Exploring the CO2 adsorption behavior via tuning the pore size of polyamine-impregnated mesoporous silica for direct air capture

Abstract

Direct air capture (DAC) has emerged as a critical negative emissions technology for addressing the escalating threat of climate change by removing CO2 directly from the atmosphere. Among various DAC approaches, solid sorbent-based systems employing amine-functionalized porous materials offer distinct advantages in terms of energy efficiency, operational flexibility, and material tunability. This study presents a systematic investigation into the structure–property relationships governing CO2 adsorption behavior in polyamine-impregnated SBA-15 mesoporous silica with controlled pore sizes. Three SBA-15 variants with small, medium, and large pore diameters were synthesized and subsequently impregnated with two representative polyamines—tetraethylenetetramine (TETA, linear) and tris(2-aminoethyl)amine (TREN, branched)—at varying loadings (15, 40, and 70 wt%). Comprehensive characterization techniques, including ATR-FTIR, N2 physisorption, SEM, TEM, and elemental analysis, were employed to evaluate the structural integrity, pore accessibility, and amine incorporation behavior. CO2 adsorption capacity and kinetics were quantified under ultradilute CO2 conditions (~400 ppm) using thermogravimetric analysis (TGA). Results reveal that both pore size and amine structure critically influence adsorption performance. At low amine loadings, adsorption capacity and rate were limited by amine immobilization on pore walls, particularly for linear TETA. Conversely, high amine loadings led to aggregation-induced diffusion limitations, especially in branched TREN systems. Optimal adsorption performance was observed at intermediate loadings (40 wt%) and with larger pore sizes, which alleviated aggregation and facilitated improved CO2 diffusion. Notably, TREN exhibited superior performance at low to moderate loadings due to enhanced molecular mobility but suffered efficiency losses at high loading levels. These findings highlight the importance of precisely tuning both the pore architecture of the support and the molecular characteristics of the impregnated amine to balance dispersion, mobility, and reactivity. The insights gained from this work provide a foundational framework for the rational design of next-generation solid sorbents tailored for efficient and scalable DAC applications.