Use of Ball Drop Casting and Surface Modification for the Development of Amine-Functionalized Silica Aerogel Globules for Dynamic and Efficient Direct Air Capture

Amine-functionalized silica aerogel globules (AFSAGs) were first synthesized via a simple ball drop casting method followed by amine grafting. The effect of grafting time on the structure and CO₂ adsorption performance of the AFSAGs was investigated. The CO₂ adsorption performance was comprehensively studied by breakthrough curves, adsorption capacity and rates, surface amine loading and density, amine efficiency, adsorption halftime, and cyclic stability. The results demonstrate that prolonging the grafting time does not lead to a significant increase in surface amine content owing to pore space blockage by superabundant amine groups. The CO₂ adsorption performance shows obvious dependence on surface amine density, determined by both the surface amine content and specific surface area, and working temperature. AFSAGs with a grafting time of 24 h (AFSAG24) with a moderate surface amine density have optimal CO₂ adsorption capacities, which are 1.78 and 2.14 mmol/g at 25 °C with dry and humid 400 ppm CO₂, respectively. The amine efficiency of AFSAG24 with low CO₂ concentrations, 0.38-0.63 with dry 400 ppm-1% CO₂, is the highest among the reported amine-functionalized adsorbents. After estimation with different diffusion models, the CO₂ adsorption process of AFSAG24 is governed by film diffusion and intraparticle diffusion. In the range of 1-4 mm, the ball size does not affect the CO₂ adsorption capacity of AFSAG24 obviously. AFSAG24 offers significant advantages for practical direct air capture compared with its state-of-the-art counterparts, such as high dynamic adsorption capacity and amine efficiency, excellent stability, and outstanding adaptation to the environment.