Hypercritical drying

Oxidative catalysis over metal oxides yields mainly HC1 and C02. Catalysts such as V203 and Cr203 have been used with some success.49 50 In recent years, nanoscale MgO and CaO prepared by a modified aerogel/hypercritical drying procedure (abbreviated as AP-CaO) and AP-MgO, were found to be superior to conventionally prepared (henceforth denoted as CP) CP-CaO, CP-MgO, and commercial CaO/MgO catalysts for the dehydrochlorination of several toxic chlorinated substances.51 52 The interaction of 1-chlorobutane with nanocrystalline MgO at 200 to 350°C results in both stoichiometric and catalytic dehydrochlorination of 1-chlorobutane to isomers of butene and simultaneous topochemical conversion of MgO to MgCl2.53-55 The crystallite sizes in these nanoscale materials are of the order of nanometers ( 4 nm). These oxides are efficient due to the presence of high concentration of low coordinated sites, structural defects on their surface, and high-specific-surface area. 

Chemical posttreatments Freeze-drying, hypercritical drying... 

Figure 2.04 Pressure-temperature diagram for hypercritical drying.

The evaporative drying process, which is cheaper and easier to use than hypercritical drying, generally leads to densified non porous materials. Large capillary forces at the liquid-vapour interface cause the gel to shrink and crack. Attempts to overcome this... 

Use supercritical (hypercritical) drying the liquid is removed above its critical temperature, T, and critical pressure, p. The values of T and for the commonly... 

Woignier T, Phalippou J, Quinson JF, Pauthe M, Laveissiere F (1992) Physicochemical transformation of silica gels during hypercritical drying. J Non-Cryst Solids 145 25-32... 

In supercritical drying (sometimes referred to as hypercritical drying), the liquid in the pores is removed above the critical temperature Tc and the critical pressure Pc of the liquid. Under these conditions, there is no distinction between the liquid and the vapor states. The densities of the liquid and vapor are the same, there is no liquid-vapor meniscus and no capillary pressure, and so there is no drying stresses. 

Since shrinkage and cracking are produced by capillary forces, Kistler [94] reasoned that those problems could be avoided by removing the liquid from the pores above the critical temperature (T ) and critical pressure (P ) of the liquid. As indicated in the phase diagram in Fig. 32, there is no longer any distinction between the liquid and vapor phases the densities become equal, there is no liquid-vapor interface and no capillary pressure. In the process of supercritical (or hypercritical) drying, a sol or wet gel is placed into an autoclave and heated along a path such as the one indicated in Fig. 32. The pressure and temperature are increased in such a way that the phase boundary is not crossed once the critical point is passed, the solvent is vented at a... 

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