Precursors organically-modified silica

An example of the appropriate application of organically-modified silica precursors is alkoxides with an alkyl group. When methyltrimethoxy- or methyl-triethoxysilane (Figure 3.2) was added in formulations to increase the hydro-phobicity of ORMOSILs, it resulted in a better enzymatic activity of lipases immobilized in the alkyl-modified silica than in a hydrophilic matrix fabricated by means ofTEOS alone [51,80,129-133]. Similarly, an increased stability of lipase from Candida antarctica B was observed after its immobilization in a silica matrix... 

Organically-modified silica precursors have three alkoxy groups. As a result, alcohol is separated after the hydrolysis. Its appearance causes a detrimental effect on biopolymers typical to that ofalkoxides. The next disadvantage of these precursors is the absence of universality. They are appropriate only in particular cases. 

Sol-gel techniques were used to prepare porous, organically modified silica materials. The introduction of organic groupings was carried out with alkoxysilanes as precursors methyl and propyl amino groups were used. The results show that high-porosity materials can be synthesized the microstructure strongly depends on reaction conditions such as composition, solvent, catalyst type, and concentration. Microstructure tailoring affects mechanical as well as adsorption properties, and custom-made materials such as abrasives and adsorbents with special properties were synthesized. 

Sol-gel matrices can also provide a chemical surrounding that favors enzymatic reactions. Lipases act on ester bonds and are able to hydrolyze fats and oils into fatty acids and glycerol. These are interphase-active enzymes with lipophilic domains and the catalytic times reaction occurs at the water-lipid interface. Entrapped lipases can be almost 100 times more active when a chemically modified silica matrix is used. The cohydrolysis of Si(OMe)4 and RSi(OMe)3 precursors provides alkyl groups that offer a lipophihc environment that can interact with the active site of Upases and increase their catalytic activity. Such entrapped lipases are now commercially available and offer new possibilities for organic syntheses, food industry, and oil processing. ... 

In some cases the heterogeneous version of a catalyst can be prepared by direct reaction of that catalyst with a suitable support material. Thus reactive Lewis acids such as aluminium chloride will react with hydroxylated materials such as silica gel to give directly bonded surface species such as -OAICI2.20 Another single-step route to the supported catalyst is via sol-gel techniques, typically to produce an organically modified mesoporous silica. This is based on the co-polymerisation of a silica precursor and an organosilicate precursor (Figure 1.5). 

An instructive example of the attainment of organically modified mesoporous silica is the synthesis [27] of hexagonal mesoporous sdicas chemically modified with en moieties via co-condensation of TEOS with two different silylating agents (i) N-[3-(trimethoxysilyl)propyl]-ethylenediamine (SiNN) and (ii) the new agent prepared from the incorporation of the en molecule into the epoxide group of the precursor 3-glycidoxypropyltrimethoxysilane (Siepox). 

Another issue posed by silica capsules has to do with the brittle nature of the glass shell that often requires (see the following text for encapsulated waxes) the use of organically modified silanes in the precursor mixture along or in place of TEOS. 

Table 3.2. Co-Precursors (CP), silylating chemicals (S), and organic modifiers (OM) used in the production of hydrophobic silica aerogels...

Ormosil ORganically Modified SILicates hybrid silica aerogels obtained by gelation of a solid network with functionalized silica precursors on which some specific organic moieties have been grafted... 

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