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Kinetics promotion

As already shown in Scheme 18, bulky groups kinetically promote the tendency of ring contraction. A comparison of the crystal structures of (Me2Si-NH)4 and the bis(tert-butyldifluorosilyl)-substituted eight-membered ring shows the following significant aspects ... [Pg.39]

In the acidic route (with pH < 2), both kinetic and thermodynamic controlling factors need to be considered. First, the acid catalysis speeds up the hydrolysis of silicon alkoxides. Second, the silica species in solution are positively charged as =SiOH2 (denoted as I+). Finally, the siloxane bond condensation rate is kinetically promoted near the micelle surface. The surfactant (S+)-silica interaction in S+X 11 is mediated by the counterion X-. The micelle-counterion interaction is in thermodynamic equilibrium. Thus the factors involved in determining the total rate of nanostructure formation are the counterion adsorption equilibrium of X on the micellar surface, surface-enhanced concentration of I+, and proton-catalysed silica condensation near the micellar surface. From consideration of the surfactant, the surfactants first form micelles as a combination of the S+X assemblies, which then form a liquid crystal with molecular silicate species, and finally the mesoporous material is formed through inorganic polymerization and condensation of the silicate species. In the S+X I+ model, the surfactant-to-counteranion... [Pg.476]

The smectite is believed to have reduced the permeability of the sands so that flow was focused through other, more permeable zones, where cementation began. Interestingly, cementation continued in these relatively clay-free sands to the point that permeability was below 1 mD, much lower than for adjacent clayey sands. Presumably, favourable nucleation kinetics promoted continued cementation of the sand beyond the point of unfavourable mass transfer properties. [Pg.270]

A kinetic model was developed from the results of catalyst screening studies that relates reaction rates to temperature, space velocity, and steam to gas ratio. A finding of kinetic modeling studies is that conversion of carbon monoxide could be enhanced in a thermal gradient compared to reactions conducted isothermally. By managing the temperature profile of a reactor, reactants can be fed at a high temperature where rapid kinetics promotes an initial approach to equilibrium. As the reaction mixture is cooled, conversion is increased due to more favorable thermodynamic driving forces. [Pg.317]

Luo, Y, and GUIs, D., 1996, Heterogeneous photocatalytic oxidation of trichloroethylene and toluene nrixtru es in ah kinetic promotion and inhibition, time-dependent catalyst activity, J of Catalysis. 163 1 -11. [Pg.167]

TMED, (CH3)2NCH2CH2N(CH3)2. B.p. 122 C a hygroscopic base which forms a hydrocarbon-soluble stable chelate with lithium ions and promotes enhanced reactivity of compounds of lithium, e.g. LiAlH4, UC4H9, due to enhanced kinetic basicity of the chelate. Used in polymerization catalysts, tetramethyl lead, TML 5 lead tetramethyl. [Pg.391]

Related results of promotion (catalysis) and inliibition of stereonuitation by vibrational excitation have also been obtained for the much larger molecule, aniline-NHD (CgH NHD), which shows short-time chirality and stereonuitation [104. 105]. This kind of study opens the way to a new look at kinetics, which shows coherent and mode-selective dynamics, even in the absence of coherent external fields. The possibility of enforcing coherent dynamics by fields ( coherent control ) is discussed in chapter A3.13. [Pg.2144]

Above pH 9, decomposition of ozone to the reactive intermediate, HO, determines the kinetics of ammonia oxidation. Catalysts, such as WO, Pt, Pd, Ir, and Rh, promote the oxidation of dilute aqueous solutions of ammonia at 25°C, only two of the three oxygen atoms of ozone can react, whereas at 75°C, all three atoms react (42). The oxidation of ammonia by ozone depends not only on the pH of the system but also on the presence of other oxidizable species (39,43,44). Because the ozonation rate of organic materials in wastewater is much faster than that of ammonia, oxidation of ammonia does not occur in the presence of ozone-reactive organics. [Pg.492]

It is established by biotesting that complexation and adsor ption ar e the most important processes promoting transformation of metal compounds in biologically and the chemically inactive forms and essential decrease their toxicity. The kinetics data have shown the maximal decrease in toxicity was observed in natural water where the complexation occurred with participation of both DOM and added HS. [Pg.27]

Special interactions of the charged reagent with the substrate can lead to kinetic complications and to exceptional substrate reactivity. For example, the strongly basic alkoxide ion promotes ionization of... [Pg.291]

Hall and Hassell (50) continued these studies with the intention of proving that possible traces of oxide dissolved in the metal play no significant role in the poisoning or promoting effects arising from hydrogen which had been presorbed during the pretreatment procedure. The catalysts were prepared in essentially the same manner as before. The kinetics... [Pg.269]


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See also in sourсe #XX -- [ Pg.21 ]




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