Synthesis of carbon monoliths with a tailored hierarchical pore structure for selective CO₂ capture

Carbon monolithic adsorbents exhibiting a hierarchical pore structure are produced via a synthesis route based on the stabilization of liquid foams followed by a carbonization step. The macro-microporous structure is achieved by the incorporation of microporous, biomass-derived activated carbon particles in the liquid foam enclosed by a cationic surfactant as stabilizer. This method yields crack-free monoliths (solid foams) with a compressive strength of the order of 20 kPa. The microstructure and the textural properties of the final solid foams have been investigated by means of Scanning Electron Microscopy (SEM) and gas adsorption. The behavior as selective CO₂ adsorbents at 25 °C has been evaluated using breakthrough experiments under simulated post-combustion conditions (16% V/V CO₂/N₂), resulting in a selectivity factor of 13 over N₂. The hierarchical pore structure of the monoliths allows a rapid transport of the gas mixture through the macropores with no appreciable pressure drop, retaining more than 90 % of the adsorption capacity (∼ 0.868 mmol/g) after several adsorption/desorption cycles. Moreover, the monolith has shown a CO₂ uptake capacity of 2.62 mmol/g under static condition at 1 bar and 25 °C. This study provides guidelines for the design of carbon-based foams decorated with carbon particles, which have morphological and textural properties that can be carefully selected for any gas-selective capture application.