CO₂ and H₂O Sorption Induced Bulk-Phase Changes of CALF-20 Captured Using In Situ Laboratory X-ray Powder Diffraction

The zinc oxalate (ox) triazolate (trz)-based MOF, Calgary Framework 20 (CALF-20), exhibits remarkable cycling stability for carbon dioxide and water adsorption and desorption and is therefore a promising candidate material for CO₂ sequestration on an industrial scale. Upon gas and vapor loading and unloading, the MOF shows pronounced structural dynamics leading to a variety of potential CALF-20 polymorphs. A systematic in situ study on CO₂ and H₂O ad- and desorption using high-resolution, laboratory X-ray powder diffraction (XRPD) shows that the CO₂-breathing behavior changes upon gas loading. A CO₂ uptake initially distorts the rectangular pore into a diamond shape. Upon further CO₂ incorporation, the breathing behavior changes, and the pore becomes more rectangular, again. At low temperatures (-70 °C), the uptake of CO₂ occurs in a core-shell mechanism, and the gas is bound strongly to the framework and cannot be removed by dynamic vacuum. During water uptake of CALF-20, two distinct hydrated phases can be identified. The overall water loading capacity is independent of temperature between 25 and 60 °C. In this paper, we demonstrate that recent advances in X-ray powder diffraction hard- and software enable a detailed investigation of the loading and breathing behavior of a crystalline MOF using laboratory equipment, turning this into easily accessible investigations.