CO2 sorption kinetics in pristine and degraded aminopolymer sorbents through fluorescent monitoring

Direct air capture (DAC) is an attractive method for mitigating global greenhouse gas emissions by removing CO₂ from the environment. Amine-based DAC systems often utilize derivatives of poly(ethylenimine) (PEI) loaded onto a mesoporous oxide support as an efficient capture medium. Gas diffusion in these systems is tightly coupled with polymer mobility, which in turn is affected by interactions with the pore wall of the support, by confinement, by the presence of co-adsorbates (moisture), and by electrostatic crosslinks that develop as a function of CO₂ chemisorption. While a better understanding of polymer mobility will be crucial for optimizing DAC systems, the development of sensitive benchtop techniques that can help disentangle the factors affecting mobility are needed. Recently, we developed a fluorescent probe molecule based on tetraphenylethylene (TPE) to study PEI mobility in mesoporous silica, and we benchmarked the probe response across a wide range of temperatures and conditions. Fluorescence intensity of the probe molecule and the shape of the emission spectra were strongly dependent on the viscosity of the supporting medium. Here, we expand on that initial work by demonstrating we can use fluorescence to monitor the kinetics of CO₂ sorption in a PEI-mesoporous oxide composite using both simulated flue gas (10% CO₂ in N₂) and 400 ppm CO₂ in N₂. Furthermore, we studied the functionality and properties of oxidatively degraded PEI, and we examined CO₂ sorption within PEI samples of different amounts of degradation. The latter results will be influential in designing more efficient DAC systems for real-world operating conditions.