Rare-earth (RE) 2,5-dihydroxyterephthalic acid (DOBDC) based metal-organic frameworks (MOFs) are promising candidates for acid gases separation and adsorption from flue gas streams, which requires a robust and stable structure. The RE clusters in these MOF structures are typically reported to be bonded by hydroxyl (OH) groups, but recent data has indicated that fluorine (F) may also be present in the RE clusters. Here, we have used periodic density-functional theory (DFT) to evaluate the thermodynamic stability of OH replacement by F in three different DOBDC MOFs. The results show that the introduction of F to RE-DOBDCs generally stabilizes the structure, but that the stabilization is site dependent. For crystals with multiple metal sites, F stabilizes the crystal at impurity levels in the metal cluster or constitutes greater than half of the RE bridge bonds. Non-uniform F addition to the structure can also result in crystal strain, making the structure less stable. The impact on the calculated IR spectra of the RE-DOBDC following F replacement is also reported as a potential signature for identification of F in RE-DOBDC metal centers. Additionally, solid-state nuclear-magnetic resonance (MAS-NMR) was used to study the presence of F in the experimentally synthesized RE-DOBDC systems. Ultimately, replacement of the OH by F in the metal cluster had minimal impact on gas binding (NO2, H2O, SO2) to the metal center. Therefore, the presence of F impurities in RE-DOBDC MOFs should not affect their use in acid gas separation applications.

Stabilization of rare earth (RE) DOBDC MOFs by fluorine

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