Unveiling\nTemperature-Induced Structural Phase Transformations\nand CO<sub>2</sub> Binding Sites in CALF-20

The increase in atmospheric\ncarbon dioxide concentration\nlinked\nto climate change has created a need for new sorbents capable of separating\nCO<sub>2</sub> from exhaust gases. Recently, an easily produced metal–organic\nframework, CALF-20, was shown to withstand over 450,000 adsorption/desorption\ncycles in steam and wet acid gases. Further development and industrial\napplication of such materials require an understanding of the observed\nprocesses. Herein, we demonstrate that conditioning as-synthesized\nCALF-20 single crystal transforms it into a different phase, γ-CALF-20.\nThe transformation resulted in significant structural changes, including\nthe binding of water molecules to Zn(II), accompanied by a reduction\nof 9% in the unit cell volume. Our experimental findings were supported\nby the energy-volume dependence of CALF-20 in the presence and absence\nof water molecules calculated from density functional theory. We have\nalso monitored the sorption process of the dominant greenhouse gas,\nCO<sub>2</sub>, on the initial phase of CALF-20 at atomic resolution\nusing in situ single-crystal X-ray diffraction under specific pressure.\nThe new understanding of CALF-20 chemistry from these studies should\nfacilitate development of novel sorbents for gas adsorption technologies.