Natural sol-gel materials

In any case, it is clear that, for its own nature, sol-gel material is suitable for the formation of many different composite materials [207] inorganic and organic polymers, as well as many different kinds of nano-objects possessing effective electrocatalytic properties, can find stable inclusion into a host matrix possessing all the characteristics previously listed. The most simple approach consists of adding the filler to the siliceous matrix after sol-gel formation. However, composite materials can also be obtained by synthesizing the sol-gel film in the presence of the precursor of the filler, e.g., a monomer [227] or a metal complex [228]. The synthesis of the composite material is finalized in a subsequent step, e.g. by a chemical oxidation or reduction, leading to polymer chains or metal nanoparticles, respectively, included inside the sol-gel matrix. 

Sol-gel chemistry offers flexible methods for the preparation of porous metal oxides such as the transition aluminas used as catalyst supports. The physical properties of sol-gel materials depend on the nature of the reactants, the rate of mixing and the conditions of drying. High surface area aluminas have been prepared from various alkoxide and salt solutions and their textures have been examined extensively. The interest in the use of these chemically prepared materials is largely for catalyst and absorbent applications [1]. The first user of alkoxide precursor was Teichner who prepared pure aluminas by the water vapour action on aluminimn methoxide [2] and reported materials with surface area of 200 m /g. Harris and Sing [3] reported gels prepared from aluminium isopropoxide with water and... 

The results of the study of diffusion rate as a function of temperature for sol-gel spheres, together with results from fused spheres run at approximately the same temperature, are presented in Table II. It is apparent that the diffusion coefficients for the sol-gel material are consistently slightly greater at the same temperature. In addition to fundamental differences in the nature of the samples, the oxygen pressure was slightly higher (30.6 mm versus... 

In addition to compositional analysis of the surface, the features of a photoelectron spectrum, particularly the binding energies, can be used to determine oxidation states, coordination numbers and the nature of the nearest neighbor atoms. Some examples of this can be seen by examining some of the applications of XPS to sol-gel materials. 

We will give emphasis on silica-based materials, which have attracted great attention, owing to the versatile chemistry of silicon compoimds, the smooth hydrolysis-condensation of the alkoxy derivatives, and the stability of the silicon-carbon bonds. Moreover, it offers easy processing of the sol-gel materials to generate films or particles with controlled size [5]. Silica is produced upon simple hydrolysis-condensation of an alkoxide precursor to form a three-dimensional covalent network, the properties of which depend on the reaction conditions (e.g., nature of the alkoxide precursor, solvent, catalyst, concentration, and temperature) (Scheme 4.1). 

Overall, the field of biomimetic sol-gel materials has proven to be a rich source of fundamental and applied developments over a wide multidisciplinary landscape. It is also an area with a great future ahead of it, which, apart from its scientific and technical interest, will contribute to the current societal and philosophical debates related to the frontiers between the natural and the synthetic worlds and the place of living matter in man-made technology. 

Similarly, a composite of hydroxyapatite and a network formed via cross-linking of chitosan and gelatin with glutaraldehyde was developed by Yin et al. [ 169]. A porous material, with similar organic-inorganic constituents to that of natural bone, was made by the sol-gel method. The presence of hydroxyapatite did not retard the formation of the chitosan-gelatin network. On the other hand, the polymer matrix had hardly any influence on the high crystallinity of hydroxyapatite. 

The ethylene glycol-containing silica precursor has been combined, as mentioned above, with most commercially important polysaccharides and two proteins listed in Table 3.1. In spite of the wide variety of their nature, structure and properties, the jellification processes on addition of THEOS to solutions of all of these biopolymers (Scheme 3.2) had a common feature, that is the formation of monolithic nanocomposite materials, proceeding without phase separation and precipitation. The syner-esis mentioned in a number of cases in Table 3.1 was not more than 10 vol.%. It is worthwhile to compare it with common sol-gel processes. For example, the volume shrinkage of gels fabricated with the help of TEOS and diglyceryl silane was 70 and 53 %, respectively [138,141]. 

One of the most promising applications of enzyme-immobilized mesoporous materials is as microscopic reactors. Galameau et al. investigated the effect of mesoporous silica structures and their surface natures on the activity of immobilized lipases [199]. Too hydrophilic (pure silica) or too hydrophobic (butyl-grafted silica) supports are not appropriate for the development of high activity for lipases. An adequate hydrophobic/hydrophilic balance of the support, such as a supported-micelle, provides the best route to enhance lipase activity. They also encapsulated the lipases in sponge mesoporous silicates, a new procedure based on the addition of a mixture of lecithin and amines to a sol-gel synthesis to provide pore-size control. 

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