An Organic Solvent-Free Route for Preparing Silica-Alkoxylated Polyethylenimine Adsorbents for CO₂ Capture

Mesoporous silica-supported polyethylenimine (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 coadsorption, 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.