Hydrophobic Silica Gels and Aerogels for Direct Air Capture: Hybrid Grafting To Suppress Water Uptake and Capillary Condensation

High Resolution Image Download MS PowerPoint Slide Direct air capture of CO 2 (DAC) is critical to achieving climate goals, requiring the removal of approximately 220 000 Mt of CO 2 by 2100. Amine-functionalized sorbents are promising for CO 2 capture but face challenges under high humidity, where water sorption reduces the CO 2 selectivity and capillary condensation may degrade the sorbent, especially the delicate silica aerogel backbone, compromising structural integrity. To address this issue, we introduce superhydrophobic silica gels functionalized by hybrid grafting of amine, long-aliphatic, and trimethylsilyl groups. These materials drastically reduce water uptake, from >50 wt % for nonhydrophobized gels to 5 wt % at 95% RH, and maintain high CO 2 /water selectivity at high relative humidity. This shows how to overcome a limitation of silica sorbents, which have shown poor performance up to now under realistic high-humidity DAC conditions. The hydrophobic functional groups shield the hydrophilic amine sites and prevent capillary condensation, preserving pore hydrophobicity, preventing pore clogging with water, and ensuring rapid CO 2 diffusion, essential for sustained performance. We demonstrate that conventional single-component and dry-condition CO 2 sorption tests overlook the critical role of moisture, underestimating the detrimental effect of presorbed water on nonhydrophobized silica gels, previously reported in the literature. The ability to apply ambient pressure drying without significant selectivity loss, along with the affordability of the silica matrix, enhances the feasibility of real-world implementation for silica-based DAC sorbents. Given the versatility of amorphous sorbents, this study paves the way for further material optimization tailored to DAC applications.