An organic solvent-free route for preparing silica-alkoxylated polyethyleneimine adsorbents for CO2 capture

Mesoporous silica-supported polyethyleneimine (PEI) and, more recently, alkoxylated PEI (APEI) are promising adsorbents for CO2 capture, displaying high adsorption capacity and selectivity. Wet impregnation is the established synthesis procedure for preparing silica-PEI. However, excessive quantities of organic solvents, particularly methanol, have invariably been used for both PEI alkoxylation and polymer mixing with silica. This study demonstrates an organic solvent-free synthesis method for 1) mesoporous silica preparation from sodium silicate solution, 2) PEI alkoxylation, and 3) the subsequent impregnation of silica-PEI using minimal water, typically with a water-to-silica mass ratio not exceeding 1.0. For large scale samples (up to 5 kg) preparation, controlled drying is essential to retain approximately 5 Wt.% moisture, preserving CO2 capture performance. APEIs can be tailored for direct air capture (30 °C) and industrial processes (50 °C) by controlling the alkoxylation chemistry and degree. Silica-APEI exhibits enhanced oxidative stability and reduced moisture co-adsorption, which extend operational lifespan and lower regeneration energy consumption. This water-based synthesis eliminates the need for excess organic solvents, such as methanol, preventing volatile organic compound (VOC) emissions, reducing drying energy consumption, and enhancing sustainability, making large-scale production commercially viable.