A geopolymer route to micro- and meso-porous carbon

Hexagonal and wormhole-type mesoporous geopolymers were developed by controlling the concentration of a structure directing agent (cetrimonium bromide, CTAB) with fixed ratios of Si/Al, KOH/(Si + Al), and H₂O/(Si + Al), and their detailed porous structures were confirmed by TEM, N₂ adsorption-desorption and X-ray diffraction measurements. The as-prepared geopolymers were then used as templates to replicate porous carbons with various structures and porosities for CO₂ adsorption. To understand the correlation between the CO₂ adsorptivity and porous structures, we tuned the porosity of the geopolymer-templated carbons by modifying the structures of the geopolymers. The porous carbons obtained from the hexagonal-type porous geopolymers were found to be composed of the aggregates of carbon nanowires exhibiting large particles, while those obtained from the wormhole-like porous geopolymers were determined to be wormhole type as well, as evidenced by TEM and X-ray diffraction studies. According to the CO₂ adsorption isotherms of the porous carbons, the aggregates of carbon nanowires exhibited the highest CO₂ adsorptivity due to their highest microporosity and largest specific surface area.