Mostly microporous in nature (with pore diameters less than 2 nm), porous carbon materials are widely used in adsorption, purification processes, chromatographic separation, etc. Although the microporosity is a highly desirable feature of carbons in many applications, e.g., in adsorption of small molecules, mesoporous carbons present distinct advantages not only for the adsorption of large molecules, but also for some new applications such as support for catalysts, and as electrodes of an electrochemical capacitor [1–4]. Development of industrial technology provides a strong demand for mesoporous carbons. An ongoing research in this field has been conducted for more than two decades. Many novel methods, such as catalytic activation [5, 6], carbonization of polymer blends and organic gels [7–11], and template carbonization [12–19], have been proposed to prepare mesoporous carbons and control their pore structure. A conventional template carbonization method is described as follows: (1) introduce an organic compound into the nano-pores of a template inorganic matrix either by impregnation of polymer solution, CVD or synthetic method; (2) carbonize the organic–inorganic composite; and (3) liberate the resulting carbons from the templates. Inorganic host matrices which can be used as templates include layered materials [12, 13], zeolites [14], porous silica [15], anodic aluminum oxides [16], etc. Such spatial regulation of the carbonization process by the nanospace makes it possible to control the structure of carbon materials at nanometer level by changing either the size or the shape of nanospace. However, the structure of the obtained carbons is undoubtedly governed by the properties of the inorganic matrix [17]. For example, the carbon resulting from poly(furfuryl alcohol) (PFA) pyrolysis between the lamellae of clays [18] is quite different from that resulting from the pyrolysis of this same precursor in zeolite [14], or in the uniform and straight channels of an anodic aluminum oxide [16]. This method leaves no options for controlling the product’s pore structure for the unchangeable fabric of the template substance. We may call the conventional template carbonization method as “inherent template carbonization method.” It is well known that the sol–gel process is a useful technique to prepare nano-composites of organic and inorganic materials with precisely controllable structure. If two sol–gel reactions, one of which is the synthesis of the template material, the other the synthesis of the carbon precursor polymer, could occur simultaneously in a reactor, the texture characteristics of both the template and the carbon precursor could be controlled synchronously by modifying the sol–gel reaction conditions. After the template framework is removed from the composite, a carbon that reflects the template framework can be obtained. The simultaneous sol–gel process, compared with the “inherent template carbonization method,” makes it possible to obtain more tunable pore structures of the resultant carbons. We may call this method as “simultaneous synthetic template carbonization method” (SSTCM). Kawashima and his co-workers, using this method, prepared mesoporous carbon from PFA/sol–gel silica hybrids by the simultaneous formation of a crosslinked PFA and the silica phase during the sol–gel process [19]. Tetraethoxy silane (TEOS) has often been used as a silica precursor for the production of nano-composites by the sol–gel process. The shape and size of silica obtained could be tuned by the sol–gel reaction conditions [20, 21]. Investigation into resorcinol-formaldehyde (RF) gels and their carbonized products indicated that their particle size and pore structure showed sharply different characteristics under different processing conditions [9–11]. To the best of our knowledge, the preparation of mesoporous carbons with a combination of the above two sol–gel reactions by simultaneous synthetic template carbonization has not been reported. The procedure of preparing carbons in this work is as follows: (1) the homogeneous mixture of TEOS, ethyl alcohol, deionized water, and HCl was added to a sealed reaction vessel and cured at 65 ◦C in a water bath for several hours to obtain the precursor sol of silica template; (2) the as-prepared sol was allowed to cool in an ice bath; (3) resorcinol (R) and formaldehyde (F) were directly solved in deionized water with a fixed molar ratio of 1:2; (4) the RF solution was added to the silica sol while stirring and the initial pH of the mixture was adjusted to 7.5 with dilute ammonia; (5) the mixture was put into a sealed bottle and cured for 7 days at 85 ◦C in a water bath to obtain mixing aquagels. During this treatment, both the gelation of TEOS and the polycondensation of RF took place; (6) after curing, the gel was removed from the sealed bottle and dried in air at 80 ◦C. Then, the mixing xerogel was further heat-treated at 1000 ◦C for 2 hr under nitrogen atmosphere to carbonize the polymer in the mixture; (7) the resultant carbon/silica composite was washed with HF to extract carbon from the silica framework; and finally (8) SSTM carbon was obtained by thoroughly washing
Read full abstract
Aug 4, 2014
Xiang Ying Chen + 4
