Chemical Science Research Frontiers 2024 74 Metal-organic frameworks (MOFs) are crystalline solids with nanosized pores, formed by infinite linkages between metal ions and organic ligands. Their design flexibility, ease of synthesis, and periodic structures make them ideal adsorbents. Understanding MOF adsorption behavior is crucial for developing new functionalities. While X-ray diffraction (XRD) has clarified molecular-level behaviors within unit cells, meso- and macro-scale properties in MOF crystals remain unresolved. Meanwhile, studying transient states during adsorption, including surface adsorption, diffusion, and pore filling, is particularly challenging as XRD averages spatial and temporal information, making it unsuitable for capturing local heterogeneities. Recent advances in hard X-ray imaging, such as XAFS-CT, enable 3D visualization of mesoscale behaviors and local chemical states, revealing defect structures in MOFs [1]. However, studies so far have focused on static heterogeneities, failing to capture dynamic adsorption processes. Here, we use 3D XAFS- CT imaging to visualize adsorption distribution across entire single MOF-74-Co crystals [2]. For XAFS-CT measurements, MOF-74-Co, featuring hexagonal one-dimensional pores and exposed metal sites, was selected (Fig. 1(a)) [3]. Pillar-shaped single crystals were prepared, treated with methanol, and desolvated under vacuum. Bulk crystallinity and porosity were confirmed by powder XRD and nitrogen adsorption isotherms at 77 K. The water adsorption isotherm at 298 K showed a stepwise profile, with a saturated capacity of 4.85 H 2 O molecules per Co ion, consistent with reported structures (Fig. 1(b)). Isosteric heat of adsorption indicated strong host– guest interactions for up to one H 2 O molecule per Co ion. Co K -edge XAFS spectra, sensitive to hydration, revealed a significant increase in white-line height at 7717 eV during hydration (Fig. 1(c)). In situ XAFS-CT measurements were conducted under controlled water vapor pressures at SPring-8 BL36XU (Fig. 2). Crystals in a glass capillary connected to a vacuum/vapor control line were scanned around the Co K -edge. 2D transmission images were captured at 181 angles in 1° increments. Original 2048 × 2048 pixel images were binned to 512 × 512 to enhance the signal-to-noise ratio. XAFS spectra were analyzed via linear combination fitting (LCF) using dehydrated and hydrated Co spectra as references. Reconstructed 3D images provided voxel-level Co density and hydration ratio (hydrated Co/all Co) information with a voxel resolution of 0.995 μ m³ (Fig. 3). Co density, proportional to the number of Co ions per voxel, remained constant regardless of vapor pressure, but inhomogeneity was observed at the tapered edges, linked to micro-displacements during adsorption/ desorption. This inhomogeneity corresponded to local crystallinity disorders. Figure 3(c) shows 3D maps of Co hydration ratio across the particle. At low vapor pressure, the crystal Unveiling adsorption distribution in a metal–organic framework crystal with 3D XAFS-CT mapping Fig. 1. (a) Crystal structures of MOF-74-Co in the hydrated and dehydrated states. (b) H 2 O adsorption isotherms of bulk MOF-74-Co at 298 K (blue plot). P 1 – P 9 on the upper axis indicate the relative pressures at which the XAFS- CT measurements were carried out. The red plot is the coordination adsorption isotherm of the Co ion, derived from XAFS-CT analyses. (c) Co K -edge XAFS spectra of MOF-74-Co in the hydrated and dehydrated states. [2] (a) (b) (c) Hydrated Co II Dehydrated –H 2O +H 2O 6 coordination b c a Octahedral 5 coordination Square-pyramidal b c a P/P 0 Energy (eV) Total Adsorption Amount of H 2 O/Co Coordinated H 2 O/Co Normalized t μ μ 0.0001 0 1 2 3 4 5 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1 