exhibited by the pH curve measured with suspended silica. Also with precipitating nickel and suspended silica a maximum in the pH level is often observed when the pH is homogeneously raised at 353 K or at a higher temperature (as shown in Fig. 18). 

The main points of interest of the structures of these polymorphs are (i) the analogies with silica and silicate structures, (ii) the presence of two interpenetrating frameworks in the most dense forms vi and vii (viii), and (iii) the ordering of the protons. Analogies with silica and silicate structures are noted in Table 15.1, namely, ice-iii with a keatite-like structure, ice-vi with two interpenetrating frameworks of the edingtonite type (p. 828), and ice-vii (and viii) with two interpenetrating cristobalite-like frameworks. In these structures, related to those of... 

The zirconium alkoxides Zr(OR)4 are reported as inactive for the epoxidation of olefins under the conditions recommended with the titanium analogs. When synthesized by reaction of Zr(CH2CMe3)4 with silica, followed by hydrolysis or calcination, a solid as active as the related Ti-based catalyst is obtained. The low selectivity for the formation of the epoxide is related to the fact that the same Zr centers catalyze both the formation and the decomposition of the epoxide.46... 

The surface chemistry of coesite and stishovite was studied by Stiiber (296). The packing density of hydroxyl groups was estimated from the water vapor adsorption. More adsorption sites per unit surface area were found with silica of higher density. Stishovite is especially interesting since it is not attacked by hydrofluoric acid. Coesite is dissolved slowly. The resistance of stishovite is ascribed to the fact that silicon already has a coordination number of six. Dissolution of silica to HaSiFg by hydrogen fluoride is a nucleophilic attack. It is not possible when the coordination sphere of silicon is filled completely. In contrast, stishovite dissolves with an appreciable rate in water buffered to pH 8.2. The surface chemistry of. stishovite should be similar to that of its analog, rutile. 

Scheme 14.15 Possible grafting mechanism of[W(=Ar)(=CH Bu)(CH5Bu)j] on silica dehydroxylated at 700°C vs silica dehydroxylated at 200°C, proposed through analogy with silsesquioxane chemistry. Intermediates that have not been observed...

On sihca added with an excess of K2CO3, such as in strongly basic solution [58] or on the MgO surface [111], the initially formed silica-supported [Ir4(CO)i2] gives sequentially [Ir8(CO)22] and [Ir6(CO)i5] Y By analogy with the Ir chemistry occurring in basic solution [58] or on the MgO surface [111], the first anionic Ir cluster formed on the silica surface with added alkah carbonates is probably [HIr4(CO)ijY... 

By achieving random dispersion of the Kaolin in the sodium silicate solution prior to formation of the silica-alumina gel, it was possible to disperse the clay crystals. They condensed somewhat perpendicular to each other and were bound together by silica-alumina gel. I therefore speculated that spray drying, during which the gel system contracts, might create a dual structure. An analogy would be a house built of cards (Kaolin), cemented together with silica-alumina gel. 

The reaction mechanism of amine deamination and disproportionation has been put forward by analogy with other eliminations, namely dehydration and dehydrochlorination [149,155], its characteristic feature being the cooperation of acidic and basic sites. In the deamination, /3-hydrogen and the NR2 group (R is hydrogen or alkyl) are eliminated by an E2-like mechanism on alumina, but by an El-like mechanism on silica-alumina. The nature of the acidic sites is not clear, protons from surface hydroxyls or aluminium ions are possible candidates. However, it seems... 

Mechanistic information is difficult to obtain when the catalytically active titanium centers are present in a dilute matrix of silica. Only few techniques can be applied, and the available information does not allow discrimination between possible mechanisms. Consequently, it is necessary in this discussion to rely on analogies with the known chemistry of titanium compounds. 

It was discussed in chapter 9 that trichlorosilylation of silica gel, thermally pretreated at 973 K, and reacted with trichlorosilane at 623 K for 1 h, has a stoichiometry factor (/) of 1.25, which means that about 30% of the chlorosilyl species are secondary and 70% are primary ones. In analogy with the equations of that paragraph, the amount of trichlorosilane (ASi) that has reacted can be calculated, if the increment of Cl groups and the stoichiometry factor are known ... 

In analogy with the trichlorosilylation of silica gel, also a secondary and/or bimolecular reaction (B) is also possible ... 

Analogous experiments with silica as the support indicate the appreciable interaction of precipitating nickel(II) ions with silica [20, 21], In the experiments represented in Fig. 12 the solutions or the suspension was kept at 298 K. The curve measured with suspended silica runs again considerably below the curve calculated from the curves recorded with water, suspended silica, and nickel nitrate alone. The interaction with the silica support causes a higher consumption of hydroxy ions at a low pH level. Burch and Flambard [22] per-... 

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