Thermodynamics, Kinetics, and Fixed-Bed Behavior of CO ₂ Adsorption on TEPA/SBA-15 under Low-CO ₂ Conditions

Amine-functionalized solid adsorbents are widely considered promising candidates for direct air capture because of their high CO 2 selectivity with good regenerability. However, under DAC-relevant low-CO 2 conditions, the coupled effects of temperature and inlet CO 2 concentration on equilibrium, kinetics, and dynamic fixed-bed performance remain insufficiently understood. In this work, 50 wt % TEPA/SBA-15 was prepared by wet impregnation and systematically evaluated under 0.04–0.20 vol % CO 2 to clarify the thermodynamic-kinetic coupling governing adsorption behavior. The equilibrium data were better described by the Freundlich model than by the Langmuir model ( R 2 > 0.99), indicating a heterogeneous adsorption surface, while the Avrami fractional-order model gave the best fit to the kinetic data. The apparent rate constant increased strongly with CO 2 concentration and slightly with temperature. Breakthrough experiments further showed that higher inlet CO 2 concentration shortened the breakthrough time, whereas higher temperature advanced breakthrough but also sharpened the breakthrough front. Analysis of the length of unused bed showed that fixed-bed utilization was controlled by the competition between thermodynamic loss and kinetic enhancement. At 0.08–0.20 vol % CO 2, heating generally reduced LUB/L and improved bed utilization, whereas at 0.04 vol % CO 2 the temperature effect became nonmonotonic. These results demonstrate that fixed-bed performance under low-CO 2 conditions should be evaluated from both breakthrough behavior and bed utilization, and provide a basis for DAC-oriented adsorbent assessment and process design.