CO2 capture performance of amine-functionalized amorphous SiO2-Al2O3 adsorbent: Insights into the support acidity

Amine-functionalized adsorbents possess considerable potential for CO₂ capture due to their high selectivity and versatility across a range of applications. However, they are susceptible to CO₂-induced chemical deactivation. Despite research efforts to synthesize supports with abundant acid sites to accommodate amines and enhance their stability, information remains sparse on how changes in surface acids impact the CO₂ adsorption performance of the resulting adsorbents. In this context, we synthesized porous amorphous SiO₂-Al₂O₃ with varying surface acidity, and impregnated with polyethylenimine (PEI). We then investigated the CO₂ adsorption performance under different temperatures, regeneration atmospheres, and humidity levels. The results indicated an optimal adsorption temperature of 75 °C and a pre-treatment temperature of 140 °C. Under these conditions, the Si/Al = 20–60 sample demonstrated the highest capture capacity, approximately 142.6 mg/g. The Avrami model proved most suitable for fitting CO₂ adsorption data across various adsorbents, providing an accurate assessment of the entire dynamic adsorption process. However, cycle stability tests revealed that Si/Al = 5–50 had the highest stability among the SiO₂-Al2O₃ adsorbents in both dry and humid conditions, due to its superior resistance to urea formation. Utilizing FT-IR, solid-state NMR, and XPS analysis, we discovered that the density of moderately strong Lewis acid sites on the surface of SiO₂-Al₂O₃ plays a crucial role in resisting urea formation, as it induces the highest degree of cross-linking reaction between PEI and the porous supports. This breakthrough offers new insights into how the surface acidity of support materials influences the stability of solid amine adsorbents for CO₂ capture.