Acid-catalyzed sol-gel process

The preferentially employed approach for the fabrication of inorganic (silica) monolithic materials is acid-catalyzed sol-gel process, which comprises hydrolysis of alkoxysilanes as well as silanol condensation under release of alcohol or water [84-86], whereas the most commonly used alkoxy-silane precursors are TMOS and tetraethoxysilane (TEOS). Beside these classical silanes, mixtures of polyethoxysiloxane, methyltriethoxysilane, aminopropyltriehtoxysilane, A-octyltriethoxysilane with TMOS and TEOS have been employed for monolith fabrication in various ratios [87]. Comparable to free radical polymerization of vinyl compounds (see Section 1.2.1.5), polycondensation reactions of silanes are exothermic, and the growing polymer species becomes insoluble and precipitates... 

Metallosilicates containing Cr, Cu or Mo have been prepared by an acid-catalyzed sol-gel process. Structural information about the silicates were obtained by elemental analysis, TGA, XRD, XRF, N2-physisorption, EPR, UVA IS and FTIR spectroscopy. The silicates are active catalysts for the oxidation of hydrocarbons with tert-butyl hydroperoxide. Catalyst stabilities with regard to metal leaching during catalytic oxidations were investigated. 

Composite Ti02 based catalysts were prepared by means of a modified acid-catalyzed sol-gel method from alkoxide precursor. The preparation was performed at room temperature as described elsewhere [5, 6]. The xerogel obtained at the end of the process was grinded into a fine powder and dried at room temperature. The powder was calcined at 400 °C in a flow of nitrogen for 2 hours to obtain the composite catalysts. Several materials were used as Ti02 supports siliceous mesoporous molecular sieves MCM-41 and MCM-48, synthesized hydrothermally according to the procedure outlined elsewhere [9] ... 

Why do acid-catalyzed and base-catalyzed sol-gel processes result in linear and branched metal oxides, respectively ... 

The fabrication of colloidal silica and optical glasses by the sol-gel process has attracted a great deal of attention (8). The process relies on the hydrolytic polycondensation reactions of alkoxysilanes, usually (EtO)4Si, in which the reactive silanols (EtO)4 Si(OH)n (n = 1-4) are formed. These then undergo acid- or base-catalyzed condensation with both water and alcohol formation, as shown in Scheme 2. 

As it was mentioned, the chemistry of the silica based sol-gel process comprises several steps. First, silicate precursor (e.g., tetraethylortosilicate TEOS or tetramethylosilicate TMOS) is mixed with water and catalyst and stirred for a few hours. This process leads to hydrolysis of the Si-O-R bonds. The hydrolysis reaction can be catalyzed by acids (HC1, HF, etc.) or bases (NH4OH, NaOH, etc.). The process is schematically described by equation ... 

Some non-silica sol-gel materials have also been developed to immobilize bioactive molecules for the construction of biosensors and to synthesize new catalysts for the functional devices. Liu et al. [33] proved that alumina sol-gel was a suitable matrix to improve the immobilization of tyrosinase for detection of trace phenols. Titania is another kind of non-silica material easily obtained from the sol-gel process [34, 35], Luckarift et al. [36] introduced a new method for enzyme immobilization in a bio-mimetic silica support. In this biosilicification process precipitation was catalyzed by the R5 peptide, the repeat unit of the silaffin, which was identified from the diatom Cylindrotheca fusiformis. During the enzyme immobilization in biosilicification the reaction mixture consisted of silicic acid (hydrolyzed tetramethyl orthosilicate) and R5 peptide and enzyme. In the process of precipitation the reaction enzyme was entrapped and nm-sized biosilica-immobilized spheres were formed. Carturan et al. [11] developed a biosil method for the encapsulation of plant and animal cells. 

