Bead aerogel

Fig. 26. Nitrogen adsorption-desorption isotherm for silica bead aerogel.

The accessibility of the primary amine functions of chitosan has been quantified in organic solvent by formation of a salicylaldimine Schiff base upon treatment with salicylaldehyde. When contacting salicylaldehyde the aerogel beads of chitosan become yellow and their UV-visible diffuse reflectance spectra exhibit the absorption band at 318 nm, characteristic of the Schiff base. Quantitative GC analysis of the remaining salicylaldehyde in the solution allows calculation of the fraction of reacted amino groups, which corresponds to the fraction of accessible sites. In the aerogel formulation, up to 70% of the amine groups are accessible [92]. 

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. 

The catalysts (MPcS chitosan) were obtained by impregnation of chitosan aerogel beads with an aqueous solution of sulfonated metal phthalocyanine. After impregnation, the solids were dried again under supercritical CO2 conditions. The textural properties are maintained and surface areas were greater than 140 m g (Table 4). 

Porous silica. Silica is commercially available in bead form (Pora-sil Merck-O-Gel Si) with a range of porosities. This is an aerogel, with a very rigid structure. It can be used in some organic solvents, but it is best used in water. It is rather highly polar, and can tend to retard polar molecules by adsorption. 

Figure 2.12. Photography (top) and scanning electron microscopy (bottom) of sylilated silica aerogel beads embedded by a polymer. Courtesy of Rigacci A.

Figure 13-10. Photograph of double glazed window filled with silica aerogel beads.

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