High-temperature supercritical drying

Supercritical alcohol drying typically induces hydrophobic properties even in nonmodified (hydrophobized) silica gels. At the high process temperatures and pressures, free silanol groups react with ethanol, leaving behind mostly alkoxy-decorated gel. This surfece chemistry, however, is not hydrolytically stable under normal ambient conditions. A TEOS-based HTSCD aerogel dried in supercritical ethanol loses its hydrophobicity within a few hours when stored in ambient air. 

---

Three different MgO samples were used in the experiments. AP-MgO with a surface area of 385 m2/g was prepared by a sol-gel technique involving high-temperature supercritical drying described in detail earlier.4,13 CP-MgO (281 m2/g) was obtained by decomposition of Mg(OH)2 prepared by hydration of commercial MgO. In both cases the final preparation step was overnight evacuation at 500°C followed by storage under ambient conditions. Their performance was compared to that of a commercial low surface area sample CM-MgO (10 m2/g). 

FIGURE 24.17 The texture of chromia catalysts (a) thermal decomposition of the precursor (macro crystalline chromia), (b) evaporative drying of the gel (microcrystalline chromia), and (c) high temperature supercritical drying of the gel (nanocrystalline chromia). (Reprinted from Rotter, H., Landau, M.V., Carrera, M., Goldfarb, D., and Herskowitz, M., Appl. Catal. B, Al, 111-126, 2004. With permission from Elsevier.)... 

FIGURE 3.3 Temperature against pressure diagram during a high-temperature supercritical drying run with methanol. 

The textural properties (surface area and porosity) of obtained samples depend considerably on the preparation conditions. However, in all cases the high-temperature supercritically dried zirconia aerogels showed larger porosity and higher surface areas compared to the aerogels dried by low-temperamre extraction with supercritical CO2. 

Furthermore, zirconia aerogels dried by low-temperature extraction with supercritical C02(l) were amorphous. However, the high-temperature supercritically dried samples contained predominantly tetragonal zirconia. 

Bedilo A F, Klabunde K J (1997) Synthesis of high surface area zirconia aerogels using high temperature supercritical drying. J Nano Mater 8 119-135... 

Casula MF, Corrias A, Paschina G (2002) FeCo-Si02 nanocomposite aerogels by high temperature supercritical drying. J Mater Chem 12 1505-1510... 

HOT supercritical drying High temperature supercritical drying process, at temperatures >260°C, in an alcohol... 

---