Aerogel resorcinol-formaldehyde gels

Other carbon sources have also been used as matrices such as multiwall carbon nanotubes (MWNTs), which led to narrower mesopore distributions than with carbon black pearls. In addition, the use of carbon fibers also allowed cylindrical mesopores to be obtained with low tortuosity. Carbon aerogel monoliths, obtained from resorcinol-formaldehyde gels after drying with COj under supercritical conditions and pyrolysis under nitrogen atmosphere at 1323 K, have also been used as templates for the generation of mesoporosity in zeolites [158]. This method presents the added advantage that the mesoporous zeolite can be synthesized as a monolith. 

Gross J, Scherer G W, Alviso C T et al (1997) Elastic properties of crosslinked Resorcinol-Formaldehyde gels and aerogels. Journal of Non-Crystalline Solids 211 132-142... 

On the other hand, the mechanical properties of monolithic carbon gels are of importance when they are to be used as adsorbents and catalyst supports in fixed-bed reactors, since they must resist the weight of the bed and the stress produced by its vibrations or movements. A few smdies have been published on the mechanical properties of resorcinol-formaldehyde carbon gels under compression [7,36,37]. The compressive stress-strain curves of carbon aerogels are typical of brittle materials. The elastic modulus and compressive strength depend largely on the network connectivity and therefore on the bulk density, which in turn depends on the porosity, mainly the meso- and macroporosity. These mechanical properties show a power-law density dependence with an exponent close to 2, which is typical of open-cell foams. 

Pekala RW, Mayer ST, Kaschmitter JL, Kong FM (1994) Carbon aerogels an update on structure, properties, and applications In Attia YA (ed) Sol-gel Process Appls, Plenum, New-York 369-377 Marie J, Berthon-Fabry S, Chatenet M, Chainet E, Pirard R, Comet N, Achard P (2007) Platinum supported on resorcinol-formaldehyde based carbon aerogels for PEMFC electrodes Influence of the carbon support on electrocatalytic properties. J Appl Electrochem 37 147-153... 

In analogy to their resorcinol-formaldehyde homologues, polyurethane wet gels are synthesized by polycondensation. Subsequently, aerogels are commonly obtained by supercritical fluid drying of the organic wet gels [23]. 

The resorcinol-formaldehyde polymer is classified as a phenolic resin. In fact, phenol-formaldehyde condensates are the first synthetic polymers introduced commercially (Bakelite) in the beginning of the twentieth century. Thus, the phenolic resin chemistry, on which there is a vast amotmt of information, can be used to understand the chemistry of the RF aerogel synthesis. In the first step, resorcinol reacts with formaldehyde to form hydro-xymethylated resorcinol in the second step, the hydroxymethyl groups condense with each other to form nanometer-sized clusters, which then crosslink by the same chemistry to produce a gel. The formation of clusters is influenced by typical sol-gel parameters such as the temperature, pH, and concentration of reactants. Though the first RF aerogel is synthesized by the aqueous polycondensation of resorcinol with formaldehyde with the use of sodium carbonate as catalyst [6, 12, 13], now several literature reports exist for aerogels made from phenol and formaldehyde [14—16]. 

Figure 36.1. Reaction mechanism for the synthesis of resorcinol-formaldehyde (RF) wet gels, aerogels, and caitxm aerogels.

Tamon H, Ishizaka H, Mikami M, Okazaki M. Porous structure of organic and carbone aerogels synthesized by sol-gel polycondensation of resorcinol with formaldehyde. Carbon 1997 35 791-6... 

Organic Materials. The sol-gel chemistry of organic materials is similar to that of inorganic materials. The first organic aerogel was prepared by the aqueous polyconclensation of resorcinol with formaldehyde using sodium carbonate as a base catalyst. 

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