One-step sol–gel synthesis of hierarchically porous, flow-through carbon/silica monoliths

Abstract

Hierarchically porous, flow-through carbon/silica bicontinuous composite monoliths with ultra-high Brunauer–Emmett–Teller (BET) surface areas and tunable porosity in micro/meso/macro-structured domains, were obtained from an efficient one-step sol–gel chemistry based on the co-assembly of organic and inorganic precursors with simultaneous polymerization-induced phase separation. Without activation, the bicontinuous composites were subjected to pyrolysis and silica removal to yield crack-free hierarchically porous carbon monoliths that have large pore volumes and high BET surface areas (∼2600 m² g⁻¹). The removal of carbon from the silica/carbon composite monolith produces a microporous silica framework (BET area ∼600 m² g⁻¹). The hierarchically porous carbon monoliths were characterized in terms of their pore morphology, flow-through porosity, phase composition, mechanical strength, structural and elemental compositions, and surface wettability. The polymer monolith was determined to be hydrophobic, whereas the carbon monolith was hydrophilic in nature. The water permeability of the carbon monolith was determined to be 12 × 10⁻¹² m², and its Young's modulus was 0.42 MPa, which suggests that this monolith could be used as a potential flow-through medium. The use of the carbon monolith as a catalytic support is demonstrated by the in situ growth of silver nanoparticles, with which the hybrid exhibits excellent catalytic activity for the reduction of 4-nitrophenol (4-NP) with NaBH₄ in an aqueous medium. The hierarchically porous carbon monoliths have a plethora of potential applications owing to their mechanical stability and transport properties throughout the monolith. The method of synthesis outlined here can be easily extended to the synthesis of monolithic oxides, such as SnO₂, TiO₂, ZnO, ITO etc.