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

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High Resolution Image Download MS PowerPoint Slide 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 CO 2 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 CO 2 and H 2 O ad- and desorption using high-resolution, laboratory X-ray powder diffraction (XRPD) shows that the CO 2 -breathing behavior changes upon gas loading. A CO 2 uptake initially distorts the rectangular pore into a diamond shape. Upon further CO 2 incorporation, the breathing behavior changes, and the pore becomes more rectangular, again. At low temperatures (−70 °C), the uptake of CO 2 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.