ABSTRACT 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 F4MIL‐140A (Ce) displays cooperative CO 2 and H 2 O 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 F x 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 CO 2 adsorption mechanism is switched off in F x MIL‐140A (Ce), leading to Langmuir‐type CO 2 isotherms. Different from F4MIL‐140A (Ce), CO 2 adsorption triggers no structural response in F x MIL‐140A (Ce), due to the reduced steric hindrance of less fluorinated linkers that make the Ce IV 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 F x MIL‐140A (Ce) and the parent F4MIL‐140A (Ce), suggesting that auxiliary adsorbate‐linker interactions play a role in inducing cooperative adsorption.
Nerli et al. (Sun,) studied this question.