Probing the influence of water on silica surface modification during amine grafting for enhancing CO2 adsorption

Abstract

Amine-grafted adsorbents are promising materials for carbon capture, but their adsorption efficiency and hydrothermal stability still require improvement. This study examines how water addition influences silica surface modification during amine grafting with (3-aminopropyl)triethoxysilane (APTES). We used 29/Si cross-polarization magic-angle spinning (CPMAS) and direct-observation 29/Si NMR spectroscopy with proton decoupling to test the common assumption that water increases the surface silanol content of silica-based sorbents. Our findings indicate a different mechanism. BET analysis and NMR results show that water promotes the hydrolysis of APTES ethoxy groups. This accelerates condensation with surface silanol sites and enables a more complete reaction between APTES and the silica surface. A reduction in Q/4 sites further supports this mechanism, suggesting that water induces structural rearrangement of surface siloxane bonds, generating additional reactive silanol groups for grafting. These mechanistic insights explain previously reported improvements observed in wet-grafted sorbents, including enhanced sorption kinetics, better thermal stability, improved regenerability, stronger CO₂ interaction, and increased selectivity. The enhanced performance is linked to the formation of more uniform siloxane bridges through hetero- and homo-condensation reactions of hydrolysed APTES molecules. Overall, this work highlights the central role of water in tuning the silica surface during amine grafting. By clarifying how water shapes the grafting mechanism, the study provides a pathway for designing more efficient and durable sorbents that can support scalable carbon-capture technologies and broader decarbonisation goals.