Discovering Inherent Characteristics of Polyethylenimine-Functionalized Porous Materials for CO₂ Capture

CO₂ capture is vital for addressing greenhouse gas (GHG)-based environmental issues worldwide. Amine-polymer/silica sorbents have been extensively studied for CO₂ capture, but the fundamental understandings of polyethylenimine (PEI) loading effect, thermal effect, and CO₂ sorption behavior are still lacking. Small-angle neutron scattering (SANS) offers promising opportunities for characterizing CO₂ sorption behavior of PEI-functionalized SBA-15. Herein, in situ SANS has been used to investigate not only PEI loading distribution but also PEI thermal swelling and temperature-dependent CO₂ sorption behavior of PEI-functionalized SBA-15. The results indicate that PEI could disperse on the mesopore surface for the sample with low PEI loading, while for the sample with high PEI loading, PEI could not only disperse on the mesopore surface but also partially fill in the mesopore as plugs. The sample with high PEI loading shows a two-stage swelling of PEI with increasing temperature from 25 to 120 °C in vacuum, in which the size of the intramolecular voids between PEI chains has no change from 25 to 75 °C but expands from 75 to 120 °C, whereas only a subtle swelling is observed up to 120 °C for the sample with low PEI loading. Besides the fact that in situ SANS successfully detects physisorbed CO₂ on the mesopore surface and chemisorbed CO₂ by the amine groups simultaneously: (1) the amount of physisorbed CO₂ increases with increasing pressure but decreases with increasing temperature, and (2) the amount of chemisorbed CO₂ has a trend of V_CO2 (75 °C) > V_CO2 (120 °C) > V_CO2 (25 °C). The thermal swelling of PEI causes dilation of intramolecular voids and thus increases the accessibility of chemisorption sites, resulting in higher CO₂ sorption capacity. Therefore, temperature and PEI swelling are essential factors for kinetic and thermodynamic controls of CO₂ capture in amine-functionalized porous adsorbents.