Probing\nAdsorption Interactions in Metal–Organic\nFrameworks using X‑ray Spectroscopy

Abstract

We explore the local electronic signatures\nof molecular adsorption\nat coordinatively unsaturated binding sites in the metal–organic\nframework Mg-MOF-74 using X-ray spectroscopy and first-principles\ncalculations. In situ measurements at the Mg K-edge reveal distinct\npre-edge absorption features associated with the unique, open coordination\nof the Mg sites which are suppressed upon adsorption of CO₂ and <i>N</i>,<i>N</i>′-dimethylformamide.\nDensity functional theory shows that these spectral changes arise\nfrom modifications of local symmetry around the Mg sites upon gas\nuptake and are strongly dependent on the metal–adsorbate binding\nstrength. The expanded MOF Mg₂(dobpdc) displays the same\nbehavior upon adsorption of CO₂ and <i>N</i>,<i>N</i>′-dimethylethylenediamine. Similar sensitivity to\nlocal symmetry is expected for any open metal site, making X-ray spectroscopy\nan ideal tool for examining adsorption in such MOFs. Qualitative agreement\nbetween ambient-temperature experimental and 0 K theoretical spectra\nis good, with minor discrepancies thought to result from framework\nvibrational motion.