Exploring the Fluorination and Hydroxylation of Pore-Space-Partitioned Metal−Organic Frameworks for C2H2/CH4 Separation

Abstract

We report three novel pore-space-partitioned metal−organic frameworks (MOFs) functionalized with fluorine and hydroxyl groups using 2,3,5,6-tetrafluorobenzene-1,4-dicarboxylic acid (F₄-BDC) and a new ligand 3,6-difluoro-2,5-dihydroxybenzene-1,4-dicarboxylic acid (F₂(OH)₂-BDC) as organic building blocks, with 1,3,5-tris(4-pyridyl)-2,4,6-triazine (TPT) as pore partition agent. With the polar fluorine and hydroxyl groups and the open metal sites being blocked by TPT, moderate molecule-framework interactions can be engineered. These three isoreticular microporous frameworks Mn-TPT-BDC-F₄ (NCKU-21), Mn-TPT-BDC-F₂(OH)₂ (NCKU-22), and Mg-TPT-BDC-F₂(OH)₂ (NCKU-23) (NCKU=National Cheng Kung University) exhibit distinct single-component gas adsorption behaviors. Although NCKU-22 uptakes a much lower amount of C₂H₂ compared to NCKU-21 and -23, dynamic breakthrough experiments show that these three materials are all capable of efficient C₂H₂/CH₄ separations. These MOFs possess moderate isosteric heat of adsorption for C₂H₂ (25.7–32.1 kJ mol⁻¹), allowing easy regeneration and energy-efficient C₂H₂/CH₄ separations.