Novel techniques to control pore structure

Recently, various novel techniques have been developed to control the pore structure of carbon materials. Since the pore structure is governed by nanotexture of carbon materials, its control has to be done during carbonization. In the following novel techniques, pore structure was established through the control of the carbonization process without any activation process. 

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. 

The carbons prepared in this way were highly microporous, even though no activation at all was included in the preparation process. In Table 11, some data on pore structure parameters of the thus prepared materials are listed. The carbon prepared by carbonization of poly(furfuryl alcohol) (PFA) followed by CVD of propylene at 973 K and then heat-treated at 1173 K for 3 h under Nj flow shows a special pore structure an exceptionally high BET surface area of 3600 m g and a large amount of micropores of about 1.5 cm g without detectable mesopores. More recently [100], the preparation has been simplified by 

Pore structure parameters of the carbons prepared by template tethnique. Courtesy of Prof. T. Kyotani of Tohoku Univ. 

Carbon precursor Template zeolite Carbonization conditions Pore structure parameters  

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Pore sizes and their distributions in adsorbents have to comply with requirements fi om different applications. Thus, relatively small pores are needed for gas adsorption and relatively large pores for liquid adsorption, and a very narrow PSD is required for molecular sieving applications. Macropores in carbon materials were found to be effective for sorption of viscous heavy oils. Porous carbons can respond to these widely ranged requirements from the applications, which is one of advantages of carbon materials even though pore size distributes in a certain range in the majority of porous carbons. Recent novel techniques to control pore structure in carbon materials (see Section 5) can be expected to contribute to overcome this limitation. 

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