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 characterisation 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-1 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.