Epoxy-modified polyethyleneimine for enhanced SO₂-resistance during CO₂ capture: Effects of propylene and butylene alkyl chains

Amine-functionalized materials have attracted considerable interest as potential candidates for the selective capture of CO₂; however, severe deterioration in the presence of even trace amounts of SO₂ in flue gases has limited their practical application. Because SO₂ mainly degrades primary amines, converting primary amines into secondary or tertiary amines can enhance their resistance to SO₂. In this study, polyethyleneimine (PEI), a widely used amine for CO₂ capture, was modified using two epoxides with different alkyl chain lengths. The PEI modified with propylene oxide (40PP) predominantly contained tertiary amines (ca. 93%), whereas the PEI modified with butylene oxide (40BP) had a similar fraction of secondary and tertiary amines. Both epoxy-modified PEI showed dramatically enhanced SO₂ resistance compared to pristine PEI. The reversibility of 40PP was more stable than that of 40BP under flue gas conditions (2000 ppm SO₂ balanced with N₂), which was attributed to the weakly conjugated SO₂ on 40PP, because tertiary amines have lower basicity than secondary amines. The SO₂ adsorption retention of 40PP and 40BP after 10 cycles of adsorption and desorption was 94% and 91%, respectively, and much higher than that of PEI (64%) under simulated post-combustion flue gas composition (400 ppm SO₂ and 15% CO₂ balanced with N₂). The adsorption–desorption mechanism was confirmed using diffuse reflectance infrared Fourier transform spectroscopy. Epoxy-modified PEI with superior reversibility even in the presence of SO₂ can contribute to effective post-combustion CO₂ capture from flue gas.