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.