Designing Nanostructured Carbon Xerogels

Abstract

Until the discovery of carbon gels in 1890, inorganic gels had dominated sol-gel literature. Traditional inorganic gels based upon hydrolysis-condensation reactions of metal alkoxides are well known as a result of their high specific surface areas and their unique morphological and structural properties (De Sousa et al., 2001). Some of the precursors used for the preparation of such inorganic gels are aluminates, titanates and borates but the most frequently employed alkoxides are alkoxysilanes, leading to the extensively investigated silica gels (Mauritz, 1998; Zareba-Grodz et al., 2004). The reaction mechanism of silica gels is based on the hydrolysis of alkoxysilanes to yield silicic acid moities, Si(OH)4, which spontaneously condensate to generate, after a sequence of specific stages, the final silica gel consisting of siloxane groups (Si-O-Si) within its framework and silanol groups (Si-OH) on its surface (Salazar-Hernandez et al., 2009). Some of the attractive features of silica gels are: the tailored textural and structural properties, abundance and low cost, high sorption capacity, very high thermal shock resistance, insolubility in most solvents and lower index of refraction compared to other inorganic gels. It is for these reasons that they are used in a wide range of industrial applications including catalysis, chromatography, drug delivery and ion exchange (Qu et al., 2008; Teng et al., 2010). It was the great interest aroused by inorganic gels, especially in the case of silica gels, in different fields of application together with the advantages associated with sol-gel methods, i.e. low temperature processing, the high homogeneity of final products and the possibility of controlling their surface properties (Houmard et al., 2009), that persuaded Pekala and coworkers to go an step further and apply this sol-gel methodology to the synthesis of organic gels (Pekala, 1989). The first organic gel was obtained by sol-gel polymerization of resorcinol and formaldehyde under alkaline conditions and supercritical drying. This produced a material called organic aerogel, consisting of interconnected colloidal particles approximately 10 nm in diameter. Properties such as low density, highly porous material and high versatility of the sol-gel process turned the carbon gel into a prominent member of the “carbon family”. Basically, an organic gel is a solid nanostructure comprised of nano-sized pores and interlinked primary particles obtained by means of polymerization reactions between hydroxylated benzenes and aldehydes, and then subjected to a drying process. The most commonly used monomers are resorcinol and formaldehyde (Al-Mutasheb & Ritter, 2003; Czzakel et al., 2005; Job et al., 2004; Tian et al., 2011a; Zhang et al., 2007; Zhu et al., 2007; Zubizarreta et al., 2008a) but, there are other potential combinations such as phenol/formaldehyde (Mukai et al., 2005a; Scherdel & Reichenauer, 2009; Teng & Wang,