Switchable cooperative CO adsorption in multivariate fluorinated MIL-140A(Ce) metal-organic frameworks

Abstract

The multivariate approach allows to achieve a controlled degree of structural heterogeneity within metal-organic frameworks (MOFs), offering a powerful means to finely tune the properties of the framework at the molecular level. In this work, we apply this concept to the MIL-140A(Ce) framework to systematically introduce multiple fluorinated terephthalate linkers (Fx-BDC2-), enabling precise modulation of the cooperative CO2 adsorption behaviour through the control of the framework's degree of fluorination. By simultaneously incorporating and varying the nature and content of Fx-BDC2- co-linkers (x = 1, 2, 3) while maintaining F4-BDC2- as a fixed component, we synthesised a large library of mixed-linker F4:Fx_MIL-140A(Ce) materials, whose nature of solid solutions was demonstrated. Across the series, we found that cooperative CO2 adsorption occurs above a threshold degree of fluorination of the framework of 75%, independently of fluorine positional isomerism, as demonstrated by the series of o-, m-, and pF2-BDC2- co-linkers, and is not determined by the content of the perfluorinated linker. The implication of the fluorination threshold on CO2/CH4 selectivity was also assessed, revealing that the onset of CO2-induced cooperative adsorption above this threshold is associated with a dramatic increase in selectivity towards CO2. Since the degree of fluorination of single-linker MOFs is intrinsically fixed, the mixed-linker strategy represents an effective route to continuously tune the framework's composition, thereby enabling the establishment of a structural design principle to control the emergence of cooperative CO₂ adsorption.