Aqueous synthesis of bimodal mesoporous carbons and carbon-silica mesostructures under basic conditions

Abstract

Carbonization of the composite materials obtained by co-condensation of phenol-formaldehyde resin and tetraethyl orthosilicate (TEOS) in a basic aqueous solution in the presence of Pluronic F127 block copolymer template yields mesoporous carbon/silica composite materials (CSX). These materials have mesopore volumes of about 0.4 cm3/g and pore sizes between 6 and 8 nm (at the maximum of the pore size distribution) that are larger than those of the ordered mesoporous carbons (OMC) synthesized by the same procedure but without TEOS. Calcination of CSX in air yields mesoporous silicas (SX) with pore sizes around 11 nm and pore volumes of about 1 cm3/g. Dissolution of the silica domains in CSX yields carbons (CX) with pore volumes and surface areas as high as 1.84 cm3/g and 1500 m2/g, respectively. These carbons feature bimodal structures with larger mesopores of about 7–8 nm (porosity of CSX) and smaller mesopores with sizes closer to 5 nm, which are created by silica dissolution. The mesopore volume of CX after silica dissolution can be tuned by varying carbonization temperature (between 500 and 850 °C) to modify the concentration of silica nanodomains in CSX without significant change in the size of mesopores in CSX. Regardless of the phenol/TEOS ratio used (between 2 and 7) all the materials have similar composition, pore size distribution and pore volumes; however, the synthesis yield decreases with decreasing amount of TEOS. The final SiO2 content in CSX does not increase with the amount of TEOS added like in the synthesis of OMC by evaporation induced self-assembly or self-assembly in acidic solutions.