Research Progress of Solid Amine Adsorbents for Direct Air Capture

Direct air capture (DAC) technology is a promising carbon removal method with great potential to address CO2 emissions and support the achievement of future carbon neutrality. The high selectivity, low energy consumption, and flexibility of solid amine adsorbents make them ideal for DAC applications. The CO2 capture capacity of solid amine adsorbents depends on the number of amine groups present on their surface. Primary and secondary amines typically require two amine groups to form a zwitterion, which enables CO2 capture via proton transfer. The capture efficiency of both types of amines is improved under humid conditions due to the assistance of water molecules. Tertiary amines are usually not used as sources of amines for solid amine adsorbents. This is due to their inability to transfer protons and their low CO2 capture efficiency. Solid amine adsorbents are often classified into four types based on their preparation methods: impregnated amine adsorbents, grafted amine adsorbents, in situ polymerization adsorbents, and composite amine adsorbents. Impregnated amine adsorbents involve directly loading amine groups onto a support material. This method is simple, resulting in a high amine loading, and typically yielding a high CO2 capture capacity. However, the weak binding between the amine groups and the support material leads to easy leaching of amine molecule during recycling, resulting in poor material stability. Grafted amine adsorbents utilize silanol groups on mesoporous SiO2 (such as MCM-41, SBA-15) to undergo a silanization reaction (grafting reaction) with amino silane reagents, or cross-coupling to attach amine groups. Chemical bonding prevents amine leaching, thereby improving material stability, which is a limitation of impregnated adsorbents. However, grafted amines are typically loaded in a single layer on the support, resulting in a lower loading capacity compared to impregnation and a relatively lower CO2 capture capacity. In situ polymerization involves fixing polymeric amine groups to the support material via covalent bonds. In situ polymerization can further increase the amine load, improve performance, and ensure material stability. Composite amine adsorbents are prepared by combining impregnation and grafting to improve the adsorption properties of the materials. Overall, improving the adsorption performance of materials and reducing preparation costs are the two key challenges for widespread applications. To achieve this, combining solid amine adsorbents with green energy, carbon dioxide resource utilization, and other systems could improve the commercial viability of the technology and facilitate its large-scale application. The development of highly efficient adsorbents and modular capture devices will be the primary focus of future research in this field.