Microporous carbon foams: the effect of nitrogen-doping on CO₂ capture and separation via pressure swing adsorption

This work investigates the effect of nitrogen doping on the porous structure and CO₂ adsorption properties of hierarchically porous carbon foams, at different temperatures and pressures. A series of carbon foams with nitrogen contents of ∼7 to 13 at % is prepared by reaction of sodium ethoxide with different amino alcohols (namely monoethanolamine, diethanolamine, and triethanolamine), followed by thermal decomposition of the reaction product. The structure, morphology, porosity and chemical composition are studied using appropriate methods. The resulting material comprises micron-scale macropores combined with unrestricted micropores and mesopores embedded in the carbon walls. It is found that both nitrogen species and ultra-micropores contribute positively to CO₂ uptake. The carbon foam with a nitrogen content of ∼7 at % (as well as an ultra-micropore volume of 0.44 cm³ g⁻¹ and a specific surface area of 1549 m² g^-1) displays the highest CO₂ uptake, adsorbing 5.14 mmol g⁻¹ at 273 K, 3.22 mmol g⁻¹ at 298 K, or 1.93 mmol g⁻¹ at 323 K. This nitrogen-doped carbon foam adsorbs more CO₂ than the undoped carbon foam reference, despite having significantly lower ultra-micropore volume and higher specific surface area. This highlights the importance of nitrogen species in CO₂ capture. However, increasing the nitrogen content leads to suppression of the micropore volume, resulting in poor CO₂ uptake. High CO₂/N₂ selectivity with good separation of CO₂ from the CO₂-N₂ gas mixture, fast adsorption via physisorption, and excellent cycling durability are also observed, pointing towards the high regenerative ability of these materials.