Precursor Reactivity

Sol-gel preparation offers excellent control of mixing because of its ability to alter relative precursor reactivity. Qualitatively, we expect good mixing when the two precursors have similar reactivities and poor mixing when they do not. As pointed out in Section 2.1.4.2.A, both hydrolysis and condensation are nucleophilic displacement reactions. Effective strategies in matching precursor reactivity thus include ... 

The ability to introduce several components into solution during the sol-gel step makes this approach especially attractive for the preparation of multicomponent oxides and bimetallic catalysts. Of course, the solution chemistry becomes more complex with additional components. But many of the concepts that we have discussed, such as matching relative precursor reactivity and changing the microstructure of the gel network, remain valid in principle. Furthermore, a promoter or an active species can be introduced the same way. There have been recent reports on the onc-step preparation of zirconia-sulfate aerogels [45] and Li/MgO catalysts [46], These samples arc active in the isomerization of n-butane and the oxidative coupling of methane, respectively. 

In some cases. Phase I metabolites may not be detected, owing to their instability or high chemical reactivity. The latter type are often electrophilic substances, called reactive intermediates, which frequently react non-enzymically as well as enzymically with conjugating nucleophiles to produce a Phase II metabolite. A common example of this type is the oxidative biotransformation of an aromatic ring and conjugation of the resulting arene oxide (epoxide) with the tripeptide glutathione. Detection of metabolites derived from this pathway often points to the formation of precursor reactive electrophilic Phase I metabolites, whose existence is nonetheless only inferred. 

After the addition of the inorganic precursor, cooperative assembly between the surfactant and the inorganic precursor takes place, leading to the formation of the mesostructure. This approach is similar to the one used by the Mobil scientists and is also the most commonly used one for the synthesis of nonsilica systems, where the control of precursor reactivity demands lower concentrations of the inorganic precursors and hence of the surfactant. In Fig. 2, the formation of a mesostructure by this approach is illustrated (route I). 

Photochemistry provides a powerful and versatile means of probing the mobility of species adsorbed on surfaces (9,10)- The basic reason for this power is that the absorption of light can produce, instantaneously on the time scale of diffusion, reactive intermediates whose chemistry is totally determined by their mobility on the surface of the porous solid. With proper selection of the reactant species, information concerning the mobility of the precursor reactive intermediates can be. locked into the structure of the stable, isolable products. In such cases, (11-15) product analysis provides a simple, yet elegant method to obtain information on the dynamics of motion of molecules adsorbed on the zeolites. 

Historically, QDs were first synthesized in glass matrices where the slow difiusion of precursors provided some measure of size control. In the last couple of decades, colloidal techniques have advanced to the point that parameters such as precursor reactivity, temperature, surfactants etc. can be independently tuned to control and regulate nanocrystal formation. This enables the synthesis of high quality solvent dispersible particles that may be further processed using simple wet-chemical methods. Qne of the earliest techniques employed to achieve this is known as the arrested precipitation method where the semiconductor growth is arrested after the... 

A reactive SPS method has been developed to prepare AlON transparent ceramics [291]. AI2O3 and AIN powder mixtures were used as precursors. Reactive SPS was conducted at temperatures between 1400 °C and 1650 °C for 15-45 min at 40 MPa... 

PEG and PEGylated proteins, including various structures of polymer precursors, reactive groups and reactions used for conjugation with proteins as well as therapeutical applications of the conjugates have been recently reviewed. The reader interested in the field of synthesis and use of PEG-protein conjugates for therapeutic application is referred to those reviews. 

Surface structures influenced by difference in precursor reactivity... 

Sol-gel process allows excellent control of the synthesised mixed oxide textural properties, such as surface area and pore volume. Within the sol-gel process, relative precursor reactivity can be used to control homogeneity [2]. 

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