Metal-Dependent Adsorption Mechanism in Perfluorinated MIL-140A Metal-Organic Frameworks

Perfluorinated metal-organic frameworks (MOFs) with MIL-140A topology display metal-dependent adsorption behavior: while the Ce^IV analogue [F4_MIL-140A-(Ce)] exhibits step-shaped CO₂ adsorption isotherms and strong affinity for H₂O, both associated with a concerted structural rearrangement of the linkers, the Zr^IV counterpart [F4_MIL-140A-(Zr)] shows a classical type I CO₂ isotherm with a lower adsorption capacity and only weak affinity for H₂O. A multitechnique approach, including solid-state nuclear magnetic resonance spectroscopy, in situ infrared spectroscopy, and variable temperature powder X-ray diffraction, provided clear evidence that this contrasting behavior arises from the absence of an open metal site on Zr^IV and the inability of Zr^IV to adopt an octacoordinated geometry, which prevent the establishment of strong metal-guest interactions.