Hydrolysis of TEOS

Hydrolysis of TEOS in various solvents is such that for a particular system increases directiy with the concentration of H" or H O" in acidic media and with the concentration of OH in basic media. The dominant factor in controlling the hydrolysis rate is pH (21). However, the nature of the acid plays an important role, so that a small addition of HCl induces a 1500-fold increase in whereas acetic acid has Httie effect. Hydrolysis is also temperature-dependent. The reaction rate increases 10-fold when the temperature is varied from 20 to 45°C. Nmr experiments show that varies in different solvents as foUows acetonitrile > methanol > dimethylformamide > dioxane > formamide, where the k in acetonitrile is about 20 times larger than the k in formamide. The nature of the alkoxy groups on the siHcon atom also influences the rate constant. The longer and the bulkier the alkoxide group, the lower the (3). 

The first step in sol-gel processing is the catalytic hydrolysis of TEOS and the second step is the polycondensation of SiOH moieties framing into silica (Scheme 3.1). In the first step of the reaction, water is present as a reactant while it is the by-product in the second step. It is likely that the molar ratio of TEOS/H2O would influence the sol-gel chemistry and hence the end properties of the resultant hybrids. The most interesting part of the sol-gel chemistry is that the catalytic hydrolysis of TEOS is an ion-controlled reaction, while polymerization of silica is not. Usually, the ionic reactions are much faster than the condensation reactions. The stoichiometric equation showing the silica formation from TEOS is presented in Scheme 3.3. 

In situ SAXS investigations of a variety of sol-gel-derived silicates are consistent with the above predictions. For example, silicate species formed by hydrolysis of TEOS at pH 11.5 and H20/Si = 12, conditions in which we expect monomers to be continually produced by dissolution, are dense, uniform particles with well defined interfaces as determined in SAXS experiments by the Porod slope of -4 (non-fractal) (Brinker, C. J., Hurd, A. J. and Ward, K. D., in press). By comparison, silicate polymers formed by hydrolysis at pH 2 and H20/Si = 5, conditions in which we expect reaction-limited cluster-cluster aggregation with an absence of monomer due to the hydrolytic stability of siloxane bonds, are fractal structures characterized by D - 1.9 (Porod slope — -1.9) (29-30). 

Time-resolved in situ Small Angle Neutron Scattering (SANS) investigations have provided direct experimental evidence for the initial steps in the formation of the SBA-15 mesoporous material, prepared using the non-ionic tri-block copolymer Pluronic 123 and TEOS as silica precursor. Upon time, three steps take place during the cooperative self-assembly of the Pluronic micelles and the silica species. First, the hydrolysis of TEOS is completed, without modifications of the Pluronic spherical micelles. Then, when silica species begin to interact with the micelles, a transformation from spherical to cylindrical micelles takes place before the precipitation of the ordered SBA-15 material. Lastly, the precipitation occurs and hybrid cylindrical micelles assemble into the two-dimensional hexagonal structure of SBA-15. 

The SANS data reveal an induction period of 5 minutes after the TEOS addition, during which the copolymer micelles do not evolve (figure 2). This induction period corresponds to the hydrolysis of TEOS, as shown by a recent complementary in-situ Raman study [3,8],... 

Blasco et al. (12,13) developed a novel method for the synthesis of Al-free Ti-beta zeolite in a fluoride medium. The Ti-beta zeolite thus obtained (Ti-beta(F)) was free of connectivity defects and was hydrophobic. The typical unseeded synthesis of Al-free Ti-beta zeolite (Ti-beta(F)) involves hydrolysis of TEOS in aqueous solutions of TEAOH (35%) and H202, followed by hydrolysis of TEOT and evaporation of ethanol and water. The water lost in the evaporation and... 

In another communication using w/o microemulsions containing a nonionic surfactant, it is shown that TEOS hydrolysis and siUca-particle growth occur at the same rate, indicating the growth of siUca particles is rate-controlled by the hydrolysis of TEOS [54], The rate of TEOS hydrolysis also depends on the surfactant concentration, which controls the molecular contact between hydroxyl ions and TEOS in solution. Because of the reaction-controlled growth mechanism, the silica-particle size distribution remains virtually same over the growth period. 

The synthesis of Si02 particles by ammonia-catalyzed hydrolysis of TEOS has been also carried out in Aerosol OT reversed micelle systems. The particles precipitated in this systems are spheres, but they have generally a broad size distribution comparing with that of nonionic reversed micelles compared to the normalized standard deviation of <10% in polyoxyethylene(5) nonylphenyl ether, that in Aero-... 

