On the Role of Linker Fluorination in the Adsorption‐Induced Structural Response of Ce IV ‐Based Metal‐Organic Frameworks

Understanding fundamental aspects of the adsorption behavior of flexible metal-organic frameworks (MOFs) is key to developing improved sorbents for gas separations. The recently reported F4_MIL-140A(Ce) displays cooperative CO2 and H2O adsorption driven by concerted rotation of the aromatic rings of the tetrafluoroterephthalate linkers, giving rise to a step-shaped isotherm. Here, we shed light on the key role played by the degree of fluorination of the linker in such a mechanism by synthesizing novel Fx_MIL-140A(Ce) (x = 2 or 3) analogs and characterizing them by gas sorption analysis, in situ powder x-ray diffraction, solid-state nuclear magnetic resonance spectroscopy, in situ infrared spectroscopy, and adsorption microcalorimetry. We found that the cooperative CO2 adsorption mechanism is switched off in Fx_MIL-140A(Ce), leading to Langmuir-type CO2 isotherms. Different from F4_MIL-140A(Ce), CO2 adsorption triggers no structural response in Fx_MIL-140A(Ce), due to the reduced steric hindrance of less fluorinated linkers that make the CeIV open metal sites accessible already at low pressure, with no need for concerted aromatic ring rotation. In contrast, the adsorption of water induces similar cooperative structural rearrangements in Fx_MIL-140A(Ce) and the parent F4_MIL-140A(Ce), suggesting that auxiliary adsorbate-linker interactions play a role in inducing cooperative adsorption.