Tuning flexibility in Metal-Organic Frameworks via linker per-fluorination: revisiting the adsorption-induced breathing of MIL-53(Al)

We present the first comprehensive investigation of the complex, multi-step adsorption-induced breathing behaviour of F4-MIL-53(Al), the recently discovered analogue of MIL-53(Al) with a per-fluorinated linker. Through a systematic characterization approach performed by combining in situ Powder X-ray Diffraction (PXRD), in situ Fourier Transform Infrared (FTIR) and Solid State Nuclear Magnetic Resonance (SS-NMR) spectroscopies with sorption analyses, we unveil the impact of fluorination on framework flexibility, adsorption properties, and phase transitions, offering fresh perspectives into the structure–property relationships governing metal-organic framework (MOF) dynamic porosity. Compared to the non-fluorinated MIL-53(Al), F₄-MIL-53(Al) exhibits a different water affinity, with uptake remaining below 1 mmol/g up to 60% relative humidity. Above this threshold, PXRD reveals a two-step expansion of the F₄-MIL-53(Al) unit cell, contrasting the typical contraction observed in MIL-53(Al). Volumetric CO2 adsorption at different temperatures displays non-hysteretic step-shaped isotherms for F4-MIL-53(Al), generated by a CO₂-induced structural expansion also confirmed by in situ PXRD analysis. These findings highlight the crucial role of fluorination in tuning host–guest interactions, modifying water affinity while preserving and revisiting dynamic porosity and, more broadly, provide new insights into the molecular-level design of responsive fluorinated MOFs for gas separation and storage.