Accelerated Testing\nof PEI-Silica Sorbent Pellets\nfor Direct Air Capture

Amine-based sorbents\nhave shown exceptional CO<sub>2</sub> uptake\nfor direct air capture (DAC). However, amine degradation is a major\nissue for this class of materials, hindering their deployment for\nlarge-scale DAC. In this study, a comprehensive evaluation of polyethylenimine\n(PEI) sorbents was conducted to understand their degradation under\nprocess-relevant environments for the DAC of CO<sub>2</sub>. A solvent-minimized\nsilica-supported PEI-sorbent powder synthesis method using centrifugal\nmixing was developed. Unlike traditional solvent-assisted impregnated\nsorbent synthesis methods, the centrifugal mixing method enabled a\n94% reduction in volatile and toxic organic solvent use in pelletized\nsorbent synthesis. The pelletized sorbents exhibited CO<sub>2</sub> adsorption capacities consistent with traditional fabrication methods\nfor PEI-based solid sorbents (about 1 mmol/g). The pelletized sorbent\ndegradation behavior was evaluated at three different regeneration\ntemperatures (80, 100, and 120 °C) under nitrogen (N<sub>2</sub>), ambient air (21% O<sub>2</sub>), and saturated dry and wet (75%\nrelative humidity (RH)) CO<sub>2</sub> environments using fixed-bed\nbreakthrough (BT) experiments. Additionally, accelerated testing (AT)\nprotocols that mimic industrial DAC conditions were developed to assess\nthe long-term stability of the PEI-silica pellets. Our results indicate\nthat the sorbent degrades rapidly (ca. 94% within 24 h) at 120 °C\nin ambient air (21% O<sub>2</sub>), demonstrating the detrimental\nimpact of oxygen when compared to an O<sub>2</sub>-free environment.\nAT performed for 100 h (equivalent to 33, 100, and 100 cycles) continuously\nat 80, 100, and 120 °C reveals that dry CO<sub>2</sub>-induced\ndegradation of the PEI-silica sorbent pellets is 30–40% and\n40–50% more than the degradation measured in wet CO<sub>2</sub> and inert (pure N<sub>2</sub>) environments.