Tuning CO₂ Selective Adsorption over N₂ and CH₄ in UiO-67 Analogues through Ligand Functionalization

Introducing functional groups into pores of metal–organic frameworks (MOFs) through ligand modification provides an efficacious approach for tuning gas adsorption and separation performances of this type of novel porous material. In this work, two UiO-67 analogues, [Zr₆O₄(OH)₄(FDCA)₆] (BUT-10) and [Zr₆O₄(OH)₄(DTDAO)₆] (BUT-11), with functionalized pore surfaces and high stability were synthesized from two functional ligands, 9-fluorenone-2,7-dicarboxylic acid (H₂FDCA) and dibenzo­[b,d]­thiophene-3,7-dicarboxylic acid 5,5-dioxide (H₂DTDAO), respectively, and structurally determined by single-crystal X-ray diffraction. Notwithstanding skeleton bend of the two ligands relative to the linear 4,4′-biphenyldicarboxylic acid in UiO-67, the two MOFs have structures similar to that of UiO-67, with only lowered symmetry in their frameworks. Attributed to these additional functional groups (carbonyl and sulfone, respectively) in the ligands, BUT-10 and -11 show enhanced CO₂ adsorption and separation selectivities over N₂ and CH₄, in spite of decreased pore sizes and surface areas compared with UiO-67. At 298 K and 1 atm, the CO₂ uptake is 22.9, 50.6, and 53.5 cm³/g, and the infinite dilution selectivities of CO₂/CH₄ are 2.7, 5.1, and 9.0 and those of CO₂/N₂ are 9.4, 18.6, and 31.5 for UiO-67, BUT-10, and BUT-11, respectively. The selectivities of CO₂/CH₄ and CO₂/N₂ are thus enhanced 1.9 and 2.0 times in BUT-10 and 3.3 and 3.4 times in BUT-11, respectively, on the basis of UiO-67. The adsorption mechanism of CO₂ in BUT-11 has also been explored through computational simulations. The results show that CO₂ molecules locate around the sulfone groups in pore surfaces of BUT-11, verifying at the molecular level that sulfone groups significantly increase the affinity toward CO₂ molecules of the framework. This provides thus an efficient strategy for the design of CO₂ capture materials.