Sol-gel preparation

Because the grafting techniques described in Section 3.5.1.2 lead to rather low loading, it is of interest to incorporate functional groups directly and in large amounts into a material via a sol-gel process. Open and patent literature report colloidal silicas [23], aerogels [24] and xerogels [14] with 3-mercaptopropyl, 2-phenylethyl, or benzyl functional groups. These materials are valuable precursors to sulfonated polysiloxanes (Section 3.3). 

Poly(sulfophenyl)siloxane (2.0 meq g ), poly(sulfobenzyl)siloxane, poly(sul-fophenylethyl)siloxane, and poly(sulfopropyl)siloxane (0.8 meq g ) were prepared by Ono et al [25]. Documented applications of these sulfonated polyorgano-siloxanes include the dehydration of alcohols, the esterification of isobutanol with acetic acid and the vapor-phase nitration of benzene. 

0 mL g pore volume. The catalyst has been tested, e. g. in the esterification of free tallow fatty acids, in the synthesis of dioctyl phthalates, and in the pro-pylation of mesitylene and anisole [26]. In the patent literature Panster and Kleinschmit also describe the synthesis of a phenylenesulfonate containing orga-nopolysiloxane (2.2-3.2 meq g ) [27]. 

---

In order to locate Ti02 mainly at the exterior surftice in the vicinity of macropores of the activated carbon, a modified sol-gel preparation method was employed. 

Kesmez, O., Erdem Camurlu, H., Burunkaya, E. and Arpac, E. (2009) Sol-gel preparation and characterization of anti-reflective and self-cleaning Si02-Ti02 double-layer nanometric films. Solar Energy Materials and Solar Cells, 93, 1833-1839. 

Figure 6 A flow diagram showing the reaction steps involved in the sol-gel preparation of Ti02 colloids...

Ren, L., Tsuru, K., Hayakawa, S. and Osaka, A. (2001) Sol-gel preparation and in vitro deposition of apatite on porous gelatin-siloxane hybrids. Journal of Non-Crystalline Solids, 285, 116-122. 

Sakka S., Kozuka H., Sol-gel preparation of coating fdms containing noble metal colloids, J. Sol-Gel Sci. Technol. 1998 13 701-705. 

To increase the surface area, the resin can be supported on porous carriers, or it can be directly incorporated into silica by a sol-gel preparation technique. Both methods have been used by Botella et al. (205), who compared several composite Nafion/silica samples with varying surface areas and Nafion loadings for isobutane/2-butene alkylation at 353 K. Furthermore, supported and unsupported Nafion samples were used. As expected, the unsupported resin with its low... 

Immobilizing DENs within a sol-gel matrix is another potential method for preparing new supported catalysts. PAMAM and PPI dendrimers can be added to sol-gel preparations of silicas " and zinc arsenates to template mesopores. In one early report, the dendrimer bound Cu + ions were added to sol-gel silica and calcined to yield supported copper oxide nanoparticles. Sol-gel chemistry can also be used to prepare titania supported Pd, Au, and Pd-Au nanoparticle catalysts. Aqueous solutions of Pd and Au DENs were added to titanium isopropoxide to coprecipitate the DENs with Ti02. Activation at 500°C resulted in particles approximately 4 nm in diameter. In this preparation, the PAMAM dendrimers served two roles, templating both nanoparticles and the pores of the titania support. 

FIGURE 7.5. Infrared spectra of CO adsorbed on two sol-gel immobilized Pt5o/Si02 catalysts. Catalyst A was prepared by adding the DENs solution at the beginning of the sol-gel preparation. For catalyst B, the DENs solution was added after the sols were allowed to form for 24 hours, but before gellation. Both catalysts were oxidized at 300°C for 16 h and reduced at 300°C for 2 h. Reprinted with permission from Appl. Catal. A. 2005, 292, 124-129. Copyright 2005 Elsevier. 

Sol-gel preparations of tetraethoxysilane can be spun into fibers once the appropriate viscosity has been achieved. These fibers are only slightly weaker than silica-glass fibers. 

