Dramatic Enhancement of Rare-Earth Metal–Organic Framework Stability Via Metal Cluster Fluorination

Abstract

Rare-earth polynuclear metal–organic frameworks (RE-MOFs) have demonstrated high durability for caustic acid gas adsorption and separation based on gas adsorption to the metal clusters. The metal clusters in the RE-MOFs traditionally contain RE metals bound by μ 3 –OH groups connected via organic linkers. Recent studies have suggested that these hydroxyl groups could be replaced by fluorine atoms during synthesis that includes a fluorine-containing modulator. Here, a combined modeling and experimental study was undertaken to elucidate the role of metal cluster fluorination on the thermodynamic stability, structure, and gas adsorption properties of RE-MOFs. Through systematic density-functional theory calculations, fluorinated clusters were found to be thermodynamically more stable than hydroxylated clusters by up to 8–16 kJ/mol per atom for 100% fluorination. The extent of fluorination in the metal clusters was validated through a 19 F NMR characterization of 2,5-dihydroxyterepthalic acid (Y-DOBDC) MOF synthesized with a fluorine-containing modulator. 19 F magic-angle spinning NMR identified two primary peaks in the isotropic chemical shift (δ iso ) spectra located at −64.2 and −69.6 ppm, matching calculated 19 F NMR δ iso peaks at −63.0 and −70.0 ppm for fluorinated systems. Calculations also indicate that fluorination of the Y-DOBDC MOF had negligible effects on the acid gas (SO 2 , NO 2 , H 2 O) binding energies, which decreased by only ∼4 kJ/mol for the 100% fluorinated structure relative to the hydroxylated structure. Additionally, fluorination did not change the relative gas binding strengths (SO 2 > H 2 O > NO 2 ). Therefore, for the first time the presence of fluorine in the metal clusters was found to significantly stabilize RE-MOFs without changing their acid-gas adsorption properties.

Fluorine extraction from organofluorine molecules to make fluorinated clusters in yttrium MOFs

“…The presence of fluorine in metal clusters can also improve their thermodynamic stability compared to hydroxy bridged clusters. 56 Fluorinated metal clusters are hydrophobic and can improve the water stability of the MOFs. 57 The fluorine in the clusters may improve the framework interaction with CO 2 that can be exploited in separations.…”

Synthesis of Fluoro-Bridged Ho3+ and Gd3+ 1,3,5-Tris(4-carboxyphenyl)benzene Metal–Organic Frameworks from Perfluoroalkyl Substances

“…The fluorinated MOF of 2 (Ho ) with PFHxA as modulator is more stable than the oxo-bridged one, 2 (Ho) with H 2 O 2 . This was predicted by Christian et al, where the fluorinated clusters were calculated to be thermodynamically more stable than hydroxylated clusters by up to 8–16 kJ/mol per atom for 100% fluorination . The extraction of fluorine from PFHxA by RE ions is potentially an important discovery in the remediation of these organic pollutants.…”

The Effect of Ligand-to-Metal Energy Transfer on the Photooxidation Performance of an Isostructural Series of Pyrene-Based Rare-Earth Metal–Organic Frameworks

“…Fluorinated modulators are required to synthesize RE-MOFs with high nuclearlty cluster nodes, as the nodes have been found to contain a mixture of bridging -OH and -F ligands. 76,77 The conditions to obtain Y-, Tb-, Dy-and Ho-CU-10 are very similar in the required amounts of 2-FBA and acetic acid, however, less solvent and the addition of HNO3 is needed for the Gd-, Er-and Tm-CU-10 isostructures. Similarly, HNO3 is needed for Yb-and Lu-CU-10, and with a higher amount of 2-FBA.…”

“…80 The low intensity of the O-H stretching peak relative to other MOFs with hydroxo-bridged cluster nodes is likely due to the possibility of a mixture of bridging -OH and -F ligands in the nonanuclear and undecanuclear cluster of RE-CU-10. 76,77 Techniques such as 1 H-NMR spectroscopy (Figure S17-S24), ICP-MS (Table S2, Figure S4 S3).…”