2 3 4 5 P 1 P 2 P 3 P 4 P 5 P 6 P 7 P 8 P 9 0.001 0.01 0.1 1 7600 7650 7700 7750 7800 7850 7900 Hydrated Dehydrated Research Frontiers 2024 75 appeared entirely blue, indicating minimal hydration. At P 2 , water molecules began coordinating with Co, significantly changing the hydration distribution. By P 3 , red regions emerged at the center. From P 4 to P 7 , hydration distribution stabilized, and near saturation pressure, the particle turned red or pale pink, indicating saturation at exposed Co sites. The histograms quantitatively illustrate H 2 O coordination within the particle. The mean hydration ratio across voxels represents the average H 2 O coordinatively adsorbed per Co ion. Plotted with the bulk isotherm (Fig. 1(b)), it revealed competition between coordination and non-coordination adsorption. Between P 1 and P 4 , coordinated H 2 O increased gradually but remained at 0.5 H 2 O/Co by P 3 , even as total adsorption reached 2.0 H 2 O/Co. From P 4 to P 7 , bulk adsorption rose rapidly to 4.0 H 2 O/Co, driven by H 2 O–H 2 O interactions, while coordination adsorption stayed constant. Eventually, both coordination and physisorption saturated, completing hydration. Microbeam X-ray diffraction at SPring-8 BL40XU confirmed single crystallinity throughout the particle, with consistent lattice parameters matching reported MOF-74-Co structures. However, diffraction spots at the edges were broader than those at the center, indicating reduced crystallinity and increased orientational disorder. The mosaicity parameter at the edge was larger than at the edge, highlighting significant local differences while preserving the overall structure. In conclusion, 3D visualization of Co hydration revealed heterogeneous H 2 O affinity within MOF-74- Co crystals, particularly at the edges where reduced crystallinity affected adsorption. These findings emphasize the importance of local crystallinity in determining adsorption properties. While ideal particles exhibit uniform adsorption, XAFS-CT imaging and microbeam diffraction captured true heterogeneities, offering critical insights for optimizing MOFs as functional materials. Hirotoshi Sakamoto Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University Email: sakamoto.hirotoshi.2k@kyoto-u.ac.jp References [1] D. Ferreira Sanchez et al. : Angew. Chem. Int. Ed. 60 (2021) 10032. [2] E. Yamada, H. Sakamoto, H. Matsui, T. Uruga, K. Sugimoto, M.-Q. Ha, H.-C. Dam, R. Matsuda, M. Tada: J. Am. Chem. Soc. 146 (2024) 9181. [3] P. D. C. Dietzel et al. : Angew. Chem. Int. Ed. 44 (2005) 6354. Fig. 3. (a) X-ray absorbance image of MOF-74-Co crystals in a capillary with a 100 μ m scale bar. (b) 3D reconstructed image of Co density of the particle, showing the voxel coordinates and the cross sections of interest. (c) 3D distribution map of the Co hydration ratio with voxel transparency of 75%. Below are the histograms of relative frequencies of the voxels classified with the Co hydration ratio in each voxel. The values in the histograms are the mean values of the Co hydration ratio. [2] Fig. 2. Schematic illustration of the XAFS-CT method to obtain three-dimensional spatial distributions of adsorption states in a single crystalline particle. [2] Projection angle, i θ 0 θ i θ Sample X-ray Rotation Vapor / vacuum Imaging unit control j θ 180 θ j-1 θ X-ray energy XAFS spectrum on each pixel Parameter extraction Parameter image CT reconstruction with different i 181 images θ 3D map of the parameter (a) (c) (b) x (46 voxels) x 1 x 23 x 46 z 163 z 91 z 20 z (182 voxels) z 1 z 182 P 9 P 8 P 7 P 6 P 5 P 4 P 3 P 2 P 1 46 μ m 46 μ m 181 μ m 46 μ m 181 μ m 0 0.809 0.727 0.600 0.592 0.587 0.576 0.541 0.381 0.100 0 0 0.4 0.5 1.0 Mean: 1 Ratio of hydrated Co (voxel) Ratio of hydrated Co
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