Water-Based Synthesis and Enhanced CO₂ Capture Performance of Perfluorinated Cerium-Based Metal–Organic Frameworks with UiO-66 and MIL-140 Topology

Reaction of cerium ammonium nitrate and tetrafluoroterephthalic acid in water afforded two new metal–organic frameworks with UiO-66 [F4_UiO-66­(Ce)] and MIL-140 [F4_MIL-140A­(Ce)] topologies. The two compounds can be obtained in the same experimental conditions, just by varying the amount of acetic acid used as crystallization modulator in the synthesis. Both F4_UiO-66­(Ce) and F4_MIL-140A­(Ce) feature pores with size <8 Å, which classifies them as ultramicroporous. Combination of X-ray photoelectron spectroscopy and magnetic susceptibility measurements revealed that both compounds contain a small amount of Ce­(III), which is preferentially accumulated near the surface of the crystallites. The CO₂ sorption properties of F4_UiO-66­(Ce) and F4_MIL-140A­(Ce) were investigated, finding that they perform better than their Zr-based analogues. F4_MIL-140A­(Ce) displays an unusual S-shaped isotherm with steep uptake increase at pressure <0.2 bar at 298 K. This makes F4_MIL-140A­(Ce) exceptionally selective for CO₂ over N₂: the calculated selectivity, according to the ideal adsorbed solution theory for a 0.15:0.85 mixture at 1 bar and 293 K, is higher than 1900, among the highest ever reported for metal–organic frameworks. The calculated isosteric heat of CO₂ adsorption is in the range of 38–40 kJ mol^–1, indicating a strong physisorptive character.