Elevated temperature CO2 capture on nano-structured MgO–Al2O3 aerogel: Effect of Mg/Al molar ratio

• Nano-structured MgO–Al2O3 aerogel adsorbents were prepared.
• Prepared adsorbents showed better CO2 capacity than commercial adsorbents.
• CO2 adsorptive performance increased with increasing basicity of adsorbent.
• Medium basicity is the key factor in determining CO2 adsorptive performance.

Nano-structured MgO–Al2O3 aerogel adsorbents (denoted as MgAl-AE-X) with different Mg/Al molar ratio (X) were prepared by a single-step epoxide-driven sol–gel method and a subsequent CO2 supercritical drying method. The effect of Mg/Al molar ratio of nano-structured MgO–Al2O3 aerogel adsorbents on their physicochemical properties and CO2 adsorptive performance at elevated temperature (200 °C) was investigated. Successful formation of flower-like nano-structured MgAl-AE-X adsorbents was confirmed by N2 adsorption–desorption isotherms and SEM analyses. The crystalline structure of MgAl-AE-X adsorbents was transformed in the sequence of Al2O3 → MgAl2O4 → MgO-MgAl2O4 with increasing Mg/Al molar ratio from 0 to 3. All the MgAl-AE-X adsorbents were found to possess weak base site and medium base site except for strong base site. In the dynamic CO2 adsorption, both the total CO2 capacity and the 90% breakthrough CO2 capacity showed volcano-shaped curves with respect to Mg/Al molar ratio, and they were decreased in the order of MgAl-AE-0.5 > MgAl-AE-1.0 > MgAl-AE-2.0 > MgAl-AE-3.0 > MgAl-AE-0. It was found that the 90% breakthrough CO2 capacity increased with increasing medium basicity of the adsorbents. Among the adsorbents tested, MgAl-AE-0.5 (Mg/Al = 0.5) adsorbent with the highest medium basicity showed the best CO2 adsorptive performance. Thus, medium basicity of nano-structured MgO–Al2O3 aerogel adsorbents served as a crucial factor in determining CO2 adsorptive performance at elevated flue gas temperature (200 °C).