CO₂ Uptake by Microporous Carbon Aerogels Derived from Polybenzoxazine and Analogous All-Nitrogen Polybenzodiazine Aerogels

The rapid rise of carbon dioxide in the atmosphere contributes to global warming and ocean acidification. Carbon capture is considered essential for keeping the atmospheric CO 2 levels from rising further. This work addresses the question of whether nitrogen or oxygen lining of the surfaces of carbon-based CO 2 absorbers is more efficient for CO 2 capture. Polybenzodiazine (PBDAZ) aerogels are carbon-aerogel precursors that were prepared recently (2023) as all-nitrogen structural analogues to well-known polybenzoxazine (PBO) aerogels. However, owing to the fact that the carbonization chemistries of both PBO and PBDAZ aerogels require a prior oxidative aromatization step in air at around 200–240 °C, both types of the resulting carbon (C) aerogels contained both oxygen and nitrogen in their structures. As a carryover from their polymeric aerogel precursors, C-PBDAZ aerogels included a higher weight percent of N (7–11%) relative to the C-PBO aerogels (5.2–5.7%) while both types of carbons included about the same amount of O (7–9%). Activation (etching) with CO 2 at 1000 °C removed more N than O, so the resulting etched carbon aerogels from either source (EC-PBDAZ or EC-PBO) contained about 8–9% of O and only 3.0–3.6% N. Postetching, most oxygen was situated in pyridonic and nitroxide sites (by XPS). Looking at the PBDAZ- and PBO-derived C and EC aerogels independently, whenever processing increased the O/N ratio within each material, the CO 2 uptake (at 273 K, 1 bar) also increased, reaching 11.5 mmol g –1 in EC-PBDAZ and 4.6 mmol g –1 in EC-PBO aerogels, starting from 7.0 mmol g –1 by C-PBDAZ and 3.0 mmol g –1 by C-PBO. Subsequently, by eliminating the relative pore volumes and surface areas as causing the different CO 2 uptakes by the two types of materials, the highest CO 2 uptake by the EC-PBDAZ aerogels was attributed to the pore sizes (diameters in the 3–4 nm range) in combination with the geometry of the CO 2 -surface adducts. EC-PBDAZ carbon aerogels showed high selectivity for CO 2 versus H 2 (up to 404:1─relevant to precombustion CO 2 capture) and high selectivity for CO 2 versus N 2 (up to 48:1─relevant to postcombustion CO 2 capture).