Carbon-templated microporous silica

The physical and chemical activation processes have been generally employed to prepare the porous carbons.18"35 However, the pore structures are not easily controlled by the activation processes and the size of the pores generated by the activation processes is limited to the micropore range only. Recently, much attention has been paid to the synthesis of meso/macroporous carbons with various pore structures and pore size distributions (PSD) by using various types of such inorganic templates as silica materials and zeolites.17,36 55... 

FIGURE 12.5 Carbon monolith synthesis using resorcinol-crotonaldehyde polymer as carbon precursor. The polymerization took place in the pores of the large silica monoliths. Silver nanoparticles could also be dispersed in the silica template and transferred to the final templated carbon materials. The carbons retained the monolithic shape of the silica template and resisted chemical activation with potassium hydroxide (KOH). After activation, carbons exhibited microporous-mesoporous structures. (From Jaroniec, M. et al., Chemistry of Materials, 20, 1069, 2008. With permission.)... 

Liu, YR. (2009) One-pot route to synthesize ordered mesoporous polymer/silica and carbon/sflica nanocomposites using poly (dimethylsiloxane)-poly(elhylene oxide) (PDMS—PEO) as co-template. Microporous and Mesoporous Materials, 124,190-196. 

The total surface area of the template carbons prepared by sucrose impregnation is significantly higher than the surface area of the corresponding silica template (Table 2), that confirms the formation of micropores during the carbonization. Just an opposite tendency is observed... 

Porous carbons were prepared using MCM-48 and SBA-15 mesoporous templates and various carbon precursors (see Chapter 3 for preparation description). Figure 8.11 displays the nitrogen adsorption isotherms at 77 K of SBA-15 and of the corresponding templated carbon obtained by carbonization of sucrose in the template. Both isotherms show a bimodal porosity in the templated carbon, mesopores are generated by the removal of the silica walls, and micropores are present in... 

As shown in Fig. 3, nitrogen adsorption isotherms of CMK-1 feature well-pronounced capillary condensation steps similar to those of ordered mesoporous silicas and indicative of high degree of mesopore size uniformity. The isotherms reveal that the CMK-1 carbon has high nitrogen BET specific surface area (1500-1800 m g ), and large total pore volume (0.9-1.2 cm g ) [14]. The adsorption capacity is comparable or larger than that of MCM-48 template. The pore-size analysis (calibrated BJH analysis) shows that typical CMK-1 has uniform mesopores about 3 nm in size, which is accompanied by a certain amount of micropores when sucrose is used as the carbon source. 

The main difference between the synthesis of MCM-41 mesoporous material and traditional synthesis of zeolite or silica molecular sieve is the use of different templates. An individual organic molecule or metal cation is used for the traditional synthesis of silica microporous molecular sieve. For example, the typical template for ZSM-5 synthesis is tetrapropylammonium ion the crystal is formed through the condensation of silicate species around the template molecule, while for the formation of MCM-41, the typical template is the assembly of large molecules containing one hydrophobic chain with more than 10 carbons. 

Zeolites were already employed as templates in the synthesis of microporous carbon with ordered structures.[247] The discovery of ordered mesoporous silica materials opened new opportunities in the synthesis of periodic carbon structures using the templating approach. By employing mesoporous silica structures as hard templates, ordered mesoporous carbon replicas have been synthesized from a nanocasting strategy. The synthesis is quite tedious and involves two main steps (i) Preparation and calcination of the silica mesophase, and (ii) filling the silica pore system by a carbon precursor, followed by the carbonization and selective removal of the silica framework. 

Lately, a fascinating strategy has been successfully developed for the preparation of ordered mesoporous carbons. The synthesis procedure of these advanced carbons consists in the infiltration of an organic precursor into the pores of silica or aluminosilicate templates, followed by the subsequent pyrolysis of the precursor and dissolution of the template framework by HF [9—12]. In another process, carbon is directly introduced in the template by a CVD method [86]. The method gives a highly ordered and interconnected network of meso- and micropores [87], where the size of carbon mesopores is defined by the walls thickness of the pristine silica matrix. Such materials are very suitable for better understanding the relationships between the porous characteristics and the supercapacitors performance [88, 89]. 

The so-called template-based technique has been found to be particularly suitable for the synthesis of carbons whose porosity is not only uniform in size and shape, but also periodically ordered in some cases. In this approach, the porous carbon is prepared through infiltration of an organic precursor into the nanochannels of an appropriate inorganic material (the template), followed by carbonization and then liberation of the resultant carbon from the template. Different nanospaces in templates have been used to confine the carbon precursors. The first templates used included, e.g., silica gel or porous glass [84,85], layered clays such as montmorillonite ortaeniolite [86,87], or pillared clays [88-90]. Several detailed reviews on this topic have been published [75,91-95] that cover the areas of microporous and, especially, mesoporous solids. Here, some illustrative examples will he presented in some detail rather than reviewing systematically the literature. 

CMK-2 [114] is an ordered mesoporous carbon obtained from sucrose as a source of carbon and SBA-1 silica as template [Fig. 34b)]. Electron diffraction showed that this carbon is composed of c s iirtercoimected with two different types of pores (meso- and micropores). CMK-3 [115] is an ordered mesoporous carbon that was synthesized using SBA-15 mesoporous silica as template and sucrose as carbon source. The structure of CMK-3 was the faithful replica of the mesoporous silica template, as revealed by XRD and TEM. This carbon had a hi BET surface area (1500 n g % and a pore size around 4.5 nm. The systematic control of pore wall thickness of hexagonal mesoporous silica templates (by varying the ratio of surfactants CwTAB and CmEOg, Fig. 35) afforded a close control of the... 

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