Zhao G, Kozuka H, Lin H, Yoko T (1999) Sol-gel preparation of Tii xVx02 solid solution film electrodes with conspicuous photoresponse in the visible region. Thin Solid Films 339 123-128... 

Bischoff BL, Anderson MA (1995) Peptization process in the sol-gel preparation of anatase Xi02. Chem Mater 7 1772-1778... 

Photocatalysis based on TiO has attracted much attention for enviromnental cleaning and antibacterial applications [1-3], In order to synthesize TiO nanoparticles, various modification of the sol-gel method have been widely used. However, sol-gel prepared TiO requires a post-calcination process for crystallization [4], which limits the applicability of TiO coatings to temperatnre resistant substrates. On the other hand, the solvothermal method, which does not need to be followed... 

Fig. 1.2 X-ray diffraction patterns of TiO powders (a) sol-gel preparation, (b) commercial TiO (Degussa P25), and (c) solvothermal preparation. (A anatase, R rutile)...

Several examples discussed below illustrate the method two of the examples have been chosen because they have interesting properties and uses, discussed later on in this book. Many other materials have been prepared by the sol-gel method, and other sol-gel preparations have been employed for the materials chosen. Therefore, these examples should be taken as illustrative and not as the main uses of the method. 

Lithium niobate is a ferroelectric material used as an optical switch. Preparation by the simple ceramic method leads to problems in obtaining the correct stoichiometry, and a mixture of phases often results. Several sol-gel preparations have been described, their advantage being the lower temperature required for the preparation and the greater homogeneity of the product. One such preparation starts with lithium ethoxide (LiOC2H5 (or LiOEt)) and niobium ethoxide Nb2(OEt)io. Each ethoxide was dissolved in absolute ethanol and the two solutions mixed. The addition of water leads to partial hydrolysis giving hydroxy-alkoxides, for example ... 

Mesoporous solids including silicas and acid-treated clays can be functionalised at their surfaces so as to provide high local concentrations of active sites. These sites can be introduced by post-modification or via sol-gel preparations. In this way a range of novel materials with useful catalytic and other properties can be prepared. One of the most valuable applications for these materials is as replacements for environmentally hazardous reagents including corrosive mineral and Lewis acids, caustic bases and toxic metallic compounds. 

The materials can be made by post-modification of preformed or commercial mesoporous solids or in some cases via sol-gel preparation of organically modified solids. The former method has been successfully used to prepare immobilised Lewis... 

In a typical sol-gel preparation of an organically modified mesoporous solid a total of 0.1 mole of silane (made up of x mole of tetrethoxysilane and 0.1 - x mole of trialkoxy(monofuntional)silane) is added to a solution of the templating amine (typically n-dodecylamine) in aqueous ethanol. The template is removed from the subsequent gel by washing with ethanol after which the material is filtered and dried. 

Endowing these polymolecular entities with recognition units and reactive functional groups may lead to systems performing molecular recognition or supramolecular catalysis on external or internal surfaces of organic (molecular layers, membranes, vesicles, polymers, etc.) [7.1-7.13, A.41] or inorganic (zeolites, clays, sol-gel preparations, etc.) [7.14-7.20] materials. 

The interaction of the metal alkoxides with the salts of carboxylic acids or with p-diketonates of other metals is especially attractive for the synthesis of bimetallic molecular precursors in the cases, when the preparation of the alkoxide of the other metal is somehow hindered or it is insoluble or irreactive under the conditions applied. This method has been widely used for the sol-gel preparation of HTSC materials (because of low solubility and reactivity of Cu(OR)2) and lead-containing ferroelectrics (in the view of difficult synthesis and low stability of Pb(OR)2). It should be mentioned that the reaction between a metal alkoxide and a functional derivative does far not always lead to the formation of a mixed-ligand bimetallic complex ... 

Attachment of a trialkoxy-Si group to the macrocycle leads to a precursor that can be mixed with TEOS in a sol-gel preparation this leads to a matrix with superior oxidative stability (116). 

Sol-gel preparation Siliceous framework (EtO)3 Si-modified porphyrin Battioni etal. (116)... 

---