Introduction of cross-linking agents to enhance the performance and chemical stability of polyethyleneimine-impregnated CO₂ adsorbents: Effect of different alkyl chain lengths

Polyethyleneimine (PEI)-functionalized silica-based CO₂ adsorbents are well known to irreversibly form urea during the regeneration process at high temperatures and under CO₂-rich atmospheric conditions. The formation of urea is a critical drawback that substantially reduces the cyclic performance of PEI-functionalized adsorbents. In this study, epoxy cross-linkers were introduced to PEI to suppress the formation of urea. Pristine PEI and epoxy cross-linked PEI were impregnated into mesostructured cellular foam silica using a dry impregnation method. In particular, epoxy cross-linkers with different alkyl chain lengths were used, and the effect of the alkyl chain length of the cross-linkers on the chemical stability and CO₂ adsorption performance of adsorbents was scrutinized. The suppression of urea formation improved as the alkyl chain length of the cross-linkers increased. The adsorbent functionalized with epoxy cross-linked PEI exhibited an approximately three times higher working capacity (0.71 mmol g⁻¹) than the adsorbent functionalized with pristine PEI (0.23 mmol g⁻¹) after 40 rapid cycles comprising adsorption at 100 °C with 15% CO₂ balanced by N₂ for 10 min and regeneration at 130 °C with 100% CO₂ for 5 min. It is believed that the alkyl chain of the cross-linkers acted as a spacer that could hinder the urea formation, enhancing the chemical stability and CO₂ adsorption performance of PEI-functionalized adsorbents.