Sol-gel procedures

Recently, Mark and co-workers also reported on organophilic silica formed by the combination of the sol-gel procedure and water-in-oil micro-emulsion method, in which methacryloyloxypropyltrimethoxysilane was used as one component of silica matrix [8]. The size of the silica particle was controlled by the content of water and emulsifier used. The surface of the particles was effectively covered with methacryloyl. organic groups. This organophilic silica is expected to be used as a novel component of composite materials. 

In the sol-gel procedure for the preparation of hybrids, polymeric acid catalysts such as poly (styrene sulfonic acid) were also used instead of hydrogen chloride [14]. The polymeric acid catalyst was effective for the preparation of hybrids at a similar level to that of hydrogen chloride catalyst. In some cases, the increased modulus was observed due to the higher extent of reaction. No difference was observed in morphologies between the hybrids prepared with polymeric and small molecule acid catalysts. The method using polymeric acid catalyst may depress the ion-conductive property, characteristic to the mobile acidic small molecules. Polymeric catalyst may also influence the rheology of the resulting hybrids. 

Poly(arylene ether ketone) and poly(arylene ether sulfone) were also tried to be incorporated into the hybrids with silica gel by means of the sol-gel procedure [19, 20], For example, triethoxysilyl-terminated organic polymer was subjected to co-hydrolysis with tetraethoxysilane. A systematic change in mechanical and physical properties of the hybrid glass has been found with the content of organic polymer and the annealing temperatures. 

The basic reactions of the sol-gel procedure are shown in Eqs. (1-3), in which the species of Si—OH are the key intermediates. These Si—OH groups are known as Bransted acids. On the other hand, the amide carbonyl groups are... 

Prepared from sodium silicate using the sol-gel procedure. Silica gel is the most widely used general purpose adsorbent for sample cleanup although it may irreversibly bind some strongly basic substances. Generally considered to be slightly acidic in character but this may vary with the method of prq ration. Activated by heating at about 180 C for 8-12 hours. 

Similarly to the above-mentioned entrapment of proteins by biomimetic routes, the sol-gel procedure is a useful method for the encapsulation of enzymes and other biological material due to the mild conditions required for the preparation of the silica networks [54,55]. The confinement of the enzyme in the pores of the silica matrix preserves its catalytic activity, since it prevents irreversible structural deformations in the biomolecule. The silica matrix may exert a protective effect against enzyme denaturation even under harsh conditions, as recently reported by Frenkel-Mullerad and Avnir [56] for physically trapped phosphatase enzymes within silica matrices (Figure 1.3). A wide number of organoalkoxy- and alkoxy-silanes have been employed for this purpose, as extensively reviewed by Gill and Ballesteros [57], and the resulting materials have been applied in the construction of optical and electrochemical biosensor devices. Optimization of the sol-gel process is required to prevent denaturation of encapsulated enzymes. Alcohol released during the... 

The first belief in the possibility of enzyme stabilization on a silica matrix was stated by Dickey in 1955, but he did not give experimental evidence, only mentioning that his experiments were unsuccessful [65]. A sol-gel procedure for enzyme immobilization in silica was first developed by Johnson and Whateley in 1971 [66]. The entrapped trypsin retained about 34 % of its tryptic activity observed in solution before the encapsulation. Furthermore, the enzyme was not released from the silica matrix by washing, demonstrating the increased stability and working pH range. Unfortunately, the article did not attract attention, although their method contained all the details that may be found in the present-day common approach. This was probably due to its publication in a colloid journal that was not read by biochemists. 

ITQ-6 functionalized by 3-aminopropyl-triethoxysilan Lipase entrapped in Si-MCM-41 prepared by sol-gel procedure... 

It is interesting to note that using the sol-gel procedure (I) the pre-formation of the rhodium diphosphine complex suppressed the formation of ligand free rhodium-cations on the silica surface. This approach gave rise to a well-defined, very selective hydroformylation catalyst. All immobilised catalysts were 10 to 40 times slower than the homogeneous catalyst under the same conditions, the sol-gel procedure yielding the fastest catalyst of this series. 

