Supercritical-drying

In a typical SCD process, the solvent can be removed in two different ways  

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Using inorganic salts as precursors, alcohol as the supercritical drying agent, and a batch process a solvent-exchange step was necessary to remove water from the gel. 

Producing aerogel-like materials without supercritical drying at all preparation of inorganic—organic hybrid materials. ... 

It is less well known, but certainly no less important, that even with carbon dioxide as a drying agent, the supercritical drying conditions can also affect the properties of a product. Eor example, in the preparation of titania aerogels, temperature, pressure, the use of either Hquid or supercritical CO2, and the drying duration have all been shown to affect the surface area, pore volume, and pore size distributions of both the as-dried and calcined materials (34,35). The specific effect of using either Hquid or supercritical CO2 is shown in Eigure 3 as an iHustration (36). 

Overview. Three approaches are used to make most sol—gel products method 1 involves gelation of a dispersion of colloidal particles method 2 employs hydrolysis and polycondensation of alkoxide or metal salts precursors followed by supercritical drying of gels and method 3 involves hydrolysis and polycondensation of alkoxide precursors followed by aging and drying under ambient atmospheres. 

Fig. 1. Crack-free monolithic titania-silica aerogel photos, (a) aerogel prepared by hi -temperature ethanol supercritical drying, (b) aerogel prepared by low-temperature CO2 supercritical drying.

An interesting recent example of successful application of the SSG process combined with ensuing supercritical drying is the design of titania-silica mixed oxides for the epoxidation of bulky olefins [16-18]. This example will be used to illustrate the opportunities the combined use of SSG and SCD provide for tailoring the chemical and structural properties of mixed oxides. 

Transition-metal mixed oxides active in combustion catalysis have been prepared by two main procedures i) classical coprecipitation / calcination procedures starting from metal nitrates and/ or alkoxides ii) preparation based on the supercritical drying of gels prepared from organic complexes (alkoxides, acetylacetonates or acetates), producing aerogels . Details on the second preparation can be found in Ref. 13. 

Boujday, S., Wiinsch, F., Portes, P., Bocquet, J.F., and Colbeau-Justin, C. (2004) Photocatalytic and electronic properties of Ti02 powders elaborated by sol-gel route and supercritical drying. Solar Energy Materials ej Solar Cells, 83 (4), 421-433. 

Summary Hydrophobic aerogels were prepared by base-catalyzed hydrolysis and condensation of RSi(OMe)3 (R = Me, Ph, PrI1)/Si(OMe)4 (1 4) mixtures in methanol, followed by supercritical drying of the obtained alcogels with methanol. The organic substituents also increase the elasticity of the aerogels. 

Torma, V., Peterlik, H., Bauer, U., Rupp, W., Husing, N., Bernstorff, S., Steinhart, M., Goerigk, G. and Schubert, U. (2005) Mixed silica titania materials prepared from a singlesource sol-gel precursor A time-resolved SAXS study of the gelation, aging, supercritical drying, and calcination processes. Chemistry of Materials, 17, 3146-3153. 

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