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 CO2 levels from rising further. This work addresses the question of whether nitrogen or oxygen lining of the surfaces of carbon-based CO2 absorbers is more efficient for CO2 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 CO2 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 CO2 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 CO2 uptakes by the two types of materials, the highest CO2 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 CO2-surface adducts. EC-PBDAZ carbon aerogels showed high selectivity for CO2 versus H2 (up to 404:1─relevant to precombustion CO2 capture) and high selectivity for CO2 versus N2 (up to 48:1─relevant to postcombustion CO2 capture).