Fabrication of bimodal mesoporous carbons from petroleum pitch by a one-step nanocasting method
Introduction
Nowadays, substantive efforts have been made to prepare various types of novel carbon nanomaterials, and tailor their nanostructures for a specific application [1], [2], [3], [4], [5], [6], [7], [8], [9]. Recently, a simple and efficient one-step nanocasting method has successfully adopted to prepare hard-templated nanoporous carbons [10], [11], [12], [13], [14], [15], [16], [17], and to modify carbon framework of soft-templated mesoporous carbons (e.g., reducing framework shrinkage, and introducing nanopores) [18], [19]. This method usually involves the formation of composites containing carbon precursors and silica through the sol–gel process. Since the reaction media for the sol–gel reaction of silica precursors such as tetraethyl orthosilicate (TEOS) and tetramethyl orthosilicate are generally water [10], [11], [12], [13], [14], [15] or ethanol–water solution [16], [17], [18], [19], only some highly hydrophilic substances like cyclodextrin [10], [12], [15], sucrose [11], [13], [14], [16], furfuryl alcohol [17], resorcinol–formaldehyde [18] and resol precursor [19] have been so far reported to be usable carbon precursors. It is believed that it is very difficult to use many frequently-used hydrophobic precursors in the field of carbon materials (e.g., pitch, polyacrylonitrile and polyvinyl chloride) as carbon source in such a hydrophilic reaction system if not impossible to achieve, because the incompatibility between hydrophobic precursor and H2O, which is indispensable during TEOS hydrolysis process, is an irreconcilable contradiction. To the best of our knowledge, the one-step nanocasting fabrication of nanoporous carbons from hydrophobic precursors has not yet been reported.
It is well-known that carbon sources usually have an important or even a decisive bearing on physical and chemical properties of carbon framework of porous carbon materials. For examples, some hydrophobic substances with graphitic building blocks (e.g., pitch, acenaphthene) usually afford carbon framework with some degree of graphitization, whereas the aforementioned hydrophilic carbon sources generally impart amorphous nature to carbon framework [20]. Obviously, the current one-step nanocasting method has obvious disadvantages because of its limitation in the diversity of carbon sources. Up to now, nanoporous carbons with hydrophobic substances like pitch as carbon source and silica as template are usually prepared by a two-step templating method, which involves two main steps: (a) obtaining a nanostructured silica template (e.g., silica gel, silica colloidal crystal and colloidal silica particle); and (b) preparing a composite of carbon precursor and nanostructured silica template, followed by carbonizing the precursor and then removing the silica framework [20], [21], [22]. This technique is successful in preparing nanoporous carbons with precise microstructures. However, as compared to the above one-step nanocasting method, such two-step process has some limitations, such as being tedious, incomplete pore filling, and formation of nonporous carbon resulting from the deposition of carbon source on the external surface of nanoporous silica template [10], [12], [16].
We report herein a novel gelation system and its implementation for the one-step nanocasting fabrication of nanoporous carbons from highly hydrophobic petroleum pitch. For such a system, a new solvent (i.e., tetrahydrofuran, THF) and a new catalyst (i.e., hydrofluoric acid, HF) have been developed to achieve the solution-sol–gel reaction of TEOS in the presence of petroleum pitch. As illustrated in Fig. 1, the overall synthetic procedure mainly includes: (1) HF-catalyzed gelation reaction of TEOS in the THF solution of pitch; (2) drying, preoxidation and subsequential carbonization of the resulting pitch/silica composite; and (3) removing silica template of the resulting carbon/silica composite. The as-prepared nanoporous carbons have a unique bimodal mesopore structure—small-sized three-dimensional wormholelike mesopores from silica gel framework and large-sized well-distributed particlelike mesopores from silica nano-agglomerates, which is believed to have superior mass transport as compared to the traditional uniform wormholelike mesoporous structure [10], [16].
Section snippets
Sample preparation
AH-90 petroleum pitch (Sinopec Corp., Guangzhou, China; softening point = 51 °C; elemental contents: 85.56 wt.% C, 10.42 wt.% H, 0.76 wt.% N, and 1.14 wt.% S) was used as carbon precursor in the experiment. The petroleum pitch was thoroughly dissolved in THF to obtain pitch/THF solution with a pitch/THF ratio of 1/5 g/ml, which is denoted as solution-A. Meanwhile, 8 ml TEOS and 1.5 ml deionized water were added into THF solvent and then 0.8 ml HF was added with stirring to form a homogeneous, colorless
Results and discussion
In general, in order to prepare nanoporous carbon materials by the one-step nanocasting method, two basic conditions must be met: (1) a proper solvent to dissolve all components; and (2) a suitable catalyst to achieve the TEOS gelation reaction in the presence of carbon precursors. Based upon this, in our first exploration, we decided to look for a proper solvent to dissolve these raw materials with considerable diversity in the hydrophilic–hydrophobic property (i.e., petroleum pitch, TEOS and H
Conclusions
In summary, we have successfully fabricated a petroleum pitch-based bimodal mesoporous carbon by a novel one-step nanocasting technique. THF and HF have been proven to be the appropriate solvent and catalyst, respectively, for the gelation reaction of TEOS and H2O. Moreover, the use amount of petroleum pitch is a key factor to obtain a uniform silica/pitch composite. The as-prepared carbon material has a unique bimodal mesoporous structure with 3.4 nm and 17.1 nm diameter mesopores. The templates
Acknowledgements
This research was supported by the Project of NNSFC (50632040, 50802116, 50972167), the Specialized Research Fund for the Doctoral Program of Higher Education (200805581014), and the Natural Scientific Foundation of Guangdong Province (8451027501001421).
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