Silica activate formation

The behavior of 1,3-cyclohexadiene in the presence of hydrogen at 170°C is very similar to that of benzene (Fig. 17), with a transient formation of acetylene. Similarly, in the presence of He, 1,3-cyclohexadiene is cracked into acetylene, and this reaction can be repeated for many successive doses of the reactant. Now, the behavior of the isomer, 1,4-cyclohexadiene, is different because in the presence of hydrogen as well as He this reactant is only cracked into acetylene (182,183). The explanation of this different evolution has been provided before 1,4-cyclohexadiene is indeed a poison for the hydrogenation of acetylene and the stepwise reaction of hydrogenolysis of 1,4-cyclohexadiene (in the presence of H2) stops with the production of acetylene. Finally, it should be mentioned that cyclohexene, either in H2 or in He, is not catalytically transformed on silica activated by hydrogen spillover. 

Titania/silica catalysts were prepared by a conventional procedure (precipitation) and a complexing-agent assisted sol-gel method. The effect of preparation methods of titania/silica catalysts on their properties and catalytic activities in the oxidation of olefins were examined. The sol-gel method gave the best dispersion of titania. In contrast, using the precipitation method, titania is deposited at the external surface of silica with formation of crystalline particles. The sol-gel catalysts are more effective for epoxidation of olefins because of the high dispersion of Ti in them. 

The value of this constant comes from the free energies of formation of the reactants and products of Eq. 7.10. The equilibrium constant defines the soluble silica activity at which gibbsite and kaolinite are in equilibrium ... 

Experimental work by others indicates that the activity ratio of alkali ions to hydrogen ions and the activity of silica are the major chemical parameters of the pore water that control whether clay minerals, zeolites, or feldspars will form at conditions that approximate surface temperatures and pressures (35, 52, 53). The formation of zeolites and feldspars is favored over clay minerals by relatively high alkali ion to hydrogen ion activity ratios and by relatively high silica activities. 

Cracking, a rupturing of carbon-carbon bonds—for example, of gas oils to gasohne—is favored by sihca-alumina, zeolites, and acid types generally. Zeohtes have pores with small and narrow size distribution. They crack only molecules small enough to enter the pores. To restrain the undesirable formation of carbon and C3-C4 hydrocarbons, zeolite activity is reduced by dilution to 10 to 15 percent in silica-alumina. 

Raman spectroscopy has provided information on catalytically active transition metal oxide species (e. g. V, Nb, Cr, Mo, W, and Re) present on the surface of different oxide supports (e.g. alumina, titania, zirconia, niobia, and silica). The structures of the surface metal oxide species were reflected in the terminal M=0 and bridging M-O-M vibrations. The location of the surface metal oxide species on the oxide supports was determined by monitoring the specific surface hydroxyls of the support that were being titrated. The surface coverage of the metal oxide species on the oxide supports could be quantitatively obtained, because at monolayer coverage all the reactive surface hydroxyls were titrated and additional metal oxide resulted in the formation of crystalline metal oxide particles. The nature of surface Lewis and Bronsted acid sites in supported metal oxide catalysts has been determined by adsorbing probe mole-... 

The Zincke reaction has also been adapted for the solid phase. Dupas et al. prepared NADH-model precursors 58, immobilized on silica, by reaction of bound amino functions 57 with Zincke salt 8 (Scheme 8.4.19) for subsequent reduction to the 1,4-dihydropyridines with sodium dithionite. Earlier, Ise and co-workers utilized the Zincke reaction to prepare catalytic polyelectrolytes, starting from poly(4-vinylpyridine). Formation of Zincke salts at pyridine positions within the polymer was achieved by reaction with 2,4-dinitrochlorobenzene, and these sites were then functionalized with various amines. The resulting polymers showed catalytic activity in ester hydrolysis. ... 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

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