Cooperative CO 2 adsorption mechanism in a perfluorinated Ce IV -based metal organic framework.

Adsorbents able to uptake large amounts of gases within a narrow range of pressure, i.e. , phase-change adsorbents, are emerging as highly interesting systems to achieve excellent gas separation performances with little energy input for regeneration. A recently discovered phase-change metal-organic framework (MOF) adsorbent is F4_MIL-140A(Ce), based on Ce IV and tetrafluoroterephthalate. This MOF displays a non-hysteretic step-shaped CO 2 adsorption isotherm, reaching saturation in conditions of temperature and pressure compatible with real life application in post-combustion carbon capture, biogas upgrading and acetylene purification. Such peculiar behaviour is responsible for the exceptional CO 2 /N 2 selectivity and reverse CO 2 /C 2 H 2 selectivity of F4_MIL-140A(Ce). Here, we combine data obtained from a wide pool of characterisation techniques - namely gas sorption analysis, in situ infrared spectroscopy, in situ powder X-ray diffraction, in situ X-ray absorption spectroscopy, multinuclear solid state nuclear magnetic resonance spectroscopy and adsorption microcalorimetry - with periodic density functional theory simulations to provide evidence for the existence of a unique cooperative CO 2 adsorption mechanism in F4_MIL-140A(Ce). Such mechanism involves the concerted rotation of perfluorinated aromatic rings when a threshold partial pressure of CO 2 is reached, opening the gate towards an adsorption site where CO 2 interacts with both open metal sites and the fluorine atoms of the linker.