Small-Pore Gallates MOFs for Environmental Applications: Sorption Behaviors and Structural Elucidation of Their High Affinity for CO₂
2020
Heymans, Nicolas | Bourrelly, Sandrine | Normand, Perine | Bloch, Emily | Mkhadder, Hassan | Cooper, Lucy | Gorman, Martin | Bouzidi, Intissar | Guillou, Nathalie | De Weireld, Guy | Devic, Thomas
The ability of two structurally related small-pore metal–organic frameworks (MOFs) to capture CO₂ was investigated by a combination of gas sorption measurements, X-ray powder diffraction (XRPD) analysis, microcalorimetry experiments, and ideal adsorbed solution theory (IAST) calculations. The title solids, formulated Mg(H₂gal) and Fe(Hgal) (H₄gal = gallic acid), are made of a naturally occurring ligand and were both prepared on the multigram scale under mild conditions. They both present very similar structures, with identical channels of ca 3.5 Å diameter, but present different amounts of acidic protons on the surface of the pores. These compounds were found to be extremely hydrophilic and exhibit moderate stability toward the water. While their ability to adsorb CH₄ and N₂ is very limited, they both adsorb significant amounts of CO₂ even at atmospheric pressure (3 and 5 mmol g–¹ at 303 K for the Fe and Mg derivatives, respectively). As a consequence, these compounds present high CO₂/N₂ and CO₂/CH₄ selectivities (380–910 and 190–460, respectively) together with good working capacities, making them of interest for the capture of CO₂ from flue gas or for landfill gas upgrading in pressure swing adsorption or vacuum swing adsorption processes. Finally, the analysis of the Mg derivative by X-ray diffraction (XRD) and adsorption microcalorimetry revealed that its high affinity for CO₂ relies on a strong and specific site of adsorption involving double hydrogen bonds between the CO₂ molecules and the acidic protons of the framework.
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