A Flexible, Perfluorinated Analog of Aluminum Fumarate Metal-Organic Framework

Herein, we report the synthesis of Al-TFS, a novel aluminum metal-organic framework (MOF) based on tetrafluorosuccinic acid (H₂TFS), of formula Al(OH)(TFS)·1.5H₂O, introducing a new member to the family of perfluorinated MOFs. The structure of the MOF, solved ab-initio from laboratory powder X-ray diffraction data, displays analogies with that of the commercially available Al-fumarate (Basolite A520). The structure is composed of 1D infinite OH-bridged AlO₆ octahedra chains connected by the dicarboxylic linkers, designing rhombic channels decorated by fluorine atoms. Upon water removal, the MOF undergoes a phase transition leading to a moderate expansion of the unit cell. Volumetric analysis revealed the presence of ultra-micropores with a size lower than 4 Å. Gas sorption measurements demonstrated for Al-TFS a slightly higher CO₂ selectivity compared to N₂ and CH₄ than the Al-fumarate analog, with peculiar shapes of the isotherms suggesting a dynamic response of the framework to CO₂ adsorption. Using different complementary techniques (in situ infrared spectroscopy, powder X-ray diffraction, solid-state nuclear magnetic resonance spectroscopy, gas/vapor sorption), and density functional theory simulations, the flexibility of the new MOF was disclosed, highlighting the significant impact of fluorination in tailoring materials with structural flexibility and peculiar adsorption properties.