(225q) Investigating the Potential of Amine-Impregnated Mesoporous Silica Materials As an Alternative CO2 Adsorbent for Space Life Support Applications

Carbon dioxide (CO₂) has the potential to act as an asphyxiant at high concentrations. It does this by lowering the normal oxygen levels in the air we breathe, which can lead to symptoms such as nausea, dizziness, and increased blood pressure and heart rate. As a result of this, the Occupational Safety and Health Administration (OSHA) has established a Permissible Exposure Limit (PEL) for CO₂ in the air at 5,000 parts per million by volume (ppmv; equal to 0.5 vol%) throughout an 8-hour work shift. Long-duration missions involved in human space exploration should ensure the astronauts' health and safety. Environmental Control and Life Support System (ECLSS) was successfully developed and implemented by the National Aeronautics and Space Administration (NASA) to provide humans with a safe breathing environment in space. The ECLSS consists of three essential elements: the Water Recovery System, the Air Revitalization System, and the Oxygen Generation System. The Air Revitalization System purifies the air circulating throughout the chamber, involving the removal of trace contaminants that are created by electronics, plastics, as well as human off-gassing (e.g., CO₂ released by the crew during respiration). The Carbon Dioxide Removal Assembly (CDRA), a four-bed molecular sieve (4BMS) system, removes CO₂ from the cabin atmosphere. The CO₂-containing air is first passed through an adsorbent bed to remove any moisture, then the dried air is passed through another adsorbent bed typically containing zeolite 5A to capture CO₂. To enable continuous CO₂ removal, one pair of beds adsorbs CO₂ while another pair is thermally regenerated. The current CDRA system has multiple limitations such as (i) removal of water vapor and CO₂ in separate adsorption beds because of the hydrophilic nature of zeolite 5A, (ii) low CO₂ adsorption, typically under 4 wt.%, requiring more frequently adsorbent regeneration, and (iii) a relatively high regeneration temperature of 200 °C resulting in high power consumption. One common adsorbent for CO₂ capture is amine-functionalized silica materials, also called aminosilicas. Aminosilicas can achieve high CO₂ uptake and selectivity. Aminosilicas adsorb more CO₂ in the presence of moisture. Additionally, moisture in CO₂-containing gas streams enhances the stability performance of aminosilicas by reducing CO₂-induced deactivation caused by urea formation. Although aminosilicas have the potential to integrate and intensify air revitalization processes, there have been few studies on their use for this purpose. Therefore, this research aims to investigate the potential use of amine-impregnated silica materials for removing CO₂ from dilute streams, including life support systems used in enclosed environments. Aminosilicas were synthesized using (i) various amines, including tetraethylenepentamine (TEPA) and branched polyethylenimine (PEI) with molecular weights of 600, 1200, and 1800; (ii) mesoporous silica supports with different particle sizes of 5, 10, and 75 microns; and (iii) amine loadings of 20, 30, 40, and 50 wt.%. The performance of aminosilicas was assessed in terms of equilibrium CO₂ uptake, adsorption kinetics, and cyclic stability using thermogravimetric analysis (TGA) at 25 °C in the presence of 5,000 ppmv CO₂ in N₂. The findings showed that CO₂ equilibrium uptake increased by enhancing amine loading. However, for all amine types, an amine loading of 50 wt.% caused slower adsorption kinetics, relative to its 40 wt.% counterpart, due to pore blockage by impregnated species. Overall, a TEPA loading of 50 wt.% achieved the highest CO₂ uptake capacity (11.30 wt.%). While silica particle size had no discernible effect on equilibrium CO₂ uptake, a larger silica particle slowed down adsorption kinetics. Given the importance of adsorbent stability on the long-term efficiency of CO₂ capture processes, 100 successive adsorption-desorption cycles were conducted on selected performant materials. While a TEPA loading of 40 wt.% exhibited favorable equilibrium CO₂ uptake and kinetics, its performance deteriorated by 72% after 100 cycles. On the other hand, PEI with a molecular weight of 600 and a loading of 40 wt.% demonstrated consistent CO₂ uptake performance over the same cycling regime. The findings of this study provide compelling evidence of aminosilica’s promising potential as alternative adsorbent materials for air revitalization.