Effect of acid catalysts on carbonization temperatures for ordered mesoporous carbon materials

Copyright © Korean Carbon Society http://carbonlett.org Highly ordered mesoporous carbon (OMC) materials have attracted much attention in many fields including catalysis, adsorption and energy storage, due to their high porosity, high surface area and controllable regular pore-structure [1-6]. Successful synthesis pathways for obtaining OMC materials include the direct synthesis of OMC materials through the organic-organic self-assembly method (soft templating process) and nanoreplication of OMC materials from mesoporous silica templates (hard templating process) [7-11]. The direct synthesis method is suitable for large-scale production of carbon materials due to its easier process compared to the nano-replication method [7,8]. However, the nano-replication method has still several advantages for synthesizing OMC materials. It is possible to synthesize various kinds of mesostructured OMC materials, which can be controlled by the mesostructure of the silica templates [12,13]. In addition, the morphology of OMC materials can also be varied with the silica templates using desired morphologies [12-14]. More importantly, various kinds of carbon precursors including sucrose, furfural, p-toluene sulfonic acid (p-TSA) and phenanthrene [14-17] can be used to control the physicochemical properties of the OMC materials, for which the nature of the carbon precursors affects the conductivity, porosity and surface functionality of the carbon frameworks [17-19]. However, nano-replication methods for OMC materials, based on a conventional thermal process at high temperature (typically above 900°C), are timeand energy-consuming processes. In order to oligomerize or polymerize the carbon precursors before the carbonization, an aging process should also be carried out in the presence of acid catalysts in low temperature ranges of 100°C–200°C. In the case of sucrose as a carbon source, sulfuric acid (H2SO4) is typically used as an acid catalyst for the dehydration and subsequent oligomerization of the carbon precursor. This is very important for preventing the infiltrated carbon precursors from draining outside the mesopores of the silica templates before the carbonization. The oligomeric carbon species are then carbonized at high temperature above 600°C. Despite using sulfuric acid which has a good carbonization ability as a carbonization catalyst, OMC with a well-developed mesostructure is not obtained at carbonization temperatures below 600°C [20]. Low carbonization temperatures (<600°C) for the synthesis of OMC materials are very important for achieving process economics and, in particular, for enhancing diversity. For various applications of OMC materials, it is necessary that heteroatom species, such as nitrogen and sulfur, are included in a final OMC product from the initial precursor mixture for the synthesis of OMC. However, carbonization temperatures above 600°C are too high to maintain the functionality of heteroatoms; thus, to achieve this aim, a low-temperature carbonization process should be addressed in the future. In the present work, we investigated the effect of acid catalysts in connection with the carbonization temperature for the synthesis of OMC materials. p-toluene sulfonic acid (pTSA, CH3C6H4SO3H), phosphoric acid (H3PO4), sulfuric acid (H2SO4), nitric acid (HNO3), hydrochloric acid (HCl), and acetic acid (CH3COOH) were used as the acid catalysts for carbonization. As a result, unlike other acid catalysts, the use of p-TSA and phosphoric acid produces OMC materials with a well-developed mesostructure at low carbonization temDOI: http://dx.doi. org/ DOI:10.5714/CL.2016. 20.066