Quantitative analysis of temperature-dependent transition of CO2 adsorption mechanisms by adsorbents – From enthalpy-driven interactions to pore-filling capture

This study investigated the CO 2 adsorption performance of seven adsorbent materials with diverse pore structures and chemical compositions over 213 K to 423 K. For zeolite-templated carbon (ZTC), coconut shell activated carbon (CSAC), MOF-199, and MIL-101, the CO 2 uptake increased rapidly in an exponential manner as the temperature decreased. In contrast, coal-based activated carbon (CBAC), 13×, and 5 A zeolite exhibited a slower, linear increase in adsorption capacity. Linear regression and statistical analyses revealed that this difference in temperature sensitivity was likely associated with a shift in the dominant CO 2 adsorption mechanism: from adsorption heat-driven interactions at above-ambient temperatures (303–343 K) to a pore-filling mechanism at sub-zero temperature conditions (213–243 K). Further analysis revealed that at 213 K, micropores with diameters up to 2.00 nm contributed to CO 2 adsorption, whereas at 243 K, only pores smaller than 1.15 nm remained effective. This indicated that as the temperature increases, the effective pore size for pore filling narrows, and above 273 K, pores larger than 0.85 nm no longer contribute to this mechanism. Moreover, density functional theory (DFT), molecular dynamics (MD) and Grand Canonical Monte Carlo (GCMC) calculations supported that the pore-filling mechanism of ZTC became significant only at relatively low temperatures (< 277 K). Nevertheless, owing to its large nonpolar carbon surface and micropore volume, ZTC exhibited a rapid increase in CO 2 uptake once the pore-filling mechanism was activated. This study quantitatively delineated the temperature-dependent transition in CO 2 adsorption mechanisms and provided quantitative guidance for selecting and designing CO 2 adsorbents optimized for different operating temperatures, particularly for sub-ambient applications such as direct air capture in cold climates.