Exploring CO2 Adsorption by Tuning the Pore Size in Polyamine-Impregnated Mesoporous Silica for Direct Air Capture

Abstract

Direct air capture (DAC) using amine-functionalized porous solid supports is a promising negative-emissions technology. However, under ultradilute CO2 conditions, the performance of supported polyamines is often governed by coupled effects of amine loading, amine topology (linear vs branched), and pore size. In this study, we investigate how mesopore size modulates CO2 adsorption behavior in polyamine-impregnated SBA-15, a structurally well-defined platform with independently tunable pore diameters. Three SBA-15 supports with increasing pore diameters (SBA-15 (S), SBA-15 (M), and SBA-15 (L)) were prepared. The variants with expanded pore size were synthesized via micelle swelling while preserving the ordered SBA-15 framework. Two short-chain polyamines─triethylenetetramine (TETA) and tris(2-aminoethyl)amine (TREN)─were impregnated at nominal loadings of 15, 40, and 70 wt %. Comprehensive characterization by ATR FT-IR, N2 physisorption, SEM, TEM, and elemental analysis confirmed the amines were successfully impregnated and revealed the accessible porosity progressively decreased with greater amine loading due to pore filling and partial blocking, while the rod-like morphology and ordered mesochannel structure remained intact. DAC-relevant uptake was evaluated by isothermal TGA under 400 ppm of CO2 in air at 30 °C, and all composites exhibited saturation-type uptake. Increasing amine loading generally increased the CO2 uptake capacity, but amine efficiency and kinetics did not scale proportionally and depended on both pore size and amine structure. Pore expansion most effectively improved accessibility and kinetics at intermediate loading (40 wt %), whereas at high loading (70 wt %) crowding- and aggregation-induced transport penalties dominated even in larger pores, particularly for branched TREN. Overall, pore size tuning provides a practical route to balance amine site density and transport accessibility to obtain optimized DAC sorbents.