X-ray multimodal computed tomography of monolithic HKUST-1 and UiO-66

Abstract

We report the multi-dimensional structural studies of monolithic metal-organic framework (MOF) materials using synchrotron X-ray multimodal computed tomography (CT). While traditional MOFs are typically obtained as microcrystalline powders, monolithic MOFs exhibit densification of phases, making them macroscopic standalone architectures. Being continuous binder-free solids, these architectures with mm- to cm-scale morphologies retain the high surface area and periodicity of their powder counterparts and hold promise for translating MOFs into functional technologies, such as those for carbon or phosphorus capture. The structural nature underpinning the formation of these bulk solids is still unknown. Diffraction/scattering computed tomography (DS-CT) is a multi-dimensional imaging technique that transcends the limitations of traditional absorption-based computed tomography by leveraging X-ray scattering to spatially map the chemical and structural identity of a material. Here, DS-CT experiments based on X-ray diffraction and pair distribution function, in tandem with X-ray fluorescence CT and absorption contrast CT, are used to study the monolithic forms of the prototypal MOFs, namely, HKUST-1, {[Cu3(1,3,5-benzenetricarboxylate)2]}n, and UiO-66, {[Zr6O4(OH)4(1,4-benzenedicarboxylate)6]}n. Furthermore, we demonstrate I2-contrast CT, wherein adsorbed I2 vapor acts as a probe molecule to highlight the changes in porosity across the bulk solids. Their properties are contextualized for their potential application in phosphorus removal from water.