Template synthesis of mesoporous carbons with tailorable pore size and porosity

Abstract

Mesoporous carbons with uniform mesopores, large surface areas (>1500 m g ) and large pore volumes (>1 cm g ) have attracted widespread attention due to their relevance for applications in emergent areas such as energy storage in double-layer supercapacitors [1], catalytic support in fuel cell electrodes [2], adsorption of bulky molecules in liquid phase [3], etc. One important method of preparing carbons with the aforementioned characteristics involves the use of mesostructured silicas materials (MSM) as templates, which allows mesoporous carbons to be obtained as inverse replicas. Recently, several researchers have reported using MSM to synthesise mesoporous carbons [4–7]. Ryoo et al. [4] were the first to describe the synthesis of mesoporous carbon by using MCM-48 silica as template. Later, other mesoporous carbons were prepared by templating various types of MSM such as SBA-15 [5], MSU-H [6] and HMS [7]. The outstanding properties of templated mesoporous carbons are related to their structural characteristics such as pore size and type of porosity (i.e. unimodal/bimodal). Normally, these characteristics are determined by the type of MSM selected as template. Consequently the properties of templated carbons can only be tailored between narrow limits. Then, one of the limitations in the replication of mesoporous carbons is the lack of control over pore size and porosity. However, although much work has been carried out on the synthesis of mesoporous carbons by templating MSM, hardly any attention has been paid to this problem. As an exception, Lee et al. [8] recently reported that by using a mixture of two surfactants at different ratios it is possible to vary the wall thickness of silicas and consequently, the pore diameter of templated carbons within the narrow range of 2.2–3.3 nm. Our purpose in this work is to be able to modulate, in a wide range, the size of primary carbon mesopores (i.e. those derived from the removal of the silica framework) by modifying the structural characteristics of the silica used as template. In addition, we show that unimodal or bimodal mesoporous carbons can be obtained from the same silica template, by simply modifying the synthesis procedure. For the synthesis of mesoporous carbons, we used as template an MSU-1 silica, which is known to possess a 3-D wormhole porous framework [9]. This type of MSM has not been used before to fabricate mesoporous carbons. The silica was synthesised in a two-step pathway as reported by Boissiere et al. [10]. Different temperatures (between 20 and 150 C) were used in the second step, the composition of the synthesis mixture being identical in all cases. Briefly, the silica source tetraethoxy-silane (TEOS, Aldrich) was added under stirring to a 0.02 M solution of a nonionic polyethylene oxide surfactant Tergitol 15–S–12 (CH3(CH2)14(EO)12, Sigma). The pH of the solution was adjusted at around 2 by the addition of HCl 0.25 M. The solution was kept in a closed Teflon vessel for 18 h without stirring at 20 C. Afterwards, a small amount of NaF was added and as a result condensation of the silica occurred suddenly. The mole ratio of the synthesis mixture was: TEOS:T–15–S– 12:NaF:HCl:H2O1⁄4 1:0.125:0.06:0.064:375. The mixture was allowed to stand for 3 days at different temperatures between 20 and 150 C. The white precipitate obtained was filtrated, dried and calcined at 600 C for 4 h. The carbons were synthesised according to the procedure reported elsewhere [11]. In a typical synthesis, the silica was impregnated with paratoluene sulfonic acid (0.5 M in ethanol). Afterwards, furfuryl alcohol was added to the silica until incipient wetness was achieved. The impregnated sample was cured in air for 12 h at 80 C to polymerise the furfuryl alcohol and to convert it into polyfurfuryl alcohol, which was then carbonised under N2 at 800 C (2 C/min). The porous carbon was obtained after dissolution of the silica framework in 48% HF at room temperature. Adsorption measurements of the calcined silica and carbons were performed using a Corresponding author. Tel.: +34-985-11-9090; fax: +34-985-297662. E-mail address: abefu@incar.csic.es (A.B. Fuertes).