Amine Sorbents for Sustainable Direct Air Capture: Long-Term Stability and Extended Aging Study

The emerging technology of direct air capture (DAC) holds promise for the extraction of CO2 from the air, offering a potential avenue to reverse climate change. However, DAC is still in its early stages of development, particularly on the sorbent material side, and the long-term stability of sorbents remains inadequately understood. This study investigates the impact of thermal degradation, hydrothermal treatment, long-term cyclic operation, and extended aging over 3 years on amine-supported sorbents and its effect on the adsorption capacity at DAC conditions. Sorbents were synthesized through various methods (impregnation, chemical grafting, and in situ polymerization) into two supports: large-pore AlMCM-41 (LPAlSi) and mesoporous silica foam (MSF). LPAlSi support demonstrates superior stability compared to MSF support under thermal and hydrothermal treatments. Impregnated sorbents with physically bonded amines exhibit the lowest stability, with a significant amine loss during all treatments, confirmed through porosity analysis, thermogravimetric analysis (TGA) decomposition, scanning electron microscopy (SEM), and Fourier-transform infrared spectroscopy (FTIR). Additionally, they experience a notable 6% decrease in CO2 uptake after 50 cycles. In contrast, chemically bonded amines through grafting and in situ polymerization display better stability due to stronger bonding, maintaining CO2 uptake despite harsh treatments. In situ-polymerized sorbents into LPAlSi exhibit remarkable stability under thermal and hydrothermal treatments, experiencing drops of 17 and 27% over 3 days, respectively. Furthermore, they demonstrate outstanding stability over 50 cycles under DAC conditions, with only a 0.3% drop in CO2 uptake. Extended aging of class III sorbent for 3 years indicates good stability in CO2 uptake, with only an 11% drop compared to impregnated ones, which showed a larger decrease over a shorter time. The presented results suggest that in situ-polymerized amines into LPAlSi are promising materials for DAC, offering good capacity and significant long-term stability. With opportunities for further sorbent optimization, there is substantial potential to deploy DAC at a scalable level and mitigate global warming effects.