Amine type and surface density govern cooperative CO2 adsorption kinetics on hierarchical porous silica

Hierarchical porous silica (HPS) with interconnected bimodal channels was synthesized using polyethylene glycol (PEG) and hexadecyl trimethyl ammonium bromide (CTAB) as dual templates and functionalized with primary (3-aminopropyltrimethoxysilane, APT) and secondary ( N -methylaminopropyltrimethoxysilane, MAP) aminosilanes to elucidate how amine type and surface density influence CO 2 adsorption performance. Structural, kinetic, and in-situ FT-IR analyses reveal that adsorption behavior is governed by the interplay between hydrogen bonding, molecular configuration, and pore accessibility. Primary amines exhibit higher intrinsic CO 2 affinity but show a volcano-type dependence on amine density, with an optimal spacing (~1.6 nm −2 ) that enables cooperative proton transfer between adjacent sites; excessive loading leads to hydrogen-bond network formation, pore constriction, and reduced diffusion. In contrast, secondary amines maintain open pore structures and exhibit a monotonic rate increase with surface density, consistent with a quasi–single-site zwitterionic mechanism. The optimized 60% MAP/HPS achieved a CO 2 uptake of 1.24 mmol·g −1 with only 7.3% capacity loss after 15 cycles, demonstrating high stability and low regeneration energy. These findings refine the conventional two-site adsorption model and establish structure–function relationships linking amine type, density, and kinetics, providing molecular level guidance for designing durable, high capacity solid amine adsorbents for post-combustion CO 2 capture.