Linker Engineering in Fluorinated CeIV-based Metal-Organic Frameworks: Impact on Cooperative CO2 Adsorption and Biogas Upgrading Potential

Understanding fundamental aspects related to the adsorption behaviour of flexible metal-organic frameworks (MOFs) is a crucial step to develop improved sorbents for gas separations. F4_MIL-140A(Ce) is a recently discovered MOF that displays a cooperative CO2 adsorption mechanism enabled 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 a cooperative mechanism by synthesising novel Fx_MIL-140A(Ce) (where x = 2 or 3) analogues and characterising them with a wide range of experimental techniques, including powder X-ray diffraction, thermogravimetric analysis, solid state nuclear magnetic resonance spectroscopy, gas sorption analysis, in situ infrared spectroscopy and adsorption microcalorimetry. We found that the removal of fluorine atoms from the linker switches off the cooperative mechanism observed in the perfluorinated prototype, leading to Langmuir-type CO2 adsorption isotherms for Fx_MIL-140A(Ce). This is accompanied to a higher CH4 adsorption capacity, which results in a progressive decrease in CO2/CH4 selectivity. Desorption of the crystallisation water coordinated to CeIV from Fx_MIL-140A(Ce) appears to induce a similar phase transition to that observed in F4_MIL-140A(Ce), suggesting that a cooperative mechanism persists when water is the adsorbate. The accessibility of CeIV sites in evacuated Fx_MIL-140A(Ce) is confirmed, although their Lewis acidity decreases at lower degrees of fluorination, due to the reduced electron-withdrawing character of the aromatic ring.