Aminopolymer-Impregnated Hierarchical Silica Structures: Unexpected Equivalent CO₂ Uptake under Simulated Air Capture and Flue Gas Capture Conditions

Poly­(ethyleneimine)-impregnated sorbents are prepared using a hierarchical silica support with bimodal meso-/macroporosity. The sorbents behave unexpectedly during CO₂ adsorption from simulated air and flue gases (400 ppm and 10% CO₂) at a fixed temperature, as compared to systems built on commonly studied mesoporous materials. The results demonstrate that (i) impregnation methods influence the efficacy of sorption performance and (ii) the sorbents show almost similar uptake capacities under 400 ppm and 10% dry CO₂ at 30 °C, exhibiting step-like CO₂ adsorption isotherms. These unusual observations are rationalized via control experiments and a hypothesized sorption mechanism. While the sorption performance near room temperature is unexpectedly identical under 400 ppm and 10% CO₂ conditions, there is an optimal temperature at each gas concentration where the uptake is maximized. The maximum sorption capacities are 2.6 and 4.1 mmol CO₂/g sorbent at the optimized sorption temperatures using 400 ppm and 10% dry CO₂, respectively. The presence of water vapor under 400 ppm CO₂ conditions further improves the sorption capacity to 3.4 mmol/g sorbent, which is the highest capacity under direct air capture conditions among known amine sorbents impregnated with a similar polymer, to the best of our knowledge.