Overcoming mass transfer limitations in cross-linked polyethyleneimine-based adsorbents to enable selective CO2 capture at ambient temperature

Abstract

New self-supported polyamine CO₂ 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 CO₂ 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 CO₂ uptake of 2.31 mmol g⁻¹ under 1 atm, 90% CO₂/Ar at 30 °C. Its CO₂/N₂ 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 CO₂ 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 CO₂ uptake enhancement is highly subject to relative humidity and CO₂ partial pressure conditions. When adsorption conditions combined low temperatures with low partial pressure CO₂, 10 : 1 (R) showed reduced uptake. Tested under breakthrough conditions representative of post-combustion conditions, at 75% RH and 40 °C, CO₂ uptake was reduced by 83% of the dry adsorption capacity. This body of work further advances the development of support-free CO₂ adsorbents for ambient temperature applications and highlights the drastic effect that relative humidity and CO₂ partial pressure have on uptake behaviour.