Research articleOrdered mesoporous carbons obtained from low-value coal tar products for electrochemical energy storage and water remediation
Graphical abstract
Introduction
Conventional porous carbon materials are obtained from carbon-rich precursors such as lignocellulosic residues or mineral coal. After the carbonization of the precursor material, the product is submitted to an activation process to further develop the porosity of the final activated carbon. Control over the activation conditions allows tuning the porous texture [1]. However, high-demanding applications (e.g., energy storage, electrocatalysis or water remediation) require materials with a very fine tuned porosity, especially in the mesopore range (2–50 nm in size).
Different techniques have been developed to approach this problem. Among them, the hard-templating method stands out as a powerful strategy for creating carbon materials with strictly controlled porosity, including ordered mesoporous carbons (OMCs). The templated carbon materials are synthesized by the infiltration of a carbon precursor into the pore network of an inorganic porous solid that acts as a template [[2], [3], [4]]. After the infiltration of the precursor, the mixture is carbonized and then the template is removed by an appropriate etching procedure. Thus, a carbonaceous material is obtained with a porous structure that is a replica of the walls of the material used as a template. By using different templates, the main pore size and the pore morphology can be accurately controlled. In the case of OMCs, the usual choice as a template is mesoporous oxides like silica. However, the success in replicating the template depends largely on the stringent control of a number of factors. A key point is the selection of a suitable carbon precursor, which must infiltrate the template homogeneously, polymerize and afford a high carbon yield after pyrolysis. Frequently used precursors are sucrose, furfuryl alcohol, acrylonitrile, phenol resin, propylene, acetylene or acetonitrile, among others [5].
Polycyclic aromatic hydrocarbons have been used as carbon precursors for the preparation of hierarchical porous carbons [[6], [7], [8], [9], [10], [11]]. These materials have been used also to infiltrate the templates in the hard-templating method [12]. The OMCs obtained using these carbon precursors exhibit better electrical conductivity than that of porous carbons obtained using more conventional carbon precursors because of the higher aromatic character of the former [13,14]. However, a comparatively small number of studies have focused on the preparation of OMCs using these carbon precursors. Acenaphthene has been employed as a precursor with Al-modified MCM-48, SBA-1, SBA-15 and KIT-6 silicas as templates for the synthesis of OMCs in an autoclave at high temperature and high pressure [14,15]. Milder synthesis procedures are a priori more attractive and have made use of naphthalene, anthracene, or pyrene dissolved in acetone as a carbon precursor and SBA-15 [12] or MSU-H [16] as a template with sulfuric acid to act as a polymerization catalyst at room temperature. A similar method has been followed to infiltrate MSU-H with phenanthrene dissolved in acetone, but using toluene sulfonic acid as a catalyst [17,18]. Anthracene has also been introduced in the pores of MCM-41 by dissolving it in dichloromethane [19]. More recently, only two studies have focused on the preparation of OMCs using polycyclic aromatic compounds. Both of them used phenanthroline dissolved in ethanol, in the presence of FeCl2 [20] and H2SO4 [21] as catalysts, and SBA-15 and mesoporous silica spheres as templates, respectively. All these previous studies made use of pure, relatively expensive polyaromatic compounds, which are solid at ambient temperature, so the infiltration of the templates must be either carried out in an autoclave at high temperatures and pressures or by dissolving the carbon precursors in a solvent (e.g., acetone), thus making the process less amenable to industrial upscaling. For example, the use of a solvent requires the repetition of the impregnation step several times to achieve a complete filling of the template pores, and the evaporation of the solvent after each step impedes an optimum impregnation.
To circumvent this issue, in the present work we have made use of creosote as a carbon precursor for the preparation of OMCs by the hard-template method. Creosote is a low-priced product obtained through distillation of coal tar in the temperature range between 240 and 350 °C, and is made up of a complex mixture of polycyclic aromatic hydrocarbons, its main components being phenanthrene, acenaphthene, naphthalene, fluorene and dibenzofuran, which collectively amount to ~50% of its total weight. Creosotes are typically employed as wood preservatives and, to the best of our knowledge, they have never been used as a precursor for the synthesis of any type of carbon material (including OMCs). Creosotes are generally liquid at room temperature or close to room temperature. This feature would avoid the need to use high temperatures or solvents when infiltrating the creosotes into hard templates, making them more convenient precursors in the synthesis of OMCs compared to pure, single-component polycyclic aromatic hydrocarbons. We demonstrate here that high quality OMCs can be efficiently prepared using such low-value, multi-component substance as the precursor, and the resulting carbons are competitive materials when used in electrochemical energy storage and water remediation applications.
Section snippets
OMC preparation
The OMCs were prepared following the hard template method as schematically depicted in Fig. SI1 (Supporting Information). First, the pores of the template were infiltrated with the carbon precursor (creosote). Then, the infiltrated template was heat-treated to obtain a composite of the template with the carbon material. Finally, the template was removed by chemical etching to give a stand-alone carbon material.
Ordered mesoporous carbons from creosote as the carbon precursor
The SBA-15 template used here exhibited a very well developed mesoporosity with a dDFT = 8.5 nm and SBET = 615 m2/g (see Fig. SI3 in Supporting Information). The initial attempts of infiltrating the SBA-15 directly with the creosote gave rise to a composite material SBA-15/creosote with ca. 30 wt% of the carbon precursor within the pores of the OMS. However, no carbon material could be retained after the carbonization step. To overcome this issue, we used a catalyst to promote the
Conclusions
Porous carbons with very well ordered mesoporosity were prepared via the hard-template method using creosote, a low-value coal tar product, as the carbon precursor. The preparation method was optimized via partial polymerization of the creosote in acidic medium, obtaining a material with a high surface area and a very narrow pore size distribution, confined mainly in the range of the mesopores. The well-developed mesoporosity made the material very promising in applications such as
Acknowledgment
This project has received funding from the Research Fund for Coal and Steel (RFCS) of the European Union (EU) under grant agreement No 709741. We also acknowledge Bilbaína de Alquitranes S.A. for providing the creosote used in this work, Cabot Corp. for the Vulcan XC72R carbon black, Kuraray Europe GmbH for the FR-20 and YP-50F activated carbons, and Silcarbon GmbH for the S300, S1030 and P835 activated carbons.
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