Carbon capture performance of amine-functionalized magnesium aluminum mixed metal oxides with high stability in flue gas

Developing highly stable solid amine materials for carbon capture is essential for achieving carbon emission reduction goals. Layered double hydroxides （LDHs） possess a highly dispersed spatial structure, and their calcined derivatives—mixed metal oxides （MMOs）—offer abundant slit-like mesopores and a broad pore size distribution, making them promising supports for preparing solid amine adsorbents. Mixed metal oxide Mg_0.55Al-O was synthesized with a Mg/Al ratio of 0.55 and compared with the commercially available mesoporous material SBA-15. A series of solid amine adsorbents with different polyethyleneimine （PEI） loadings were prepared via the impregnation method, the CO₂ adsorption–desorption properties and stability were investigated under simulated flue gas conditions （the mixed gas of CO₂ and N₂ with φ(CO₂) is 15%）. Mg_0.55Al-O exhibits a high density of mesopores with pore diameters ranging from 5 to 20 nm, its pores are gradually filled with increasing PEI content. When the PEI loading exceeds 33%, the Mg_0.55Al-O-based adsorbents demonstrate higher CO₂ adsorption capacity than the PEI-functionalized SBA-15. Both types of adsorbents reach their maximum CO₂ uptake at 67% PEI. In-situ FTIR reveals that Mg_0.55Al-O- PEI adsorbents form carbamic acid adsorption species during CO₂ adsorption, enhancing the reaction efficiency between CO₂ and amine groups and thus improving amine efficiency. Thermogravimetric analysis at 75 ℃ under the condition that the mixed gas of CO₂ and N₂ with φ(CO₂) is 15% for 60 minutes shows CO₂ uptakes of 3.44 and 2.81 mmol/g for Mg_0.55Al-O-67PEI and SBA-15-67PEI, respectively. Unlike the hydrogen bonding between PEI and SBA-15, strong electrostatic interactions between PEI and Mg_0.55Al-O contribute to superior thermal stability. High-temperature desorption experiments confirm that Mg_0.55Al-O-67PEI retains its adsorption activity after treatment at 300 ℃. After 110 adsorption–desorption cycles, it still maintains a CO₂ capacity of 3.28 mmol/g, indicating excellent stability of the amine groups during long-term cyclic operation.