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. The hierarchical porous silica was functionalized with primary amine (APT) and secondary amine (MAP), revealing distinct silica adsorption behaviors between the two adsorbents. The primary amine-functionalized adsorbent achieved optimal CO 2 capture efficiency at approximately 50% amine loading, whereas the secondary amine counterpart exhibited peak performance at around 60% loading. Kinetic analysis based on the pseudo-first-order kinetic model indicated a volcano-shaped relationship between the adsorption rate constant and the density of primary amine groups, in contrast to a monotonic relationship observed for secondary amine groups. This divergence is attributed to differences in their adsorption mechanisms. These findings refine the conventional dual-site adsorption model and establish a structure-function relationship correlating amine type, loading density, and adsorption kinetics. • Mechanistic Insight: Elucidates the influence of amine type (primary vs. secondary) and surface density on CO 2 adsorption kinetics, revealing a volcano-type dependence for primary amines and a monotonic increase for secondary amines. • Superior Performance: The optimized secondary amine-functionalized sorbent (60% MAP/HPS) exhibits a high CO 2 adsorption capacity of 1.24 mmol g -1 under dilute CO 2 conditions (5 vol%, 40 °C). • Enhanced Stability & Regenerability: The 60% MAP/HPS sorbent exhibits outstanding cyclic stability, maintaining 92.7% of its adsorption capacity after 15 adsorption-desorption cycles, significantly outperforming primary amine-based counterparts. • Refined Adsorption Model: Challenges the conventional two-site model by demonstrating that cooperative proton transfer plays a critical role for primary amines, whereas secondary amines follow a more accessible quasi-single-site zwitterionic mechanism.