Comparison of CO2 adsorption/desorption behaviors of grafted solid amine materials with tunable polyethyleneimine chain lengths

Abstract

A series of polyethyleneimine (PEI)-grafted polystyrene (PS) resins, designated PS-LPEI-y (where y represents the average degree of polymerization of PEI, with values of y = 1.7, 2.8, 3.5, 4.2, 5.5), were synthesized via controlled ring-opening polymerization of 2-methyl-2-oxazoline. The materials were evaluated for CO₂ adsorption, revealing that PS-LPEI-3.5 exhibited the highest uptake at 25 °C. In contrast, materials with higher amine loadings demonstrated reduced CO₂ uptake due to PEI aggregation, which hindered CO₂ diffusion. At elevated temperatures, this aggregation effect was mitigated, allowing materials with longer PEI chains to outperform those with lower amine densities. Kinetic modeling indicated that the more heavily aminated materials exhibited higher activation energies for both CO₂ adsorption and desorption, while their adsorption heats, as measured by calorimetry, were lower. Detailed characterizations revealed that the different adsorption/desorption behaviors correlated with the morphological properties of the adsorbents. Mechanistic calculations suggested favorable carbamic acid formation, accompanied by hydrogen bonding interactions with adjacent ethyleneimine units.