Development of new fluorinated Cerium based Metal-Organic Frameworks (MOFs) with MIL-140A topology as solid adsorbents for biogas upgrading

Biogas upgrading involves CO2 removal from raw biogas to produce biomethane, a sustainable biofuel well aligned with international climate goals. Therefore, the development of efficient and cost-effective CO2 separation technologies to scale-up the production of biomethane is imperative [1]. In this context, an appealing candidate for a real life application in carbon capture is F4_MIL - 140A(Ce), an ultramicroporous Metal-Organic Framework (MOF) based on Ce(IV) and tetrafluoroterephthalic acid as organic linker. This system exhibits a non-hysteretic step-shaped CO2 adsorption isotherm, characterized by a steep uptake increase at low pressure (0.2 bar) at 298 K due to a phase-transition occurring upon CO2 adsorption, whose molecular origin is attributed to a cooperative CO2 mechanism that involves the concerted rotation of fluorinated aromatic rings [2,3]. To better understand the influence of fluorination of ligands (in terms of symmetry, steric hindrance and fluorine functionalization degree) on the step-shaped adsorption isotherm of F4- MIL_140A(Ce), we herein report a ligand engineering approach targeting the MIL-140A(Ce) topology (MIL stands for Materials of Institute Lavoisier) and involving terephthalic linkers with different degree of fluorination and isomerism. To this end, we synthesised novel Fx-MIL_140A(Ce) MOFs by means of both an acetonitrile-based solvothermal synthesis and a milder methanol/water mixed solvent approach. The former route led to highly crystalline MOFs with the major drawbacks of the presence of unreacted linkers and fluoride ions trapped in the pores (suggesting partial decomposition of the ligand during synthesis), while the latter allowed clean and phase-pure materials to be obtained, although with lower crystallinity. The MOFs were characterized by solid-state nuclear magnetic resonance techniques that shed light on their local structure, as well as gas adsorption measurements. No step-shaped CO2 isotherm was observed, unravelling a strong relationship between the fluorination degree of the linker and the adsorption behaviour of the resulting materials. These results open the way to deeper experimental and computational investigations into structure-property relationship which will guide the design of such advanced materials and rationalize their separation performance. References: [1] R. Murano, et al. Energies, 2431, 1-14 (2021). [2] R. D'Amato, et al. ACS Sustainable Chem. Eng., 7, 394-402 (2019). [3] M. Cavallo, et al. J. Mater Chem. A., 11, 5568-5583 (2023).