Aerogel surface area

As previously described, the parameters which affect the properties of the hydrogel affect the texture of the aerogel. Chitosan aerogel surface area can be doubled but the benefit of such an increase, which is clear in the first catalytic test (98% conversion, 12 h), becomes a drawback in the successive re-uses of the catalyst (second run 47% conversion). Therefore a balance has to be met between dispersion of the material (surface area) and the mechanical stabiUty of the chitosan beads. 

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). 

Thus, the porosity of an aerogel is ia excess of 90% and can be as high as 99.9%. As a consequence of such a high porosity, aerogels have large internal surface area and pore volume. 

Both xerogels and aerogels are characteristically high surface area materials (surface areas normally exceed 500 m2/g). Unlike wet gels, many uses exist for dried gels due to their high surface areas and small pore sizes (typically, < 20 nm diameters). Examples include catalyst supports (12.). ultrafiltration media (18), antireflective coatings (19-20), and ultra-low dielectric constant films. (Lenahan, P. M. and Brinker, C. J., unpublished results.)... 

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. 

Titania-silica aerogels possess very high surface areas (600 to 1000 m2/g) and large pore volumes (1 to 4 cm3/g), and thereby have attracted considerable interest for photocatalysis. A number of studies have shown that titania-silica intimate mixtures exhibit enhanced UV photocatalytic activity compared with pure titania [180-183],... 

Since both aerogels and xerogels have high surface areas and small pore diameters they are used as ultrafiltration media, antireflective coatings, and catalysts supports. Final densi-fication is carried out by viscous sintering. 

Aerogels are highly porous materials where the pore sizes are truly on a molecular level, less than 50 nm in diameter. This gives a material with the highest known internal surface area per unit weight. The surface area of 1 oz. is equal to 10 football fields, over 1000 m in 1 g. 

Aerogels that are about 2-5 nm in diameter have large surface/volume ratios on the order of 10 m and high specific surface areas approaching 1000 m /g. Such large surface/volume ratios make the surface particularly active and potential materials as catalysts, absorbents, and catalyst substrates. 

Cao Y, Hsu JC, Hong ZS, Deng JF, Fan KN (2002) Characterization of high-surface-area zirconia aerogel synthesized from alcohothermal and supercritical fluid diying techniques. Catal Lett 81 107-112... 

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