Accelerated Testing of PEI-Silica Sorbent Pellets for Direct Air Capture

Amine-based sorbents have shown exceptional CO₂ uptake for direct air capture (DAC). However, amine degradation is a major issue for this class of materials, hindering their deployment for large-scale DAC. In this study, a comprehensive evaluation of polyethylenimine (PEI) sorbents was conducted to understand their degradation under process-relevant environments for the DAC of CO₂. A solvent-minimized silica-supported PEI-sorbent powder synthesis method using centrifugal mixing was developed. Unlike traditional solvent-assisted impregnated sorbent synthesis methods, the centrifugal mixing method enabled a 94% reduction in volatile and toxic organic solvent use in pelletized sorbent synthesis. The pelletized sorbents exhibited CO₂ adsorption capacities consistent with traditional fabrication methods for PEI-based solid sorbents (about 1 mmol/g). The pelletized sorbent degradation behavior was evaluated at three different regeneration temperatures (80, 100, and 120 °C) under nitrogen (N₂), ambient air (21% O₂), and saturated dry and wet (75% relative humidity (RH)) CO₂ environments using fixed-bed breakthrough (BT) experiments. Additionally, accelerated testing (AT) protocols that mimic industrial DAC conditions were developed to assess the long-term stability of the PEI-silica pellets. Our results indicate that the sorbent degrades rapidly (ca. 94% within 24 h) at 120 °C in ambient air (21% O₂), demonstrating the detrimental impact of oxygen when compared to an O₂-free environment. AT performed for 100 h (equivalent to 33, 100, and 100 cycles) continuously at 80, 100, and 120 °C reveals that dry CO₂-induced degradation of the PEI-silica sorbent pellets is 30-40% and 40-50% more than the degradation measured in wet CO₂ and inert (pure N₂) environments.