Hierarchically Porous Silica, Carbon and Metal Oxide Monoliths – Synthesis and Characterization

Abstract

Silica monoliths exhibiting a three-modal, hierarchical pore structure have successfully been prepared via sol-gel processing. Monolithic bodies with interconnected macropores in the μm range are a result of phase separation and gelation kinetics both being controlled by the addition of a hydrogen bonding polymer. The textural mesopores in the 10-20 nm range originate from voids between silica particles. Furthermore, the particles exhibit internal mesoporosity with pore diameters in the 2-4 nm range as a result of supramolecular templating by a cationic surfactant. These silica monoliths have been used as hard templates to prepare nanocast carbon monoliths exhibiting a three or four-modal porosity by one-step impregnation of furfuryl alcohol as carbon precursor. The combined volume and surface templating, together with controlled synthesis of the starting silica monoliths used as the scaffold, enables a flexible means to simultaneously control the pore size on several length scales. The carbon monoliths are positive replicas of the micrometer length scale, while they are negative replicas on the nanometer scale. In a similar fashion it is possible to prepare cobalt oxide replicas with a bimodal porosity. This approach has to the best of our knowledge never been reported previously. Furthermore, it is shown that the method can be generalized to encompass other metal oxides as well. These include tin and manganese oxides, which are difficult to prepare by direct synthesis. The different modes of porosity are arranged in a hierarchical structure-withinstructure fashion. This arrangement is optimal for materials requiring a high surface area in combination with a low pressure drop, such as catalysts, HPLC columns, and pressure sensors. The nanocasting route makes it possible to tailor the pore structure depending on the requirements for different applications.