LaMer, Monomer Addition Growth Model. Most of the recent publications (13,18,37,43-45) concerning the Stober silica precipitation describe a first-order hydrolysis of TEOS as the rate-limiting process in the silica particle precipitation. The second reaction step, the condensation reaction, was found to be faster by at least a... 

Based on the change in intensity of the Si-O-C band at 967 cm-1, the rate of hydrolysis of TEOS was determined to be first order with respect to the alkoxide concentration ... 

In principle, silica growth kinetics may be controlled by (1) slow release of monomer via alkoxide hydrolysis in the particle-free reverse micelles, (2) slow surface reaction of monomer addition to the growing particle, and (3) slow transport processes as determined by the dynamics of intermicellar mass transfer. There is strong experimental evidence to support the view that the rate of silica growth in the microemulsion environment is controlled by the rate of hydrolysis of TEOS (23,24,29). Silica growth kinetics can be analyzed in terms of the overall hydrolysis and condensation reactions ... 

The [Pt(NH3)4](HC03)2 nanofibers are stable under alkaline conditions in which the pH value may vary between 8 and 11. Under acidic condition, however, the precipitates are unstable, and, therefore, the hydrolysis of TEOS can only be performed in alkaline medium. The alkalinity of the solution (optimum of pH 8.5-9) is controlled by the addition of several ml of 0.2 to 0.8 N ammonia solution. In more alkaline solutions the formation of non-structured Si02 particles is enhanced due to the higher rates of hydrolysis of TEOS and condensation, i.e. more Si02 seeds form and grow in the solution without being in contact with the precipitated [Pt(NH3)4](HC03)2 nanofibers. 

The pH at which precipitation is carried out has a major influence on the structure of the products obtained. For example, the amorphous silica obtained by hydrolysis of TEOS under acidic conditions has a surface area of 900 m2/g and is weakly crosslinked, with an asymmetric Si—O stretching vibration at 1030 cm-1, whereas the hydrolysis of TEOS obtained under neutral or basic conditions produces a silica having a surface area of 400 m2/g which is strongly crosslinked, with the asymmetric Si—O stretching vibration at 1100 cm-1 (Schraml-Marth et al., 1992 Miller et al., 1994 Liu et al., 1994). 

A method was developed to increase the formation of Si—O—Ti bonds it involves the partial hydrolysis of TEOS followed by the addition of the... 

Another method described for the synthesis of TS-1 involves the use of colloidal Si02 and tetrapropylammonium peroxo titanate (Taramasso et al., 1983). Indeed, TS-1 can be produced by this method, but subsequent experience has shown that it has the serious drawback of impurities contained in colloidal Si02, particularly Al3 +. These impurities are incorporated into the crystalline product and modify the catalytic properties, as discussed in Section V. It is certainly possible to obtain pure colloidal Si02 by hydrolysis of TEOS, but in this case the method does not offer advantages over the use of metal alkoxides. 

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. 

Jada [27] followed the acid catalyzed hydrolysis of TEOS by NMR. He generated the water in situ by the reaction of acetic acid with the ethanol solvent. He was able to follow the changes in the concentrations of the CH2 and CH, of the TEOS and the OH of the ethanol. These changes led him to the conclusion... 

To better understand this solgel procedure, the synthesis of silica microspheres using the Stobe-Fink-Bohn (SFB) method [46] and a modification of the SFB method [47-49] are excellent examples. The SFB method consists of the hydrolysis of TEOS (Si(C2H50)4) in ethanol, methanol, //-propanol, or //-butanol in the presence of ammonia as a catalyst [46], In Table 3.3, the batch composition for the synthesis of silica microspheres, by means of TEOS in an alcohol (methanol or isopropanol) in the presence of ammonia as a catalyst, with and without double-distilled water (DDW) in the synthesis media, is presented [47],... 

In a recent study the IT-catalyzed rearrangement of allyl (3,5-di-terr-butylphenyl) ether was used as selective probe for the outer surface activity.44 The best passivation results were obtained by treatment of the Beta crystals with tetraethyl orthosilicate (TEOS). Slow hydrolysis of TEOS by traces of water present in the pores of the zeolite provides a thin, porous layer of amorphous silica and leads to complete outer surface passivation on 1 p,m crystals. 

The purpose of selecting XMou-SiOj as the inorganic precursor is that they are chemically active toward siloxanes. At first the hybrid XMo,-Si02 sols were prepared via hydrolysis of TEOS in the presence of XMo,under acidic conditions. A little amount of nitric acid was added to the solution in order to increase the solubility of XMo, and still remained the mono lacunary structures. Second, the sols of the hybrid materials were introduced into the spaces between the templating PS sphere arrays (with suction applied) subsequently, a solid hybrid material skeleton was constructed via in situ sol-gel transformation. The PS templates were removed by extraction with toluene solution. 

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