Molecular basket sorbents polyethylenimine–SBA-15 for CO₂ capture from flue gas: Characterization and sorption properties

A series of molecular basket sorbents consisting of SBA-15 loaded with different amounts of polyethylenimine (PEI) have been systematically studied, focusing on their characterization by small angle X-ray scattering (SAXS), N₂ physisorption, scanning electron microscopy (SEM), and thermogravimetric analysis (TGA) and their CO₂ sorption properties. The meso-structure of SBA-15 was retained after PEI loading. When the PEI loading was no more than 50 wt.%, PEI was dispersed and filled inside the pore channels of SBA-15. Higher PEI loading resulted in a part of the loaded PEI coating on the external surface of the SBA-15 particles and thus caused the agglomeration. The effects of sorption temperature and PEI loading on the CO₂ capacity of the PEI/SBA-15 sorbents were examined over a fixed-bed flow system using a model flue gas. The relationship between the sorption performance and the sorbent structure has been discussed. CO₂ diffusion in the PEI bulk may play a crucial role for determining the CO₂ sorption capacity and sorption/desorption rate at low temperature. With the increase of the sorption temperature, CO₂ diffusion can be accelerated and more amine sites may be exposed and accessible for CO₂ sorption. As a result, the sorption capacity increases. However, at further higher temperature, e.g., 100 °C, the sorption capacity decreases due to the dominant desorption. In addition, the sorption behavior of SBA-15 and PEI-50/SBA-15 at high CO₂ pressures (up to 50 bars) has been studied. The results showed that CO₂ sorption over PEI/SBA-15 occurs mainly via chemisorption even under pressurized conditions.