Overcoming mass transfer limitations in cross-linked polyethyleneimine-based adsorbents to enable selective CO 2 capture at ambient temperature

New self-supported polyamine CO 2 adsorbents are prepared by cross-linking branched polyethyleneimine (PEI) with 2,4,6-tris-(4-bromomethyl-3-fluoro-phenyl)-1,3,5-triazine (4BMFPT). Controlling the degree of cross-linking to ensure abundant free amine functionalities while maintaining a structure conducive to efficient mass transfer is key to accessing high CO 2 adsorption and fast kinetics at ambient temperature. The polyamine-based adsorbent, PEI-4BMFPT, 10 : 1 (R), is composed of spherical particles up to 3 μm in diameter and demonstrates fast CO 2 uptake of 2.31 mmol g −1 under 1 atm, 90% CO 2 /Ar at 30 °C. Its CO 2 /N 2 selectivity, predicted by the ideal adsorbed solution theory is 575, equalling that of highly selective metal–organic frameworks. Based on humidified thermogravimetric analysis, it was observed that the presence of water promotes CO 2 uptake capacity of 10 : 1 (R) to 3.27 mmol g −1 and results in strong chemisorption; likely by formation of ammonium carbonate and bicarbonate species. It is observed that CO 2 uptake enhancement is highly subject to relative humidity and CO 2 partial pressure conditions. When adsorption conditions combined low temperatures with low partial pressure CO 2 , 10 : 1 (R) showed reduced uptake. Tested under breakthrough conditions representative of post-combustion conditions, at 75% RH and 40 °C, CO 2 uptake was reduced by 83% of the dry adsorption capacity. This body of work further advances the development of support-free CO 2 adsorbents for ambient temperature applications and highlights the drastic effect that relative humidity and CO 2 partial pressure have on uptake behaviour.