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 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). The removal of fluorine also results in a higher CH4 adsorption capacity and, therefore, in a progressive decrease of CO2/CH4 selectivity. In situ infrared spectroscopy using CO as a probe confirmed that CeIV sites are accessible in Fx_MIL-140A(Ce), but their Lewis acidity decreases at lower degrees of fluorination. Adsorption microcalorimetry revealed that the CeIV sites are accessible to CO2 even at low loading and that a higher degree of fluorination leads to a stronger interaction with CO2. 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, likely because of its ability to establish hydrogen-bonding like interactions with the linker. The collective evidence points towards reduced steric hindrance around the most favourable adsorption sites for CO2 as the main factor responsible for the absence of a cooperative adsorption mechanism in Fx_MIL-140A(Ce) MOFs.