Exceptionally High CO2 Adsorption at 273 K by Microporous Carbons from Phenolic Aerogels: The Role of Heteroatoms in Comparison with Carbons from Polybenzoxazine and Other Organic Aerogels

Phenolic aerogels containing oxygen and other polymeric aerogels containing both oxygen and nitrogen (polybenzoxazine and a polyamide‐polyimide‐polyurea co‐polymer) are converted to carbon aerogels (800 °C/Ar), and are etched with CO₂ (1000 °C). Etching opens closed pores and increases micropore volumes and size. Heteroatoms are retained in the etched samples. All carbon aerogels are evaluated as CO₂ absorbers in terms of their capacity and selectivity toward CH₄, H₂, and N₂. CO₂ adsorption capacity is linked to microporosity. In most cases, monolayer coverage of micropore walls is enough to explain CO₂ uptake quantitatively. The interaction of CO₂ with micropore walls is evaluated via isosteric heats of adsorption, and is stronger with carbons containing only oxygen heteroatoms. The adsorption capacity of those carbons (5–6 mmol g^?1) is on par with the best carbon and polymeric CO₂ adsorbers known in the literature, with one exception however: etched carbon aerogels from low‐density resorcinol‐formaldehyde aerogels show a very high CO₂ uptake (14.8 ± 3.9 mmol g^?1 at 273 K, 1 bar) attributed to a pore‐filling process that proceeds beyond monolayer coverage, whereas surface phenoxides engage in a thermally neutral carbonate forming reaction (surface‐O^– + CO₂ → surface‐O–(CO)–O^–) that continues until micropores are filled.