(703f) Improving the Hydrothermal Stability of Amine-Grafted MCM-41 Silica Via Incorporation of Aluminum into the Structure of the Support

In recent years, steam regeneration of CO₂ adsorbents has gained momentum, and is currently used in the near-commercial CO₂ capture technology developed by Svante. Steam acts as both the heat source and the purge gas and is particularly suitable for CO₂ capture in industrial plants where low-heat steam is typically available as waste heat. Steam regeneration is advantageous since it is relatively easy to recover pure CO₂ via steam condensation, following the desorption step. While steam regeneration is conceptually interesting, the exposure of amine-modified silica materials to steam lowers their performance and stability owing to amine leaching, structural degradation of the support, and/or amine deactivation due to certain metal impurities in industrial steam. These shortcomings indicate the need for developing amine-modified silica materials with higher hydrothermal stability. Ordered mesoporous silica materials such as MCM-41 and pore-expanded MCM-41 (PE-MCM-41) possess unique combination of high surface area, tunable pore size with narrow distribution, and high pore volume. However, these materials typically suffer from low hydrothermal stability, limiting their use in cyclic CO₂ adsorption-desorption processes involving steam regeneration. Previous reports indicated that aluminum incorporation into silica framework improves its hydrothermal stability. Despite their high potential, however, reports on the use of amine-modified Al-MCM-41 in cyclic CO₂ adsorption-desorption processes involving steam regeneration remain extremely limited, warranting further investigation. To that end, the impact of aluminum incorporation into the structure of MCM-41 silica on its adsorptive properties and hydrothermal stability during CO₂ capture was studied. Large-pore Si-MCM-41 and Al-MCM-41 (Si/Al = 15) supports with similar pore sizes were synthesized using a novel one-pot synthesis technique developed by our group. Amine functionalization of the supports was conducted using dry and wet grafting of triamine. Amine-tethered Al-MCM-41 adsorbents exhibited higher CO₂ uptakes and faster CO₂ adsorption kinetics relative to their Si-MCM-41 counterparts. Following one three-h steam stripping cycle, significant deterioration of adsorptive properties was observed for Si-MCM-41, particularly at adsorption temperatures of 25 and 50°C. Al-MCM-41, however, showed unprecedented hydrothermal stability with no significant drop in CO₂ uptake and adsorption kinetics. Following steaming, N₂ adsorption measurements at 77 K showed significantly higher reductions in the BET surface area of grafted Si-MCM-41 samples compared to their Al-MCM-41 counterparts. It is proposed that aluminum incorporation into the structure of the support may reduce the restructuring of the grafted materials upon exposure to steam, enhancing their hydrothermal stability.