Thermal calcinated porous silica

A), which corresponds to a very high surface concentration of sUanols (4.8 groups/nm ). According to some estimations, thermally calcinated porous silica may have small sections of its surface in the form of microcrystalline p-kristobalite (approximately 10-15%), and the formation of bridged (difunctional) bonding could be observed only on this type of surface. 

Zhdanov (Institute of Silicate Chemistry, the U.S.S.R. Academy of Sciences, Leningrad) (154) showed (1949) that the adsorption of water vapor by SiC>2 (porous glasses, silica gels) strongly depends on the temperature of the preliminary thermal treatment of the adsorbent. Calcination of 300-500 °C resulted in a sharp decrease in the adsorption of H2O at low values of pressure over initial pressure p/p0 (<0.3), and the adsorption isotherms were found to be irreversible. On the other hand, the adsorption isotherms of water on silica subjected to calcination in vacuo at <500 °C (but after the sample was kept in contact with water vapor or liquid water at room temperature) again became reversible that is, the adsorption activity of Si02 was restored. 

The template removal step, needed to achieve porous materials, is one of the most critical points. In contrast to silica, other compositions are usually more sensitive to thermal treatments and calcination can result in breakdown of the mesostructures. Hydrolysis, redox reactions, or phase transfonnarions to the thermodynamically preferred denser crystalline phases account for this lower thermal stability. Many of the transition metal-based mesostruetured materials synthesized in the presence of cationic surfactants collapse during thermal treatments. The poor thermal stability observed could be due to the different 0x0 chemistry of the metals compared to silicon. Several oxidation states of the metal centers may be responsible for oxidation and/or reduction during calcination. In addition, incomplete condensation of the framewoik is possible. 

Aerogels composed of 60% silica and 40% zirconia were calcined in air, at 400°C, 800°C and 1000°C. After thermal treatment, the porous texture of the samples have been analyzed by mercury porosimetry (Fig. 11-15) (Pirard, 1997c). The three samples are irreversibly densified by isostatic pressure in the whole pressure domain, from 0.01 to 200 MPa. The data analysis (Fig. 11-16) has been done using equation (11-7), with a constant k estimated at 48 nm MPa°-, by nitrogen adsorption-desorption isotherm analysis. The volume distributions versus pore size obtained show that the pore volume decreases for all pore sizes during aerogel calcination at increasing temperatures. This is... 

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