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 degrees C/Ar), and are etched with CO2 (1000 degrees 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, H-2, and N-2. 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.