Uncovering the dynamic CO2 gas uptake behaviour 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 to mitigate CO2 emissions by capturing and storage. The zinc-triazole-oxalate-based metal-organic framework CALF-20 demonstrates high CO2 capacity, low H2O affinity, and low CO2 adsorption heat, enhancing energy efficiency while maintaining stable performance over multiple adsorption/desorption cycles. This study examines CO2 adsorption in CALF-20. Using the combination of positron annihilation lifetime spectroscopy (PALS), in situ-Powder X-ray Diffraction (PXRD) analyses, and gas adsorption experiments, we elucidate the CO2 adsorption mechanisms in CALF-20 under various temperatures, and humidity levels, simulating ambient conditions. The variable temperature PALS experiments demonstrate that CO₂ molecules are spatially localized within the CALF-20 cages, leaving temperature- and pressure-dependent gaps between them. PALS results indicate that CO2 initially spreads across cage centers, 1D chains, and ultimately adheres to pore walls. Interestingly, positronium intensity, which increases with CO₂ adsorption pressure, closely aligns with the Langmuir-Freundlich isotherm and reflects gas uptake behaviour. Moreover, we explore the adsorption characteristics of relative humidity (RH) and humid CO₂ in CALF-20. At low RH in pure humidity run, water molecules are sparsely adsorbed within the framework, forming isolated clusters or small oligomers with minimal hydrogen bonding. Above 35% RH, water molecules begin to form interconnected hydrogen-bond networks that fill the cages, significantly altering the material's free volumes. In humid CO₂ experiment, competitive interactions between CO₂ and water are observed, where CO₂ initially disrupts the propagation of water, but at higher RH, water molecules form more extensive hydrogen-bond networks. This competition influences both the cage and inter-granular spaces, with the latter becoming larger and more flexible as water fills the framework. The synergy between in situ-PALS, in situ-PXRD, and gas adsorption techniques provides a comprehensive understanding of CALF-20's potential for efficient CO2 capture under varying environmental conditions.