Figure 3.7. The concept of chemistry in iutcrphascs visualisation of two different ways to obtain such a material, sol-gel procedure using different reagents (left) and silica grafted materials with sufficiently long spacers (right). Solvent occupies the space between the spacers...

Lindner et al. carried out detailed studies on the use of the sol-gel procedure to obtain new materials as suitable matrices [45]. A co-condensation of Si(OR)4, Si(OR)3-spacer-Si(OR)3 and Si(OR)3-spacer-ligand resulted in new hybrid materials, the properties of which strongly depended upon the spacer and the ratio of the different components used. Most of these materials were characterised with IR, EXAFS, and EDX spectroscopy, scanning electron microscopy and solid state NMR spectroscopy. Solid state 31P NMR was used to quantify the mobility of the interphases the changes in line-... 

Lindner et al. also prepared catalysts for the hydroformylation of olefins using a sol-gel-procedure to obtain rhodium complexes 5a-c (Figure 3.9). The bifunctional cocondensation agent (MeO)3Si(CH2)6Si(OMe)3 and a carbonylhydrido-... 

This sol-gel procedure is an elaboration on well established entrapment methods [29], but with the added advantage of stability and better flow properties. Interestingly, none of the examples presented thus far demonstrate competitive behavior between multiple ligands (i.e. displacement) in the FAC analysis of trimethoprim and pyrimethamine a reversed order of elution based on is described, but this could simply be due to the shift towards an on-rate limited situation for higher affinity compounds, as described earlier. Erosion of dynamic competition between ligands could occur if the sol-gel allows convective mixing of the entrapped protein however the bimodal pore structure of these materials would... 

In the sol-gel process, ceramic polymer precursors are formed in solution at ambient temperature shaped by casting, film formation, or fiber drawing and then consolidated to furnish dense glasses or polycrystalline ceramics. The most common sol-gel procedures involve alkoxides of silicon, boron, titanium, and aluminum. In alcohol water solution, the alkoxide groups are removed stepwise by hydrolysis under acidic or basic catalysis and... 

MTO has been heterogenised inside the porous systems of hybrid silica matrices [59,60]. Thus, it has been supported on silica functionalised by polyethers [59]. These polyethers are covalently attached to a silicate particle using sol-gel procedures and act as the solvent for the catalyst (Scheme 14). 

Although tetraalkoxysilanes are usually applied as silica precursors in sol-gel procedures, the oligomeric type precursors can also be used instead. Ethyl silicate 40 (ES 40) is the most common commercial form of ethoxypolysiloxane available at prices competitive to those of TEOS/TMOS. Hence its application in the synthesis of composite adsorbents could be economically attractive provided that this would has little or no adverse effect on their final properties. The objective of this work was to test an alternative, cost-effective preparation procedure making use of the low-cost reagents ES 40 and 95wt% EtOH and to compare the properties of the adsorbents obtained with those of their counterparts similar in composition, yet synthesized with the use of TEOS. 

The hybrid xerogels exhibiting remarkable adsorption capacities of water vapor can be prepared in a cost-effective way using the modified one-step, sol-gel procedure with ethyl silicate 40 as silica precursor. 

The sol-gel procedure enables encapsulation of enzymes in optically transparent, porous silicate matrices, under mild room-temperature conditions. The small pores prevent microbial degradation and, due to the biomolecule size, they will not diffuse out of the polymer network. The physical encapsulation avoids self-aggregation effects as well as protein unfolding and denaturalization. At the same time, the catalytic activity is maintained as the enzymes are able to react with small substrates that can transfer across or within the support, assuring continuous transformations [75]. 

Ghanem, A. The utility of cyclodextrins, sol-gel procedure and gas chromatography in lipase-mediated enantioselective catalysis kinetic resolution of secondary alcohols. PhD Thesis, University of Tubingen, 2002. 

Chen et al. [152] prepared a Pt/Al203 catalysts by different sol-gel procedures, either by depositing the carrier sol and the catalyst solution simultaneously or subsequently on to micro channels made of silicon wafers. 

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