日本ゼオライト学会　刊行物 Publication of Japan Zeolite Association

Metal–organic frameworks (MOFs), recognized by the 2025 Nobel Prize in Chemistry, are porous crystalline materials with promising applications in adsorption, separation, and catalysis. However, adsorption behavior at the level of individual crystal particles remains insufficiently understood. In this study, we visualized the coordination adsorption process of water molecules in MOF-74-Co crystals by combining X-ray absorption fine structure (XAFS) spectroscopy with three-dimensional X-ray computed tomography (XAFS-CT) imaging. In situ 3D-XANES data obtained under controlled relative pressures for the first time revealed a heterogeneous distribution in which coordination adsorption on Co sites proceeds preferentially in the crystal core while being delayed at the edges. Furthermore, microbeam X-ray diffraction measurements demonstrated that this adsorption heterogeneity originates from local variations in crystallinity. These findings elucidate the competitive behavior between coordination and physisorption within the pores and the spatial dependence of adsorption inside the crystal, providing a new methodology for understanding the adsorption properties of MOFs.