Uncovering the Dynamic CO2 Gas Uptake Behavior of CALF-20 (Zn) under Varying Conditions via Positronium Lifetime Analysis.

Carbon dioxide (CO2) is a major greenhouse gas contributing to global warming. Adsorption in porous sorbents offers a promising method for CO2 capture and storage. The zinc-triazole-oxalate-based Calgary framework 20 (CALF-20) demonstrates high CO2 capacity, low H2O affinity, and low adsorption heat, enabling energy-efficient and stable performance over multiple cycles. This study examines CO2 adsorption mechanism in CALF-20 using positron annihilation lifetime spectroscopy (PALS), in situ powder X-ray diffraction (PXRD), and gas adsorption experiments under varying temperatures and humidity levels. Variable-temperature PALS experiments demonstrate that CO₂ molecules are spatially localized within the CALF-20 cages, leaving temperature- and pressure-dependent gaps. CO2 begins at cage centers, forming 1D chains, and ultimately adheres to pore walls. Interestingly, positronium intensity correlates with the Langmuir-Freundlich isotherm, reflecting gas uptake behavior. Moreover, under pure relative humidity (RH), water molecules form isolated clusters or small oligomers at low RH, transitioning to hydrogen-bonded networks above 35 %RH, significantly altering free volumes. In humid CO₂ conditions, competitive interactions arise: CO₂ initially disrupts water propagation, but higher RH leads to extensive water networks filling the framework. The synergy between in situ-PALS, in situ-PXRD, and gas adsorption techniques provides comprehensive insights into CALF-20's potential for efficient CO2 capture under varying conditions.