Switching Off Cooperative CO2 Adsorption in Fluorinated CeIV-based Metal-Organic Frameworks by Linker Engineering

Understanding fundamental aspects related to the adsorption behaviour of flexible Metal-Organic Frameworks (MOFs) is a key step to develop improved sorbents for gas separations. F4_MIL-140A(Ce), a recently discovered MOF, displays a cooperative CO2 adsorption mechanism 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 organic linker in such mechanism by synthesising novel Fx_MIL-140A(Ce) (x = 2 or 3) analogues and characterising them with a wide range of experimental techniques, including in situ powder X-ray diffraction, solid state nuclear magnetic resonance spectroscopy, gas sorption analysis, in situ infrared spectroscopy and adsorption microcalorimetry. We found that reducing the degree of fluorination of the linker switches off the cooperative adsorption mechanism observed in the perfluorinated prototype, leading to conventional Langmuir-type CO2 isotherms for Fx_MIL-140A(Ce) and lower CO2/CH4 selectivity. No structural response is triggered by CO2 adsorption in Fx_MIL-140A(Ce), despite the availability of similar adsorption sites to those found in perfluorinated F4_MIL-140A(Ce), i.e., CeIV open metal sites generated upon removal of the coordinated water found in the as-synthesised MOFs. Reduced steric hindrance around these sites in Fx_MIL-140A(Ce) MOFs makes them accessible already at low pressure, thus bypassing the concerted ring rotation underpinning the cooperative CO2 adsorption mechanism.