1. Synthesis of resorcinol-formaldehyde xerogels

The most common way of synthesizing organic gels is via the polymerization reaction between resorcinol (R) and formaldehyde (F), using water as solvent and a basic catalyst as the reaction promoter [1–5]. This polymerization reaction consists of two stages: an addition reaction (Figure 1) and a condensation reaction (Figure 2).

Resorcinol is a benzyl compound with two hydroxyl groups at positions 1 and 3, which allow formaldehyde to be added at positions 2, 4 and 6 (see Figure 1) [6, 7]. In the presence of a basic catalyst, the ionization of the resorcinol occurs through the abstraction of hydroxyl hydrogens, resulting in resorcinol anions. Resorcinol anions are more reactive than resorcinol itself, which

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© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

curing, where the remaining hydroxyl groups continue to undergo polymerization, thereby

Carbon Xerogels: The Bespoke Nanoporous Carbons http://dx.doi.org/10.5772/intechopen.71255 71

The formation of the nodules, their growth and cross-linking between polymer chains depend on the concentration of each of the reagents used. The final polymeric structure will depend on the values of these variables. This is of great importance since it is the polymeric structure that determines the physical and chemical properties of the materials and, therefore, their adapt-

After the gelation and curing processes, i.e., the polymerization and cross-linking reactions, a stable three-dimensional polymer is obtained. However, the polymer is imbibed in the reaction media. Usually, the steps mentioned above (i.e., polymerization and curing) occur in covered containers [9, 15–17]. This means that all the reaction media or solvent used is covering the polymer, filling all the pores of the structure. The next step of the process is to eliminate the solvent in order to obtain a dry polymer with all its pores available. Three drying methods are found in the literature [4, 6, 18]: (i) supercritical drying, (ii) cryogenic drying and (iii) subcritical drying, giving rise to three different kinds of gel commonly referred to as aerogels,

The most common method of solvent elimination is by supercritical drying [8, 12, 14, 16, 19–25]. This method consists in exchanging the solvent used for the synthesis, by CO2 under supercritical conditions, (i.e. under high values of pressure and temperatures), and then eliminating the CO2 as a gas simply by changing the operating conditions (i.e., reducing the pressure). If water has been used as the solvent, as in the present case, it must first be replaced by an organic solvent due to the high solubility of the CO2 in water. The main advantage of this procedure is that the polymeric structure is not subjected to any surface tension forces, as the final solvent, CO2, is eliminated in gas phase. In this way, the previously tailored porosity is preserved. However, the several solvent exchanges and the high pressures required for the supercritical drying make this

option expensive and too long. The gels obtained by this method are known as aerogels.

Figure 3. Scheme of different ways of eliminating the solvent during the synthesis of resorcinol-formaldehyde gels.

increasing the number of cross-linkages between the nodules [14].

ability to the requirements of a specific application.

cryogels and xerogels, respectively, (Figure 3).

Figure 1. Addition reaction between resorcinol and formaldehyde.

Figure 2. Polycondensation reactions during the synthesis of resorcinol and formaldehyde gels.

favors the addition of formaldehyde leading to the formation of hydroxymethyl derivatives, as shown in Figure 1 [8, 9]. These hydroxymethyl derivatives are the monomers that are necessary for polymerization to occur. At the same time as the addition reaction, the condensation reaction takes place, in which the hydroxymethyl derivatives lose OH groups to form a benzyl cation (Figure 2) [10]. This cation reacts with a benzene ring of another molecule by bonding the rings with methylene and ether bridges [11, 12], giving rise to the formation of polymeric particles, known as nodules. The nodules are aggregated and cross-linked by polymer chains to form a three-dimensional porous network in a liquid medium (Figure 2).

The appearance of the nodules decreases the fluidity of the precursor mixture. The time required from the start of the reaction to the point where the solution starts to lose fluidity, i.e., when the gel begins to form, is known as the gelation time, whereas the exact moment at which the gel is obtained is called the gelation point [13]. After reaching this point of gelation, the solid structure that is immersed in the liquid continues to evolve. This stage is called curing, where the remaining hydroxyl groups continue to undergo polymerization, thereby increasing the number of cross-linkages between the nodules [14].

The formation of the nodules, their growth and cross-linking between polymer chains depend on the concentration of each of the reagents used. The final polymeric structure will depend on the values of these variables. This is of great importance since it is the polymeric structure that determines the physical and chemical properties of the materials and, therefore, their adaptability to the requirements of a specific application.

After the gelation and curing processes, i.e., the polymerization and cross-linking reactions, a stable three-dimensional polymer is obtained. However, the polymer is imbibed in the reaction media. Usually, the steps mentioned above (i.e., polymerization and curing) occur in covered containers [9, 15–17]. This means that all the reaction media or solvent used is covering the polymer, filling all the pores of the structure. The next step of the process is to eliminate the solvent in order to obtain a dry polymer with all its pores available. Three drying methods are found in the literature [4, 6, 18]: (i) supercritical drying, (ii) cryogenic drying and (iii) subcritical drying, giving rise to three different kinds of gel commonly referred to as aerogels, cryogels and xerogels, respectively, (Figure 3).