The Stober method is also known as a sol-gel method [44, 45], It was named after Stober who first reported the sol-gel synthesis of colloid silica particles in 1968 [45]. In a typical Stober method, silicon alkoxide precursors such as tetramethylorthosili-cate (TMOS) and tetraethylorthosihcate (TEOS), are hydrolyzed in a mixture of water and ethanol. This hydrolysis can be catalyzed by either an acid or a base. In sol-gel processes, an acidic catalyst is preferred to prepare gel structure and a basic catalyst is widely used to synthesize discrete silica nanoparticles. Usually ammonium hydroxide is used as the catalyst in a Stober synthesis. With vigorous stirring, condensation of hydrolyzed monomers is carried out for a certain reaction time period. The resultant silica particles have a nanometer to micrometer size range. 

These reactions are responsive to both acid and base catalysis, and can be manipulated to give a variety of silica products, e.g., discrete particles, monolithic gels, films, and fibers. This technique of materials synthesis via alkoxide hydrolysis has become known as sol-gel processing (17). It should be noted, however, that under certain conditions, gelation may be confined only to the interior of discrete particles (base-catalyzed systems), while the sol may consist of polymeric networks rather than individual particles (acid-catalyzed systems). 

Silica Gels. The acid-catalyzed alkoxide sol-gel process produces gels (17). Frib-erg and coworkers (40-50) pioneered the extension of this process to silica synthesis in microemulsions both aqueous and nonaqueous microemulsions were used. For aqueous microemulsions, experiments were conducted mostly with the SDS/ pentanol/water/acid system. A representative flow diagram is shown in Figure 2.2.9. The nonaqueous microemulsion systems utilized included CTAB/decanol/ decane/formamide and AOT/decane/glycerol (44-46,49,50). The experimental approach followed the sequence nonaqueous microemulsion preparation, water addition, and then TEOS addition. 

While it is possible to use TMOS in the sol-gel process or in coating binders, the by-product of the hydrolysis is methanol. The toxicity of methanol makes TMOS unsuitable, so it must be converted into TEOS before it can be used. The conversion is carried out by reacting TMOS with ethanol. The reaction is catalyzed by both acids and bases. Since acid catalysis is used in the industrial preparation, this is the reaction that we have concentrated on. 

With the advent of the sol-gel process for making ceramic glasses, the eighties saw a large increase in the study of the hydrolysis of alkoxysilanes. Most of these studies deal with the hydrolysis of TEOS [21, 22]. Blum and Ryan [25] studied the acid catalyzed hydrolysis of TEOS by following the formation of ethanol by gas chromatography. They were chiefly concerned about the effect different quantities of water had upon the reaction. They found that when they increased the water four-fold, the reaction still took the same amount of time to complete. 

The kinetics of the acid catalyzed hydrolysis of ethoxysilanes has been studied. Each of the silanes that were used had a phenyl or para-substituted phenyl group attached to the silicon atom. This permitted a study of the linear free energy relationships of this reaction. The reaction is of interest because of its role in silane coupling agent chemistry, in the preparation of zinc-rich silicate coatings, in the sol-gel process and in the preparation of silicones in general. 

Extensive research on sol—gel processing of silicic acid esters, eg, tetraethoxysilane (TEOS), Si(OC2H5)4, in alcohol—water mixtures has ducidated silica polymerization in nonaqueous solvents (54,55). The relative rates of hydrolysis and condensation depend on the substrate, water, and catalyst (acid or base) concentrations these rates determine the polymer structure. For example, acid-catalyzed hydrolysis of TEOS at low water concentrations produces linear polymers. These solutions yield fibers upon spinning. Conversely, high H20 /TEOS ratios favor the formation of highly cross-linked polymers under... 

Second, it has already been mentioned that base catalyzed reactions are different from acid catalyzed reactions. Some preliminary observations in this study were that an acid such as HCl drove the hydrolysis reaction while impeding gelling. Then a base such as NH4OH limited hydrolysis which made gelling impossible. However a salt such as NH4CI postponed hydrolysis, but this was quickly followed by gelling. In a crude way, it can be suggested that to speed up the overall sol-gel process, initial treatment should be with acid followed by a final treatment with base. 

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