Non-hydrolytic processes

The sol-gel methodology can also be applied to non-hydrolytic processes. The preparation of methylsilsesqui-oxane-titanium oxide hybrid by the etherolysis-condensation of a mixture of methyltrichlorosilane and metal chloride,353 354 and by the condensation of preformed poly(methylsilsesquioxane) carrying Si-OH and Si-OCHj groups with titanium(iv) -butoxide,355 has been reported. 

Acosta S., Corriu RJ.P., Leclercq D., Hubert Mutin P., Vioux A. Monophasic pre-mullite gels prepared by a non-hydrolytic process. Mater. Res. Soc. Symp. Proc. 1994 346 345-350 Alder K.I., Sherrington D.C. Synthesis and characterisation of Ti(IV) and Zr(IV)-containing elastomeric polysiloxane networks a possible route to interesting heterogeneous catalysts. J. Mater. Chem. 2000 10 1103-1111... 

The dinuclear Ti conqjlex assembles spontaneously on the silica surface via a non-hydrolytic alkoxide condensation reaction which generates the Ti-O-Ti bridge as well as C3H6 (16). Confirmation of this condensation process was obtained via the exposure to Ti(0 Pr)4 of a silica fidly loaded (i.e., with no unreacted hydroxyl groups) with a mononuclear titanium alkoxide coir lex. The latter was prepared by grafting of Ti(NEt2)4 followed by ligand metathesis with PrOH, eq 2. The alkoxide condensation reaction proceeds quantitatively (13), eq 3 ... 

Most industrial enzymatic processes refer to reactions conducted by hydrolases in aqueous medium for the degradation of complex molecules (often polymers) into simpler molecules in conventional processes with limited added value (Neidelman 1991). Reasons underlying are clear since hydrolases are robust, usually extracellular and have no coenzyme requirements, which makes them ideal process biocatalysts. Enzyme immobilization widened the scope of application allowing less stable, intracellular and non-hydrolytic enzymes to be developed as process biocatalysts (Poulsen 1984 D Souza 1999), as illustrated by the paradigmatic case of glucose isomerase for the production of HFS (Carasik and Carroll 1983) and the production of acrylamide from acrylonitrile by nitrile hydratase (Yamada and Kobayashi 1996). 

Both stages are controlled by condensation chemistry that can include, as a first step, hydrolysis of hydrated metal ions or metal alkoxide molecules [6, 7] (hydrolytic sol-gel processing). The condensation chemistry in this case is based on olation/oxolation reactions between hydroxylated species. The hydroxylated species for further condensation can be formed also by a non-hydrolytic route, that is, by reactions between metal chlorides and alcohols with electron-donor substituents [8]. The nonhydrolytic sol-gel processing may... 

PMMA-Si02-Zr02 nanohybrid films were prepared successfully using a novel non-hydrolytic sol-gel process and, owing to their excellent optical transparency combined with high thermal stability, were stated to have high potential in applications for optical devices at elevated temperatures [307]. 

It is well known that various parameters (e.g. solvent, pH, immobilization, chemical modification and temperature) can have an effect on the enantioselectivity of enzyme-catalysed processes. Most studies in this respect have been carried out on hydrolytic enzymes, especially lipases, esterases and proteases [28]. Recent reports, especially those involving non-hydrolytic enzymes, are discussed below. 

Constrained glycosides as conformational probes for non-hydrolytic biochemical processes... 

Commonly two main sol-gel methods are used. The first one involves aqueous solutions of salt precursors in which the formation of hydroxyl species, then their polycondensation, is induced by varying the pH. The second method consists of generating the hydroxyl species by hydrolyzing metal-organic molecular precursors, such as alkoxide derivatives, most often in alcohohc solution. Therefore the terms sol-gel and water are closely related, and the term non-hydrolytic sol-gel sounds incongruous. However, the desire to explore new condensation reactions in low-temperature processes where kinetics control the structure and the texture of the products has prompted some interest in non-hydrolytic sol-gel methods over the last decade. Some conprehensive reviews have been pubhshed on the topic (Vioux, 1996, 1997 Hay, 2001). This chapter aims at outlining the main results reported in the area, including in the most recent period. 

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