The most common method of solvent elimination is by supercritical drying [8, 12, 14, 16, 19–25]. This method consists in exchanging the solvent used for the synthesis, by CO2 under supercritical conditions, (i.e. under high values of pressure and temperatures), and then eliminating the CO2 as a gas simply by changing the operating conditions (i.e., reducing the pressure). If water has been used as the solvent, as in the present case, it must first be replaced by an organic solvent due to the high solubility of the CO2 in water. The main advantage of this procedure is that the polymeric structure is not subjected to any surface tension forces, as the final solvent, CO2, is eliminated in gas phase. In this way, the previously tailored porosity is preserved. However, the several solvent exchanges and the high pressures required for the supercritical drying make this option expensive and too long. The gels obtained by this method are known as aerogels.

favors the addition of formaldehyde leading to the formation of hydroxymethyl derivatives, as shown in Figure 1 [8, 9]. These hydroxymethyl derivatives are the monomers that are necessary for polymerization to occur. At the same time as the addition reaction, the condensation reaction takes place, in which the hydroxymethyl derivatives lose OH groups to form a benzyl cation (Figure 2) [10]. This cation reacts with a benzene ring of another molecule by bonding the rings with methylene and ether bridges [11, 12], giving rise to the formation of polymeric particles, known as nodules. The nodules are aggregated and cross-linked by polymer chains

The appearance of the nodules decreases the fluidity of the precursor mixture. The time required from the start of the reaction to the point where the solution starts to lose fluidity, i.e., when the gel begins to form, is known as the gelation time, whereas the exact moment at which the gel is obtained is called the gelation point [13]. After reaching this point of gelation, the solid structure that is immersed in the liquid continues to evolve. This stage is called

to form a three-dimensional porous network in a liquid medium (Figure 2).

Figure 2. Polycondensation reactions during the synthesis of resorcinol and formaldehyde gels.

Figure 1. Addition reaction between resorcinol and formaldehyde.

70 Porosity - Process, Technologies and Applications

Figure 3. Scheme of different ways of eliminating the solvent during the synthesis of resorcinol-formaldehyde gels.

Under cryogenic drying, the solvent used must be frozen and then eliminated by sublimation. As in the first case, when water is the dissolvent used for the synthesis, before the freezing process, it must be replaced by an organic solvent in order to avoid the formation of ice crystals inside the polymer structure, which would lead to the uncontrolled formation of megalopores or voids [18, 26]. The gels obtained in this drying process have high pore volumes [18] and are known as cryogels. Despite the fact that this procedure is more affordable than supercritical drying, it is still expensive due to the need for using exchanging solvents, very low operating temperatures and numerous long steps.

The performance of the third drying method is based on the direct evaporation of the solvent and the generation of gels known as xerogels. Unlike the two cases mentioned above, under subcritical drying conditions, a liquid-vapor interphase takes place. This interphase generates high superficial tensions that may cause the collapse of the porous structure of the gel. When water is the solvent used during the synthesis of the gel, one possible solution to this problem can be to replace the water by another solvent with less surface tension, such as acetone or cyclohexane. However, some authors [18, 27–32] have demonstrated that a suitable choice of operating conditions during the drying stage minimizes the shrinkage of the gel structure. In other words, the porosity of the gel is preserved without the need for solvent exchanges. For this reason, subcritical drying is the cheapest, easiest and fastest method, and consequently the most upscalable alternative for producing gels on a large scale.

are not independent of each other. Therefore, in order to be able to predict how changes in more than one of them will affect the porosity of the resultant xerogel, it is necessary to know

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The complexity of the problem can be explained briefly as follows. Resorcinol is responsible for the formation of the nodules or clusters and so the greater the amount of resorcinol that is used, the more the clusters that will be generated, while formaldehyde strengthens the gel by generating a structure that is more branched and/or interconnected. The volume of solvent added affects the distance between the nodules and so the greater the volume of solvent that is used, the more segregated the nodules will be, while the pH influences the speed of the reaction, i.e., the higher the pH is, the faster the resorcinol anions will be formed and consequently more nodules of small size will be created. All of these differences in the polymer structure affect the final porosity of the RF gel. Finally, the type of catalyst and the composition of the formaldehyde solution also influence the porosity formed by the RF gel [34]. However, as mentioned above, it is obvious that the modifications in porosity brought about by changes in the pH of the precursor solution, for example, will not be the same if the amount of solvent also changes. This is exemplified in Figure 5, where scanning electron microscope photographs, and the sketches in the insets, show for instance how increasing the pH of the precursor solutions from 5 to 5.8 leads to a polymeric structure with larger nodules (and hence wider

Interestingly, the mesoporosity (or macroporosity) of the organic xerogels is preserved with only slight variations when thermal treatments such as carbonization and/or activation are applied [39]. The meso- or macroporosity of the carbon xerogels that is designed before and formed during the synthesis of the organic xerogel persists in the carbon xerogel, occasionally with a slight shrinkage of the pores when the carbonization or activation temperature is higher than 900C [40–42]. However, as this shrinkage is a function of the temperature and heating rate, it can easily be predicted. Therefore, by taking this into account, it is possible to predesign

the mesoporosity (or macroporosity) of the carbon xerogels before they are produced.

the way in which they are interrelated, which is not a straightforward task [34, 37, 38].

Figure 4. Schematic representation of pore formation in RF xerogel synthesis.

pores) if the dilution is increased at the same time from 5.7 to 11.7.
