Water/alkoxide ratio

Silica membranes have also been studied by several investigators for use in gas separation and membrane reactors. They arc thermally very sublc up to about 500°C. Sintering and densification temperatures of silica membranes depend on the water/alkoxide ratio in the sol-gel process for making the membranes (Langlct et al., 1992]. Crystallization of amorphous silica particles in the membranes takes place at temperatures around 1,000°C [Larbot et al., 1989]. However, pore growth can gradually... 

The effect of the water/alkoxide ratio (hydrolysis ratio) on the surface area is almost the same in the two conditions. At low hydrolysis ratio, about 4, the hydrolisis is not favoured, however the particles association is favoured. As a consequence cross-linking is favoured and the surface area decreases. At hydrolysis ratio higher than 10 the diluition could lead to a... 

Many factors influence the kinetics of the hydrolysis and condensation reactions in the sol-gel process, which include the water/alkoxide ratio, temperature, the nature of solvent, and so forth [107]. 

In a typical hydrolytic xerogel synthesis, the silicon and titanium alkoxide precursors are mixed with the solvent in the presence of an acid or base catalyst under vigorous stirring at ambient condition. Typically applied solvents are alcohols, such as ethanol or isopropanol, due to their ability to effectively homogenize the metal precursors. Since Si(OR)4 is by far less reactive toward hydrolysis and condensation reactions than Ti(OR)4, phase separation is expected, as already discussed [23,42]. As one possibility to reduce the difference in reactivity of the various alkoxides, Yoldas proposed to prehydrolyze the less reactive silicon alkoxide prior to the addition of the titanium alkoxide [26]. Keeping in mind that the water/alkoxide ratio is the key parameter, which is typically kept below 2 to increase the time for gelation, thus favoring Si—O—Ti bond formation, this route is pursued in many of the reported studies [27,45]. 

The structures of sol-gel-derived inorganic polymers evolve continually as products of successive hydrolysis, condensation and restructuring (reverse of Equations 1-3) reactions. Therefore, to understand structural evolution in detail, we must understand the physical and chemical mechanisms which control the sequence and pattern of these reactions during gelation, drying, and consolidation. Although it is known that gel structure is affected by many factors including catalytic conditions, solvent composition and water to alkoxide ratio (13-141, we will show that many of the observed trends can be explained on the basis of the stability of the M-O-M condensation product in its synthesis environment. 

It has been shown that the spinnability occurs in the silicon alkoxide solution when an acid is used as catalyst and the water content is small at less than 4 in the water to metal alkoxide ratio. 

Nevertheless, the main parameter that affects the course of hydrolysis reaction is the molar ratio of reactants (i.e., the water to metal alkoxide ratio h = [H2OJ [M(OR)J). This h value determines the composition and the properties of the hydroxides and oxides formed on hydrolysis and thus allows them to be obtained in different forms of powders, films, glasses, and fibers. 

T3.8.2.3 Obtained in Microemulsion Ammonium hydroxide was added to a solution containing Igepal Co520 (surfactant) and cyclohexane, and the mixture was shaken to obtain a microemulsion. Solutions of zirconium n-propoxide in acetylacetone and n-butanol (l 0.5 3) and of tetraethyl orthosilicate in water and ethanol (1 1 4) were mixed for 3 h, and the mixture was added to the microemulsion with stirring, and hydrolyzed for 3 d. The final concentrations of alkoxides were 0.0075 M, the water surfactant molar ratio was 0.8, and the water alkoxide molar ratio was 10. The powder was washed with acetone and heated at 900°C for 2 h. 

The sol-gel titania-silica mixed oxide contained 20 wt% titania. The catalyst was synthesized imder acidic conditions [5]. The acidic hydrolysant was added to an isopropanohc solution of tetraisopropoxytitanitam(IV) modified by acetylacetone (molar ratio alkoxide acetylacetone =1 1) and tetramethoxysilicon(IV). The water alkoxide acid molar ratio was 5 1 0.09. The resulting gel was dried by semicontinuous extraction with supercritical COg at 40 °C and 240 bar, and stored in a closed vessel under Ar. After calcination in flowing air at 600 °C, the BET smface area was 648 m g and the specific pore volume 2.9 cm g". Details on the synthesis and characterization of the sol-gel titania-sihca by means of FTIR, Raman and UV-vis spectroscopies, Ng-physisorption, XRD, XPS, TEM and thermal analysis have been reported previously [5, 6, 14, 15]. 

The sm-face area and pore volumes of alumina samples were found to increase with increase in base to alkoxide molar ratio. Samples with ageing duration of 16 h and higher base / alkoxide, solvent ratios have mesopores whereas samples with no aging duration have only micropores. A highly distinct pore size distribution obtained for all the samples where conditions are standardised in terms of ageing duration, base / alkoxide mole ratio and IPA / water mole ratio. 

G, Oskam, A, Nellore, R, L, Penn, and P, C, Searson, The growth kinetics of Ti02 nanoparticles from titanium(lV) alkoxide at high water/titanium ratio, J. Phys. Chem. B, 107 ... 

Dopant molecules uniformly distributed throughout matrix particles diffuse through the huge microparticle inner porosity. Also in this case, the release velocity can be varied from mg/hours to mg/month by changing parameters such as the water-to-alkoxide ratio, pH, alkoxide concentration, and matrix hydrophobicity. 

As in the case of inorganic silica systems [Eqs. (8)-(ll)], both hydrolysis and condensation of alkoxides may be acid- or base-catalyzed [9,24,37]. The relative contributions of the four reactions—hydrolysis, olation, oxolation, and alcoxolation—determine the physical and chemical characteristics of the resulting oxide material. These contributions are in turn determined by the nature of the metal alkoxide (i.e., nature of the metal atom, nature of the alkyl groups) and the characteristics of the chosen experimental conditions (i.e., the water/alkoxide molar ratio, the nature and concentration of the catalyst, the nature of the solvent, and temperature). A major factor is the hydrolysis ratio, h, defined as the water/alkoxide molar ratio ... 

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